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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?
Yes, that is called running extra fast. And then falling, with the same momentum. Unless there are two grounds, with different relativities. Like with a treadmill: you run relatively to the treadmill, but you are stationary relatively to the ground under it, because you run at exactly the same speed as the treadmill moves in the other way (hopefully for you…).
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.
C. A train flies out of the blue 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.
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.
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
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
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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.
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.
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.
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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.
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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
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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.
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.
You can tell it’s been a long time since anyone played the game.
“Momentum, a function of mass and velocity, is conserved between portals. In layman’s terms, speedy thing goes in, speedy thing comes out.” -GlaDOS
Now it’s time for an all-out flame war with anyone who even took a moment to consider the utter nonsense that is A. ΔV goes in, ΔV comes out. And the bundle of wage slaves has velocity (and mass, therefore momentum) with respect to the portal.
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
Conservation of momentum says B I would think. From the protal’s reference frame, the people are moving fast toward it.
Conservation of momentum would suggest A, otherwise an outside observer would see momentum generated from nowhere right?
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.
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.
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.
If you jump into a portal, YOU have momentum, these people are stationary and therefore have no momentum, the answer is A.
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.
gonna go with a - the people aren’t moving when they go in, so they won’t be moving when they come out
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.
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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.
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.
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.
Can portals exist on a moving object?
Not in the regular game, there are mods/fan games that allow it though.
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”.
The people are possessed of no kinetic energy, the train cannot physically act upon the people since the portal is intangible. There’s no way for the train to transfer any kinetic energy to the people, and there’s no other force that could act on the people. No kinetic energy going in = no kinetic energy coming out.
From the perspective of the train, the people have just as much kinetic energy as the people think the train has. Again, you’re acting as if there is one absolute frame of reference - there isn’t. Physics just doesn’t work the way you think.
This is why all your comments in this thread are wrong, they have one simple logical issue: the people on the tracks aren’t “stationary”. They are stationary to the ground, but not to the train. It’s not correct to say “the train is moving and the people are stationary”, because it’s also just as correct to say “the people are moving and the train is stationary”. Physically both are true at the same time, that’s what general relativity is really about. You can’t look at the scene and decide “only one reference frame is valid”, that would break all of physics.
Absolutely correct. We learn this in basic static as dynamic physics.
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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.
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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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.
