Photons, mass, and gravity

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Photons, mass, and gravity

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1richardbsmith
Jan 21, 2010, 10:48pm

Reading a little above my pay grade, it struck me that if a photon cannot escape a black hole then it is effected by gravity. But photons are theoretically massless and demonstrated limit of 10e-51 grams or so.

Is this a relativistic energy mass equivalency?

2daschaich
Jan 24, 2010, 3:05am

One of the key differences between general relativity and newtonian gravity is that in GR, gravity affects everything with energy, not just mass. Thus the photon can be massless (this is not a relativistic energy mass equivalency), but still be affected by gravity.

A demonstration that light is indeed affected by gravity was one of the most famous early tests of general relativity,
http://en.wikipedia.org/wiki/Sir_Arthur_Eddington#Relativity

3richardbsmith
Jan 24, 2010, 6:02am

good to see you post again.

I was familiar with that observational confirmation of relativity, and considered the possibility that it might be the answer to this question. It is always intimidating to ask scientists what is probably a simple question.

Thanks. Always glad to read your comments.

4daschaich
Jan 24, 2010, 1:59pm

Even apparently simple questions often have deeper aspects, or connections to deeper issues.

While I'm not an expert on the history, I have heard that the fact that energy gravitates really surprised physicists of the time. Apparently Einstein and others initially expected general relativity (or, really, a Lorentz-invariant theory of gravity) to be a trivial extension of special relativity (a Lorentz-invariant theory of electromagnetism). The fact that energy gravitates is a major reason this is not the case.