Current Events > I have a physics/portal related question.

(+) Yeah! (+)

https://gamefaqs.gamespot.com/a/user_image/0/6/7/AAZuMeAAEI6L.png

The top platform is moving down with a portal on it at a high speed.
Does the cube on the platform shoot out at the red trajectory, or just slide off the angled platform like the purple trajectory

My heart says A but my brain says B

I think it would just plop

Red. Even though the portal is moving in our frame of reference, in the reference frame of the portal the cube is moving. Speedy thing goes in, speedy thing comes out

purple

this sounds like one of the reasons portals don't already exist

VampireCoyote posted...

I think it would just plop

Depending on friction, it may just pop out at an angle and stay there.

Plop

teep_ posted...

Speedy thing goes in, speedy thing comes out

but a stationary thing went in

Theres no momentum

It's a trick question, because you can't put portals on moving surfaces

Kloe_Rinz posted...

Theres no momentum

This, so purple. If you dropped the cube Into the portal hole it would fire out more like red. The portal dropping on the stationary block won't make it move.

UnholyMudcrab posted...

It's a trick question, because you can't put portals on moving surfaces

But every surface is technically moving.

Mistere_Man posted...

But every surface is technically moving.

Is it moving relative to the other portal, though?

Trumble posted...

Is it moving relative to the other portal, though?

On a microscope level, yes

the bottom platform would have to be launching the cube up, because all you're pressing down onto the cube is space from the other portal. there's no mass, nothing to cause work. until the top portal meets the bottom platform, the cube has stable ground to remain stationary on even as the portal passes over it. only the cube's own weight on the slope would cause movement.

the portal meeting the ground (therefore meeting the other portal) honestly seems like an impossibility anyway, because it would mean the flat bottom platform existing in one space has suddenly become the surface of the sloped object in another space, essentially destroying the first portal and creating an impossible object with conflicting geometry existing in two places at once

tl;dr: this is why portals don't move

https://gamefaqs.gamespot.com/a/user_image/1/4/5/AAZ4uYAAEI7Z.png

I'm opting for the third option (in yellow), on the grounds that it would be funny.

This is my answer that I definitely didn't steal from the physics subreddit.

I think 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 and conservation of energy really easy to get tripped up by.

To explain: 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 space-time 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, in particular, 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. To be specific, 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" (i.e., 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.

TL;DR: (B)

UnholyMudcrab posted...

It's a trick question, because you can't put portals on moving surfaces

Isn't there a part where you use moving portals and a laser to cut some pipes?

MarbyIsBack posted...

https://gamefaqs.gamespot.com/a/user_image/1/4/5/AAZ4uYAAEI7Z.png

I'm opting for the third option (in yellow), on the grounds that it would be funny.

I love u bagelman

pegusus123456 posted...

This is my answer that I definitely didn't steal from the physics subreddit

Wha

Link?

The block itself has no inertia, relative to a reference of "everything". It'll either stay where it is, or slide off and plop.

Why are the smart people I know saying red trajectory though ;^;

ApherosyLove posted...

Why are the smart people I know saying red trajectory though ;^;

Because they were too smart to waste their time playing Portal. For us gamers, the answer is obviously yellow.

Heres the problem I just came up with for the people saying its the blue trajectory:
Imagine you had the same setup as this one, except that over the exit portal you had some kind of ceiling or pad that measures force through impact at a lower height then the cube.
Now if you SLAMMED the portal over the cube it would be transposed to the exit one at the same speed you were smashing down, meaning that it would smack into this ceiling before it came all the way out of the portal.
If you subscribe to the idea of the blue trajectory, then the cube can't have any force by coming out of the portal. So the question would remain: where did this force come from then??

cadcrafter posted...

So the question would remain: where did this force come from then??

gravity

cadcrafter posted...

Heres the problem I just came up with for the people saying its the blue trajectory:
Imagine you had the same setup as this one, except that over the exit portal you had some kind of ceiling or pad that measures force through impact at a lower height then the cube.
Now if you SLAMMED the portal over the cube it would be transposed to the exit one at the same speed you were smashing down, meaning that it would smack into this ceiling before it came all the way out of the portal.
If you subscribe to the idea of the blue trajectory, then the cube can't have any force by coming out of the portal. So the question would remain: where did this force come from then??

Holy shit. I think you cracked it for us laymens.

ApherosyLove posted...

Holy shit. I think you cracked it for us laymens.

No I take it back, this doesn't explain anything.
The force comes from the thing that measures force. It would simply crush the box between itself and the platform the box is on.

I have a completely different answer. The cube never leaves the left platform. You'd have to place the portal under the cube for it to move.

ApherosyLove posted...

No I take it back, this doesn't explain anything.

lol

cadcrafter posted...

Heres the problem I just came up with for the people saying its the blue trajectory:
Imagine you had the same setup as this one, except that over the exit portal you had some kind of ceiling or pad that measures force through impact at a lower height then the cube.
Now if you SLAMMED the portal over the cube it would be transposed to the exit one at the same speed you were smashing down, meaning that it would smack into this ceiling before it came all the way out of the portal.
If you subscribe to the idea of the blue trajectory, then the cube can't have any force by coming out of the portal. So the question would remain: where did this force come from then??

Okay, let's look at this with a diagram.
Your device or whatever is the green box. If the orange portal went all the way down, the pink box would hit the green box and measure its force.
https://gamefaqs.gamespot.com/a/user_image/9/9/2/AAZuMeAAEJIo.png

But because of how portals work, the following representation would also be accurate:
https://gamefaqs.gamespot.com/a/user_image/9/9/3/AAZuMeAAEJIp.png

Here you can see that the "force" is coming from the box being crushed between the device and the platform that the box is on. The force itself is dictated by how strong the piston that holds the orange portal is.

Cut a hole in a sheet of newspaper, and drop it over whatever cube-ish object you have nearby.
Observe how much momentum the cube gains.

Im still not really sure how to answer my own question yet, but I definitely dont think its just the box being crushed, or not in the way we'd usually think of it at least.

ApherosyLove posted...

Okay, let's look at this with a diagram.
Your device or whatever is the green box. If the orange portal went all the way down, the pink box would hit the green box and measure its force.
https://gamefaqs.gamespot.com/a/user_image/9/9/2/AAZuMeAAEJIo.png

But because of how portals work, the following representation would also be accurate:
https://gamefaqs.gamespot.com/a/user_image/9/9/3/AAZuMeAAEJIp.png

Here you can see that the "force" is coming from the box being crushed between the device and the platform that the box is on. The force itself is dictated by how strong the piston that holds the orange portal is.

Questionmarktarius posted...

Cut a hole in a sheet of newspaper, and drop it over whatever cube-ish object you have nearby.
Observe how much momentum the cube gains.

The problem I have with modeling the question in this way is that it models it as if the blue portal is moving, but the whole crux of the issue is that the other end of the portal is stationary. You would only gain the same effect of putting the cube through a loop if the blue portal was moving backwards an equal amount as the orange one

cadcrafter posted...

The problem I have with modeling the question in this way is that it models it as if the blue portal is moving, but the whole crux of the issue is that the other end of the portal is stationary. You would only gain the same effect of putting the cube through a loop if the blue portal was moving backwards an equal amount as the orange one

The "orange portal" is merely the top side of the newspaper.

Questionmarktarius posted...

The "orange portal" is merely the top side of the newspaper.

Right, that I can see but the blue portal (the other side of the newspaper) is moving an equal amount. In the actual question its stationary

cadcrafter posted...

In the actual question its stationary

And it still is. The cube's own inertia is 0, which is the opposite of "speedy thing".

Wouldn't the top part slamming into the bottom generate force?

MedeaLysistrata posted...

Wouldn't the top part slamming into the bottom generate force?

Why

Purple. The cube itself has no momentum.