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Photon's gravity
Posted: Tue Nov 16, 2010 10:22 pm
by v13
Hello dear APODers,
I came with a thought experiment which I'm not sure how to answer, so I'm asking your help:
Do photons cause gravity? I mean: If you create some very-very energetic photons (a beam of very energetic photons), will they attract matter?
I bet that possible answers are "yes" and "no". So:
- If Yes: What's the deal of the propagation? Since gravity propagates with the speed of light, the propagation of photons' gravity along their movement is a mind mess. Photons will always catch-up with their "gravitational wave", causing a constant "sonic boom". If so, shouldn't this result in an ever increasing gravitational force?
- If No: Isn't it statistically possible for a black hole to convert part of its mass to photons, leaving it with less than the critical mass and explode in the most energetic "supernova" ever seen?
Re: Photon's gravity
Posted: Tue Nov 16, 2010 11:45 pm
by 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.
Rob
Re: Photon's gravity
Posted: Wed Nov 17, 2010 12:24 am
by Chris Peterson
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.
Re: Photon's gravity
Posted: Wed Nov 17, 2010 2:31 am
by neufer
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?
Re: Photon's gravity
Posted: Wed Nov 17, 2010 2:41 am
by rstevenson
Oh yeah,
that tensor.
Rob
Re: Photon's gravity
Posted: Wed Nov 17, 2010 4:48 am
by neufer
rstevenson wrote:
Oh yeah,
that tensor.
Just be thankful I didn't mention then most "
offal" tensor of them all:
Riemann–Christ
offel tensor as written using Christ
offel symbols:
The
Ricci(-Ticci) tensor is related to the Riemann–Christ
offel tensor:
Re: Photon's gravity
Posted: Wed Nov 17, 2010 9:52 am
by v13
So, if photons cause gravity, how can the gravity propagate along the path of photon's propagation?
Re: Photon's gravity
Posted: Wed Nov 17, 2010 1:24 pm
by neufer
v13 wrote:
So, if photons cause gravity, how can the gravity propagate along the path of photon's propagation?
Photons, like other particles, cause gravity by exchanging
virtual gravitons
mostly perpendicular to the path of the photon's propagation.
Even gravitons cause gravity by exchanging
virtual gravitons
mostly perpendicular to the path of the graviton's propagation.
In any event, such
virtual particles simply do not obey the laws of real particles:
Re: Photon's gravity
Posted: Tue Nov 23, 2010 5:21 pm
by dougettinger
neufer wrote:v13 wrote:
So, if photons cause gravity, how can the gravity propagate along the path of photon's propagation?
Photons, like other particles, cause gravity by exchanging
virtual gravitons
mostly perpendicular to the path of the photon's propagation.
Even gravitons cause gravity by exchanging
virtual gravitons
mostly perpendicular to the path of the graviton's propagation.
In any event, such
virtual particles simply do not obey the laws of real particles:
Re: Photon's gravity
Posted: Tue Nov 23, 2010 5:32 pm
by dougettinger
Please excuse the previous submission. I hit the wrong button at the wrong time.
Virtual particles are an invention to explain force fields. Neither photons or gravitons are virtual particles because they can be detected. So how is a graviton detected ?
I have some questions about virtual lives as shown. When you receive undeserved wealth why does your life path go for a loop? Is a conservation law violated ? What generally happens after the third post-doctorate ? Does the 2.1 children depict a possible quantum entanglement of one birth of a child ?
Doug Ettinger
Pittsburgh, PA
