What is the current understanding of the nature of photons?

[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 So your mathematics of claiming that dF is zero is no longer valid!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

<br /> a_p^{\dagger}|0\rangle<br />

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

 

[lightarrow]

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.

So, if a photon had mass, its momentum and its energy should be infinite, in that case.

 

[Mentz114]

Hi Bright,

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

(my emphasis).
In this case, you are not doing science. You must have experimentally testable hypotheses. How do you propose to find out what's happening between measurements, without making a measurement? It's a logical conundrum.

M

 

[Bright]

How do you propose to find out what's happening between measurements, without making a measurement?

I like this question so much!
Unfortunately, I have no clear answer yet ...

My first idea... if photon does not exist between measurements, its evergy does not exist as well. That means NO conservation of energy.

If we believe in conservation of energy, we MUST admit that energy of photon DOES EXIST between measurements. If energy exists between measurements, in what form it exist? The evident answer is: "In form of vulgar photon"
Thus, it seems to me that right now we just proved that photon DOES EXIST between measutements.



If you do not like this proof, I am redy to prove that Mentz114 exists ONLY when he is online. But when we see OFFLINE after his nick, that means that Mentz114, (AND EVEN THE INDIVIDUAL BEHIND THIS NICK!), is not available on the PhysicsForum, and even is not available on the Earth and is not available in the Universe.

Moral: in addition to measurements, there are another possibilities to verify existence. For example, the CONSERVATION LAWS.

Another idea. Assume on Jan. 2, 2008, at 9:00 am, we proved Pythagorean theorem. Then on Jan 10, 2008, at 9:00, we again proved this theorem. My question is: what about this theorem for period of time from Jan 3 to Jan 9? Was it valid for that period of time? According to YOUR consideration, Pythagorean theorem not only was not verified for that period of time, it even did not existed at all between couple of its actual proofs.

 

[Bright]

So, if a photon had mass, its momentum and its energy should be infinite, in that case.

If a photon had REST mass, its momentum and energy should be infinite at v = c

 

[lightarrow]

My first idea... if photon does not exist between measurements, its evergy does not exist as well. That means NO conservation of energy.
If we believe in conservation of energy, we MUST admit that energy of photon DOES EXIST between measurements. If energy exists between measurements, in what form it exist? The evident answer is: "In form of vulgar photon"
Thus, it seems to me that right now we just proved that photon DOES EXIST between measutements.

You started with "If we believe in conservation of energy..." and you finish with "we just proved that photon DOES EXIST". My question is: "believing" is enough as "prove"?

Moral: in addition to measurements, there are another possibilities to verify existence. For example, the CONSERVATION LAWS.

Faith on conservation laws is not physics. Conservation laws are valid as long as they are experimentally valid.

 

[lightarrow]

If a photon had REST mass, its momentum and energy should be infinite at v = c

Can you please write down the equations defining photon's energy and momentum as functions of your inertial mass?

 

[Bright]

You started with "If we believe in conservation of energy..." and you finish with "we just proved that photon DOES EXIST". My question is: "believing" is enough as "prove"?
Faith on conservation laws is not physics. Conservation laws are valid as long as they are experimentally valid.

OK, forget about "believing in conservation of energy"...

Conservation of energy is a law of physics and all of us never heard about its violations.
So, starting with experimentally established law of conservation energy, we can prove that photon exists between measurements. Otherwise law of conservation of energy is wrong.

 

[Bright]

Can you please write down the equations defining photon's energy and momentum as functions of your inertial mass?

No, I cannot, because photon's energy and momentum ARE NOT functions of inertial mass.
They are functions of frequency only

E = h\nu

p = h\nu / c

 

[lightarrow]

No, I cannot, because photon's energy and momentum ARE NOT functions of inertial mass.
They are functions of frequency only

E = h\nu

p = h\nu / c

So, where is the physical meaning of your "inertial mass"?

 

[Bright]

So, where is the physical meaning of your "inertial mass"?

The couple of expressions for E and p do not describe ALL PROPERTIES of photons. There are many other properties of photon not included in these expressions, for example interference, difraction etc.

We may 'feel' physical meaning of "inertial mass" when we are trying to change absolute speed of photon. But photon is RESISTING so much. We cannot change its absolute speed even a little bit. That means its inertial mass (tangential component of it) is very large, or even infinity.

 

[lightarrow]

The couple of expressions for E and p do not describe ALL PROPERTIES of photons. There are many other properties of photon not included in these expressions, for example interference, difraction etc.

We may 'feel' physical meaning of "inertial mass" when we are trying to change absolute speed of photon. But photon is RESISTING so much. We cannot change its absolute speed even a little bit. That means its inertial mass (tangential component of it) is very large, or even infinity.

So, your "infinite inertial mass of a photon" is just another name for "light's speed doesn't change"? Why the need of another name, then?

 

[Mentz114]

We may 'feel' physical meaning of "inertial mass" when we are trying to change absolute speed of photon. But photon is RESISTING so much. We cannot change its absolute speed even a little bit. That means its inertial mass (tangential component of it) is very large, or even infinity.

Wrong logic. You can't apply a force to a photon, so discussing it's inertial mass is a waste of time. It is wrong to apply calssical mechanics, eg F=ma, to photons, as you've been told a dozen times.

Classical reasoning applied to the bending of light around the sun does not agree with experiment. I think that demonstrates the futility of doing it.

 

[Bright]

So, your "infinite inertial mass of a photon" is just another name for "light's speed doesn't change"?

Yes. it is! At last, lightarrow was the first who understood what I am saying...

Why the need of another name, then?

Because if we use the PREVIOUS name, we must think why we cannot change light speed.
But if we use the NEW name, we must not think why we cannot change light speed.

This just a way of unification of physical concepts... We just MINIMIZED number of concepts, which are necessary to explain everything we can observe in nature.

 

[Bright]

Wrong logic. You can't apply a force to a photon, so discussing it's inertial mass is a waste of time.

Wrong statement. In my post about reflection photons from MOVING MIRROR (my post #82 in this topic), I EXPLAINED HOW TO APPLY A FORCE TO PHHOTON. When we move mirror, we must apply ADDITIONAL FORCE. In result, energy of photons becomes larger after reflection from moving mirror.

It is wrong to apply calssical mechanics, eg F=ma, to photons, as you've been told a dozen times.

I never applied classical mechaniks, eg F=ma, to photon. I just used F=ma TO EXPLAIN WHAT IS INERTIAL MASS IN NONRELATIVISTIC CASE. Then I suggested that anybody give relativistic generalization.

Classical reasoning applied to the bending of light around the sun does not agree with experiment. I think that demonstrates the futility of doing it.

Yes, I know, the difference from GR is two times, if I am not mistaken... Again, I did not use classical reasoning for photon, it was jast NONRELATIVISTIC explanation of the tern INERTIAL MASS.

 

[Mentz114]

Bright,
energy is not a relativistic invariant, so it is measured differently from different frames. This includes the frequency of light. This accounts for your 'force', so in my opinion your first premise is wrong.

Can you explain how light is lensed by gravity if it has 'infinite inertial mass' ?

M