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Combining two different internet debates
(sopuli.xyz)
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this post was submitted on 18 Aug 2023
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Speedy thing goes in. Speedy thing comes out.
Although it kind of depends how fast the tram is going.
No, the speed of the pedestrians relative to the portal is not zero.
But the pedestrians have no momentum, only the portal has momentum.
Replace the portal with a normal open doorway, or that gameshow "hole in the wall", when someone passes through the hole, they aren't launched out of the other side, they're still just standing there as the door passes around them.
They have momentum though. They're on the earth (presumably), which is yeeting through space at a rapid pace. Portals must respect relative speed, otherwise everything would fly out due to their speed.
No your wording was correct, do not apologize.
If we assume portals obey https://en.wikipedia.org/wiki/Galilean_invariance, then there's no difference between "the tram|portal is moving and the people are stationary" and "the tram|portal is stationary and the people are moving." The outcome should be the same.
You mean A. The people are not speedy things, they are stationary things.
The whole thing would just be relative to the portal, though. Relative to the portal, they come in fast and out fast.
Not relative to the train. From the trains perspective, they are moving towards the train with the same speed they see the train moving.
There is no single correct reference frame. All reference frames are equally correct. If you want to argue that something is stationary, you have to explain what it is stationary relative to. There is no absolute "stationary".
The people are possessed of no kinetic energy, the train cannot physically act upon the people since the portal is intangible. There's no way for the train to transfer any kinetic energy to the people, and there's no other force that could act on the people. No kinetic energy going in = no kinetic energy coming out.
From the perspective of the train, the people have just as much kinetic energy as the people think the train has. Again, you're acting as if there is one absolute frame of reference - there isn't. Physics just doesn't work the way you think.
This is why all your comments in this thread are wrong, they have one simple logical issue: the people on the tracks aren't "stationary". They are stationary to the ground, but not to the train. It's not correct to say "the train is moving and the people are stationary", because it's also just as correct to say "the people are moving and the train is stationary". Physically both are true at the same time, that's what general relativity is really about. You can't look at the scene and decide "only one reference frame is valid", that would break all of physics.
Absolutely correct. We learn this in basic static as dynamic physics.