What is the current understanding of the nature of photons?

[dst]

P.S. There are ANNIHILATION operators in quantum field theory that can destroy photon.
But, I never heard about any operators that could STOP photon.

http://www.hno.harvard.edu/gazette/1999/02.18/light.html

 

[Bright]

http://www.hno.harvard.edu/gazette/1999/02.18/light.html

That example with Bose-Einstein condensate is very interesting. But everything depends on interpretation.

Consider an example, when light is slowed not 20 billion times, as in your citation, but only 1.5 times, as it happen in glass.

Sped of light according to regular, commonly accepted understanding is

v = c / n

where n is refractive index of material.

Now, let us recollect a piece of Feynman explanation of refraction.
In his explanation, in materials the speed of light is still = c.
But, material generates secondary waves, so superposition of initial and secondary waves LOOKS LIKE SLOW wave.

The similar happens in cited recent experiment. The behaviour of light in Bose-Einstein condensate is very complex. There is no stright trajectories for light propagation anymore, as it was in vacuum. Light is absorbed and emitted billion times before it leaves the area of BE condensate. The total effect of such absorbtion-emission again LOOKS LIKE SLOW wave.

 

[lightarrow]

Actually, we are not able TO STOP photon. Because its inertial mass is infinity.

But, we can DESTROY photon. If a photon would be detected by any device, it would not exist after detection. That means we would destroy both photon itself and its infinite inertial mass.

Bottom line. Photon cannot be stopped until it exists, but it can be destroyed. :)

What is "inertial mass" in a photon? How could it be infinite? It's mass is zero, it's energy divided by c^2 is finite, so what are you talking about?

 

[dst]

That example with Bose-Einstein condensate is very interesting. But everything depends on interpretation.

Consider an example, when light is slowed not 20 billion times, as in your citation, but only 1.5 times, as it happen in glass.

Sped of light according to regular, commonly accepted understanding is

v = c / n

where n is refractive index of material.

Now, let us recollect a piece of Feynman explanation of refraction.
In his explanation, in materials the speed of light is still = c.
But, material generates secondary waves, so superposition of initial and secondary waves LOOKS LIKE SLOW wave.

The similar happens in cited recent experiment. The behaviour of light in Bose-Einstein condensate is very complex. There is no stright trajectories for light propagation anymore, as it was in vacuum. Light is absorbed and emitted billion times before it leaves the area of BE condensate. The total effect of such absorbtion-emission again LOOKS LIKE SLOW wave.

That's true, it just gets repeatedly absorbed and retransmitted but that's the closest we'll get to "slowing light". Everyone has already acknowledged that the speed of light is constant no matter what.

I can't see why a massless particle would have infinite inertial mass unless inertial mass was inversely proportional to mass of a particle and that's obviously a retarded concept.

 

[lightarrow]

You are right. There is a weak point in my proof. And I do not know about conservation of the gravity at the moment.

Ok! Below is a better proof. It is well established, that black holes has only mass (and sometimes momentun).

Instead of the box we have black hole. We drop proton and antiproton toward black hole. These proton and antiproton ADD mass to the original mass of black hole. Now, assume that inside black hole two our particles annihilate. This process CANNOT reduce mass of resulting (original black hole plus two our particles) mass of black hole. So, this is a proof that photons have gravitational mass.

You don't need a black hole: when a couple electron/positron (with total momentum = 0) annihilates, it generates a couple of gamma photons of identical energy traveling in opposite directions and the mass of the system of the two photons is exactly = the total energy of the initial couple electron/positron.

But one single photon has mass = 0.

 

[ZapperZ]

Actually, we are not able TO STOP photon. Because its inertial mass is infinity.

But, we can DESTROY photon. If a photon would be detected by any device, it would not exist after detection. That means we would destroy both photon itself and its infinite inertial mass.

Bottom line. Photon cannot be stopped until it exists, but it can be destroyed. :)

You say destroy, I say stop. The end effect is the same, i.e. the momentum is gone or transfered. Would you care to tell me what happen if I transfer the momentum of something with an infinite inertial mass?

Zz.

 

[Bright]

What is "inertial mass" in a photon? How could it be infinite? It's mass is zero, it's energy divided by c^2 is finite, so what are you talking about?

In physics, there are TWO masses.
One mass, M1, is in definition of gravitational potential

Potential ~ M1 / r

This is gravitational mass.
Another mass is in acceleration formula

a = F / M2

This is inertial mass.
Many experiment confirmed that with GREAT precision

M1 = M2

But, all the experiments were performed only for regular bodies, not for fields.
So, may be for fields, like photon

M1 is not equal M2

As for photon, it travels only with speed c, and never slower or faster (restrict themselsf for simplicity only considering vacuum). So, we cannot change absolute value of photon speed. Than can be interpreted as INFINITE photonic INERTIAL mass.

 

[Bright]

I can't see why a massless particle would have infinite inertial mass unless inertial mass was inversely proportional to mass of a particle and that's obviously a retarded concept.

I am not sure that inertial mass was inversely proportional to mass of a particle...
maybe there are TWO different definition of the INERTIAL mass. Could you please give any link to definition of the inertial mass which is inversely proportional to mass of a particle?

 

[Bright]

P.S. I just found in Wikpedia definition of inertial mass.
http://en.wikipedia.org/wiki/Mass
It seems to me that it is NOT inverse proportional to the mass of particle, but just equal to that in the Newtonian law F = ma

 

[Bright]

... the mass of the system of the two photons is exactly = the total energy of the initial couple electron/positron.

But one single photon has mass = 0.

Sorry, I do not understand...
Mass of two photons is NOT equal to zero. OK, well...
Why a single photon has mass = 0?

 

[dst]

I am not sure that inertial mass was inversely proportional to mass of a particle...
maybe there are TWO different definition of the INERTIAL mass. Could you please give any link to definition of the inertial mass which is inversely proportional to mass of a particle?

I'm saying it is difficult to believe that a photon could have an infinite inertial mass unless inertial mass is inversely proportional to rest mass.

 

[Bright]

You say destroy, I say stop. The end effect is the same, i.e. the momentum is gone or transfered. Would you care to tell me what happens if I transfer the momentum of something with an infinite inertial mass?

Zz.

OK! It seems to me that we nedd THREE masses for every particle.
Two of them, well known in hystory of physics, are inertial and gravitational masses.
The third is MOMENTAL (associated with moment) mass

The MOMENTAL mass of photon is not infinite and is equal to its gravitational mass (something like homework / c^2 )

But, inertial mass of photon is still infinity. And there is NO conservation of inertial mass. When photon is destroyed (or stopped, as you prefer), the inertial mass just DISSAPEAR, but not transformed to another body like detector of photon.

 

[Bright]

I'm saying it is difficult to believe that a photon could have an infinite inertial mass unless inertial mass is inversely proportional to rest mass.

OK! If you give such a definition of inertial mass, then it would be easy to believe...
but I do not understand why such a definition is necessary.

Wikpedia says about inertial mass the following:
"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. "

Photon resists SO MUSH when we try change its velocity, when we try make it larger or smaller than c. That means its inertial mass is VERY large.

 

[jcsd]

OK! If you give such a definition of inertial mass, then it would be easy to believe...
but I do not understand why such a definition is necessary.

Wikpedia says about inertial mass the following:
"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. "

Photon resists SO MUSH when we try change its velocity, when we try make it larger or smaller than c. That means its inertial mass is VERY large.

Whilst a photons speed is fixed at c, it's very easy to change it's velocity (speed is the magnitude of velocity), by changin it's direction.

We're talking relativity here, so an all-encompassing definition of inertial mass is not obvious as F=ma no longer holds true. Howvwre any reasonably sensible defitnion of inertial mass does not diverge for a photon.

 

[Bright]

Whilst a photons speed is fixed at c, it's very easy to change it's velocity (speed is the magnitude of velocity), by changin it's direction.

OK! Inertial mass of photon has TWO components. One tangential to its velocity. This component equals to infinity. And another component is normal to its velosity. This component of the inertial mass is FINITE.

 

[peter0302]

I don't know if this is in any way significant or noteworthy (or maybe just obvious) but if you take the formulas

E=pc
p=ymv (y=gamma)

And

E=ymc^2 (y=gamma)

You get

ymvc=ymc^2

v=c

In other words, anything with zero mass must travel at the speed of light irrespective of reference frame.

 

[Riogho]

It a result of the theory of relativity that a photon has momentum, but
have zero mass. One of the worrisome aspects of relativity and quantum
mechanics is that we must abandon our graphic notions of Newtonian
mechanics, and also some of the notions of classical electromagnetic theory
of Maxwell. The problem you are asking about is only one of several notions
that has to be abandoned. In addition, there is the "fact" that the photon
has angular momentum -- but from the classical picture, "something" has to
be spinning. What's spinning? And an electron orbiting a proton in the
hydrogen atom should spiral into the proton emitting electromagnetic
radiation, but it doesn't. Why not? The "why not" to all the above is that
our classical picture just does not correspond to observation, so we are
forced to reject our "picture" and adhere to the "observation" and not the
other way around.

 

[Riogho]

Also, if you think of a photon as a ripple in the electromagnetic field, which is all it is. Then of course it has no rest mass. If the photon were to be still, it wouldn't be a ripple now would it, and it would not exist. So in order to exist at all, it must be moving.

 

[Mephisto]

i am quite pleased with the answer that it is a ripple in the electromagnetic field, especially since i prefer to think about the field as an actually existing physical entity, not just a mathematical abstraction. This thread is starting to go a little off topic.

 

[lightarrow]

In physics, there are TWO masses.
One mass, M1, is in definition of gravitational potential

Potential ~ M1 / r

This is gravitational mass.
Another mass is in acceleration formula

a = F / M2

This is inertial mass.

That equation is wrong in the relativistic case.

Many experiment confirmed that with GREAT precision

M1 = M2

But, all the experiments were performed only for regular bodies, not for fields.

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.

So, may be for fields, like photon

M1 is not equal M2

As for photon, it travels only with speed c, and never slower or faster (restrict themselsf for simplicity only considering vacuum). So, we cannot change absolute value of photon speed. Than can be interpreted as INFINITE photonic INERTIAL mass.

That's a wrong deduction; if you destroy the photon when you detected it, this doesn't mean that you could'n slow it down; it could mean that you have *completely* slowed it down. Anyway, the "infinite inertial mass of a photon" is wrong just for what ZapperZ wrote you: what would be the momentum of an infinite mass object? And even if you postulated a non-infinite mass and a non-infinite momentum for the photon, all this wouldn't match with the equation that jtbell wrote you:

$$(mc^2)^2 = E^2 - (pc)^2$$

 

[lightarrow]

Sorry, I do not understand...
Mass of two photons is NOT equal to zero. OK, well...
Why a single photon has mass = 0?

1. A single photon's mass is zero because if it wasn't then we could apply:

E = (mc^2)/sqrt[1 - (v/c)^2]

and v = c --> E = infinite (which is not).

2. The mass of a system of two identical energy photons not traveling exactly in the same direction is not zero because in that case you can find a reference frame in which they travel in opposite direction and with equally opposite momentum, so the system's momentum is zero. Then, applying:

(mc^2)^2 = E^2 - (cp)^2

you have (mc^2)^2 = E^2 and so m =/= 0.

 

[ZapperZ]

OK! It seems to me that we nedd THREE masses for every particle.
Two of them, well known in hystory of physics, are inertial and gravitational masses.
The third is MOMENTAL (associated with moment) mass

The MOMENTAL mass of photon is not infinite and is equal to its gravitational mass (something like homework / c^2 )

But, inertial mass of photon is still infinity. And there is NO conservation of inertial mass. When photon is destroyed (or stopped, as you prefer), the inertial mass just DISSAPEAR, but not transformed to another body like detector of photon.

Don't you ever feel that you're making this up as you go along?

While there is no conservation of "mass", there is still a conservation of momentum. You're digging even deeper into the hole with this. You haven't addressed how we could easily stop or transfer (or even detect) something with moviing (at c no less) having an "infinite inertial mass". Show me how a Compton effect experiment is consistent with this. So far, all you have done have been simply giving some hand-waving statements without justification based on empirical evidence.

I would also like to remind you to review the https://www.physicsforums.com/showthread.php?t=5374" that you have agreed to. Pay particular attention to the issue of speculative, unverified personal theory.

Zz.

 

[lightarrow]

Assume the frequency of electromagnetic field is w and energy of the ripple is E

Now, if E < hw (for instance E = homework / 10 )
then I do not believe that ripple is a photon.

If E = hw then MAYBE that ripple is a photon.

If E = 1.2345 * hw then I have no idea what that ripple is.

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.

 

[Mentz114]

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.

 

[Bright]

While there is no conservation of "mass", there is still a conservation of momentum. You're digging even deeper into the hole with this. You haven't addressed how we could easily stop or transfer (or even detect) something with moviing (at c no less) having an "infinite inertial mass".

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".

2. We start with simplified situation, when all processes are elastic and particles are not destroyed.

3. Consider an electron first. When its speed is very slow, its resistance to changing its state of motion when a force is applied is very weak, and according definition, its inertial mass is small and equal to its regular mass.

4. When speed of electron approaching to the speed of light, for example v = 0.999999999*c, it becomes very difficult to change absolute value of its speed. We may apply huge force for long time, but change of its speed would be mizerable. For example v = 0.999 999 999 5*c instead of v = 0.999 999 999*c. in this case its resistance to changing its state of motion when a force is applied is very strong, and according definition, its inertial mass is large.

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.

6. Now, my question is. What is INERTIAL MASS of the object, described in section #5? . Note: use definition of inertial mass from Wikpedia or from another source.

Possible answers:
1. Mie = 0
2. 0 < Mie < infinity
3. Mie = infinity
4. Anything else

Show me how a Compton effect experiment is consistent with this.

In Compton effect experiment we may change photon direction, wavelenght, but its absolute speed remains the same v = c.

 

[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.