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dwnielsen
2009-Aug-11, 06:21 PM
This question may be very naiive, but..

Is it proven by experiment that gravity is absolutely tied to mass?
By this, I mean, is it possible that a massless particle (such as a photon) could produce a gravitational effect far too small to detect?
You may be wondering why I would ask this, or why this might occur. I am thinking that perhaps it is the low-level informational content that could create the gravitational effect. In this case, it is not mass per se, but the informational content of the mass quantity, that would be related to gravity.

Now I am probably mincing words here; what do I mean by information content? That is, an observable ordering or such. I hope I am clear enough in this question.

Argos
2009-Aug-11, 06:29 PM
This question may be very naiive, but..

Is it proven by experiment that gravity is absolutely tied to mass?

Short answer: yes, yes, and yes.

Jeff Root
2009-Aug-11, 08:48 PM
Sorry Argos, but one of those should be "no".

It is proven by a combination of observation and reasoning that gravity is
absolutely tied to energy, not just mass. Mass is a form of energy. It is,
in fact, the densest form of energy. So a small amount of mass has a lot
of energy, and therefore a (relatively) lot of gravity. Other forms of energy
are less compact, so it is almost impossible to get much gravity from them.
Light has a "mass equivalent" which can be calculated to tell you how much
gravity is associated with it.

-- Jeff, in Minneapolis

Argos
2009-Aug-11, 09:29 PM
I concede. ;)

dwnielsen
2009-Aug-11, 10:24 PM
Thanks, Argos & Jeff.

Really? So light, even moving at relative c can be said to have a gravitational effect? Is that right?

Ilya
2009-Aug-11, 10:30 PM
Yes. Tiny gravitational effect, but it is there. And in theory, if enough light occurred simultaneously in a small space, there could be large gravitational effects, up to and including formation of a black hole. There is even a word for such (theoretical!) black hole made of light alone -- Kugelblitz (http://en.wikipedia.org/wiki/Kugelblitz_(astrophysics))