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Does light have weight and mass? 
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#1
Jan2206, 08:45 AM

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i'm just wondering, does light have weight and mass? if so, what is the weight and mass of light?



#2
Jan2206, 09:34 AM

P: 400

photons dont have a rest mass, they do however have momentum, [tex]E=hf=pc[/tex]
where f is the freq of the light, h is plank's constant, p is the momentum and c is the speed of light. i dont know GR so well, but there is an effect of space bending near mass which makes it look as if light is attracted by gravity... someone else here might know more and elaborate on this... i also know of an experiment which showd that if you emmit light upwards from earth, it undergoes redshift as if it had to invest energy for the potential energy of graviation it gained. 


#3
Jan2206, 07:31 PM

P: 52

Light doesn't have rest mass (it can only travel at c) but it has energy and GR says that energy and mass curves and warps spacetime; although a photon would only curve it very very slightly i guess you could say that it does have some kind of mass ONLY in the sense that it curves the fabric of spacetime other than that it has no rest mass



#4
Jan2206, 08:57 PM

P: 2,251

Does light have weight and mass?
i dunno GR super well either, but one of the "equivalence" thought experiments of Einstein was that of a person standing in a spaceship that is accelerating in the direction that his/her head is pointing by 9.8 m/s^{2} (the acceleration of gravity of the earth), then, according to the equivalence hypothesis, that person cannot tell the difference between that and standing on the earth's surface. now if a beam of light came in through a window of the accelerating space ship, even though that beam is really travelling a straight line, it would appear to the occupant standing in the space ship to be curving "downward" (toward his/her feet) because he/she is accelerating "upwardly". now if the equivalence principle is true, than the corresponding observer standing on the earth's surface would see the same downward curvature of a corresponding beam of light implying that gravity is acting on it as if it has weight. 


#5
Jan2206, 09:24 PM

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P: 17,210

I considered this question quite a bit earlier. I could probably dig up my math somewhere. My idea was to (theoretically) get a perfectly reflecting box and trap the light in there so I could try to weigh it. I worked out the math based on the momentum of the photons and found that it had extra inertia and weight exactly as though it were filled with a gas of the mass you would expect from E = mc^2.
Dale 


#6
Jan2206, 10:24 PM

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I'm not so convinced about equivalency either. Light may bend around a massive object simply because space is warped. It's also possible that a strong gravtiational field attracts some form of dark matter or energy (stuff we can't detect) that simply refracts the light. 


#7
Jan2206, 10:28 PM

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The other issue with photons, is that for them, the speed of light is not constant from their frame of reference. Each photon "sees" it's own speed of light as zero, but "sees" other photons from other beams as nonzero.



#8
Jan2206, 10:41 PM

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#9
Jan2206, 10:58 PM

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You wouldn't even need light. I could just weigh a known weight and see if the weight decreased with altitude within the spaceship. 


#10
Jan2206, 11:28 PM

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#11
Jan2206, 11:43 PM

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Yeah, but my point is that given accurate enough instruments, you can tell the difference between gravity and acceleration. I don't like the space ship analogy because there is a way to detect the difference in this case, even with single beam. Just coat the interior with mirros and the rate of bend changes as the beam move towards or away from a gravity source.
Equivalency should be explained as follows: that an infinitely thin beam of light bends the same amount in a gravity field as it would if observed from an accelerating reference point where the acceleration would generate the same force on objects as the gravitaional field. If the beam isn't inifitely thin, then the difference in strength of the gravitational field would spread the beam a bit. This also ignores the fact that during acceleration, time and distance dialation occurs, and this would affect the results as well. Probably just easier to state that light is affected by gravity by the same amount that mass would be if the mass could travel at the speed of light. 


#12
Jan2206, 11:45 PM

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#13
Jan2206, 11:49 PM

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I don't even need accurate instruments. I can see if I'm standing on the Earth or out in a spaceship accelerating. It's obvious you can tell the difference. I don't think that's the point here. The point is that light is affected by gravitational fields.
Now if you're referring to the general concept of equivalency, which doesn't involve light at all. Then the effects of on a mass from a gravitation source is the equivalent of acceleration. But still there's the issue that gravitation fields vary in strenth depending on distance from the center of the source (and assuming your outside the surface of the source). Would the gravity field from an ifinitely large flat plane do the job? 


#14
Jan2306, 12:09 AM

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#15
Jan2306, 12:25 AM

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Even so, I can see if I have motion relative to other objects, like the stars (except you took away my windows). 


#16
Jan2306, 12:38 AM

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#17
Jan2306, 01:16 AM

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Ok, I get your point, but to the Apollo 10 crew, approaching the earth at 24,861 mph, it was pretty clear to them that they were rapidly approaching the earth. They stated that they could see the rate of apparent increase in size of the earth as they got close. Unlike the shuttle, this is pretty close to escape velocity and if they messed up on aiming for reentry, it was going to be a very long and lonely ride.



#18
Jan2306, 03:51 AM

P: 2,954

"weight" of the object. You can't support a beam of light, only a gas of photons in a box may be weighed. The result is the weight of the box + the weight of the gas. Pete 


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