B since all movement is relative.
This was a triumph.
A, since portals cannot transfer momentum from the tram to the victims.
To put it another way, if you were standing and the portal was pushed towards you by a tram, do you think you would be launched out of the other side at that rate?
There might be some increase in momentum as the part of you that went through the portal first gets pushed forward by the parts of you that get pushed forward after, but it’s not going to be as dramatic as the momentum you would have received being hit by the tram.
Most likely you would stumble forward and fall down or have to catch yourself.
Portals maintain velocity. Velocity is relative. Therefore the velocity they maintain is the relative velocity of the portal and the subject. Any other way and there would be no consistent way to pass any moment when passing through a portal.
After thinking about it longer then i care to admit I think i finally agreed with you.
As you said it is all relative, from the prospective of the moving portal. You could say it isnt moving at all but the entire world around it is moving, thus when people enter the portal from the portal’s prospective they people are the ones moving and will continue moving when they exist.
Yes, join the relativity gang
The reason this is so confusing with different answers is that the portals don’t really exist, so inherently whether you say a or b is gonna depend on assumptions. In game they aren’t allowed to move so we have nothing to base it on to match game physics.
Here’s my take, momentum is a product of velocity. Velocity needs a reference frame. Without it, there’s no real difference in saying the portal has a velocity of 0 and the people tied up have a the velocity and therefore momentum, or the other way around. If we assume velocity with respect to the portal is what matters and is the momentum carried forward, then it should be B. If it’s relative to the earth or tied up people, then A.
If portals did not apply a transient vector to your momentum then you could not pass through a portal.
Take for instance the many times Chell jumps through a portal. Her momentum is maintained as she passes through the portal, allowing her and her robot legs to do truly stupendous feats of gravity assisted acrobatics.
If Chell was stationary and the portal fell on her, she would not be launched out of the other side with the momentum of the portal, she would just find herself sticking out of the other side of the portal.
Similarly, if Chell were to ride a moving platform up into an overhead portal, we would expect the top half of Chell’s body to pop out of the portal without being accelerated by anything other than the moving platform on the bottom of her feet.
Therefore, unless there is some strange unknowable physics that we will not be able to discover until we develop portals of our own, the most likely outcome is that the victims on the tram would not gain any momentum as the portal was pushed into them, and they would plop out safely on the other side.
You can say you can expect, but you really can’t, because if you’re talking about momentum you’re talking about velocity and you need a reference frame to define velocity and therefore momentum. Let’s pick the sun for instance with the assumptions of A. So if we just have one portal pointing one direction and one portal pointing up and chell walks in, you should blast out straight up at 66,000 mph plus the speed she was walking then. I think you could make the reference Frame to earth and try and get a, but that would create problems too.
I think B, velocity relative to the moving portal, would be the only way to maintain some kind of consistency in game if you were going to have moving portals. Your examples are most consistent with B. A portal falls on chell, how fast does she come out? The speed the portal fell on her of course. And then she stops going out once the portal stops moving because it hit the ground and has stopped moving and they no longer have any relative difference in velocity. You could also say in the platform example that the platform was sitting still and the portal was moving down, you would emerge out the portal at the speed the first portal was moving down. Both should be equally valid ways if you want to maintain some consistency. But all of this is probably why they don’t allow moving portals in the first place.
In the end though these are definitely strange unknowable physics, portals don’t exist, so really you could make the game however you please, either one is perfectly valid, you could just say any velocity on the other side is whatever it was in relationship to the earth before going through, but that’d be weird, because how fast do the people move out of A then? Do they fly out at the speed of the moving portal and then suddenly stop mid air and plop straight down? If you’re not moving faster than a moving portal does is become brick wall and smash you out of the way so you don’t gain any velocity in relation to earth so A can be maintained? There’s no way to test it in the current games. Hence the endless arguing. But I think B would be most consistent and allow for some really interesting puzzles though, especially if you had two moving portals! Or maybe 3d portals that can sit in the air and allow full movement through them in any direction to help make it possible. Portal 3? In VR with depth perception to accommodate?
The only speed that should be relevant is the object’s speed relative to the portal. Anything else is a distraction. The physics don’t care if you are hurtling at it or it is flying at you, both scenarios are equivalent. The only way to maintain conservation of momentum is to assume your exit speed relative to the exit portal equals your entrance speed relative to the entry portal.
If it did work the other way, well it wouldn’t assuming your exit speed is equal to your initial speed, relative to the exit. That means your speed is 0 as you “exit.” This leaves us with two possibilities. Either you are smashed into a 2d plane and physics gets very concerned, likely forming a teeeeeny tiny black hole. Or the incoming matter behind the first bits will push the first layers through, which, will just wind up back at the starting point, as they will cascade into each other at a speed defined by the speed of the blue portal, being indistinguishable from the projectile interpretation.
it’s relative to the earth or tied up people, then A.
If it is relative to the earth, they would be crushed at an atomic level.
Imagine the trolley-portal is passing around a tape measure at 10m/s. The tape measure is stationary on the earth. After 10 seconds, 100m of tape has entered the portal in a straight line. For me to have 100m of tape in a straight line at the exit, the end of the tape has to be moving away from the portal at 10m/s. Given that “crushed into a singularity” is not an option, we can assume the velocity cannot be relative to earth, and must be relative to the portal.
C, it combines the victim into a horrible overlapping monster of body parts
Or as I like to call it: Tuesday
If I stick my arm in a stationary (relative to earth surface) portal, I expect my arm to stick out of the exit portal. If the exit portal is moving at 10m/s over the earth, I expect my arm to also be moving 10m/s over the earth. My arm is stationary relative to the portal, but the portal is moving.
If that portal is moving toward a standing person and I make a fist, I expect my fist to hit that person at 10m/s. I am stationary relative to the earth; they are stationary relative to the earth, but my fist is moving at 10m/s relative to the earth. From their perspective, I punched them. From my perspective, they ran into my fist.
If I look through the portal, I will see them approaching me at 10m/s. They will see me inside the portal, approaching them at 10m/s. When the portal passes around them, they will not feel any change in their velocity, they will just collide with me immediately after the portal passes around them. To them, the earth will seem to suddenly be moving at 10m/s.
All motion is relative. To understand how the people will move, we need to look at them relative to the portal. If the trolly is moving at 5 m/s relative to the ground, then the people are moving at 1 m/s relative to it. So they enter the portal moving at 1 m/s and exit at the same speed.
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I think Portal solved this conundrum by saying portals can’t move.
Energy is relative when there is a frame of reference.
When the tram-portal is the frame of reference, the person has the energy. And speedy thing goes in, speedy thing comes out.
Using Portals canon, the person cannot be the frame of reference (ie 0 energy), because the portal has to move for that scenario - which is Portal-ly impossible. So the person has to come flying out.
If you break Portals canon and say that portals can move, then then the person would likely be super-compacted (matter transporting on top of existing matter) into a singularity or just destroyed.Portal breaks its own canon on Portal 2’s neurotoxin implosion scene though.
Portals can move along the plane of the portal in that scene, but never forwards or backwards
It’s canon that they portaled between earth and moon. For a portal to be stationary relative to both, it has to be moving relative to its opposite end.
They totally can move though. In one of the puzzles there’s a button that makes part of a wall angle itself so that you can propel yourself forward, and the portal on it will move.
May be remembering wrong but I thought any wall movements like that one had to be done before you shoot a portal onto it, because if you do beforehand it’ll poof away when you move the wall.
It has been a while, so I may also be remembering wrong so 💀
I remember that.
It’s a timed puzzle.
The wall moves on a button push, and moves back after an amount of time.
The portal is destroyed when the panel moves.
There’s literally nothing in the universe that is ‘stationary’ so the entire concept is flawed.
I mean, portals are flawed.
Anything moving through a protal experiences acceleration, unless the exit-portal is at the instantaneously-exact position of the entrance portal.
There has to be rules and limits that are ignored if portals are to exist, which is what the hypothetical situation is presentin5
Frames of reference matter. Whether the train or the people moving happened it doesnt matter to the portal. There is net movement and the momentum is the mass of the person moving x the speed of the train.
Imagine the train was moving the speed of light. If the person exiting the other end of the portal wasnt coming out at the speed of light their body would come out like a soup. All the atoms in their body compressing to escape at some randomly low speed.
A portal is as another commenter has framed it, essentially a hula hoop with a different space on the other side of it.
It doesn’t matter how fast a hula hoop falls over your body. You are not going to be launched out of the other side of the hula hoop even if the hula hoop is moving at the speed of light.
If the hula hoop is moving at the speed of light you are more likely to be killed by the shockwave of all of the atoms in front of the hula hoop compressing to adapt to the sudden intrusion of a lightspeed object with Mass, in which case it is very likely that you would pop out of the other side as some sort of soup, but that would not be because of your interaction with the portal inside of the hula hoop, or the acceleration of the hula hoop itself but rather the acceleration of the things around the hula hoop as it moved through space.
When im talking about speed of light i am assuming it will be in a perfect vacuum. If this was in ambient under normal conditions, a train going the speed of light would ionize all the air around it causing insane levels of heat.
So with the thought of it moving in a vacuum, if you look at the portal on a frame by frame basis every nano second you would see either
-
1 nano second in his entire body is within some imaginary dimension between the 2 spaces
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The body gets infinitely squeezed in 1 space turning them into a mini black hole
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They leave the portal at the same rate they came in
These are my 3 options, i dont see how it can be any other way.
I think that the velocity of the tram has nothing to do with the velocity the people it is running over until it actually runs them over and transfers momentum to them.
The portal puts a gap in between the tram and the victims, so there is no physical contact to transfer momentum. Momentum is a physical property, it cannot be transmitted without contact.
Therefore, in a frictionless vacuum the people must keep their original velocity and momentum regardless of the speed of the portal or whatever is pushing it forward.
If it worked the other way, Chell could not have leapt off of a ceiling and been launched out of the other side in the game. If portals transmit momentum without touch, then Chell would have first impacted an unmoving object with the same force as hitting the floor.
You can’t have it both left moving objects fly though unimpeded keeping their original momentum and also have unmoving objects suddenly gain momentum from a moving portal.
The portal does not affect momentum, it is a break in momentum. Momentum does not transfer across portals.
The momentum stays with the object that passes through the portal.
Quantum states can jump long distances without particle interaction, this includes energy transfer (you just need a measurable latent possibility for particle interaction!)
https://www.discovermagazine.com/the-sciences/first-demonstration-of-energy-teleportation
https://www.nature.com/articles/ncomms7811
The scifi answer that makes sense is that the space curvature around portal and shifting the object between two frames of reference (perhaps through frame dragging effects / gravity-wave like space warping) imparts momentum and it is conveyed from the portal as it is the thing which has to put in energy to correctly warp space for the object transfer.
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Or, to say it in Glados’ words: “Speedy thing comes in, speedy thing goes out”.
I’m making a note here: huge success
It’s hard to overstate my satisfaction
Violates conservation of momentum unless the portal/train slow down.
Yes, but portals violate basic physics anyway.
A portal that faces downwards into another portal is effectively a perpetual motion machine. Drop a ferromagnetic object into the loop and wrap some wires around the loop, now you have an infinite electric generator.
yeah, all movement is relative, if it was B then the relative movement between the people and the train would have changed, if it’s A then it’s conserved
Portal would fail due to being placed on moving object
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It’s all relative.
Tell that to einstein
Oh wait
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.
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.
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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.
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.
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.
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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.
Obviously A
starts world war 3
Yeah definitely A. The momentum of the object going through the portal matters not the objective that has the portal on it.
But how would the objects get on the other side then? The receiving side isn’t moving, so the objects essentially need to be pushed through the portal at the speed at which the train is moving, resulting in B. The only way A could work would be both portals moving at the same relative speed.
I see it like throwing a hoola hoop around something. The object will pass through without having its speed affected.
But with a hula hoop, both sides of the “portal” are moving at the same speed, so the momentum of the subject is unchanged. That’s not the case presented in this post
That’s a good point.
Here’s how I always phase it. Imagine you have a shovel and you are using that shovel to flick some dog shit into your neighbours garden.
With no portal the shit hits the shovel and you flick it, transfering the speed of the shovel into the turds. You stop the shovel and the turds fly away.
Now imagine the shovel has a big rusty hole in it. So it’s like a n shape. No portal yet. You go to flick the dog dumps but you just pass straight over them with the hole and the dumps go nowhere. The dumps have gained no momentum because nothing touched them and transferred that to them.
Now put a portal on the end of the shovel. As you sweep it over the cack has anything touched them? Has any object transferred it’s momentum to the dog eggs? No, so the dumps just gently tumble out of the other side of the portal.
Imagine there being 2 portals on the shovel one on the front and one on the back Whatever passes through remains in the same place. THIS is the actual analoge for the rusty hole. Entering the first portal the shit gains the speed of the shovel but since the exit portal is also traveling with the same speed but facing the opposite direction, it effectively cancels out the previous speed gain. Imagine it like the first portal adding one unit of speed to it and the exit one adding minus one unit of speed because of the opposite direction.
So now with one portal on the shovel and the other somewhere on a wall the shit gets accelerated and you will feel a force acting on the shovel that decelerates it’s swing.
Velocity is relative and this is why it works. If velocity wasn’t relative then Einstein would be wrong and many of the effects we see in the real world could never happen.
Nuh-uh. The gift from the rump don’t make no jump
Saying no doesn’t make it false. If you have a proper counter argument then present it
I didn’t have a proper argument to begin with dude. It’s a meme about video game concepts and I’m talking about flicking shite with a shovel. What made you think this was a serious debate?
fair enough but there is no reason not to debate something on a meme sub
But even in the image example, how would that work? The people have no momentum, they wouldn’t flop out, just fall back through, and then stay half in half not. According to you, air resistance wouldn’t even push them out because as soon as it goes through the portal the air is not moving relative to the people. I think your blatant lack of respect for relativity is unnerving and gets super confusing very quickly.
Nuh-uh. Flicking dog turds
dog eggs
🤮
I think that’s a very strong argument and a great metaphor, but you forget relativity.
All reference frames are valid - you could say the Earth and the people are moving and the train is stationary, you could say the train is moving and the earth and people are stationary, or you could say they each have a vector moving around the sun or anything else
But when you travel through a portal, the only valid reference frames are you and the entry portal. Your momentum relative to the Earth doesn’t matter - why would it? You can open a portal to the moon and jump through, and we see momentum is preserved. The Earth isn’t a special reference frame, it’s just the most noticeable one.
So let’s pick the reference frame of someone on the track. Let’s look through the portal and say there’s a sign on the other side - as it approaches, you’d see a sign approaching you through the portal. Relative to you, through the portal the sign is moving at 30mph. The portal passes over you - you haven’t moved, but you enter a new reference frame, a frame in which the Earth and everything on it is moving at 30mph
Speed, and by extension momentum, is relative. I’m sorry, but Einstein got you.
Except the momentum changes with portals most of the time anyway. Momentum is a vector, not just a scalar, meaning momentum has both a scalar and a direction component. And that direction component usually must be conserved as well. But portals change the direction of momentum all the time relative to the orientation of the entering and exiting portals. If the direction of the momentum of the object is relative to the orientation of the portal, then it makes as much sense that the scalar of the momentum would be relative to the velocity of the portal as well.
Energy is not conserved either, which is why the infinite falling box arrangement means the box keeps accelerating downwards gaining kinetic energy even though it started out with a much smaller, finite potential energy. Portals and conservation do not mix well.
As the people on the track are moving at an accelerated speed of 0 m/s, normally a train would apply the full force of the train moving to the
meat bagshuman ethic problems on the tracks.As newton’s first law states F = m*a, or Force = Mass * acceleration
F = x * 0 = 0 N of force
thus, they could just plop out as if falling after having a chair removed.
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?
But portals can create energy. Put one above the other face to face and drop an object into the bottom one, it now has infinite potential energy.
… fuck. You’re absolutely right. All my theorems - flushed down the drain.
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.
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.
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.
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.
Fuck, you just made me question the whole thing. Cave Johnson must be turning in his grave.
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
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.
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.
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
Interesting. It would be the same as if the trolley hit them directly.
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.
Now you’re thinking with
portalsphysics!The energy would come from the trolley.
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.
This guy is thinking with portals!
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
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 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?
B. Since there is relative velocity between the orange portal and the target, the momentum is conserved and they will launch.
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.
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
Relative to the train, and by extension the portal, the people are moving towards it at the same speed as the train relative to the ground, since the people are tied to the ground. I’m gonna work with the definition of momentum that equals it to the velocity of an object times its mass, and with the assumption that the portals conserve mass and momentum of the objects during teleportation, or with negligible losses. Having found that the momentum stays constant, and given the mass before and after teleportation is constant, the velocities relative to the portal are gonna be constant too. (p1=p2 <=> mv1=mv2 <=> v1=v2). And since the velocity of the people relative to the portal is the velocity is the train relative to the ground, and the velocity of the train relative to the ground is far bigger than the velocity of the people relative to the ground, the answer is gonna be B, where the people shoot out of the portal with great speed.
If the people actually go into the portal and not under it that is.
If that is so, the train or portal would have to lose its momentum for the transfer to happen otherwise you’d be generating more relative velocity after the portal. I can’t imagine portals transfering monentum, only maintain it.
Think of it as a pole entering the portal, the end will have to exit at high speeds and so it will need to drag the rest of it out at that speed
That might be the crux of it.
If you replaced the people with a pole on a roller, you wouldn’t expect the pole to get sucked into the portal or to roll towards the tram as it advanced, right?
That is what would happen if people were launched out of the other side of portal. The part of the pole that has been launched would drag the remainder of the pole with it.
But that wouldn’t happen.
The pole would just lay there until the tram passed it by, so the answer must be A. There’s no momentum added to the pole as the tram passes it by. The only thing that changed is the location of the pole.
You almost arrived at the correct answer. The problem is that would happen. More specifically we know that four fundamental interactions can pass through the portals, e.g. a thing that is whole before passing through portals is still whole after passing through them, though for any pair of particles, atoms, neutrons and protons etc. there was a moment in time where they were separated by the portal. If you imagine the portal moving towards the pole at some speed and in the middle of it suddenly stopping, the momentum of the part on the other side would slightly pull on it, and the pole would still be moving at a reduced speed. The momentum of the second part cannot disappear and will pull on the first part. The only other reasonable option is that the pole gets split, but that obviously is not the case. So B it is
If the pole is entering the portal at the rate of 60 miles per hour, it must exit the portal at the same rate. After a minute, 1 mile of pole has entered the portal, and 1 mile has exited it. If it exits more slowly than it enters, where is the missing part of the pole?
And if it exits at the speed it enters, does it lack momentum despite clearly being in motion outside the second portal? Does it magically halt when all of it has passed through?
The front part of the pole wouldn’t pull the back part of the pole more so than in any normal contiguous space. If you send a pole flying from the front and catch it mid-flight from the back stopping its motion, you’ll have to apply a force opposite in direction to the motion of the pole, and by Newton’s third law (every action has an equal and opposite reaction) it’s gonna pull you towards the direction it’s moving by reactionary force while decelerating.
In the case if moving portals, it might be a bit confusing, but what it comes to teleportation through the portals, the portals are absolutely stationary the world around them moves. And in the case only one of the two portals move relative to the ground, not only does the world move relative to both portals, it also deforms in a non-euclidean manner. That is why the pole that was stationary relative to the ground suddenly started moving after coming out the portal. And yes, it would require massive amounts of energy for the portal to function like that and keep its own momentum relative to the ground after teleporting things, but tbh that’s a woe for Aperture science, not mine 🙂
Your did the math! 😁
They didn’t do the math.
They just mentioned some of the formulas.
It’s the difference between reading a book and knowing the book exists.
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
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.
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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 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.
Yeah, but that’s because both sides of the window are traveling at the same speed. If the blue portal was on the other end of the tram, they’d plop.
If you strap a camera to the window, it will appear as if the object launches from the camera’s perspective.
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“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.
Momentum is relative too, since it’s equal to mass times velocity (in classical mechanics, of course)
Can portals exist on a moving object?
Not in the regular game, there are mods/fan games that allow it though.
There is one segment in Portal 2 where you need to cut some tubes that transport neurotoxin, and for that small segment only, the game allows moving portals. https://youtu.be/OrAHvenjZpA
it’s specifically scripted for that though, normally the engine just freaks out
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gonna go with a - the people aren’t moving when they go in, so they won’t be moving when they come out
Correct. If you do a ring toss the stick doesn’t shoot into the air.
This can’t apply because unlike the portals, both sides of the ring are moving at the same speed/direction.
The ring is not touching the thing passing through it to impart any forces. The object passing through carries only what energy it takes with it.
Then where does the energy to displace the air on the blue side of the portal coming from?
Yes they are though, it’s the exact same premise.
What force is generated where on the other side of the portal, it’s a hole in space, it doesn’t transfer anything.
Speedy thing goes, speedy thing comes out. Nothing gets transferred to or from the hole.
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.
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Except it is wrong
By that logic you have to account for the earths speed as it moves through the universe… That doesn’t sound accurate.
