MadSci Network: Astronomy

Re: Is it possible to generate and receive an gravitational field variation ?

Date: Wed Feb 20 18:11:25 2002
Posted By: Benjamin Monreal, Grad student, Physics, MIT
Area of science: Astronomy
ID: 1012754004.As

Hello Daniel,

All of the effects and processes you describe are - from a strictly physical standpoint - possible. A gold nucleus does generate a gravitational field. A moving gold nucleus does generate a changing field, which can exert a gravitational force on objects very far away. So, you ask, "could such a system be used as a transponder?"

Unfortunately, I think that you have designed the weakest transponder imaginable. :) Some numbers to illustrate: The gravitational acceleration caused by any mass M, a distance R away, is 6x10^-11 * M/R^2 meters per second per second. 6x10^-11 ... that's 0.006 billionths of a m/s/s, caused by a 1 kg mass 1 meter away. (Note that I'm using the American "billion", which is 10^9, or 1000000000) If you want to use a single gold nucleus, that's 3x10^-25 kilograms ... causing 0.3 billionths of a billionth of a millionth of a m/s/s of acceleration. That is an unimaginably small acceleration. So if you put two gold atoms 1 meter apart and wanted them to fall together gravitationally, it would take on the order of 10^18 years, or a billion times longer than the lifetime of the Universe.

It's amazing how weak gravity is. It's really one of the big mysteries of modern physics - why did nature choose to make this one force so ridiculously feeble? With some examples (from the "fun factoids" school of science education) you can perhaps get a sense of how feeble it is:

Perhaps your mistake was looking at density rather than mass. The gravitational force between two objects, when they are reasonably far apart, depends only on the mass, not on the density. Nuclei may be dense, but on human scales, they're still extremely light, and thus are very poor sources of gravitational waves.

Good question, hope this answer has been interesting. Gravitational physics is a neat topic, and a great challenge to both experimenters and theorists. If you want to learn more, the LIGO webpage linked above makes good reading, as does Brian Greene's book "The Elegant Universe".

-Ben Monreal

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