Tuesday, June 16, 2009

Gravity Meshes are Impossible

One invention for the sake of making the filming easier in Sci-fi shows like Star Trek are gravity webs, meshes, or floors, that provide gravity in environments like space that shouldn't have any. This makes it easier to film, because the "space" environment can be shot in a basement in hollywood, instead of having to actually take the actors into low earth orbit, which would be significantly more expensive.
Such a device would have massive utility. Not only could you walk in space instead of float, and travel between the stars without your bones weakening into goo, but you could exercise with them (the gym has DOUBLE GRAVITY today), make transportation easier (the packages now weight NEGATIVE whatever because there's a reversed gravity mesh in the truck's ceiling), or just carnival style novelty (as in, hey, let's walk on the walls! Because we can!).
However, it can't happen. The best way to prove to me that it can't happen is to build a perpetual motion machine with it, so I'm going to do exactly that. Please enjoy this flagrant violation of the laws of thermodynamics.
It circles around...
As you can see in the diagram, the very large ball falls onto the turning wheel, imparting the energy it got from gravity and thereby providing the extractable energy. It then falls to the lower ramp, moving to the other half of the device. At this place, gravity is reversed, so it "falls" to the top of the machine, where it hits the ramp that moves it back to the other side. Oh hey, gravity is reversed again due to the absence of the mesh in this part, so it falls again, back to the wheel. This would endlessly produce power, against all reason.
So obviously the gravity meshes either have to use more power to function than all the falling that happens in their area, or they just can't happen. In either case, Sci-Fi fans everywhere pout at this rude intrusion by reality. Then they go back to enjoying their fandom, because since when does television, movies, or books have to in any way correspond to reality? Wait up, guys.
Thanks to D. K. Wolfe for the illustration.


Viktor Lofgren said...

You're able to extract energy because gravity meshes, the way you describe them, create a non-conservative force (and thus a non-conservative potential, which in turn breaks conservation of energy).

If you reformulate the description of the mesh to create a conservative gravitational field, this will solve itself.

On a side-note, an interesting discussion that came up a while back is how the gravitational force lines would appear in Valve's video game Portal. I did a rough sketch of the resulting forces, and came to the conclusion that the premise of that game creates some seriously messed up physics.

themadengineer said...

I suppose then that the mesh would work if arranged in a conservative pattern, but your average sci-fi ship isn't. Star Trek, the original example, had their saucer-and-platform ships that almost assuredly aren't conservative.
Although some genres are worse. Star Wars has air in space.
Definitely one could wreck all kinds of havoc with Portal's physics.

Viktor Lofgren said...

It's possible to salvage Star Trek-style gravity if you introduce some changes to the "charge carriers" of gravity.

If you create two new forms of particles, one with the gravitational equivalent to positive electrical charge -- attracting matter, creating a positive G field, and one with the gravitational equivalent of negative electrical charge -- repulsing matter, creating a negative G field, and sandwich a room with it (that is, negative charge on the ground, positive on the ceiling), you would get an uniform gravitational field inside the room the same way you get a an uniform electrical field in a plate capacitor.

You could also go on adding more floors according to the same principle without messing up things too badly. It wouldn't be perfect (the field near the hull would be pretty wonky), but it would approximate the Star Trek model of artificial gravity.

Viktor Lofgren said...

Uh, I may have just designed that with upside-down gravity. It should be positive down, negative up.

themadengineer said...

Gravity "particles" are a good explanation. And the charge reversal would explain all those weird accidents in the starship gym. You know, the ones where people fell up into the ceiling?

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