You must log in or register to comment.
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.
Interesting way to look at it, but I still dont see where the force is acting on the object going through the portal. The object is not in motion and will stay in that state unless something acts upon it, so where is the energy coming from to act on the object?
To make it clear from the start: I agree 100% with B - there has to be movement, because without it, people wouldn’t come out of the portal at all. And if there is a movement, then the only reasonable speed would be that of the train.
But: Your question about the energy is still interesting. It must come from somewhere. And I think, the only source, from which it can come, is the train. That is, the train would lose energy and therefore slow down.
The portal moves towards the people. It’s a hole. Momentum won’t transfer from nothing as the hole is the one moving.
So, how would the people come out of the portal without movement?
Because the portal is moving them through it
Like how you would move through a hoop if it passed through you, it’s just a door through space
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 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 tr, 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.
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.
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.
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.
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.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.
-
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.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.
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
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.
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.
I’m making a note here: huge success
Or, to say it in Glados’ words: “Speedy thing comes in, speedy thing goes out”.
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
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.
Someone watched a YouTube video and think they know what they are on about…
Portals don’t transfer energy, there is zero energy transferred to the people they are simply moving from one space to another
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.
That YouTube video was wrong, doesn’t even make sense
It’s a hole, no energy gets transferred
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.
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 bagshuman ethic problems on the tracks.As newton’s first law states F = m*a, 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.
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.
But even in the image example, how would that work? The people have no momentum, they wouldn’t flop out, just fall back through, and then stay half in half not. According to you, air resistance wouldn’t even push them out because as soon as it goes through the portal the air is not moving relative to the people. I think your blatant lack of respect for relativity is unnerving and gets super confusing very quickly.
Nuh-uh. Flicking dog turds
Imagine there being 2 portals on the shovel one on the front and one on the back Whatever passes through remains in the same place. THIS is the actual analoge for the rusty hole. Entering the first portal the shit gains the speed of the shovel but since the exit portal is also traveling with the same speed but facing the opposite direction, it effectively cancels out the previous speed gain. Imagine it like the first portal adding one unit of speed to it and the exit one adding minus one unit of speed because of the opposite direction.
So now with one portal on the shovel and the other somewhere on a wall the shit gets accelerated and you will feel a force acting on the shovel that decelerates it’s swing.
Velocity is relative and this is why it works. If velocity wasn’t relative then Einstein would be wrong and many of the effects we see in the real world could never happen.
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.
I see it like throwing a hoola hoop around something. The object will pass through without having its speed affected.
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
With the tennis racket analogy both portals would be moving. In the thought experiment from the image just one is moving, resulting in an unaccounted for momentum, unless the people shoot out the blue portal
deleted by creator
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.
The ring is not touching the thing passing through it to impart any forces. The object passing through carries only what energy it takes with it.
Then where does the energy to displace the air on the blue side of the portal coming from?
Yes they are though, it’s the exact same premise.
What force is generated where on the other side of the portal, it’s a hole in space, it doesn’t transfer anything.
Speedy thing goes, speedy thing comes out. Nothing gets transferred to or from the hole.
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.
in my interpretation of how portals work - by joining space together - moving through a portal doesn’t involve infinite speed because you haven’t moved - the portals have just changed the space you occupy.
a bit like the inertialess/ Alcubierre drive where you travel faster than light without breaking any laws because you’re not moving at all in your space, it’s just that the space you occupy changes.
in the reasoning the people hurling towards the portal only appear to do so, once they pass through the portal they’ll be as immobile as they were before entering it.
the minute physics video is fun though. love their stuff.
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.
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.
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)
deleted by creator
It’s two dimensional in the sense that the surface of the portal is a plane, through which things pass.
So as things pass through the portal, conservation of momentum is either preserved or it isn’t, with respect to a constant observer. What happens as they partly enter the portal in both of these situations?
If momentum is preserved, and they have zero momentum going in to the portal, then they are motionless as they exit the portal. There is nothing to cause your hand to move out of the way for your arm. Scaled down to the atomic level, you become a paste.
So you say that your hand moves out of the way because it is connected to your arm. The fact that it moves out of the way fast enough to make room for your arm means that it has velocity, and therefore momentum. The momentum means that it (and you) would get launched into the air, but conservation of momentum was violated.
There is no scenario where you exit the portal motionless but intact.
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.
I just don’t agree that’s how it would work. You can’t gain momentum simply by passing through a portal. The portal cannot create momentum. The object passing through has no kinetic energy going in, it can’t have kinetic energy coming out. It would exit the portal at the velocity of the first portal, as the entry portal passes over the object, and then the object would drop to the ground.
There is no way that it works without breaking even more laws of physics than the game. So you’re right, you can’t gain momentum. Nor can you be deposited intact on the other side of the portal.
But of the options, the one you described seems the least likely. I keep telling you exactly how it wouldn’t work, and rather than addressing the concerns you just say “no”.
We can agree that you can partially enter a portal, so you can put your hand in and only your hand comes through the other side. So now tell me: how does your hand move out of the way for your arm to come through, without moving? Because if it moves, then it has gained momentum, which you’ve explicitly said doesn’t happen.
deleted by creator
Why?
Where does the energy even come from?
A hole/portal doesn’t create or generate energy it just passes things through.
Just think of it as a hole across space because that is exactly what a portal is.
deleted by creator
No energy is every transferred as a result of a portal
You fly in the air if you drop in one because you are carrying momentum downwards that suddenly translates to upwards
You are sat in the floor, a portal flies towards you. You are sat at the floor at the end, you had no momentum going in and no momentum going out
deleted by creator
Zero fast. There is no energy being transferred to the people, they would plop out and push into each other as they are forced through.
If you blocked the stationary portal then the portal moving would essentially just be a wall, no one would go though.
This whole relative thing makes no sense, energy isn’t just created because it’s observed by someone else, the door is moving not the people so them sitting there won’t suddenly be catapulted going through a moving portal, where is that energy created?
Your wind question is confusing.
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.
Is that guy wearing blue and black jeans or white and gold?
Depends on if he’s saying Laurel or Yanny.
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
Here is an alternative Piped link(s): https://piped.video/OrAHvenjZpA
https://piped.video/OrAHvenjZpA
Piped is a privacy-respecting open-source alternative frontend to YouTube.
I’m open-source, check me out at GitHub.
it’s specifically scripted for that though, normally the engine just freaks out
Portal would fail due to being placed on moving object
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.
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.
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.
This platform deserves a community solely dedicated to trolley memes.
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.
Can someone put this in a lambda function for me so I can kill myself trying to figure it out?
