As far as the game engine is concerned, "stationary" is relative to the... other comparatively big thing: the former big thing is moving very quickly around the latter, even if it looks stationary because it has a lower angular velocity
In that situation both portals (the inside and the outside window frame) have equal, but opposite velocities and therefore cancel out any momentum from the person.
When dealing with OP's scenario, only one portal is moving.
The problem with this analogy is that both sides of the window are at the same point in space so there is no pressure differential. What if the blue portal was at the bottom of the Mariana's Trench? What force displaces the water to place the cube on the other side?
It wouldn't. Same as if the other side of the portal was a brick wall. It wouldn't go through or would be smooshed by whatever force is driving the portal.
I guess the odd thing is, if the orange portal can be moved, the pressure from the brick wall would be directed at the attachment point of the orange portal, otherwise why wouldn't the orange portal keep moving, since it is not "touching" anything?
If the blue portal was deep under water, the pressure would force the water out the orange portal.
For anyone who doesn't know, Optozorax on YouTube has made a finite element analysis simulation of gravity ontop of his portal simulation so we can get an answer to the question. The answer is both. Both happen depending on how fast the portal is moving and the where the second portal is placed in the gravity well.
Option B. Portals already break a couple of laws of physics and can reverse entropy, but I like to think you should look at the speed of an object relative to the portal.
Frame of reference. If a person is hit by a train, from their perspective, they were standing still and the train was moving. But a person in the train can stand up and walk around without difficulty, they can stand on one foot and balance, they can jump up and down and not go flying away; so they are standing still from the trains perspective, it's everyone and everything outside the train that's moving.
What happens isn't a transfer of momentum or energy or velocity; it's a change in the frame of reference. That can't happen in real life and breaks a lot of different laws of physics, so people are having trouble wrapping their brain around it.
The portals are a frame of reference. Both the IN and OUT portals have their own, separate frame of reference. The player's reference doesn't actually factor into it. When you go through a portal, you are switching from the IN frame of reference, to the OUT frame of reference. And in this situation, the IN frame of reference has you flying toward the portal at whatever speed it's approaching you.
If anything, it's rapid exit from the portal would cause an increase in pressure on the top of the cube, causing it to stay in place. The cube still isn't moving regardless of the velocity relative to the moving platform.
At what point in time would the cube start moving away from the platform it's sitting on, and what forces act on the cube?
Also shit changes frame of reference all the time irl, and portals a could be thought of as wormholes, which which could theoretically exist IRL.
Alright, since you didn't like my last explanation for some reason, let me try again.
You know how when you stick your hand out of a moving car, you feel a massive rush of wind, even if it's not windy out? Ok, so imagine sticking a portal out of a moving car, what do you think happens to the air? Does it come to a dead stop after it exits the portal? Why not? The air itself wasn't moving, only the car with the portal was.
Solids in this case will work the same way. It doesn't matter if a solid is moving toward the portal, or the portal is moving toward it, what matters is the relative velocity.
From the orange portals frame of reference, the cube, and everything else in the world, is rushing toward it. So the cube (as well as the entire world around it) has a velocity relative to the orange portal.
But only the cube goes through the portal, the rest of the world does not come with it. At that point, the frame of reference changes. You can think of it like this: The cube has an velocity of X with respect to reference frame Y; once it leaves the portal, it keeps that velocity X but the reference frame changes from Y to Z. The velocity doesn't disappear.
Portal 2 (story mode) only has laterally moving surfaces, that inherently don't cause things to enter the portal. The moon portal is stationary in the engine.
Yeah, but those are moving from side to side. To my recollection, you never see a portal in a wall that’s moving in a direction that’s perpendicular to the plane it’s in.
The speed of one thing is the same speed of the other if you change the reference point. So the cube has momentum but the moving platform does not. It’s more about having a velocity in respect to the portal it’s going in or coming out of or an average of both. Like when you jump into a portal, you come out faster on the other side. Now what if that out portal was traveling at the same rate? Would you be traveling just outside the portal or double speed (in relation to the ground and static objects in the same room)?
Better rage bait answer: portals can't move relative to each other. If they do, the one that is moving 'pops' and disappears from the surface. So this scenario is impossible inside the parameters of the game, so it's not worth thinking about.
(The devs realized a lot of stuff could get horribly broken if portals could move, so they can't move)
I think they experimented and wanted them to be able to move in more places. But with engine limitations they only left it in one scripted place where there are no physics objects that can interact with it.
Even stationary portals break conservation of energy unless there's some unseen location changing energy to compensate, but yes, it gets more complicated with relative movement. In lore though, it is established that portals can move with hugely different velocities relative to each other at least with enough distance when you
::: spoiler spoiler
shoot one portal to the moon
:::.
Momentum/velocity is not an intrinsic property of an object, it is a relative property between two objects in a frame of reference.
Behind the yellow portal is the part of the universe where the blue portal is. The yellow portal, and therefore the space where the blue portal is, has momentum relative to the cube.
Put yourself in the frame of reference of the cube. You see the yellow portal come toward you. Imagine there are objects on the other side of that portal. You see those objects rush toward you through the portal. Why would the objects just stop dead when you came through the portal? (If they did just stop, it would be as though you hit a wall, and you wouldn't actually go through the portal at all.)
Agreed. Also consider the duration of the time that the portal is passing over the cube. It is moving at speed, the entire way through the portal and out the other side and then it just stops dead immediately once through? That would hardly be a conservation of momentum anyway from any frame of reference and does not make any intuitive sense.
This exactly. Consider an object that isn't a point mass (like the cube) going through. As the first atoms go through: if they just stopped relative to the portal boundary, that would suggest the next layer of atoms need to exert force on the first layer to push through. But portals in the game Portal do not have any resistance to objects passing through them.
I think the game treating portals as fuzzy holes in reality is more just a limitation or oversight in the physics engine. It completely ignores relativity and makes the portals no longer logically consistent and pointless to argue about. Velocity and angle conservation relative to the portal is established elsewhere though, so running with that gives you the "fly up" result.
To the space outside of the blue portal. Moving the orange portal doesn't drag the whole surroundings, so assuming both places are on the same planet, the momentum of the box in relations to both places is the same
Well, it doesn't in the only source of knowledge we have: the game Portal. I don't know enough about gravitational forces to have a coherent theory on it, gravitational waves moving through space-time is a bit beyound my real understanding. But in the game if you put a portal on a ceiling, it doesn't "suck" you until you're throuth it, so there it isolates gravity somehow.
If you stick to the premise that the portals exist in a universe with GR, then we can pay much know that space-time is locally flat in and around at least the middle parts of the portal. We know this because things don't get fucked up when they're going through.
Moving the orange portal kinda does ‘drag the whole surroundings’. It moves an entire universe.
The cube is stationary relative to the universe of the platform it’s on, but it has momentum relative to the universe through the portal. The question is which universe is the cube in? What is the boundary of the portal surface?
Portals aren't real and so work however you want them to, but people rarely consistently apply their models.
I'd say A given how they work in the games though. Only the object's momentum is conserved. The portal doesn't transfer momentum. There may be a slight breeze coming out of the blue end as the column of air in front of the portal-plate is pushed in, but the cube is just going to flop out.
The object's momentum is preserved. Movement of the entrance portal does nothing. Think of it like stepping though a door, except you're Buster Keaton and the door hole is falling on you.
There is no such thing as an absolute inertial frame of reference. If you are in free fall on earth, you would experience the same sensation as being in space. The only things that would tell you otherwise are the air resistance hitting you and what you can see.
So no, you would not be able to tell the universe is moving around you unless something else in the universe hit you.
The first part made sense, then you contradicted yourself at the end^^ If the universe moves around you, you could literally see things moving. It would be the same as you moving relative to everything else
Think about it more fundamentally. Instead of a person with senses, think of an object experiencing forces. Thats what an inertial reference frame means.
But senses are just ways for an object to experience forces.
I know if I'm hanging upside down because my senses are telling me my body is being pulled down from my head. I'm feeling the gravitational force pulling me down.
An object moving in the universe is indistinguishable to the universe moving around an object. To any observer or to the object itself the forces experienced are the same
well, in my opinion it is a bit of a paradox. i think you are partially correct.
your door analogy may be right from the perspective of the platform on which the cube is. however, one flaw is that the whole space is moving, not just the door. as soon as the cube enters that door, it will be inside that space, and if you consider the cube part of it, moving that space "downward", then due the cube being held by the platform, you could consider the cube moving "upward" in the space behind the orange portal. if you consider preserving momentum there, then as soon as the apparatus stops moving, the cube should not stop its momentum and move further up in the space.
or look at it like this: if you are behind the orange portal, meaning you are looking at the blue portal, you will see a cube moving towards you. then as soon as the platform hits the wall of the orange portal, the preserved momentum should make the cube "fly".
the change of perspective leads to two different conclusions, therefore i think it is a paradox.
the problem sort of is that there are two "spacetimes" that are kind of being changed. one could interpret it as both of these effects "pulling" against each other since when the cube has partially passed the portal, one part of the cube is "in one of the spacetimes", ant the other part in the other. ultimately though, the cube then would actually fly because in the limit (infinitesinally close to when the platform hits the portal wall) the cube is completely on the other side of the portal, so from the view of that spacetime, the cube does get pushed and should fly.
i don't even remember if it is possible in the video game for walls with portals that move. a video game would probablt have some kind of "center of mass", like a single vector of coordinates where describing the position of the object. there is no notion of "partially" being somewhere (except for collision boxes, but i don't mean that). so also from that view the box would fly. however, in most video games (with or without portals), if a platform moves up and suddenly stops, the object won't travel further in that direction anyway, especially not ulwards (even sideways usually not).
As for the portals moving in the game, portals are destroyed if the panel they're on is moved except for one specific part, where you can put a portal on a panel moving laterally to get a laser to cut the tubes connecting to the neurotoxin tank. That part needed to be explicitly programmed to allow it, and it's not possible without cheating to physically reach that portal.
Whats the difference between an object moving and everything else moving? For an observer on the sun every one of us has huge momentum which would be preserved when entering a portal.
Momentum is always relative to some frame of reference. Momentum 'toward' the portal would be conserved and mirrored through the other portal.
The difference in this case is that the object has 0 momentum and the portal doesn't impart its momentum to the cube because you can't collide with the portal. There is no force acting on the object to make it move.
That is not how either relativity or momentum work. The cube is still inert and the portal's movement would have no effect on it unless it has some acting force that pushes it when passing through. Even if the exit portal was moving, the cube would remain stationary unless it had nothing to rest upon and gravity pulled it down.
What if the portal on the moving platform had a wall right in front of it, so moving though the portal would cause the box to run into the wall. You'd have a scenario where the box wouldn't be able to move through the portal, but you'd have energy being generated by the moving arm "pushing down" on the box. The only way that could work is if the moving portal is transferring energy to the box, and therefore require option B.
Although that being said, portals break the laws of physics, so there probably can't be a "right" answer.
And if the blue portal is at a different angle than the orange portal, then… well, how fast is the milky way traveling through the galaxy relative to the universe?
But reference frame changes as the cube passes through the portal. The piston would feel resistance as its momentum is transferred to the cube to conserve it.
You would need the portal panties to make it work. Portal A would be on a wall, or whatever object you want to thrust towards, and Portal B would be on the panties aimed at your opening. When you thrust forward at portal A, it would exit Portal B, and since Portal B is stationary (panties are magical, yo) to your opening, the pillar would rise from the portal and enter the opening.
You could probably make it work with some surfaces that aren't facing each other, since technically genitals don't sit at 180 on the pelvis. It would probably take the porn star level of size though to make it work.
If we're only supposed to do ragebait answers, then anyone who passed 3rd grade science would be able to tell you the answer is obviously B
If you actually played the game, and paid attention in physics class in highschool, then you would know the answer is clearly A, and everyone who says it is B really needed the simplified explanation from GLaDOS to understand how to play the game.
Ragebait successful. College physics here. The only correct answer is B. You can only get A if the box is securely anchored to the pillar then released after the portal stops.
I love the games, but you can't trust a game physics engine to properly track what's going to happen with moving and especially accelerating portals. They (usually) track an entire object from one reference point against one absolute world grid.
Think of a portal as moving a plane of particles from one location to another. An entire object passing through a portal is just a bunch of successive planes being moved. One particle plane entering the portal plane on one side must displace the previous particle plane at the same rate on the exit side. Therefore, an object's speed through a portal must remain consistent relative to the portal on both sides, and with nothing to magically cancel that inertia, that motion will continue even after the object has completely left the portal.
GLaDOS specifically states "Momentum, a function of mass and velocity, is conserved between portals."
If scenario B was correct (which it is not) then if the portal was put on an object lighter than the box and then quickly moved over the box, the box would be launched at a speed equal to the speed of the lighter object, while it has more mass, this is not a conservation of momentum.
The best way to explain movement through portals is that momentum is only conserved from a perspective that encompasses all of the objects involved, this is true for regular physics as well actually. Consider this, what if the portal was shoved very quickly over the box down to it's halfway point? The obvious answer is that the box would only appear halfway out of the other portal, but if you subscribe to the "logic" that leads to answer B, then the box is actually sucked up by the portal, while the actual logical and intuitive answer is that only half the box sticks out of the portal, and it stays where it was placed.
Portals aren't objects, they don't have mass, or even a velocity, they are a shortcut in space, and that was what GLaDOS was explaining when she said that momentum was conserved. Although B seems to initially make sense, it is not correct from a game lore perspective, nor from an actual physical perspective.
It's not about whether portals have mass or not. It's about the box's velocity relative to the space on the other side of the portal. In the game, as you move toward a portal, you also move toward the objects on the other side of the portal. Therefore you have velocity relative to those objects, and that's the momentum that is conserved as you pass through.
This would suggest that portals in the game Portal are flat paintings, but it's not. As you move around or toward the portal, your distance and angle to the objects on the other side of the portal move in your frame of reference. Therefore you have velocity relative to those objects. Not to the portal itself, not to the universe behind the portal as a point mass, but to the spatially separated objects on the other side of thr portal individually.
This would suggest that portals in the game Portal are flat paintings
No, it doesn't imply that portal are flat paintings at all.
As you move around or toward the portal, your distance and angle to the objects on the other side of the portal move in your frame of reference.
Yes, but only technically so because those distances and angles change outside of the portal. Your path through the portals isn't real, it's a shortcut through space. This is why an object that is moved through a portal has momentum, but an object that a portal is moved over does not, the two scenarios are not the same when you have a frame of reference that includes the space on the other side of the portal. They are the same if you ignore that space in your frame of reference, but then any conclusion you come to about things outside that frame of reference are invalid.
Yes, but only technically so because those distances and angles change outside of the portal.
Ah, so you're saying Portals are not flat paintings, they're flat TV screens which project a representation of the universe behind them like based on where the observer is. Upon impact with an object they then store the objects momentum relative to the TV screen, teleport the object to another point in the universe, and then transfer the momentum to the teleported object. Got it!
One minor inconsistency is that in the Portal universe, objects transition smoothly through portals and can stop halfway through. But that's okay since we're just making shit up anyway.
This is why an object that is moved through a portal has momentum, but an object that a portal is moved over does not, the two scenarios are not the same when you have a frame of reference that includes the space on the other side of the portal.
Circular argument. You're using your preferred conclusion as evidence for your flawed reasoning.
They are the same if you ignore that space in your frame of reference, but then any conclusion you come to about things outside that frame of reference are invalid.
Velocity is a relative property between two objects. There's no "I'm moving toward the portal, therefore I have momentum" versus "The portal is coming toward me, therefore I have no momentum".
Lol. This will be fun. Glados' explanation is still correct, but momentum being conserved between portals (or "across portals" as I'd rather phrase it) is exactly what gets you answer B.
"A perspective that encompasses all of the objects involved" is not a thing in reality. No point in space cares about another point in space beyond the forces felt from that direction transmitted through spacetime, and the frame of reference following the left portal moving down and the frame of reference following the stationary right portal are both equally valid and moving relative to each other over the timeframe of the cube transitioning through the portal. I don't agree that the game is pushing a different model, but if it actually is, then that opens up so many (more) holes in physics that we may as well be debating magic.
I actually addressed the portal stopping halfway in another comment here which I won't bother to copy here, but the summary is that you shouldn't think of the box as one item but a mass of particles connected by internal forces. Your answer is indeed "obvious" and intuitive, but accelerating portals do funny things with frames of reference that means intuitive isn't always right and is the reason this topic always creates the disagreement it does.
As the portal comes down across the top half of the cube with constant relative velocity V and the appropriate momentum for said velocity, all those portaled particles will emerge from the other side of the portal with the same relative velocity V and same associated momentum compared to that side of the portal. Whether the particles maintain velocity relative to the portal because of something intrinsic to how space works at the portal boundary or because particles coming in forcibly displace particles going out in equal measure, the result is the same, fast in = fast out.
When the descending portal suddenly stops halfway, the particles already portaled through don't "know" this. They've still got the momentum of being sent through moments earlier reinforced by all the particles around them doing the same thing, so they "desire" to keep moving away from the portal. The particles of the bottom half of the box are similarly "ignorant" of all the shenanigans above and "want" to maintain the zero momentum they've had all along as measured by most of the objects in the scene.
This means the top and bottom halves of the box, despite never losing conservation of momentum, suddenly don't align with each other. The conflict of the top half "wanting" to keep moving away from the portal at velocity V and the bottom half "wanting" to stay where it is will resolve in one of two ways:
If the cube's internal forces are strong and "springy" enough to accommodate the disagreement, forces will propagate and equalize throughout the whole cube effectively averaging all "desired" motion out and the cube will fly out of the right portal at velocity ~1/2 V.
If the internal forces are not enough, the two halves of the cube will separate from each other, either cleanly at the portal boundary if the momentum disagreement massively overpowers internal forces or more messily and violently if it barely overpowers them. Obviously we don't have portals in real life to visualize this, but you can kind of get a feel for the effect through planetary bodies breaking up at Roche limits.
Edit: Wording the overall effect another way: Portals moving relative to each other effectively link two frames of reference to be one and the same over the portal boundary. If one portal changes velocity, the two frames of reference cease to align. This can cause conservation of momentum to appear to have been violated externally while actually being maintained in all the internal reference frames objects actually experienced.
perspective that encompasses all of the objects involved” is not a thing in reality.
Actually it is, that's exactly what a perspective, or refence is, it's either this, or you are arguing that there is a universal reference point. There isn't a universal reference point though, all motion is relative to other objects. The problem with using B as the result of this experiment is it draws a conclusion on the relative motion of the box, without considering the relative motion of everything on the other side of the portal. B only makes "sense" in a system where only the position and motion of the box and the orange portal are considered, any conclusion regarding motion outside this system are invalid when you use a perspective thar excludes everything except the box and the orange portal.
Maybe I'm misinterpreting your use of the phrase, but it seems you're arguing from applied Newtonian mechanics where objects define a reference frame. A reference frame in that sense is a useful convention but doesn't actually signify something in reality, hence my comment. Objects don't care about their velocity relative to other objects or about reference frames suddenly shifting as measured that way. An object's motion is encoded as an entirely local property in spacetime itself, AKA its worldline. This distinction between object-based reference frames and local reference frames is usually not worthwhile, because they're usually entirely consistent with each other. The convention just happens to break in situations where worldlines cease to be continuous and consistently differentiated, as in the case of portals.
An object’s motion is encoded as an entirely local property in spacetime itself, AKA its worldline.
Worldline is just a way to describe how an object or point moves in spacetime, since relatively makes everything kinda fucky and hard to follow using newtonian laws of motion, it does not appear to be a replacement for an inertial frame of reference.
Edit: by your argument of what a worldline is, then we would expect A to happen, since there is no motion component "encoded" in the worldline of the cube anyway.
Lol. Serious /uj question: Are you deliberately misreading me to do the rage bait thing? I'm using "worldline" in the first sense you gave. I'll avoid it and reword the actual main point as this is good practice for me anyway:
Local spacetime is the ultimate thing all velocity maps onto, so any examination of this problem needs to respect spacetime as the primary driver of behavior. Answer A ignores the spacetime the cube is being moved into on the exit side of the portal as a real thing it must maintain inertia within (the critical detail also overlooked in the "Buster Keaton" answers).
If momentum is conserved then neither A or B is correct, the box should be an infinity thin sheet. If the portal is half way through the box if it maintains it's thickness the box has to be moving at the speed of the portal out of it.
Except it's not moving out of the portal, the portal is moving over the box. I get it when people say that it doesn't matter if the portal is moving, or of the box is moving, and they are right, up to a point. As soon as you need to account for the movement on the other side of the portal, you have to include that space in your frame of reference, and relative to that space, the velocity of the box is 0.
As I explained here...
The best way to explain movement through portals is that momentum is only conserved from a perspective that encompasses all of the objects involved, this is true for regular physics as well actually.
I've played the game, you maintain speed whether you jump or fly in using your jet pack, no idea who this GlaDOS character is or what that funky cube is though. Abyss is the best map.
The question is: Is it the velocity of the object relative to the portal that matters or the velocity of the object in some other frame of reference?
In the later case, we must ask how the portals at different angles don’t send objects flying off at the speed of the milkyway’s travel through the universe.
The answer that requires the least assumptions and justifications is B.
In addition https://www.youtube.com/watch?v=DydIhwLrbMk which tries to make portals act "real" by treating them as spacetime with an unusual (but not mathematically disallowed) geometry.
Yes, they are fictional, so they behave however Valve (and fan works) want them to behave, but trying to bridge the gap between fictions and current physics models and be a fun and enlightening experience.
If we could test it, I think it would be helpful to put portals on either side of a very light, plastic disk in an attempt to see if it works like a hula hoop or not. If we drop the hoop over the cube, would the hoop:
Just land on top of the cube because the hoop has far less energy than a much more massive cube traveling at the same velocity?
Drop around the cube, such that the cube is just sitting on top of the exit portal?
Launch the cube into the air with an initial velocity equal to that of the hoop when the cube entered the portal? If so, where does all the additional energy required to accelerate the cube come from if the hoop's momentum is far less than the cube's momentum at the same velocity?
Since nobody seemingly posted link to this absolute ... Idk, gold? It's about actually simulating space(-time?) with portals:
https://youtube.com/@optozorax_en
I see offspec's comment is mentioning the guy, yeah.
Tldr answer is complicated.
...
Ah, ragebait answers only. Ugh, worm! Arrrr! I am so angry roar grrrr.
a.
while the portal travels down it collects air, increasing air pressure on the other side, meaning once it reaches the cube, the increased air pressure has mad the air solid so the cube cant fly through it.
The cube's mass is x (eg one gram) and its velocity is zero, therefore its momentum (m*v) is zero. Objects in motion tend to stay in motion, while objects at rest (e.g. momentum of zero), tend to stay at rest.
The thing which is moving, is the portal. At best, the event horizon of it reaches the leading-end then the trailing-end of the cube at the speed the portal is moving over the depth of the cube. If the portal moves faster, that distance is covered faster, and the full cube is entirely on the other end of the other portal faster [than a slower moving portal].
Since going through the portal is a frictionless and forceless action, the speed at which the portal fully envelops the cube would neither add nor remove momentum upon it. And if the cube did have its own momentum it would be maintained, but since it does not, the cube would not move until another force, like gravity, acted upon it.
I don't think this can work. Think through it in several steps rather than just one. Once the cube just started entering the portal, a part of it will already be on the other side, as close to the 2nd portal as possible. When the portal continues moving, more of the cube needs to be on the other side, so it "pushes" the first part of the cube further away. The cube imparts momentum onto itself.
Another way to look at it is using reference frames. It really doesn't make a difference whether the cube is moving towards the portal or the portal towards the cube. It only matters what the difference of velocity is. The difference of velocity must be maintained on the other side of the portal, and since the 2nd portal is stationary, the cube must move.
Saying "the cube imparts momentum onto itself" sounds like it would violate the law of conservation of energy. The 'energy can't be created or destroyed' theory, so where did it actually come from? Did the cube suck force going at one vector (the portal) and convert it into the opposite vector? That feels like it violates inertia. And since momentum is mass*velocity, that means the mass of the portal would play into the math changing the cube's own momentum. And I'd honestly assume the portal's mass is zero, which still maintains it wouldn't impart any energy onto the cube, right?
Secondly, regarding inertia from another standpoint, I have a thought experiment: let's say the portal is coming at the cube blazingly fast, then immediately stops halfway through enveloping the cube, so it's just chilling 50/50 on each side. By your statement of "it pushes the first part of the cube..." That would mean one half would have momentum at the same time the other half, not yet through the horizon, would still have none. Maybe so much so that (1) the cube would be pulled the rest of the way through even through the portal had stopped because half the atoms of the cube suddenly had momentum for some reason, or (2) the cube would rip in half (brittle cube I guess, but let's ignore the material considerations for simpliciry) because half the cube now suddenly has momentum while the other half still has none.
So I think I understand your perspective, but I can't yet get around what seems to be a few lingering problems. All the same, I'm enjoying the discussion, thanks net-neighbor!
Conservation of energy is already broken by portals. Consider the portals in a frictionless tube (it could be frictioned, but that would be less fun) that use the earth's gravity to accelerate the cube to the speed of light. If we are take the cube's effect on the earth as non-negligible, then the earth has also accelerated a significant bit in the 'upwards' direction that the tube has. Simply run to the other side of the earth, put the 'exit' portal aiming towards the same direction as the 'upwards' of the tube, in this case directly at the earth, and now you have accelerated the earth twice as much in that 'upwards' direction with no apparent gain or loss of energy (and also have obliterated the earth, but let's not talk about a cube's mass hitting the earth at c.
As for the portal suddenly stopping while moving blazingly fast, sure, it's a good quandary. It seems insane to think of the cube suddenly gaining momentum, but it's also insane to think that a particle (in this case the portion of the cube's atoms that are through the portal already) that is moving at speed somehow has no momentum.
A. Object momentum relative to the world is retained thru portals regardless of the speed of the portal surface or the portal itself. The portal is only transmitting matter from A to B, it does not impart energy of any kind.
If the portals don't impart energy, you wouldn't gain speed when you put one directly above another and drop through forever. The higher portal increases your potential energy.
What if the entire room were rigged to a piston, the orange portal were held still and the room forced upwards at speed. Would you see scenario B then?
If so, how do the portals know which to choose? Remember that neither portal is actually stationary: they're both orbiting the sun at 30km/s, etc.
Its not the portal doing that though. It's gravity. Your own momentum moves you through the portal while gravity is acting upon you; the portal is nothing but a doorway.
It doesn't matter what's "doing" it; the energy has to come from somewhere - gravity is not a source of free energy. In the real world, you can't build a perpetual motion machine using gravity. With portals, you could pour water through a pair of vertical portals and use it to drive a waterwheel, and the waterwheel could, say, drive a generator and power a light. It would keep going forever. The light requires energy to work - where is it coming from?
Without the portal, an object tossed down a mineshaft would accelerate until, assuming it were deep enough, it reached the centre of the earth. Then its potential energy would be zero, and it would stop accelerating - it would have reached its maximum kinetic energy and minimal potential energy, and start trading speed for height, until it reached a maximum height (at the other side of the world) and oscillated back and forth. If you tried to use this to drive a waterwheel and power a lightbulb, each time the object hit the wheel it would slow down. That reduction in speed would translate to a reduction in energy, and a reduction in maximum height, until eventually the object would stop being able to turn the wheel.
That doesn't happen in the portal scenario, because the portal moves objects up, increasing their potential energy, no matter what.
Neither. The cube is not making contact with the portal at all, since the portal is on a stationary block with some speedy lines drawn on it.
Speedy lines don't nake thingy go fast - everyone witb a gel-hairstyle knows that.
The answer is definitely B (see other comments), but I think the more interesting question is, how does it affect the portals themselves and the surfaces they're attached to?
When the portals are attached to a solid unmovable wall (as in the games), we can pretty much ignore the forces acting upon the wall as they will just be automatically cancelled out by the normal force. But the same does not apply for portals attached to detached panels. Let's say that instead of a piston pushing down, the portal is attached a free-falling panel with the portal facing down.
We are inclined to assume that as the portal falls onto the cube, it feels no force, as if it fell through air or through a vacuum. But is that really the case? Maybe instead, the falling portal experiences resistance from the cube? Or even, actually, an accelerating force?
Let's assume conservation of momentum at each end of the portal. That is, in the general case: if the portal (which we assume has no mass) is attached to an object with mass M and velocity V, and "swallows" a cube or other object with mass m and velocity v, and at the end of this interaction the portal-surface object with mass M has velocity V', then we expect:
M × V' = (M × V) + (m × v)
Mass is not conserved because the cube was transferred to the other portal.
Solve for V':
V' = V + (m/M) v
This is a strange result. The cube's momentum is imparted into the portal, but the result would be different depending on our reference frame. From a reference frame tracking the cube pre-interaction, v = 0 (by definition) and so V' = V, the portal-surface object has no change in velocity. But in any other reference frame, it does.
Physics can't be affected by reference frame (thanks, relativity) so this analysis must be wrong.
Okay, after thinking about this a while, I've decided that conservation of momentum just doesn't work, unless the portal actually absorbs the mass of every object passing through it, which would probably cause its own set of contradictions.
Instead, I think forces that act through the portal are applied to the portal. What do I mean by this? Well, take this classic case of a cube floating halfway through two floor portals:
The system has reached equilibrium, and nothing is moving. Both halves of the cube are being pulled down by gravity, but cancel each other out by applying a normal force on each other right at the portal interface. What I'm claiming is that this normal force is applied to the portals, which ultimately transfer it to the floor. If this entire floor was a scale, it would measure the calibrated weight of the Weighted Storage Cube.
But this is the static analysis. What happens when objects are in motion? To be continued...
So we might think: a cube falling through a floor portal and flying out another floor portal right next to it, kinda looks like how a bouncy ball bounces off the floor, and should apply forces on the floor similar to that - an elastic collision. Unfortunately, this brings us right back to conservation of momentum, which didn't work out. But maybe I just did it wrong? Maybe, if done more carefully, conservation of momentum can work without being dependent on the reference frame?
I genuinely don't know yet. Time to bring out the calculus.
We have:
M = the mass attached to the portal (e.g. the floor)
V = the velocity of M
V' = dV/dt = the acceleration of M (assuming no other forces)
F = MV' = the force applied on the portal
m' = dm/dt = the rate of mass entering the portal
v = the velocity of the mass entering the portal
If we assume conservation of momentum (our old nemesis), the force applied on the portal should be the same as the momentum taken from the mass entering the portal:
F = m'v
But now we're back to the case from before, where the choice of reference frame changes the outcome! We're not having that. So I hereby decree:
The force applied upon a portal by a mass entering it must be calculated by conservation of momentum in the intertial reference frame of the portal itself at the instant under examination.
This gives us, effectively: F = m'(v-V)
Now THAT'S something we can work with!
Note that this formula also works for the exit portal. In that case, m' is negative.
A curious result follows.
For the entry portal, m' is positive, and (v-V) is a vector pointing into the portal. (This must be the case, otherwise the mass would be exiting the portal). Therefore, the force would be pushing the portal into the wall.
For the exit portal, m' is negative, and (v-V) points out of the portal, therefore the portal again would be pushed into the wall.
The result: whenever anything passes through a portal, both ends always get pushed in the same direction.
So, back to my thought experiment.
If a portal attached to a free-falling panel falls onto a cube, the cube's entry into it would brake its fall to some extent. However, this wouldn't be the only force in play.
As I said in my previous comment, forces applied between the two halves of an object going through a portal, get applied to the portal itself. In practice, this mostly means tension and compression forces. In the static analysis from before, it was compression.
As the panel is falling onto the cube, a part of the cube is already out the other side and moving away the portal. If the panel has slowed, this would apply a pulling force (tension) on the part of the cube that still hasn't made it in. This tension force actually compels the portal, and the panel it's attached to, to speed up towards the ground.
So, in total, would the panel be slowed, accelerated, or unaffected? Maybe the forces cancel out and it just continues on its normal freefall? I don't know yet. Would need to do more math.
The "ragebait answers only" in the OP is the only thing stopping me from actually raging at everyone saying A. I am giving them all the benefit of the doubt, they're deliberately saying the wrong answer.
Everyone saying B missed the assignment and answered accurately.
because of that law it is B ;). the cube "teleports" into the reference frame of after the portal and in this frame it is highly in motion.
it's like you trying to jump on to a moving train, you first need to match the trains velocity (speaking relative to earth), then you are at rest in the trains frame.
this kind of thoughts is what makes special relativity so cumbersome.
This is Einstein's elevator thought experiment but with portals.
B
Also, if the cube has to emerge out of the blue portal top first it has momentum moving out of the blue portal, as seen by the blue portals frame of reference. It can't emerge with zero momentum or it wouldn't move relative to the blue reference frame.
Einstein's elevator explains why acceleration can mimic gravity. It doesn't, by itself, explain why a stationary cube would emerge with a large sideways velocity. That conclusion depends on additional assumptions about how moving portals transfer momentum.
Yeah it does explain it if you use your brain for like half a second. Is the orange portal moving down to the cube or is the cube moving up towards the orange portal? We have the reference frame of the cube so it looks like the portal is approaching the cube.
What you also conveniently forgot to mention is his thought experiment also explains relativity where you can't tell whether it's acceleration due to the elevator moving up, or gravity pressing you down in the elevator while it's stationary.
Since you can't tell the difference between the two scenarios the cube should only do one thing in both scenarios to be consistent with relativity. If it doesn't it breaks it...
Was going to post the correct Answers (A) but then looked at the post and though na. But if you want to know more about portals and how they would work in real life check this out. Great guy.
If you had a table with a cylinder, with a ping pong ball in it, and a circular weight dropped around the top of the cylinder that hits the table, the ping pong ball will bounce due to the forces of the weight hitting the table. Enough energy would be transferred through the impact, that would cause the ball to seemingly jump up in the cylinder.
As the cube enters the portal, it will take on energy from the piston, slowing the piston down in the process, and that energy will propel it out. If it enters the portal fast, it has to exit the portal fast.
Regardless of the how the portal works, if the cube enters the portal at a certain rate, it has to exit the portal at the same rate and everything else has to follow.
The cube doesn't interact with the piston, but it does interact with the portal and the portal is fixed to the piston. The same way that when I hit a nail with a hammer, I feel the resistance of the nail though the hammer handle. I don't touch the nail, but the force is transferred through the hammer and in the hypothetical, the force would be transferred through the portal.
The hypothetical does not bring up the possibility that the portal has infinite sharpness at it's edges and would cut though the piston head or the possibility that relativity is somehow involved and would allow the cube to exit the portal at a different rate that it enters. Wormholes moving at different rates is a theoretical form of time travel, but not at these speeds.
The cube doesn’t interact with the piston, but it does interact with the portal and the portal is fixed to the piston.
The portal isn't an object, the cube doesn't interact with it either. Saying you interact with the portal is the same as saying the hole in a lasso is an object that you interact with.
If the portal is being pushed down by the piston, that means it interacts with piston. A portal isn't a lasso because lasso's exist and have known rules to them. The only things we can know about portals is what's presented in the hypothetical.
Sure, because the portal is affixed to the piston, but you should note that the portal is not affixed to the box. The box doesn't interact with the piston, or the portal, in the same way it doesn't interact with a hoola-hoop that is passed around it.
If the box enters the portal at a certain rate, it has to exit the portal at the same rate. Unless there's relatively involved, that has to be true and everything else follows.
How is the portal affixed to the piston? Whatever made up system that affixes the portal to the piston would also allow it to interact with the box.
The box has to exit at the same rate it enters and the law of conservation of mass and energy dictates that no new energy is created and it has to come from somewhere. If there is no way for the energy to be transferred from the portal to the box, then simply, portals cannot exist.
It's doesn't matter really, this is a red herring.
The box has to exit at the same rate it enters...
There is the problem with your logic. The box doesnt move, there is a shortcut in space that moves around it.
the law of conservation of mass and energy dictates that no new energy is created and it has to come from somewhere
EXACTLY!! A is the only scenario where energy (or more accurately, momentum) is conserved in this case. If you want to think about why that is, see my previous post where I extrapolated on a case where a lighter object with a portal on it is moved over the box.
if the portal was put on an object lighter than the box and then quickly moved over the box, the box would be launched at a speed equal to the speed of the lighter object, while it has more mass, this is not a conservation of momentum.
It pushes off of the bottom platform, the cube accelerates by pushing off of itself (if it didn't it would flatten into a pancake). And because there's a downward force on the bottom of the cube it pushes down on the platform
I'm not sure what you're saying. You mean the piston and portal would compress the platform? Even if that's the case, that still means force is being transferred through the portal.
The momentum the cube gains comes from the platform. The piston "can't tell" the cube is there, it experiences the same forces if the cube is there or not
You can't put a portal on a moving platform, it has to stop moving first, so neither can happen... Unless the moving surface is a celestial body at least the size of The Moon, I guess?
If you come out stationary wont you become a pancake of all your z axis compressed into a single infinitely thin plane? Also i think when the game calculates this it treats you as a point travelling through the portal plane and adds the plop worth of momentum as a compensation to not break the physics. Also if you think about it a lot of different situations already lead to extra momentum out of nowhere(classic portal above and below infinite loop as an example) so the portal in this case adding momentum doesnt break physics cause portals already break it. Also theres a problem of some people answering with the game physics in mind and other with the real world interpretation of it so this really is the perfect ragebait.
I mean yeah thats true but that still means the cube would heat up or something and then you could use that to generate infinite energy. The point is you can accelerate something to a speed without putting in the energy needed to accelerate it.
The cube wouldn’t necessarily heat up much while approaching terminal velocity.
Its close to infinite energy because you never have to reach the ground, so you technically have infinite potential energy but when you convert that potential energy into kinetic or some other type of energy, the vast majority of that potential will disappear instantly
Edit: in other words, you could generate incredibly large bursts of energy but not sustained energy with this kind of portal loop
Terminal velocity is only reached because the object (or player) is colliding with the air. Momentum is still being added. It's just also transferring out into the air. Terminal velocity just happens to be where the two rates match. You can kind of think of the object as being a fan blade running on zero power. It's technically a source of infinite energy, but it might be hard to extract from that exact setup. If we could do this in the real world, we'd tweak it to just be water in a vertical pipe with a turbine extracting the free momentum.
Get a hula hoop attach a camera to it. Drop hula hoop over something with it being in the middle. Did thing in hula hoop take off? No. Thing had no momentum. It's velocity versus relative velocity.
You need to cut the hula hoop in half. The bottom moves and the top is stationary off to the side. The cube pops out at the same speed the bottom half is falling, but because the top half of the hula is stationary, the cube seems like it's moving upwards.
This has been really heavily debated before and the unanimous answer is B IRL and A for portal.
How does the cube move out of the blue portal? If you ignore the orangep portal entirely and consider just looking from the side as the cube emerges from the blue portal, the top has to be moving extremely fast. The cube needs to be moving relative to the blue portal at the same speed as the orange portal is descending or it doesn't work.
You can also draw a force balance diagram and you get the same result...
But you're welcome to disagree with me! That's part of the wonder of the internet.
B would be correct if this existed in real life and had to be compatible with general relativity. If we took this exact same setup and moved it into space, how could you tell if the cube is moving towards the portal or the portal is moving towards the cube? You also can't use the blue portal as a reference point since it could also be moving independantly from the orange portal.
It would be A because it's the orange portal that's moving, not the cube.
The only way it would be B is if the portals suck since the cube doesn't have any momentum to begin with. The game clearly shows that this is not how portals work.
Think about how you come out of the second portal, your head comes out first and the rest of your body follows. Your head has to have a velocity coming out of the portal or the rest of your body will smash into you. So either you come out flying or you're smashed to a pulp.
You don't even need to think about reference frames for the answer to be B, because each layer is being displaced by the layer behind it. Take the limit to go from a discreet to continuous domain and you exit the portal at the same speed you entered, which is exactly the speed of the falling portal.
Has to be, for the particles to clear the second portals entry they have to be moving at a relative velocity to the first portal at the same rate the first portal is moving
Assuming the black platform does not fit through the orange portal, and the moving panel with the orange portal stops on impact (which is what appears to be true because on the blue portal side we see the surface of the black panel), then the cube is no longer moving relative to the orange portal and therefore has no momentum. Once the cube is entirely on the blue side of the portal it will be affected by the gravity on that side and slide down as if on a ramp (A).
C. The moving portal platform breaks the black platform and the whole thing goes through the portal.
Jokes aside, B.
Devoid of any other external forces, in the cube's reference frame there's functionally no difference between it moving towards the portal or the portal moving towards it at a constant acceleration.
Ergo the cube should be pushed out the other side at the same momentum that the system had prior to the collision, and not be simply plopped out as in answer A.
It depends. It looks like the orange portal is being moved by a hydraulic cylinder (aka laminar flow), which means the relative positions and velocities of things moving through the orange portal are conserved, and A is correct.
If instead that's (yuck) an air fired cylinder... Then it's going to be B.
If the movement is handled by a threaded rod, then the box will emerge from the blue portal spinning, and (perhaps contrary to expectations) if the orange portal is moving because of gravity, then the bouyant force of the box progressing through the portal will stop it from falling further, and the box will end up just less than halfway (because of the angle) through the blue portal.
And as a matter of fact the fastest possible velocity you can get is with a hydraulic cylinder because of the immense amount of force it applies using the orange portal.
If you used magnetic levitation to move the orange portal the box would never make it through, because it's made of metal and will be attracted to the levitation equipment.
I don't know enough about space-time. Does distorting space transfer energy to the matter in that space? How about time?
I think it must be A, out of the options.
Otherwise the future of man is less 'sustained ftl travel', and more 'giant space slingshot'.
the cube isn't fully lined up with the portal so the collision with the edge of the portal/platform causes some source shenanigans and ends up being option b
Imagine if the cube was actually a long rod and the orange portal start with the column half way inside it. Then the orange portal moves, in this case it would be very odd for the rod to jump in this scenario like in case b, the thing you would expect would be for the rod to stay still (besides tipping sideways a bit) consistent with case a.
If you considered the cube to be 1m^3 , the time at which the cube enters the orange portal t1, and the time at which the cube fully exist the blue portal t2, then the speed of the cube relative to the world outside of the blue portal would be 1m / (t2-t1).
Assuming the cube maintains this speed after exiting the blue portal, it would behave as in scenario B.
Most likely option is it flops out but only if the angle is sufficient for the cube to overcome its centre of gravity and topple. And do so before the orange portal rises again. Also depends if the downward orange portal directly touches the base or not, since there might be a bit of a "lip" to overcome too.
This only works if you treat the cube as one object anchored by a small part. In reality, objects are a bunch of particles each with their own momentum held together by forces between each particle usually strong and springy enough to cancel out differences in momentum. This is why all objects tear apart if enough uneven force is applied on them.
If the portal stops partway through the cube, the stationary bottom part on one side of the portal and the top part still carrying the momentum of recently exiting the portal will have very different competing momentums. If the cube's internal forces win out, the overall momentum will average out, and the cube will fly up with a speed proportional to however much of it got pushed through the portal before it stopped. If the competing momentums win out, the cube will break into both stationary and flying parts.
If the box left the blue portal at a slower speed than it entered the yellow one, the top of the box would be moving slower than the bottom of the box, which would compact the box dramatically during the transition, but we can see that its size is unchanged and it is still square.
The top of the box is pushed rapidly out of the blue portal by the bottom of the box being thrust into the yellow portal. (The bottom of the box is pulled rapidly up into the air by the top of the box which is already moving quickly.)
281 replies
Stop stealing all my upvotes
You asked a question and they gave an answer. Why wouldn’t they get the upvotes?
I've just never had a comment on my post get more upvotes than the post itself.
Am angy
and upset I didn't think of posting that image in the first placePerfectly understood the assignment, 10/10, no notes.
There'd be no cube but yes
In the first moment of exit, it might look like this though
You can't place a portal on a moving surface in either game, so neither.
you can in one specific spot in portal 2!
Two, if you count... the big one at the end
it doesn't look like it's moving though. and considering how many things in portal 2 move when they look stationary or vice versa...
As far as the game engine is concerned, "stationary" is relative to the... other comparatively big thing: the former big thing is moving very quickly around the latter, even if it looks stationary because it has a lower angular velocity
pretty obvious tbh
So now we just need to build this test in there and we are set
So the cube is just smashed?
It's a practical answer which is slightly upsetting because I wanted more discourse.
No, cubes cannot be destroyed by crushing. The portal disappears when the surface starts moving and then the cube stops the surface's descent.
what about the portal on the moon?
Well, everything is moving or not moving, relatively
Someone should write a thesis on it (or whatever it’s called). Could call it something like The Relativity Theory.
It's unstable and will dissipate on its own when the relative displacement between them will be too great.
Skill issue
You'd make a horrible scientist.
It said ragebait answers only ¯\_(ツ)_/¯
If it is a question of real life physics, then B would be correct
Damnit, you are correct, my bad.
Scientists know when to use existing knowledge.
This was answered decades ago
In that situation both portals (the inside and the outside window frame) have equal, but opposite velocities and therefore cancel out any momentum from the person.
When dealing with OP's scenario, only one portal is moving.
wikipedia.org/wiki/Steamboat_Bill,_Jr.
Nearly a century ago. Fun fact, this movie inspired Steamboat Willie, the very first appearance of Micky Mouse.
The problem with this analogy is that both sides of the window are at the same point in space so there is no pressure differential. What if the blue portal was at the bottom of the Mariana's Trench? What force displaces the water to place the cube on the other side?
It wouldn't. Same as if the other side of the portal was a brick wall. It wouldn't go through or would be smooshed by whatever force is driving the portal.
I guess the odd thing is, if the orange portal can be moved, the pressure from the brick wall would be directed at the attachment point of the orange portal, otherwise why wouldn't the orange portal keep moving, since it is not "touching" anything?
If the blue portal was deep under water, the pressure would force the water out the orange portal.
According to a gravitationally accurate simulation a guy did for Youtube, the correct answer is_
... ah, ragebait answers only. Well, mission accomplished, probably >:3
For anyone who doesn't know, Optozorax on YouTube has made a finite element analysis simulation of gravity ontop of his portal simulation so we can get an answer to the question. The answer is both. Both happen depending on how fast the portal is moving and the where the second portal is placed in the gravity well.
I'm not sure if it's the alt text or simply the post's body, but it reads "Ragebait answers only"
Option B. Portals already break a couple of laws of physics and can reverse entropy, but I like to think you should look at the speed of an object relative to the portal.
Speedy thing go in, speedy thing come out. Clearly explained in game.
But In this case the cube has no speed, the platform does, so it wouldn't just suddenly start moving.
It has speed relative to the portal though. Everything has some speed because we're orbiting the sun, etc
The cube does have a non zero speed, it just depends what frame of reference you are measuring it by.
Frame of reference. If a person is hit by a train, from their perspective, they were standing still and the train was moving. But a person in the train can stand up and walk around without difficulty, they can stand on one foot and balance, they can jump up and down and not go flying away; so they are standing still from the trains perspective, it's everyone and everything outside the train that's moving.
What happens isn't a transfer of momentum or energy or velocity; it's a change in the frame of reference. That can't happen in real life and breaks a lot of different laws of physics, so people are having trouble wrapping their brain around it.
The portals are a frame of reference. Both the IN and OUT portals have their own, separate frame of reference. The player's reference doesn't actually factor into it. When you go through a portal, you are switching from the IN frame of reference, to the OUT frame of reference. And in this situation, the IN frame of reference has you flying toward the portal at whatever speed it's approaching you.
If anything, it's rapid exit from the portal would cause an increase in pressure on the top of the cube, causing it to stay in place. The cube still isn't moving regardless of the velocity relative to the moving platform.
At what point in time would the cube start moving away from the platform it's sitting on, and what forces act on the cube?
Also shit changes frame of reference all the time irl, and portals a could be thought of as wormholes, which which could theoretically exist IRL.
Alright, since you didn't like my last explanation for some reason, let me try again.
You know how when you stick your hand out of a moving car, you feel a massive rush of wind, even if it's not windy out? Ok, so imagine sticking a portal out of a moving car, what do you think happens to the air? Does it come to a dead stop after it exits the portal? Why not? The air itself wasn't moving, only the car with the portal was.
Solids in this case will work the same way. It doesn't matter if a solid is moving toward the portal, or the portal is moving toward it, what matters is the relative velocity.
From the orange portals frame of reference, the cube, and everything else in the world, is rushing toward it. So the cube (as well as the entire world around it) has a velocity relative to the orange portal.
But only the cube goes through the portal, the rest of the world does not come with it. At that point, the frame of reference changes. You can think of it like this: The cube has an velocity of X with respect to reference frame Y; once it leaves the portal, it keeps that velocity X but the reference frame changes from Y to Z. The velocity doesn't disappear.
What if the blue portal is at the bottom of the Marianas Trench?
Niether. Moving surfaces break portals instantaneously.
Portal 2 has moving surfaces, with the cop-out that it's accelerating surfaces that cause the portal to break.
::: spoiler spoiler No matter that you literally put a portal on a different planet... :::
Portal 2 (story mode) only has laterally moving surfaces, that inherently don't cause things to enter the portal. The moon portal is stationary in the engine.
Portals don’t stay on moving platforms.
But if they did, it would be A as the cube itself has no momentum.
WHAT DID YOU NOT UNDERSTAND ABOUT RAGEBAIT. ANSWERS. ONLY.
... Uh
u mad?
So really I saved this comment and you're all welcome.
It seemed to work.
Except that one time when you use portals to cut the neurotoxin pipes.
Yeah, but those are moving from side to side. To my recollection, you never see a portal in a wall that’s moving in a direction that’s perpendicular to the plane it’s in.
The speed of one thing is the same speed of the other if you change the reference point. So the cube has momentum but the moving platform does not. It’s more about having a velocity in respect to the portal it’s going in or coming out of or an average of both. Like when you jump into a portal, you come out faster on the other side. Now what if that out portal was traveling at the same rate? Would you be traveling just outside the portal or double speed (in relation to the ground and static objects in the same room)?
I'm giving so many people Fs on this assignment because you failed to read all the instructions.
The correct answer is B due to the graphical conservation of speed lines.
I did it deliberately.
Lemmy reads more than most but people aren't good at reading
I think any answer is a ragebait answer because nobody here understands physics
That's definitely a ragebait answer
Judging by the most highly voted answers on physics.stackexchange, it's also true.
Trick question. Fun answers are not ragebait. Serious answers (correct or incorrect) are ragebait. You get an F.
If the downward moving portal hits the platform and there is some elastic rebound, then the object will bounce outward slightly.
Rage bait answer: Portals can't exist, it is from a child's game.
Better rage bait answer: portals can't move relative to each other. If they do, the one that is moving 'pops' and disappears from the surface. So this scenario is impossible inside the parameters of the game, so it's not worth thinking about.
(The devs realized a lot of stuff could get horribly broken if portals could move, so they can't move)
Except that they can move in that one level with gas and lazers )
Scripted exception
Honestly the worst decision they could have made while writing. I'm still kinda' mad about it and I don't know why.
I think they experimented and wanted them to be able to move in more places. But with engine limitations they only left it in one scripted place where there are no physics objects that can interact with it.
Even stationary portals break conservation of energy unless there's some unseen location changing energy to compensate, but yes, it gets more complicated with relative movement. In lore though, it is established that portals can move with hugely different velocities relative to each other at least with enough distance when you ::: spoiler spoiler shoot one portal to the moon :::.
B.
Momentum/velocity is not an intrinsic property of an object, it is a relative property between two objects in a frame of reference.
Behind the yellow portal is the part of the universe where the blue portal is. The yellow portal, and therefore the space where the blue portal is, has momentum relative to the cube.
Put yourself in the frame of reference of the cube. You see the yellow portal come toward you. Imagine there are objects on the other side of that portal. You see those objects rush toward you through the portal. Why would the objects just stop dead when you came through the portal? (If they did just stop, it would be as though you hit a wall, and you wouldn't actually go through the portal at all.)
Agreed. Also consider the duration of the time that the portal is passing over the cube. It is moving at speed, the entire way through the portal and out the other side and then it just stops dead immediately once through? That would hardly be a conservation of momentum anyway from any frame of reference and does not make any intuitive sense.
This exactly. Consider an object that isn't a point mass (like the cube) going through. As the first atoms go through: if they just stopped relative to the portal boundary, that would suggest the next layer of atoms need to exert force on the first layer to push through. But portals in the game Portal do not have any resistance to objects passing through them.
"speedy thing goes in, speedy thing comes out."
So stationary thing goes in, stationary thing comes out.
Speedy or stationary relative to what?
To the portal if course. That's why it comes out speedy at the other end.
But the game treats the portals as holes in reality. If a whole falls on you do you suddenly pop up out of the other side? No.
It would plop. It would only fly up if the platform the cube was on moved up towards the portal.
I think the game treating portals as fuzzy holes in reality is more just a limitation or oversight in the physics engine. It completely ignores relativity and makes the portals no longer logically consistent and pointless to argue about. Velocity and angle conservation relative to the portal is established elsewhere though, so running with that gives you the "fly up" result.
> stationary
> comes
Choose one. There is no "plop", all of the cube has to emerge on the blue portal's plane
The object going into the portal has no momentum so it would be A.
No momentum in relation to what? Momentum doesn't exist in isolation.
Momentum relatively to the source game engine grid.
"speedy thing go in, speedy thing come out."
So A.
As I said, there is no “speed” in isolation. Speed is a relative property of an object and a reference frame (i.e. another object, like e.g. a planet)
In this particular case it is speed with respect to the fixed reference frame of the game.
To the space outside of the blue portal. Moving the orange portal doesn't drag the whole surroundings, so assuming both places are on the same planet, the momentum of the box in relations to both places is the same
As Optozorax taught us, gravity must act through the portal.
Well, it doesn't in the only source of knowledge we have: the game Portal. I don't know enough about gravitational forces to have a coherent theory on it, gravitational waves moving through space-time is a bit beyound my real understanding. But in the game if you put a portal on a ceiling, it doesn't "suck" you until you're throuth it, so there it isolates gravity somehow.
If you stick to the premise that the portals exist in a universe with GR, then we can pay much know that space-time is locally flat in and around at least the middle parts of the portal. We know this because things don't get fucked up when they're going through.
At this point, I consider Optozorax’ research the primary source of how portals work. If the games disagree, they're wrong.
Moving the orange portal kinda does ‘drag the whole surroundings’. It moves an entire universe.
The cube is stationary relative to the universe of the platform it’s on, but it has momentum relative to the universe through the portal. The question is which universe is the cube in? What is the boundary of the portal surface?
Portals aren't real and so work however you want them to, but people rarely consistently apply their models.
I'd say A given how they work in the games though. Only the object's momentum is conserved. The portal doesn't transfer momentum. There may be a slight breeze coming out of the blue end as the column of air in front of the portal-plate is pushed in, but the cube is just going to flop out.
The object's momentum is preserved. Movement of the entrance portal does nothing. Think of it like stepping though a door, except you're Buster Keaton and the door hole is falling on you.
I brought this up on another comment that's for some reason now deleted but, if the universe moves around you, would you not perceive it as movement?
There is no such thing as an absolute inertial frame of reference. If you are in free fall on earth, you would experience the same sensation as being in space. The only things that would tell you otherwise are the air resistance hitting you and what you can see.
So no, you would not be able to tell the universe is moving around you unless something else in the universe hit you.
The first part made sense, then you contradicted yourself at the end^^ If the universe moves around you, you could literally see things moving. It would be the same as you moving relative to everything else
Seeing things counts as something hitting you: photons.
True. But then thats not the exception
Think about it more fundamentally. Instead of a person with senses, think of an object experiencing forces. Thats what an inertial reference frame means.
But senses are just ways for an object to experience forces.
I know if I'm hanging upside down because my senses are telling me my body is being pulled down from my head. I'm feeling the gravitational force pulling me down.
An object moving in the universe is indistinguishable to the universe moving around an object. To any observer or to the object itself the forces experienced are the same
Exactly. Seeing was just an example. The whole universe moving around an object would cause that object to experience forces.
well, in my opinion it is a bit of a paradox. i think you are partially correct.
your door analogy may be right from the perspective of the platform on which the cube is. however, one flaw is that the whole space is moving, not just the door. as soon as the cube enters that door, it will be inside that space, and if you consider the cube part of it, moving that space "downward", then due the cube being held by the platform, you could consider the cube moving "upward" in the space behind the orange portal. if you consider preserving momentum there, then as soon as the apparatus stops moving, the cube should not stop its momentum and move further up in the space.
or look at it like this: if you are behind the orange portal, meaning you are looking at the blue portal, you will see a cube moving towards you. then as soon as the platform hits the wall of the orange portal, the preserved momentum should make the cube "fly".
the change of perspective leads to two different conclusions, therefore i think it is a paradox.
the problem sort of is that there are two "spacetimes" that are kind of being changed. one could interpret it as both of these effects "pulling" against each other since when the cube has partially passed the portal, one part of the cube is "in one of the spacetimes", ant the other part in the other. ultimately though, the cube then would actually fly because in the limit (infinitesinally close to when the platform hits the portal wall) the cube is completely on the other side of the portal, so from the view of that spacetime, the cube does get pushed and should fly.
i don't even remember if it is possible in the video game for walls with portals that move. a video game would probablt have some kind of "center of mass", like a single vector of coordinates where describing the position of the object. there is no notion of "partially" being somewhere (except for collision boxes, but i don't mean that). so also from that view the box would fly. however, in most video games (with or without portals), if a platform moves up and suddenly stops, the object won't travel further in that direction anyway, especially not ulwards (even sideways usually not).
As for the portals moving in the game, portals are destroyed if the panel they're on is moved except for one specific part, where you can put a portal on a panel moving laterally to get a laser to cut the tubes connecting to the neurotoxin tank. That part needed to be explicitly programmed to allow it, and it's not possible without cheating to physically reach that portal.
Whats the difference between an object moving and everything else moving? For an observer on the sun every one of us has huge momentum which would be preserved when entering a portal. Momentum is always relative to some frame of reference. Momentum 'toward' the portal would be conserved and mirrored through the other portal.
The difference in this case is that the object has 0 momentum and the portal doesn't impart its momentum to the cube because you can't collide with the portal. There is no force acting on the object to make it move.
Of course it has momentum relative to the portal.
That is not how either relativity or momentum work. The cube is still inert and the portal's movement would have no effect on it unless it has some acting force that pushes it when passing through. Even if the exit portal was moving, the cube would remain stationary unless it had nothing to rest upon and gravity pulled it down.
Correct. If a stationary door/portal has no affect on the cube's velocity than that's also true for a moving door/portal.
More like a hole in the wall with you standing perfectly still while the wall moves past you.
Except the wall has no mass or width.
It would move really fast through the portal then slide down from stand still. So yea B is correct.
What if the portal on the moving platform had a wall right in front of it, so moving though the portal would cause the box to run into the wall. You'd have a scenario where the box wouldn't be able to move through the portal, but you'd have energy being generated by the moving arm "pushing down" on the box. The only way that could work is if the moving portal is transferring energy to the box, and therefore require option B.
Although that being said, portals break the laws of physics, so there probably can't be a "right" answer.
Momentum is conserved when passing through a portal. In the reference frame of the out portal, the object has no momentum, so it plops out.
Speedy thing did not go in
Speedy thing goes in, speedy thing comes out.
Cube had no speed going in, therefore no speed going out. The portal speed is irrelevant
The cube has a speed of about 30km/s around the sun. You want to measure speed relative to the blue portal, but why that one and not the orange one?
And if the blue portal is at a different angle than the orange portal, then… well, how fast is the milky way traveling through the galaxy relative to the universe?
But reference frame changes as the cube passes through the portal. The piston would feel resistance as its momentum is transferred to the cube to conserve it.
I choose A because that would mean I could use the portal to fuck myself in the ass easier.
"Plop"
Trying to conceptualize what that would be like is making my head hurt.
You would need the portal panties to make it work. Portal A would be on a wall, or whatever object you want to thrust towards, and Portal B would be on the panties aimed at your opening. When you thrust forward at portal A, it would exit Portal B, and since Portal B is stationary (panties are magical, yo) to your opening, the pillar would rise from the portal and enter the opening.
Of all the days to be literate....
...
I'm glad this is one of them
Let's make sure you feel the same about your eyes: https://e621.net/pools/20220
I don't think I'm gonna click that.
I hadn't considered panties, but I was thinking about how you'd just be chasing yer pecker around trying to back up on that thing.
It's like Zeno's arrow, but kinky.
You could probably make it work with some surfaces that aren't facing each other, since technically genitals don't sit at 180 on the pelvis. It would probably take the porn star level of size though to make it work.
A strap-on would prolly do the trick.
If we're only supposed to do ragebait answers, then anyone who passed 3rd grade science would be able to tell you the answer is obviously B
If you actually played the game, and paid attention in physics class in highschool, then you would know the answer is clearly A, and everyone who says it is B really needed the simplified explanation from GLaDOS to understand how to play the game.
Ragebait successful. College physics here. The only correct answer is B. You can only get A if the box is securely anchored to the pillar then released after the portal stops.
I love the games, but you can't trust a game physics engine to properly track what's going to happen with moving and especially accelerating portals. They (usually) track an entire object from one reference point against one absolute world grid.
Think of a portal as moving a plane of particles from one location to another. An entire object passing through a portal is just a bunch of successive planes being moved. One particle plane entering the portal plane on one side must displace the previous particle plane at the same rate on the exit side. Therefore, an object's speed through a portal must remain consistent relative to the portal on both sides, and with nothing to magically cancel that inertia, that motion will continue even after the object has completely left the portal.
Incorrect. The answer is A, and here is why.
GLaDOS specifically states "Momentum, a function of mass and velocity, is conserved between portals." If scenario B was correct (which it is not) then if the portal was put on an object lighter than the box and then quickly moved over the box, the box would be launched at a speed equal to the speed of the lighter object, while it has more mass, this is not a conservation of momentum.
The best way to explain movement through portals is that momentum is only conserved from a perspective that encompasses all of the objects involved, this is true for regular physics as well actually. Consider this, what if the portal was shoved very quickly over the box down to it's halfway point? The obvious answer is that the box would only appear halfway out of the other portal, but if you subscribe to the "logic" that leads to answer B, then the box is actually sucked up by the portal, while the actual logical and intuitive answer is that only half the box sticks out of the portal, and it stays where it was placed.
Portals aren't objects, they don't have mass, or even a velocity, they are a shortcut in space, and that was what GLaDOS was explaining when she said that momentum was conserved. Although B seems to initially make sense, it is not correct from a game lore perspective, nor from an actual physical perspective.
It's not about whether portals have mass or not. It's about the box's velocity relative to the space on the other side of the portal. In the game, as you move toward a portal, you also move toward the objects on the other side of the portal. Therefore you have velocity relative to those objects, and that's the momentum that is conserved as you pass through.
Well, the box's velocity relative to the space on the other side of the portal is 0, so that is why it does not move.
That's what this statement means.
This would suggest that portals in the game Portal are flat paintings, but it's not. As you move around or toward the portal, your distance and angle to the objects on the other side of the portal move in your frame of reference. Therefore you have velocity relative to those objects. Not to the portal itself, not to the universe behind the portal as a point mass, but to the spatially separated objects on the other side of thr portal individually.
No, it doesn't imply that portal are flat paintings at all.
Yes, but only technically so because those distances and angles change outside of the portal. Your path through the portals isn't real, it's a shortcut through space. This is why an object that is moved through a portal has momentum, but an object that a portal is moved over does not, the two scenarios are not the same when you have a frame of reference that includes the space on the other side of the portal. They are the same if you ignore that space in your frame of reference, but then any conclusion you come to about things outside that frame of reference are invalid.
Ah, so you're saying Portals are not flat paintings, they're flat TV screens which project a representation of the universe behind them like based on where the observer is. Upon impact with an object they then store the objects momentum relative to the TV screen, teleport the object to another point in the universe, and then transfer the momentum to the teleported object. Got it!
One minor inconsistency is that in the Portal universe, objects transition smoothly through portals and can stop halfway through. But that's okay since we're just making shit up anyway.
Circular argument. You're using your preferred conclusion as evidence for your flawed reasoning.
Velocity is a relative property between two objects. There's no "I'm moving toward the portal, therefore I have momentum" versus "The portal is coming toward me, therefore I have no momentum".
Lol. This will be fun. Glados' explanation is still correct, but momentum being conserved between portals (or "across portals" as I'd rather phrase it) is exactly what gets you answer B.
"A perspective that encompasses all of the objects involved" is not a thing in reality. No point in space cares about another point in space beyond the forces felt from that direction transmitted through spacetime, and the frame of reference following the left portal moving down and the frame of reference following the stationary right portal are both equally valid and moving relative to each other over the timeframe of the cube transitioning through the portal. I don't agree that the game is pushing a different model, but if it actually is, then that opens up so many (more) holes in physics that we may as well be debating magic.
I actually addressed the portal stopping halfway in another comment here which I won't bother to copy here, but the summary is that you shouldn't think of the box as one item but a mass of particles connected by internal forces. Your answer is indeed "obvious" and intuitive, but accelerating portals do funny things with frames of reference that means intuitive isn't always right and is the reason this topic always creates the disagreement it does.
As the portal comes down across the top half of the cube with constant relative velocity V and the appropriate momentum for said velocity, all those portaled particles will emerge from the other side of the portal with the same relative velocity V and same associated momentum compared to that side of the portal. Whether the particles maintain velocity relative to the portal because of something intrinsic to how space works at the portal boundary or because particles coming in forcibly displace particles going out in equal measure, the result is the same, fast in = fast out.
When the descending portal suddenly stops halfway, the particles already portaled through don't "know" this. They've still got the momentum of being sent through moments earlier reinforced by all the particles around them doing the same thing, so they "desire" to keep moving away from the portal. The particles of the bottom half of the box are similarly "ignorant" of all the shenanigans above and "want" to maintain the zero momentum they've had all along as measured by most of the objects in the scene.
This means the top and bottom halves of the box, despite never losing conservation of momentum, suddenly don't align with each other. The conflict of the top half "wanting" to keep moving away from the portal at velocity V and the bottom half "wanting" to stay where it is will resolve in one of two ways:
If the cube's internal forces are strong and "springy" enough to accommodate the disagreement, forces will propagate and equalize throughout the whole cube effectively averaging all "desired" motion out and the cube will fly out of the right portal at velocity ~1/2 V.
If the internal forces are not enough, the two halves of the cube will separate from each other, either cleanly at the portal boundary if the momentum disagreement massively overpowers internal forces or more messily and violently if it barely overpowers them. Obviously we don't have portals in real life to visualize this, but you can kind of get a feel for the effect through planetary bodies breaking up at Roche limits.
Edit: Wording the overall effect another way: Portals moving relative to each other effectively link two frames of reference to be one and the same over the portal boundary. If one portal changes velocity, the two frames of reference cease to align. This can cause conservation of momentum to appear to have been violated externally while actually being maintained in all the internal reference frames objects actually experienced.
Actually it is, that's exactly what a perspective, or refence is, it's either this, or you are arguing that there is a universal reference point. There isn't a universal reference point though, all motion is relative to other objects. The problem with using B as the result of this experiment is it draws a conclusion on the relative motion of the box, without considering the relative motion of everything on the other side of the portal. B only makes "sense" in a system where only the position and motion of the box and the orange portal are considered, any conclusion regarding motion outside this system are invalid when you use a perspective thar excludes everything except the box and the orange portal.
Maybe I'm misinterpreting your use of the phrase, but it seems you're arguing from applied Newtonian mechanics where objects define a reference frame. A reference frame in that sense is a useful convention but doesn't actually signify something in reality, hence my comment. Objects don't care about their velocity relative to other objects or about reference frames suddenly shifting as measured that way. An object's motion is encoded as an entirely local property in spacetime itself, AKA its worldline. This distinction between object-based reference frames and local reference frames is usually not worthwhile, because they're usually entirely consistent with each other. The convention just happens to break in situations where worldlines cease to be continuous and consistently differentiated, as in the case of portals.
Worldline is just a way to describe how an object or point moves in spacetime, since relatively makes everything kinda fucky and hard to follow using newtonian laws of motion, it does not appear to be a replacement for an inertial frame of reference.
Edit: by your argument of what a worldline is, then we would expect A to happen, since there is no motion component "encoded" in the worldline of the cube anyway.
Lol. Serious /uj question: Are you deliberately misreading me to do the rage bait thing? I'm using "worldline" in the first sense you gave. I'll avoid it and reword the actual main point as this is good practice for me anyway:
Local spacetime is the ultimate thing all velocity maps onto, so any examination of this problem needs to respect spacetime as the primary driver of behavior. Answer A ignores the spacetime the cube is being moved into on the exit side of the portal as a real thing it must maintain inertia within (the critical detail also overlooked in the "Buster Keaton" answers).
If momentum is conserved then neither A or B is correct, the box should be an infinity thin sheet. If the portal is half way through the box if it maintains it's thickness the box has to be moving at the speed of the portal out of it.
Except it's not moving out of the portal, the portal is moving over the box. I get it when people say that it doesn't matter if the portal is moving, or of the box is moving, and they are right, up to a point. As soon as you need to account for the movement on the other side of the portal, you have to include that space in your frame of reference, and relative to that space, the velocity of the box is 0.
As I explained here...
I must applaud your diligent rage baiting, well done
I've played the game, you maintain speed whether you jump or fly in using your jet pack, no idea who this GlaDOS character is or what that funky cube is though. Abyss is the best map.
The question is: Is it the velocity of the object relative to the portal that matters or the velocity of the object in some other frame of reference?
In the later case, we must ask how the portals at different angles don’t send objects flying off at the speed of the milkyway’s travel through the universe.
The answer that requires the least assumptions and justifications is B.
Exactly. What would that 'other frame of reference' be and how would the portal know about it?
How bout the other portal?
Could be. But then it would make sense for the frame of reference for the second portal to be the first one and the result would be the same.
https://www.youtube.com/watch?v=B19nlhbA7-E
Not providing a link to alternative platforms is my ragebait
In addition https://www.youtube.com/watch?v=DydIhwLrbMk which tries to make portals act "real" by treating them as spacetime with an unusual (but not mathematically disallowed) geometry.
Yes, they are fictional, so they behave however Valve (and fan works) want them to behave, but trying to bridge the gap between fictions and current physics models and be a fun and enlightening experience.
It went through the portal at speed, so it should exit the portal at speed. That’s my take.
If we could test it, I think it would be helpful to put portals on either side of a very light, plastic disk in an attempt to see if it works like a hula hoop or not. If we drop the hoop over the cube, would the hoop:
Just land on top of the cube because the hoop has far less energy than a much more massive cube traveling at the same velocity?
Drop around the cube, such that the cube is just sitting on top of the exit portal?
Launch the cube into the air with an initial velocity equal to that of the hoop when the cube entered the portal? If so, where does all the additional energy required to accelerate the cube come from if the hoop's momentum is far less than the cube's momentum at the same velocity?
Since nobody seemingly posted link to this absolute ... Idk, gold? It's about actually simulating space(-time?) with portals: https://youtube.com/@optozorax_en I see offspec's comment is mentioning the guy, yeah. Tldr answer is complicated.
...
Ah, ragebait answers only. Ugh, worm! Arrrr! I am so angry roar grrrr.
Fuck that's a cool channel, holy moly. Thanks dawg!
Neither, I've already gleefully incinerated that fucking cube before it got to this point.
Both portals stationary: object velocity unchanged.
Both portals moving at the same speed: object velocity unchanged.
Entry portal moving: object pushed out at the same speed it entered.
Exit portal moving faster than object entered: object explodes.
a. while the portal travels down it collects air, increasing air pressure on the other side, meaning once it reaches the cube, the increased air pressure has mad the air solid so the cube cant fly through it.
Now there's a high quality ragebait answer
My take:
The cube's mass is x (eg one gram) and its velocity is zero, therefore its momentum (m*v) is zero. Objects in motion tend to stay in motion, while objects at rest (e.g. momentum of zero), tend to stay at rest.
The thing which is moving, is the portal. At best, the event horizon of it reaches the leading-end then the trailing-end of the cube at the speed the portal is moving over the depth of the cube. If the portal moves faster, that distance is covered faster, and the full cube is entirely on the other end of the other portal faster [than a slower moving portal].
Since going through the portal is a frictionless and forceless action, the speed at which the portal fully envelops the cube would neither add nor remove momentum upon it. And if the cube did have its own momentum it would be maintained, but since it does not, the cube would not move until another force, like gravity, acted upon it.
So, A.
I don't think this can work. Think through it in several steps rather than just one. Once the cube just started entering the portal, a part of it will already be on the other side, as close to the 2nd portal as possible. When the portal continues moving, more of the cube needs to be on the other side, so it "pushes" the first part of the cube further away. The cube imparts momentum onto itself.
Another way to look at it is using reference frames. It really doesn't make a difference whether the cube is moving towards the portal or the portal towards the cube. It only matters what the difference of velocity is. The difference of velocity must be maintained on the other side of the portal, and since the 2nd portal is stationary, the cube must move.
Hmm, maybe? But two things to (re) consider:
Saying "
the cube imparts momentum onto itself" sounds like it would violate the law of conservation of energy. The 'energy can't be created or destroyed' theory, so where did it actually come from? Did the cube suck force going at one vector (the portal) and convert it into the opposite vector? That feels like it violates inertia. And since momentum is mass*velocity, that means the mass of the portal would play into the math changing the cube's own momentum. And I'd honestly assume the portal's mass is zero, which still maintains it wouldn't impart any energy onto the cube, right?Secondly, regarding inertia from another standpoint, I have a thought experiment: let's say the portal is coming at the cube blazingly fast, then immediately stops halfway through enveloping the cube, so it's just chilling 50/50 on each side. By your statement of "
it pushes the first part of the cube..." That would mean one half would have momentum at the same time the other half, not yet through the horizon, would still have none. Maybe so much so that (1) the cube would be pulled the rest of the way through even through the portal had stopped because half the atoms of the cube suddenly had momentum for some reason, or (2) the cube would rip in half (brittle cube I guess, but let's ignore the material considerations for simpliciry) because half the cube now suddenly has momentum while the other half still has none.So I think I understand your perspective, but I can't yet get around what seems to be a few lingering problems. All the same, I'm enjoying the discussion, thanks net-neighbor!
Conservation of energy is already broken by portals. Consider the portals in a frictionless tube (it could be frictioned, but that would be less fun) that use the earth's gravity to accelerate the cube to the speed of light. If we are take the cube's effect on the earth as non-negligible, then the earth has also accelerated a significant bit in the 'upwards' direction that the tube has. Simply run to the other side of the earth, put the 'exit' portal aiming towards the same direction as the 'upwards' of the tube, in this case directly at the earth, and now you have accelerated the earth twice as much in that 'upwards' direction with no apparent gain or loss of energy (and also have obliterated the earth, but let's not talk about a cube's mass hitting the earth at c.
As for the portal suddenly stopping while moving blazingly fast, sure, it's a good quandary. It seems insane to think of the cube suddenly gaining momentum, but it's also insane to think that a particle (in this case the portion of the cube's atoms that are through the portal already) that is moving at speed somehow has no momentum.
Or, this, but mind blown --> The exit portal would be the thing that moves backwards to maintain the zero momentum of the cube 😅
A. Object momentum relative to the world is retained thru portals regardless of the speed of the portal surface or the portal itself. The portal is only transmitting matter from A to B, it does not impart energy of any kind.
If the portals don't impart energy, you wouldn't gain speed when you put one directly above another and drop through forever. The higher portal increases your potential energy.
What if the entire room were rigged to a piston, the orange portal were held still and the room forced upwards at speed. Would you see scenario B then?
If so, how do the portals know which to choose? Remember that neither portal is actually stationary: they're both orbiting the sun at 30km/s, etc.
Its not the portal doing that though. It's gravity. Your own momentum moves you through the portal while gravity is acting upon you; the portal is nothing but a doorway.
It doesn't matter what's "doing" it; the energy has to come from somewhere - gravity is not a source of free energy. In the real world, you can't build a perpetual motion machine using gravity. With portals, you could pour water through a pair of vertical portals and use it to drive a waterwheel, and the waterwheel could, say, drive a generator and power a light. It would keep going forever. The light requires energy to work - where is it coming from?
Without the portal, an object tossed down a mineshaft would accelerate until, assuming it were deep enough, it reached the centre of the earth. Then its potential energy would be zero, and it would stop accelerating - it would have reached its maximum kinetic energy and minimal potential energy, and start trading speed for height, until it reached a maximum height (at the other side of the world) and oscillated back and forth. If you tried to use this to drive a waterwheel and power a lightbulb, each time the object hit the wheel it would slow down. That reduction in speed would translate to a reduction in energy, and a reduction in maximum height, until eventually the object would stop being able to turn the wheel.
That doesn't happen in the portal scenario, because the portal moves objects up, increasing their potential energy, no matter what.
A, momentum is in the portal, not the cube
Neither. The cube is not making contact with the portal at all, since the portal is on a stationary block with some speedy lines drawn on it. Speedy lines don't nake thingy go fast - everyone witb a gel-hairstyle knows that.
EVERY GIT KNOWS FAST LINES AIN'T DA TRICK! RED IZ WOT MAKES THINGS GO FAST!!!
Next you’ll claim stink lines don’t make something stinky.
These heathens! Arguing against centuries of line faith! Draw heat wave lines around them and make them burn up!
neither, no portal rules were given. Please create an Aperture support ticket.
The real answer is the enemies we made along the way
The answer is definitely B (see other comments), but I think the more interesting question is, how does it affect the portals themselves and the surfaces they're attached to?
When the portals are attached to a solid unmovable wall (as in the games), we can pretty much ignore the forces acting upon the wall as they will just be automatically cancelled out by the normal force. But the same does not apply for portals attached to detached panels. Let's say that instead of a piston pushing down, the portal is attached a free-falling panel with the portal facing down.
We are inclined to assume that as the portal falls onto the cube, it feels no force, as if it fell through air or through a vacuum. But is that really the case? Maybe instead, the falling portal experiences resistance from the cube? Or even, actually, an accelerating force?
Let's assume conservation of momentum at each end of the portal. That is, in the general case: if the portal (which we assume has no mass) is attached to an object with mass M and velocity V, and "swallows" a cube or other object with mass m and velocity v, and at the end of this interaction the portal-surface object with mass M has velocity V', then we expect:
M × V' = (M × V) + (m × v)
Mass is not conserved because the cube was transferred to the other portal.
Solve for V':
V' = V + (m/M) v
This is a strange result. The cube's momentum is imparted into the portal, but the result would be different depending on our reference frame. From a reference frame tracking the cube pre-interaction, v = 0 (by definition) and so V' = V, the portal-surface object has no change in velocity. But in any other reference frame, it does.
Physics can't be affected by reference frame (thanks, relativity) so this analysis must be wrong.
... I'll keep thinking on this.
Okay, after thinking about this a while, I've decided that conservation of momentum just doesn't work, unless the portal actually absorbs the mass of every object passing through it, which would probably cause its own set of contradictions.
Instead, I think forces that act through the portal are applied to the portal. What do I mean by this? Well, take this classic case of a cube floating halfway through two floor portals:
(sorry, can't do better than ASCII art right now)
The system has reached equilibrium, and nothing is moving. Both halves of the cube are being pulled down by gravity, but cancel each other out by applying a normal force on each other right at the portal interface. What I'm claiming is that this normal force is applied to the portals, which ultimately transfer it to the floor. If this entire floor was a scale, it would measure the calibrated weight of the Weighted Storage Cube.
But this is the static analysis. What happens when objects are in motion? To be continued...
So we might think: a cube falling through a floor portal and flying out another floor portal right next to it, kinda looks like how a bouncy ball bounces off the floor, and should apply forces on the floor similar to that - an elastic collision. Unfortunately, this brings us right back to conservation of momentum, which didn't work out. But maybe I just did it wrong? Maybe, if done more carefully, conservation of momentum can work without being dependent on the reference frame?
I genuinely don't know yet. Time to bring out the calculus.
We have:
M = the mass attached to the portal (e.g. the floor)
V = the velocity of M
V' = dV/dt = the acceleration of M (assuming no other forces)
F = MV' = the force applied on the portal
m' = dm/dt = the rate of mass entering the portal
v = the velocity of the mass entering the portal
If we assume conservation of momentum (our old nemesis), the force applied on the portal should be the same as the momentum taken from the mass entering the portal:
F = m'v
But now we're back to the case from before, where the choice of reference frame changes the outcome! We're not having that. So I hereby decree:
The force applied upon a portal by a mass entering it must be calculated by conservation of momentum in the intertial reference frame of the portal itself at the instant under examination.
This gives us, effectively: F = m'(v-V)
Now THAT'S something we can work with!
Note that this formula also works for the exit portal. In that case, m' is negative.
A curious result follows.
For the entry portal, m' is positive, and (v-V) is a vector pointing into the portal. (This must be the case, otherwise the mass would be exiting the portal). Therefore, the force would be pushing the portal into the wall.
For the exit portal, m' is negative, and (v-V) points out of the portal, therefore the portal again would be pushed into the wall.
The result: whenever anything passes through a portal, both ends always get pushed in the same direction.
So, back to my thought experiment.
If a portal attached to a free-falling panel falls onto a cube, the cube's entry into it would brake its fall to some extent. However, this wouldn't be the only force in play.
As I said in my previous comment, forces applied between the two halves of an object going through a portal, get applied to the portal itself. In practice, this mostly means tension and compression forces. In the static analysis from before, it was compression.
As the panel is falling onto the cube, a part of the cube is already out the other side and moving away the portal. If the panel has slowed, this would apply a pulling force (tension) on the part of the cube that still hasn't made it in. This tension force actually compels the portal, and the panel it's attached to, to speed up towards the ground.
So, in total, would the panel be slowed, accelerated, or unaffected? Maybe the forces cancel out and it just continues on its normal freefall? I don't know yet. Would need to do more math.
I legitimately love how much effort you put into this.
On a shitpost of all things. It's just beautiful.
If the universe moved around you would other people not perceive you moving? Would that not be indistinguishable to movement?
But it's not exerting force, it's teleporting.
This is excellent rage bait
"Speedy thing goes in, speedy thing comes out" as the cube isn't a speedy thing (no speed lines) going in, it won't be a speedy thing coming out.
The "ragebait answers only" in the OP is the only thing stopping me from actually raging at everyone saying A. I am giving them all the benefit of the doubt, they're deliberately saying the wrong answer.
Everyone saying B missed the assignment and answered accurately.
Where do the facts begin and the ragebait ends? Who knows, that's the fun.
The piston moves throught the portal and the portal hovers in the air.
It would probably be A because of Newton's first law of motion.
because of that law it is B ;). the cube "teleports" into the reference frame of after the portal and in this frame it is highly in motion.
it's like you trying to jump on to a moving train, you first need to match the trains velocity (speaking relative to earth), then you are at rest in the trains frame.
this kind of thoughts is what makes special relativity so cumbersome.
A 🔥
This is Einstein's elevator thought experiment but with portals.
B
Also, if the cube has to emerge out of the blue portal top first it has momentum moving out of the blue portal, as seen by the blue portals frame of reference. It can't emerge with zero momentum or it wouldn't move relative to the blue reference frame.
But in portals physics the answer is A.
Einstein's elevator explains why acceleration can mimic gravity. It doesn't, by itself, explain why a stationary cube would emerge with a large sideways velocity. That conclusion depends on additional assumptions about how moving portals transfer momentum.
Yeah it does explain it if you use your brain for like half a second. Is the orange portal moving down to the cube or is the cube moving up towards the orange portal? We have the reference frame of the cube so it looks like the portal is approaching the cube.
What you also conveniently forgot to mention is his thought experiment also explains relativity where you can't tell whether it's acceleration due to the elevator moving up, or gravity pressing you down in the elevator while it's stationary.
Since you can't tell the difference between the two scenarios the cube should only do one thing in both scenarios to be consistent with relativity. If it doesn't it breaks it...
Was going to post the correct Answers (A) but then looked at the post and though na. But if you want to know more about portals and how they would work in real life check this out. Great guy.
https://youtu.be/1zgBwQVr-wI
and his answer: https://optozorax.github.io/portal/?scene=speed_model
It's a superposition of both states until you make a measurement
A
Now what happens if you put it on a treadmill?
A, if you also teleport gravity
https://youtu.be/DydIhwLrbMk?t=1561
Ragebait aside the answer is B.
If you had a table with a cylinder, with a ping pong ball in it, and a circular weight dropped around the top of the cylinder that hits the table, the ping pong ball will bounce due to the forces of the weight hitting the table. Enough energy would be transferred through the impact, that would cause the ball to seemingly jump up in the cylinder.
Physics are weird.
nice ragebait answer, but I'm not falling for it
I reject your reality and substitute my own.
The correct answer is C: the cube plops out and the object to which the blue portal is affixed flies off in the southwest direction.
B.
As the cube enters the portal, it will take on energy from the piston, slowing the piston down in the process, and that energy will propel it out. If it enters the portal fast, it has to exit the portal fast.
Why would it take the energy of the piston? It never interacts with the piston at all.
Regardless of the how the portal works, if the cube enters the portal at a certain rate, it has to exit the portal at the same rate and everything else has to follow.
The cube doesn't interact with the piston, but it does interact with the portal and the portal is fixed to the piston. The same way that when I hit a nail with a hammer, I feel the resistance of the nail though the hammer handle. I don't touch the nail, but the force is transferred through the hammer and in the hypothetical, the force would be transferred through the portal.
The hypothetical does not bring up the possibility that the portal has infinite sharpness at it's edges and would cut though the piston head or the possibility that relativity is somehow involved and would allow the cube to exit the portal at a different rate that it enters. Wormholes moving at different rates is a theoretical form of time travel, but not at these speeds.
The portal isn't an object, the cube doesn't interact with it either. Saying you interact with the portal is the same as saying the hole in a lasso is an object that you interact with.
If the portal is being pushed down by the piston, that means it interacts with piston. A portal isn't a lasso because lasso's exist and have known rules to them. The only things we can know about portals is what's presented in the hypothetical.
Sure, because the portal is affixed to the piston, but you should note that the portal is not affixed to the box. The box doesn't interact with the piston, or the portal, in the same way it doesn't interact with a hoola-hoop that is passed around it.
If the box enters the portal at a certain rate, it has to exit the portal at the same rate. Unless there's relatively involved, that has to be true and everything else follows.
How is the portal affixed to the piston? Whatever made up system that affixes the portal to the piston would also allow it to interact with the box.
The box has to exit at the same rate it enters and the law of conservation of mass and energy dictates that no new energy is created and it has to come from somewhere. If there is no way for the energy to be transferred from the portal to the box, then simply, portals cannot exist.
It's doesn't matter really, this is a red herring.
There is the problem with your logic. The box doesnt move, there is a shortcut in space that moves around it.
EXACTLY!! A is the only scenario where energy (or more accurately, momentum) is conserved in this case. If you want to think about why that is, see my previous post where I extrapolated on a case where a lighter object with a portal on it is moved over the box.
It pushes off of the bottom platform, the cube accelerates by pushing off of itself (if it didn't it would flatten into a pancake). And because there's a downward force on the bottom of the cube it pushes down on the platform
I'm not sure what you're saying. You mean the piston and portal would compress the platform? Even if that's the case, that still means force is being transferred through the portal.
The momentum the cube gains comes from the platform. The piston "can't tell" the cube is there, it experiences the same forces if the cube is there or not
You can't put a portal on a moving platform, it has to stop moving first, so neither can happen... Unless the moving surface is a celestial body at least the size of The Moon, I guess?
What!? What if the platform is a train on Earth?
Portals don't exist so the piston crushes the cube
If you come out stationary wont you become a pancake of all your z axis compressed into a single infinitely thin plane? Also i think when the game calculates this it treats you as a point travelling through the portal plane and adds the plop worth of momentum as a compensation to not break the physics. Also if you think about it a lot of different situations already lead to extra momentum out of nowhere(classic portal above and below infinite loop as an example) so the portal in this case adding momentum doesnt break physics cause portals already break it. Also theres a problem of some people answering with the game physics in mind and other with the real world interpretation of it so this really is the perfect ragebait.
Does that actually loop infinitely? Don’t you just reach a terminal velocity and then maintain that momentum?
I mean yeah thats true but that still means the cube would heat up or something and then you could use that to generate infinite energy. The point is you can accelerate something to a speed without putting in the energy needed to accelerate it.
The cube wouldn’t necessarily heat up much while approaching terminal velocity.
Its close to infinite energy because you never have to reach the ground, so you technically have infinite potential energy but when you convert that potential energy into kinetic or some other type of energy, the vast majority of that potential will disappear instantly
Edit: in other words, you could generate incredibly large bursts of energy but not sustained energy with this kind of portal loop
But that doesnt really matter cause my argument of it breaking physics still stands. Little infinite energy and a lot is the same.
Terminal velocity is only reached because the object (or player) is colliding with the air. Momentum is still being added. It's just also transferring out into the air. Terminal velocity just happens to be where the two rates match. You can kind of think of the object as being a fan blade running on zero power. It's technically a source of infinite energy, but it might be hard to extract from that exact setup. If we could do this in the real world, we'd tweak it to just be water in a vertical pipe with a turbine extracting the free momentum.
you know only engineer/computer science nerds post on lemmy because nobody has called out that the box looks like the box from hellraiser.
no matter what you do to it, you'll probably end up in hell.
A if it's a cube. B if it's beans.
Edit: Obligatory bean tax
The top of that masher is clearly protecting the cube from the rain, so I'm not sure how the rain is expected to shoot the cube out as in option B.
A.
A
In this example, the object wasn’t moving. It has no momentum to continue moving. Instead, the world around it moved. So A
Movement is relative.
You need a second reference point for movement to exist.
To the (for us) moving portal, the cube was moving. And it would keep that momentum.
Get a hula hoop attach a camera to it. Drop hula hoop over something with it being in the middle. Did thing in hula hoop take off? No. Thing had no momentum. It's velocity versus relative velocity.
If the universe on the other side of the hula hoop moved with it, the thing did take off in reference to that universe.
A person in that universe would see a thing rapidly approaching the hula hoop from the other side, before it getting launched through.
You need to cut the hula hoop in half. The bottom moves and the top is stationary off to the side. The cube pops out at the same speed the bottom half is falling, but because the top half of the hula is stationary, the cube seems like it's moving upwards.
I hope that makes sense
Yeah if this existed IRL B is the correct answer. If you tried this in portal the video game A is the answer.
If it were in real life it would be A. And if the game engine allowed you to move portals, it would also be A.
This has been really heavily debated before and the unanimous answer is B IRL and A for portal.
How does the cube move out of the blue portal? If you ignore the orangep portal entirely and consider just looking from the side as the cube emerges from the blue portal, the top has to be moving extremely fast. The cube needs to be moving relative to the blue portal at the same speed as the orange portal is descending or it doesn't work.
You can also draw a force balance diagram and you get the same result...
But you're welcome to disagree with me! That's part of the wonder of the internet.
Correct.
It’s like dropping a hula hoop over it in this scenario. There is nothing to generate its motion.
Using portals rules yeah A is the correct answer.
B would be correct if this existed in real life and had to be compatible with general relativity. If we took this exact same setup and moved it into space, how could you tell if the cube is moving towards the portal or the portal is moving towards the cube? You also can't use the blue portal as a reference point since it could also be moving independantly from the orange portal.
B
The cube gets sucked into the portal with the same force the platform in A is getting sucked upwards.
It would be A because it's the orange portal that's moving, not the cube.
The only way it would be B is if the portals suck since the cube doesn't have any momentum to begin with. The game clearly shows that this is not how portals work.
Think about how you come out of the second portal, your head comes out first and the rest of your body follows. Your head has to have a velocity coming out of the portal or the rest of your body will smash into you. So either you come out flying or you're smashed to a pulp.
You don't even need to think about reference frames for the answer to be B, because each layer is being displaced by the layer behind it. Take the limit to go from a discreet to continuous domain and you exit the portal at the same speed you entered, which is exactly the speed of the falling portal.
This is clearly an unbirth scenario and the A and B things are unrelated. Either way, your baby is companion cube and GladOs is the father.
Thanks for coming to my Ted X talk.
There's a guy on YouTube with excellent portal simulations that's been working on the theory of portal physics for a while named optozorax
The entire pedestal the cube is on comes through the blue portal and the cube slides to the floor
B. Because it's relative.
Also, because Cave Johnson is your dad.
Has to be, for the particles to clear the second portals entry they have to be moving at a relative velocity to the first portal at the same rate the first portal is moving
B. this is because of Galilean relativity i think maybe
I'd say a mix of both. There's inertia, but there's also the gravitational force when the cube gets to the other side.
[email protected]
Assuming the black platform does not fit through the orange portal, and the moving panel with the orange portal stops on impact (which is what appears to be true because on the blue portal side we see the surface of the black panel), then the cube is no longer moving relative to the orange portal and therefore has no momentum. Once the cube is entirely on the blue side of the portal it will be affected by the gravity on that side and slide down as if on a ramp (A).
C. The moving portal platform breaks the black platform and the whole thing goes through the portal.
Jokes aside, B.
Devoid of any other external forces, in the cube's reference frame there's functionally no difference between it moving towards the portal or the portal moving towards it at a constant acceleration.
Ergo the cube should be pushed out the other side at the same momentum that the system had prior to the collision, and not be simply plopped out as in answer A.
What? That cube will definitely get crushed, no idea what's up with A or B but that cube will get crushed and I'm looking forward to it
Real world B. In-game, A
It depends. It looks like the orange portal is being moved by a hydraulic cylinder (aka laminar flow), which means the relative positions and velocities of things moving through the orange portal are conserved, and A is correct.
If instead that's (yuck) an air fired cylinder... Then it's going to be B.
If the movement is handled by a threaded rod, then the box will emerge from the blue portal spinning, and (perhaps contrary to expectations) if the orange portal is moving because of gravity, then the bouyant force of the box progressing through the portal will stop it from falling further, and the box will end up just less than halfway (because of the angle) through the blue portal.
And as a matter of fact the fastest possible velocity you can get is with a hydraulic cylinder because of the immense amount of force it applies using the orange portal.
If you used magnetic levitation to move the orange portal the box would never make it through, because it's made of metal and will be attracted to the levitation equipment.
I don't know enough about space-time. Does distorting space transfer energy to the matter in that space? How about time? I think it must be A, out of the options.
Otherwise the future of man is less 'sustained ftl travel', and more 'giant space slingshot'.
the cube isn't fully lined up with the portal so the collision with the edge of the portal/platform causes some source shenanigans and ends up being option b
It should be a.
Imagine if the cube was actually a long rod and the orange portal start with the column half way inside it. Then the orange portal moves, in this case it would be very odd for the rod to jump in this scenario like in case b, the thing you would expect would be for the rod to stay still (besides tipping sideways a bit) consistent with case a.
If you considered the cube to be 1m^3 , the time at which the cube enters the orange portal t1, and the time at which the cube fully exist the blue portal t2, then the speed of the cube relative to the world outside of the blue portal would be 1m / (t2-t1).
Assuming the cube maintains this speed after exiting the blue portal, it would behave as in scenario B.
Most likely option is it flops out but only if the angle is sufficient for the cube to overcome its centre of gravity and topple. And do so before the orange portal rises again. Also depends if the downward orange portal directly touches the base or not, since there might be a bit of a "lip" to overcome too.
A, but it would have the average of the two states.
It gets stuck in between, with part of the cube not being fully outside, it hangs on the edge.
This only works if you treat the cube as one object anchored by a small part. In reality, objects are a bunch of particles each with their own momentum held together by forces between each particle usually strong and springy enough to cancel out differences in momentum. This is why all objects tear apart if enough uneven force is applied on them.
If the portal stops partway through the cube, the stationary bottom part on one side of the portal and the top part still carrying the momentum of recently exiting the portal will have very different competing momentums. If the cube's internal forces win out, the overall momentum will average out, and the cube will fly up with a speed proportional to however much of it got pushed through the portal before it stopped. If the competing momentums win out, the cube will break into both stationary and flying parts.
divide by zero
A
Everyone else is wrong. (Ragebait.)
Speed of approach = speed of separation.
If the box left the blue portal at a slower speed than it entered the yellow one, the top of the box would be moving slower than the bottom of the box, which would compact the box dramatically during the transition, but we can see that its size is unchanged and it is still square.
The top of the box is pushed rapidly out of the blue portal by the bottom of the box being thrust into the yellow portal. (The bottom of the box is pulled rapidly up into the air by the top of the box which is already moving quickly.)
More importantly, what if I straddle the blue one?
Img pls?
Portals aren't real, you know.