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Is that guy wearing blue and black jeans or white and gold?
Depends on if he’s saying Laurel or Yanny.
B since all movement is relative.
This was a triumph.
I’m making a note here: huge success
It’s hard to overstate my satisfaction
Violates conservation of momentum unless the portal/train slow down.
Yes, but portals violate basic physics anyway.
A portal that faces downwards into another portal is effectively a perpetual motion machine. Drop a ferromagnetic object into the loop and wrap some wires around the loop, now you have an infinite electric generator.
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
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.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
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
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.
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.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 💀
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.
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
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1 nano second in his entire body is within some imaginary dimension between the 2 spaces
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The body gets infinitely squeezed in 1 space turning them into a mini black hole
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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.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.
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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.
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C, it combines the victim into a horrible overlapping monster of body parts
Or as I like to call it: Tuesday
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.
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?
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.
it’s relative to the earth or tied up people, then A.
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.
yeah, all movement is relative, if it was B then the relative movement between the people and the train would have changed, if it’s A then it’s conserved
Or, to say it in Glados’ words: “Speedy thing comes in, speedy thing goes out”.
A, the people are traveling towards the portal, not the other way around.
If they were falling/running at the portal it would be different.
Here the portal is moving forward towards them, they have no momentum.tk travel through the portal.
If the train drives slow enough that is takes 3s between when your head gets through and your feed are trough, it also needs to take 3s on the other side or you are ripped to pieces or squashed.
Now if it takes 0.1s, you also have to come out in this time and will have a velocity, the same as the train.
How long it takes makes no difference. In your story any non-instant teleportation would “rip” you to pieces
Yes, if the speed you go in wouldn’t the speed you go out, you’d be ripped apart.
In portal (the whole point of this joke), it happens instantly an works like walking through a door. If your hand is through, it’s at the other side and the rest of your body isn’t.
If you would travel at the same direction and speed of the train, you could step through and be stationary at the other side. If you stand still and the train travels to you, the only “logical” answer is that you fly out the other side or be ripped apart.
a portal is supposed to be like a hole that you go thru except you end up somewhere else, if i pass a hole over you, would you feel anything? A
Except in that scenario both portals are moving if they act like a moving hole. Imagine a hula hoop, except it’s 2 portals connected back to back. If I passed a hula hoop over you, you’d be going into the bottom at the same velocity that you are coming out the top, therefore momentum is preserved. You’re moving at the exact same velocity in reference to both of the portals
This is my favorite post in a while.
Thank you!
Truthfully it was pretty low effort to make, but I appreciate it anyways.
Speedy thing goes in, speedy thing comes out
E: I was just quoting GladOS… Not really thinking about the actual physics!
Yeah but the thing isn’t moving the portal is, and the energy has to come from somewhere if the portal makes the thing go fast.
The energy would come from the trolley. The people would launch out at approximately the same speed as the trolley interacts with them and the trolley would slow down in response to how much kinetic energy was transferred to the people.
Now you’re thinking with
portalsphysics!This is correct. The motion of the people is relative to the Portal. It doesn’t matter if the trolley is accelerating the Portal towards them or something is accelerating them towards the Portal. Therefore they accelerate out of the other side with some retained momentum. Technically it probably resembles something in between pictures A and B.
This reminds me of the experiment about whether an airplane could take off from a treadmill.
Only if you assume the people will experience friction against the portal.
If they would accelerate to the speed of the train within the time it takes them to go through it, they’d experience very high pressure change against the due to one part of body accelerating faster than the other. This would cause the bodies to explode out the portal
Interesting. It would be the same as if the trolley hit them directly.
Yes, as I noted elsewhere in the thread, the part of the body exiting the portal will experience inertia as it enters into the space outside the second portal and it will be forcefully pushed by the next part of the body heading into the first portal and thus imparting momentum to the parts ahead.
If this momentum has to be taken from anywhere its from the portal itself and by extension the train.
The energy would come from the trolley.
Has the trolley come to a complete halt, or even showed down? If not, then either no energy has been transferred to the people and they just flop out, or we’ve just invented perpetual motion.
This guy is thinking with portals!
No moving objects are entering… lol
Then they can’t enter at all and have to be flattened by the portal, because they must have motion too exit the other portal
How can it not be b? Every situation in the Portal games is already exactly like this, but with the portal fixed to a slab that moves with the rotation of the Earth, whereas in the drawing the portal moves as the sum of earth rotation + the movement of the train.
C. A train flies out of the blue portal
I think it’s A because I assume a portal stitches two points in space to each other.
So if I have a surface A and B with a portal ‘]’ in the middle A0 A1 ] A2 A3. B0 B1 [ B2 B3
A portal creates a new surface
A0 A1 ][ B2 B3
And if you move the portal the new surface changes.
A0 A1 A2 ][ B3
Speed is distance over time. When a portal moves the object that passed through the portal stays stationary. Let’s say I am standing on B2. When the portal advances I find myself standing on A2 , have i moved? No the environment has changed but i am still in the same relative position with respect to the portal surface. No distance travelled so no speed.
I say its B, because if we jump in a portal we fly out of the other one. Now the difference is here the portal is moving and not the person, but in physics we are only interested in the motion of two bodys relative to each other. If you are standing on the train you would see the portal as stationary and the people coming towards it. Because the object/person enters the portal with speed it also comes out with the same speed.
If you jump into a portal, YOU have momentum, these people are stationary and therefore have no momentum, the answer is A.
There is no momentum without saying relative to what. A train is moving relative to the people not on the train. The earth is moving relative to the sun. But if you are on earth the sun is moving and you are stationary and your viewpoint is not wrong! The reason is, that there is no underlying space we are moving relative to. Maybe this helps: Imagine a train in a train. Driving with the same speed but in the opposite direction. Now from the outside the inner train is not moving at all, but the outer train is. From the inside the inner train is moving and the world around them is. If there is now a portal inside the outer train and the inside train is passing through. What happens. Nothing because the inside train is not moving or it shoots out because it is moving?
But the object doesn’t enter at speed here it’s stationary. If a hoop is thrown at an object and passes around the object the object is still stationary. The speed of the portal relative to the object does not impart movement on the object.
The quicker the portal moves the quicker the object appears on the other side but for the object to shoot out it means energy is being transfered from the car to the object.
If you’re right then what happens If the two portals are moving at equal and opposite speeds?
The portal is a connection of two spaces, but the spaces themselves are not moving. It’s hard to say though because some sort of force would have to push you into place, but momentum is conserved so… Who knows.
B.
Speedy thing goes in. Speedy thing comes out.
Although it kind of depends how fast the tram is going.
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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.
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No your wording was correct, do not apologize.
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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.
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”.
Absolutely correct. We learn this in basic static as dynamic physics.
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.
The whole thing would just be relative to the portal, though. Relative to the portal, they come in fast and out fast.
A if the trolley is going slow
B if the trolley is going fast.
Can portals exist on a moving object?
Not in the regular game, there are mods/fan games that allow it though.
There is one segment in Portal 2 where you need to cut some tubes that transport neurotoxin, and for that small segment only, the game allows moving portals. https://youtu.be/OrAHvenjZpA
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it’s specifically scripted for that though, normally the engine just freaks out
I think B and maybe it’s easier to explain my reasoning with a more dramatic example. instead of people on a track, maybe the trolly is heading towards a 50 foot horizontal pole. when the trolly comes to the pole at 90mph, the pole is not moving. but after the trolley’s portal has “swallowed” 40 feet of the pole, all 40 feet of that pole are exiting the portal at 90mph, being pushed by the 10 feet of pole that the trolly is still “swallowing”, so the momentum of 40feet worth of pole would continue to launch the remaining mass of the pole out of the portal and it would be launched out instead of flopping to the ground.
if we go back to our people on a track example, I think this would also kill them as for people on either side of the portal, it would feel like they’re being ran into at 90mph by the people on the other side of the portal.
did that all make sense?
Yes, I also thought of the pole example. It would feel like a wave of gravity suddenly shifting around them. I do think it would be less dangerous than a direct collision at the same speed, but only by a little - the danger with sudden deceleration isn’t the fact that it’s sudden, but that it is almost always uneven, it rips things inside you apart or collapse them together. But the portal would be a very clean uniform wave of pressure moving over you, so it could still cause damage by compression over one dimension (in the direction of portal movement) but most other forms of impacts will be missing
B
Lets say the train is moving with 10 meters per second. That means that the people will enter the portal with 10 meters per second. Therefore, they will leave the other portal with, you guessed it, 10 meters per second. Henceforth, they will be traveling with 10 meters per second after leaving the portal. 10 meters per second.
How many meters per second was that again?
The would be moving at 10 meters per second in regards to the train that didn’t touch them, the same as the were before the train got close enough to touch them.
Think of it the other way. If the went into the stable portal and came out I’m from of a moving portal, what would happen?
The portal would move forward and swallow them up and spit them back out the way they came in.
They would not have accelerated in the process. They wouldn’t fly out the portal they just walked in at the speed of the train. The train didn’t touch them so it can’t transmit any of its momentum to them.
Where will the energy that accelerates the people come from?
The train.
As the object enters the first portal then the inertia of the far end of the object that is forced to pass through will need to be accelerated in the space outside the second portal to pass through, acceleration which is induced by being pushed by the other end of the object still outside the first portal.
In other words, pushing a portal onto an object pushes that object with half the speed of the portal. This will likely require energy put into the portal itself to maintain it, which needs to come from the train.
That was an idea I’d floated above as well.
The train never touches the people.
The portal touches them first, moving where the people are.
The portal is moved by the train
If the other portal is on back of the train, they they will stay at rest(might be displaced to back at lengthOfTheTrain distance
Like that episode of myth busters where they fired a canon backwards out of a moving vehicle
