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Full Video: https://www.youtube.com/watch?v=zDAE9A_1NA4
What if you try to sandwich a cube between two portals?
Does the cube disappear?
Do the portals bounce or stop?
Or something else?
Imagine instead that you extend a piston through the blue portal.
Eventually it comes back and hits itself from behind.
The piston can't keep extending into the portal because there is no "inside" of the portal - "inside" the blue portal is "outside" the orange portal, and vice versa.
At this point one of three things could happen.
If the piston is weak it should crumple, the same as if it were pushing against a solid wall.
If the piston is rigid and the portals are super heavy, then the piston will just stop expanding (since it can't lengthen any more without the portals moving farther apart).
But the orange portal is clearly "pushing" the piston out of it to the right, so Newton's "equal and opposite" law of motion means the piston is pushing the orange portal to the left - "kind of like what happens when you throw a ball".
Similarly, the blue portal is somehow absorbing the momentum of the piston entering it, and so should be pushed to the right - like what happens when you catch a ball.
So if the portals aren't heavy (and aren't on super heavy objects), then the logical (if weird) conclusion is that the portals themselves get pushed apart!
Minute Physics provides an energetic and entertaining view of old and new problems in physics -- all in a minute!
Created by Henry Reich
The portals moving assume they provide any resistance at all, which shouldn’t be the case, since they are portals.
Why would the portal interact with the piston? The portal isn't an object. It is a 2d plane that connects to positions. The piston would extend until it hits its casing and then stop/break.
All of these questions always overthink the portals. They're just holes. That's it.
Your last theory seems to imply that a portal will move if you run through it too fast. They also require a surface, and won't phase through one.
This is like saying if I ran through a doorway fast enough the doorway would rip itself out of the wall flying the other way. Portals are not acting upon the things passing through them they are closer to a whole in the fabric of spacetime that things can move through.
Neat thing with the piston, but let's get back to finding out what will happen if two portals sandwich a cube.
As the great Tyler Folse says, “No”.
Seems a lot of us don't think the portal interacts with the cube. Maybe MinutePhysics has never played the game which, if memory serves, clearly states "speedy thing goes in, speedy thing goes out," and which allows the player to hover between portals.
I disagree that "the orange portal is pushing the piston." The piston is pushed by the cube. Similarly, the piston doesn't push the blue portal. The piston is pushed through the portal holes, by the cube. When the cube pushes the piston toward the right, conservation of momentum requires the cube to move toward the left. When the piston eventually reaches the left side of the cube, the momentum of the piston and the equal & opposite momentum of the cube cancel out (assuming no bounce and nothing breaks).
A portal shouldn't be "pushing" or "pulling" anything. It's just a window to another space. That would be like saying putting your arm out your car window is your window pushing your arm outward...
Why is nobody considering any structural forces of the objects themselves? The portals are simly a discontinuity in the coordinate system. A piston pushing itself through a portal would be like if the arm curved around as it extended. Once it meets itself, the internal stress increases, first causing elastic deformation, then plastic deformation, then buckling. Its like if you were to put nothing into one of those oddly satisfying crusher videos. The piston itself would fail. Same with the moving portals. The cube would meet itself, then begin compressing itself, deforming and then breaking. The assumption of portal is that you can create space coordinate discontinuities, not that objects have infinite strength.
Your last theory implies that the piston is strong enough to reshape the dimensions of the space around it (meaning the piston is comparable to a black hole). But whatever i guess.
No, because portals aren't objects. The moving end of the piston is pushing off of the stationary end, which is pushing off of whatever it is fixed on. Then you are asserting that the blue portal needs to apply a force to the object moving through it for momentum to be preserved. This is not the case. Ignoring the fact that the portals cannot push you in the same way a doorway can't, neither the mass, nor the velocity of the piston head changes, so momentum is preserved, no extra force needed.
A door opening isn’t pushed in the opposite direction when you walk through it. The piston isn’t touching the portal, it’s going through it. Why would an opposite force be subjected to something not in contact with the piston?
The sandwiched box would get crushed by itself. This thought experiment is analogous to bringing two more identical boxes on either side closer and closer with infinite force. But since the two boxes on either side are just parts of the original box, the entire box will collapse due to infinite compression.
"the orange portal is pushing the piston out to the right" No no no. Neither portal pushes anything. They connect the two locations. They exert no forces. The piston on the left is not being pushed by the portal, it is being pushed by the piston on the far right. The Newtonian equal-opposite reaction stays within the piston, even across the portal line.
Newton’s third law is pretty much just conservation of momentum, and since the material that moves through the portal keeps its momentum we don’t need the portals to receive any force/momentum.
No. Imagine the piston being a circle. If you push it out far enough it hits itself and stops. That's what the portals do: Connect two points in space like a door. The portals do nothing. They don't interact with the objects passing through in any way. Force is conservated, hence the "portal on ceiling over portal on floor" acceleration because of gravity and prolonging the way down indefinitely. If force is fully conservated as seen, there is transfer of firce to the portal, meaning NO INTERACTION.
The piston is not exerting any force on the portal. The two options are the piston either exerts enough compressive force to crush itself or it doesn't. For the original problem the cube would get crushed as the space between the portals shrinks. There is no other option.
If we're talking about just a cube then the portals would be as close as the width of the cube. Cuz as soon as the cube started going through the portal it would start hitting the back of the cube before it goes into the other portal so it can only get as close as the width of the cube