Is there direct evidence photons attract gravitationally?

[ensabah6]

Not sure where better to place this thread, here or gr.

Is there direct evidence photons attract one another gravitationally? I am aware of the arguments that GR couples to the energy-stress tensor so photons should attract one another gravitationaly but I wonder if Gravity needs an inertial mass to act as a kind of charge analogous to electric charge in E&M.
So just as photons do not have electric charge, they don't have "gravitational" charge in the form of mass?

 

[javierR]

Not sure where better to place this thread, here or gr.

Is there direct evidence photons attract one another gravitationally?

No.

... I wonder if Gravity needs an inertial mass to act as a kind of charge analogous to electric charge in E&M.
So just as photons do not have electric charge, they don't have "gravitational" charge in the form of mass.

But there's evidence that light and massive objects gravitate (light if deflected by stars e.g.). If photons weren't "charged", how would this occur?

If they do self-interact due to gravity then why isn't this make QED non-re normalizable.

Because the gravitational interaction is not part of QED. QED is the interaction of photons
and particles that interact via photon exchange.

 

[hamster143]

That would be a vanishingly tiny effect that is impossible to detect at this level of technology (a 100 watt lightbulb emits 10^-12 g worth of photons every second ...) We do know that photons experience gravitational pull from massive objects, just like everything else.

Photons' gravitational self interaction is an important component of the Big Bang theory, the universe would evolve differently if they weren't attracting each other. And since the Big Bang theory seems to work well, that could be evidence in favor of this assertion. One would have to redo the math and see what predictions come out differently ...

 

[Naty1]

Photons' gravitational self interaction is an important component of the Big Bang theory, the universe would evolve differently if they weren't attracting each other.

Can anyone explain this comment??

 

[hamster143]

Can anyone explain this comment??

Really simple ... for the first 50 000 years or so after the Big Bang, radiation density exceeded matter density. Expansion during this period is modeled using GR equations, which assume that photons gravitationally interact with each other. In a sense, photons are the driving force behind this expansion (which is a curious paradox - gravity is an attractive force, but universe nevertheless expands!) If photons don't REALLY interact, GR is not applicable and our expansion timeline is off.

And expansion timeline affects numerous other things, from CMB temperature, to relative densities of basic isotopes, to large-scale galaxy structure ...

 

[humanino]

for the first 50 000 years or so after the Big Bang, radiation density exceeded matter density.

Which quantum aspect of the radiation is necessary in this period of the Big Bang ? I mean, why can I not treat the radiation classically ? If I can, is it fair to say that "photons" have been indirectly tested to attract each other ?

 

[Vanadium 50]

I should point out that there is no direct evidence that marbles attract gravitationally. However, nobody doubts they do. Why should photons be different? We know that they respond to a gravitational force, and conservation of momentum requires that this force be balanced. Isn't that good enough?

 

[hamster143]

I should point out that there is no direct evidence that marbles attract gravitationally. However, nobody doubts they do. Why should photons be different? We know that they respond to a gravitational force, and conservation of momentum requires that this force be balanced. Isn't that good enough?

Gravitational attraction of macroscopic objects ("marbles") was directly observed in the lab as early as 1798 by Cavendish.

It is a good point that photons respond to a gravitational force from macroscopic objects ... HOWEVER ... we could imagine a world where photons gravitate to baryons, and baryons gravitate to other baryons, but photons don't gravitate to other photons.

Or, in GR language, maybe we could write a complicated version of Einstein's field equation that replaces the full stress-energy tensor with the part that only contains massive fields. There's an obvious problem that such a stress-energy tensor would no longer be divergenceless .. but maybe there are ways around it.

But that would most likely be in vain due to the Big Bang argument above.

 

[Vanadium 50]

Gravitational attraction of macroscopic objects ("marbles") was directly observed in the lab as early as 1798 by Cavendish.

No, gravitational attraction of a marble by a very large mass has been directly observed in 1798. That tells you that the marble is attracted to the large mass, not the other way around. The marble-marble force is also unmeasured.

Of course, I don't doubt that marbles have gravitational fields - it's the OP's requirement of direct evidence that I think is unreasonably strong. Isn't conservation of momentum good enough to close the loop? It is for marbles.

 

[Naty1]

I wonder if Gravity needs an inertial mass to act as a kind of charge analogous to electric charge in E&M.

An interesting question: but as others have implied gravity seems to be a unique "force"...it effects ALL known particles and energies (kinetic, potential,heat,etc) and even pressure. Photons on the other hand seem to mediate only the electromagnetic force between charged particles while the strong and weak forces also have their unique, less wide ranging effects...

I don't believe anything is immune from gravity...not even spacetime!
Is the Higgs boson(s) immune...I'm not sure...there are lots of different theoretical models.

 

[njinear]

i have some questions about this, maybe you folks can educate me.

einstein says energy carries gravity, but he makes the exception for gravity itself, that the tensor has an entry of zero for gravity's gravity. since gravity does not have gravity itself and it can carry energy away (seems pretty well proven by binary star decay) then the gravity in the universe is not a conserved quantity, with significant cosmological implications since energy is more often emitted than absorbed.

the fact that photons are deflected by gravity doesn't mean they generate gravity. the deflection is due to the shape of spacetime, the photons are following the geodesic. we would expect gravity waves to do the same thing, because if for no other reason, there is no straight line EXCEPT the geodesic.

further, if a photon for example has gravity, it seems like a bit of a problem getting "gravitons" in some quantum version of gravity to be "entrained" by the photons.

as i understand it the gravity of photons is still not established. we know for sure that the nuclear force causes gravity from decay weight differences in fission and fusion. not sure about electromagnetic static fields, whether chemical energy has been proven for example to have mass.

perhaps massless particles do not carry gravity, just massive particles and static (nongravity?) fields. we already have an example of this with gravity waves themselves. if photons don't have gravity, then when they are absorbed for example as chemical energy, one would presume they must "reinstate" the gravity.

seems like there are a lot of loose ends here.

 

[Drakkith]

So is the overall view in science that Photons have their own gravity, or that they do not but are attracted by it?

 

[cragar]

Look at this thread and this article titled on the gravitational field of light.
http://authors.library.caltech.edu/1544/1/TOLpr31a.pdf
https://www.physicsforums.com/showthread.php?t=174805
And it also depends on if they are parallel or anti-parallel if they will be attracted .
As you will see from the paper

 

[Dr Lots-o'watts]

Is there direct evidence photons attract one another gravitationally?

Suppose we send two light pulses traveling parallel to each other towards the moon and they are reflected back. It they attract each other, the distance that separates them when they are detected back here on Earth should be smaller than the distance between the two sources.

Can anyone calculate the change in separation from an existing "photon-photon gravity" theory?

 

[Drakkith]

Suppose we send two light pulses traveling parallel to each other towards the moon and they are reflected back. It they attract each other, the distance that separates them when they are detected back here on Earth should be smaller than the distance between the two sources.

Can anyone calculate the change in separation from an existing "photon-photon gravity" theory?

Also, if light did attract itself, wouldn't light from far away stars and galaxies look...different after thousands, millions, or billions of light years?

 

[Researcher X]

Doesn't attraction necessitate a change in velocity, something impossible for light?

 

[russ_watters]

Light most certainly can show a change in velocity (via change in direction) - just not a change in speed.

 

[njinear]

i have a problem with the 1931 paper that it assumes what it is trying to prove, that is, that the mass energy tensor applies to photons. and really, it doesn't deal with photons, it deals with a "beam", which to say the least is not descriptive of what is really going on. the proof would have to be for an individual photon.

photons are deflected by gravity but this doesn't men they generate gravity. they have to "deflect" because space is curved.

so it seems that the question of photons gfenerating a grav field is an open question, and if they do it results in a number of problematic consequences.

 

[cragar]

In relativity it says that mass energy or pressure bends space-time .
And if i have an electron positron annihilation and it produces photons , It seems to me that the gravitational fields of the electron and the positron would be transferred to the photons .

 

[Pythagorean]

What about N3? If a photon is being pulled by a large mass, shouldn't it be pulling back [strike](even if orders of magnitude lighter?)[/strike]

EDIT: scratch "orders of magnitude lighter"

 

[njinear]

cragar you are right that generally any mass/energy should bend space time. but i am questioning if this is right. for example it is not clear what the gravitational field from a light speed particle would be like. also, imagine a 2 slit interference experiment where the photon could go through either slit-- and maybe the detector is a mile away. which way does the gravitational field go?

i mean basically there are fundimental problems with photons or any free particle cqarrying gravity (except maybe gravitons). that's my problem.

 

[Redbelly98]

Doesn't attraction necessitate a change in velocity, something impossible for light?

No, attraction necessitates a change in momentum. Also, as Russ pointed out...

Light most certainly can show a change in velocity (via change in direction) - just not a change in speed.

I'll just point out that if a photon or beam of light is heading directly towards or away from the object that is attracting it, then even its velocity would not change. However, the momentum (and frequency, and wavelength) will change.

 

[JDługosz]

Or, in GR language, maybe we could write a complicated version of Einstein's field equation that replaces the full stress-energy tensor with the part that only contains massive fields.

An early test of GR was the displacement of stars during a solar eclipse. Now, we have much more accurate measurements using radar, grazing the surface of the sun.

Meanwhile, GR supplied a correction to the precession of Mercury, due to more subtle effects than just the mass. So, do the radar measurements show that photons, not just planets, are affected in the full GR manner?

 

[njinear]

no, the deflection of light does not mean photons generate gravity. same as iron is attracted to a magnet but in itself is not particularly magnetic. weak but instructive metaphor. but light gets deflected because the star warps space. the photon, as far as it is concerned, just travels in a straight line. no evidence the photon makes gravity itself

 

[Dr Lots-o'watts]

The issue of photon gravitation may be related to hypothetical photon mass, so using the classical gravitation law, and a hypothetical upper limit for an x-ray photon mass from Ef=mc^2, I get that two x-ray photons going to the moon and back (see my other post), initially separated by d1 = 1 mm, come back separated by d2 = (1mm minus about Planck length). Although these elementary equations most likely don't apply, I believe this could be considered as a crude approximation for the maximum deflection.

Not very encouraging for eventual experimental evidence.

 

[Redbelly98]

Not very encouraging for eventual experimental evidence.

Indeed. To minimize diffraction effects to smaller than a Planck length, the beam diameters would have to be orders of magnitude larger than the 1 mm separation of the two beams!

 

[njinear]

really hard to measure. but it has much wider philosophical and cosmological implications: since significant amounts of mass are evaporating from the universe, the gravity would be decreasing. and the fundamental energy-mass tensor is not right.

 

[Vanadium 50]

i have a problem with the 1931 paper that it assumes what it is trying to prove

When someone points out that they have found an elementary flaw in a paper that thousands of people have studied for decades, alarm bells go off. Which do you think is more likely - Einstein made a grade-school mistake and 1000's of people missed it? Or that there is something you don't understand?

If it helps, that paper doesn't attempt to prove anything. It's a derivation - it shows that the theory predicts how light behaves, a prediction that can be compared with experiment. (As it happens, successfully)

for example it is not clear what the gravitational field from a light speed particle would be like. .

Of course it is. G = 8pi T. T₀₀=T₁₁ and all other terms are zero. Now you know G.

no, the deflection of light does not mean photons generate gravity. same as iron is attracted to a magnet but in itself is not particularly magnetic. weak but instructive metaphor. but light gets deflected because the star warps space. the photon, as far as it is concerned, just travels in a straight line. no evidence the photon makes gravity itself

First, iron is magnetic, and indeed, must be magnetized itself to be attracted to a magnet.

Second, the fact that photons fall plus conservation of momentum requires that photons "make gravity".

since significant amounts of mass are evaporating from the universe

Evidence, please?

 

[inflector]

No, gravitational attraction of a marble by a very large mass has been directly observed in 1798. That tells you that the marble is attracted to the large mass, not the other way around. The marble-marble force is also unmeasured.

Of course, I don't doubt that marbles have gravitational fields - it's the OP's requirement of direct evidence that I think is unreasonably strong. Isn't conservation of momentum good enough to close the loop? It is for marbles.

and

Second, the fact that photons fall plus conservation of momentum requires that photons "make gravity".

This seems like a classical Newtonian perspective to me so I must be missing something. Classically, in the case of the marble, it seems that you'd have a force with equal and opposite action and reaction from the change in the momentum vector due to the curvature.

Can you explain how conservation of momentum applies to a photon following a geodesic under general relativity? Is it that you still measure the momentum vector in flat spacetime (Minkowski space)? Otherwise, how is there any change in the momentum vector if it is following what is analogous to a straight line through curved geometry?

 

[Vanadium 50]

This seems like a classical Newtonian perspective to me

What's wrong with that? GR doesn't dispense with momentum conservation.

 

[njinear]

vanadium is right, in the far field the change in momentum for the photon must be balanced against a very small change in the velocity of the body it bends around, so free photons have gravitational fields.

however it is very poor form to assume that since others didnt see something it must not be true.

since they have gravity then what happens when a photon wave function splits at a beamsplitter? one path could include bending around a star and the other just shoots into space. now what do you do with the gravity? or in any attempt to work "gravitons" into a quantized version of gravity, how does the graviton and the photon couple together? still seems like there are lots of loose ends.

 

[inflector]

What's wrong with that? GR doesn't dispense with momentum conservation.

I'm not suggesting that GR doesn't conserve momentum. What I'm trying to figure out is where the change in momentum comes from that needs to be conserved/offset.

If a photon goes through a geodesic where does it have a change in momentum? Isn't it just taking the unaccelerated path? How does that change the photon's momentum such that this change needs to be offset by a change in the momentum of something else?

 

[JDługosz]

When you start bending space and time, the classic definition is meaningless. See http://en.wikipedia.org/wiki/Momentum#Four-vector_formulation" as a starting point.

 

[njinear]

inflector good question. i am thinking that in the far field (away from significant gravitational fields) if the photon is moving in a different direction then the momentum difference between inbound and outbound directions has to be balanced against a comeasurate change in momentum from the large gravitating body, which implies the photon must have chaged the momentum of the larger body.. so it must generate gravitation. there does seem to be an issue here, that the photon is merely following a straight line in space, not accelerating in any normal sense. but i am not willing to give up conservation of momentum.

 

[inflector]

When you start bending space and time, the classic definition is meaningless. See http://en.wikipedia.org/wiki/Momentum#Four-vector_formulation" as a starting point.

That addresses a side issue, it seems to me, that one need consider the Lorentz factor when looking at relativistic momentum. How does the four-vector have anything to do with a change in momentum for a photon following a geodesic?

Is the issue here that momentum is conserved in a frame-dependent way? Just like energy is conserved in a frame-dependent way? i.e. that from the photons frame it is just going straight so there is no change in momentum but from the perspective of an observer the photon curves and therefore there is a need for an offset to the change in the photon's momentum vector?

This doesn't really make sense to me either, I'm just trying to get a handle on this concept.

The real conceptual problem here to me is that this all acts very much like a field like EM rather than what general relativity is supposed to be (a curvature in spacetime). If energy can be transferred to spacetime itself through the act of curvature, this means that spacetime acts like it is springy in some ways.

Or is it really the case that general relativity is just a field theory like EM and that all this talk of curved geometries is just a mathematical approach to describing the field equations?

 

[njinear]

im with you on this. it is one of the issues that seem pretty gnarly once we allow photons generate gravitational fields. so in a certain sense it is a bit of a paradox. but from the external inertial frame observer outside the field, momentum must have been transferred to the gravitating body by the photon.

 

[inflector]

Maybe it should be a strong clue the equations for GR are called the Einstein Field Equations.

I had always thought that the use of field was derivative of the curvature of spacetime but maybe the field is the fundamental concept and the curvature of spacetime is just a convenient mathematical tool.

Any GR specialists care to clarify?

 

[JDługosz]

Is the issue here that momentum is conserved in a frame-dependent way? Just like energy is conserved in a frame-dependent way? i.e. that from the photons frame it is just going straight so there is no change in momentum but from the perspective of an observer the photon curves and therefore there is a need for an offset to the change in the photon's momentum vector?

Momentum is not conserved when you have curved spacetime. So, to understand anything you have to use full GR. The conservation is a consequence of isotropy, so it only holds in the limit of flat Minkowski spacetime.

 

[Darryl]

Very interesting, My "amateur" understanding, is that gravitation is the attraction between mass, for something to be effected by gravitation it has to be in the presense of a mass. Yes, that mass changes the shape of the space around it, so a 'straight line' is not straight, but curved or warped.

Photons, light, travel at the speed of light, therefore cannot have mass, otherwise they would require infinite energy to 'accellerate" the photon to the speed of a photon.
Which to my untrained mind makes no sense.

From the photons perspective, there is no time, and no distance. To me, I see a photon (from the photon) as the instant it comes into existence it recombines with mass and ceases to be a photon.

Photons also travel at the 'speed of gravity' or gravity travels at the speed of light, so I can't see how there can be interaction between photons and gravity. except from a external frame of reference, and what you then observe is the change of shape of space (due to gravity) and the photons traveling in that changes space in straight lines.

If the level of energy in a photon had an effect on the gravitational attraction, then would you not see different images of distant objects when looking at different light spectrums. Due to the lower 'bending' or gravition effect on the higher energy photons?

Would you get less, or can you detect less or more gravitational lensing if you observe the magnified object at RF frequencies, light, and x-Ray/Gamma ray energies?

That would confirm that higher energy photons are bent less by a large mass that lower energy photons.. But I do not think this has been observed ?

We are so lucky to live in such an interesting Universe, with lots of interesting things to find out about..

 

[cragar]

Darryl mass ,energy or pressure bends space-time .

 

[Darryl]

Darryl mass ,energy or pressure bends space-time .

I don't know, I would be interested to know what energy that does not have mass would effect gravity (therefore bend space).

The only energy I can think of that is not massive is electromagnetic, and the point of this question is an energetic particle like a photon affected by mass ?

Or does it travel in a straight line, along curved space.

and Pressure, I would have to consider pressure to be mass, (something compressed), or if its massless, its an electromagnetic or electric or magnetic field pressure.

Or gravitation pressure (in intensity), we allready know gravity effects gravity, but does gravity affect massless particles that travel at c, that experience zero time (they travel at c).

We know gravity warps space, so a gravity pressure would be a specific bening of its local space, so gravity indirectly changes the path of the photon, to an outside observer. But the photon itself would see no (to me) experience no interaction with the gravity directly, its just traveling in what is for it a straight line.

If gravity travels at the speed of light, and mass curves space with gravity, and a photon can only travel at one set speed, then for it to maintain that set fixed speed, it has to travel along the curve of space/time to maintain a constant c. ! mabey :)

Q.

Gravitational lensings ? I am trying to visulise the shape of the space/time bending around a super massive object, with a light source behind it, (gravitational lens), and trying to work out if the lens would be concave or convex, if the mass changes space time the most in the center of the mass and less on the outside, so the gravity and space/time bending would be too strong for photons to 'pass through the lens' (a black hole), but going further out away from the centre of mass would be a point where the rate of space bending would allow photons to take a 'short cut' through a point in space where a straigh line is a curve so for a photon to stay at a constant speed, it has to change paths, to stay in space/time..

space, and length to me is not constant, it depends on where you are, and how fast your going !.

If you were on the surface of a black hole, with a tape measure and measured out 1 meter, it would be the same 1 meter as you can measure out on earth, but if you could take both of those lengths to a third location, (with its own space/time) the two lengths would be very different.

But when you are on the black hole or on the Earth you would see no difference in length.

So it would appear to me that mass has gravity, and gravity changes the shape and size of space, light speed is constant and to remain constant it has to travel a straight line through space that is varying in shape.

 

[brainstorm]

einstein says energy carries gravity, but he makes the exception for gravity itself, that the tensor has an entry of zero for gravity's gravity. since gravity does not have gravity itself and it can carry energy away (seems pretty well proven by binary star decay) then the gravity in the universe is not a conserved quantity, with significant cosmological implications since energy is more often emitted than absorbed.

Would it be ridiculous to say that gravity tends to consume energy, by converting PE into KE and ultimately regurgitates it as EM radiation? I don't think this process fails to conserve mass/energy and although I've never heard of conservation of force, I think it may do that too. After all, EM radiation is really just magnetic/electric fields being transmitted at C, right? So why wouldn't the gravitational force of all the fuel consumed by a star not be conserved in the total EM radiation generated in the process?

the fact that photons are deflected by gravity doesn't mean they generate gravity. the deflection is due to the shape of spacetime, the photons are following the geodesic. we would expect gravity waves to do the same thing, because if for no other reason, there is no straight line EXCEPT the geodesic.

there is no straight line except the geodesic, and the relative straightness of those seems to be due to relations between relative mass, momentum, inertia, and gravity. I am curious if physics will eventually explain spacetime as a product of energy, where mass emerges from energy and is attributable to it along with inertia. I could see how the emergence of inertia from energy could be responsible for the relative straightening/flattening of spacetime that allows energy to move linearly at all, but I don't know whether BBT already addresses this or whether some other physics may go in this direction or already is.

further, if a photon for example has gravity, it seems like a bit of a problem getting "gravitons" in some quantum version of gravity to be "entrained" by the photons.

Could the relationship between gravitons and photons be analogous to that between photons and electrons? By this I mean that the gravitons emitted by low-intensity EM waves could be negligible but as EM waves intensify to the point of matter-forming capacity, they would emit more gravitons. This is of course assuming that some level of EM waves can somehow "congeal" into particles with inertia - but how else could matter (have) form(ed) considering that BBT seems to be convinced that energy preceded matter in the universe?

Intuitively, I think it will turn out that gravity and EM force exist as a single hybrid force at levels of energy-density of a black hole or greater. If so, my question would be why that force differentiates into matter/inertia/gravity and EMism with the emergence of spacetime. Sorry if this post sounds speculative, but I find these questions directly implicated by BBT as far as I understand it so far.

 

[Vanadium 50]

Would it be ridiculous to say that gravity tends to consume energy, by converting PE into KE and ultimately regurgitates it as EM radiation?

I'm afraid so.

Could the relationship between gravitons and photons be analogous to that between photons and electrons?

I'm afraid not.

Intuitively, I think it will turn out that gravity and EM force exist as a single hybrid force at levels of energy-density of a black hole or greater.

This is overly speculative.

 

[brainstorm]

I'm afraid so.

Do you even understand what I meant?

I'm afraid not.

Well, if it was it could explain a potential mechanism for why/how photons would radiate linearly in higher gravity fields and coagulate under their own gravitation in the absence of stronger gravity. Likewise, there is a relationship between gravity and pressure that generates photons with increasing frequency and a similar relationship could exists between photons and gravity where increasing inertia results in more concentrated gravity-emissions. The question would be under what conditions does/would radiation "congeal" to the point of exhibiting inertia. Is your only basis for refusing to consider this the fact that you understand photons as fundamentally massless and you recognize no possible connection between EM energy and matter-formation?

This is overly speculative.

Let me ask you a question, then. What possible behavior could EM radiation exhibit in a BH-level gravitational field where it cannot escape gravity? If it can't move linearly, what would light do? Wouldn't it have to propagate intensively instead of extensively? Similarly, without being able to move, how would electrons generate magnetism? So if electromagnetism was prevented due to stasis of the particles involved, how would that force be able to be expressed except as gravitation?

 

[Darryl]

"What possible behavior could EM radiation exhibit in a BH-level gravitational field where it cannot escape gravity?"

For the photon, it has no concept of time, or distance, the photon will in a BH grav field, travel at the speed of light, in a straight space/time line.

If for you as an 'observer' see that space/time you are seeing it from your space/time/gravity.

The photons don't slow down, they can't by definition slow down, they travel at the speed of photons, or the speed of light, regardless of where they are.

Distance, is an amount of traveled in a specific time period, at a specific speed.

So distance is a relationship between time and speed.

To calculate the arrival time of an object, you use its speed and the distance in that calculation, if you travel at a certain speed for a certain time, you can calculate the distance.

But if you travel at that speed for a longer or shorter time, the distance will vary.

Time slows down by gravity, because gravity changes the shape of space/time,

So as your local time gets slower, (with higher gravity), the distance you can travel is less if you are traveling at a fixed speed.

So to vary your distance you can vary your speed, (if you can) or you can vary the rate that time "ticks".

Thats what happens with light in a black hole, that light is still traveling at the speed of light the but equation for its speed uses a value for time that is much much slower than is experienced say on Earth.

So if time is going REALLY slow, evening traveling at the speed of light gets you no where.

the higher the gravity the slower time goes, on a black hole time is going so slow, that from an external frame of reference "from outside" its looks black, and light can't escape.

But for the photons in a BH, I would say they travel in a straight line they do not experience or even 'feel' gravity, and they are as far as they are concerned traveling in a straight line, and the speed of light..

IF there is matter in the path of the photon, it will be absorbed by that matter, so its the space/time line curves into the surface of the BH you would expect that photons would (from the outside) travel backwards, when in fact due to the ultra slow time, would make the direction of the photon when emmitted to be in the direction of the mass, because that is the straight line space time path for that photon that HAS to travel at a constant speed, regardless of how fast time goes.

At a point away from the BH will be a point of space/time where time is going fast enough for the photon to make forward progress (from external frame of reference).

Then as gravity continues to decrease, its local rate of time will continue to increase, so the distance traveled by the photon in unit time is high enough for light to be seen from the outside.

Its not the photon speeding up, its time speeding up, the photon travels at one speed and one speed only, if it does not travel at that speed it ceases to be a photon. Which is what happens when a photon strikes an object.

For the photon there is NO time, therefore NO distance,

If a photon has no time and no distance, there is nothing for it to react too,

So if you had a two photon universe, no gravity or matter, and those two photons just happened to be coherent or sitting next to each other.

Neither of the photons experiece time, and they travel at a constant speed, I am sure from an outside observer in an infinite length of time, the two photons would be in the exact same relative position as when they started.

So the photon that you see in your telescope after traveling in space for 13 billion years, as created at the instant it was detected in your telescope !. If your the photon that is.

So a photon, at the speed of light experiences zero time, so the instant its created its destroyed, but that 'instant' can be an infinite time ! I think..

So it to me has to do with photons having to travel at one fixed speed, and the knowledge that gravity changes time.
And if you change speed or the rate of time (or time itself) then distance will change.
With light you can't change speed, but you can change local time, so if you slow time up enough no matter what speed you travel you won't get anywhere, if time stands still, as you can't have infinite speed, but a fixed speed that is why you have places where time is so slow that light can't get anywhere, its not until further out where time is running faster where light can go enough distance to propagate outwards.

So if your traveling at 300,000Km/s but a second for you is a million years for me on earth, then for me, you are only traveled 300,00Kms in 1 million years.
But for you it was only a second gone by on your watch.

So really its all about time and a fixed speed, if something has a fixed speed but travels through space where the rate of time varies that will cause that photon to appear to travel in a curve, and appear to be gravitationally attacked to mass, but its not, its just changing course to stay at a constant speed in varying time.

So a gravition lens is really a gravity induced time lens.

 

[Galap]

No.But there's evidence that light and massive objects gravitate (light if deflected by stars e.g.). If photons weren't "charged", how would this occur? Because the gravitational interaction is not part of QED. QED is the interaction of photons
and particles that interact via photon exchange.

note that because we have not been able to discern any experimental difference between inertial mass, passive gravitational mass (affects magnitude of force felt in a gravitational field), and active gravitational mass (affects magnitude of force EXERTED on other masses), it doesn't mean that there are no situations (like photons) in which they can be different.

that being said, it is pretty well accepted that all three of these things are equivalent for all phenomena.

to answer OP, no we have not experimentally observed gravitational interactions between 2 photons.

 

[JDługosz]

Photons also travel at the 'speed of gravity' or gravity travels at the speed of light, so I can't see how there can be interaction between photons and gravity.

The "correspondence principle" of acceleration being the same as gravity. Given that idea as a starting point, look at a beam of light crossing the elevator -- it seems to drop. This strange prediction was found to be true.

 

[cragar]

If I am in a black hole and i shine a laser out towards the event horizon, Would the photon get red shifted to zero. Assuming it got shot straight out and didn't get bent.

 

[rwjst4]

Look at this thread and this article titled on the gravitational field of light.
http://authors.library.caltech.edu/1544/1/TOLpr31a.pdf
https://www.physicsforums.com/showthread.php?t=174805
And it also depends on if they are parallel or anti-parallel if they will be attracted .

The paper is not theoretical proof that photons attract gravitationally. This is because the authors begin with the assumption that a photon (i.e. a "pencil" or beam of photons) has a gravitational field. So to say that photons attract one another from this article is circular reasoning.

 

[GeorgeDishman]

I've read through all the replies in this thread and I don't think there is a clear answer yet. It might be more relevant in the GR forum but I don't want to start a duplicate thread.

A long time ago in a forum far, far away, I read that while light and matter interact, two photons do not interact gravitationally according to GR. The graphic some way down this article seems to back that up though the text doesn't mention it:

http://plus.maths.org/content/os/issue29/features/kalmus/index

However, if two "boxes of photons" were placed some distance apart in deep space, I would expect them to accelerate towards each other in the same way as any other masses. The form of the energy in the box should be irrelevant.

So if two photons pass on perpendicular paths (say in the x and y directions respectively) and at the crossing point of the paths, they are separated by a small amount in the z direction, would GR predict that their paths would be deflected or not?

Would that for example be expected to cause some blurring of quasar images due to interaction with the photons of the CMB?