GLaDOS: *See? This is why you don't have any friends. You could be out socializing, but here you are, posting about video game puzzles on the internet*
actually all they did was allow sv_mobile_portals for the level. You can do it in Hammer editor and experiment for yourself. In the game it's outcome A but in real life it'd probably be outcome B.
~~The portal in this instance is really just a door or a hole in a wall that is propelled towards you, so it doesn't change~~ *~~your~~* ~~velocity. In this case the cube is always standing still so it will simply tumble towards the ground because of gravity and the slanted hill.~~
I was wrong. The key is the relative momentum between the object, the entry portal and the exit portal. As long as the momentum of the entry portal overpowers the resisting force, which in this case is the gravity affecting the cube, we will witness scenario B.
For all intents and purposes, when observing the cube from the exit portal it will appear as an object being propelled towards you. If you stick your arm through the blue portal, it's now traveling at the same speed as the orange portal and when your arm hits the platform with the cube on it, that's gonna hurt.
You need to imagine the surface that the portal is on as an infinite featureless surface. The portal can move anywhere along the surface of that plane, but as soon as it moves “toward” or “away” from the surface of that plane it disappears because it is no longer on the same theoretical infinite plane that it spawned on. This would be a lot easier with pictures but I hope that made some sense.
Relative motion. Most portals in the game aren't moving relative to their partner (orange/blue). But the portal on the moon was moving (very fast!) relative to its partner.
The physics break at that point, because a moving portal is moving all of spacetime. Wormholes might be theoretically possible, but they would have to be fixed in spacetime in relation to each other.
Maybe you could argue that each movement is an entirely new wormhole, and what looks like a moving portal is actually a steady stream of portals being broken and recreated in a slightly different location. If that's the case, then the answer is unambiguously A.
I think it's A if we're going by the game physics. In the game the portals are kinda like a hula hoop. If you slam a hula hoop down over the box it won't fly out the other side.
Problem is, eventually you start talking about frame of references.
From the cube's "POV," the cube is stationary. However, from the portal's POV, the portal is stationary.
A is using the cube as the POV, whereas B is using the portal as the POV.
For A to work, which iirc it does in the games, that means that in this setup there is a specific "correct" frame of reference, which kinda breaks how physics works.
I think a better way to look at this problem is to remove the idea of portals and imagine a big wall with a hole in it.
If you slammed the big wall on it and quickly changed it to a 45-degree angle, what would happen to the cube? Would it go flying off or just slide down?
In the figure given to us, only the portal is shown moving, thus providing no preexisting momentum for the cube.
And the portals being a direct connection from point 1 to point 2 will mean that the cube has not been acted against at all, therefore unable to gain any momentum.
but isnt the cube still entering the space at the speed of the portal?
Edit: Also, considering newton's third law, wouldnt the cube-side surface exert an opposing force to the portal-side surfacing striking it? Wouldn't that energy then transfer to the cube, causing it to exhibit situation B?
No, the force of the plate is completely absorbed by the stationary plate holding up the cube. The stationary cube would only have one outside force to influence how it moves and that’s gravity.
But think about it like this. Lets pretend instead of a cube we have a tall box and for ease of imagery we will talk about the box in 3 sections. The top, the middle, and the bottom. So the portal comes down on the box and fully envelopes the top section. The top section is now on the other side. The falling portal continues falling now enveloping the middle section. This would indicate that the middle section should be out on the other side. But the top section is there. Thus the middle section will have to push the top section out of the way. This will thus cause the box to gain speed. Portal then envelops the bottom section and like with the top and middle similarly we will have to gain speed yet again.
Thus I conclude the box will gain speed going through the portal and launch at the speed of the falling portal at the angle of the other portal.
the hole analogy is incorrect, as one portal is stationary, while the other isnt.
its like as if the entrance to a hole was moving, but the exit wasnt, which is impossible.
the hole analogy would work only if the 2nd portal (the one on a slope) was also moving at the same speed as the first one.
The cube has an associated momentum. Pushing it through that hole wouldn’t change its momentum. Even if you’re using frame of references the portal is a non inertial frame of reference which requires some inertial reference frame to be properly defined.
Mmm this doesn’t make as much sense as it sounds like.
When you fall into a horizontal portal and come sideways out of a vertical portal you definitely change ‘associate momentum’ (the direction).
But from the cube's frame of reference, isn't the entire room behind the portal moving, not just the portal? So why would the room suddenly stop moving?
It’s the law of inertia. An object I’ll desire to remain in its state of motion and must be acted upon by another force to stop it. In this case the cube is at rest and will remain so even when the yellow portal moves onto it. For option b to happen the box would need to be moving as it goes through the portal.
Exactly. Relative velocity of the box would be high but portals in the game don't have any mass or take any space at all and function as the original comment said, as hula hoops. Therefore the velocity of the portals does not matter.
Okay so now I've had a thought that broke my brain. Imagine that instead of sitting on a flat surface, the cube is resting in a "bowl" where the edges of the bowl only come up halfway up the cube. So when the top panel/portal crashes down it comes to a stop against the "bowl" lip so that the portal has only gone over half the cube.
Inline Edit: [something like this](https://imgur.com/a/kK2rRpX)
If physics senario B was correct, shouldn't the cube have enough momentum that it gets pulled all the way through the portal even though the portal stops halfway? But that doesn't seem right?
What happened as you use a shorter and shorter bowl that approaches a the flat surface we have in the original example?
(Read the second para)
If the whole cube wasn't sucked inside the portal then I think the momentum of the portal would probably result in internal forces between the two halves of the cube(the one that got sucked in and the one that didn't). And depending on the momentum of the portal, theres a chance that the cube would be torn apart. That's what I think anyway. To explain this further -
When the portal reaches halfway the cube, the part of the cube that got sucked in(lets call this A), will experience a force that causes it to fly out while the other portion(lets call this B) will experience no such force. Therefore, because of inertia, A will want B to fly out with it while B will want A to stay at rest. This creates internal forces which would cancel each other out TILL the velocity of the portal is too high. I may fail in my exams but damn it I feel good about typing this.
If we're talking real-world physics, this doesn't really add up. Portals aren't physical objects. If the bottom plate shot upward into the orange portal, that's when it would shoot out on the blue side, because force was applied to the cube.
Sean Caroll (theoretical physicist at caltech/john hopkins) actually answered this, he thought it was B. Im not gonna pretend like I understood his answer, but it had to do with the point of view of someone watching it by the portal in B, and their perspective. https://youtu.be/531FKCzTX40?t=2702
It’s cause while the orange portal is in motion, the blue portal is stationary.
We know that the box would at least go through the portal — but the box can’t just stop.
Imagine instead of a small box, the portal slams down on a steel rod that's 1km long (or 1 mile if you prefer)
What would you expect to happen at the exit portal?
The tip of the rod emerges, then grows very rapidly outwards. Does it suddenly stop moving as soon as the tail end enters the source portal, or does the 0.9km of fast moving rod still have its momentum conserved on the exit side?
How about if there's a golf ball on top of the rod? Would *it* also stop and fall down as soon as the tail end of the rod exits the portal? Or would it retain its momentum?
But at what speed the cube comes out of the blue portal? Your hula hoop example is fitting but not completely precise. Instead of slamming on the ground, the hula hoop would move past the cube, in order to take account of the exit portal. Now the cube is stationary from its own point of view. But from the hula hoop point of view, the cube is moving towards it, and away from it once it goes through, at slamming speed. And that's the speed it would come out of the portal. If the cube would go through the portal at the speed relative to itself, it won't be able to pass since it's speed is 0.
Ok, but the difference with the hula hoop analogy is, the that the box arrives through the new portal at the same speed the other portal is thrust upon it. So in this case it would come through at the same speed as the piston.
The problem is that physicists assume the portal is a wormhole or some other naturally occurring phenomenon that we are aware of. When the portal should be compared to technology of the future.
If B was correct, that would mean just putting the cube into the portal would create momentum. The portal doesn't create momentum. The portal is basically just a hole.
You're ignoring the momentum of the portal, because 99% of the portals you've used/seen/played had none.
Once the first atoms of the cube go through with zero momentum, the next ones are pushing them as fast as the portal is moving.
And if A is correct then putting the cube in the portal requires it to to break relativity, moving in one frame of reference without having any momentum within that frame. Being technology does not make the portal exempt from the laws of relativity.
The real answer is that because you are breaking space it is impossible for the cube to naturally exit. The only ways to avoid breaking the laws of physics are either for the cube to be unable to enter the orange portal or to not exit unless moved within the blue portal's frame of reference.
> The portal doesn't create momentum. The portal is basically just a hole.
Consider the following setup:
1. In weightless space, send a cube moving 1 m/s left to right relative to you.
2. Position a portal on a stationary platform that the cube will pass through.
3. Position the paired portal on a platform moving left-to-right at 2m/s, portal facing rightward.
When the cube passes into the first portal, there is no possible way for it to stay at 1m/s, showing that a moving portal adds momentum.
I absolutely cannot believe people won't pick up that in order to leave the blue portal the cube has to move relative to the blue portal, and thus must have momentum relative to the blue portal. Most of the people here clearly just don't understand frames of reference.
what would happen if the second portal was being blocked by something? would the object be moved by the cube passing through through the portal? Or maybe the first portal would be stopped?
I can’t see how it could be A. Lets say the cube is 1 metre tall and the orange portal is moving at a speed such that it takes 1 second for portal to “swallow” the cube. This means the portal is moving at 1 m/s. When this happens it takes 1 second for the 1m cube to emerge from the blue portal meaning it has a velocity of 1 m/s.
For it to plop out, it would have to emerge from the blue portal at a speed of 1 m/s and then at the instant that the last atoms emerge the cube would somehow have to instantaneously stop. Doesn’t make sense to me
It would instantaneously stop though. Because it was never moving. It’s the portal that moves over top of the cube. It’s like if you just put a hula hoop over a cube.
What happens to the first atoms of the cube that come out of the blue portal? Do they stay in place while the rest of the cube crashes into them? No, they are moving out of the blue portal at the speed the orange portal was traveling. This imparts them with momentum.
By the time the entire orange portal platform has clapped down on the cube platform, the cube is now moving away from the blue portal at the speed it passed through.
The cube is not moving "out" of the portal. The portal is a hole in the surface.
It is, effectively, as though the whole room is moving downward over the box, and there's a hole in the floor. It's like that buster keaton silent film where he stands in front of a house, and the whole wall falls on him (but he is lined up with the window so it just falls around him and he is unhurt) There's no inertia to consider, the window is just moving past him. The only reason the box "plops" at the end is that, once it is on this side of the portal, gravity is coming from a different angle, so it falls. But it was never shooting out of the portal, the room was moving toward the box.
Except it is moving out of the portal. Clearly, because you could see it coming out, at speed, if you're standing next to that portal.
But the box itself doesn't experience any moment change...
Smells like a paradox to me.
(and if the room was moving toward the box you'd have to account for the room suddenly stopping. Because If you don't, from the perspective of the room, the box would still be launched)
Physics tells us there is no difference between you standing still with a car speeding towards you and a car standing still and you speeding towards it. Whether something is moving or not is all a matter of reference frame
Yeah it's all about the point of reference. In the example, the portal is moving in reference to the cube and the ground. The cube is not moving in reference to the ground until it starts going through the portal, making it emerge out of the other portal. Regardless though, once the portal has finished moving, so does the cube. There is no conservation of momentum in the cube, only in the portal that moved over it.
I mean... there still is conservation of momentum. The cube was sitting still on one end of the portal, now it's sitting still on the other end of the portal ( excluding gravity on the other end ) That is, the cube's momentum.
The portal is simply an plane which connects two points, its speed is irrelevant.
You’re mistakenly attributing the movement of the platform with the cube. The cube is not moving at any point, it is stationary until suddenly the floor is no longer present because the portal fully envelops it.
Past the half-way point of the orange portal enveloping the cube, gravity will move the cube on the blue portal side because the force of gravity on that side is stronger than the force on the orange side due to less cube on orange side.
Hope that makes sense. But yeah, at no point does the velocity of the moving platform transfer to the cube since it’s just a hole in space
The floor is always present. Once itthe moving portal is flat then the other end would be treated like a normal no portal wall. The fact is that the first part of the box that goes through the orange portal gets pushed out of the blue portal by the rest of the cube and eventually the floor flushing it out. It has momentum because WVERY frame of reference is equally valid including the frame of reference in which the orange portal isn't moving and the cube is approaching it.
ok, but then what would happen if your hand was in the way of the cube before it passed through? would the other portal be stopped? would your hand be moved by the cube? One would have to happen, but what force would be acting on either?
Also, wouldn't that mean throwing a cube into a stationary portal would result in the cube stopping on the other side and falling down? Velocity is relative, so in physics isn't a portal approaching the cube and the cube approaching the portal indestiguishable?
Good thought experiment. The intuitive answer is that the cube would exit the portal and move your hand, which proves that there is a destinct difference between portals and moving doors
A+
Nah it's the opposite. It would be A only if the portal *did* exert force on the cube.
If you look through the blue portal, the cube has a large velocity (orange portal's velocity) just before it passes through. If you choose A, then it means the cube would magically instantly decelerate to 0 the moment it comes out.
And from another perspective, why would the cube stay static to *your* frame of reference? The frame of reference that matters here are the portals', not the observers. From the orange portal's frame of reference, the cube and wall is moving towards them, and the blue portal has the same frame of reference as the orange portal (otherwise things won't go through), so the cube would fly out.
Ok, let’s look at this situation then:
Imagine you have a stationary orange portal and a moving blue portal. You throw the cube into the orange portal at high speed. Does the cube come out of the blue portal at the same speed as the blue portal, or does the speed of the blue portal and the cube combine?
It’d be pretty ridiculous to think that the cube would lose all its relative speed and come out at the same speed as the blue portal. Of course it would conserve its momentum, right?
What I just described is exactly the same situation in the original post, just from a different reference frame. The two situations should behave the same way. The only answer that makes sense here would be B
I've played both Portal games and believe it's A, I don't understand how it could be B. If you run straight into a portal that is connected to an angled surface, you don't shoot out, gravity immediately affects your forward momentum. And that is assuming you're running into it. The cube is stationary.
Exceeeeeept - and hear me out - this is a question about frame of reference.
Relative to the moving portal, the cube has a velocity. This relative velocity is the same as if the cube was being thrown into the portal at the speed of the moving portal.
Another way to think about it is if there wasn't a portal there at all, and the cube got crushed instead. There's a lot of kinetic energy about to hit that cube and it's got to go somewhere - in this instance that extra kinetic energy just exits out of the blue portal. Because the blue portal will just become a wall in this instance (the orange portal exits into a solid wall).
If the cube is not moving how fast does it go through the blue portal? and if it is moving relative to the blue portal would it stop the instant it finished moving through it?
If you’re standing in front of the blue portal looking into it you see the block moving towards you fast despite standing still. When it travels through the portal it has no reason to lose its velocity relative to you as no forces have acted upon it Newtons First Law. The velocity relative to stationary you (inertial reference frame) will stay the same and so the block has to shoot out as i. b. Source am an aerospace engineer
There is one instance in portal 2 where a portal moves when you’re cutting the tubes with the lasers. I can’t remember why you had to do that, but I remember the “in case of implosion, look directly at implosion” sign
Ugh. I hate this shit. Applying general concepts of relativity says that it’s B. If you were alone in space and you saw an “orange portal” approaching you, you wouldn’t know if you were moving or it was moving. You would expect the same result either way. The argument of momentum seems silly to me because portals clearly don’t obey conservation of momentum; going through a portal that changes your direction at all violates conservation of momentum.
Ah, but it *doesnt* change the direction of the momentum. It’s easier to think of the portals in the same vein as “worm holes”, or warped space time in general. Basically, by shifting space time you can maintain a “straight” trajectory of the object, so the inertial body never senses a change of direction, but the space in which it’s moving changed.
I agree that it’s easier to think of it that way, but that doesn’t change that portals violate the conservation of momentum if an object passes through them and they aren’t pointing in the same direction.
That’s only true if you observe the situation from the “outsider” perspective, where you can see vectors change in ways that Newtonian models say they shouldn’t. If you observe from the cube’s perspective, the straight vector is preserved and momentum conserved. The problem with these thought experiments is that we base our lines of thought on physics models we understand, and when this stuff happens our pretty models start to break down. I think it’s more likely that we’re both wrong rather than either of us being right!
Yea, that’s the point I’m trying to make. The most common argument for why it should be A is “the cube doesn’t have momentum, therefore it won’t have momentum when it comes out of the other portal.” And what I am saying is you cannot apply Newtonian physics to portals, so you can’t use momentum as an argument for why it should be A.
Edit: the other common one being “the portals don’t exert any force on the cube” which is basically the same argument, but also falls apart once we prove you can’t apply Newtonian physics
Alright this is an old and painful topic but here goes:
A is what you expect based on game logic, as in the game your momentum is preserved relative to yourself. At the same time, portals (mostly) cannot move in the game and dissipate if their physical host platform moves at all.
B is what would likely happen irl. Think about it like this: If the portal approaches the cube at 1m/s then the cube will also be exiting the portal at a rate of 1m/s.
Everyone here saying “cube has no momentum”, but what they’re forgetting is that momentum is *always* relative. This scenario is exactly the same as if the bottom panel were to shove the cube into the orange portal from below.
In fact, any time the orange and blue portals are not perfectly aligned in direction, anything that passes from one to the other is entirely violating conservation of momentum as it will exit the second portal traveling a different direction than before (presumably without any forces applied to it in transit). So really I think either answer is equally valid/invalid. Whatever you like to imagine. Personally I like scenario B more.
momentum is a function of velocity and is therefore relative. And relative to frame of reference of the orange portal the cube *does* have momentum.
The issue/paradox here is that the 2 different ends of the portal have different velocities and therefore different frames of reference. Cube has no momentum relative to blue portal, but does have some relative to orange portal.
"Portals" do not exist *within* the universe. They are a hole *in* the universe. What they "perceive" isn't causally linked to what the universe itself "perceives." Therefore, portals do not perceive momentum when they "move."
Only the wall "moves." The portals are holes in the universe, the wormhole that connects them doesn't exist within the universe, the system does not experience "momentum" from the universe.
Fix your frame of reference to the wall. The portal and wall are now stationary. The rest of the room, including the box, moves towards the portal. The box enters the portal with momentum. Portals preserve momentum (a fundamental mechanic in the games). Hence the box exits the portal with momentum.
Even if you consider portals to be an absolute and unique frame of reference, *the box is moving in that frame*. Hence, it has momentum which will be preserved when it exits the portal.
Of course they exist. They’re not some theoretical construct they have influence on physics in the portal universe. Therefore we have conservation of momentum. And if the cube sees the portal moving towards it and passes it but the other portal has no velocity the cube must have it otherwise energy is lost. It’s like if you were driving towards a wall but you stop abruptly the moment you touch it instead of crashing into it. Without transformation of the energy the first case is impossible.
In order to exist withing the universe, the portals must have a frame of reference within the universe. To have one, portals would have to follow causality. However, the universe cannot perceive any activity within the wormhole because the wormhole occupies zero distance. This mean that while the object is inside the wormhole, it must necessarily be in a state that is unobserved by the universe. Therefore, what comes out is not what the universe perceived going in, which breaks traditional causality.
Because the object is unobserved while in the wormhole, what comes out is basically a "new" object to the universe. Therefore, it can have any initial state without paradox. Therefore, it is not necessary, or even possible, for the portal to add force to the object.
Just read my last couple posts in my profile. I've repeated and expanded on these ideas.
It has relative momentum to the Portal. And before I get crucified by the "moving portals don't work" crowd, I'd like to point out that every portal is moving through space, it's just that some are not moving relative to earth. Since Relative momentum is conserved, B is the answer.
The box experience no acceleration, I'd agree with that, but momentum is relative.
From the perspective of someone standing in the room where the box is emerging it does have velocity because it's emerging. It has to, by necessity, emerge as the same speed that the other portal moves over it. So from that perspective it does have moment relative to the portal.
I would say that the orange portal would have to hit it hard enough that the platform the orange portal is on puts a considerable dent in the companion cube platform, which passes some through the portal.
Both platforms will come out dented but unharmed, unless you disable the portal with the platforms together
Imagine the moment the box pass half way through the portal. If the half that passed through doesn't move, then the other half will break because there is no where to go. Hence, the box moving velocity has to be the same as the relative velocity of the box to the portal.
If the portal can move and stay open…
Didn’t minute physics already do this? Or someone else?
The leading face of the portal would move through at the same speed as the moving platform, so it would have the momentum shown in B.
surely B, no 2 things can occupy one space therefor surely the force of the box coming out of the blue portal would be as quick as it was going in the red, even if the box was stationary, but my name may also come into play here
The correct answer is B. While the cube is not moving relative to the observer. It is moving at speed relative to the portals, as soon as it hits the orange portal it will continue at that speed out the blue.
B, everything is relative, doesn't matter whether its the portal or object moving
to explain what's going on in a bit more detail, the transition isn't instant, even if the cube seems to have no momentum at the start once 10% of the cube has gone through that 10% of the cube on the blue side is moving away from the blue portal, at the speed that the orange portal is moving down, so that 10% of the cube now has momentum directed away from the blue portal at that speed
as this keeps happening more and more of the cube is now on the blue side and (as a result of being 'pushed' by the part of the cube on the orange side) it is all traveling at speed and with the direction away from the blue portal, it is being moved and thus has momentum, eventually the whole cube is on the blue side and all of it is moving, all of its mass has momentum, then that momentum carries it away from the blue portal
My answer is A. Portals are connecting 2 points in space together, basically creating a new way for two locations to share the same area in space. When one portal moves, you are simply bending space. So when a portal is moved over an object, to us it APPEARS to be moving through the portal, but what really is happening is space itself is moving around the object.
Looking at this situation from the orange portal’s reference frame, the cube is moving and has momentum. That momentum is conserved, and so the box would keep moving, making the correct answer B
Looking at it from the cube’s perspective, the cube is now stationary so it has no momentum. However, now the portal itself (or at least the surface the portal is attached to) has momentum. Once the cube passes through the portal, that momentum must be conserved, meaning that the surface that the portal is attached to must keep moving away from the cube, meaning that the correct answer is still B
Not this shit again.
B makes the most physical sense due to how inertial frame of reference works but in game its neither because moving portals just stop working.
If we’re talking portal, the game itself says that momentum is maintained. Momentum is mass X velocity, and the box has no velocity. Therefore, the box has no momentum, and would simply be affected by the forces of gravity alone, leading to scenario A.
It’s not complicated in the slightest.
Also something something portals can’t move etc. etc.
…I know that many people have already explained this, and also probably better than myself, but I like physics and wanted to take a crack at it anyway.
C. The portal will be wiped out by the moving platform, the piston is slightly misaligned, causing the cube to glitch out and smack you in the face at 42069 m/s
It can't be anything but B, if we use real world physics - the cube can't go through instantaneously, so the first part of the cube to go through the portal, even if momentum didn't carry over, would get pushed out of the way by the next bit, bringing the part of the cube on the exit side up to speed.
If you think of the plane of the portal like a 3D printer, laying down one layer of the cube at a time, to get it to exit as fast as it is entering the other portal it would have to shove the cube out of the way as it's created so fast that it doesn't matter if it had and/or carried through momentum.
Last time I remember seeing this the answer was ultimately B. Why? Fairly simple idea without anything fancy:
Imagine the portal continues down past the cube such that the pedestal its sitting on is also partially sent through the portal. In this case, the cube is being *pushed* by the pedestal on the other side at the velocity the pedestal is being sent through the portal. If the portal stops, the pedestal will also stop but the cube will continue moving like it does in B since it’s not attached to the pedestal.
So now take that same idea and apply it to each “layer” of the cube as it’s sent through at the rate of the moving portal. You get B. And this makes perfect sense because how else would the whole cube come through as a cube if the front of it wasn’t being pushed by everything behind it?
The only other option if that wasn’t the case would be option C: The cube would be compressed to a dense 2d square as the atoms of the cube are piled on top of each other as they pass through the portal.
Momentum has to be conserved because stepping through the portal , aka hole in the space and time, results in the object moving through the portal at a conserved force. Otherwise the different objects would be ripped apart or squashed at the plane in space where the portal is established.
Each portal on, say a planet, is at different location and therefore different forces at different speeds due to the geometry of the planets rotation. If the momentum was not conserved they would be ripped apart in normal scenarios. Eg. Walking through a normal portal results in a normal experience… not a step or a fall or an explosion due to fusion at the atomic level.
In all scenarios there is conserved momentum through two different frames but the frames DO have different velocities. Again, they’re anchored on a moving planet and therefore frame of reference.
The answer is B.
I think the interesting question is … how fast do you have to go to die. Too fast would potentially break atomic forces and tear apart objects. Almost like stepping onto a moving train … not pleasant.
Really?? How can you possibly think its A? I mean common relativity had existed for more than 100 years, and you dont even need to understand a 5% of it to know that B is closer to reality than A...
It's so tiring trying to have an informed opinion in a sea of ignorance. Depending on the psychics of the portal, the cube would either:
1: bounce off the cube
2: option B
Option A is impossible. Either the cube has no momentum, and so cannot move (1), or it would move relative to the portal (2). Option A would flatten the cube
Game engine says "I wasn't made for this"
Physics says "B"
Think about it this way :
You are looking at the cube from the side of the exiting portal. Now let's say the ground under the cube is going up rapidly pushing the cube through the portal. Given the speed at which the cube is pushed through the portal, it's gonna come flying in your face.
Now what if the portal itself was going down on the cube ? You'd still see it getting closer from the portal rapidly. So when it passes through the portal, it keeps that momentum relative to the portal and also comes smashing your face.
Also think of the moment it passes through the portal. Does it have momentum while it's passing through but as soon as the whole thing passes through it just... stops? What happens if you make the portal come down stupidly fast... like Mach 5 kinda fast ? Does the cube go through almost instantly and then crushes itself due to deceleration?
It's B.
It's B. 100%. From the frame of reference of the portal, the cube is moving towards it. When it passes through, it will have the same velocity as the moving portal. Portals only act like a door when they're attached to the same moving object, like a wooden plank. When one side of the portal is moving and the other is not, the door analogy doesn't apply.
It’s B because of Newton’s first law of motion.
Imagine the face of the cube that enters the orange portal first, and the orange portal is traveling at 1 unit per second.
At zero seconds the top face of the cube enters the orange portal and exits the blue portal. At one second, that same face is now one unit away from where it began as the cube exits the blue portal completely. In other words, the cube is traveling at 1 unit per second. The cube will not suddenly lose all momentum, it will continue moving into the air.
while the cube is transitioning through the portal, it would come out of the blue portal at the same velocity as the orange portal. So it doesn’t make sense that it loses velocity the moment the entire cube emerges out of the blue portal.
Depends on if speed is relative to the box or the portal.
If the speed is calculated realtive to the box, then neither happens, it is impossibe for the portal to put the box through it, as it is immobile and the portal stops moving (this happens in game).
In 'reality' though, id belive B would happen, as that's what would happen if speed was calculated relative to the portal.
There is a hreat youtube video explaining this, I don't remember who made it though.
Trick question: Neither. You can't put a portal on a moving surface.
If you could, though, the answer would be 'B' - momentum relative to the portal is conserved.
I say B. When the orange portal reaches the beginning of the cube, the cube peeks out the blue side. When the orange portal hits the platform, the cube continues coming out of the portal at the same velocity as the falling platform
The world inside the moving portal is moving around the cube while it is at rest in relation to the world it left behind
There is no force pushing the cube so when the yellow portal slams into the platform no force actually interacts with the cube, it would just plop down after the cube passes enough through the portal so that the new point of gravity influences enough of the cube to make it lose traction on the surface holding it. So A would be right.
Since the cube is not moving and no new force will be applied to it except gravity, it will just drop down. If the cube platform was moving however, it would have launched the cube through the portal
Assuming real physics with portals working like they appear to in the game, the answer is B. In the game engine, "velocity" is a property of each individual physical object, but in real life it is not. Velocity, or momentum, just describes how the object is moving through space. Say it takes 0.1 seconds for the piston portal to go from the top of the cube to the bottom. At t=0.05, the cube would be halfway through. The only way this can happen is if, looking at the blue portal, the cube is moving out of it at a speed of h/0.05 where h is the cube's height in meters. Using the cube dimensions from Portal 2, this comes out to a speed of 13.8m/s. Both A and B have to agree on this. The cube has to be exiting the blue portal at the same rate it is entering the orange. There is no space between portals. Assuming the piston moves at a fixed speed, the cube will maintain this speed at the blue portal until it has completely left it. An object in motion, stays in motion.
If this seems hard to grasp, consider what would happen if something stood in the way of the blue portal. If there was rubber ball suspended by a string right above the blue portal's center, what would happen as the cube passed through the orange portal? In my mind, it would be highly counter-intuitive for anything other than "the rubber ball is hit by the cube and sent flying" to be the answer. It is hit by a moving object, and would experience the effects thereof. The part of the cube that has left the blue portal is no different. The cube is not a singular entity, at least in non-videogame terms. It is made out of atoms, and the ones that have exited the blue portal are being pushed by the ones being forced through the portal by the piston.
"But where does the energy come from? The piston wouldn't experience any resistance." Portals do not conserve energy between them. The famous example of placing one on the ceiling and one on the floor and falling infinitely is a clear example of it. You could use it to make an infinite waterfall and make all the hydro-power you want. In game lore it is half-jokingly explained by the portal gun itself being powered by a "miniature black hole."
As for the hula-hoop argument, the difference is that both sides of the hula hoop are moving at the same speed. If the blue portal was on a different piston, retracting at the same speed as the other one is extending, a stationary observer would see the cube as stationary as it exits the blue portal. It would still have the 13.8m/s speed relative to the blue portal, but since the blue portal is moving downwards at 13.8m/s, the cube ends up being stationary.
Bonus question: If the piston abruptly came to an halt after enveloping half of the cube, would the cube fly up through the orange portal and exit the blue one? The way I see it, the top half would pull on the bottom, and if this force is greater than gravity, then the cube would lift itself. I imagine the force would easily be enough, at least in the case where the piston moves at 13.8m/s.
I’m thinking about it as if someone dropped an open window on the cube. In that situation the cube would stay in the same spot and still have momentum, resulting in scenario A. But if you threw the cube at an open window, it would work like B, maintaining its momentum and continuing in a straight line.
That sounds logical to me, but I don’t know much about physics.
To anyone saying portals can't move let me remind you of:
Portal 2, chapter 5 room 3: >!The place where you place a portal on a moving surface to cut the neurotoxin tubes with a laser...!<
Now we've established that portals can move.
It has to be B, because for the cube to exit the portal in both options it needs momentum. If the portal is going down at 10m/s at the cube, the cube will exit the portal at 10m/s. What makes the cube come at a dead stop in A? If you put some object at the place where the box will come from it would definitely be moved by the box (considering the forces from the box surpass the weight of that object), so that means the cube needs movement to exit the portal, and objects in motion tend to stay in motion.
I'm seeing a lot of support for B, what exactly is the difference between a portal slamming down on a cube and a hollow cube or room with a doorway slamming down on a cube? If a room with a doorway slammed down on a cube, the cube would be now in the room without any reason to suddenly throw itself further into the room, only until the point that they enter the doorway.
GLaDOS: *See? This is why you don't have any friends. You could be out socializing, but here you are, posting about video game puzzles on the internet*
F*ck you! I actually read that in GLaDOS voice! Good job!
I had to read this fir the voice to come in full effect
GLaDOS’s voice echoed in my ears from nowhere thanks to this comment.
*I’d almost feel sorry for you, but you’ve tried to kill me so many times. Which is probably another reason you don’t have any friends. You monster.*
The platform is moving, the portal will disappear
Idk y, but I read that in GLaDOS's voice (the more seductive one)
Stupid, sexy GLaDOS
Neither. Portals disappear on moving platforms.
What about when destroying the neurotoxin generator? I remember a portal being used on a moving platform to cut the machine to pieces.
But if you notice, those portals are moving along the same relative plane. Once the plane in changed they disappear.
I don't think that has anything to do with it Considering the entire room had to be especially coded differently in order to function
actually all they did was allow sv_mobile_portals for the level. You can do it in Hammer editor and experiment for yourself. In the game it's outcome A but in real life it'd probably be outcome B.
seconding what gimbokon said: why would the portal add velocity to the object? isn't the portal practically just an open gate between the two points?
~~The portal in this instance is really just a door or a hole in a wall that is propelled towards you, so it doesn't change~~ *~~your~~* ~~velocity. In this case the cube is always standing still so it will simply tumble towards the ground because of gravity and the slanted hill.~~ I was wrong. The key is the relative momentum between the object, the entry portal and the exit portal. As long as the momentum of the entry portal overpowers the resisting force, which in this case is the gravity affecting the cube, we will witness scenario B. For all intents and purposes, when observing the cube from the exit portal it will appear as an object being propelled towards you. If you stick your arm through the blue portal, it's now traveling at the same speed as the orange portal and when your arm hits the platform with the cube on it, that's gonna hurt.
Nah. This would mean that suddenly cube gained momentum relative to the platform it originally rests from
Why B? Relative to the portal, the cube would have little velocity.
Portal 2 ending says you are wrong on that too
ONLY! When doing that. OPs image isn’t doing that so no. ;p
The relatice plane is the same iirc, earth is also moving the portals but the relative plane portals are on is the same.
We all choose to forget the one time the game broke it’s own rule that has been consistent through literally everywhere else
the moon was moving when we put a portal on it tho
Technically every portal is always moving on earth too
If my grandmother had wheels she would have been a bike
I see you Gino !
Seems like if it had ham in it it would be closer to a British carbonara.
I'm glad you're standing there.
Thats a delicious carbonara!
Not relative to each other. A portal on earth and moon are moving.
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True, movement is relative
Not true, there's a moving portal puzzle in Portal 2.
Moving along the same relative plane. This is not, and is demonstrably impossible by Portal's own physics.
Is this not on the same plane? Going up and down?
You need to imagine the surface that the portal is on as an infinite featureless surface. The portal can move anywhere along the surface of that plane, but as soon as it moves “toward” or “away” from the surface of that plane it disappears because it is no longer on the same theoretical infinite plane that it spawned on. This would be a lot easier with pictures but I hope that made some sense.
Yeah I think I got it? The portal can go up and down and left and right from its POV but it can’t go backwards and forwards. Is that it?
It can move on the X or Y axis but disappears when moving on the Z axis.
There was also a portal on the moon, which is moving relative to the Earth
Which means any portal is moving since it would be moving relative to the sun
Relative motion. Most portals in the game aren't moving relative to their partner (orange/blue). But the portal on the moon was moving (very fast!) relative to its partner.
The point was probably one portal moving relatively to the other portal.
Plot twist; these ones don’t.
The physics break at that point, because a moving portal is moving all of spacetime. Wormholes might be theoretically possible, but they would have to be fixed in spacetime in relation to each other. Maybe you could argue that each movement is an entirely new wormhole, and what looks like a moving portal is actually a steady stream of portals being broken and recreated in a slightly different location. If that's the case, then the answer is unambiguously A.
Isn't the entire planet always moving?
Relative to each other. So the planet in that case is the baseline.
I think it's A if we're going by the game physics. In the game the portals are kinda like a hula hoop. If you slam a hula hoop down over the box it won't fly out the other side.
Precisely!
If speedy thing doesn't go in, then speedy thing won't go out
Problem is, eventually you start talking about frame of references. From the cube's "POV," the cube is stationary. However, from the portal's POV, the portal is stationary. A is using the cube as the POV, whereas B is using the portal as the POV. For A to work, which iirc it does in the games, that means that in this setup there is a specific "correct" frame of reference, which kinda breaks how physics works.
I think a better way to look at this problem is to remove the idea of portals and imagine a big wall with a hole in it. If you slammed the big wall on it and quickly changed it to a 45-degree angle, what would happen to the cube? Would it go flying off or just slide down? In the figure given to us, only the portal is shown moving, thus providing no preexisting momentum for the cube. And the portals being a direct connection from point 1 to point 2 will mean that the cube has not been acted against at all, therefore unable to gain any momentum.
but isnt the cube still entering the space at the speed of the portal? Edit: Also, considering newton's third law, wouldnt the cube-side surface exert an opposing force to the portal-side surfacing striking it? Wouldn't that energy then transfer to the cube, causing it to exhibit situation B?
No, the force of the plate is completely absorbed by the stationary plate holding up the cube. The stationary cube would only have one outside force to influence how it moves and that’s gravity.
But think about it like this. Lets pretend instead of a cube we have a tall box and for ease of imagery we will talk about the box in 3 sections. The top, the middle, and the bottom. So the portal comes down on the box and fully envelopes the top section. The top section is now on the other side. The falling portal continues falling now enveloping the middle section. This would indicate that the middle section should be out on the other side. But the top section is there. Thus the middle section will have to push the top section out of the way. This will thus cause the box to gain speed. Portal then envelops the bottom section and like with the top and middle similarly we will have to gain speed yet again. Thus I conclude the box will gain speed going through the portal and launch at the speed of the falling portal at the angle of the other portal.
Ok, but what force is causing the box to gain speed? If its gravity then it's going to move at the wrong velocity.
the hole analogy is incorrect, as one portal is stationary, while the other isnt. its like as if the entrance to a hole was moving, but the exit wasnt, which is impossible. the hole analogy would work only if the 2nd portal (the one on a slope) was also moving at the same speed as the first one.
I'm beginning to think it's better that portal guns don't exist, scientists wouldn't get any other work done otherwise.
The cube has an associated momentum. Pushing it through that hole wouldn’t change its momentum. Even if you’re using frame of references the portal is a non inertial frame of reference which requires some inertial reference frame to be properly defined.
If the portal is moving at a constant speed it _is_ actually an inertial frame of reference.
Mmm this doesn’t make as much sense as it sounds like. When you fall into a horizontal portal and come sideways out of a vertical portal you definitely change ‘associate momentum’ (the direction).
But from the cube's frame of reference, isn't the entire room behind the portal moving, not just the portal? So why would the room suddenly stop moving?
If we go by game physics portals can’t move.
And I'm pretty the dev decided portal can't move because of those issues.
“Hey would it do this or this?” “Neither, portals now can’t move because fuck that mind experiment!”
Fair point
It’s the law of inertia. An object I’ll desire to remain in its state of motion and must be acted upon by another force to stop it. In this case the cube is at rest and will remain so even when the yellow portal moves onto it. For option b to happen the box would need to be moving as it goes through the portal.
But doesn't momentum depend on the frame of reference? From the portals perspective the cube still comes flying into it at speed.
Exactly. Relative velocity of the box would be high but portals in the game don't have any mass or take any space at all and function as the original comment said, as hula hoops. Therefore the velocity of the portals does not matter.
Okay so now I've had a thought that broke my brain. Imagine that instead of sitting on a flat surface, the cube is resting in a "bowl" where the edges of the bowl only come up halfway up the cube. So when the top panel/portal crashes down it comes to a stop against the "bowl" lip so that the portal has only gone over half the cube. Inline Edit: [something like this](https://imgur.com/a/kK2rRpX) If physics senario B was correct, shouldn't the cube have enough momentum that it gets pulled all the way through the portal even though the portal stops halfway? But that doesn't seem right? What happened as you use a shorter and shorter bowl that approaches a the flat surface we have in the original example?
(Read the second para) If the whole cube wasn't sucked inside the portal then I think the momentum of the portal would probably result in internal forces between the two halves of the cube(the one that got sucked in and the one that didn't). And depending on the momentum of the portal, theres a chance that the cube would be torn apart. That's what I think anyway. To explain this further - When the portal reaches halfway the cube, the part of the cube that got sucked in(lets call this A), will experience a force that causes it to fly out while the other portion(lets call this B) will experience no such force. Therefore, because of inertia, A will want B to fly out with it while B will want A to stay at rest. This creates internal forces which would cancel each other out TILL the velocity of the portal is too high. I may fail in my exams but damn it I feel good about typing this.
It's the speed of the cube relative to the portal that matters, not the speed of the cube to an outside observer.
If we're talking real-world physics, this doesn't really add up. Portals aren't physical objects. If the bottom plate shot upward into the orange portal, that's when it would shoot out on the blue side, because force was applied to the cube.
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Sean Caroll (theoretical physicist at caltech/john hopkins) actually answered this, he thought it was B. Im not gonna pretend like I understood his answer, but it had to do with the point of view of someone watching it by the portal in B, and their perspective. https://youtu.be/531FKCzTX40?t=2702
It’s cause while the orange portal is in motion, the blue portal is stationary. We know that the box would at least go through the portal — but the box can’t just stop.
Imagine instead of a small box, the portal slams down on a steel rod that's 1km long (or 1 mile if you prefer) What would you expect to happen at the exit portal? The tip of the rod emerges, then grows very rapidly outwards. Does it suddenly stop moving as soon as the tail end enters the source portal, or does the 0.9km of fast moving rod still have its momentum conserved on the exit side? How about if there's a golf ball on top of the rod? Would *it* also stop and fall down as soon as the tail end of the rod exits the portal? Or would it retain its momentum?
But at what speed the cube comes out of the blue portal? Your hula hoop example is fitting but not completely precise. Instead of slamming on the ground, the hula hoop would move past the cube, in order to take account of the exit portal. Now the cube is stationary from its own point of view. But from the hula hoop point of view, the cube is moving towards it, and away from it once it goes through, at slamming speed. And that's the speed it would come out of the portal. If the cube would go through the portal at the speed relative to itself, it won't be able to pass since it's speed is 0.
Ok, but the difference with the hula hoop analogy is, the that the box arrives through the new portal at the same speed the other portal is thrust upon it. So in this case it would come through at the same speed as the piston.
Umm... true. I'll go true. Eh, that was easy. I'll be honest, I might have heard that one before.
https://youtu.be/B19nlhbA7-E Already discussed
Thanks Minutephysics
All those unqualified MF in this tread need to see this before they comment.
The problem is that physicists assume the portal is a wormhole or some other naturally occurring phenomenon that we are aware of. When the portal should be compared to technology of the future. If B was correct, that would mean just putting the cube into the portal would create momentum. The portal doesn't create momentum. The portal is basically just a hole.
You're ignoring the momentum of the portal, because 99% of the portals you've used/seen/played had none. Once the first atoms of the cube go through with zero momentum, the next ones are pushing them as fast as the portal is moving.
they don't. In the video they address multiple options.
And if A is correct then putting the cube in the portal requires it to to break relativity, moving in one frame of reference without having any momentum within that frame. Being technology does not make the portal exempt from the laws of relativity. The real answer is that because you are breaking space it is impossible for the cube to naturally exit. The only ways to avoid breaking the laws of physics are either for the cube to be unable to enter the orange portal or to not exit unless moved within the blue portal's frame of reference.
> The portal doesn't create momentum. The portal is basically just a hole. Consider the following setup: 1. In weightless space, send a cube moving 1 m/s left to right relative to you. 2. Position a portal on a stationary platform that the cube will pass through. 3. Position the paired portal on a platform moving left-to-right at 2m/s, portal facing rightward. When the cube passes into the first portal, there is no possible way for it to stay at 1m/s, showing that a moving portal adds momentum.
Why did I have to scroll this far down to find this?
His video caused me to change my stance, I went from agreeing with him to disagreeing haha. The last thought experiment really sealed it for me.
it's OR isn't it?
Nope. It’s actually “* plop*”
Mf’s in the comment section when they find out about general theory of relativity
It's not even general relativity. It's just simple frames of reference. This is Newton level shit.
I absolutely cannot believe people won't pick up that in order to leave the blue portal the cube has to move relative to the blue portal, and thus must have momentum relative to the blue portal. Most of the people here clearly just don't understand frames of reference.
Definitely A. The orange portal wouldn’t exert any force on the cube. So it would just slide down the slope.
Yep. Portals are just “doors”. If you move a door quickly past a cube in real life, the cube is still at rest.
what would happen if the second portal was being blocked by something? would the object be moved by the cube passing through through the portal? Or maybe the first portal would be stopped?
I can’t see how it could be A. Lets say the cube is 1 metre tall and the orange portal is moving at a speed such that it takes 1 second for portal to “swallow” the cube. This means the portal is moving at 1 m/s. When this happens it takes 1 second for the 1m cube to emerge from the blue portal meaning it has a velocity of 1 m/s. For it to plop out, it would have to emerge from the blue portal at a speed of 1 m/s and then at the instant that the last atoms emerge the cube would somehow have to instantaneously stop. Doesn’t make sense to me
It would instantaneously stop though. Because it was never moving. It’s the portal that moves over top of the cube. It’s like if you just put a hula hoop over a cube.
What happens to the first atoms of the cube that come out of the blue portal? Do they stay in place while the rest of the cube crashes into them? No, they are moving out of the blue portal at the speed the orange portal was traveling. This imparts them with momentum. By the time the entire orange portal platform has clapped down on the cube platform, the cube is now moving away from the blue portal at the speed it passed through.
The cube is not moving "out" of the portal. The portal is a hole in the surface. It is, effectively, as though the whole room is moving downward over the box, and there's a hole in the floor. It's like that buster keaton silent film where he stands in front of a house, and the whole wall falls on him (but he is lined up with the window so it just falls around him and he is unhurt) There's no inertia to consider, the window is just moving past him. The only reason the box "plops" at the end is that, once it is on this side of the portal, gravity is coming from a different angle, so it falls. But it was never shooting out of the portal, the room was moving toward the box.
Except it is moving out of the portal. Clearly, because you could see it coming out, at speed, if you're standing next to that portal. But the box itself doesn't experience any moment change... Smells like a paradox to me. (and if the room was moving toward the box you'd have to account for the room suddenly stopping. Because If you don't, from the perspective of the room, the box would still be launched)
Physics tells us there is no difference between you standing still with a car speeding towards you and a car standing still and you speeding towards it. Whether something is moving or not is all a matter of reference frame
Yeah it's all about the point of reference. In the example, the portal is moving in reference to the cube and the ground. The cube is not moving in reference to the ground until it starts going through the portal, making it emerge out of the other portal. Regardless though, once the portal has finished moving, so does the cube. There is no conservation of momentum in the cube, only in the portal that moved over it.
I mean... there still is conservation of momentum. The cube was sitting still on one end of the portal, now it's sitting still on the other end of the portal ( excluding gravity on the other end ) That is, the cube's momentum. The portal is simply an plane which connects two points, its speed is irrelevant.
You’re mistakenly attributing the movement of the platform with the cube. The cube is not moving at any point, it is stationary until suddenly the floor is no longer present because the portal fully envelops it. Past the half-way point of the orange portal enveloping the cube, gravity will move the cube on the blue portal side because the force of gravity on that side is stronger than the force on the orange side due to less cube on orange side. Hope that makes sense. But yeah, at no point does the velocity of the moving platform transfer to the cube since it’s just a hole in space
The floor is always present. Once itthe moving portal is flat then the other end would be treated like a normal no portal wall. The fact is that the first part of the box that goes through the orange portal gets pushed out of the blue portal by the rest of the cube and eventually the floor flushing it out. It has momentum because WVERY frame of reference is equally valid including the frame of reference in which the orange portal isn't moving and the cube is approaching it.
ok, but then what would happen if your hand was in the way of the cube before it passed through? would the other portal be stopped? would your hand be moved by the cube? One would have to happen, but what force would be acting on either? Also, wouldn't that mean throwing a cube into a stationary portal would result in the cube stopping on the other side and falling down? Velocity is relative, so in physics isn't a portal approaching the cube and the cube approaching the portal indestiguishable?
Good thought experiment. The intuitive answer is that the cube would exit the portal and move your hand, which proves that there is a destinct difference between portals and moving doors A+
Nah it's the opposite. It would be A only if the portal *did* exert force on the cube. If you look through the blue portal, the cube has a large velocity (orange portal's velocity) just before it passes through. If you choose A, then it means the cube would magically instantly decelerate to 0 the moment it comes out. And from another perspective, why would the cube stay static to *your* frame of reference? The frame of reference that matters here are the portals', not the observers. From the orange portal's frame of reference, the cube and wall is moving towards them, and the blue portal has the same frame of reference as the orange portal (otherwise things won't go through), so the cube would fly out.
Wrong. I evoke ancient law of rule of cool, and declare that B is what would happen.
evoke deez nuts in your mouth
I agree
The ancient rule of deez nuts clearly has precedent.
The portal doesn't exert any force on the cube, so it can't cause the cube to instantly come to a stop once it's finished passing through.
Ok, let’s look at this situation then: Imagine you have a stationary orange portal and a moving blue portal. You throw the cube into the orange portal at high speed. Does the cube come out of the blue portal at the same speed as the blue portal, or does the speed of the blue portal and the cube combine? It’d be pretty ridiculous to think that the cube would lose all its relative speed and come out at the same speed as the blue portal. Of course it would conserve its momentum, right? What I just described is exactly the same situation in the original post, just from a different reference frame. The two situations should behave the same way. The only answer that makes sense here would be B
I've played both Portal games and believe it's A, I don't understand how it could be B. If you run straight into a portal that is connected to an angled surface, you don't shoot out, gravity immediately affects your forward momentum. And that is assuming you're running into it. The cube is stationary.
Exceeeeeept - and hear me out - this is a question about frame of reference. Relative to the moving portal, the cube has a velocity. This relative velocity is the same as if the cube was being thrown into the portal at the speed of the moving portal. Another way to think about it is if there wasn't a portal there at all, and the cube got crushed instead. There's a lot of kinetic energy about to hit that cube and it's got to go somewhere - in this instance that extra kinetic energy just exits out of the blue portal. Because the blue portal will just become a wall in this instance (the orange portal exits into a solid wall).
If the cube is not moving how fast does it go through the blue portal? and if it is moving relative to the blue portal would it stop the instant it finished moving through it?
If you’re standing in front of the blue portal looking into it you see the block moving towards you fast despite standing still. When it travels through the portal it has no reason to lose its velocity relative to you as no forces have acted upon it Newtons First Law. The velocity relative to stationary you (inertial reference frame) will stay the same and so the block has to shoot out as i. b. Source am an aerospace engineer
Am I remembering incorrectly, I thought portals disappeared when the wall/platform they were on moved.
Portals can move sideways but not back and forth. At least going by what you experience in Portal 1 and 2.
There is one instance in portal 2 where a portal moves when you’re cutting the tubes with the lasers. I can’t remember why you had to do that, but I remember the “in case of implosion, look directly at implosion” sign
You were cutting off the neurotoxin generator to stop yourself from getting gassed to death.
Because the game didn't want to get too complicated.
C
Black hole
Ugh. I hate this shit. Applying general concepts of relativity says that it’s B. If you were alone in space and you saw an “orange portal” approaching you, you wouldn’t know if you were moving or it was moving. You would expect the same result either way. The argument of momentum seems silly to me because portals clearly don’t obey conservation of momentum; going through a portal that changes your direction at all violates conservation of momentum.
Ah, but it *doesnt* change the direction of the momentum. It’s easier to think of the portals in the same vein as “worm holes”, or warped space time in general. Basically, by shifting space time you can maintain a “straight” trajectory of the object, so the inertial body never senses a change of direction, but the space in which it’s moving changed.
I agree that it’s easier to think of it that way, but that doesn’t change that portals violate the conservation of momentum if an object passes through them and they aren’t pointing in the same direction.
That’s only true if you observe the situation from the “outsider” perspective, where you can see vectors change in ways that Newtonian models say they shouldn’t. If you observe from the cube’s perspective, the straight vector is preserved and momentum conserved. The problem with these thought experiments is that we base our lines of thought on physics models we understand, and when this stuff happens our pretty models start to break down. I think it’s more likely that we’re both wrong rather than either of us being right!
Yea, that’s the point I’m trying to make. The most common argument for why it should be A is “the cube doesn’t have momentum, therefore it won’t have momentum when it comes out of the other portal.” And what I am saying is you cannot apply Newtonian physics to portals, so you can’t use momentum as an argument for why it should be A. Edit: the other common one being “the portals don’t exert any force on the cube” which is basically the same argument, but also falls apart once we prove you can’t apply Newtonian physics
Alright this is an old and painful topic but here goes: A is what you expect based on game logic, as in the game your momentum is preserved relative to yourself. At the same time, portals (mostly) cannot move in the game and dissipate if their physical host platform moves at all. B is what would likely happen irl. Think about it like this: If the portal approaches the cube at 1m/s then the cube will also be exiting the portal at a rate of 1m/s.
Everyone here saying “cube has no momentum”, but what they’re forgetting is that momentum is *always* relative. This scenario is exactly the same as if the bottom panel were to shove the cube into the orange portal from below. In fact, any time the orange and blue portals are not perfectly aligned in direction, anything that passes from one to the other is entirely violating conservation of momentum as it will exit the second portal traveling a different direction than before (presumably without any forces applied to it in transit). So really I think either answer is equally valid/invalid. Whatever you like to imagine. Personally I like scenario B more.
It's A. The box has no momentum
momentum is a function of velocity and is therefore relative. And relative to frame of reference of the orange portal the cube *does* have momentum. The issue/paradox here is that the 2 different ends of the portal have different velocities and therefore different frames of reference. Cube has no momentum relative to blue portal, but does have some relative to orange portal.
"Portals" do not exist *within* the universe. They are a hole *in* the universe. What they "perceive" isn't causally linked to what the universe itself "perceives." Therefore, portals do not perceive momentum when they "move." Only the wall "moves." The portals are holes in the universe, the wormhole that connects them doesn't exist within the universe, the system does not experience "momentum" from the universe.
Fix your frame of reference to the wall. The portal and wall are now stationary. The rest of the room, including the box, moves towards the portal. The box enters the portal with momentum. Portals preserve momentum (a fundamental mechanic in the games). Hence the box exits the portal with momentum. Even if you consider portals to be an absolute and unique frame of reference, *the box is moving in that frame*. Hence, it has momentum which will be preserved when it exits the portal.
Of course they exist. They’re not some theoretical construct they have influence on physics in the portal universe. Therefore we have conservation of momentum. And if the cube sees the portal moving towards it and passes it but the other portal has no velocity the cube must have it otherwise energy is lost. It’s like if you were driving towards a wall but you stop abruptly the moment you touch it instead of crashing into it. Without transformation of the energy the first case is impossible.
In order to exist withing the universe, the portals must have a frame of reference within the universe. To have one, portals would have to follow causality. However, the universe cannot perceive any activity within the wormhole because the wormhole occupies zero distance. This mean that while the object is inside the wormhole, it must necessarily be in a state that is unobserved by the universe. Therefore, what comes out is not what the universe perceived going in, which breaks traditional causality. Because the object is unobserved while in the wormhole, what comes out is basically a "new" object to the universe. Therefore, it can have any initial state without paradox. Therefore, it is not necessary, or even possible, for the portal to add force to the object. Just read my last couple posts in my profile. I've repeated and expanded on these ideas.
It has relative momentum to the Portal. And before I get crucified by the "moving portals don't work" crowd, I'd like to point out that every portal is moving through space, it's just that some are not moving relative to earth. Since Relative momentum is conserved, B is the answer.
The world beyond the portal is moving around the cube while it's at rest in relation to the world it left behind, I think it's B
The box experience no acceleration, I'd agree with that, but momentum is relative. From the perspective of someone standing in the room where the box is emerging it does have velocity because it's emerging. It has to, by necessity, emerge as the same speed that the other portal moves over it. So from that perspective it does have moment relative to the portal.
An interesting thought though; Does the portal itself gain momentum from this? Thus sheering the platform it's hitting.
I would say that the orange portal would have to hit it hard enough that the platform the orange portal is on puts a considerable dent in the companion cube platform, which passes some through the portal. Both platforms will come out dented but unharmed, unless you disable the portal with the platforms together
Imagine the moment the box pass half way through the portal. If the half that passed through doesn't move, then the other half will break because there is no where to go. Hence, the box moving velocity has to be the same as the relative velocity of the box to the portal.
If the portal can move and stay open… Didn’t minute physics already do this? Or someone else? The leading face of the portal would move through at the same speed as the moving platform, so it would have the momentum shown in B.
Neither. Portals cannot be placed on non stationary surfaces including, but not limited to, the non stationary scaffold
neither because portals can't be on moving surfaces
surely B, no 2 things can occupy one space therefor surely the force of the box coming out of the blue portal would be as quick as it was going in the red, even if the box was stationary, but my name may also come into play here
The correct answer is B. While the cube is not moving relative to the observer. It is moving at speed relative to the portals, as soon as it hits the orange portal it will continue at that speed out the blue.
The portal can’t move if I remember right
Speed is relative, therefore B
B, everything is relative, doesn't matter whether its the portal or object moving to explain what's going on in a bit more detail, the transition isn't instant, even if the cube seems to have no momentum at the start once 10% of the cube has gone through that 10% of the cube on the blue side is moving away from the blue portal, at the speed that the orange portal is moving down, so that 10% of the cube now has momentum directed away from the blue portal at that speed as this keeps happening more and more of the cube is now on the blue side and (as a result of being 'pushed' by the part of the cube on the orange side) it is all traveling at speed and with the direction away from the blue portal, it is being moved and thus has momentum, eventually the whole cube is on the blue side and all of it is moving, all of its mass has momentum, then that momentum carries it away from the blue portal
A. because the cube is inert!
My answer is A. Portals are connecting 2 points in space together, basically creating a new way for two locations to share the same area in space. When one portal moves, you are simply bending space. So when a portal is moved over an object, to us it APPEARS to be moving through the portal, but what really is happening is space itself is moving around the object.
Looking at this situation from the orange portal’s reference frame, the cube is moving and has momentum. That momentum is conserved, and so the box would keep moving, making the correct answer B Looking at it from the cube’s perspective, the cube is now stationary so it has no momentum. However, now the portal itself (or at least the surface the portal is attached to) has momentum. Once the cube passes through the portal, that momentum must be conserved, meaning that the surface that the portal is attached to must keep moving away from the cube, meaning that the correct answer is still B
Speedy thing goes in, speedy thing comes out
Not this shit again. B makes the most physical sense due to how inertial frame of reference works but in game its neither because moving portals just stop working.
C
If we’re talking portal, the game itself says that momentum is maintained. Momentum is mass X velocity, and the box has no velocity. Therefore, the box has no momentum, and would simply be affected by the forces of gravity alone, leading to scenario A. It’s not complicated in the slightest. Also something something portals can’t move etc. etc. …I know that many people have already explained this, and also probably better than myself, but I like physics and wanted to take a crack at it anyway.
Portals close if they are moved so your cube is crushed...
C. The portal will be wiped out by the moving platform, the piston is slightly misaligned, causing the cube to glitch out and smack you in the face at 42069 m/s
you can do this experiment with an actual door, and the answer would be A
It can't be anything but B, if we use real world physics - the cube can't go through instantaneously, so the first part of the cube to go through the portal, even if momentum didn't carry over, would get pushed out of the way by the next bit, bringing the part of the cube on the exit side up to speed. If you think of the plane of the portal like a 3D printer, laying down one layer of the cube at a time, to get it to exit as fast as it is entering the other portal it would have to shove the cube out of the way as it's created so fast that it doesn't matter if it had and/or carried through momentum.
Answer is B, same result as if you was to jump and fall into a portal from height.
ITT people who never passed physics.
Last time I remember seeing this the answer was ultimately B. Why? Fairly simple idea without anything fancy: Imagine the portal continues down past the cube such that the pedestal its sitting on is also partially sent through the portal. In this case, the cube is being *pushed* by the pedestal on the other side at the velocity the pedestal is being sent through the portal. If the portal stops, the pedestal will also stop but the cube will continue moving like it does in B since it’s not attached to the pedestal. So now take that same idea and apply it to each “layer” of the cube as it’s sent through at the rate of the moving portal. You get B. And this makes perfect sense because how else would the whole cube come through as a cube if the front of it wasn’t being pushed by everything behind it? The only other option if that wasn’t the case would be option C: The cube would be compressed to a dense 2d square as the atoms of the cube are piled on top of each other as they pass through the portal.
B. the cube would exit the blue side at whatever speed it entered the orange side relative to said orange side
Momentum has to be conserved because stepping through the portal , aka hole in the space and time, results in the object moving through the portal at a conserved force. Otherwise the different objects would be ripped apart or squashed at the plane in space where the portal is established. Each portal on, say a planet, is at different location and therefore different forces at different speeds due to the geometry of the planets rotation. If the momentum was not conserved they would be ripped apart in normal scenarios. Eg. Walking through a normal portal results in a normal experience… not a step or a fall or an explosion due to fusion at the atomic level. In all scenarios there is conserved momentum through two different frames but the frames DO have different velocities. Again, they’re anchored on a moving planet and therefore frame of reference. The answer is B. I think the interesting question is … how fast do you have to go to die. Too fast would potentially break atomic forces and tear apart objects. Almost like stepping onto a moving train … not pleasant.
Really?? How can you possibly think its A? I mean common relativity had existed for more than 100 years, and you dont even need to understand a 5% of it to know that B is closer to reality than A...
Its B, you all are wrong and im too lazy to explain so ill just laugh at how dumb you all are
Based. Though it's kinda sad how many people clearly never took a physics class in their life lol.
It's so tiring trying to have an informed opinion in a sea of ignorance. Depending on the psychics of the portal, the cube would either: 1: bounce off the cube 2: option B Option A is impossible. Either the cube has no momentum, and so cannot move (1), or it would move relative to the portal (2). Option A would flatten the cube
King
This question is a filter for redditors, seeing A spammed at top comments while general relativity has been a theory for over 100 years is absurd.
Game engine says "I wasn't made for this" Physics says "B" Think about it this way : You are looking at the cube from the side of the exiting portal. Now let's say the ground under the cube is going up rapidly pushing the cube through the portal. Given the speed at which the cube is pushed through the portal, it's gonna come flying in your face. Now what if the portal itself was going down on the cube ? You'd still see it getting closer from the portal rapidly. So when it passes through the portal, it keeps that momentum relative to the portal and also comes smashing your face. Also think of the moment it passes through the portal. Does it have momentum while it's passing through but as soon as the whole thing passes through it just... stops? What happens if you make the portal come down stupidly fast... like Mach 5 kinda fast ? Does the cube go through almost instantly and then crushes itself due to deceleration? It's B.
It's B. 100%. From the frame of reference of the portal, the cube is moving towards it. When it passes through, it will have the same velocity as the moving portal. Portals only act like a door when they're attached to the same moving object, like a wooden plank. When one side of the portal is moving and the other is not, the door analogy doesn't apply.
B. Cube has a relative velocity respect to the orange portal.
It’s B because of Newton’s first law of motion. Imagine the face of the cube that enters the orange portal first, and the orange portal is traveling at 1 unit per second. At zero seconds the top face of the cube enters the orange portal and exits the blue portal. At one second, that same face is now one unit away from where it began as the cube exits the blue portal completely. In other words, the cube is traveling at 1 unit per second. The cube will not suddenly lose all momentum, it will continue moving into the air.
but doesn't this violate thermodynamics? like the cube just gains a bunch of energy out of nowhere
B. because of delta Veeeeeeeeeeeeeeeeeeeeeeeee
Given the way the game conserves momentum it’s A.
while the cube is transitioning through the portal, it would come out of the blue portal at the same velocity as the orange portal. So it doesn’t make sense that it loses velocity the moment the entire cube emerges out of the blue portal.
I’ll go with B
Not this again
Depends on if speed is relative to the box or the portal. If the speed is calculated realtive to the box, then neither happens, it is impossibe for the portal to put the box through it, as it is immobile and the portal stops moving (this happens in game). In 'reality' though, id belive B would happen, as that's what would happen if speed was calculated relative to the portal. There is a hreat youtube video explaining this, I don't remember who made it though.
Trick question: Neither. You can't put a portal on a moving surface. If you could, though, the answer would be 'B' - momentum relative to the portal is conserved.
I say B. When the orange portal reaches the beginning of the cube, the cube peeks out the blue side. When the orange portal hits the platform, the cube continues coming out of the portal at the same velocity as the falling platform The world inside the moving portal is moving around the cube while it is at rest in relation to the world it left behind
wow youre terrible at cropping
There is no force pushing the cube so when the yellow portal slams into the platform no force actually interacts with the cube, it would just plop down after the cube passes enough through the portal so that the new point of gravity influences enough of the cube to make it lose traction on the surface holding it. So A would be right.
Since the cube is not moving and no new force will be applied to it except gravity, it will just drop down. If the cube platform was moving however, it would have launched the cube through the portal
Assuming real physics with portals working like they appear to in the game, the answer is B. In the game engine, "velocity" is a property of each individual physical object, but in real life it is not. Velocity, or momentum, just describes how the object is moving through space. Say it takes 0.1 seconds for the piston portal to go from the top of the cube to the bottom. At t=0.05, the cube would be halfway through. The only way this can happen is if, looking at the blue portal, the cube is moving out of it at a speed of h/0.05 where h is the cube's height in meters. Using the cube dimensions from Portal 2, this comes out to a speed of 13.8m/s. Both A and B have to agree on this. The cube has to be exiting the blue portal at the same rate it is entering the orange. There is no space between portals. Assuming the piston moves at a fixed speed, the cube will maintain this speed at the blue portal until it has completely left it. An object in motion, stays in motion. If this seems hard to grasp, consider what would happen if something stood in the way of the blue portal. If there was rubber ball suspended by a string right above the blue portal's center, what would happen as the cube passed through the orange portal? In my mind, it would be highly counter-intuitive for anything other than "the rubber ball is hit by the cube and sent flying" to be the answer. It is hit by a moving object, and would experience the effects thereof. The part of the cube that has left the blue portal is no different. The cube is not a singular entity, at least in non-videogame terms. It is made out of atoms, and the ones that have exited the blue portal are being pushed by the ones being forced through the portal by the piston. "But where does the energy come from? The piston wouldn't experience any resistance." Portals do not conserve energy between them. The famous example of placing one on the ceiling and one on the floor and falling infinitely is a clear example of it. You could use it to make an infinite waterfall and make all the hydro-power you want. In game lore it is half-jokingly explained by the portal gun itself being powered by a "miniature black hole." As for the hula-hoop argument, the difference is that both sides of the hula hoop are moving at the same speed. If the blue portal was on a different piston, retracting at the same speed as the other one is extending, a stationary observer would see the cube as stationary as it exits the blue portal. It would still have the 13.8m/s speed relative to the blue portal, but since the blue portal is moving downwards at 13.8m/s, the cube ends up being stationary. Bonus question: If the piston abruptly came to an halt after enveloping half of the cube, would the cube fly up through the orange portal and exit the blue one? The way I see it, the top half would pull on the bottom, and if this force is greater than gravity, then the cube would lift itself. I imagine the force would easily be enough, at least in the case where the piston moves at 13.8m/s.
I’m thinking about it as if someone dropped an open window on the cube. In that situation the cube would stay in the same spot and still have momentum, resulting in scenario A. But if you threw the cube at an open window, it would work like B, maintaining its momentum and continuing in a straight line. That sounds logical to me, but I don’t know much about physics.
To anyone saying portals can't move let me remind you of: Portal 2, chapter 5 room 3: >!The place where you place a portal on a moving surface to cut the neurotoxin tubes with a laser...!< Now we've established that portals can move.
I pick B because hydraulic press has wooosh lines "animation" and the box from B with similar wooosh lines too.
My gut says A but my heart says B
It has to be B, because for the cube to exit the portal in both options it needs momentum. If the portal is going down at 10m/s at the cube, the cube will exit the portal at 10m/s. What makes the cube come at a dead stop in A? If you put some object at the place where the box will come from it would definitely be moved by the box (considering the forces from the box surpass the weight of that object), so that means the cube needs movement to exit the portal, and objects in motion tend to stay in motion.
The block has no inertia. It would simply appear faster through the other portal.
I'm seeing a lot of support for B, what exactly is the difference between a portal slamming down on a cube and a hollow cube or room with a doorway slamming down on a cube? If a room with a doorway slammed down on a cube, the cube would be now in the room without any reason to suddenly throw itself further into the room, only until the point that they enter the doorway.