[Rincewind (OP)]

We all know light can be affected by gravity.  Does this mean that a ray of light has a gravitational attraction on massive bodies?  

And does this in turn mean that heat (internal kinetic energy) increases a body's gravitational field, as well as relative velocity?

 
I was just wondering if the effect of various forms of energy (EM, heat, kinetic) on gravity has been tested much, and if anyone could point me in the direction of any resources, or even take a shot at explaining it in your own words.

[ukmicky]

--------------------------------------------------------------------------------
 
 

quote:  We all know light can be affected by gravity. Does this mean that a ray of light has a gravitational attraction on massive bodies?

Andrew light isn't affectrd by gravity its the space that light travels through that gravity affects



Michael                 HAPPY NEW YEAR                    

[Rincewind (OP)]

okay, so does light affect space then? 8l

And apply all the rest of the questions to that (how does energy affect space?)

[ukmicky]

Hi andrew i remember you now, hpoe life's treating you well. and in answer to your question. No

Michael                 HAPPY NEW YEAR                    

[Rincewind (OP)]

How sure are you?  

Life's treating me very well, thanks:)  I hope the same for you.

[Rincewind (OP)]

I mean, for example, a body's kinetic energy does affect it's gravitational effect, that's why mass->infinity as velocity->Speed of Light

And the internal heat of a body is the kinetic energy of its particles, so heat would also affect a body's mass, ay.

[Soul Surfer]

I am particularly interested in the answer to this question myself and have not yet managed to totally satisfy myself whether the mass increase assocated with kinetic energy increases the gravitational effect of a particle.

I have been searching the web for years to find something on relativistic orbit theory ie what to the orbits of objects look like when their velocities become relatavistic but I have found nothing to indicate that they are significantly different.  (perihelion rotation apart)

This lack of any information leads me to believe that relativistic mass increase does produce increased gravitational attraction and this I think would result in orbits looking very much the same in a non relativistic or relatavistic binary.

To return to your original question this implies that as bodies get very hot their gravitational effect inctreases compared with their rest mass if the body was cold. but I'm not sure what the effective temeperature would have to be for a 1% increase.  My guess is many millions of degrees.

Another piece of evidence is that the people who describe extreme energy processes describe gravity becoming on a par with the other forces under extreme conditions and the only way I can see this hapening is that the kinetic energy increases gravitational effect as well as effective mass.

Remember also the individual masses of the quarks that make up a proton have been measured to be significantly less than the proton itself and a considerable part of the mass of the proton is the kinetic energy of the quarks rattling around inside of it.  the gravitational effect of the proton is therefoore determined by its "rest mass" which includes the kinetic energy of thw quarks.

I think that i have now managed to convince myself  []

Learn, create, test and tell
evolution rules in all things
God says so!

[another_someone]

quote:Originally posted by Soul Surfer

I am particularly interested in the answer to this question myself and have not yet managed to totally satisfy myself whether the mass increase assocated with kinetic energy increases the gravitational effect of a particle.

I have been searching the web for years to find something on relativistic orbit theory ie what to the orbits of objects look like when their velocities become relatavistic but I have found nothing to indicate that they are significantly different.  (perihelion rotation apart)

I may be going off beam here, but the above example questions the effect of special relativity upon gravity.  If gravitational effect is effected by relativistic effects, then would not gravity be effected by gravity itself (i.e. the relativistic effect of an object under the influence of a gravitational field might alter the gravitational pull of the object itself).  Would this not mean that two objects under the mutual effect of each others gravitational field would have a combined gravitational pull that was subtly different than the sum of the individual gravitational field of the separate objects?

[Soul Surfer]

Yes that is exactly what I was talking about but the effects do not change the orbit much because the incereased gravitational effect cancels out the relatavistic mass increase if this didn't happen things would accelerate less as the got faster when they are falling into an area with an intense gravitational field because of the relatavistic mass increase

Learn, create, test and tell
evolution rules in all things
God says so!

[another_someone]

quote:Originally posted by Soul Surfer

Yes that is exactly what I was talking about but the effects do not change the orbit much because the incereased gravitational effect cancels out the relatavistic mass increase if this didn't happen things would accelerate less as the got faster when they are falling into an area with an intense gravitational field because of the relatavistic mass increase

Learn, create, test and tell
evolution rules in all things
God says so!

But in any case, when something a body orbits another, or is free falling, in both cases there is no nett gravitational effect, and thus I would expect no relativistic effects should ensue.

It is what something is held in position against a gravitational force (e.g. sitting atop a planet, held up by non-gravitational forces, but experiencing the downward force of gravity) that one would have relativistic effects.

I assume this to be true because of the supposed equivalence between acceleration and gravity, and thus if one does not feel the gravity/acceleration, then it can be having no effect.  An object in stable orbit does not actually have the sense of having any force applied to it.

[Solvay_1927]

But surely an object must be experiencing a force if it's in orbit - otherwise it would not move in an orbit, it would be moving in a straight line.  (Or have I misunderstood what you're saying, George?)

[another_someone]

quote:Originally posted by Solvay_1927

But surely an object must be experiencing a force if it's in orbit - otherwise it would not move in an orbit, it would be moving in a straight line.  (Or have I misunderstood what you're saying, George?)

Is not an object moving in orbit is following the curvature of the space, and thus following the shortest path (not necessarily a straight line, because a straight line is only the shortest path in a euclidean space).

For someone sitting inside a spacecraft in orbit around a planet, without being able to look outside, does not know whether the spacecraft is in orbit, or in freefall, or as if the gravity of the planet had been switched off.  The forces he perceives in all three scenarios are the same.  Thus, as I understand General Relativity, the three scenarios must be absolutely equivelent.

[Solvay_1927]

That's not my understanding. The astronauts can do experiments that will tell them whether they're orbiting or not, can't they? (Whereas no experiment will differentiate between being stationary and moving in uniform linear motion.)

Or have I misunderstood my physics?

[gsmollin]

quote:Originally posted by Rincewind

Q1: We all know light can be affected by gravity.  Does this mean that a ray of light has a gravitational attraction on massive bodies?  

A1: Couching the answer in your original terms, the answer is "yes", but it is not measurable. Photons have mass, given by m=E/c*c, so they experience the same gravitational forces as massive objects, and also generate a gravitational field of their own, just like massive particles. However, the gravitational force is so weak, no one has ever been able to verify this attraction experimentally.

Q2: And does this in turn mean that heat (internal kinetic energy) increases a body's gravitational field, as well as relative velocity?

A2: Also, yes, but we can't measure it for the same reason as above. It would take more heat than we can produce at this time to give elementary particles enough energy to change their masses. We can measure the mass increases than come from high relative velocities in particle accelerators, however. These effects are well documented.
 
Q3: I was just wondering if the effect of various forms of energy (EM, heat, kinetic) on gravity has been tested much, and if anyone could point me in the direction of any resources, or even take a shot at explaining it in your own words.

A3: I don't have a reference available. Certainly any good atomic physics book will discuss relativistic mass change. However, as I said, the gravitational forces are too weak to measure.


"F = ma, E = mc^2, and you can't push a string."

[Soul Surfer]

gsmollin.  Sorry but you've got it wrong there in answer 1.  Photons travel at the speed of light (by definition and by precise observation) and cannot have a rest mass because (except for some very wierd conditions) you can't stop them.  They do however have momentum (because when they hit something and bounce off it reacts just as if massive particle had hit it) and that may be what you are thinking about when you describe them as having mass.  

The simple non relatavistic formula for the momentum of an object is mass x velocity the momentum of a photon is h.nu/c or E/c  which is just as if it had a mass of E as you said but it is not related to a gravitiational mass and photons ignore each other completely gravitationally only being attracted by the gravitational field of massive objects.

I've just done some fag packet calculations on how hot things have to get before the relativistic mass increase becomes significant.

The velocity of typical gas molecules in air at room temperature (300 deg K)  is around 0.5 km/sec  (hydrogen is a bit quicker so lets call it 1km/sec.
The velocity of light is 300,000 km/sec and temperature is proportional to the square of the velocity of the molecules.  molecule velocities have to get over half the speed of light before there is a significant increase in mass say around 10%.so lets say the molecule velocities need to be at least 100,000 times quicker before there is any increase that means that the temperature must be 300-400 times room temperature or around 100,000 degres kelvin.  

The only place you will find temeperatures like this is well down into the middle of stars where the trmperatures reach millions of degrees kelvin

Learn, create, test and tell
evolution rules in all things
God says so!

[Mad Mark]

If research on the relativistic mass increase associated with kinectic energy has not been researched enough then are models of galaxies incorrect?
It could imply that those stars in the outlying sectors of the galaxies exert more gravity than those with less kinetic mass increase further in.
Would this negate the need for dark matter to hold the structure together.

Tomorrow lies outside our universe without it there would be no tomorrow.

[Rincewind (OP)]

That's the exact reason I asked the original question Mark - you beat me to it!  Dark energy as well - it's cold and quiet out there in between galaxies.

The thing is, in relativity energy is basically mass, but I wonder if maybe energy has a different gravitational effect than the equivalent amount of matter.  I mean it's not like we can tell how much of the sun's gravity comes from its mass and how much from its heat, we can just measure the net force.

Has anyone got the answer or thought about it?

Thanx for the calculations btw soulsurfer, it would have taken me ages to work that out (I don't have enough equations or facts accesible, ie in my brain).

[Rincewind (OP)]

OK, that sounds a bit weird because everyone knows the most fundamental particle of matter is a photon.  But maybe there're layers, or something?  Light is affected a bit, sub atomic particles a bit less. Atoms a bit less than that (per unit energy, that is).  The 'knot' in energy that makes it into matter (allows it to have mass at rest) decreses the gravitational effect.

[philo]

The energy and momentum of light also generates curvature of space-time so according to theory it can attract objects gravitationally.

newbielink:http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html [nonactive]

[philo]

Gravity is produced by energy, and since massive objects have such a large amount of energy (remember Einstein), the largest gravitational effects come from massive objects. But energy (ALONE, i.e. massless photons - my comment), such as the photons from light, also produce gravity.

newbielink:http://www.fnal.gov/pub/inquiring/virtual/aas_transcript-01-29-04.html [nonactive]