Memes @lemmy.ml

1y ago

Combining two different internet debates

310 comments

B since all movement is relative.
This was a triumph.
- 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.
  
  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.
  
  C, it combines the victim into a horrible overlapping monster of body parts
  
  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.
  
  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.
  
  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.
  
  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.
- Or, to say it in Glados' words: "Speedy thing comes in, speedy thing goes out".
- 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.
- 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

Portal would fail due to being placed on moving object
- The world is moving, checkmate.
  
  It's all relative.
  
  I would imagine that the relative motion between the entry/exit portal would be more important than the absolute motion of the two portals.
  
  So if portals didn't have a distance maximum, assuming that they twist through some higher dimension or into an alternate universe and back or something like that, it would make sense that you could open a portal on Earth and on Mars and anything you push through that portal would maintain its velocity relative to Earth.
  Which could result in some hilarious events where things basically detonate the instant they are pushed through as they are slammed into the surface of Mars at potentially ten of thousands of miles an hour depending on the Earths and Mars' relative velocities.
  Despite that, there would also undoubtedly be times where their velocities synchronize due to their varying rotational locations and orbital velocities around the solar system, during which times you could conceivably quite easily step from Earth to Mars in a single go.
  The safe thing to do though would be to decant from the Earth into a portal that is in orbit around Mars far enough away that at the worst you would experience some relatively gentle abrasion from the smattering of hydrogen atoms in the space surrounding Mars and then parachute down from orbit.
- Except for that one section in Portal 2 /s
  
  Why the /s?
  It's true. Obviously it makes for simpler puzzle design plus was easier to ignore the full capability (even the version in 2 seems to just work enough to allow the set-piece), so it seems silly to use developer limitation as a gotcha.

It needs to be 2. Otherwise all the people will materialize inside eachother. In fact, everyone will be deposited onto the 2-dimensional pane of the blue portal itself, like an infinitely thing coat of paint, absolutely smearing them.
Think about it. As your fingertips enter the orange portal, they materialize at the entrance of the blue portal. Then your wrist enters the orange portal, where does it materialize at the blue portal?
If your fingers shift to make room, then that has imparted momentum and it's option B.
If you continue to materialize on the other side of the portal like a mirror image, then for all intents and purposes the blue portal is also moving at the same speed as the orange portal, even if orange ring appears still.
If your fingertips don't have momentum and your wrist materializes at the portal, then your wrist is occupying the same space as your fingertips. Congratulations, you're now a paste.
For whatever reason I feel more willing to break conservation of momentum than I am to
- Good explanation.
  This has the interesting implication that the relative speed between the portals is "added" to whatever goes through it.
  Example: the blue portal is on a train running with the same speed in opposite direction. The people-bundle would instantaneously be accelerated to twice the speed of each of the trains. (This becomes a real headscratcher if you were able to put the portals in a particle accelerator)
- Yeah I really think you've misunderstood some things. An infinitely thin coat of paint? Are you familiar with the mechanics of the Portal games?
  It would be like dropping a hula hoop over a basketball. Regardless of how fast the hoop falls, the basketball still just sits there.
  
  I really think you didn't read my full comment, because I explained the problem with this exact scenario.
  First, in your hoolahoop example both sides of the hoop are moving with the same velocity (this is essentially option 3 I described). But the entire thought experiment is "what if the two sides didn't move with the same velocity"
  If you've played the game, you know that you don't instantly teleport when you touch the portal, you can be half in the portal. This means that when something enters the portal, it is deposited on the surface of the other portal. So as your arm enters the portal, your hand needs to move out of the way to make space for your arm.
  If your hand doesn't move out of the way to make room for your arm (it is still because it has the same momentum that it had when it entered) then your arm will materialize in the same space as your hand. Now scale that down to the atomic level, if the atoms of your fingertips don't move for the next atoms, everything will be deposited in a 1 atom thick film.
  If your hand does move out of the way fast enough to make room for your arm, then it is moving at the same speed that the train was moving. Your momentum from that speed would fling you into the air.
  In no scenario do you just pop out intact but motionless.

Obviously A
starts world war 3
- Yeah definitely A. The momentum of the object going through the portal matters not the objective that has the portal on it.
  
  But how would the objects get on the other side then? The receiving side isn't moving, so the objects essentially need to be pushed through the portal at the speed at which the train is moving, resulting in B. The only way A could work would be both portals moving at the same relative speed.
  
  Here's how I always phase it. Imagine you have a shovel and you are using that shovel to flick some dog shit into your neighbours garden.
  With no portal the shit hits the shovel and you flick it, transfering the speed of the shovel into the turds. You stop the shovel and the turds fly away.
  Now imagine the shovel has a big rusty hole in it. So it's like a n shape. No portal yet. You go to flick the dog dumps but you just pass straight over them with the hole and the dumps go nowhere. The dumps have gained no momentum because nothing touched them and transferred that to them.
  Now put a portal on the end of the shovel. As you sweep it over the cack has anything touched them? Has any object transferred it's momentum to the dog eggs? No, so the dumps just gently tumble out of the other side of the portal.
  
  Speed, and by extension momentum, is relative. I'm sorry, but Einstein got you.
  
  Except the momentum changes with portals most of the time anyway. Momentum is a vector, not just a scalar, meaning momentum has both a scalar and a direction component. And that direction component usually must be conserved as well. But portals change the direction of momentum all the time relative to the orientation of the entering and exiting portals. If the direction of the momentum of the object is relative to the orientation of the portal, then it makes as much sense that the scalar of the momentum would be relative to the velocity of the portal as well.
  Energy is not conserved either, which is why the infinite falling box arrangement means the box keeps accelerating downwards gaining kinetic energy even though it started out with a much smaller, finite potential energy. Portals and conservation do not mix well.
  
  As the people on the track are moving at an accelerated speed of 0 m/s, normally a train would apply the full force of the train moving to the ~~meat bags~~ human ethic problems on the tracks.
  As newton's first law states F = ma, or Force = Mass acceleration
  F = x 0 = 0 N of force
  thus, they could just plop out as if falling after having a chair removed.

I think it has to be A. You figure that if it were B, the people on the track would suddenly be traveling at a high velocity, but the train's velocity wouldn't be impacted at all, since there was no impact between the train and the people. Wouldn't this mean that the portal had created energy, which is impossible?
- But portals can create energy. Put one above the other face to face and drop an object into the bottom one, it now has infinite potential energy.
  
  ... fuck. You're absolutely right. All my theorems - flushed down the drain.
  
  Think of a portal as a door, if someone brings an open door up to you (idk maybe it's on wheels or something) and you go through it, you don't suddenly fly through the frame.
  
  Another violation is that they conserve speed, not velocity. Put 2 portals 90° apart. Travel into the first perpendicular to the surface. You'll exit the second perpendicular to its surface. That means you accelerated to change direction, which takes energy. Portals don't conserve momentum or energy.
  
  No way, the portal displaces space meaning it just allows gravity to work unimpeded adding more kinetic energy to the object. The potential energy during a "falling cycle" is infinite but infinitely removed when the spacial disruption is broken.
- In classical physics you would be right, but in modern physics there is no standard frame of reference. It's equally correct to think that the people are still and portal is moving as it is to think that portal is still and people go in it immn fast speed.
  Regardless, people and portal have large speed difference going in, so there will be large speed difference going out.
  
  Fuck, you just made me question the whole thing. Cave Johnson must be turning in his grave.
- This is how I always look at it. The portals don’t actually move what is behind them, they are just a portal to that place, so there is no momentum to impart
- The matter has to move through the portal at the speed of the train, and it won't suddenly lose all momentum when it's done being pushed through. B imo.

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.
  
  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

Conservation of momentum says B I would think. From the protal's reference frame, the people are moving fast toward it.
- The portal is a hole. The hole is moving. The conservation of momentum is the hole moving as it continues to move along the track. If the people start moving, where does that momentum come from?
  Imagine a tennis racket with no strings. Two portals are stretched across the space the strings would normally be, back to back, one orange one blue. If you threw a ball in the air as if you were going to serve and swung the racket, the ball would pass straight through the portals as if they weren't there and would fall straight down due to gravity. The ball maintains its conservation of momentum, and the tennis racket holding the portals also maintains its conservation of momentum as it swings through the air. There is no force applied by a hole.
  
  Lets say the tennis racket has 2 portals. One in the front and one in the back. When you swing the racket, the front portal moves forwards with some speed V. The portal on the back is moving backwards with the same speed, so -V (same speed V, but in opposite direction). A stationary ball, suspended in mid-air would have 0 speed. The racket portal approaches the ball at speed V, so the ball has a relative speed V to the racket. The portal on the back has a speed of -V and ven you combine that with the ball's speed of V, we get -V+V=0. And so the ball stays put. The portals in the image are not both in motion. The front portal is approaching the people with a speed of V and so the relative speed of the people to the portal is V. The exit portal has a speed of 0, relative to the people. When the people go through the portal, their speed is 0+V=V, meaning they get launched out the exit portal with the same speed the entrance portal hit them.
- Conservation of momentum is based on Newton's first law which states "a body at rest tends to stay at rest" so that would imply A. not B.
  Those dudes were just chilling, and would still be laying there chilling.
  
  Yeah but the momentum is relative to the portal.
  If the blue exit portal was behind the wagon and so moving at the velocity of the orange entry portal, then I would agree that it's A because they move at the same velocity and in the same direction.
  But since the blue exit portal is static and the orange one is moving, the people will enter the portal at a relative velocity to the portal which will be transferred to the blue one. Meaning B will occur.
  If the portals were on two wagons going in the opposite directions at the same X velocity, then the people would enter at X relative velocity and exit at 2X velocity.
  
  Right, in perspective of the initial orange portal the people are moving. They aren't at rest compared to the portal. The portal is at rest.
- Conservation of momentum would suggest A, otherwise an outside observer would see momentum generated from nowhere right?

B. Since there is relative velocity between the orange portal and the target, the momentum is conserved and they will launch.
- The only way B is correct is if the people are launched as a pink paste from the forces resulting from the instant acceleration, and if the trolley also at least slows
  
  Launched with a little more entropy.
- In simpler terms, speedy thing goes in, speedy thing comes out
  
  Except the people aren't moving so aren't speedy
  No momentum goes in, no momentum comes out

Relative to the train, and by extension the portal, the people are moving towards it at the same speed as the train relative to the ground, since the people are tied to the ground. I'm gonna work with the definition of momentum that equals it to the velocity of an object times its mass, and with the assumption that the portals conserve mass and momentum of the objects during teleportation, or with negligible losses. Having found that the momentum stays constant, and given the mass before and after teleportation is constant, the velocities relative to the portal are gonna be constant too. (p1=p2 <=> mv1=mv2 <=> v1=v2). And since the velocity of the people relative to the portal is the velocity is the train relative to the ground, and the velocity of the train relative to the ground is far bigger than the velocity of the people relative to the ground, the answer is gonna be B, where the people shoot out of the portal with great speed.
If the people actually go into the portal and not under it that is.
- If that is so, the train or portal would have to lose its momentum for the transfer to happen otherwise you'd be generating more relative velocity after the portal. I can't imagine portals transfering monentum, only maintain it.
  
  Think of it as a pole entering the portal, the end will have to exit at high speeds and so it will need to drag the rest of it out at that speed
- Your did the math! 😁
  
  They didn't do the math.
  They just mentioned some of the formulas.
  It's the difference between reading a book and knowing the book exists.

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.
- Because the rule that's literally in the game
  "Speed thing goes in, speedy thing comes out"
  Something isn't moving goes in, it won't move coming out. A hole having momentum won't transfer it to what passes through the hole.
  Basic stuff
  
  But, "speedy" relative to what? Relative to the walls of the room your are inside? What if you are in a falling elevator? Relative to the rotating surface of the earth? To the center of the solar system? "Relative to the portal" is the only answer to that question that makes sense.
  
  But if we use coordinates relative to the orange hole, the whole world, including the rails and people on them, is moving, and the people are moving towards the hole with a speed of the train.

I believe it should be A. People aren't moving, and the portal doesn't carry momentum. At most people would be appearing on the other side with very little delay between eachother resulting in the most recently teleported person violently pushing away the last one.
- Of course the yellow/orange portal carries momentum. That's why it should be B.
  
  Its just a hole though. If you have a tennis racket with no strings and swing it over something stationary the object doesnt move
- I think it'd be B. It has to exit the portal at the speed it entered or you end up with a scunched up human or a stretched out human.

gonna go with a - the people aren’t moving when they go in, so they won’t be moving when they come out
- Correct. If you do a ring toss the stick doesn't shoot into the air.
  
  This can't apply because unlike the portals, both sides of the ring are moving at the same speed/direction.
- my brother came up with a great analogy - say you’re falling and there’s a portal below you also falling, just slightly slower than you. when you eventually fall through it - do you come out falling slowly or quickly?
  it would have the be quickly. even though you and portal are moving slowly relatively to each other, your individual momentum is conserved, the movement of the portal is irrelevant.
  
  Except there is no concept of "individual momentum," it's all relative to something. Not to mention, technically speaking, any specific reference point that isn't the blue portal will actually show it has infinite speed as it instantly moves from one spot to another. I think the most intuitive answer is to imaging standing in front of the blue portal, and look through it. From your perspective, the victims are being hurled at you, propelled by the ground. As soon as they go through the portal, no linger being in contact with the ground, they are effectively projectiles. By no means a hard proof, but this video has a compelling argument for that interpretation.
  
  Except it is wrong
  
  By that logic you have to account for the earths speed as it moves through the universe... That doesn't sound accurate.

B. Velocity is relative.
- but relative to what? assuming portals work similarly to windows, if I take a hoop/window and place it quickly over an object, that object won't launch in the opposite direction
  
  if I take a hoop/window and place it quickly over an object
  Then the velocity of the object relative to the "exit" of the hoop would be the same as the velocity of the object relative to the "entrance" of the hoop, which is option B.
  In your analogy, option A would mean the object has a relative velocity of entering the hoop but suddenly no relative velocity exiting it, so the object magically starts following the hoop.
  
  Yeah, but that's because both sides of the window are traveling at the same speed. If the blue portal was on the other end of the tram, they'd plop.
  
  If you strap a camera to the window, it will appear as if the object launches from the camera's perspective.
- "Velocity is relative" doesn't really apply here. The question is momentum, the pedestrians have none, the portal will pass right around them. Imagine the exit portal is on the back of the train, it would be as if a large hollow tube was passing around the pedestrians, they would still be laying there stationary.
  
  Momentum is relative too, since it's equal to mass times velocity (in classical mechanics, of course)

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

The train is not moving. The rest of the world is moving underneath it. Therefore, by the principle of "speedy thing go in, speedy thing come out", the people will be launched.

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.

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?
- 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
- 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.
- 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.

Discussed this with some friends and the view we came to is that your momentum relative to both the portal and your surroundings is preserved (which explains how you could portal to the moon and not get liquefied by the difference in rotational momentum between earth and the moon). The portal speeds you up or slows you down depending on local conditions on the other side to preserve your relative momentum. This would, logically, indicate that energy is created or destroyed depending on the difference, which (to me) means that 'portals' technically exist outside our universe as a concept and are therefore not subject to conservation of energy.
- not subject to conservation of energy
  That's already the case simply because you can go through a portal from the bottom of a cliff to the top of a cliff without doing work; this obviously lets you build a perpetual motion machine.
  (Do gravitational fields pass through portals? If you try to go through a portal from earth to deep space, will Earth's gravity pull you back?)

Is that guy wearing blue and black jeans or white and gold?
- Depends on if he's saying Laurel or Yanny.

B for sure. Consider a long pole (stationary relative to the track) entering the portal at the front of the trolley, it would leave the portal at the speed the trolley is moving.
- Yes but it wouldn't be possessed of any momentum, it only appears to be moving because the train is moving. As soon as it cleared the portal it would drop straight to the ground.
  
  But it gains the momentum when it exits. It's moving at it exits the blue portal. Meaning it has momentum at the exit point.

There's a reason why in the game you could never put a portal on a moving surface
- Except when you use the laser tp destroy the tanks of neurotoxin. Always annoyed me.

i still can't believe people think it's A
- Portal 2 even had sloped portal surfaces. Technically it's not a or b but b is the closest.
  
  But the orange portal is moving. The game code works more like A (it bugs out and the object bounces off the portal surface, but it uses a world-fixed coordinate frame that would match A for behavior). A (Newtonian) relativistic coordinate system would match B. For everything with non-moving portals A & B are equivalent.
- Do you even portal, bro?

Unfortunately, this isn't testable in Portal because portals can't be affixed to moving surfaces.
I would assume the people just plop out fine since they would retain their momentum (which is nil), and the portal's own momentum wouldn't be applied to them. But God damn it I wish I could just make a Portal map with a moving portal and see.
- Considering that portals are quite literally linked in a spatial manner, it would make sense that they physically cannot move independantly. Moving the orange portal would also move the blue portal. Or from a different perspective: the portals are always fixed in space, but their surrounds can move.
  But that does not make the question shown here untestable. It just means the output portal will have a velocity of it's own.
  How to test: place 2 portals next to each other on a wall. Then apply propulsion gel in front of the orange portal. And finally move yourself at high speed through the orange portal.
  If your speed is unchanged after exiting the blue portal, but your velocity has been inverted with respect to the direction that the wall is facing, we can conclude option B must hold.
- If we assume local relativity, their momentum, which would then be relative to the orange portal (the one which they will interact with), wouldn't be "nil". It is pretty clear to me that both portals have different relativities, and therefore, would clearly lead to case B.
  
  Then I could imagine a sport where you have a racquet with a portal on it, and you swipe at a suspended ball, with a target somewhere beyond the other portal.
  
  Think of a portal like a doorway or just a frame of one. Stepping through it seamlessly puts you on the other side. If you were standing still, and the door went around you, would any force be exerted upon you, causing you to move? That's why I think it would be A. Nothing is applying any force to you, you're just basically teleporting. You may not even actually fully emerge from the portal, being trapped on the threshold, since no force is moving you beyond the point where the two ends meet.
- Trouble is even if you could, all that would show is what would happen under the developer's implementation of the concept in their simulation, not what would happen if portals were real and you tried this which is really the spirit of the question.
  EDIT: Actually mate, if you want to know what the game does with it, looks like a few people actually managed to experiment with this https://www.youtube.com/watch?v=S85nudR6D-Y ages back. Disappointing result, again it just shows what the game would do though.

This is all irrelevant, portals can't be placed on moving objects.
- The earth is a moving object, so all portals are moving through 3d space at all times.
  
  It's probably dependent on the reference frame of the first portal, or the portal gun.
- that is a game engine limitation, not an in-universe physics limitation.
  
  Games and real life are the same! Now if you'll excuse me I'm going to jump off a bridge then use an energy drink to heal my broken bones.
- In Portal 2 you place portals on moving surfaces to cut the neurotoxin pipes.

B) The train transfers momentum to the things that pass through the portal. If it wouldn't, the portal couldn't exist since you need a certain amount of energy to displace the air on the other end.
- Fun fact, the portal doesn't exist.
  
  Not with that attitude!
- I don't agree. Inertia and momentum are not affected by a portal, even a moving one. If so, their legs would be stretched as they were partially through the portal till the body was being jerked away from the neck causing an even greated relative velocity compared to entering the portal.
  
  Let's say the train moves at 100mph, therefore your body needs to exit the other portal at 100mph. Otherwise it would be compressed. What happens to the energy of this movement if you just stop on the other end?

there's a video with a great explaination about this: https://youtu.be/B19nlhbA7-E
- Also this one!
  “I rewrote Portal from scratch and solved the Portal Paradox”
  
  Here is an alternative Piped link(s): https://piped.video/ao1qVi5Qp3Y
  Piped is a privacy-respecting open-source alternative frontend to YouTube.
  I'm open-source, check me out at GitHub.
- Here is an alternative Piped link(s): https://piped.video/B19nlhbA7-E
  Piped is a privacy-respecting open-source alternative frontend to YouTube.
  I'm open-source, check me out at GitHub.

Dont they need to be hovering mid air for either to happen? I am trying to imagine how would a moving portal teleport a person laying down without teleporting the ground beneath him. I think neither a or b would happen, I think they would be draggen on the ground and splattered, but if I HAVE to choose, I say A is more likely. Because they are laying and not hovering I dont think they will be launched.
- I agree, in the portal game they would be scraped/blended on the bottom of the support of the portal attached to the train. If they were in fact hovering in line with the portal I'm leaning towards, an object in motion will stay in motion. Given portal orange is moving and blue is stationary, objects entering the portal will exit at the same velocity and b would probably happen in game.  
- However, the portal is moving. So if we look at this relative to the portal, they would moving into the portal. I imagine they would get shot out along with the rails. Of course, they’ll eventually plop back down, so it really could be A if the trolley was moving quite slow.

This platform deserves a community solely dedicated to trolley memes.

Can someone put this in a lambda function for me so I can kill myself trying to figure it out?

This is my favorite post in a while.
- Thank you!
  Truthfully it was pretty low effort to make, but I appreciate it anyways.

This reminds me of when somebody set up the Trolley Problem for their toddler with those little wooden toy trains. The kid put the one guy on the track with the several guys and then plowed through all of them with glee.

B.
Speedy thing goes in. Speedy thing comes out.
Although it kind of depends how fast the tram is going.
- 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".

I don’t think portals carry the momentum with them. It’s as if the train is a moving tunnel. So A.
- Yeah but the exit isn't moving WITH the tunnel.
  
  So?
  Holes don't transfer energy, speedy thing in, speedy thing out.

I love this comment section. Perfect balance of people actually trying to discuss and people getting pissy because their answer can be the only right one.
I don't know shit but when I try and picture it happening in my head I can only see A being correct, think Doctor Strange portals. When he moves a portal the person comes out stationary

C. A train flies out of the blue portal

B, but only their bellies; the portal is above most of their bodies, and their heads and ankles will get cut off for being off even the track.

If it's A, then it figures to me that the blue portal would enact some force on the structure on which it's placed when the tied up people plop out

This comment section reminds me a bit of these cringe powerscalers, discussing Saitama vs Son Goku in the comments of a YouTube video.

A if the trolley is going slow
B if the trolley is going fast.

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.
  
  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.
  
  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?
  
  If you jump into a portal, YOU have momentum, these people are stationary and therefore have no momentum, the answer is A.

Someone take this a step further and make the train an airplane on a treadmill
- My original idea for this involved two tracks (like the original trolley problem) and which track you chose (for minimum casualties) would depend on your interpretation of the physics problem.
  Unfortunately it ended up super complicated and I made this simple one instead.
  But adding the train on a treadmill is a great way to overcomplicate it further if I ever go for the complicated version.

The only way it could be B in this universe is if the train also decelerates equivalent to how the people accelerate. If the people accelerate and the train maintains velocity you've created energy in a closed system.
- Portals don't abide by the laws of physics. Portal above + portal below = infinitely falling object (and thus infinite kinetic energy)
  
  Portals don't abide by physics, but people still do. Even in the infinite fall scenario there isn't infinite energy because the object accelerating is still subject to terminal velocity, and it's change in momentum comes from gravity. For the people to change their velocity, there has to be energy imparted onto them. My theory is that the train would have to slow.
- Objects coming out of the portal in "a" have to have velocity coming out (they don't magically appear outside of the portal, they move out inch by inch/ cm by cm). So in a you would actually have double the deceleration on the train because it has to accelerate people leaving the portal and then instantly decelerate them once they have fully exited the portal.

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.

I've seen this debate about the outcome of the moving portal. I'm pretty certain that because of inertia, and the people aren't moving, they will just plop out the other side. Think of it like moving a hoola hoop through the people. That's basically what the portal is.
The hoola hoop has inertia and is moving, but it doesn't actually come in contact with the people, so it passes right around them. There's no way for the people to have instant acceleration because the porta did, otherwise it'd be like them hitting a brick wall and they would probably explode
- Fully disagree on concept. The thing with the portal is solid matter moves through at a constant rate.
  If you have a portal moving towards you, at say 2 MPH and you are holding your arm out, From the other side of the portal, they would see your arm coming out at 2mph. Any other result would involve compressing or stretching your arm.
  Lets say further here. a man dangling his legs off the back of a moving cart. Man is moving 20mph forward, portal trolley is chasing him at 22MPH. As his feet pass through the portal, they would start coming out the other end of the portal at 2mph.
  As a result I concur that the only logical exiting of a portal. |travler velocity - Entry portal velocity| + exit portal velocity = objects exit velocity.
  Now the real physics debate would be what happens if 2 portals were moving forward and someone was in the path of one. The only logical conclusion that fits my mind there, is instant compression
  
  Yeah, the imparted momentum has to be something like the averaged vector between the two, which does cause compression if you enter a pair of portals both moving your direction.
- The problem with your hoola hoop example is that it keeps moving after you go through it so that you have the same relative velocity. However, in the portal example, the exit portal is stationary, so in order to stay the same relative velocity to it when you exit is to speed up yourself, as in B.
- Momentum is relative.
  If you say the portal is stationary, and the person is moving then B makes sense. However, this is just changing the frame of reference from following the tram to following the person.
  Changing the frame of reference (from tram to person, or from person to tram) doesn't change the velocity/momentum/energy (it's just the person is moving towards the tram or the tram is moving towards the person).
  The acceleration the person would experience is likely similar to if the person just gets hit by the tram, however in Portals canon it is nonexistent.
  Because, as you say, the person is accelerated. However, the acceleration when using the tram as the frame of reference is still 0 when you account for the rules of physics a portal would break. Even changing the direction of travel would be acceleration.
  Like, if the in-portal and the out-portal were back-to-back where there was absolutely 0 distance between them, anything passing through the portal would experience 0 acceleration - no change in direction, it might as well be a standard hoola-hoop.
  If the portals were side-by-side facing the same direction, anything passing through the portal would experience twice the acceleration of running into a wall - like bouncing a ball off a wall. Once going into the portal (forward motion) and once coming back (backwards motion), because the object has to completely reverse it's velocity.
  And considering that things going into portals do not get damaged (and chel doesn't lose health) it's fair to consider that objects observe 0 acceleration.
  But portals moving is outside of the Portals canon because it highlights that an object experiences acceleration when passing through a portal. And the acceleration is (or is near) instantaneous. And the object does not suffer from this.

Suppose the blue portal is sitting upright on the tracks facing directly into the orange portal (parallel to it) from some distance. We will of course neglect gravity and most physical laws.
Option B: The people shoot out of the blue portal, eventually reaching the orange portal again after a finite time (exactly halfway between the portals). At which point, their velocity relative to the train is double what it originally was, and they shoot out of the orange portal again twice as fast. Since the people are faster than the train, they will hit the train before it covers half of the remaining distance; and so this all happens again, with the people's velocity now increasing to triple the initial value. And it happens again, and again, until relativistic effects take over and the velocity is no longer approximately additive. In other words, the people accelerate to an appreciable fraction of the speed of light, regardless of the starting velocity.
Option A: The people plop out of the portal and eventually get smashed between the train and the portal wall in a satisfying and physically plausible fashion.
Bonus Option C: In option B, it is unspecified if the resultant velocity of the people is equal to the velocity of the people relative to the train, or equal and opposite to the velocity of the train relative to the people. This difference becomes meaningful at the relativistic speeds we achieved, and I implicitly assumed the latter. In the former case, the people are eventually carrying ~100% of the energy of the system and therefore doubling it every time they pass through the portal, and time dilation be damned, for an instant they achieve infinite energy.
Now suppose the blue portal is just a centimeter behind the orange portal, opening the other direction, so anything that goes in one almost doesn't even seem to have teleported. When the people pass through the orange portal, they appear on the other side; inside the train.
Option B: As the people pass into the portal, they instantly shoot backward, as if the train grabbed and threw them behind itself.
Option A: The people simply pass through the portal, as if it weren't there at all.

Suppose the blue portal is instead aligned parallel to and facing the ground. Maybe a 18" off the ground, a little higher than a person is wide. Additionally, the person is standing upright on the track.
In the above scenario, with the ground rushing at the person, does it suddenly "stop," with the person gently falling onto the ground? This is the same problem, I suppose, but from a different perspective.
Now, what if that blue portal is instead only 6" off the ground? Is the person embedded in the ground, or does the universe crash?
- I think you would collide with the ground like you were falling face-down, and is there isn't room you'll simply remain half-in half-out. At that point, your front half is still relative to the ground, but the back half is moving with the train, that way your velocity is zero in relation to both portals.

The Portal can't move front or back, only "to it's sides"

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

A
No, I will not elaborate. Fight me.
- boom pow crack

A.
its the train that has velocity. The people who enter the portal will not be moving?
Its like that buster keaton clip where he stands still and the side of the house falls down around him(well.. sort of)
- The train has absolutely no velocity relatively to the orange portal. The people are moving relatively to the orange portal.
  
  If the ground disappears from under your feet at 60 miles per hour, the moment you start falling are you falling at 60 miles per hour?

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