# Planck's Nobel Address - Photon direction

1. Apr 17, 2005

### Andrew Mason

Although Max Planck introduced the concept of light quanta in 1900, he did not accept it as real until much later. (Even as late as 1913 he refused to believe it, apparently (see: http://physicsweb.org/articles/world/18/4/2).

By 1920 he had accepted that light quanta were real and posed the following question (in his Nobel address given in June 1920):

"There is in particular one problem whose exhaustive solution could provide considerable elucidation. What becomes of the energy of a photon after complete emission? Does it spread out in all directions with further propagation in the sense of Huygens' wave theory, so constantly taking up more space, in boundless progressive attenuation? Or does it fly out like a projectile in one direction in the sense of Newton's emanation theory? In the first case, the quantum would no longer be in the position to concentrate energy upon a single point in space in such a way as to release an electron from its atomic bond, and in the second case, the main triumph of the Maxwell theory - the continuity between the static and the dynamic fields and, with it, the complete understanding we have enjoyed, until now, of the fully investigated interference phenomena - would have to be sacrificed, both being very unhappy consequences for today's theoreticians."​

Has this question ever been completely answered?

AM

2. Apr 17, 2005

### dextercioby

HINT:Question:Were lasers invented in 1920?

Daniel.

3. Apr 17, 2005

### Andrew Mason

In stimulated emission, that would be the case, just on the basis of conservation of momentum. I think that was Einstein's reasoning when he introduced (about 3 years before Planck's speech) the concept of stimulated emission of light quanta. But what about non-stimulated random emission? If the photon is emitted in one direction only, how does momentum get conserved?

AM

4. Apr 17, 2005

### dextercioby

Does the photon have a momentum (3 vector part of the energy momentum 4vector) and is this 4 momentum conserved if the photon is free (doesn't interact with any other particle) ?

In 1920 the physicists were not convinced by the corpuscular structure of em.radiation.I hope u know why.

So i hope it's clear now.

Daniel.

5. Apr 17, 2005

### Andrew Mason

Clear as mud. The photon has momentum E/c but no rest mass, so no 3 momentum. Is it conserved if it does not interact? Doesn't it have to be?

Let me ask the question another way.

A relativistic electron encountering a strong magnetic field perpendicular to its direction of motion, will emit a photon (synchrotron radiation). In the rest frame of the electron, what will the direction of the photon be?

AM

Last edited: Apr 18, 2005
6. Apr 17, 2005

### dextercioby

This is a joke,right? For a free photon the momentum 4-vector is $$p^{\mu}=\left(\omega,\vec{k}\right)$$

,where,because of its lack of mass,$\omega=\left|\vec{k}\right|$

Of course it is,do you think i've said otherwise?

I'll get back to you on this one (if no one else answers it),as soon as i document myself on synchrotron radiation in QED.

Daniel.

7. Feb 9, 2010

### rehevkor5

From the wikipedia article on light: "the momentum p of a photon is also proportional to its frequency and inversely proportional to its wavelength."

However, like you I am also interested in how light can have momentum, and how this impacts the probabilities which add together to determine where a photon ends up.

8. Feb 9, 2010

### conway

There are many unanswered questions in quantum mechanics but it is interesting that Planck's question has in fact been answered. How can the quantum be "in the position to concentrate energy upon a single point in space such as to release an electron from its atomic bond"? The answer is that the electron itself behaves in ways that no one could imagine only a hundred years ago. And that once the true nature of the electron was understood (1926) it eventually became clear that the phenomenon in question, namely the release of an electron from its atomic bonds, could indeed be understood in terms of the classical electric fields of Maxwell. The "concentration of energy" which worried Planck and others so much is in fact a natural consequence of the interaction of ordinary electromagnetic energy with the distributed wave functions responsible for electric charge.

9. Feb 10, 2010

### PhilDSP

There seems to be a secret locked up inside the general QM concept which very few have dared to try to unlock. That is that the flow of energy can and does in some circumstances becomes focused geometrically to create in effect a vortex in at least one dimension which generates a self-propelled traveling wave. Spinning objects with EM fields that vary angularly can generate that type of energy flow.

Last edited: Feb 10, 2010