Dear Hossein,

I think that you need a theory of quantum gravity to investigate your question properly. General Relativity is a classical theory and, in quantum field theory on curved spacetime, the gravitational field couples to a regularized expectation value of the stress-energy-momentum tensor. It does not directly couple to the underlying quantum field operators. The picture of photons as discrete packets of energy being emitted and absorbed by charged particles is just an interpretation of the results of quantum electrodynamics.

Best regards,

David

Dear David

You are right and I agree with you. In fact theoretical physics needs a theory of quantum physics. As you know the energy-momentum tensor of EM field, in fact mean that electromagnetic fields can also change the geometry of spacetime. Anything with energy and momentum affects the geometry of spacetime because; the gravitational field equations exhibit a coupling of spacetime geometry to energy-momentum.

A photon becomes energy-laden by revolving. We know this because the electromagnetic fields around a "ray of light" are electromagnetic waves not static fields. Relativistic ally, the electromagnetic field generated by a photon is much stronger than the associated gravitational field. Further it is not clear at the present time whether the gravitational field of an energy-laden photon is static or oscillatory. It is not understood how the photon generates two sets of fields (electromagnetic and gravitational) of so different intensities. This is an enigma.

Let's take a new look at behavior of electromagnetic wave in a gravitational field; it can help for resolving this enigma. As we know an electromagnetic wave has form of two vertical electricity field and magnetic field. As General Relativity predicted and experiments show, the frequency of photons changes in a gravitational field. Remember Mossbuaer Effect. By reconsider to these ideas, we are able take a different and new result: 1- Energy of photon increases and according relativity mass-energy, its mass increases too. 2- According to Mossbuaer Effect and W=∆mc2, when gravity force acts on photon, mass (energy) of photon increases. 3- A part of gravity work converts to electricity energy and other part of gravity work converts to magnetic energy.

Increasing and decreasing of photon’s energy were doing part by part. And photon’s energy comes of its electricity energy and magnetic energy, so in red-shift and blue-shift; electricity and magnetic energy of photon do change. So, intensity of electricity field and magnetic field change by gravity effect.

Zoom on an electromagnetic wave. There are two electric field and magnetic field. These fields move with linear speed of c. But they have other motion that the paths of these motions are changeable. Blue and red shifts of photon on gravitational field show that there is a grand relationship between gravity and electromagnetic wave. If we do resolve this relationship, then we will able answer many of physical mysteries that present theories are not answer them.

With Best Regards

Best regards,

maybe: (this is just a lecture key) imagine that light is an oscillation of "something" that is constrained on both its ends, namely bound between source and destination. To put it another way the photon is not emitted from the emitting that shoots into space towards infinity, the photon is emitted when already "knows" its destination.

A constrained oscillation of the electromagnetic field.

At this point you can conveniently associate the dilation of the frequency for the wave with the concomitant distortion of the metric that indicates the "size" of the space in which the same oscillation exists.

I don't see the usefulness of such a definition, the photon does not "Know". It is emitted with a given frequency in a given direction with speed c. It might be polarized

but otherwise it has no properties useful for "knowing". its goal. Also, the goal may have moved away when the photon arrives at that location.

@Matts yours is a pragmatic vision, while mine is philosophical: imagine the simplest of situations. 2 electrons that scatter exchanging a photon: our reason leads us to say that the photon as a gauge boson is emitted and "after" is received, right?

Only after this phenomenon the 2 electrons are kept into contact and therefore aware of each other. But before then either ignore the presence of the other.

If so, why at a certain moment of their lives they (one of two) must emit a photon?

So

according to your view of things then it would be possible to emit a photon, leaving him free to go and let it continue to infinity; and then maybe it would be possible to create along the way (what is left to run) some object able to receive it and let the photon is then revealed?

You can do any experiments with photons you like and describe it pragmatically. You don't need any philosophy for that.

@Matts Roos: if you look carefully at these 2 experiments, having in mind my point of view, maybe you can reconsider the whole perspective.

http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

here it results easily the observation that a photon never travels but it is already everywhere in its path , as constitued of an infinite aleph2 amount of virtual photons

moreover, if you want to support an argumentation that does not imply the fact that the photon is both here and there [i.e. it lives only in the massive-charged matter, in what we call emitter and receiver], then you have to be able to give a location to the photon. But this is impossible, either prohibited by relativity than by quantum field theory

then

http://en.wikipedia.org/wiki/Purcell_effect

here you can see how a photon, without a contestual absorber can't be emitted. And this is possible only if a photon knows where its destination will be.

You can't emit a photon, then let it travel in space, and then capture it with an appropriate detector: in this way you could not give an answer to the first of two experiments that I showed you, the Delayed choice experiment

V. Fulcoli

Your statement a photon never travels but it is already everywhere in its path is wrong. Photons emitted from distant stars, say supernovae, will be observed in a distant future because they travel over real distances with the velocity c.

Yes you are able to give a location to the photon, those from a supernova explosion travel as a light front until they are observed.

@Matts Roos

It is a perverted corpuscular idea of a photon we get from our daily lives that leads to all the questions about an hypotetical photon motion and speed.

The answers are not to say simple, but rather questions themselves should have no place if you understand what the photon really is (or what it isn’t).

Note that the photon is from the bosons family.

The Photon carries the light just as the gluon “carries” the strong force, or the Higgs boson “creates” the mass.

They are used in physical models and proved themselves useful as proxies in interactions (for the lack of better alternatives), but outside of the equations they make very little sense.

No one have ever observed a boson for example.

The only way to know where  a photon is,  is to get it absorbed by an electron – but then there is no photon any more, and it is still a question if there was one to begin with.

All we know is that one electron somewhere emitted a portion of energy and then another somehow received it. There is definitely no ball of light traveling through the space between them, as was demonstrated by the double-slit experiment all the way back in 1800′s, so the question “how can photon travel” actually became invalid long before it could have come to any of our heads.

Dear V. Fulcoli, I think you could get clarification of your thoughts by discussion with elders at La Sapienza (I have also been a guest professor there) rather than to start a quarrel of words in RG.

@Roos: if you want to make it easy, then make it easy, but in that way it doesnt work