[Robro (OP)]

What is the mechanism that allows gravity to latch onto a photon?

[MikeS]

What is the mechanism that allows gravity to latch onto a photon?

I don't believe it does.  A photon travels in a straight line but follows a geodesic in curved space-time.
Gravity does not directly affect the course of a photon, neither does it cause a photon to gain nor loose energy.

[imatfaal]

Rob - here is a nice simple explanation
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961102.html
As Mike says the photon follows a geodesic (which is the shortest distance between two points) this is not a straight line when spacetime itself is curved.  Mass and energy distort the actual geometry of spacetime - it is this distortion that einstein proposed and formalised in GR to explain gravity.


Mike - a photon in deep space that is not energetic enough to interact with a certain electron may by the time it is at the earth's surface be energetic enough to interact.  I know the mechanism is more complicated than the simple heuristic that many believe - but be that as it may, it is a fact that by any method of measurement the energy will have increased due to a wavelength blue shift.  Please keep your more speculative ideas and explanation to New Theories and do not include them as a first answer to questions on the main Physics board

[lightarrow]

What is the mechanism that allows gravity to latch onto a photon?

If the question is: "how can a gravitational force affect the motion of a massless body", you can see it this way: let an object fall on the ground (in the void). Does its accelerated motion depend on its mass?
What happens with an object which mass tends to zero? Does its acceleration change?

[Kryptid]

Photons could be considered to have mass based on the principle of mass-energy equivalence.

[lightarrow]

Photons could be considered to have mass based on the principle of mass-energy equivalence.

Photons are massless. If you think they have mass, explain how an object with non zero mass m can have finite energy moving at c, since its momentum p is:

p = m/sqrt[1 - (v/c)²]

Put v = c and tell me how much is p.

[Kryptid]

They have no rest mass, but since they are never at rest...

This is a tricky one, because I've read in more than one place that objects with finite energy density should generate gravitional fields. I've seen suggestions that concentrating enough high-energy light inside of its own Schwarzchild radius should allow for the creation of a black hole. Light can also transfer momentum to other objects (solar sail technology).

Perhaps what I should have said is that light behaves as if it has mass.

[lightarrow]

They have no rest mass, but since they are never at rest...

The term "rest mass" is not a good one, exactly for that reason.
The correct term is "invariant mass" which now is simply called "mass". "Invariant" means that it doesn't change with the frame of reference. A photon has zero invariant mass.

This is a tricky one, because I've read in more than one place that objects with finite energy density should generate gravitional fields. I've seen suggestions that concentrating enough high-energy light inside of its own Schwarzchild radius should allow for the creation of a black hole.

Any physical system which is *stationary* in a given frame of reference, has a mass which depends on its energy content (m = E/c²), so if the system is a fixed region of space in the void which is crossed by a light beam, then in that moment it acquires mass, even if the light beam hasn't.
This has been discussed several times in this forum.

Light can also transfer momentum to other objects (solar sail technology).

And this has nothing to do with light's mass. Even this has been discussed several times in this forum (this even recently).

Perhaps what I should have said is that light behaves as if it has mass.

Ok, but if you don't specify in which exact conditions (see up) it behaves "as if", it's not useful, on the contrary, it makes confusion.
Light has zero mass.

[Pmb]

Photons are massless. If you think they have mass, explain how an object with non zero mass m can have finite energy moving at c, since its momentum p is:

p = m/sqrt[1 - (v/c)²]

Put v = c and tell me how much is p.

That formula is derived with the assumption that v < c. The momentum for  a photono is P = mc where the m is the inertial.

Here is a nice quote from Feynman

From the Feynman Lectures Vol -I page 7-11
Section entitled Gravitation and Relativity

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.

[lightarrow]

We have already discussed that and we didn't agree. About what Feynman writes, I have explained the concept in my previous post. You are one of the few left who still talk of relativistic mass.

[MikeS]

This is a tricky one, because I've read in more than one place that objects with finite energy density should generate gravitional fields. I've seen suggestions that concentrating enough high-energy light inside of its own Schwarzchild radius should allow for the creation of a black hole.

As I see it, light having no mass, can not generate (or directly interact with) a gravitational field.  Therefore it does not have a Schwarzchild radius so could not create a black hole.  Light can only 'appear' to interact with a black hole because the black hole curves space-time geodesics.  However, it could be argued that the Schwarzchild radius of light is the same as that of the universe but in that case 'light' is not the sole defining character.

[Pmb]

We have already discussed that and we didn't agree. About what Feynman writes, I have explained the concept in my previous post. You are one of the few left who still talk of relativistic mass.

I posted what I did because the source of the confusion is the use of the term mass without actually nailing down its definition. Someone above thought that the photon doesn't interact with a gravitational field because "light has no mass" meaning that the proper mass of a photon is zero. Light is affected by a g-field because light has passive gravitational mass.

As far as who uses what - I won't be changing my ressponses to fit with what "most" people do. Only what I think is the best way to explain something. E.g. in particle physics most of them use mass to mean propr mass. However that is not the same in GR where a lot of people use mass to mean gravitational mass, just like the first post since it was about the interaction of photons and gravity.

[Pmb]

MikeS - Light can generate a gravitational field.

[lightarrow]

MikeS - Light can generate a gravitational field.

We are discussing about photons. Can a photon generate a gravitational field?

[Pmb]

MikeS - Light can generate a gravitational field.

We are discussing about photons. Can a photon generate a gravitational field?

yes.

[JP]

MikeS - Light can generate a gravitational field.

We are discussing about photons. Can a photon generate a gravitational field?

yes.

Anything with energy/momentum should be able to do so, and a photon has both. 

By the way, a lot of the mass in matter is caused by the binding energy in the nucleus, and to a far lesser extent by electromagnetic binding energies between atoms.  All this binding energy can be described in terms of gluons/photons, which are massless.

[MikeS]

MikeS - Light can generate a gravitational field.

We are discussing about photons. Can a photon generate a gravitational field?

yes.

Anything with energy/momentum should be able to do so, and a photon has both. 

By the way, a lot of the mass in matter is caused by the binding energy in the nucleus, and to a far lesser extent by electromagnetic binding energies between atoms.  All this binding energy can be described in terms of gluons/photons, which are massless.

I think it is debatable whether a photon has momentum.
Is this a mainstream answer, as I understand the situation, where mass comes from is still open to question.


Why would anything with energy be able to generate a gravitational field?

[imatfaal]

MikeS - Light can generate a gravitational field.

We are discussing about photons. Can a photon generate a gravitational field?

yes.

Anything with energy/momentum should be able to do so, and a photon has both. 

By the way, a lot of the mass in matter is caused by the binding energy in the nucleus, and to a far lesser extent by electromagnetic binding energies between atoms.  All this binding energy can be described in terms of gluons/photons, which are massless.

I think it is debatable whether a photon has momentum.
Is this a mainstream answer, as I understand the situation, where mass comes from is still open to question.


Why would anything with energy be able to generate a gravitational field?

It is not really debatable that photons have momentum - can be quantified as either p=E/c or p=hbark (where k is the wave vector). 

The mass of fundamental particles is still up for debate - see the lhc's search for the higg's boson - but we know that much of the mass of composites from small (protons) to large (macro molecules) comes from the binding energy. 

energy is equivalent to mass - we calculate the spacetime curvature with the stress energy tensor within einsteins field equations.  you might just as well ask why mass is able ...  physics models and explains at lower and more basic levels - but the eternal verities are left to philosophers and drunks

[MikeS]

MikeS - Light can generate a gravitational field.

We are discussing about photons. Can a photon generate a gravitational field?

yes.

energy is equivalent to mass - we calculate the spacetime curvature with the stress energy tensor within einsteins field equations.  you might just as well ask why mass is able ...  physics models and explains at lower and more basic levels - but the eternal verities are left to philosophers and drunks

The fact that energy is equivalent to mass is irelevant.  It does not mean they can be thought of as being the same.  In most ways they are the complete opposites of each other.  Mass is heavy, photons are light.

[MikeS]

 Someone above thought that the photon doesn't interact with a gravitational field because "light has no mass" meaning that the proper mass of a photon is zero. Light is affected by a g-field because light has passive gravitational mass.

So which is the correct interpretation and why?
A photon can interact with a gravitational field because it has 'passive' mass.  Can you please explain the term 'passive' as it implies it has no effect?
A photon can not interact with the gravitational field.  It only appears to because it follows the geodesics of curved space time as caused by gravity.
One is obviously wrong.