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time traveller d
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i'm just wondering, does light have weight and mass? if so, what is the weight and mass of light?
i don't know 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/s2 (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 traveling a straight line, it would appear to the occupant standing in the spaceship 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.fargoth said:photons don't 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 don't 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...
Ok except for one minor detail. Use two beams coming in at different altitudes in the space ship. Since gravity changes with distance from the center of mass, if's it gravity affecting the beams, then one of them bends more than the other (may nead a really "tall" spaceship to dectect this, but I'm talking hypothetical with infinitely accurate measurements). If it's acceleration, then both beams bend at the same rate.rbj said:now if a beam of light came in through a window of the accelerating space ship, even though that beam is really traveling a straight line, it would appear to the occupant standing in the spaceship 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.
Jeff Reid said:Ok except for one minor detail. Use two beams coming in at different altitudes in the space ship. Since gravity changes with distance from the center of mass, if's it gravity affecting the beams,...
... then one of them bends more than the other (may nead a really "tall" spaceship to dectect this, but I'm talking hypothetical with infinitely accurate measurements). If it's acceleration, then both beams bend at the same rate.
I'm not so convinced about equivalency either. Light may bend [in the region of] 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.
but it's not, I'm referring to the case of a very tall spaceship resting on a large planet. The beams won't bend by the same amount in this case.this accelerating spaceship is out in the middle of nowhere...Originally Posted by Jeff Reid
Ok except for one minor detail. Use two beams coming in at different altitudes in the space ship. Since gravity changes with distance from the center of mass, if's it gravity affecting the beams,...
Jeff Reid said:but it's not, I'm referring to the case of a very tall spaceship resting on a large planet. The beams won't bend by the same amount in this case.
You wouldn't even need light. I could just weigh a known weight and see if the weight decreased with altitude within the spaceship.
Jeff Reid said:Yeah, but my point is that given accurate enough instruments, you can tell the difference between gravity and acceleration.
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 dilation 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.
Ok I sit corrected, but such fixed gravitation fields don't exist in real life.nowhere in this EP is the assumption of a varying graviational field (such as inverse-square). it's a perfectly uniform graviational field.
Jeff Reid said: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).
what if your source of gravity is an infinite plane of mass? would the strength of gravity be different for different distances from that infinite plane?
Jeff Reid said:Yes, from a point source, it's 1/r^2, from an infinitely long line it's 1/r, and from an infinitely wide plane it's constant.
Even so, I can see if I have motion relative to other objects, like the stars (except you took away my windows).
Since, by definition, mass is defined as the ratio of momentum to speed it follows that since light has momentum and speed it therefore must have a non-zero mass. However the proper mass (intrinsic mass) is zero. The force of gravity acts of light if that's what you were wondering. To measure the weight of something you must be able to support the object in a gravitational field. The force required to support the object is called thetime traveller d said:i'm just wondering, does light have weight and mass? if so, what is the weight and mass of light?
Jeff Reid said: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 re-entry, it was going to be a very long and lonely ride.
Robine said:Back to the original question. Light has no rest mass, hence no weight.
Well a photon is at rest relative to itself.rbj said:how did you draw that conclusion? is light "at rest" in any reference frame?
Jeff Reid said:Well a photon is at rest relative to itself.
Photons only exist at the speed of light.
A good answer is http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html" .time traveller d said:i'm just wondering, does light have weight and mass? if so, what is the weight and mass of light?
Jeff Reid said:Do we even know if there is really such a thing as a photon?
... or is it just another case of a digital universe, there is a smallest electrical charge, a smallest step in kinetic energy in electrons, so maybe there's a just smallest amount of light energy, and it's not particle related at all?
No, light does not have weight or mass. In physics, weight is defined as the force exerted on an object due to gravity, and mass is the amount of matter an object contains. Since light has no mass and is not affected by gravity, it does not have weight or mass.
Light is made up of particles called photons, which have zero rest mass. This means that they have no mass even when they are stationary. Additionally, the speed of light is constant, and according to Einstein's theory of relativity, objects with mass cannot reach the speed of light. Therefore, light cannot have mass.
Although light has no mass, it can still be affected by gravity. This is because gravity affects the fabric of spacetime, and light follows the curvature of spacetime. This is known as gravitational lensing, where light is bent by the gravity of massive objects.
Since light does not have mass, it is not affected by the properties of matter like friction or resistance. This allows light to travel through a vacuum at a constant speed of approximately 299,792,458 meters per second, making it the fastest thing in the universe.
Currently, there is no evidence to suggest that light can have mass. However, some theories suggest that photons may acquire a tiny amount of mass in certain extreme conditions, such as in the early universe or near black holes. But for now, it is widely accepted in physics that light has no mass.