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What is the opposite of gravity?
- Jiing Huey (age 12)
Actually, as far as we know, gravity doesn't have an opposite.
(republished on 07/12/06)
Follow-Up #1: Does anti-matter have anti-gravity?
Does that mean then that anti-matter has gravity?
- Lucy (age 17)
That's a very interesting question. The answer is we don't know.
From an experimental point of view the obvious thing to do is to drop a particle and an anti-particle at the same time and see if one moves up and the other moves down due to gravity. Unfortunately, it is an extremely difficult experiment to perform. The problem is that the gravitational force is so weak that stray electrical and magnetic forces swamp that of gravity. A very clever experimenter, William Fairbanks of Stanford University, worked for many years on it but was unable to solve the stray field problem. He even encased his apparatus inside of a superconducting lead balloon; it didn't fly. Even electrically neutral particles, like anti-neutrons, have magnetic moments that are affected by magnetic field gradients.
From a theoretical point of view it is more or less agreed that anti-gravity doesn't exist. If it did there might be cosmological consequences. However the debate is still ongoing. For some details of the arguments, take a look at:http://en.wikipedia.org/wiki/Gravitational_interaction_of_antimatter#The_antimatter_gravity_debate
My take is a little different from Lee's. The arguments that gravity remains attractive for antimatter seem overwhelming. All sorts of inconsistencies show up if you try to change that. The arguments that the matter-antimatter force is repulsive look extremely feeble to me. I think this question is about as settled as any questions get.
(published on 03/25/10)
Follow-up on this answer.