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- Thread starter neh4pres
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1) gravitational time dilation should slow light (think refractive index).

2) it isnt said to be massless

- #3

Fredrik

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Photons are massless. Massless particles always move with speed c on the straightest possible path through space-time, but the geometry of space-time around near a star is such that the corresponding path through space is actually a curved path in space.

I think there are circular paths around a black hole that light can actually follow (in principle), but those orbits are unstable. If the photon isn't going in exactly the right direction, it would either fall in or move away from the black hole.

I think there are circular paths around a black hole that light can actually follow (in principle), but those orbits are unstable. If the photon isn't going in exactly the right direction, it would either fall in or move away from the black hole.

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if light can be affected by gravity then why is it said to have no mass?

Quote from en.wikipedia.org/wiki/Relativistic_mass: "If an object is moving at the speed of light, it is never at rest in any frame. In this case the total energy of the object becomes smaller and smaller in frames which move faster and faster in the same direction. The**rest mass** of such an object is zero, and the only mass which the object has is **relativistic mass**-- a quantity which depends on the observer."

Photons don't have rest mass like subatomic particles (protons, neutrons, electrons), but do have relativistic mass, which is why light is affected by gravity.

.. if a black holes gravity was perfectly fixed wouldent there be a infinitaly small layer where light orbits in a perfect sphere around the black hole??

I am not sure what you mean by 'perfectly fixed gravity', but light can get trapped in a circular orbit by the gravity of a black hole based on the trajectory of the light entering it. See this: en.wikipedia.org/wiki/Blackhole#Photon_sphere

Quote from en.wikipedia.org/wiki/Relativistic_mass: "If an object is moving at the speed of light, it is never at rest in any frame. In this case the total energy of the object becomes smaller and smaller in frames which move faster and faster in the same direction. The

Photons don't have rest mass like subatomic particles (protons, neutrons, electrons), but do have relativistic mass, which is why light is affected by gravity.

.. if a black holes gravity was perfectly fixed wouldent there be a infinitaly small layer where light orbits in a perfect sphere around the black hole??

I am not sure what you mean by 'perfectly fixed gravity', but light can get trapped in a circular orbit by the gravity of a black hole based on the trajectory of the light entering it. See this: en.wikipedia.org/wiki/Blackhole#Photon_sphere

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- #5

Fredrik

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The red stuff should be included too. This is the reason I've never liked this explanation.Photons don't have rest mass like subatomic particles (protons, neutrons, electrons), but do have relativistic mass, which is why light is affected by Newtonian gravity in the framework ofspecialrelativity.

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actually, you're also wrong about the energy not escaping. energy is slowly leaked out of black holes, and the black holes themselves destingerate after immensly long period of time.

An, the light affected by gravity is explained in the local frame/closed rocket acceleration thought expiriment done by einstein. it also explains y planets close to the sum have time differences in their orbits which can't be explained unless it's the theory of genral relativity.

An, the light affected by gravity is explained in the local frame/closed rocket acceleration thought expiriment done by einstein. it also explains y planets close to the sum have time differences in their orbits which can't be explained unless it's the theory of genral relativity.

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DrGreg

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Remember the old experiment attributed to Galileo where he dropped heavy and light objects off the Tower of Pisa and they all fell at the same rate? The effect of gravity is independent of the mass of the object (for tiny objects near huge planets) so even something with zero mass should (and does) accelerate the same way.if light can be affected by gravity then why is it said to have no mass?

It is possible for light to orbit a black hole, but how would it get there? Any light approaching the hole would be on the wrong trajectory to orbit -- it would either fall in or else pass by and escape. Something (e.g. a collision) would have to move it onto a different trajectory. Only a small number of photons would get captured this way -- and after capture could later escape by the same mechanism -- so there wouldn't be an intense "belt" round the hole.and sience light is affected by gravity.. if a black holes gravity was perfectly fixed wouldent there be a infinitaly small layer where light orbits in a perfect sphere around the black hole?? this light would never be seen because it will never escape or plummet but would probably be intence enough to vaporize anything that passes through it...

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Remember the old experiment attributed to Galileo where he dropped heavy and light objects off the Tower of Pisa and they all fell at the same rate? The effect of gravity is independent of the mass of the object (for tiny objects near huge planets) so even something with zero mass should (and does) accelerate the same way.

It is possible for light to orbit a black hole, but how would it get there? Any light approaching the hole would be on the wrong trajectory to orbit -- it would either fall in or else pass by and escape. Something (e.g. a collision) would have to move it onto a different trajectory. Only a small number of photons would get captured this way -- and after capture could later escape by the same mechanism -- so there wouldn't be an intense "belt" round the hole.

Think of it this way. If an observer went to the point outside a black hole where the gravity was at the right amount that photons could orbit. Can you see light. Yes. and if you move along this sphere so as to position a specific star at they proper angle so that its photons are being caught you would see them enter their orbit. the preceding was only for thought. Light comes from every star at every conceivable angle, so of course you should have a steady flow filling this sphere of light. Read about the photon sphere on this page http://en.wikipedia.org/wiki/Black_hole

- #9

DrGreg

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Think of it this way. If an observer went to the point outside a black hole where the gravity was at the right amount that photons could orbit. Can you see light. Yes. and if you move along this sphere so as to position a specific star at they proper angle so that its photons are being caught you would see them enter their orbit. the preceding was only for thought. Light comes from every star at every conceivable angle, so of course you should have a steady flow filling this sphere of light. Read about the photon sphere on this page http://en.wikipedia.org/wiki/Black_hole#Photon_sphere

Light hitting the photon sphere from outside wouldn't normally get captured, it would pass straight through it, spiralling into the black hole. Only those photons that are already in orbit within the photon sphere are at the right angle to remain in orbit. For a photon to be captured it would have to change direction, perhaps by collision, or by the influence of another massive object passing by. More likely, a photon would be produced as a result of particles colliding just as they pass through the photon sphere. This would be only a slow trickle of photons being added.

More importantly the orbits are "unstable" which means only those photons travelling in

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if light can be affected by gravity then why is it said to have no mass?

Quote from en.wikipedia.org/wiki/Relativistic_mass: "If an object is moving at the speed of light, it is never at rest in any frame. In this case the total energy of the object becomes smaller and smaller in frames which move faster and faster in the same direction. Therest massof such an object is zero, and the only mass which the object has isrelativistic mass-- a quantity which depends on the observer."

Photons don't have rest mass like subatomic particles (protons, neutrons, electrons), but do have relativistic mass, which is why light is affected by gravity.

This is incorrect. Light is affected by gravity, not because it has relativistic mass.

The reason is space-time is curved and light has to follow the curved path, called geodesic

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Does this mean that http://uk.answers.yahoo.com/question/index?qid=20080418035618AACWgjD&show=7" is wrong?

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Does this mean that http://uk.answers.yahoo.com/question/index?qid=20080418035618AACWgjD&show=7" is wrong?

If you agree with her/him, you will get only half of the bending, which Einstein did for his first try.

You will get the same answer using Newton's theory of gravity.

After Einstein developed GR, he use GR to calculate the deflection of light, he got the right answer which is twice as large. This is due to curved space

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everybody has mass

we can calculate his total energy from

the equation:

E=mc^2

if light has mass

we can calculate his Energy from: E=mc^2

and his speed is v=c

when a mass is moving a speed of light it will be infinited

that means E=infinite energy (by Joule) and that impossible of course

so....we calculate Energy of light from E=hv

h=planck constant

v=frequency of light electromagnetic waves

so the light has body with no mass! (Specifically the photon)

we can calculate his total energy from

the equation:

E=mc^2

if light has mass

we can calculate his Energy from: E=mc^2

and his speed is v=c

when a mass is moving a speed of light it will be infinited

that means E=infinite energy (by Joule) and that impossible of course

so....we calculate Energy of light from E=hv

h=planck constant

v=frequency of light electromagnetic waves

so the light has body with no mass! (Specifically the photon)

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Why do you think that is a contradiction?if light can be affected by gravity then why is it said to have no mass?

Mass affects the curvature of spacetime. The worldline of unaccelerated obects follow the geodesics of spacetime. It is not the case that if they have no mass they can ignore those geodesics.

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photons belong to the bosons

the bosons have spin=0,1,2...........

like gluons (weak nuclear force)

i dont know how photons affect by gravitational fields...

i didnt read the General Relativity to answer this...

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Actually it is quite correct. Both Einstein and Feynman stated this explicitly themselves.This is incorrect. Light is affected by gravity, not because it has relativistic mass.

That is not a reason. That is a description. And even that description is in accurate since its possible to have a gravitational field in the absense of spacetime curvature (e.g. a uniform gravitational field has zero spacetime curvature). It is important to understand that light has gravitational mass because without that knowledge one has no reason to believe that light is deflected in a gravitational field. The fact that light has gravitational mass is the reason Einstein was led to postulate the equivalence between a uniform gravitational field and a uniformly accelerating frame of reference.The reason is space-time is curved and light has to follow the curved path, called geodesic

I recommend that you look this up in Eintein's book "The Evolution of Physics" by Einstein and Infeld. I'm out of town for the next week or two so I can't tell you where in that book Einstein says that. When I get back home I can give you the precise reference and quote.

Best wishes

Pete

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Fredrik

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1. We can prove that a massive particle at rest has an energy mc

2. We can define a velocity-dependent "relativistic mass" such that the energy of a massive particle is

3. This suggests that

4. A photon must have some energy, so if we were right in step 3, then light should be affected by Newtonian gravity too.

This doesn't really have anything to do with GR. Saying that the world line of a ray of light is a null geodesic is definitely a much better "explanation" of why the path of light is bent by a heavy object than the idea that it's the energy/c

I'm also surprised that you're characterizing the GR explanation as a "description" and the SR+Newton explanation as the "reason" why gravity bends light.

If the relativistic mass of a photon is the reason why it's affected by gravity (as defined by GR), then its world line would be a time-like geodesic, not a null geodesic.

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I disagree of course. The assertion "only made sense" implies that the arguement for it can be proven to be wrong and I certainly don't believe that is true. In fact I know of no valid arguement which could possibly prove it wrong. Especially since it makes perfect sense. And just because that reasoning is what led to GR it does't mean that it can or should be dismissed after its discovery. Each of us is different and as such we all think differently. We all arrive at ideas along different paths. To say that one way of thinking is right and another, equally valid line of thinking, is wrong is a very wrong statement. In fact it is for that reason that both Feynman and Einstein presented those arguements.Pete, thinking about it that way only made sense before GR was discovered.

Let me ask you this: If it indeed makes no sense then why do you think Einstein and Feynman explained the defection of light using the mass-energy equivalence arguement? Let me quote them

This same sentiment was expressed by Feynman in Feynman Lectures Vol - I, page 7-11. Section entitled Gravitation and RelativityBut there is, fortunately, a grave fault in the reasoning of the inside observer, which saves our previous conclusion. He said: “A beam of light is weightless and, therefore, it will not be affected by the gravitational field.” This cannot be right! A beam of light carries energy and energy has mass.

One feature of this new law is quite easy to understand is this: In Einstein relativity theory, anything which has energy has mass -- mass in the sense that it is attracted gravitationaly. Even light, which has energy, has a "mass". When a light beam, which has energy in it, comes past the sun there is attraction on it by the sun.

Of course it does. What possible reason could you have for saying otherwise?The reasoning would have to go something like this:

1. We can prove that a massive particle at rest has an energy mc^{2}.

2. We can define a velocity-dependent "relativistic mass" such that the energy of a massive particle isalwaysmc^{2}.

3. This suggests thatmaybeit's E/c^{2}that should go into Newton's law of gravity, not the rest mass.

4. A photon must have some energy, so if we were right in step 3, then light should be affected by Newtonian gravity too.

This doesn't really have anything to do with GR.

By the way, Einstein never proved that a body at rest has energy. When Einstein first derived the expression E = mc

Why do you consider that a "better" explanation??Saying that the world line of a ray of light is a null geodesic is definitely a much better "explanation" of why the path of light is bent by a heavy object than the idea that it's the energy/c^{2}that should go into Newton's law of gravity instead of the mass.

In any case it can't be taken as an explaination since it is a description of the phenomena and not an explaination. Its quite wrong to consider things like this as "explanations" since laws of physics are formulated, not to explain the phenomena in nature that we observe, but to

Me? I wish I could take the credit but this is due to Einstein and not myself. Eddington himself explained that GR is not an explanation of gravity but a description of it. This must be kept in mind so that one has a good understanding of the relationship between mass and gravity in both its active and passive aspects.I'm also surprised that you're characterizing the GR explanation as a "description" and the SR+Newton explanation as the "reason" why gravity bends light.

Because timelike geodesics are for particles which have a non-zeroIf the relativistic mass of a photon is the reason why it's affected by gravity (as defined by GR), then its world line would be a time-like geodesic, not a null geodesic.

I hope you're not confusing the notion of rest mass with the gravitational mass of light?

By the way. The

Best wishes

Pete

ps - By the way. One can go on and on about this and never get past semantics and opinions. If this thread gets to that point then I'll bow out.

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- #19

DrGreg

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I think some of the contributions to this thread are missing the point. If we are talking about a **tiny** mass near a **huge** mass (say a 1 kg mass near a planet) then the mass of the small object is irrelevant. A 1 kg mass, a 1 g mass, or a 1 nanogram mass all behave identically -- they all follow the same geodesics generated by the large object and we can ignore any minuscule spacetime curvature introduced by the small object. By that logic, a zero-proper-mass object should also behave identically (the only difference being a difference in velocity). It doesn't matter whether that object has some other sort of "mass" or not. For example, a high-energy high-frequency photon follows the same path as a low-energy low-frequency photon (in vacuum), energy being a frame-dependent concept.

The mass of the "small" object only becomes relevant when it is much larger, e.g. another planet.

The mass of the "small" object only becomes relevant when it is much larger, e.g. another planet.

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Exactly!I think some of the contributions to this thread are missing the point. If we are talking about atinymass near ahugemass (say a 1 kg mass near a planet) then the mass of the small object is irrelevant. A 1 kg mass, a 1 g mass, or a 1 nanogram mass all behave identically -- they all follow the same geodesics generated by the large object and we can ignore any minuscule spacetime curvature introduced by the small object. By that logic, a zero-proper-mass object should also behave identically (the only difference being a difference in velocity). It doesn't matter whether that object has some other sort of "mass" or not.

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This is the essential difference between Newton theory and GR

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This is the essential difference between Newton theory and GR

To a local observer the speed of light is constant everywhere in the gravitational filed and to a coordinate observer who has the "big picture" the speed of light speeds up as it climbs out of a gravitational field. I think Newton thought of a photon as a "corpuscle" (with mass?) so presumably Newton would have predicted the opposite (the light would slow down as it climbs out of a gravitational field).

- #23

Fredrik

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The assertion is that gravity bends lightThe assertion "only made sense" implies that the arguement for it can be proven to be wrong and I certainly don't believe that is true. In fact I know of no valid arguement which could possibly prove it wrong.

I have a hard time believing that it did.And just because that reasoning is what led to GR...

The reason is of course that we're obviously talking about Newtonian gravity in the framework of special relativity. This doesn't even suggest that space-time might be curved. There certainly is no obvious connection to GR. The insight that's needed to take the step from SR to GR is that objects in free fall follow geodesics, and that there might be a way for this to remain true even when gravity is included. I really don't see how the idea that maybe E/cOf course it does. What possible reason could you have for saying otherwise?

Do you realize that you're asking why I consider GR a better theory than SR+Newton? (And that the double question marks suggests that you think it's absurd to do so). GR is a better theory than SR+Newton for the same reasons that "the Earth is round" is a better theory than "the Earth is flat". (It agrees with experiments to a higher degree).Why do you consider that a "better" explanation??

That's not the only reason why I consider GR better, but it's the only one that I'm sure is objectively true. I think these may be valid reasons too, but there is some subjectivity in all of them: GR is simple and beautiful. It's just one theory, and not two theories glued together like SR+Newton. It doesn't require action at a distance.

This is junior high school semantics, and I'm very familiar with it. What you're saying is certainly true in the sense that any explanation leads to more questions, so nothing can really be explained. "Description" may be a more appropriate word than explanation, but if we're going to be that pedantic we might as well eliminate the word "explanation" altogether, since there are no better explanations around for...not to explain the phenomena in nature that we observe, but to describe the phenomena...

Anyway, you seem to have misunderstood which one of your statements in #16 I objected to. I don't mind so much that you described GR as a "description" rather than an "explanation". That's just a matter of semantics. The thing is, you did this immediately after saying that light is affected by gravity

I'm not saying that you don't understand these things well enough. I'm just saying that you chose your words very poorly.

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I don't see how that would be neccesary. Seems to me that you're using Newtonian intuition. In any case the speed of light

This is the essential difference between Newton theory and GR

For now I'd like to retract that since I only saw Einstein state that explicitly in his book which was written much later. He did however base the equivalence principle on the fact that all matteer falls at the same rate regardless of its mass. If Einstein believed that light didn't have mass then I see no reason for him to state the equivalence principle. In Einstein's 1907 paper in which he announced the equivalence principle he argued for the deflection of light based on the variation of speed with gravitational potential. I believe that Einstein derived the fact that matter moves on geodesics by using the equivalence principle.Fredrik said:I have a hard time believing that it did.

There is nothing in any comment that I've read or posted that would indicate that we're talking about Netonian gravity. And the presence of tidal forces is only required when there are tidal gradients. In, e.g., a uniform gravitational field there are no tidal forces and thus no spacetime curvature. Yet there is a gravitational field in which light and all other matter is affected by gravity.Fredrik said:The reason is of course that we're obviously talking about Newtonian gravity in the framework of special relativity. This doesn't even suggest that space-time might be curved.

Then I guess there is no reason for me to continue.Fredrik said:This is junior high school semantics, and I'm very familiar with it. ...

Anyway, you seem to have misunderstood which one of your statements in #16 I objected to. ...

I'm not saying that you don't understand these things well enough. I'm just saying that you chose your words very poorly.

Pete

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You mentioned spacetime isn't curved in a uniform gravitational field. What's that? Seems to me like anything producing a gravitational field inherently causes a 'gravitational gradient' with distance. The only circumstance under which I can imagine a uniform field is at the center of a symmetrically oriented and moving group of bodies, like for instance a tetrahedron shape of 4 equal-mass bodies all slowly traveling, spinning and co-orbiting together, where the uniform field would be at the very center, equidistant from each, although for 4 bodies to be oriented that way, much less to have equal mass, would be a long damn shot. Also the 'center' would be a 'snapshot' and not really legitimate. Is there any other way to get a uniform field and thus 'flat' spacetime?

-Gerrit

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