If photons in motion have no mass, then how can gravity bend light ?

Question

Would like to see some reference for any explanations. Either light doesn't bend or moving photons DO have mass. One of them must be wrong, yes?

Answer 1

Basically because "gravity" consists of changes in the shape of space. The path of the photons isn't really bending, it's just following the shape. To us it looks like it's bending because we can't perceive the shape of space *other than* by looking at what it does to light.



Remember that Newton didn't describe how gravity worked - he just made an observation about what it seemed to be doing.

Answer 2

There are two ways of explaining this. One way is the way Richard Feynman explained it in his Lectures and that is because since a photon has energy it has an equivalent mass. The second is to use Einstein's equivalence principle which says that a particle will fall at a rate which is independent of the mass of the particle. Mind you that the general relativity explanation has nothing to do with spacetime curvature because you can have a gravitational field in the absence of spacetime curvature (i.e. tidal forces) such as in a uniform gravitational field. In fact that's the first example of a gravitational field Einstein used. Think of what an observer in an accelerating frame sees as a photon flies past. For him the photon's trajectory is bent down as if there's a gravitational force acting on it.

Answer 3

Mark -



The way that Einstein thought about this was in his classic thought experiment that resulted in the Equivalence Principle. You may be aware that gravity and acceleration are equivalent. You can think of this in the following terms:



A space ship (Einstein used an elevator) with no windows and no instruments sits on the surface of the earth. The pilot stands on the floor and feels 1G acceleration pressing against the soles of his feet due to the gravitational field of the earth. (ADDED: If he drops an object - any object - it will fall at 9.8 m/sec^2 toward the floor).



The same ship accelerates at 1G in empty space, far from any star or galaxy. The pilot again feels 1G against his feet due to the acceleration of the spacecraft. (ADDED: A dropped object will again fall at 9.8 m/sec^2 to the floor).



With no windows and no instruments, the pilot has no way to tell the difference between these two conditions. The insight that Einstein provided is that he cannot tell the difference because THERE IS NO DIFFERENCE. Gravity and acceleration are two ways of looking at the exact same thing.



Now, think of the pilot shining a flashlight across the cockpit as he accelerates upward. If the acceleration is extreme enough, or if the space ship is vast enough, the pilot will see that as he accelerates upward, he will leave the light beam slightly behind. It will not hit the far wall of the cockpit at the same height at which he is holding the flashlight. In his accelerated frame of reference, he will see the light beam curve downward.



Since gravity is equivalent to acceleration, gravity will have the same effect and will therefore also bend the light beam downward. The "mass" or lack of mass of the photons does not play a role at all. It is the way the universe is affected by gravity.

Answer 4

This is the classic question addressed by Einstein when trying to find a way to verify his Theory of Relativity that proposes that gravity distorts the fabric of time-space rather than acting like a simple Newtonian force which is inversely proportional to distance. The light must still travel through space-time and so even though it is not accelerated by the gravity as an object with mass would be, its direction is still deflected by the distortion of space-time due to the gravitational field. Einstein's Theory was a departure from Newtonian Laws of Motion which predicted that just as you have stated, since the photons had no mass, they would be undeflected by gravity. Proof of the deflection of starlight by the gravitational field of the Sun measured during a total solar eclipse was the first definitive verification of Einstein's revolutionary Theory.

Answer 5

"If something gets pulled by gravity it means it has mass" No. It means the photon is affected by the shape of space-time. The real problem in a black hole is the stretching of space-time itself as it falls into the singularity (for a simple black hole, it is either a point or a circle, depending). The rate at which the "fabric" of space stretches inside a black hole is such that the distance between the photon and the event horizon keeps increasing, even though the photon is moving outwards at the speed of light (relative to its local space).

Answer 6

A photon can move a solar sail so it must have mass.

A photon not in motion can't exist,it must be in a state of acceleration or at terminal velocity.

Answer 7

I just wanna quickly refer to a Black Hole, which in fact Can bend light.

If light can be absorbed or shifted, we can say it has been Bended.

Black Holes have an immense gravitational attraction, and you didn't say How much gravity, so if this is what you're talking about, Yes, Gravity Can bend light.

Answer 8

something like a galaxy with a large mass 'bends' the space around it.... a photon traveling thru the bend area appears to curve as it follows it's path thru the 'lens'..... just as a satellite can be 'slingshotted' around a planet, so, too, can light be bent around a massive gravity 'dip' in the fabric of space.....

Answer 9

"...One of them must be wrong, yes?..."



No, not wrong at all. Think for a moment about what gravity really is--a change in the shape (..geometry..) of spacetime itself. Now try to think of anything that doesn't exist within spacetime. Disregarding concepts and ideas about other dimensions, multiple universes, etc.,. there is nothing that isn't embedded in spacetime.



Now think of a space probe that's moving through space without any thrust of any kind being applied. The path it follows *must* conform to the geometry of spacetime. If it didn't then it wouldn't be in this universe.



Light is no exception. It *must* follow the geometry (..shape..) of spacetime, and if that's warped, altered, changed, distorted in any way then light too must conform to that spatial shape.

Answer 10

it's not light bending, it's the fourth dimension, Einstein's space-time , that is doing the bending. light just follows it's straight path, but that path appears bent because the space it is traveling through is bending

Answer 11

No. It is Newton's theory of gravity that is wrong. It was superseded in 1913 by Relativity. Surely you've heard of it.

Answer 12

when particles at motion poccess kinetic energy

kinetic energy=p^2/2m

p=momentum

m=mass

so moving photon do have mass

in case of black hole also light is being absorbed and light doesnot bend

Answer 13

Why don't you read a physics book that explains it, instead of asking the ordinary, everyday NON-PHYSICISTS on this forum? Do you really, truly expect to get a good answer here?

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Answer 14

Photons do not have a rest mass, but they do have mass depending on their momentum.

E=h/mv

E=mc^2

See?



The sun looses mass. How? Energy is emitted through photons.



In a fusion or fission reaction mass is lost through the emission photons, gamma rays.

PS- So far no-one has gotten it right. Please, if you don't know what you are talking about please don't post. You are making it harder for the asker to get a correct answer.