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Q & A: equivalence

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Most recent answer: 05/15/2010
Would the gravitational pull off two different objects differ with substance? For example, if two spherical objects have the same mass but one is made out of hydrogen and one is made out of helium, would their gravitational pull differ? Also, gravitons, how is it theorized that they pull everything towards them, how are they said to work?
- Evan (age 16)
United States
So far as anyone has been able to tell by experiment, the gravitational pull depends only on mass, not any other properties at all. The whole form of the theory of gravity (General Relativity) would unravel if that were not true. Then (going back to a Bohr-Einstein argument) the uncertainty principle would unravel and thus the whole form of quantum mechanics. So we're really confident that this equivalence holds.

As for the second part of your question, it's a good question but a little out of my league. I'll try to get a colleague to help with an update.

Mike W.

Some time ago, I was asked the same sort of your second question. Actually, it was "Does an anti-particle fall up in a gravitational field?".   I wasn't sure so I asked my guru and he said no and is due to the fact that the force transmitter, the graviton, has spin two.  Spin two force transmitters have attracive forces only, unlike the photon which has spin one and has both attracive and repulsive forces depending on the relative sign of the charge of the two particles. You need several courses in quantum field theory in order to figure this out.   I only had one. 


(published on 05/15/2010)

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