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.
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.
Do we agree that the massive block with the blue portal is not moving? So, imagine we are standing right in front of it when it happens - what happens exactly? How do the people appear?
The people appear because the orange portal is moving towards them, the people then, like through a moving door, just pop out the other side.
It’s a door way through space, not a momentum giving portal.
Can you not see the orange portal moving? They don’t jump through it, this carrying their own momentum through. They are stationary through it so coming out they will also be stationary
Of course I see the orange portal moving. So we agree the orange portal is moving, the blue is stationary.
So, when you’re saying “the people just pop out the other side”. What does that mean? Do they pop out one by one? So one moment, no one is there, the next moment one person spawns and after that the third person spawns? Or do they spawn altogether? Or does each person come from the portal centimeter by centimeter, atom by atom - pushing through the portal?
That’s not what I mean. Let’s just look at one person: the person will not just appear as a whole at the blue portal, right? First there will be, I don’t know, the left arm, then the left side of the torso, the middle part… and so on. That’s what I meant with centimeter by centimeter, atom by atom - pushing through the portal. Would you agree?
At least that’s how it’s in the game. It’s not that you are touching one portal, despawn and appear at the other portal, right?
Yeah they kinda lose ground under them as the portal goes through them, they aren’t pushed, they are moved through space. No additional force or momentum is ever applied.
In the game you aren’t pushed through ever, you walk through or fall through them. You appear at the other side of the door.
So, you say, they are moving without speed. Which could only mean when one person touches the portal in one millisecond, his complete body just disappears completely from the rails and in the same millisecond his body appears behind the other portal.
That’s not how portals in the game work. That’s rather how the transporter in Star Trek works.
Who even said anything about teleporting the whole body?
The portal is moving into him so as the parts of him go through they appear on the other side like a door
Think of it as a portal falling on you and another on the floor next to you, as it falls your head goes through and your head appears through the portal on the ground and the rest of the body goes through as it falls until the portal lands on the floor and you’re standing on that same floor but in the other place as the portals are still connected.
Nobody said it’s teleporting the whole body. But that was my only explanation of how something can move without speed.
But great, that’s what I wanted to clarify the whole time. I think my explanations were too complicated, sorry.
So, taking your example:
When the portal falls on me - as soon as it reaches my neck. Only my head is coming out of the other portal, literally right above the ground. Correct?
As the portal keeps falling down, more and more of my body comes out of the other portal until my whole body is out of it. Correct?
Which leads to the fact that my head was moving from right above the ground to the height of my body. Correct?
So my head moves upwards. And since it takes time for my head to move upwards, we can measure the speed of my head moving upwards. Wouldn’t you agree?
Yeah the falling down portal does help a lot with explanation imo
we can measure the speed of my head moving upwards. Wouldn’t you agree?
Fair point, we can measure that, it would match the speed of the portal falling. It still won’t launch the person as it isn’t transferred but you can certainly measure the speed of the portal using it.
That’d be gravity. But the difference is that we are not moving (at least not relative to the ground) before entering the portal, so gravity just keeps us at the bottom.
After entering the portal, we are having a movement to the speed of the entering portal as we already established. So, gravity has yet to work against this motion (which will, of course, eventually keep us on ground again because gravity does its thing. But temporarily, and depending on the actual speed, we might have a bit of air time in this scenario).
I have my doubts, that this will convince you now. However, I gotta say, it was a lot of fun discussing this completely speculative topic with you. Also, I give you, that - while I’m still 100% sure that B is the only sensible solution for this question - you helped me better understand why people might think A is correct.
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.
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.
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.
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.
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 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.
If we assume portals obey 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.
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.
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?
There is no momentum without saying relative to what. A train is moving relative to the people not on the train. The earth is moving relative to the sun. But if you are on earth the sun is moving and you are stationary and your viewpoint is not wrong! The reason is, that there is no underlying space we are moving relative to. Maybe this helps: Imagine a train in a train. Driving with the same speed but in the opposite direction. Now from the outside the inner train is not moving at all, but the outer train is. From the inside the inner train is moving and the world around them is. If there is now a portal inside the outer train and the inside train is passing through. What happens. Nothing because the inside train is not moving or it shoots out because it is moving?
but 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.
that’s true relative to the hoop, but relative to the ground the velocity would stay zero. otherwise, relative to the ground, the object would gain velocity without any force being applied to it.
Technically, relative to the ground the object becomes moving infinitely fast as soon as it enters the portal. I think a more intuitive answer can be found by replacing a nice discrete object like a box or group of people with a long pole that enters the portal lengthwise. Obviously, it’s going to have to be exiting the other portal at whatever speed the first portal is moving. The out speed should always be the same as the relative speed of the object to the entrance portal, it’s the only thing that makes sense, and also the only way to appease conservation of momentum.
“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.
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
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)
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.
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?
No, the dude is right - if they’re moving “zero fast” upon exiting the portal, then that means they’re either a 2 dimensional paste on the surface of the portal (dead), or they were entirely transported upon contact (cannot partially enter a portal) which is explicitly not how the portals in the game work.
This is because: if your hand goes into the portal and appears on the other side, it must move out of the way to make room for your arm. Because it is moving, it has velocity, which means it has momentum. If it doesn’t move out of the way, then you’re now 2 dimensional, and dead.
Also, in portal, energy is absolutely created. Every time you portal to a higher place, you gain potential energy that you didn’t have before, without losing any of the other kinds of energy that you had.
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.
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
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