Suppose the blue portal is sitting upright on the tracks facing directly into the orange portal (parallel to it) from some distance. We will of course neglect gravity and most physical laws.
Option B: The people shoot out of the blue portal, eventually reaching the orange portal again after a finite time (exactly halfway between the portals). At which point, their velocity relative to the train is double what it originally was, and they shoot out of the orange portal again twice as fast. Since the people are faster than the train, they will hit the train before it covers half of the remaining distance; and so this all happens again, with the people’s velocity now increasing to triple the initial value. And it happens again, and again, until relativistic effects take over and the velocity is no longer approximately additive. In other words, the people accelerate to an appreciable fraction of the speed of light, regardless of the starting velocity.
Option A: The people plop out of the portal and eventually get smashed between the train and the portal wall in a satisfying and physically plausible fashion.
Bonus Option C: In option B, it is unspecified if the resultant velocity of the people is equal to the velocity of the people relative to the train, or equal and opposite to the velocity of the train relative to the people. This difference becomes meaningful at the relativistic speeds we achieved, and I implicitly assumed the latter. In the former case, the people are eventually carrying ~100% of the energy of the system and therefore doubling it every time they pass through the portal, and time dilation be damned, for an instant they achieve infinite energy.
Now suppose the blue portal is just a centimeter behind the orange portal, opening the other direction, so anything that goes in one almost doesn’t even seem to have teleported. When the people pass through the orange portal, they appear on the other side; inside the train.
Option B: As the people pass into the portal, they instantly shoot backward, as if the train grabbed and threw them behind itself.
Option A: The people simply pass through the portal, as if it weren’t there at all.
Unfortunately, this isn’t testable in Portal because portals can’t be affixed to moving surfaces.
I would assume the people just plop out fine since they would retain their momentum (which is nil), and the portal’s own momentum wouldn’t be applied to them. But God damn it I wish I could just make a Portal map with a moving portal and see.
If we assume local relativity, their momentum, which would then be relative to the orange portal (the one which they will interact with), wouldn’t be “nil”. It is pretty clear to me that both portals have different relativities, and therefore, would clearly lead to case B.
Think of a portal like a doorway or just a frame of one. Stepping through it seamlessly puts you on the other side. If you were standing still, and the door went around you, would any force be exerted upon you, causing you to move? That’s why I think it would be A. Nothing is applying any force to you, you’re just basically teleporting. You may not even actually fully emerge from the portal, being trapped on the threshold, since no force is moving you beyond the point where the two ends meet.
Then I could imagine a sport where you have a racquet with a portal on it, and you swipe at a suspended ball, with a target somewhere beyond the other portal.
Considering that portals are quite literally linked in a spatial manner, it would make sense that they physically cannot move independantly. Moving the orange portal would also move the blue portal. Or from a different perspective: the portals are always fixed in space, but their surrounds can move.
But that does not make the question shown here untestable. It just means the output portal will have a velocity of it’s own.
How to test: place 2 portals next to each other on a wall. Then apply propulsion gel in front of the orange portal. And finally move yourself at high speed through the orange portal.
If your speed is unchanged after exiting the blue portal, but your velocity has been inverted with respect to the direction that the wall is facing, we can conclude option B must hold.
Trouble is even if you could, all that would show is what would happen under the developer’s implementation of the concept in their simulation, not what would happen if portals were real and you tried this which is really the spirit of the question.
EDIT: Actually mate, if you want to know what the game does with it, looks like a few people actually managed to experiment with this www.youtube.com/watch?v=S85nudR6D-Y ages back. Disappointing result, again it just shows what the game would do though.
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…).
Except in that scenario both portals are moving if they act like a moving hole. Imagine a hula hoop, except it’s 2 portals connected back to back. If I passed a hula hoop over you, you’d be going into the bottom at the same velocity that you are coming out the top, therefore momentum is preserved. You’re moving at the exact same velocity in reference to both of the portals
This reminds me of when somebody set up the Trolley Problem for their toddler with those little wooden toy trains. The kid put the one guy on the track with the several guys and then plowed through all of them with glee.
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.
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.
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.
Only if you assume the people will experience friction against the portal.
If they would accelerate to the speed of the train within the time it takes them to go through it, they’d experience very high pressure change against the due to one part of body accelerating faster than the other. This would cause the bodies to explode out the portal
Yes, as I noted elsewhere in the thread, the part of the body exiting the portal will experience inertia as it enters into the space outside the second portal and it will be forcefully pushed by the next part of the body heading into the first portal and thus imparting momentum to the parts ahead.
If this momentum has to be taken from anywhere its from the portal itself and by extension the train.
Has the trolley come to a complete halt, or even showed down? If not, then either no energy has been transferred to the people and they just flop out, or we’ve just invented perpetual motion.
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 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.
The issue with being able to get infinite amounts of energy out of a portal can be solved by thinking with entropy.
If you only look at the entropic system of the portal and the object falling through it, say for instance a magnet inside of a vacuum tube that is perfectly abutted to both ends of the portal, the energy that you are able to extract from the system using a set of coils would come from gravity, and that gravity energy would come from the Earth.
In that case, one of two things would have to happen, either the mass of the Earth would decrease by being converted into energy, or two, the amount of energy needed to maintain the portal would be equal to or greater than the amount of energy that you're capable of extracting from it with an infinitely falling magnet and coil.
I think that the tram would not have to slow down, and that there would be no additional momentum added to the victims as they pass through the portal, and any energy that is lost in those two equations would come from the energy needed to maintain the portals operation.
You see, shit like this is why I love theoretical discussions about impossible crap. Somewhere else in this thread we use the idea that a moving portal can impart momentum in combination with the idea that portals need to maintain a minimum energy level to explain why moving portals collapse in a way that would at least be satisfying to someone reading a sci-fi novel. It’s now my head canon.
Objects coming out of the portal in “a” have to have velocity coming out (they don’t magically appear outside of the portal, they move out inch by inch/ cm by cm). So in a you would actually have double the deceleration on the train because it has to accelerate people leaving the portal and then instantly decelerate them once they have fully exited the portal.
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.
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.
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.
I think it has to be A. You figure that if it were B, the people on the track would suddenly be traveling at a high velocity, but the train’s velocity wouldn’t be impacted at all, since there was no impact between the train and the people. Wouldn’t this mean that the portal had created energy, which is impossible?
This is how I always look at it. The portals don’t actually move what is behind them, they are just a portal to that place, so there is no momentum to impart
I mentioned this elsewhere but this line of reasoning may have a huge flaw, and that flaw is that energy has to be spent to maintain the portals operation. We do not currently know what the relationship is between the amount of mass that is moved through a portal and how much energy it takes to keep the portal operational.
So when you take into account the total entropy of the system you also have to include the entropy of the earth and the entropy of the power systems that maintain the portals.
As I said earlier, if you were to put a magnet inside of a vacuum tube that was welded to itself through the portal and then wrapped coils around that tube to drain the electromagnetic energy from the falling magnet, the energy that you were extracting from the system would come from either converting the mass of the planet into energy or it would be a total net loss as the amount of energy needed to maintain the portal would be greater than any amount of energy you could extract from the system no matter how fast the magnet inside of it was moving or how perfectly configured your coils were.
After all, as the magnet approaches the speed of light eventually its mass would be come equal to or greater than the mass of the planet, and that would cause the portal to lift the Earth towards itself.
However, coils on electromagnetics exert electromotive braking Force, and when you account for e that Force you can prevent the magnet from reaching luminal speeds, but I still don't think you're going to have an over-unity device.
No way, the portal displaces space meaning it just allows gravity to work unimpeded adding more kinetic energy to the object. The potential energy during a “falling cycle” is infinite but infinitely removed when the spacial disruption is broken.
By moving an object laterally into the portal falling loop, you would do no work and increase the potential energy of the object to effectively infinity. You would be creating energy.
I wouldn’t consider that the portal, itself, imparting the energy, though. It’s just facilitating an environment where an object can fall infinitely. The portal is outputting the same momentum that is inputted to it. The actual increase of energy happens while the object is falling between the portals.
The portal teleports an object to a position in space with high potential energy, while apparently spending no energy of its own. This action creates energy.
Think of a portal as a door, if someone brings an open door up to you (idk maybe it’s on wheels or something) and you go through it, you don’t suddenly fly through the frame.
If the door is moving 1 meter per second, you are relatively moving 1 m/s towards the door even if you are stationary on the ground. You pass and, although you are still stationary to the ground, you are still moving 1 m/s in the same direction relative to the door. The door is the frame of reference, not the ground.
But that would signify there is an impact? And that you are crashing at 1m/s, if you don’t enter in contact? If I’m in my house, I am not moving at 130km/h from the highway near my house?
If you are looking at someone through the portal, they will appear to be standing on a parade float. They are standing still on a surface, but that surface is coming at you.
You won’t feel any change in momentum as the portal passes around you, but the ground will be suddenly moving under you.
Yes but relative to you the door is moving away and you’re stationary. In this example of the portal coming towards you, therefore upon paasing through the other side of the frame, the other portal, is moving away from you.
In this diagram, it’s assumed that the person is the frame of reference, therefore I believe A to be the correct outcome.
Another violation is that they conserve speed, not velocity. Put 2 portals 90° apart. Travel into the first perpendicular to the surface. You’ll exit the second perpendicular to its surface. That means you accelerated to change direction, which takes energy. Portals don’t conserve momentum or energy.
The matter has to move through the portal at the speed of the train, and it won’t suddenly lose all momentum when it’s done being pushed through. B imo.
In classical physics you would be right, but in modern physics there is no standard frame of reference. It’s equally correct to think that the people are still and portal is moving as it is to think that portal is still and people go in it immn fast speed.
Regardless, people and portal have large speed difference going in, so there will be large speed difference going out.
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