Gravity bends light but how, photons have no mass?

[nightvision]

This was a question on the 700th episode of 'Sky at Night' that I thought highlighted interesting differences in 'our' understanding. I use the word 'our' loosely to reference what appeared to be differences between the guest positions (unspoken but you could cut the silence with a knife!) and reactions to the question.

I have some thoughts but it would be good to hear the views of the astronomy community.

Tony.

[michael.h.f.wilkinson]

Actually, photons have no rest mass, they do have energy, so they have mass, thus, they are affected by gravity. Another way of looking at it is that light does NOT bend, it is space itself that is not straight. Any large mass distorts space around it, and light is simply traveling along the shortest possible path (which is no longer a straight line, but still the least curved path).

[nightvision]

Hi Michael, I note the dual reasoning:

1. Photons have energy, so they have mass

2. Space is curved by gravity and so the path of light is curved

Both make sense, if you had to pick only one which would it be?

Duality is something that has always bugged me, 'it's a particle and it's also a wave' , counter intuitive but seems widely accepted.

Tony.

[michael.h.f.wilkinson]

Hi Michael, I note the dual reasoning:

1. Photons have energy, so they have mass

2. Space is curved by gravity and so the path of light is curved

Both make sense, if you had to pick only one which would it be?

Duality is something that has always bugged me, 'it's a particle and it's also a wave' , counter intuitive but seems widely accepted.

Tony.

The first is the special relativity version of affairs. It acknowledges the existence of photon mass, and lets Newtonian gravity do its job. This does not account for the observations and is therefore wrong in this form. The second is the view of general relativity, and does predict the observed curvature. You could reformulate Newtonian gravity to obtain the results of general relativity, but at the expense of getting inconsistent results when viewing from an accelerated frame of reference (i.e. the measurements would be wrong for observations from a rocket with its motor running). Thus the second formulation fits the observations better.

This does not mean it is right;)

[Theinvoker]

The general ideas people have put are correct but there is a simpler way to put it.

Gravity bends space-time. We are pretty happy with that fact now days even if some may think adjustments need making to relativity. When the space is 'bent' (I'm sure most have seen the heavy ball on an elastic sheet analogy) you make a curve in space-time.

Light always follows what is known as null geodesic Geodesic (general relativity) - Wikipedia, the free encyclopedia In simple terms this is the shortest route from on place to another. If you think of a plane flying from London to New York, the shortest path is NOT a straight line. In fact it is a curved line to the north that allows for the curvature of the Earth. In the same way when light enters the curved space-time caused by gravity, the shortest path is no longer a straight line, but is curved. Therefore light is just following the shortest path avaliable.

Hope that helps

[michael.h.f.wilkinson]

The general ideas people have put are correct but there is a simpler way to put it.

Gravity bends space-time. We are pretty happy with that fact now days even if some may think adjustments need making to relativity. When the space is 'bent' (I'm sure most have seen the heavy ball on an elastic sheet analogy) you make a curve in space-time.

Light always follows what is known as null geodesic Geodesic (general relativity) - Wikipedia, the free encyclopedia In simple terms this is the shortest route from on place to another. If you think of a plane flying from London to New York, the shortest path is NOT a straight line. In fact it is a curved line to the north that allows for the curvature of the Earth. In the same way when light enters the curved space-time caused by gravity, the shortest path is no longer a straight line, but is curved. Therefore light is just following the shortest path avaliable.

Hope that helps

Dead right, I should have said space-time is bent, not space:icon_salut:

[ronin]

Isn't it mass that bends space-time not gravity?

And is it not the bending of space-time that we interpret as gravity.

[Theinvoker]

Isn't it mass that bends space-time not gravity?

Yes sorry I should have put mass

[themos]

Not just mass, that's just the "electric" effect of gravity. There's also the "magnetic" effect but it is weaker. Pressure also acts as a gravity source and light certainly exerts pressure so light will make gravity.

Now, to the question of whether light "feels" gravity. Everything in spacetime feels gravity because gravity is a distortion in spacetime.

The way I picture it is that everything tries to go forward in time (or it just feels like that to us, whatever). In the absence of gravity, a body that is standing still next to us, will stay at the same distance as it travels forward in time. In the presence of a distortion in spacetime, some of that forward motion in time will "leak" into a motion in space and we will see it accelerate, that is, "fall".

[michael.h.f.wilkinson]

Any acceleration actually IS gravity according to general relativity. This answered the long-standing question of why "heavy" mass was the same as "inertial" mass.

[nightvision]

... double post!

[nightvision]

Themos, that's a great mind model, really helps me grasp what we believe is going on.

Interested in your views on this intuitive model I have been playing with for many years:

Layman perspective only (with engineering background) -

Space-time is inferred but not as yet tangible?

If space-time exists is it possible that it behaves like a fluid?

If it is a fluid could it then flow into matter?

Could this flow induce a density gradient in proximity to matter?

Could the density gradient then provide the properties for refraction?

Could this refraction account for gravitational lensing?

If space-time exists both inside and outside matter then why do we need a photon to be a particle? Aren't waves far more efficient propagators than accelerated particles?

It could be taken that the above implies that gravity is a manifestation of space-time density and the flow (or stretching) that occurs in proximity to matter and not the inate property of matter. Gravity in this model originates from space-time and pushes in to all matter in most cases from all directions, local asymmetry maintains orbital models etc. With this model any large-scale asymmetry in universal space-time would be likely to drive expansion.

Tony.

[michael.h.f.wilkinson]

Actually, the bending of light can be described by another notion of general relativity: the slowing of time in strong gravity. As time slows to the outside observer, light slows to the same observer, just like in a dielectric material, i.e., just like in refraction. The differential slowing of light gives the bending effect.

[themos]

fluids as fundamental constituents went out a long time ago, I'm afraid.

Refraction and gravitational lensing are easily explained by the "light takes the least time" principle, which follows naturally from the full quantum picture of photons in the short-wavelength approximation.

Try to follow the first two videos here

The Vega Science Trust - Richard Feynman - Science Videos

[michael.h.f.wilkinson]

fluids as fundamental constituents went out a long time ago, I'm afraid.

Refraction and gravitational lensing are easily explained by the "light takes the least time" principle, which follows naturally from the full quantum picture of photons in the short-wavelength approximation.

Try to follow the first two videos here

The Vega Science Trust - Richard Feynman - Science Videos

Having said that, quantum mechanics and general relativity are not good bedfellows. I am not aware of any theory casting gravity into QM.

[Photosbykev]

I'm looking for an optical table that is 186,000 miles long and a laser that will project a beam that far so I can test the idea that a dropped object and the laser beam will both hit the table one second later.

I might have a few problems keeping the table level.

[michael.h.f.wilkinson]

I'm looking for an optical table that is 186,000 miles long and a laser that will project a beam that far so I can test the idea that a dropped object and the laser beam will both hit the table one second later.

I might have a few problems keeping the table level.

Maybe there is one on Ebay

[Photosbykev]

Maybe there is one on Ebay

plenty of dropped objects on ebay

[michael.h.f.wilkinson]

plenty of dropped objects on ebay

LOL :D

[Karlos]

Not just mass, that's just the "electric" effect of gravity. There's also the "magnetic" effect but it is weaker. Pressure also acts as a gravity source and light certainly exerts pressure so light will make gravity.

Now, to the question of whether light "feels" gravity. Everything in spacetime feels gravity because gravity is a distortion in spacetime.

The way I picture it is that everything tries to go forward in time (or it just feels like that to us, whatever). In the absence of gravity, a body that is standing still next to us, will stay at the same distance as it travels forward in time. In the presence of a distortion in spacetime, some of that forward motion in time will "leak" into a motion in space and we will see it accelerate, that is, "fall".

Yes light does feel gravity, in the extremes of a black hole, but due to it tiny tiny tiny minute mass (Which in the case of photons is the energy) it requires extreme gravity to make any significant effects to light.