Anti-Gravity

[ydoaPs]

That's what I've been trying to ask.

 

That thing where you put a heavy object like a bowling ball on a trampoline sheet' date=' then roll marbles. The marbles come to the bowling ball. I think of anti-gravity like puting a bowling ball [b']under[/b] the trampoline sheet. I was just wondering how such a thing would be possible, and if it turns out not to be, are there any alternatives to this?

 

thus the problem with the rubber-sheet analogy. it needs gravity to describe how gravity works.

[noz92]

Why? Space time will bend around every object. I just recently learned that gravity is not a pulling force. I'm sitting on my chair not because gravity is pulling me into it, but because spacetime is pushing me into it.

[Severian]

thus the problem with the rubber-sheet analogy. it needs gravity to describe how gravity works.

 

No it doesn't. All it requires is that the objects are stuck to the sheet (which is reasonable). The moving objects don't bend toward the gravitational attractors because they are lower down, but because the space is curved. If they travel in a straight line, they will still bend towards the large masses because a 'straight line' does.

 

Edit: So the previous poster's idea of having a big ball under the sheet pushing up wouldn't create antigravity - it would produce normal gravity, becaues the important thing is not whether parts of the space are 'higher' than others, it is whether the space is curved or not.

[noz92]

But would't curving spacetime away from the object create some sort of anti-gravity?

[1veedo]

...So does that mean it's impossible? Or at least in relativity?

[5614]

by "it" do you mean curving space time?

 

if so space time is curved around black holes.

[Severian]

But would't curving spacetime away from the object create some sort of anti-gravity?

 

Well, presumably one could curve spacetime 'by hand' in such a way that it appears that one body is gravitationally repelling another, but this is not possible within the framework of GR.

[ydoaPs]

No it doesn't.

 

ok. no gravity. why does the ball bend the sheet? what force acts on it?

 

try rubber sheet in 0g, and it won't work.

[1veedo]

ok. no gravity. why does the ball bend the sheet? what force acts on it?

 

try rubber sheet in 0g, and it won't work.

Yeah, and this is also the reason why putting a ball under it would not make it bend the other directions:

 

It would "pull down" in much the same way a ball on top would.

 

This is why, I think, in relativity it is impossible to have anti gravity.

 

Ok, I know. It's just an analogy, but still. I've never heard of an object the curvs space-time in the opposite way. Actually, all references to "curvature" all all in the same direction...

[noz92]

ok. no gravity. why does the ball bend the sheet? what force acts on it?

 

try rubber sheet in 0g' date=' and it won't work.[/quote']

 

No force has to act on it. Every object curves spacetime for some reason that I don't know of. And objects just follow the bent path in spacetime.

 

The only big problem with the sheet analogy is that it only gives an object two dimensions to move in, whereas real spacetime gives objects four dimensions. I have seen a computer generated model like this, but I doubt I'd be able to find it.

[Crash]

simply put the opposite of gravity is no gravity at all, this is how i see it (correct me if im wrong) like the trampoline analogy the ball under pushing it upwards would still be normal gravity, the only opposite i can see with a basic comprhension that breaks no laws is anti-gravity would be like a flat smooth trampoline that has no distortions, hope it helps noz. Actually can anyone see a flaw in this, i mean its no theory dosent make definite predictiopns but it seems to hold true

[Sayonara]

The rubber sheet analogy isn't supposed to involve actual forces, which is why it's an analogy. The idea is that mass always moves towards the sheet (and it doesn't really matter why). So a ball "under" it will move in the opposite direction to a ball on top of it.

 

Where that ball "is" in spatial terms, though, is anyone's guess.

[ed84c]

Well for a Rubber sheet to have 2 sides, it must be 3d aposed to 2d sheet (remember the 'dents' as sayo pointed out analogalistic and hence do not make the sheet 3d) so that means we have an extra dimention.

 

I personally believe, this means threfore sayo, that the ball is on the other side of space with 4 spacial dimentions, if you get my meaning.

 

Rubber Sheet= Spacial Dimentions-2 + Time

If rubbersheet is now 3d then we are adding another dimention (not the same as the one taken off)

 

Difficult concept to visualise, i know.

[Sayonara]

You are trying to explain the analogy by changing it.