Hey Veeky Forums, I know this contains a video game reference but /v/ is in a tizzy over which scenario is correct...

But portals are not fixed in space, related be to another observer, they are moving, with whatever planet they are on. Put a portal on a fixed surface extending outward toward the edge of our atmosphere. Now imagine space ship, relattively static in space around it. When the globe rotates, and the portal captures the space ship, you are still faced with OP's conundrum

You just changed it to now a a force is acting on the cube.
Good job at accomplishing nothing.

/thread

Those people are retarded.

No.

From the orange portal's perspective, the cube is pushed through it at quite a high speed. Scenario B assumes it maintains this speed.

Though I suppose the actual answer to the problem is "portals don't real"

The most confusing thing about the picture is that when portals are around, there's no such thing as a global inertial reference frame. This makes arguments that rely on conservation of momentum really easy to get tripped up by.

Let's say you want to argue (A) based on conservation of momentum. To make that kind of argument you first choose inertial coordinates on spacetime and then argue that the momentum of the block before and after it passes through the portal in those coordinates is equal. I think the intuitive thing is to use the coordinates "as shown in the picture"; these coordinates are discontinuous at the portal, where as you pass through the plane of the portal there is a sudden rotation, translation, and velocity shift in your coordinates.

Alternatively, someone else might come along and choose coordinates that go smoothly through the portal, but are discontinuous somewhere between the left and right sides of the picture. Choose coordinates on the left side of the picture such that the orange portal is not moving with respect to us, and on the right side choose coordinates "as shown" (such that the blue portal is also not moving). There's no reason to believe these coordinates are any "fundamentally" worse than the ones that are discontinuous at the portal, but everyone agrees that in these coordinates the block does something like (B) (since on the left side we're seeing it fly into the orange portal).
How does one distinguish between these two cases? The fundamental difference between these coordinate systems is that in the latter case, the block does not pass through the region of space-time where our inertial coordinates are discontinuous, so classical conservation of momentum should hold. In the former case, there's no reason to expect that conservation of momentum should hold, since we haven't made a choice of coordinate system encompassing everything interesting about the system that looks anything like classical Newtonian physics.

TLDR: (B)

From orange frame of reference, the cube is not moving. The orange portal has the same reference frame of the cube. So it's still a

No you aren't, it's still the same problem, you just have a static ship instead of a static block and a moving portal on a planet instead of a moving piston. there's still no momentum transfer.

Portals are just different perspectives in space.
Like opening your front door and seeing your backyard.

Perspectives cannot transfer energy.
Therefor A

I'm responding to
>portals don't work on movingly surfaces

Say I set up a normal door frame on a track, and then stood in front of a wall at the other end of the track. If the door starts accelerating down the track towards me, am I suddenly shot forward when the frame passes over me? No, of course not. Same thing with portals, so A.

>A moves at 40mph to envelope B and is ejected at C which is moving at 40mph for a net change of 0
This is a door

>A moves at 40mph to envelope B and is ejected at C which is moving at 0mph for a net change of 40
This is a non existent portal anons are sperging about.

The numbers must be equal not counting gravity and wind resistance.

Its B. Only trolls and retards would say otherwise.

A is correct.

If B were correct that entire game would need to be redone.

However, neither A nor B can even work in that game since portals can't move like that.

B is obviously correct.

If the piston slams down on the cube at 10 meters per second, then 10 meters of cube enter the portal per second. That means 10 meters of cube had better come OUT as well, otherwise the cube would get squished by passing through the portal, which the picture makes clear definitely does not happen. And if 10 meters of cube come out of the portal per second, that means the cube has a velocity of 10 meters per second relative to the exit portal -- that's what velocity is.

Nobody has been able to justify why "A" because it's not "A". Go ahead, try to prove it, you will fail.

And no, the answer is not "There is no answer lol".

In the game, objects come out of the portal the same speed that they went in. I can prove the answer is (B).

Let's look at an atom on the top face of the cube. Also for simplicity, let's say that the cube is 1 cubic meter and the portal is travelling at 1m/s. We are not even going to look at the blue side of the portal, we will focus on what is happening on the orange.

Now the atom on top of the cube hits the blue portal at t = 0. Now let's calculate that atom's distance from the orange exit portal.

Well, after 0.5 seconds we know that the blue portal has half-way consumed the cube (0.5m). So where is the other 0.5 meters of the cube? Sticking out of the orange portal. So what is the atom's distance from the orange portal? Easy, it's 0.5m.

After 0.5s, we have gone from a distance of 0m to 0.5m. Brilliant, this means that the atom is travelling at 1m/s on the other side of the portal! And it will keep at that speed unless acted on by an outside force, that is how atoms behave. So that atom will keep traveling at 1m/s UNLESS some of the other atoms of the cube are holding it back via electrical forces.

So how do we know that the top atom won't get held back by any of the other atoms? Easy. We just proved that the top atom on the cube travels at 1m/s when it exits the portal. This also holds true for every other atom in the cube. So in total, all atoms of the cube are traveling at 1m/s away from the orange portal. Therefore the whole cube is travelling at that speed, therefore the answer is b. Otherwise the entire cube would have to slam to an abrupt halt just after the last atom exits the portal, and this violates inertia.

Just think of it intuitively: If you throw a ball in a portal at 3 m/s it will come out at 3 m/s.

Think about it this way. As far as the orange portal is concerned, a cube has just emerged at X speed and shows no signs of stopping.

A is correct. Relative to the portals, cube is moving fast, so it quickly appears. But relative to ground/earth the cube is not moving, so it falls to the ground after quickly appearing. The portals add zero momentum. Otherwise there would be some kind of inertia that had to be of overcome when the portal moves over the cube as the piston instantly accelerates the cube. This would generate a ton of heat, etc. But these affects are never touched upon by the game or anyone that ever discusses this question because it is assumed these affects are not relevant, because everyone knows A is correct

in one location in Portal 2 you have to place a portal on a moving surface

Nice long wrong answer, faggot.

>In the game...
portals don't move.

Think about it this way. The piston instantaneously halts after 75% of the cube passes through the portal. Are you going to have the 25% of the cube still under the piston magically jump off the platform suddenly?

you're not displaced in space like you would be when going through the portal
also, both the exit and the entrance are moving in the same frame of reference relative to you and at the same velocity

if its going fast enough, yes

It's obviously B.
Anyone saying otherwise is a turbo brainlet.

obviously A is correct. The cube has no initial velocity and the portal passing around it does not impart it any velocity as it moves around the cube. The stationary cube remains stationary (before it slides off the second platform).

The top 75% has quite a bit of momentum at this point and will pull the remaining 25% through.

It's unanswerable because such a fucking device doesn't exist you /v/edditors

>In the game, objects come out of the portal the same speed that they went in.
Wrong, objects come out of the portal with the same MOMENTUM that they went in relative to the exit portal.

the portal is not a force acting on the cube

and the same mass, so that means it's the same speed you faggot imbecile

and assuming that what went in has the same mass as what came out it will have the same velocity. unless the "portal" transforms some of the kinetic energy into angular momentum but I have never played the stupid game this question is about so whatever.

The answer is still A, because even if the moving portal sees the object with a speed V, it also sees the stationary exit portal with a speed V, and thus the velocity of the object relative to the exit portal is 0.

if portals didn't move you'd be right
after that, speed is relative depending on which one you're looking at, so your idea on what happens is ridiculous

imagine the portal stopped halfway
according to you the box will still be thrown out and lifted for some reason

your shitty idea of mechanics of a fucking videogame is inconsistent, so
>there's no answer lol

>The answer is still A
obviously

but we are assuming a coordinate transformation accomplished by the portal upon the cube without respect to the displacement of the matter in the location of the cubes materialization. You must presuppose annihilation of the matter in the location of the cubes materialization (impossible) or the whole thing is impossible.

This is the first argument I have seen in many discussions on this topic that actually changed my mind and caused me to update my thoughts. Good job, user. Credits are deserved.

I maintain that answer A is obviously wrong, but I am now in doubt about answer B as well. The physics of portal-with-movable-portals is one for which it is not clear whether it can mathematically exist as an extension of Newtonian physics. While it was obvious to me that A would not be a consistent extension of Newtonian physics, I thought that B could be; but now I am more inclined to say that neither A nor B works as a consistent extension of real-world physics. Meaning that either (1) the behavior would be something unlike either A or B; or (2) a physics of moving-portals can be an extension of Newtonian physics, but the high-level behavior of objects therein is NOT an extension of the high-level behavior of objects in Newtonian physics; or (3) the extended physics is contradictory in its entirety.

The discussion managed to change my mind. Well done, user, and thank you.

>I said "there's no answer lol" was wrong but now I realize it was right
video game mechanics are usually shitty and inconsistent.

No, that's oversimplifying things.

Most people saying "there is no answer" are deferring to the fact that (1) the game doesn't let you know this so the question is moot, or (2) the fact that this isn't real OR supported by the game means we can't get more experimental evidence, or (3) portals aren't real so the question is silly. I don't support any of these; exploring the nontrivial consequences of a hypothetical extension of physics is entirely legitimate.

There have been some people claiming the scenario is inconsistent (that is to say, incompatible with any consistent physics); but they are not in this thread, and I have never seen anyone offer an *explanation* of that, just a flat claim to dismiss the argument. (And no, a reference to the impossibility in the game does not count, for it is the inconsistency of an extended hypothetical physics that we are discussing, not a property of the game.) This is the first time ever I have seen an argument causing me to take seriously the possibility (I'm not entirely convinced yet) that no such extended physics might be possible.

then how did the fucking cube exit the portal faggot
jesus christ so many faggots in here acting like they know shit when this scenario is intentionally ambiguous
before entering blue, the cube's particles have no momentum wrt the frame of reference of the room
upon entering blue, the particles exiting orange are imparted momentum since they're FUCKING MOVING wrt the room
wrt the frame of reference of the blue (and orange) portals the cube is moving

kill yourself portals aren't even fucking real and this is just one reason why

so your idea is everyone was wrong because nobody bothered to explain it to you?
honestly I call people in threads like this idiots and tell them to fuck off with the unanswerable videogame "physics" bullshit pretty often
you shouldn't assume so much about what people who don't say the same as you know. they might know more.

>then how did the fucking cube exit the portal faggot
Displaced by the cube's own weight normal to the surface of the inclined plane the orange portal.

>t. IQ of 80 with no imagination or abstract reasoning faculties

that doesn't change the fact that the particles that have moved through to the other side have been imparted momentum in some fashion
whether or not the portal itself imparted the momentum doesn't change the fact

most people saying that "there is no answer" weren't deferring to any of your retarded arguments
it's really easy to see why the scenario is self-contradictory and your inability to see this while maintaining superiority is hilarious

What would happen now?

so objects which move wrt an inertial frame don't have momentum?
thanks for the enlightening idea, i'll be sure to collect my Nobel this year

l-lewd.

Jesus Christ I'm amazed at how retarded this board can be sometimes.

>exits through portal B with (obviously) non-zero velocity with respect to the room, and thus the Earth
>remains stationary
this is really activating my almonds

>we can only talk about things that physically exist in reality guys
>the entirety of mathematics is bullshit guys please believe me those things don't even exist in reality all mathematicians are liars
Stop breathing my oxygen.

No, you continue to move with the same velocity, relative to the doorframe, unless some force acts upon the doorframe or you. It's B.

This is literally physics and not math. Discussing retatded vydia shit has no purpose other than pure cringe, because the only thing we know it's that it makes something teleport magically into some place. It's you retatded faggots that are always analysing comic book shit and just being a neckbeard autistic manchildren faggot.

what now, Veeky Forums ?

i'm at a pure loss

I see the scenario this same way, if the portals can move relative to each other then you have to consider everything behind the portal to be like a second universe moving along with it, otherwise if you put portals on entities moving at very different speeds (like the moon, as a totally random example wink wink), by maintaining your velocity relative to the entrance portal you would go flying and splat against a wall whenever you passed through them

moreover, I think if the portals can change the direction of your movement based on their relative angle they should also change your speed based on relative speed

>"This is literally physics and not math"
>theoretical physics isn't a thing
Stop posting any time.

>Theoretical physics is math

where's my grant money

...

shouldn't the surface behind the portal actually collide between itself when the two portals also collide ?

This is wrong too.

Momentum magnitude is conserved, not direction.

the tricky part of this scenario is keeping in mind that there is nothing behind the portals, everything behind the orange is what's in front of the blue and visa versa

No, the tricky part is even getting as far as supposing the portals move, which they don't. The moving portal question has no basis in either virtual or physically reality.

Portals dont work on moving surfaces.
You can write 2 more walls of text that doesn't make it right.

i made that picture because it's the only "lore" friendly paradox you can actually make.

portal can move but not forward or backward, only up/down/left/right. the game also handles portals that move forward and backward but they're never shown.

ie: ur wrong & stupid, stupid.

what about the orange portal ? it's not shown in the picture but it should definetely come out of the blue portal

So, uh, forgive me for asking supid questions. My intro physics classes were quite a while ago now.

I feel like a lot of people here are focusing on Newton's First Law and while I think that's interesting I also know that the interactions here are a bit too complex for me to say what's going on with that in this particular instance.

However, I'm just thinking about Newton's Third Law.

So based on Newton's Third Law we know that if the cube is sitting on the platform it starts on that it must be pushing down on the platform with a force equal to the relative force of gravity and the platform is pushing the cube up with the same force, right?

But then when the portal passes over the cube there force gravity exert's on the cube changes direction instantaneously and becuase of that the cube's center of gravity relative to the platform shifts to it's corner instead of it's center.

So would that cause the cube to sort of try to rotate then? Because the force it's exerting on the platform goes from being evenly distributed across it's surface to being weighted to one side of it's surface on a gradient. Like, would the change cause the forces to suddenly become unequal and thus cause the top edge of the cube to be pushed forward and cause it to rotate?

>And object in motion tends to stay in motion and an object at rest tends to stay at rest.

>~Isaac Newton

the orange portal never enters the blue portal, it doesn't do anything

portals aren't exist, aspie

First of all, assuming a break through were made and we could create portals, would portals be flat as depicted in this game or would they be 4th dimensional or maybe 3rd?

Isn't the point of them that they're two-dimensional interconnected holes?

But if they're connected, would they really be two-dimensional? If you can directly travel through them, would that define them as 3rd dimensional or 4th dimensional?

Can you actually travel through things with a second dimensional nature?

if they really are 2D, wouldn't the edge be able to cut through anything ?

Can 2D objects interact with 3rd or 4th dimensional objects? Or can they only be displayed?

Define "cut"

It does, the energy has to come from somewhere

The cube has inertia that must be overcome to accelerate the cube. There must be a reactionary force due that that inertia. All the usual physical mechanics must still be present and balanced. All the portal does is bend space-time between two regions. For all the cube is concerned it has not moved, space just bent around it, but the cube never accelerated. Momentum is concerned.

The only way B would be right is if blue portal was on the equator and orange was at a different latitude, then the cube would maintain it's higher momentum from the angular velocity it had on the equator with respect to the new space. And even then, it wouldn't be the portal giving the cube momentum, but the orange portal falling away/behind the cube

HEY GUYS! DID YOU HEAR THESE user'S?!

THEY SAID PORTAL'S AREN'T REAL!

Technically, gravity would get really wonky when you get close to the portals, because now the Earth is nearly equally close to the cube above as it is below.

I thought about that but in the end I gave up and decided that gravity just wouldn't pass through a portal or shit will get fucked really quickly.

The cube wasn't accelerated, though, from the perspective of the orange portal it was always moving.

...

in order for a paradox not to be created the exit portal has to be moving at the same velocity as the entrance portal. the moving exit portal then deposits the stationary cube in the different location.

A. The cube is not moving through the portal, the portal is moving around the cube.

I wish someone would just make a gif of someone putting a cube on a raised platform through a hula hoop already.

It's not real, so really, whatever you would like to happen is fine

Probably best answer in this thread, only one which got no (you)s

Contributing with my take on this, more precisely why is this not tractable with classical mechanics.

btw the "portals violate energy conservation laws anyway" is not a good argument. This is only because of the way gravity is dealt with in the game, you could have a gravity potential that works with portals. And gravity is not even necessary here.

Right, so in the orange reference frame, the plinth isn't moving. In the blue reference frame, the plinth is moving upward and then stops. So that means that every part of the system that goes through the portal is given motion as soon as it goes through that portal.

If the cube remains static after passing through the portal, it'll just plop down. However, if you consider the smasher continuing to push downward so the portal consumes the plinth, the plinth will clearly be moving upward in the blue reference frame until the smasher stops. That essentially makes the plinth into a pinball plunger. Guess that makes it B.

The earth moves
>inb4 they don't move relative to each other
In the ending of the second game you put a portal on the moon.

Wow. That is some ancient bait. I'm impressed that somebody animated it.

Point out 1 ACTUAL EXAMPLE of something that is inconstant with portals.

Yes everyone knows portals aren't real. But the rules are very well established, and you can extend these rules to moving portals with zero inconsistency. This extension of rules can only happen in one logical way, we have shown what that way is. Getting the answer requires thinking with portals, something the game tries to teach you to do.

I'm sitting here and you still haven't pointed out a single inconsistency. (Inconsistent with PORTALS, we already know portals can make perpetual work machines.) But within the logic of moving portals there is nothing inconsistent with B.

Its A, simply because otherwise you could make a perpetual motion machine for infinite energy