What is the current understanding of the nature of photons?

[Bright]

Hi Bright,

there are no free photons. The only time light shows particle-like properties is during absorption or creation. At this time, and only at this time, the energy becomes localised, and acquires definate momentum, like a particle. When light is not interacting with matter, there are no photons. It's all explained in Einsteins 1916 paper which is translated and published in 'Sources of Quantum Mechanics' edited by van der Waerden, Dover 1970.

Right!
Now we may forget about photons when they are not interacting with matter. However, their speed remains the same v = c. And we cannot change it.

According this picture, photon existed when it was emitted by one atom. Then photon did not existed for a couple of seconds. Then photon again come into being when it had been absorbed by another atom located 600 000 000 km apart from the first atom.

 

[Bright]

I don't understand what you mean. If an electromagnetic field has frequency w, every ripple has energy E = hw, it can't have more nor less; for this reason we say that a photon is "a quantum" of the EM field.

In Quantum Mechanics energy of each ripple is exactly E = hw
In Classical mechanics there is NO such restriction on the ripple energy.

 

[Bright]

Inertial mass of photon may have TWO components:
1. Normal to its velocity.
2. Tangential to its velocity.

We may easily change direction of photon. That means there is nothing unusual with NORMAL component of photon inertial mass.

But we cannot change absolute value of photon speed. That means something is wrong with TANGENTIAL component of photon inertial mass. One possible explanation is that this component of inertial mass becomes infinity.

Anyway, let us try answer the question. If an object is resisting so much to attempts to change absolute value of its speed, what is its inertial mass?

 

[Bright]

That equation is wrong in the relativistic case.

You are right. In relativistic case another definition of inertial mass should be used

The first great achievement of Einstein's General Relativity Theory (where M1 = M2 is postulated) was to correctly predict the deviation of light from stars by the sun. So that equality is confirmed for electromagnetic field too.

I am not sure that in that experimant (deviation of light from stars by the sun) the equivalence M1 = M2 was the primary question.

... what would be the momentum of an infinite mass object?

If its speed v > 0, momentum vould be infinity.
but if only inertial mass is infinity and regular mass is finite, then everything is OK. Momentum is finite.

 

[Astronuc]

5. Consider photon. We may try whatever we can do in order to change absolute value of its speed, however, absolute value of its speed remains the same v = c. That means its resistance to changing its state of motion when a force is applied is EXTREMELY strong.

In Compton effect experiment we may change photon direction, wavelength, . . . .

There appears to be slight inconsistency here.

 

[lightarrow]

In Quantum Mechanics energy of each ripple is exactly E = hw
In Classical mechanics there is NO such restriction on the ripple energy.

Certainly, but answering to Riogho's statement that a photon is a ripple in the electromagnetic field, you wrote:

"Assume the frequency of electromagnetic field is w and energy of the ripple is E.
Now, if E < homework (for instance E = homework / 10 )
then I do not believe that ripple is a photon.
If E = homework then MAYBE that ripple is a photon.
If E = 1.2345 * homework then I have no idea what that ripple is".

 

[Bright]

There appears to be slight inconsistency here.

You are right! Let me make some correrctions.

State of motion in this case has at least three component:
1. Wavelength
2. Direction
3. Absolute value of speed.

As for #1 and #2 - we can change those easily.
As for #3 - we CANNOT CHANGE IT

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

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

 

[Bright]

Certainly, but answering to Riogho's statement that a photon is a ripple in the electromagnetic field, you wrote:

"Assume the frequency of electromagnetic field is w and energy of the ripple is E.
Now, if E < homework (for instance E = homework / 10 )
then I do not believe that ripple is a photon.
If E = homework then MAYBE that ripple is a photon.
If E = 1.2345 * homework then I have no idea what that ripple is".

Yes!
When I wrote that, I meant CLASSICAL PHYSICS.
In the other words, in classical physics we may have objects (like ripple with energy E = homework / 10) that cannot even exist in QUANTUM physics.

 

[lightarrow]

The first great achievement of Einstein's General Relativity Theory (where M1 = M2 is postulated) was to correctly predict the deviation of light from stars by the sun. So that equality is confirmed for electromagnetic field too.

I am not sure that in that experimant (deviation of light from stars by the sun) the equivalence M1 = M2 was the primary question.

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.

... what would be the momentum of an infinite mass object?

If its speed v > 0, momentum vould be infinity.
but if only inertial mass is infinity and regular mass is finite, then everything is OK. Momentum is finite.

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

 

[lightarrow]

Yes!
When I wrote that, I meant CLASSICAL PHYSICS.
In the other words, in classical physics we may have objects (like ripple with energy E = homework / 10) that cannot even exist in QUANTUM physics.

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

 

[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.