A question about gravitons&photons

[startic84]

Hi. i have a question.Hope i could get some answers.

Say i have 2 identical light emitting bodies placed next to each other, side by side, somewhere in space. Will the photons and gravitons given out by both the bodies collide in mid space? If so, what are the quantum mechanical(since both photons&gravitons are so small)consequences of such an even happending? Also, will the gravitons and photons pass through each other if they are about to experience a head-on collision?

 

[turin]

As far as I know, photons do not carry charge. If they do not, then they should not interact with each other, since photons interact with charge. I'm pretty sure about this. E&M radiation (photons) obey the superposition principle, which basically results in nothing happening to "interacting" photons.

I'm not so sure about gravitons. I think that gravitons interact with mass energy. I seem to have gotten conflicting ideas regarding whether or not gravitons are energy. I have read in books such as Ohanian's "Gravitation and Spacetime," MTW's "Gravitation," and Rindler's "Relativity: Special, General, and Cosmological," as well as a few papers, that the gravitational field itself gravitates. However, I have been hearing from a few people, whose oppinions I do respect, that this is not the case. Or maybe I'm just confused.

 

[GRQC]

Gravitons and photons will interact only if there is a gravity-EM crossterm in the Lagrangian (i.e. a non-zero scattering amplitude).

However, more to the point: we don't know if there is such a thing as a graviton, and current theories of quantum gravity are shaky at best.

I'm not aware of the possible results from M-theory or LQG offhand.

 

[startic84]

To GRQC, "Gravitons and photons will interact only if there is a gravity-EM crossterm in the Lagrangian (i.e. a non-zero scattering amplitude)."

Put in layman's term...what does it mean?

I don't think I'm getting an answer from the replies. My question is, what happends when a photon and a gravitnon are on a head-on collision? Just like two billiard balls, when they collide you could tell me they are deflected away in some directions set by the initial conditions.

 

[Stingray]

He's basically stating a more specific version of "they interact if the theory allows an interaction between them. Since we don't have a theory for gravitons, who knows."

 

[turin]

Originally posted by startic84
... what happends when a photon and a gravitnon are on a head-on collision?

I would bet that they do not act very much like two billiard balls, just because they are wave-quanta. I think that a graviton and a photon would interact (my rather uneducated opinion) because the graviton interacts with energy (I think) and the photon is energy.

Here's my guess:
The photon and graviton are on a collision course. They meet. The graviton is an amount of gravitational field, which is basically a geometrical disturbance. Since the photon must behave geometrically, then the graviton essentially gets to tell the photon what to do. So, in the aftermath (overly dramatic term), there would be a photon and no graviton. The path of the photon would be diverted loyal to the graviton's last dying wish (the thing about bosons, as I understand it, is that they only get to tell other particles what to do once, and then they die, kind of like a bee sting).

 

[p-brane]

Originally posted by startic84
Hi. i have a question.Hope i could get some answers.

Say i have 2 identical light emitting bodies placed next to each other, side by side, somewhere in space. Will the photons and gravitons given out by both the bodies collide in mid space? If so, what are the quantum mechanical(since both photons&gravitons are so small)consequences of such an even happending? Also, will the gravitons and photons pass through each other if they are about to experience a head-on collision?

I think that photons and gravitons only exist as plankt-like quantifications that make it easier to study waves of light or gravity. The photon is the quantum approach to measuring and experimenting with light and the graviton, so far, is the quantum approach to measuring and experimenting with gravity. Someone please correct me if I am mistaken!

 

[startic84]

I was wondering...Maybe gravitons and photons are composed of radeically totally different "substances"...If not for that, there'd be chaos in the world should light and gravity get jumbled around, somehow. Take for instance, the light I'm shining at you suddenly becomes a gravity wave thus pulling you toward me.

What do you all think?

 

[Nereid]

analogies

Another way to think about it is 'which of the four fundamental forces does X feel?' The photon is the carrier of the EM force, and it's unaffected by the weak or strong nuclear force. A beam of photons would not interact with a beam of particles whose only interactions were weak or strong force ones, for example neutrinos.

As GRQC said, since there's no theory of quantum gravity, we can only speculate on the properties of the graviton (indeed, it may not even exist). The only things limiting your speculation are the results of experiments and observations; if your speculation requires that photons exhibit frequency-dependent gravitational lensing (for example), then we can be pretty sure your speculations are wrong (within the limits of what we've observed, of course).

Where might any photon-graviton interactions be observed? In colliding/coalescing neutron stars; in the merger of two galactic nuclei (supermassive black holes); other examples?

What about the gravitational effects of a photon? (not to be confused with the effects on a photon of a gravitational field). How much energy does a photon need to have before we could detect the gravitational influence of this photon? Certainly far, far more energy than even the most energetic photons from the LHC. However, nature regularly produces photons with as much energy as a baseball pitched by a Yankee, and we've long been able to measure the gravitational influence of a baseball-sized mass.

Anyone care to guess when we'll see the first reports of observations of the gravitational effects of very energetic photons?

 

[p-brane]

Originally posted by startic84
I was wondering...Maybe gravitons and photons are composed of radeically totally different "substances"...If not for that, there'd be chaos in the world should light and gravity get jumbled around, somehow. Take for instance, the light I'm shining at you suddenly becomes a gravity wave thus pulling you toward me.

What do you all think?

It is a popular observation that light bends when it come in contact with gravity. Gravity remains unaffected by light or in as much as we have not studied the effects of light on gravity, all things being equal, the two forces, light and gravity, can be, and by all accounts are, equal.

 

[TeV]

What do you mean by term "collision" of photon and graviton at all?
Particle interaction is one thing,effects of G fields another.

 

[TeV]

Originally posted by startic84
Hi. i have a question.Hope i could get some answers.

Say i have 2 identical light emitting bodies placed next to each other, side by side, somewhere in space. Will the photons and gravitons given out by both the bodies collide in mid space? If so, what are the quantum mechanical(since both photons&gravitons are so small)consequences of such an even happending? Also, will the gravitons and photons pass through each other if they are about to experience a head-on collision?

Simply put yes,they will pass through each other if they are to experience a "head- on collision".
Interaction is another thing.Photons have zero rest mass and can interact only with particles having charge and nonzero rest mass (can't interact with neutrons or neutrinos).

 

[startic84]

Thank you TeV for your response.

Could you substantiate on what you've posted? From where did you learn about what you'd typed? For eg. , "Simply put yes,they will pass through each other if they are to experience a "head- on collision. Interaction is another thing.Photons have zero rest mass and can interact only with particles having charge and nonzero rest mass..."

It would help in my understanding if you could elaborate more on what you have typed.

 

[PRodQuanta]

what he is saying is that a photon will have no effect whatsoever on the graviton. A photon, which has no mass, when at rest, therefore no rest mass, will only interact with a particle with a particle with mass at rest, and has a charge (i.e.-electron has a (-) charge). He is saying, also, that a graviton has no mass at rest, and has not charge, since he says they will not interact. I hope this explains what he is saying a little better. Now, if he is correct or not, I don't know.

Now I was wondering, on the topic about how gravity bends light. Can there be a recipricol effect? Can bending a beam of light create a gravitational field? Just a question.

Paden Roder.

 

[Nereid]

PRodQuanta wrote: Now I was wondering, on the topic about how gravity bends light. Can there be a recipricol effect? Can bending a beam of light create a gravitational field? Just a question.

Think of a high energy photon, say 1 TeV (10¹² eV). If it were a massive particle at rest, what would its mass be? Could such a particle bend light? Now, to recap, we have a photon with an energy of 1 TeV; what is the magnitude of the gravitational field it creates?

Now you're an experimenter. What's the smallest mass you can detect, using only the mass's gravitational influence? How energetic would a photon need to be before you could detect its gravitational field (in principle)?

[Edit: fixed typo]

 

[Stingray]

You usually don't have just one photon. Its probably better to ask whether any conceivable beam of light (a collection of many low energy photons) could have a measurable gravitational field. I think a few grams per cubic centimeter is technically detectable now.

The energy density of an electromagnetic field goes as E^2. Having the equivalent of a gram of matter in a cubic centimeter would require E around 10^16 V/m if I didn't make a calculator error!

In principle though, light does generate a gravitational field.

 

[Kit]

can we put it in this way?[?]

if graviton really exists, a photon will probably interact with a graviton coz light is bended by the gravtational field...

but if they do not collide each other, how do they interact?

moreover, does gravition interact with all things which have mass or energy?

 

[Nereid]

Originally posted by Kit
can we put it in this way?[?]

if graviton really exists, a photon will probably interact with a graviton coz light is bended by the gravtational field...

but if they do not collide each other, how do they interact?

moreover, does gravition interact with all things which have mass or energy?

Since no one has observed a graviton, and there are no solid theories from which to derive the properties of 'the graviton', you are free to speculate on how any such particle will (or will not) interact with photons.

Stingray wrote: You usually don't have just one photon.

CANGAROO (http://icrhp9.icrr.u-tokyo.ac.jp/) is one project which counts cosmic ray photons one at a time; in particular TeV photons detected via the atmospheric Cherenkov technique. GLAST (http://glast.gsfc.nasa.gov/) will detect photons with energies up to 300 GeV directly.

 

[TeV]

Originally posted by startic84
Thank you TeV for your response.

Could you substantiate on what you've posted? From where did you learn about what you'd typed? For eg. , "Simply put yes,they will pass through each other if they are to experience a "head- on collision. Interaction is another thing.Photons have zero rest mass and can interact only with particles having charge and nonzero rest mass..."

It would help in my understanding if you could elaborate more on what you have typed.

Both massless particles,free "boson" of G field and free boson of EM field,if they are to meet HEAD-ON vise will not interact,since they travel at speed of light in oposite directions (considered as stright line in Euclidian space).Any exchange of impulse or energy is impossible under these circumstances.Please,find me reference frame where interaction occurs in this case!

 

[TeV]

Originally posted by Kit
can we put it in this way?[?]

if graviton really exists, a photon will probably interact with a graviton coz light is bended by the gravtational field...

but if they do not collide each other, how do they interact?

moreover, does gravition interact with all things which have mass or energy?

Relativity predicts so called gravity waves.
In any form of quantizied theory of gravity that would be just bunch of free gravitons.
I undoubtly believe gravity waves exist.
But G relativity doesn't need quanta to explain interaction of G field.You may say it is treated as sort of illusion there,not necessary to be treated as sort of field at all.
It is a just local change of spacetime curvatore and objects do not fall,beams of light do not "bend",but they follow the shortest path in a curved higher dimensional space.

 

[p-brane]

Eeghads, I'm thick as a plank.

Sorry, I spelt Plank's name wrong. I also looked up the word Photon and found that is is a small "packette" of an electromagnetic wave of light in any form. This would include ultraviolet, x-ray and other forms of light including, if my p-brane is still functioning, radio waves.

It still seems to me, and my p-brane, like the measurement that is a photon is more like an "inch" than it is an actual physical particle since it is more for the convenience of measuring light and studying its properties that the photon has come about as a term and concept.

Its a little like being hit by a down-pour of rain and saying "there are "inches" of water hitting me", in my opinion. Any corrections to this approach are invited.

 

[turin]

Originally posted by p-brane
It still seems to me ... like the measurement that is a photon is more like an "inch" than it is an actual physical particle since it is more for the convenience of measuring light and studying its properties that the photon has come about as a term and concept.

Its a little like being hit by a down-pour of rain and saying "there are "inches" of water hitting me", in my opinion. Any corrections to this approach are invited.

That sounds about right. Except that you have to imagine a ruler that only allows inch measurements, like the ruler doesn't even exist between the inches. Better yet (a more faithful analogy correction), imagine that the rain piles up on the ground in sudden steps of inches, so that, if you watched the flood gauge, the water level would suddenly jump from tick mark to tick mark with no time in between.

One other point: the photon is not just a matter of convenience. The photoelectric effect demonstrates that it is physical.

 

[p-brane]

Originally posted by turin
That sounds about right. Except that you have to imagine a ruler that only allows inch measurements, like the ruler doesn't even exist between the inches. Better yet (a more faithful analogy correction), imagine that the rain piles up on the ground in sudden steps of inches, so that, if you watched the flood gauge, the water level would suddenly jump from tick mark to tick mark with no time in between.

One other point: the photon is not just a matter of convenience. The photoelectric effect demonstrates that it is physical.

Of course light is physical. Is there a defined contrast between a vacuumous field and the photon? Or is there a considerable gradient of electromagnetic activity observed when defining the edge of an electromagnetic wave and the field of the vacuume?

 

[turin]

Originally posted by p-brane
Of course light is physical.

I meant that the photon - that is, the discretization of the energy of light - is physical. That is, nature plays along with the notion of the photon. Whether or not light travels in photons, I don't know, but whenever it interacts, it interacts in photons. Gamma particles make this very pronounced, almost like billiard balls.

Originally posted by p-brane
Is there a defined contrast between a vacuumous field and the photon? Or is there a considerable gradient of electromagnetic activity observed when defining the edge of an electromagnetic wave and the field of the vacuume?

I don't understand.

 

[p-brane]

Originally posted by turin
I meant that the photon - that is, the discretization of the energy of light - is physical. That is, nature plays along with the notion of the photon. Whether or not light travels in photons, I don't know, but whenever it interacts, it interacts in photons. Gamma particles make this very pronounced, almost like billiard balls.

If you could site a study that backs your statement here that would be helpful.

Originally posted by turin
I don't understand.

What you've stated above seems to begin to answer my question. When I stated:

"Or is there a considerable gradient of electromagnetic activity observed when defining the edge of an electromagnetic wave and the field of the vacuume?"

I meant is there a defining edge between a single unit of light (photon) and the medium through which it is traveling?

The medium could well be atmosphere, it could be gravitational waves and it could be a vacuum with little or no other influences present.

 

[turin]

Originally posted by p-brane
If you could site a study that backs your statement here that would be helpful.

Well, the inaugural example of the physicality of the photon to which I was introduced was that of black body radiation. This is the phenomenon for which Planck introduced his famous, essential constant to relieve the UV catastrophe of the spectrum. Not too long afterwards, Einstein came along and explained the puzzling photo-electric effect while at the same time attributing credibility to Planck's constant, a rather bold move, but not the last made by Einstein. I can't site the details of these specific studies (i.e. how you could dig up the "official" documentation), but these two preliminary examples are ubiquitously accepted in the physics community, and, if you have any faith in textbooks, that would be the place to look (almost any "modern physics" text will have at least a reference to these two famous examples). In fact, they are in some respect considered to be the part of the primary reason for the invention of QM (along with Bohr's H-atom angular momentum hypothesis). I have seen with "my own eyes" (well, not literally, that would be impossible) the corpuscular nature of gamma particles in several radiation experiments. Again, for this I cannot site a specific study (other than my own personal study), so I appologize.

Originally posted by p-brane
... is there a defining edge between a single unit of light (photon) and the medium through which it is traveling?

If you mean this spatially, then I would say no, but then, more specifically, I would say that I'm not sure. Not that I've heard of. In fact, I think that, in order to literally have a single free photon, it must extend over all space as a uniform plane wave (the wave function, that is, should have complete uncertainty in position and a perfectly well defined momentum).

 

[p-brane]

Originally posted by turin
Well, the inaugural example of the physicality of the photon to which I was introduced was that of black body radiation. This is the phenomenon for which Planck introduced his famous, essential constant to relieve the UV catastrophe of the spectrum. Not too long afterwards, Einstein came along and explained the puzzling photo-electric effect while at the same time attributing credibility to Planck's constant, a rather bold move, but not the last made by Einstein. I can't site the details of these specific studies (i.e. how you could dig up the "official" documentation), but these two preliminary examples are ubiquitously accepted in the physics community, and, if you have any faith in textbooks, that would be the place to look (almost any "modern physics" text will have at least a reference to these two famous examples). In fact, they are in some respect considered to be the part of the primary reason for the invention of QM (along with Bohr's H-atom angular momentum hypothesis). I have seen with "my own eyes" (well, not literally, that would be impossible) the corpuscular nature of gamma particles in several radiation experiments. Again, for this I cannot site a specific study (other than my own personal study), so I appologize.




If you mean this spatially, then I would say no, but then, more specifically, I would say that I'm not sure. Not that I've heard of. In fact, I think that, in order to literally have a single free photon, it must extend over all space as a uniform plane wave (the wave function, that is, should have complete uncertainty in position and a perfectly well defined momentum).

The questions that startic84 asked have spawned these investigations into the nature of light and gravity. Light is slightly better understood than gravity and with questions like startic84's, we look a little longer at the interactions and compositions of light and gravity. Overall, the answers do not seem to be forth-coming as yet. Billiard balls? Waves? Uncertainty remains the standard answer.

What does come to mind after kit's post is that gravity will trap and diminish the energy of light waves. This comes after the studies on black holes (very strong sources of gravity) and their effect on light. To assign a physical property to gravity is premature. What we observe is gravity's effect on what is physical. It is not certain that we can observe gravity as a separate entity made up of units like "gravitons".

 

[Nereid]

p-brane wrote: The questions that startic84 asked have spawned these investigations into the nature of light and gravity. Light is slightly better understood than gravity and with questions like startic84's, we look a little longer at the interactions and compositions of light and gravity. Overall, the answers do not seem to be forth-coming as yet. Billiard balls? Waves? Uncertainty remains the standard answer.

Er, light is well understood, and there are plenty of references to the key experiments confirming that our understanding is correct. Some examples:
wave-particle duality
More on wave-particle duality, with lots of links
Double slit experiment
http://www.aip.org/physnews/update/505-3.html describing a single experiment which shows both the wave and particle nature of light

 

[turin]

Originally posted by p-brane
Overall, the answers do not seem to be forth-coming as yet. Billiard balls? Waves? Uncertainty remains the standard answer.

If you feel that these answers are unsatisfactory or inadequate/incomplete, then you may be in for some disappointment as science (physics) plods along in intimate agreement with the radically non-classical consequences of QM.

Originally posted by p-brane
To assign a physical property to gravity is premature.

I think that gravity is physical. How else would you account for the effect on the physical universe?

 

[startic84]

Greetings to all.

I have had great pleasure in reading all of your posts. Immensely eye-opening! From what i have read it seems that different people have different opinions on certain concepts/ideas about the nature of light/gravity. I feel that this phenomenon is understandable since topics like light/gravity still have their certain areas left un-discovered and un-explained. As far as i am aware,

One last thing. From what i see, it is ostensible that the question i asked in the first post of this thread isn't answered - or am i not proficient enough to decipher the information that truly is the answer staring right at me? It seems that there is not even a unanimous agreement to the concepts of light/waves here, not to mention the answer to the question about their interaction. My highest qualification is A Levels, something equivalent to the qualification of a high school graduate. I studied physics at A levels too. So, many times i don't really grasp the ideas revolving around here. Such a pity because i know I'm missing out on alot. Well, with my knowledge it's impossible to contribute something uselful here. My reason for being here stems more from the yearn to seek answers than to answer questions.

Okay, to the crux of this post now, i seem to get the impression that photons and gravitons wouldn't interact, right? Does this mean they will whizz pass each other like ghosts? Then again, gravity bends light. Doesn't this mean there exists an interaction between those two class of particles when brought together? I am confused...anybody care to enlighten?

 

[Loren Booda]

Speculation:

Bosons such as photons or gravitons are indistinguishable among their own species. Photons contribute to energy density though, as would gravitons, and thus interact through spacetime curvature. Gravitons, being quadrupole, also warp spacetime by inducing complementary gravity waves.

 

[Stingray]

Originally posted by startic84
Okay, to the crux of this post now, i seem to get the impression that photons and gravitons wouldn't interact, right? Does this mean they will whizz pass each other like ghosts? Then again, gravity bends light. Doesn't this mean there exists an interaction between those two class of particles when brought together? I am confused...anybody care to enlighten?

Once again, your question is ill-defined because gravitons do not even exist in any accepted physical theory. They are hypothesized to exist "at some point" with properties unknown until then. Anyone who says they know what gravitons do is either pushing their pet theory, or doesn't know what they are talking about.

In fact, the usual concept of a quantum particle makes very little sense even in quantum field theory formulated in a classical curved spacetime. This is a little-appreciated fact by the particle physicists who've only bothered to learn how to calculate cross-sections for their particle accelerators.

 

[Nereid]

startic84 wrote: I feel that this phenomenon is understandable since topics like light/gravity still have their certain areas left un-discovered and un-explained.

There are surely many, many things about light and gravity which remain un-discovered ... however, until we 'discover' them - through an experiment here on Earth, or observation of something in a galaxy far, far away (and a long, long time ago) - we can't even begin to 'explain' them!

AFAIK, there are no experiments involving light and gravity (separately or together) which are 'un-explained' in terms of general relativity (GR, the best theory of gravity that we have) or quantum electro-dynamics (QED, the best theory involving light that we have).

To drive home Stingray's point: photons have a home in QED; there are no gravitons in GR.

Now for the caveat (there's always a caveat in science; if there weren't, it wouldn't be science!): When it comes to observational cosmology, there is some data which doesn't seem to fit the best cosmological theories we have today; however QED and GR have to be reconciled for any decent cosmological theory, but no one can claim much success with that yet.

 

[TeV]

Originally posted by Stingray
Once again, your question is ill-defined because gravitons do not even exist in any accepted physical theory. They are hypothesized to exist "at some point" with properties unknown until then. Anyone who says they know what gravitons do is either pushing their pet theory, or doesn't know what they are talking about.

In fact, the usual concept of a quantum particle makes very little sense even in quantum field theory formulated in a classical curved spacetime. This is a little-appreciated fact by the particle physicists who've only bothered to learn how to calculate cross-sections for their particle accelerators.

Properties of free graviton should not differ than that of free traveling gravity wave if quantuum field of gravity and GR are on the course to unite one day.We don't know yet if this is possible.
In the same fashion It is not usual to talk about radio wave as photon and vise versa.But this is basically the same thing-sometimes show particle and sometimes wave characteristics depending on the energy.One way or another I would propose the guy who asked orginal question to study "interaction" of localised traveling gravity wave (as described in GR) and TEM in Minkowski STR reference frame.Frontal collision in one line is without exchange of impulse ,and energy and direction of propagation if observed from spacetime points just before and after " interaction".This is what says GR about it.That was startlic original question wasn't it?Other cases of collision "angle" always result in both changed clock times and direction of propagation paths.
Althought "interaction" cannot be treated in usual sense since spacetime characteristics are altered by the presence of gravity wave itself.

 

[TeV]

I forgot to stress that free graviton due to the all the mentioned and reasons is NOT observable or detectable as a PARTICLE at all.