this post was submitted on 18 Aug 2023
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A, since portals cannot transfer momentum from the tram to the victims.
To put it another way, if you were standing and the portal was pushed towards you by a tram, do you think you would be launched out of the other side at that rate?
There might be some increase in momentum as the part of you that went through the portal first gets pushed forward by the parts of you that get pushed forward after, but it's not going to be as dramatic as the momentum you would have received being hit by the tram.
Most likely you would stumble forward and fall down or have to catch yourself.
Portals maintain velocity. Velocity is relative. Therefore the velocity they maintain is the relative velocity of the portal and the subject. Any other way and there would be no consistent way to pass any moment when passing through a portal.
After thinking about it longer then i care to admit I think i finally agreed with you.
As you said it is all relative, from the prospective of the moving portal. You could say it isnt moving at all but the entire world around it is moving, thus when people enter the portal from the portal's prospective they people are the ones moving and will continue moving when they exist.
Hmm tlnit that i typed this out I feel like i didn't do a very good job. Owell the answer is B.
Yes, join the relativity gang
The reason this is so confusing with different answers is that the portals don't really exist, so inherently whether you say a or b is gonna depend on assumptions. In game they aren't allowed to move so we have nothing to base it on to match game physics.
Here's my take, momentum is a product of velocity. Velocity needs a reference frame. Without it, there's no real difference in saying the portal has a velocity of 0 and the people tied up have a the velocity and therefore momentum, or the other way around. If we assume velocity with respect to the portal is what matters and is the momentum carried forward, then it should be B. If it's relative to the earth or tied up people, then A.
If portals did not apply a transient vector to your momentum then you could not pass through a portal.
Take for instance the many times Chell jumps through a portal. Her momentum is maintained as she passes through the portal, allowing her and her robot legs to do truly stupendous feats of gravity assisted acrobatics.
If Chell was stationary and the portal fell on her, she would not be launched out of the other side with the momentum of the portal, she would just find herself sticking out of the other side of the portal.
Similarly, if Chell were to ride a moving platform up into an overhead portal, we would expect the top half of Chell's body to pop out of the portal without being accelerated by anything other than the moving platform on the bottom of her feet.
Therefore, unless there is some strange unknowable physics that we will not be able to discover until we develop portals of our own, the most likely outcome is that the victims on the tram would not gain any momentum as the portal was pushed into them, and they would plop out safely on the other side.
The only speed that should be relevant is the object's speed relative to the portal. Anything else is a distraction. The physics don't care if you are hurtling at it or it is flying at you, both scenarios are equivalent. The only way to maintain conservation of momentum is to assume your exit speed relative to the exit portal equals your entrance speed relative to the entry portal.
If it did work the other way, well it wouldn't assuming your exit speed is equal to your initial speed, relative to the exit. That means your speed is 0 as you "exit." This leaves us with two possibilities. Either you are smashed into a 2d plane and physics gets very concerned, likely forming a teeeeeny tiny black hole. Or the incoming matter behind the first bits will push the first layers through, which, will just wind up back at the starting point, as they will cascade into each other at a speed defined by the speed of the blue portal, being indistinguishable from the projectile interpretation.
You can say you can expect, but you really can't, because if you're talking about momentum you're talking about velocity and you need a reference frame to define velocity and therefore momentum. Let's pick the sun for instance with the assumptions of A. So if we just have one portal pointing one direction and one portal pointing up and chell walks in, you should blast out straight up at 66,000 mph plus the speed she was walking then. I think you could make the reference Frame to earth and try and get a, but that would create problems too.
I think B, velocity relative to the moving portal, would be the only way to maintain some kind of consistency in game if you were going to have moving portals. Your examples are most consistent with B. A portal falls on chell, how fast does she come out? The speed the portal fell on her of course. And then she stops going out once the portal stops moving because it hit the ground and has stopped moving and they no longer have any relative difference in velocity. You could also say in the platform example that the platform was sitting still and the portal was moving down, you would emerge out the portal at the speed the first portal was moving down. Both should be equally valid ways if you want to maintain some consistency. But all of this is probably why they don't allow moving portals in the first place.
In the end though these are definitely strange unknowable physics, portals don't exist, so really you could make the game however you please, either one is perfectly valid, you could just say any velocity on the other side is whatever it was in relationship to the earth before going through, but that'd be weird, because how fast do the people move out of A then? Do they fly out at the speed of the moving portal and then suddenly stop mid air and plop straight down? If you're not moving faster than a moving portal does is become brick wall and smash you out of the way so you don't gain any velocity in relation to earth so A can be maintained? There's no way to test it in the current games. Hence the endless arguing. But I think B would be most consistent and allow for some really interesting puzzles though, especially if you had two moving portals! Or maybe 3d portals that can sit in the air and allow full movement through them in any direction to help make it possible. Portal 3? In VR with depth perception to accommodate?
If it is relative to the earth, they would be crushed at an atomic level.
Imagine the trolley-portal is passing around a tape measure at 10m/s. The tape measure is stationary on the earth. After 10 seconds, 100m of tape has entered the portal in a straight line. For me to have 100m of tape in a straight line at the exit, the end of the tape has to be moving away from the portal at 10m/s. Given that "crushed into a singularity" is not an option, we can assume the velocity cannot be relative to earth, and must be relative to the portal.
C, it combines the victim into a horrible overlapping monster of body parts
Or as I like to call it: Tuesday
All motion is relative. To understand how the people will move, we need to look at them relative to the portal. If the trolly is moving at 5 m/s relative to the ground, then the people are moving at 1 m/s relative to it. So they enter the portal moving at 1 m/s and exit at the same speed.
If I stick my arm in a stationary (relative to earth surface) portal, I expect my arm to stick out of the exit portal. If the exit portal is moving at 10m/s over the earth, I expect my arm to also be moving 10m/s over the earth. My arm is stationary relative to the portal, but the portal is moving.
If that portal is moving toward a standing person and I make a fist, I expect my fist to hit that person at 10m/s. I am stationary relative to the earth; they are stationary relative to the earth, but my fist is moving at 10m/s relative to the earth. From their perspective, I punched them. From my perspective, they ran into my fist.
If I look through the portal, I will see them approaching me at 10m/s. They will see me inside the portal, approaching them at 10m/s. When the portal passes around them, they will not feel any change in their velocity, they will just collide with me immediately after the portal passes around them. To them, the earth will seem to suddenly be moving at 10m/s.
I think Portal solved this conundrum by saying portals can't move.
Energy is relative when there is a frame of reference.
When the tram-portal is the frame of reference, the person has the energy. And speedy thing goes in, speedy thing comes out.
Using Portals canon, the person cannot be the frame of reference (ie 0 energy), because the portal has to move for that scenario - which is Portal-ly impossible. So the person has to come flying out.
If you break Portals canon and say that portals can move, then then the person would likely be super-compacted (matter transporting on top of existing matter) into a singularity or just destroyed.
Portal breaks its own canon on Portal 2's neurotoxin implosion scene though.
Portals can move along the plane of the portal in that scene, but never forwards or backwards
It's canon that they portaled between earth and moon. For a portal to be stationary relative to both, it has to be moving relative to its opposite end.
They totally can move though. In one of the puzzles there's a button that makes part of a wall angle itself so that you can propel yourself forward, and the portal on it will move.
May be remembering wrong but I thought any wall movements like that one had to be done before you shoot a portal onto it, because if you do beforehand it'll poof away when you move the wall.
It has been a while, so I may also be remembering wrong so 💀
I remember that.
It's a timed puzzle.
The wall moves on a button push, and moves back after an amount of time.
The portal is destroyed when the panel moves.
There's literally nothing in the universe that is 'stationary' so the entire concept is flawed.
I mean, portals are flawed.
Anything moving through a protal experiences acceleration, unless the exit-portal is at the instantaneously-exact position of the entrance portal.
There has to be rules and limits that are ignored if portals are to exist, which is what the hypothetical situation is presentin5
Frames of reference matter. Whether the train or the people moving happened it doesnt matter to the portal. There is net movement and the momentum is the mass of the person moving x the speed of the train.
Imagine the train was moving the speed of light. If the person exiting the other end of the portal wasnt coming out at the speed of light their body would come out like a soup. All the atoms in their body compressing to escape at some randomly low speed... actually it might make a tiny black hole on the other end as the atoms compress infinitely.
A portal is as another commenter has framed it, essentially a hula hoop with a different space on the other side of it.
It doesn't matter how fast a hula hoop falls over your body. You are not going to be launched out of the other side of the hula hoop even if the hula hoop is moving at the speed of light.
If the hula hoop is moving at the speed of light you are more likely to be killed by the shockwave of all of the atoms in front of the hula hoop compressing to adapt to the sudden intrusion of a lightspeed object with Mass, in which case it is very likely that you would pop out of the other side as some sort of soup, but that would not be because of your interaction with the portal inside of the hula hoop, or the acceleration of the hula hoop itself but rather the acceleration of the things around the hula hoop as it moved through space.
When im talking about speed of light i am assuming it will be in a perfect vacuum. If this was in ambient under normal conditions, a train going the speed of light would ionize all the air around it causing insane levels of heat.
So with the thought of it moving in a vacuum, if you look at the portal on a frame by frame basis every nano second you would see either
1 nano second in his entire body is within some imaginary dimension between the 2 spaces
The body gets infinitely squeezed in 1 space turning them into a mini black hole
They leave the portal at the same rate they came in
These are my 3 options, i dont see how it can be any other way.
I think that the velocity of the tram has nothing to do with the velocity the people it is running over until it actually runs them over and transfers momentum to them.
The portal puts a gap in between the tram and the victims, so there is no physical contact to transfer momentum. Momentum is a physical property, it cannot be transmitted without contact.
Therefore, in a frictionless vacuum the people must keep their original velocity and momentum regardless of the speed of the portal or whatever is pushing it forward.
If it worked the other way, Chell could not have leapt off of a ceiling and been launched out of the other side in the game. If portals transmit momentum without touch, then Chell would have first impacted an unmoving object with the same force as hitting the floor.
You can't have it both left moving objects fly though unimpeded keeping their original momentum and also have unmoving objects suddenly gain momentum from a moving portal.
The portal does not affect momentum, it is a break in momentum. Momentum does not transfer across portals.
The momentum stays with the object that passes through the portal.
Quantum states can jump long distances without particle interaction, this includes energy transfer (you just need a measurable latent possibility for particle interaction!)
https://www.discovermagazine.com/the-sciences/first-demonstration-of-energy-teleportation
https://www.scientificamerican.com/article/nil-communication-how-to-send-a-message-without-sending-anything-at-all/
https://www.nature.com/articles/ncomms7811
The scifi answer that makes sense is that the space curvature around portal and shifting the object between two frames of reference (perhaps through frame dragging effects / gravity-wave like space warping) imparts momentum and it is conveyed from the portal as it is the thing which has to put in energy to correctly warp space for the object transfer.