What is the current understanding of the nature of photons?

[physicist888]

simply, photon is a particle without a mass.
as debroglie supposed, that every particle moving with a velocity v it will creat a wave in the medium wher's it's moving.
if we apply this concept on the photon, we could easily find the wave associated to the photon, which people call it electromagnetic wave.
finally, we ask the question, is the photon a particle ? or a wave?
what I am going to say that ths photon is the both. in many experiments, its demonstrated that photon is a particle ( photoelectric for example); in another experiments ( interference of lighe for example) show that photons ( light ) is a wave.
so what debroglie say, that photon has a duality. its a particle which creat a wave in the medium of propagation. so it could behave like a particle or an electromagnetic wave.
in nuclear physics, its more simple to treat photon as a particle.
in quantum physics, its more complicated, cause we need to know the structure of atoms, so we treat photon as a wave which interact with the wave associated to the electrons inside the atoms and give us a results about the form of energy levels inside the atom.spectrum.

 

[Bright]

I mean that GR correctly predicted light deviation of stars and the GR theory itself is constructed on the equivalence M1 = M2; if that equivalence would be wrong for light, as you say, then the actual deviation should be different to the predicted one, IMHO.

In that experiment and in that calculations based on GR theory, PHOTONS CHANGES ITS DIRECTION ONLY. So, only normal (perpendicular) to its velocity component of inertial mass was involved. But there is nothing unusual with that component.

However, they even did not think that TANGENTIAL component of inertial mass can be not equal to NORMAL component. So, they were concerned and veryfied equivalence only for NORMAL component of inertial mass and regular mass.

And what is "regular mass" and how do you define "momentum"?

I use definition from regular college textbook on physics.

Inertial mass, as soon as it is not equal to "normal" mass, cannot be used in ALL expressions for momentum etc.

Tangential Inertial Mass can ONLY be used in very special expressions that describe how an object is resisting to our attempts to change its absolute velocity.

 

[Bright]

...and a photon is a quantum object, not a classical one.

OK! Photon is a quantum object, but a RIPPLE OF EM FIELD can be both quantum and classical one... )

 

[Bright]

so what debroglie say, that photon has a duality. its a particle which creat a wave in the medium of propagation. so it could behave like a particle or an electromagnetic wave.

I would rather say that photon looks like a wave when it propagates, and it looks like a particle when it is absorbed or emitted.

P.S. But I'm afraid that Mentz114 may say now that photon exist ONLY when it interacts with matter. So, there is no wave at all, and photon is definitely a particle.

 

[Mentz114]

Hi Bright,
trying to define properties for something that defies description in clasical terms, always leads to the logically inconsistent and circular discussion we see in this thread.

But it is good to let your imagination have a good run now and then.

Happy new year.

 

[ZapperZ]

Let us try another way. Instead of answering questions about INERTIAL MASS OF photon, I will ask a question about that.

1. We start with a definition from Wikpedia http://en.wikipedia.org/wiki/Mass
"Inertial mass is a measure of an object's resistance to changing its state of motion when a force is applied. An object with small inertial mass changes its motion more readily, and an object with large inertial mass does so less readily".

Already, this is INVALID for a photon. You need NO FORCE to cause it to move!

Thus, your starting premise that you base all of your subsequent argument is faulty.

In photoemission, a photon is completely absorbed by the system. There are zero indication that such a thing has an infinite inertial mass because the momentum of the photon (which, btw, is how one measures an inertial mass in the first place) is negligible. All the description of the photoelectron spectroscopy includes nothing on the initial momentum of the photon. Is this an indication of something having an infinite inertial mass?

I strongly suggest you drop referring to Wikipedia (I'm the wrong person to impress by using Wikipedia as a source) and read the Usenet FAQ on the "mass" of a photon. Even the controversial use of "relativistic mass" doesn't result in something having an infinite value for the photon.

Zz.

 

[Astronuc]

That is why I introduced TWO components of the Inertial Mass.

Normal component (FINITE) corersponds to #2
Tangential component (INFINITY?) corersponds to #3

How does one reconcile Infinite Inertial Mass in the Tangential direction with the deflection of the photon in Compton scattering? How is Infinite Inertial Mass in one direction and finite in the other reconcile with what we know about mass?

 

[Bright]

Hi Bright,
trying to define properties for something that defies description in clasical terms, always leads to the logically inconsistent and circular discussion we see in this thread.

But it is good to let your imagination have a good run now and then.

Happy new year.

You are right... however, quantum terminology has its own logically inconsistent points, which, maybe not so clear as classical ones, but still available.

Happy new year... :)

 

[Bright]

How is Infinite Inertial Mass in one direction and finite in the other reconcile with what we know about mass?

I am very sorry, but it does not reconcile absolutely with what we know about mass. Because what we know about mass is:
1. Inertial mass = Gravitational mass
2. There are no components of inertial mass, only scalar inertial mass.

How does one reconcile Infinite Inertial Mass in the Tangential direction with the deflection of the photon in Compton scattering?

If we consider Compton scattering as an INSTANT process, then there is a logical problem, because tangential component of photon velocity changes. But it cannot be changed because on infinite tangential inertial mass.

But, if we consider Compton process as SLOW process, then everything is OK. Because very small change of velocity in normal direction (dV) may step by step change direction of photon, but INSTANT tangential velocity remains the same. Change of velocity would be of the order (dV), and change of tangential velocity would be of the order (dV)^2. If dV->0, then (dV)^2 is negligeble and there are no logical problems.

 

[Bright]

I strongly suggest you drop referring to Wikipedia

No problem... )
I looked for definitions of inertial mass in other sources... all of them say that inertial mass is an object's resistance to changing its velocity (or speed, or state of motion)

Already, this is INVALID for a photon. You need NO FORCE to cause it to move!

Yes, one need NO FORCE to cause it to move, because as soon as photon is created, it already HAS speed v = c.

Now, my question is: What can we say about RESISTANCE of photon to changing its absolute velocity? We can kill (destroy) photon, but UNTILL IT EXISTS, we cannot change its absolute velocity.

My second question is: Can we describe such RESISTANCE of photon (UNTILL IT EXISTS) to changing its absolute velocity in the terms of INERTIAL MASS?

My third question is: If we can describe that in the terms of inertial mass, then what is the value of inertial mass of the photon?

 

[ZapperZ]

You have turned this thread into a question about a photon's mass. We have had TONS of such threads already.

Until you can actually define and then describe how an "inertial mass" is measured, and then show that such a definition is applicable to a photon, then this whole mess is moot and not relevant to the OP.

Zz.

 

[Bright]

Until you can actually define and then describe how an "inertial mass" is measured, and then show that such a definition is applicable to a photon, then this whole mess is moot and not relevant to the OP.

With great pleasure. Thanks for good question.

Let us start with simple situation, and then, if necessary, make a generalization for relativistic case.

In non-relativistic case in order to find INERTIAL mass, we should just apply a FORCE to an object and make measurements of the force and the ACCELERATION. Then we can find INERTIAL mass from expression

$$F = Mi * a$$

Now the question is how we can apply a force to photon. A photon is electricaly neutral and I do not want deal with gravitation. OK! I have an idea. Consider a MIRROR, which reflects 100% of all incident particles, does not matter electrons, photons, neutrino, superstrings, Higgs bosons etc. If the mirror is in rest, absolute speed of reflected particles is the same as speed of incident particles.

Now we move the mirror toward incident particles at small speed Vm. If speed of the incident electrons in non-relativistic case was Ve, then after reflection from moving mirror the speed would be (Ve + Vm).

In the case of electrons the force is comparatevely small, in the case of protons larger, that means the inertial mass of protons is larger than that of electrons. Everything works!

Now consider photons. When mirror is in rest, we already should apply some force to it in order keep it in rest. If the mirror is moving toward incident photons, this force should be larger. Let the difference between these forces be dF. Now, what is change of ABSOLUTE speed of photons in case of moving mirror? It is zero! So, additional acceleration of photons after we started to move mirror is ZERO. Substitution of our measured FINITE additional force and ZERO additional acceleration in NEWTONIAN expression gives:

$$dF = Mi * da$$

$$dF = Mi * 0$$


Since dF is finite according to our measurements, and da = 0, we immediately have result: Mi must be infinity.

Thus, we applied a REAL FINITE ADDITIONAL force to the mirror when we started to move it toward incident particles. In case of electrons the change of its absolute speed may be big. In case of protons and the same force and the same density of proton beam the change of absolute speed will be smaller, that means inertial mass of protons is larger than that of electrons. In case of photons we applied EXACTLY THE SAME ADDITIONAL force when we started to move mirror, but change of the absolute speed of photons is ZERO. That means inertial mass of photons is extremely large. I believe it is infinity.

Now we need generalization of above theory for relativistic case.

 

[ZapperZ]

You have a very strange understanding of classical mechanics.

What is that "F"? It is the force exerted on the mirror. This is equivalent to the change of momentum, dp/dt of the photon.

So are you telling me that the mirror experience no force whatsoever due to light bouncing off it? Before you answer that, you might want to look up on why a photoemission phenomena cannot happen in free electron gases, and on what principle a "solar sail" work on. This applies even if you move the mirror.

Zz.

 

[Bright]

What is that "F"? It is the force exerted on the mirror. This is equivalent to the change of momentum, dp/dt of the photon.

So are you telling me that the mirror experience no force whatsoever due to light bouncing off it?

1. If speed of mirror is zero and NO light bouncing off it, force = 0
2. If speed of mirror is zero and light bouncing off it, force = F1
3. If mirror is moving toward incident photons, force = F2

A difference dF = F2 - F1 is an ADDITIONAL force, which is necessary to move mirror.

In cases of electrons or protons this ADDITIONAL force actually accelerates particles.
In case of photons this force is not zero. We do some work. Energy of the light bouncing off the mirror is bigger than that of incident light. However, change of absolute speed of photons is zero.

This is equivalent to the change of momentum, dp/dt of the photon.

YES! when we started to move mirror, we sactually increased the momentum of the photons, but NOT their speed!

 

[ZapperZ]

1. If speed of mirror is zero and NO light bouncing off it, force = 0

Really! Who told you that? What happened to the light's momentum? It went from p to 0. You're telling me that the mirror feels no recoil? Did you just violate the conservation of momentum?

Zz.

 

[Bright]

Really! Who told you that? What happened to the light's momentum? It went from p to 0. You're telling me that the mirror feels no recoil? Did you just violate the conservation of momentum?

Zz.

When I say
"1. If speed of mirror is zero and NO light bouncing off it, force = 0"
I mean NO light bouncing off it AND no light INCIDENT on it. NO LIGHT AT ALL. ONLY mirror in the rest. No photons.

Of course, if the mirror absorbs all photons, the force is not zero. But I do not consider such case. I said in the beginning that mirror reflects 100% of EVERYTHING.

P.S.
ZapperZ, Happy New Year!
Have a glass of good French shampaigne (for 10 000 francs for a bottle) ))))))))

 

[ZapperZ]

When I say
"1. If speed of mirror is zero and NO light bouncing off it, force = 0"
I mean NO light bouncing off it AND no light INCIDENT on it. NO LIGHT AT ALL. ONLY mirror in the rest. No photons.

Of course, if the mirror absorbs all photons, the force is not zero. But I do not consider such case. I said in the beginning that mirror reflects 100% of EVERYTHING.

Again, you are making things up as you go along. And this is simple classical mechanics.

So why a mirror reflecting everything 100% not have a force? Again, do simple classical mechanics on a ball with velocity v hitting a surface and bounce with velocity -v. Do you think there's no momentum transfer here onto the mirror?

Edit: When the mirror moves, there will be a DOPPLER SHIFT on the reflected light, which changes its WAVELENGTH and thus, it's momentum, when compared to the unmoving mirror.

Zz.

 

[Bright]

Again, do simple classical mechanics on a ball with velocity v hitting a surface and bounce with velocity -v. Do you think there's no momentum transfer here onto the mirror?

In this case momentum is NOT zero, force is NOT zero

So why a mirror reflecting everything 100% not have a force?

If such a mirror reflects anything, the force is NOT zero.
If such a mirror reflects NOTHING, not electrons, nor photons, the force is zero.

 

[Bright]

Edit: When the mirror moves, there will be a DOPPLER SHIFT on the reflected light, which changes its WAVELENGTH and thus, it's momentum, when compared to the unmoving mirror.

You are absolutely right. Momentum of REFLECTED photons will be bigger than momentum of initial photons. In this experiment we applied some force, we do some work, we increased momentum of photons, but we failed to increase their speed, because their INERTIAL mass is infinity

 

[ZapperZ]

You are absolutely right. Momentum of REFLECTED photons will be bigger than momentum of initial photons. In this experiment we applied some force, we do some work, we increased momentum of photons, but we failed to increase their speed, because their INERTIAL mass is infinity

That is not your original premise! Your original premise argued that there's no change in force! I've just showed you that there is a small change in force when compared to the situation of unmoving mirror. So your mathematics of claiming that dF is zero is no longer valid! There's no infinite m.

Zz.

 

[Bright]

That is not your original premise! Your original premise argued that there's no change in force! I've just showed you that there is a small change in force when compared to the situation of unmoving mirror. So your mathematics of claiming that dF is zero is no longer valid!

I'am sorry, probably that was misunderstanding. I never said dF = 0
When we started to move mirror, we have to apply ADDITIONAL force (if mirror reflects anything)

However, if there is no electrons or photons to reflect and mirror reflects NOTHING (no light, no electrons) then this additional force is not necessary.

In my first post on MOVING mirror (post #82) I said:
"If the mirror is moving toward incident photons, this force should be larger. Let the difference between these forces be dF."

 

[ZapperZ]

I'am sorry, probably that was misunderstanding. I never said dF = 0
When we started to move mirror, we have to apply ADDITIONAL force (if mirror reflects anything)

However, if morror reflects NOTHING (no light, no electrons) this additional force is not necessary.

I give up. This is getting absurd.

BTW, Mathematicians will go nuts looking at your argument that you can deduce directly that dF = infinity*zero is finite.

You might also want to consider the logic of your argument with neutrinos. Considering that we originally thought that it had no mass and moves exactly at c, by your premise it should have an infinite mass. It took a lot of effort to find out that, instead, it had the tiniest of mass. Now how can that be that there is such an abrupt transition from "infinity" to "tiny"? It went from having an infinite mass to almost no mass based on your argument. You don't find this rather outrageous?

Zz.

 

[Bright]

BTW, Mathematicians will go nuts looking at your argument that you can deduce directly that dF = infinity*zero is finite.

dF is finite because of measurements.
da = 0 because of measurement
Mi = ?
(dF = Mi * da)

OK, well, instead of dF = Mi * da, consider another expression:

1 / Mi = da / dF

now, da = 0, dF is finite,
then ( 1 / Mi ) = 0
Is everything OK with mathematics?

You might also want to consider the logic of your argument with neutrinos. Considering that we originally thought that it had no mass and moves exactly at c, by your premise it should have an infinite mass.

Case of neutrino is more difficult, because until now nobody knows if their speed is "c" or something very close to "c". And nobody knows if their rest mass is exactly ZERO or just very small.

By the way, I never told about INFINITE MASS, I told about infinite INERTIAL mass only.

 

[ZapperZ]

Then you need to cite references that allow you to separate out these various masses and how they somehow are not the same, especially in reference to the covariant mass in relativity. You should also look at the relativistic equations and figure out why F=ma that you are using is invalid for a photon.

Zz.

 

[jostpuur]

I'm confused. It's a particle and it's also a wave; I'm actually starting to think that photon is just a distortion of spacetime or something... It's a small electro-magnetic field, where one always induces the other and thus this ever-changing electromagnetic pulse travels through space.

A photon at least is not a wave packet of a classical electromagnetic field. A photon is a quantum mechanical concept, not classical.

Do physicists actually have a well accepted idea of what a photon ACTUALLY is?

IMO, no. We would need to have a well accepted idea of what the relativistic quantum mechanics is, in order to know what photons are. But there is no well accepted understandable relativistic quantum mechanics.

I'm also thinking that as photon comes near an electron, the way it gets absorbed would be that somehow the electric field interacts with the electrons electric field and it then pulls/pushes that electron and converts this energy stored in the photon's electric field into the kinetic energy of an electron. Or something like that. I don't know. I haven't exactly figured out emittance of a photon yet.

Physicists are capable of calculating something with photon emissions and absorptions, but not surprisingly, I don't understand much of it. For example Peskin & Shroeder book has a section "particle creation by a classical source" in the first chapter about Klein-Gordon field. All QFT books probably discuss something about it. Feel free to try figuring out what they are saying.

How right/wrong am I? What is the accepted view?

If you have an opinion, that has a clear meaning, chances are it's wrong.

The accepted view is to use different wave equations or other mathematical constructions without well defined interpretations.

 

[jostpuur]

As far as I know the most stringent definition is something like "an excitation of a tempo-spatial mode".
Basically, every system "contains" an infinite number of electromagnetic modes and each mode can be occupied by 0 or more excitations that we call photons
This is somewhat analogues to other phenomena in e.g. acoustics. There is an inifinte number of acoustic modes in a room, when you clap your hands some modes are excited but when the the room is silent no mode is occupied (but they are still there).

An harmonic oscillator of frequency v,excited to an energy level n is equivalent to n photons.

Yeah... I'll try to put the same thing in more "axiomatic style" (perhaps less intuitive at the same time). Mainstream information: "A quantum mechanical field can be thought to be an infinite dimensional quantum mechanical harmonic oscillator, and then the excitation states are interpreted as being the momentum eigenstates of relativistic quantum mechanical particles." (A detail: The infinite amount of dimensions correspond to the different wave modes, which there are also infinitely)

I suppose all physics knowing people agree with this? If somebody disagrees, then he/she could explain it in more detail.

Mephisto, if nobody disagrees with that greatly, you can consider that as some kind of "safe" piece of information.

But the problems don't end there. It is so easy to define states in some abstract vector space by raising operators like this

$$a_p^{\dagger}|0\rangle$$

but such states are easily left completely disconnected from the physical reality. Here's some simple questions that IMO seem to be tabu subjects.

Can photons be described by spatial wave functions?

If they cannot, then how do we deal with the spatial probability densities?

If they can, is the time evolution of the relativistic wave function given by some PDE? If yes, then what is the PDE? If not, then how is the time evolution given?

 

[Bright]

Then you need to cite references that allow you to separate out these various masses and how they somehow are not the same, especially in reference to the covariant mass in relativity.

You are right. I need to cite references... but what can I do if there is NO such references yet... until now all scientists believed that all masses are equal. And only since this topic was started, we got some preliminary ideas about situations when they may be NOT equal to each other... :)

You should also look at the relativistic equations and figure out why F=ma that you are using is invalid for a photon.

Again you are right. I think correct relativistic approach may change some expressions, but the main idea would be the same: When we move mirror that reflects photons, we do some work, we increase photon energy, but we cannot increase their speed. Thus, photons resist so hard against increasing their speed. Btw, relativistic electrons as well resist hard against increasing their speed. And we know, that inertial mass of relativistic electrons is very large. However, photons resists even harder. That means photonic INERTIAL mass is larger than that of relativistic electrons.

 

[Bright]

A photon at least is not a wave packet of a classical electromagnetic field. A photon is a quantum mechanical concept, not classical.

We would need to have a well accepted idea of what the relativistic quantum mechanics is, in order to know what photons are. But there is no well accepted understandable relativistic quantum mechanics.

...

The accepted view is to use different wave equations or other mathematical constructions without well defined interpretations.

jostpuur, I agree with you 99%!

Today, the best theory of atomic and nuclear phenomena is QFT.
But it is TOO abstract.
It is good when we nedd to calculate stationary levels of energy or probabilities of some transitions. However QFT refuses give us any explanations, any understandable models etc. The only thing the QFT can do is gives us set of rules, like do this and do that and you will have correct value of energy levels of this or that system.

I think we can say that QFT is a theory of stationary eigenvalues (levels of energy), but it is not a theory of PHENOMENA, not a theory of POCESSES, not theory of what is going on, but a theory about what kind of energy spectrum and probabilities may be obtained in experiments.

 

[jostpuur]

Oh! We should have asked Mephisto if he has the same problem also with the non-relativistic electrons. Mephisto, do you find non-relativistic electrons equally puzzling? If so, then perhaps my, f95toli's, and quantumfireball's posts were not ideal. Even once the non-relativistic electrons are understood, the photons still remain problematic. In order to deal with a one problem at the time, one should start with the non-relativistic electrons.

I hope Bright does not end up getting the thread locked. Perhaps moving the relativistic force problem to another thread would be more appropriate?

 

[ZapperZ]

You are right. I need to cite references... but what can I do if there is NO such references yet... until now all scientists believed that all masses are equal. And only since this topic was started, we got some preliminary ideas about situations when they may be NOT equal to each other... :)

Then you are making things up as you go along, which is what I've claimed all along. And you are also making your own personal theory, which is a no-no if you've read the PF Guidelines.

Again you are right. I think correct relativistic approach may change some expressions, but the main idea would be the same: When we move mirror that reflects photons, we do some work, we increase photon energy, but we cannot increase their speed. Thus, photons resist so hard against increasing their speed. Btw, relativistic electrons as well resist hard against increasing their speed. And we know, that inertial mass of relativistic electrons is very large. However, photons resists even harder. That means photonic INERTIAL mass is larger than that of relativistic electrons.

No, the main idea is NOT the same. F=ma is not valid for relativistic dynamics. This is because p is not just mv since the total energy is now the quadratic sum of momentum term and proper mass term for an arbitrary particle. And guess what the proper mass is for a photon? It is only then are you able to take into account such dynamics, not simply by haphazard guesswork like this.

Zz.

 

[Bright]

I hope Bright does not end up getting the thread locked. Perhaps moving the relativistic force problem to another thread would be more appropriate?

I like this idea!

However, I think first we should define precisely (if that is possible) an object, we are talking about. That object is PHOTON. I'm going to start a new topic "Models of Photon" and discuss at least THREE models of photon, which are Schrodinger, Heisenberg and QFT models.

 

[lightarrow]

With great pleasure. Thanks for good question.

Let us start with simple situation, and then, if necessary, make a generalization for relativistic case.

In non-relativistic case in order to find INERTIAL mass, we should just apply a FORCE to an object and make measurements of the force and the ACCELERATION. Then we can find INERTIAL mass from expression

$$F = Mi * a$$

Now the question is how we can apply a force to photon. A photon is electricaly neutral and I do not want deal with gravitation. OK! I have an idea. Consider a MIRROR, which reflects 100% of all incident particles, does not matter electrons, photons, neutrino, superstrings, Higgs bosons etc. If the mirror is in rest, absolute speed of reflected particles is the same as speed of incident particles.

Now we move the mirror toward incident particles at small speed Vm. If speed of the incident electrons in non-relativistic case was Ve, then after reflection from moving mirror the speed would be (Ve + Vm).

In the case of electrons the force is comparatevely small, in the case of protons larger, that means the inertial mass of protons is larger than that of electrons. Everything works!

Now consider photons. When mirror is in rest, we already should apply some force to it in order keep it in rest. If the mirror is moving toward incident photons, this force should be larger. Let the difference between these forces be dF. Now, what is change of ABSOLUTE speed of photons in case of moving mirror? It is zero! So, additional acceleration of photons after we started to move mirror is ZERO. Substitution of our measured FINITE additional force and ZERO additional acceleration in NEWTONIAN expression gives:

$$dF = Mi * da$$

$$dF = Mi * 0$$


Since dF is finite according to our measurements, and da = 0, we immediately have result: Mi must be infinity.

Thus, we applied a REAL FINITE ADDITIONAL force to the mirror when we started to move it toward incident particles. In case of electrons the change of its absolute speed may be big. In case of protons and the same force and the same density of proton beam the change of absolute speed will be smaller, that means inertial mass of protons is larger than that of electrons. In case of photons we applied EXACTLY THE SAME ADDITIONAL force when we started to move mirror, but change of the absolute speed of photons is ZERO. That means inertial mass of photons is extremely large. I believe it is infinity.

Now we need generalization of above theory for relativistic case.

If, as you say, the photon had a non 0 (tangential) mass, there would be a simple explanation to the fact that you have a force with no acceleration, in that case: as it has already been written, F = m*a is not valid in the relativistic case. If acceleration a has the same direction of a body whith mass m and velocity v, the right formula is more simple and it is:

F = m*a/sqrt[1 - (v/c)^2]

Now send v --> c and tell me what happens in that formula.
If v --> c faster than how m --> 0, the term m/sqrt[1 - (v/c)^2] goes to infinity.

 

[muppet]

My understanding of the answer to the OP: QFT treats particles as secondary, and some fairly abstract 'fields' as being primary. These fields permeate all space, and are pretty much defined in terms of the interactions with the field that particles we observe experience. In this picture, the photon is the quantum of the electromagnetic field; it represents the fact that all of this field's interactions come in discrete 'lumps'- integral mupltiples of Planck's constant (or of Planck's reduced constant).
What this actually 'means' is something that no-one actually knows for sure. Some arguments are made that the photon actually has no existence outside of interactions which we can measure; it exists solely as a "field of potentialities" which has the potential to become localised at a particular point upon an act of measurement being performed. The standard 'interpretation' is that the ontology (the 'true nature', irrespective of our knowledge) of the photon is unknowable, as the act of measurement combines that which we would measure passively with measuring apparatus that even in principle cannot avoid altering whatever independent existence the photon may have.

see here if you want to read more about QFT from someone who actually understands it properly

 

[Bright]

F = m*a/sqrt[1 - (v/c)^2]

Now send v --> c and tell me what happens in that formula.
If v --> c faster than how m --> 0, the term m/sqrt[1 - (v/c)^2] goes to infinity.

It is easy to say Now send v --> c , but hard to do...
because we can do that, for example, for an electron, since electron EXISTS at v --> c .
Hovewer, photon do not exist at v --> c , photon exist ONLY at v = c
So, I am not sure that we may use LIMIT formalism for photon.

 

[Bright]

Some arguments are made that the photon actually has no existence outside of interactions which we can measure;

OK! Great! This is the END of Science.
QFT had been developed because somebody wanted a theory that explained ONLY measurements and ignored the nature BETWEEN measurements. As soon as such very special and limited theory had been created, we accepted it as an ULTIMATE truth. But we forget, that nonexistence between measurements was our original intent to simplify theory and avoid difficult questions about wave collapse, interpretations etc.

The standard 'interpretation' is that the ontology (the 'true nature', irrespective of our knowledge) of the photon is unknowable, as the act of measurement combines that which we would measure passively with measuring apparatus that even in principle cannot avoid altering whatever independent existence the photon may have.

I think if we accept QFT as absolute truth, then we MUST accept that 'standard interpretation' in order to avoid logical problems and inconsistencies with QFT.

But, if we have a choice to forget about QFT, we may think whatever we want about existence of photons between measurements