Antigravity in General Relativity

[rogerl]

Antigravity is impossible in General Relativity because gravity is not a force but a consequence of curved spacetime. But if you can stop the time component, geodesic motion would stop as well and gravity would be nulled. So is Antigravity in General Relativity simply about stopping the time. Or is this impossible since if you cut the time off, the worldline would simply cease? But we haven't discovered the theory of quantum gravity yet. So we can't say that as time cease to function, matter (or quantum object) won't move anymore. Matter moving while time is stopped may be antigravity. What do you think? Pls. refute this.

 

[bcrowell]

Antigravity is impossible in General Relativity because gravity is not a force but a consequence of curved spacetime.

This is incorrect. GR is compatible with gravitational repulsion. However, we observe that most forms of matter under most conditions we know of satisfy various energy conditions: http://en.wikipedia.org/wiki/Energy_condition

 

[rogerl]

This is incorrect. GR is compatible with gravitational repulsion. However, we observe that most forms of matter under most conditions we know of satisfy various energy conditions: http://en.wikipedia.org/wiki/Energy_condition

Hmm... how can that be. All matter so far observed obeys GR. You mean matter can be engineered such that it can automatically repulse gravity (without any known propulsion), How?

 

[Bill_K]

The Principle of Equivalence says that any small test object, regardless of its composition, follows a geodesic. Which is to say it is attracted to other objects. Any statement to the contrary is in conflict with general relativity and in conflict with experiment.

 

[Forestman]

There currently are experiments going on at NASA using superconductors to produce antigravity. It has been found that matter placed above a ring of superconductive material while the material is in the process of superconducting will cause the matter placed above it to lose weight. I don't doubt GR, however there is always something new to learn about nature.

 

[ZapperZ]

There currently are experiments going on at NASA using superconductors to produce antigravity. It has been found that matter placed above a ring of superconductive material while the material is in the process of superconducting will cause the matter placed above it to lose weight. I don't doubt GR, however there is always something new to learn about nature.

There is no such thing right now. This is the Podkeltnov effect that was done a long time ago, and no positive results came out of it. You need to get your information updated.

Zz.

 

[Forestman]

My information came form the show Mythbusters. It was the one where they were trying to see if the lifters produced any real anti gravitational force. Which they found that the lifters did not, but instead produced a form of ion thrust which allowed them to float. Anyway though during the show they admitted that experiments involving antigravity using superconductors were being done at NASA.

 

[bcrowell]

This is incorrect. GR is compatible with gravitational repulsion. However, we observe that most forms of matter under most conditions we know of satisfy various energy conditions: http://en.wikipedia.org/wiki/Energy_condition

Hmm... how can that be. All matter so far observed obeys GR. You mean matter can be engineered such that it can automatically repulse gravity (without any known propulsion), How?

Obeying GR (the Einstein field equations) is not the same thing as obeying an energy condition. For example, the existence of the cosmological constant violates the strong energy condition, but it's consistent with GR.

The Principle of Equivalence says that any small test object, regardless of its composition, follows a geodesic.

Agreed. So for example, we can't have an electrically neutral test body that will accelerate away from the Earth's surface when released from the Earth's surface.

Which is to say it is attracted to other objects.

But this doesn't follow. For example, Reissner–Nordström repulsion doesn't violate the equivalence principle (although the interpretation of it is controversial).

The equivalence principle says that if small, uncharged particle A is repelled by something, then any other small, uncharged particle B must also be repelled by it.

 

[ZapperZ]

My information came form the show Mythbusters. It was the one where they were trying to see if the lifters produced any real anti gravitational force. Which they found that the lifters did not, but instead produced a form of ion thrust which allowed them to float. Anyway though during the show they admitted that experiments involving antigravity using superconductors were being done at NASA.

And you didn't pay any attention to when such NASA experiment was done but simply parrotted what you heard on TV as your primary source?

That Mythbusters show was also old! Please do not relay pieces of information of which you know nothing about. It forces the rest of us to come in and do the dirty work of having to clean up after you to correct the misinformation.

Zz.