It is proven. Gravity expands at speed of light

[shade]

Originally posted by GP


They don't have mass? Or don't have rest mass?

Shouldn't they have a mass based on their energy?

euhm if the formulas of the theory of relativity are true then both statements mean the same thing.
(m=(m0/sqrt(1-(v/c)²)) )
... a second hint would be that they move at speed of light...every particle that moves at that speed must have restmass=0 (otherwise their mass would become infinite)...

On the other hand, it might be pretty interesting to have photons with mass/restmass different of zero and moving at the speed of light .

read my following post.

I'm studying my second course Quantum physics
but it doesn't say much about photons.(the course Relativity on the other hand uses the photons a lot...they are used as example for mass-less-fields(and it seems to work)

Shade

[Rogan Josh]

Originally posted by GP


They don't have mass? Or don't have rest mass?

Shouldn't they have a mass based on their energy?

When physicists say 'mass' they invariably mean what you call 'rest mass'. So yes, I mean that they don't have rest mass.

It is a bit of a shame that kids are taught in school that a particle's mass increases with velocity, since it is really not a good way of looking at it. It is much better to think of the mass as fixed (independent of the velocity) and just have the relation between energy and velocity non-linear (actually, velocity isn't the best thing to think of either - momentum is better).

So the energy is E= m c^2 /sqrt(1-v^2/c^2) where m is mass and c is the speed of light. This is a nonlinear relation, and is the best way to think of it.

If you were to define the mass by E=m c^2, then the mass would increase with energy according to:

m= m0/sqrt(1-v^2/c^2)

so it would increase with energy.

(Really it is best to use E^2=m^2 c^4 + p^2 c^2 as SD wrote earlier)

[TCO]

Thanks dude.

[Dauphin]

Originally posted by GP


They don't have mass? Or don't have rest mass?

Shouldn't they have a mass based on their energy?

A question that I want to have answered experimentally is if photons have a gravitational mass (e.g If a matter and anti-matter planet collided, turning everything to photons, what would happen to the gravitation field that they were producing? Does it remain linked to the photons?). If the photons do retain the net effect of the field, then presumably they have an inertial mass as inertial and gravitational masses are equal.

Back on subject, its all about terminology and how you define things. Photons don't have mass, they have momentum and energy from which a mass equivalent can be deduced. It tends to be better not to consider it in terms of mass though.

[Zero]

Originally posted by Rogan Josh

E= 1/2 m v^2 /(1-v^2/c^2)

Why not just type "gamma" instead of writing out the whole v^2/(1-v^2/c^2)? Does anyone know how to do that funky symbol?

actually its been awhile and forgot what gamma is... but theres no need to scare us with drawn out formulas and make it look simpler~

EDIT:Wait was it 1/(1-v^2/c^2)?

[Craig P.]

gamma = g

[TCO]

Originally posted by Sagacious Dolphin


A question that I want to have answered experimentally is if photons have a gravitational mass (e.g If a matter and anti-matter planet collided, turning everything to photons, what would happen to the gravitation field that they were producing? Does it remain linked to the photons?). If the photons do retain the net effect of the field, then presumably they have an inertial mass as inertial and gravitational masses are equal.

Back on subject, its all about terminology and how you define things. Photons don't have mass, they have momentum and energy from which a mass equivalent can be deduced. It tends to be better not to consider it in terms of mass though.

Or just a positron/electron combination.

[TCO]

Originally posted by Rogan Josh


When physicists say 'mass' they invariably mean what you call 'rest mass'. So yes, I mean that they don't have rest mass.

It is a bit of a shame that kids are taught in school that a particle's mass increases with velocity, since it is really not a good way of looking at it. It is much better to think of the mass as fixed (independent of the velocity) and just have the relation between energy and velocity non-linear

Ok. So just so that I have this all down. If I accelerate a mass to relativistic speeds (say an electron in the f orbital of a uranium atom), does it's mass change in terms of gravitational effects? I.e. will it exert a force based on its rest mass or on its "relativistic mass"?

[Rogan Josh]

Originally posted by GP


Ok. So just so that I have this all down. If I accelerate a mass to relativistic speeds (say an electron in the f orbital of a uranium atom), does it's mass change in terms of gravitational effects? I.e. will it exert a force based on its rest mass or on its "relativistic mass"?

Yes. Gravity comes from the energy - not the mass (or to be exact the energy-momentum tensor). It is just that for normal slow moving objects most of the energy is in the mass, so we tend to think of gravity as being generated by the mass.

[Dauphin]

Originally posted by Rogan Josh


Yes. Gravity comes from the energy - not the mass (or to be exact the energy-momentum tensor).

Is that true of potential energy aswell? I guess it must be.

[Rogan Josh]

Originally posted by Sagacious Dolphin
Is that true of potential energy aswell? I guess it must be.

hmmmm..... yes, I think so, but I don't think you would look at it in that light. GR is all about curving space-time so I suppose you are really asking does that curvature of space-time where the source is sitting affect the source's ability to curve spacetime.

I suppose that answer must be yes, but I would have to sit down and calculate the energy-momentum tensor to see......

[TCO]

Originally posted by Rogan Josh


Yes. Gravity comes from the energy - not the mass (or to be exact the energy-momentum tensor). It is just that for normal slow moving objects most of the energy is in the mass, so we tend to think of gravity as being generated by the mass.

hmmm. Well. I guess it is just a question of terms. And I don't want to bog the physics down into philosiphy. But that seems to make it more reasonable to call it mass. Harder to accerate and gives a greater gravity force...walks like a duck, quacks like a duck.

[TCO]

Originally posted by Sagacious Dolphin


Is that true of potential energy aswell? I guess it must be.

Does a compressed spring have more gravity than a relaxed one? Anybody ever test this?

[shade]

Originally posted by GP


Does a compressed spring have more gravity than a relaxed one? Anybody ever test this?

euhm ... now I think of it...potential energy isn't about gravity...it's about the forcefield you're in(wich is in most examples the gravitational field)...but you could also calculate the potential energie of a electric particle in an ellectric field.

Shade

[Dauphin]

Originally posted by Rogan Josh


hmmmm..... yes, I think so, but I don't think you would look at it in that light. GR is all about curving space-time so I suppose you are really asking does that curvature of space-time where the source is sitting affect the source's ability to curve spacetime.

I suppose that answer must be yes, but I would have to sit down and calculate the energy-momentum tensor to see......

If you have a system with all the energy oscillating between kinetic and potential, presumably there should be no variation in gravitational field strengths observed from a suitable distance. Else you have a problem with conserving gravitational potential energy (?).

This leads onto the point - how much of a gravitational effect would the gravitational potential energies of distance galaxies have?