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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
Conservation of momentum would suggest A, otherwise an outside observer would see momentum generated from nowhere right?
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.
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.
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.
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This is my favorite post in a while.
Thank you!
Truthfully it was pretty low effort to make, but I appreciate it anyways.
Can someone put this in a lambda function for me so I can kill myself trying to figure it out?
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.
What in the world is going on in this image?
Portal themed morality decision test meme.
Ohhh now I see the portals
B. Since there is relative velocity between the orange portal and the target, the momentum is conserved and they will launch.
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
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.
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.
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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 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.
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.
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…).
A if the trolley is going slow
B if the trolley is going fast.
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.
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).
Ah, I see what you are saying. They apply the velocity of the object again after teleporting rather than the difference between the velocities of the portal and the object. Thus the velocity of the train would be ignored. Well, B is wrong simply because the game engine doesn’t rotate characters in the teleport because that would ruin character physics. So B is wrong twice.
Yeah, most game engines like Portal’s uses absolute speed relative to the coordinate system (which doesn’t change when the coordinate values change), in addition Portal technically doesn’t actually implement “wormhole type” portals and instead superimpose a clone of the polygons near both portals behind the other (to preserve expected object collision behavior around the portal) plus doing tricks with virtual cameras, so if you fixed the bugs with moving portals then it would be A.
But if you implemented proper relativistic physics with proper wormhole type portals you’d get B.
Do you even portal, bro?
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.
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?
We also need to consider the air that is in front of the train as it moves forward. That air is going into the portal at the rate the train moves forward and therefore coming out the other portal at that rate.
Fun fact, the portal doesn’t exist.
Not with that attitude!
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.
The world is moving, checkmate.
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I would imagine that the relative motion between the entry/exit portal would be more important than the absolute motion of the two portals.
Portal 2 ends with you (Chell) placing a entry portal on Earth vs an exit portal on the Moon.
That means the portals were ~2236 mph (aka Mach3) relative to each other.
Hmm well if an object passed through that portal and it wasn’t moving ~2236mph relative to the surface of the moon, then I guess the question from the OP has been answered already haha.
Wouldn’t that provoke all air in the Earth to get sucked to the Moon due to the difference in atmospheric pressure?
Vacuum doesn’t suck. The atmosphere on Earth would push air through the portal with a pressure of about 1 bar.
Yes, but the flow rate is only so much. It was only open for a short time before Glados closed it.
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.
It’s all relative.
Tell that to einstein
Oh wait
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
