08 August 2005

Gravity -- Just Another Theory?

The most fascinating thing about gravitation is that, while we all experience it, and engineers can estimate its force accurately enough to build bridges that don't fall down and send rockets to Saturn, we just don't know how it works.
"Gravitation is the tendency of masses to move toward each other. . . . Exactly why two masses separated in space have a gravitational attraction to one another remains largely unknown . . . ." (From Wikipedia article)
Although Isaac Newton formulated the equations that quantify gravity which are used for most practical purposes, he also recognized that his formulation of "action at a distance" didn't explain anything and was inadequate (Principia Mathematica, 1687). (See Wikipedia article on Action At A Distance.)

Newton Was Only Close

Newton's theory of gravity, and formulas for calculating its effects, doesn't accurately account for several observations (for example it gives an erroneous value for the precession of the perihelion of Mercury). Einstein's General Theory of Relativity (1915) does accurately account for these observations. The Einstein field equations can calculate the relationship between the presence of mass/energy/momentum and the curvature of spacetime.

So we know how to calculate and predict gravity's effects, using either Newton's laws or Einstein's equations as appropriate. But we still don't know how gravity works. We have an accepted standard description (General Relativity), but no mechanism.

It is still as Newton said:
I wish we could derive the rest of the phenomena of nature by the same kind of reasoning from mechanical principles; for I am induced by many reasons to suspect that they may all depend upon certain forces by which the particles of bodies, by some causes hitherto unknown, are either mutually impelled towards each other, and cohere in regular figures, or are repelled and recede from each other; which forces being unknown, philosophers have hitherto attempted the search of nature in vain.
Even Einstein's concept of warping of spacetime doesn't explain how masses warp spacetime. As with other forces, gravity can be regarded as a wave or a particle, and gravitational waves have been detected. (just this year: Star and Telescope article.) (Also see Wikipedia on Gravitational Radiation.)

The Gravitational "Force"

Home-made Cavendish experiment
The gravitational "force" is very weak compared to electro-magnetic forces. (Consider that a small magnet can pick up a paper clip while the entire Earth is pulling it the other direction by gravity.) However, it is easy to demonstrate that even small objects have gravitational attraction to each other. This was done quantitatively in the Cavendish torsion bar experiment (first carried out in about 1798, while John Adams was the second president of the United States and Napoleon was invading Egypt.) You can perform this experiment yourself, if you have some time on your hands and a suitably quiet space. (Or you can just read about someone else doing it here.)

The Current Theory

So in a sense we do understand how gravity works. The current hypothesis is that mater emits "gravitational waves" when it accelerates (but how?). Those waves affect the curvature of spacetime as they pass through it.

Exactly how gravity works is of great interest because it is gravity which will determine whether the universe keeps expanding for ever or eventually recollapses.

What Is A "Theory"?

One reason I wrote this post is that I am puzzled. We don't even have a solid "theory of gravitation" (there are competing theories), and the theories we do have don't explain everything. Newton's theory, which is commonly taught as fact, is known to be wrong.

On the other hand, the theory of evolution by natural selection is much better supported by experiment and observation, explains much about life, and continues to impress scientists with its ability to generate, employ and explain new and amazing information, for example from molecular biology.

Yet some people would insist we should teach "alternatives theories" to evolution. Where are their alternative theories regarding gravitational attraction?

Or maybe this is just a biologist's point of view. Do physicists see the situation differently?


Additional Information

Amusing, satirical post on gravitational "theory" here.

The image of the magnet and paper clips is form http://learningideasgradesk-8.blogspot.com/2011/01/how-strong-is-your-magnet.html

We are still exploring gravity.

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