But it's still A, no?

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Only if you use a weird physics model like in the third picture. Under real physics plus portals or game physics that model objects like the second picture, you'd end up with B. If you want A, you need some way to get the weighted storage cube to suddenly stop moving when it finishes moving through the portal. What causes the dog to stop moving in your picture?

Well I'm assuming that the portal works like a hole. So it would be the equivalent of running a hula hoop around the object, hence why it wouldn't move.

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@Val: That the surface that is pushing the portal down on him stopped as well.

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because the portal is against a surface now that the box was sitting on?

'asdlkfja';ds

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I'm still the Queen of Dicks

That's the thing. You're basically putting an object through a glassless window frame. The object won't get shot in the air no matter how hard you pass the object through it because the surface the object is sitting on it's stationary.

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And even if it wasn't, if once the rfame stopped when colliding with the surface, unless there is a force of massive proportions between the frame and the surface, there would be no transfer of force, so the object will still be stationary. And considering portals are holes without physical attributes, no matter how strong the object passes through it, if the surface is not pushing the object itself, it should remain stationary. At least that's how i'm picturing it.

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From the POV of the blue portal, it's like the dog is standing on a platform moving quickly upward that suddenly stops as the dog passes through a hula hoop.

@DaCrum: But both sides of a hula hoop are in the same reference frame. Each side of the portal here is in a different reference frame, such that an abject at rest in one frame is in motion with respect to the other.

@Mirak: But the dog is not glued to the surface it was sitting on. Immediately before the portal stops, the dog is moving quickly upward. What would cause it to stop moving?

I'm not sure what the correct english term is, but i picture it like a portal being the equal of a plastic "hula hoop". The hula hoop is glued to one surface that is moving, but it's not like the dog is moving quickly upward because all it's doing is poking it's head through the hole after you passed the "hula hoop" through him, maybe it will be fast and disorienting for the dog's vision, but not fast enough for the dog to be launched into the air because it's like letting a hula hoop fall through the dog until it hits the ground the dog is resting at. Man im having a difficult time speaking physics in english

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Doing just fine buddy

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I'm going to make another animated example tomorrow to further explain what i mean because i think i'm getting close to what i truly wanted to convey, my point is that i think we're focusing too much on the surface the portal is placed onto and the strenght of the portal passing through the dog instead of the surface the dog is resting at and the light composition of the portal itself.

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Have another look at this image. Save it and look at it frame by frame if you can.

As in frames 2 and 3, as soon as the dog's particles have passed through the portal, they have acquired a nonzero speed (given the opening presentation, I'm assuming this is a pretty good clip, enough to launch the dog if that's what the physics calls for). This change in speed does not require a force or energy input because we're effectively just looking at things from a new frame of reference. In fact, it may be more appropriate to say that the dog acquired a nonzero speed relative to the blue portal's reference frame as soon as the orange portal started descending (a change which makes portal physics especially impossible due to the crazy amount of energy created). However, if you want to make the dog stop moving with respect to the blue portal now that it's moving with respect to the blue portal, you need some application of force. One solution would be to glue the dog to the platform it was sitting on, so that the tensile strength of the glue would arrest the dog's motion (let's assume that the dog can survive the forces involved physically intact.) But that's not part of the original question, and another force that could provide the deceleration has not been proposed.

In short: all reference frames are valid. Start at the blue portal's POV and see if your assessment makes sense.

Edit: I prefer the blue portal's reference frame because the object in question is entirely on that side of the portal when the debatable physics takes place. If you want to use the orange portal's reference frame for what happens on the other side of the portal, we need to get into how to evaluate what happens on the other side of an accelerating portal. We can invoke General Relativity for that, but it ends up as a more convoluted version of what I've been saying for the blue reference frame.

Looking at Val's examples I realise that I instinctively use the particulate model for such problems. Having done university level physics my perspective might be a little one-sided on the issue.

The bottom line here is, that looking at it as a hula-hoop will not give you the correct answer, because a hula hoop has both "portals" always stationary with respect to one another. This is not the case here. One portal moves the other is still. But in physics all is relative, so whether it's portal moving and overtaking the object, or the object moving and flying into the stationary portal, these are equivalent situations, and only depend on the frame of reference you choose to use. This is why this example is so impossible, because we have a passage between two different reference frames. Try to consider it this way: yellow portal and blue portal are in different universes/realities. The yellow portal is flying towards the object. If you look into the stationary blue portal while the yellow portal is moving, you will see the object moving towards you. It's simple. A stationary object in the yellow portal's world is a moving object in the blue portal world, simply because one of the frames of reference taken from the portals' points is moving. There is no reason for the object to stop after passing through the blue portal. Even if the yellow portal stops moving. That's because the cube is no longer in any way a part of the yellow portal's reality, so changes in that won't affect it.


Here's an even more difficult problem. What if the yellow portal suddenly stops when it reaches the mid-point of the cube, so that half the cube has gone through it?

In my way of thinking using the same principles, half the particles of the cube have gone through and out the blue portal, and they have a momentum and velocity relative to the blue portal, but they are attached to the same mass of molecules that are stationary and in yellow portal's reference frame. What I think would happen then would be the cube bouncing up once in the yellow portal's frame. Whether it jumps high enough to get through the blue portal entirely can be debated, it depends on the velocity the yellow portal had when moving. But if it doesn't move too fast then moving the yellow portal down and then stopping it half-way through the move will make the cube jump up a little, further into the yellow portal, then probably fall back down because gravity on the yellow portal's side would pull it back, and since in blue portal's reference frame gravity would pull the cube partially in the same direction (as in back into the portal) it would fall back on the platform staying half-way through the two portals.

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That gets into the General Relativity stuff I mentioned, because we need to figure out what happens to the far side of a portal when the portal accelerates. I propose explaining it by treating the far side of the portal as a non-inertial reference frame subject to universe-wide forces that account for the (from the reference frame of the other side of the portal) non-inertial behavior of everything there when the portal accelerates. For example, in my example images I proposed 10m/s for the descent of the orange portal. Suppose it stops in 1/100 second, so the orange portal experiences 1000m/s/s acceleration for 1/100 second. From the reference frame in which the orange portal had been descending at 10m/s and is now stopped, the entire universe seen through the orange portal just underwent the same 1000m/s/s acceleration as the portal itself. We can debate about how the force would be applied near the portal itself*, but I don't think that it matters for your scenario because the force should be symmetric with respect to the portal. If that's the case, a weighted storage cube halfway through the portal should acquire an upward velocity of half that of the orange portal's initial downward velocity. Whether this is enough to launch it depends on the particulars of the situation.

*We could suppose that the force is applied 100% to everything on one side of the portal and not at all to everything on the other side, but I suspect that a more "realistic" scenario would be a gradation with 50% intensity at the portal boundary, mostly tapering off within a portal radius or so, in a manner of a minimal surface connecting two planes of different height.

Anything more to say about portals?

I wish they were real.

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Well, wormholes are supposedly possible according to General Relativity. They would conserve energy though, which would mess up a lot of the cool stuff portals do.

Weekly discussion 22 (8/1-8/8): The most important question of all

Should we build a Death Star? Tenacious D makes a compelling case. Thoughts?

Currently, I don't think mankind is ready for that kind of responsibility.

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Mankind? Responsibility?
Please, we can't even invent something without having the tiniest amount of misuse attached.

On a more serious note though, it is a curious question as to whether we are ever prepared to build a weapon as fine and terrible as a Death Star. But before we step into that, for what purpose would it serve?

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It would fight evil space squid, stimulate the economy, and secure the future of humanity.

Aren't we already building the Enterprise?

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Who will fund this project though? Tax dollars? Private enterprise? Would we really want to give Warren Buffett a planet destroyer?

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The moment we build it is the moment we destroy earth.

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My realism workshop Updated April 24, 2013: Not the best

Well, we'd probably need most of the resources on Earth to build it, so eh.

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