10-01-2003 21:50									#61
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quote: 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 .

quote: 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

	10-01-2003 21:57									#62
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quote: 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)

	10-01-2003 22:07									#63
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Thanks dude.

	10-01-2003 22:08									#64
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quote: 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.

	10-01-2003 22:13									#65
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quote: 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)?

	11-01-2003 00:06									#66
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gamma = g

	11-01-2003 01:02									#67
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quote: 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.

	11-01-2003 01:10									#68
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quote: 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"?

	11-01-2003 02:21									#69
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quote: 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.

	11-01-2003 02:32									#70
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quote: 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.

	11-01-2003 03:50									#71
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quote: 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......

	11-01-2003 04:30									#72
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quote: 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.

	11-01-2003 04:44									#73
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quote: 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?

	11-01-2003 04:52									#74
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quote: 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

	11-01-2003 05:02									#75
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quote: 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?

	11-01-2003 06:55									#76
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quote: Originally posted by shade 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

The potential enrgy involved in different combinations of nucleons has measurable mass effects. Think chemical would be the same but real small.

	12-01-2003 11:40									#77
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Lars:

It's an assumption based on many, many posters on space.com who vaguely remembered or had heard about the S o L changing theory.

It always came back to van Flandern, usually to parts of his article copied onto different sites with the name changed to stop people from immediately disregarding it (Yes, Creationists ARE that low).

He starts with his explanation of why the Michelson-Morely experiment was wrong and goes from there, and he has a chart of different 'experiementally derived' values for c over time.

Maybe you have read something else I haven't heard of, but in 99% of posts similar to yours, the culprit was ultimately van Flandern, usually 'repackaged' to fake credibility, which fools a lot of people.

	12-01-2003 11:48									#78
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Great Clarification on 'energy creates gravity', RJ. Thanks.

	12-01-2003 12:07									#79
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quote: Originally posted by Seeker Lars: It's an assumption based on many, many posters on space.com who vaguely remembered or had heard about the S o L changing theory. It always came back to van Flandern, usually to parts of his article copied onto different sites with the name changed to stop people from immediately disregarding it (Yes, Creationists ARE that low). He starts with his explanation of why the Michelson-Morely experiment was wrong and goes from there, and he has a chart of different 'experiementally derived' values for c over time. Maybe you have read something else I haven't heard of, but in 99% of posts similar to yours, the culprit was ultimately van Flandern, usually 'repackaged' to fake credibility, which fools a lot of people.

Maybe Flandern's stuff is just modified? copies of stuff he found somewhere. Maybe he even got it from credible sources, but there's no references on his site?

	12-01-2003 12:20									#80
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True, true, and double true!

In which case you could have seen something earlier that he mangled, or possibly something he used, and then a piece of his stuff that was used by something else, like a telephone game of pop. sci.

	12-01-2003 17:52									#81
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quote: Originally posted by GP Or just a positron/electron combination.

Sure, but the idea of matter & anti-matter planets colliding is more fun for the minds eye.

	12-01-2003 22:38									#82
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quote: Originally posted by Sagacious Dolphin Sure, but the idea of matter & anti-matter planets colliding is more fun for the minds eye.

As a nuclear geek, I prefer the positron death spiral (oh...the sad fate of being born into an electron world.) Mix in some Compton scattering and I'll really get off.

	13-01-2003 01:13									#83
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Btw, are cosmologists already positive on the fact that there is more matter than antimatter?

Could it be that there is a place where galaxies of anti matter escape into the vast distance of space, and matter is considered to be something of an exotic feat?

	17-01-2003 01:40									#84
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Speed of Gravity Results 'Incorrect,' Physicist Says

http://space.com/scienceastronomy/g...eed_030116.html

	17-01-2003 02:52									#85
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quote: Originally posted by Eli Speed of Gravity Results 'Incorrect,' Physicist Says http://space.com/scienceastronomy/g...eed_030116.html

I feel very justified in pushing for a tangible explanation of what was actually measured. Whether it is a business thang or a science thang, it often pays to unravel the ball of yarn to understand exactly what is going on.

It sounds like the matter is still in dispute. I would love to at least get a geometric desciption of what was measured.