by neufer » Wed Nov 17, 2010 2:31 am
Chris Peterson wrote:rstevenson wrote:
One quick thing I can say, and then I'll duck while others more knowledgeable answer. ...
1. Gravity is a consequence of mass, and photons have no rest mass - so, no gravity as a result of photons.
That's absolutely true, but the concept of rest mass is a matter of theory, not practice. Photons are never at rest, so they do have an effective mass. That mass is determined by E=mc^2, or by calculating the photon momentum p = h / lambda, and then using the classical p = mv with v=c. It is because photons have momentum, and therefore an equivalent mass, that things like solar sails work, or that radiation can affect the orbits of astronomical bodies.
One of Eddington's photographs
of the total solar eclipse of 29 May 1919,
confirming Einstein's theory that light "bends"
Note that since Eddington observed that the sun attracts photons
then it naturally follows that those same photons must also attract the sun.
-----------------------------------------------------------------------------------------------
Technically, gravity is a consequence of the momentum energy tensor
(i.e., the symmetric stress-energy tensor):
Got it?
[quote="Chris Peterson"][quote="rstevenson"]
One quick thing I can say, and then I'll duck while others more knowledgeable answer. ...
1. Gravity is a consequence of mass, and photons have no rest mass - so, no gravity as a result of photons.[/quote]
That's absolutely true, but the concept of rest mass is a matter of theory, not practice. Photons are never at rest, so they do have an effective mass. That mass is determined by E=mc^2, or by calculating the photon momentum p = h / lambda, and then using the classical p = mv with v=c. It is because photons have momentum, and therefore an equivalent mass, that things like solar sails work, or that radiation can affect the orbits of astronomical bodies.[/quote]
[float=right][img]http://upload.wikimedia.org/wikipedia/commons/thumb/3/37/1919_eclipse_positive.jpg/260px-1919_eclipse_positive.jpg[/img]
One of Eddington's photographs
of the total solar eclipse of 29 May 1919,
confirming Einstein's theory that light "bends"[/float]Note that since Eddington observed that the sun attracts photons
then it naturally follows that those same photons must also attract the sun.
-----------------------------------------------------------------------------------------------
Technically, gravity is a consequence of the momentum energy tensor
(i.e., the symmetric stress-energy tensor):
[img]http://upload.wikimedia.org/math/3/6/3/363506ba8f803a8bba92b6ed98fff3f4.png[/img]
[img]http://upload.wikimedia.org/math/0/1/c/01cccfad1943e6725b8cc3436f70cf5a.png[/img]
[img]http://upload.wikimedia.org/math/c/1/d/c1d9d487cf6d3b331ea27689a9a47fcf.png[/img]
[img]http://upload.wikimedia.org/math/6/6/8/668572dc41cb2731fc9d077259b5bdb1.png[/img]
[c]Got it?[/c]