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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.
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.
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 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
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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.
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)
Your entire comment is predicated off one singular assumption.
In fact, everyone will be deposited into the 2- dimensional pane of the blue portal itself
It just ain’t two dimensional. Ever.
They’re two halves of a hole. Neither are two dimensional. They are three dimensional. If one is open and the other isn’t, it doesn’t turn back to 2D. The third dimension is just blocked, like a cylinder that’s been filled to the brim. This is proven by the game itself and the ability to have an object “split” between the two portals. If you remove one portal it doesn’t split the object in half. It merely pushes it back out.
Removing the thought of it being two dimensional, which makes no sense to begin with, and you’re left with the argument OP made. Do they get fired out or just pushed out by simple momentum?
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.
Is that guy wearing blue and black jeans or white and gold?
Speedy thing goes in, speedy thing comes out
E: I was just quoting GladOS… Not really thinking about the actual physics!
Yeah but the thing isn’t moving the portal is, and the energy has to come from somewhere if the portal makes the thing go fast.
This guy is thinking with portals!
The energy would come from the trolley. The people would launch out at approximately the same speed as the trolley interacts with them and the trolley would slow down in response to how much kinetic energy was transferred to the people.
This is correct. The motion of the people is relative to the Portal. It doesn’t matter if the trolley is accelerating the Portal towards them or something is accelerating them towards the Portal. Therefore they accelerate out of the other side with some retained momentum. Technically it probably resembles something in between pictures A and B.
This reminds me of the experiment about whether an airplane could take off from a treadmill.
Now you’re thinking with
portalsphysics!
No moving objects are entering… lol
Then they can’t enter at all and have to be flattened by the portal, because they must have motion too exit the other portal
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.
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.
B.
Speedy thing goes in. Speedy thing comes out.
Although it kind of depends how fast the tram is going.
But therein lies the argument. Speedy thing goes in then speedy thing goes out.
However the speed of the pedestrians is zero. The tram is the speedy thing.
So does the object going in count as speedy when it’s the portal that’s moving?
I mean all of this was literally answered in Portal 2 with panels that had moving portals but it’s a fun thought project.
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.
B. Velocity is relative.
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 you strap a camera to the window, it will appear as if the object launches from the camera’s perspective.
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
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.
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.
There’s a reason why in the game you could never put a portal on a moving surface
A if the trolley is going slow
B if the trolley is going fast.
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…).
