[daveshorts (OP)]

I got asked this quesiton on e-mail:

Have you tested that photons attract a planet? If not, why do you think they do?
 I hope you can answer my questions,

I personally haven't, I was never that kind of physicist, however astronomers have noticed gravitaional lensing, this is where light is deflected by a massive body, like a planet or a star producing a distorted view behind.

I am fairly sure that the gravitational force from a bunch of photons has not been measured but

A photon has momentum so it it is changing in direction it must have experienced a force, and as far as we know forces are allways equal and opposite, as otherwise all sorts of strange things could happen. like you could build a rocket that needed no reaction mass and thus created energy.

eg If you had two mirrors with light bouncing between them attached onto one side of a mass, the photons are definitely attracted to the mass so they will hit the outer mirror less hard than the inner one creating a net force, if the mass is not attracted to the photons the whole object will experience a net force and accelerate.
[diagram=46_0]
This seems unlikely and I am sure it has consequences that could be measured. So the odds are photons attract masses the same amount as they are attracted to the masses.

[lightarrow]

According to GR, not only mass curves space, energy and momentum too:

G_mu_nu = 8π T_mu_nu.

G_mu_nu is the curvature tensor (it gives the space-time curvature)
T_mu_nu is the stress-energy-momentum tensor (it gives the mass, energy and momentum density)

So light do produce a gravitational field.

http://en.wikipedia.org/wiki/Einstein_equation

[Soul Surfer]

A very interesting proposition although in earth type gravitiational fields the effect woould be extremely small.

[Atomic-S]

If you could focus enough light is a small enough space, maybe it would self-disappear into a black hole.

[ghh]

T_mu_nu is the stress-energy-momentum tensor (it gives the mass, energy and momentum density)

So light do produce a gravitational field.

I did this in my "one particle" (see new theories)
the gravitational force exerted by a photon is proportional to h/c^2=m. so mm/r^2 does not become significant unless the wavelength is very small r = ~10^-21 metres (light is 10^-6)
Graham

[McQueen]

You are right man, gravity is just photons!!! Just think about it the gravitational force is 10⁴⁰ times less than the electromagnetic force. Have you any conception of what that number means??? It has to do with some kind of virtual photon alignment and nothing else!!!

[another_someone]

If you could focus enough light is a small enough space, maybe it would self-disappear into a black hole.

The trouble is that the black hole that is that small would evaporate/explode almost as quickly as you create it.

The other problem is that you would need a very short wavelength of EM radiation to be able to keep the photons contained in the space - that is a great deal of energy (although that higher energy would also have a greater mass).

One interesting question arising from this is, given the weakness of gravity, at what energy would the gravity of a photon become significant (although I would suspect that photons at that energy would have sufficient energy to be creating matter/anti-matter pairs)?

[DoctorBeaver]

If you could focus enough light is a small enough space, maybe it would self-disappear into a black hole.

One interesting question arising from this is, given the weakness of gravity, at what energy would the gravity of a photon become significant (although I would suspect that photons at that energy would have sufficient energy to be creating matter/anti-matter pairs)?

I would imagine the answer would be the Planck mass. I say that because, as far as I'm aware, it would have to act at the Planck scale in order for the EM force & gravity to interact directly, and that means you would need Planck scale mass/energy.

Now tell me I'm a dumbo and I've got that totally wrong  []

[Soul Surfer]

It is generally accpted that there is an upper limit to photon energy when the energy of the photon effectivley vanishes into a black hole but I can't remember exactly what that energy is but its around the planck length and time