Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: daveshorts on 17/12/2006 12:12:41
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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.
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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
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A very interesting proposition although in earth type gravitiational fields the effect woould be extremely small.
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If you could focus enough light is a small enough space, maybe it would self-disappear into a black hole.
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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
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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!!!
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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)?
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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 [:(]
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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