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Combining two different internet debates
(sopuli.xyz)
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this post was submitted on 18 Aug 2023
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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
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?
Yes the blue portal isn't moving
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?
It's on a slant so they will just fall down and yes on top of each other one by one
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.
Yes. And you said it correctly: they are moved through space. Now tell me: at which speed are they moved through space?
No speed at all, the portal doesn't give or create energy and they were stationary as they passed through
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
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.
But now, when you have a moving object (my body) we need a force to stop that momentum. Where does that force come from?
The force that's keeping you on the ground from the other portal
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.
Have a nice one :)
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?