Ragebait answers only
this post was submitted on 26 Jul 2026
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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".
Nope, they aren't flat screens either. They are shortcuts in space. How many more times will I need to state this for you?
It's okay, we can terminate this interaction here.
Agreed.
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).
No, I've been serious since I acknowledged that I was supposed to ragebait people.
Okay, let's step through this slowly for you, if local spacetime is what all velocity maps to, then it sounds like each object has its own velocity that is independent of other objects (IE, in a universe with a single object in it, it could have a velocity), correct?
Yes, it would have velocity, or at least all the mechanical properties of it. That velocity would be undefined from our perspective since all possible ways you could define it (0, 5 m/s to the right, etc.) would be equally valid depending on the coordinates you choose to put at rest, but that's a measurement problem, not anything intrinsic to reality.
The important bits are: 1) inertia exists as long as you have the minimum ingredients, an object in spacetime, 2) all possible inertial reference frames are still meaningfully distinct from each other and stay as such, 3) any change in velocity of the object would show up in all possible inertial reference frames, and 4) any such change will stay logically consistent regardless of which inertial reference frame you choose to measure the velocity change in.
Okay, so since we have established (in your framework of understanding) that the cube has its own velocity, if the portal is moving towards the cube at 1m/s, how fast is the cube moving?
I think I see exactly where you're going with this, but let's just say the cube is defined to be at rest with the portal approaching at 1 m/s.
Alright. Cube is moving at 0 m/s (or at rest) and the portal moves over the cube. How fast is the cube moving?
0 m/s relative to the chosen inertial reference frame, 1 m/s relative to the portal entry, and 1 m/s relative to the portal exit.
Woah woah woah woah. I thought relative movements were not real. We're talking in terms your worldlines here.
This was you BTW...
So please, I ask again, how fast is the box moving when the portal is passed over it at 1 m/s?
Sorry. Lol. I used some shorthand I didn't think I'd need to defend based on earlier statements, but for clarity:
Movement relative to other objects is in itself not a real thing.
Movement relative to local spacetime geometry is always real.
Spacetime geometry is usually continuous.
The usual continuity of spacetime allows for the convention of defining movement relative to other objects with the underlying connecting spacetime element left unstated.
Breaking any underlying assumptions about spacetime continuity also breaks the convention, but the convention is still valid across spaces where the underlying assumptions are not broken.
Expanding out my last statement:
The box is initially moving 0 m/s in the inertial reference frame we defined to be at rest.
The portion of the box that has not yet crossed the portal boundary is moving 1 m/s in the inertial reference frame the entry portal is currently at rest in (valid due to the continuity in spacetime across the measurement between them).
The portion of the box that has already crossed the portal boundary is moving 1 m/s in the inertial reference frame the exit portal is currently at rest in (valid due to the continuity in spacetime across the measurement between them).
There are additional possible declarations to make if the portal boundary is a non-zero volume instead of a plane of instantaneous transition, but the games' interpretation of portals constrains that to either be zero or close enough to zero to not matter for the end result. Let me know if you want me to expand on that too.
Bit of a gish gallop, but it looks like you are dropping the argument about relative movement not being real, so situation A becomes the only logical choice because the relative movement of the cube to the other side of the portal is the same, regardless of how fast the portal is moving.
Except it's not moving, it has the appearance of movement, but the actual velocity of the box is 0 m/s. If the box suddenly gained the velocity of the portal, then momentum is no longer conserved, which is directly in contradiction to how the game explains movement through portals.
I'll chalk that up to me not being as clear as I should have been from the start. Please consider my position to have always been the last point: Motion relative to other objects = convention not directly based on something real but usually useful due to building on things almost always true. Motion relative to local spacetime = the actual real thing on which everything else builds. So...
I'm not disagreeing with this. To break it down further though, mass is just an inherent property of an object, and its velocity is fundamentally expressed in the object's relation to local spacetime.
Declaring an "actual velocity" sounds more like an argument against relativity (again for clarity, the fundamental spacetime geometry based relativity I was never intending to call into dispute). The critical detail you skipped over is that the 1 m/s velocity is seen in the inertial reference frame in which the portal is at rest.
Since this has started retreading the same points, I'll just go ahead and jump to where I think our understandings actually diverge, how portals relate to spacetime.
Your argument seems to boil down to: The box starts at 0 m/s in its local spacetime, and the portal is a simple hole on top of reality with no power to change this, so the box must end at 0 m/s in the same local spacetime.
My argument boils down to: Portals can momentarily link different inertial reference frames within spacetime together, so the box can seamlessly transition between the two resulting in an apparent momentum change while the actual inherent momentum doesn't change.
It's worth adding that static portals already do the same thing. Momentum and velocity have a direction component, not just speed. When an object enters a portal going one direction and comes out in another direction as measured against the wider environment, that's also an apparent momentum change.
Really, my argument boils down to what is stated about motion through portals, momentem is conserved. Momentum is only conserved within a fram of reference, two balls bouncing off of eachother in an isolated box make no change in the total momentum of the system... But as soon as you consider that the box is on a truck that is accelerating, then the total momentum for the box is not conserved from the perspective of something inside the box.
If you simplify the system to just the orange portal and the box, then from that perspective, it makes no difference if the box is moving, or if the portal is moving. As soon as you need to account for movement on the other side of the portal, then frame of reference has to include that space in your initial assessment of what is actually moving, otherwise you're making predictions on a system that you haven't fully accounted for. If you take away the portal, then the relative velocity between the box, and the area that would have been the exit of the portal, would be 0. Add back in your portal, and the wave it around randomly, and the relative velocity is still 0. Move the portal over the box, and the relative velocity is still zero, because as stated in the game, momentum is conserved for objects passing through the portal. It doesn't matter how fast the portal is going, the relative movement stays the same, which in this case, is 0.
Ah, if that's how you've been interpreting the explanation, I can see why we've been at an impasse. Portals as shown absolutely do seem to violate conservation of momentum in the more global sense.
You don't even need a complicated set up to do it. Put a portal in empty space, rotate the plane of the exit 90 degrees, then send a box through at a simple constant speed. Assuming the universe isn't compensating in other ways*, the total momentum of the universe will be different before and after the box went through, so there's no environment encompassing the portal interactions that will satisfy the zero condition. Only each object's local experience will keep the condition.
*I can think of a few options for this bit:
The entire universe somehow accelerates the opposite way in response, a fun idea, but it has no meaningful consequences.
Portals behave more like wormholes as scientifically defined. They have mass themselves and, if free-floating, move in reaction to mass sent through them in a way that preserves total momentum. This is enough to run with though. The games' portals always have to be anchored to other mass. This leaves room for the idea that the portals actually do want to move in response to objects passing though them. They just transmit that force into the mass they're anchored to, and that force propagating out is where the momentum conservation actually resolves.
Pretty sure the momentum from the perspective of the box stays the same. The game doesn't explicitly say that momentum for the entire system stays the same, just that "Momentum, a function of mass and velocity, is conserved between portals.” so rotating the exit of the portal in a different direction changes the momentum of the entire system (room, portals, box, etc..), but the box never actually experiences a change in momentum. Maybe a better way to put it, is that the absolute momentum of the box never changes when it passes through a portal.
How exactly are you defining monentum here? "The box sees the entire universe move at velocity V backwards both before and after passing through the portal." This is probably at least approximately true, but total momentum of the universe minus the box would have to be exactly zero to make this technically correct. "The box never perceived a change in its own momentum." This is 100% true.
Then why are we still talking about this? Lol. I thought that was one of the main points of your position.
Correct, the box never experiences a change in momentum. The box exists in one inertial reference frame and, in going through the portal, is deposited into a different reference frame. Two inertial reference frames that differ in orientation are equally as distinct as two inertial reference frames that differ in velocity. The same logic applies to both.
You're describing what I labeled as "inherent," so I'll take it.
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