I What is a Photon? - Physics Basics Explained

[maline]

"It is clear that the field has the same values for coordinates x and times t which satisfy the relation t—(x/c) = const, that is,
x = const +ct."
This is the most basic description of a wave, and it is very, very different from "the size of the photon is transferred to the next grid location to the right and made either slightly larger or slightly smaller, depending on the phase of the photon".
For one thing, there is no "made slightly larger or smaller". Also, plane waves, by definition, fill (at least) the entire YZ plane at each moment! They do not resemble "balls of energy".

You are looking sideways on a 2d, grid of oscillators

Ah, we're getting somewhere! This is a major improvement over calling "a photon" one "harmonic oscillator".

 

[vanhees71]

Thank you for the reference. On page 108, in The wave equation section:
I feel there is nothing new or different here. The animation at http://www.animatedphysics.com/games/photon_cavity.htm does this exactly. You are looking sideways on a 2d, grid of oscillators. The first part of the animation is the representation of one photon. At each step, the size of the photon is transferred to the next grid location to the right and made either slightly larger or slightly smaller, depending on the phase of the photon. The time interval used here is 0.1 femtoseconds (10^-15), if you use the STEP button, it advances frame by frame.

Argh. Of course, I meant Landau/Lifshitz vol. IV about quantum electrodynamics. Sorry for the confusion.

 

[edguy99]

Argh. Of course, I meant Landau/Lifshitz vol. IV about quantum electrodynamics. Sorry for the confusion.

On page 5 of Landau/Lifshitz vol. IV, Talking about Quantization of the Free Electromagnetic Field:

With the purpose of treating the electromagnetic field as a quantum object, it is convenient to begin from a classical description of the field in which it is represented by an infinite but discrete set of variables. This description permits the immediate application of the customary formalism of quantum mechanics. The representation of the field by means of potentials specified at every point in space is essentially a description by means of a continuous set of variables. ...

On page 11, Introducing Photons:

These formulae enable us to introduce the concept of radiation quanta or photons, which is fundamental throughout quantum electrodynamics. We may regard the free electromagnetic field as an ensemble of particles each with energy ω (= ħω) and momentum k (=nħω/c). The relationship between the photon energy and momentum is as it should be in relativistic mechanics for particles having zero rest-mass and moving with the velocity of light. ... The polarization of the photon is analogous to the spin of other particles; ... It is easily seen that the whole of the mathematical formalism developed in §2 is fully in accordance with the representation of the electromagnetic field as an ensemble of photons; it is just the second quantization formalism, applied to the system of photons. ...

And Continues:

The plane waves ... may be treated as the wave functions of photons having given momenta k and polarizations e^α.

I agree completely. A photon is a plane wave traveling through space at the speed of light.

Also, today is the birthday of Erwin Schrödinger (ca. 1887–1961). A great quote:

The world extended in space and time is but our representation.

 

[Nugatory]

The plane waves ... may be treated as the wave functions of photons having given momenta k and polarizations e^α.

I agree completely. A photon is a plane wave traveling through space at the speed of light.

"The plane wave may be treated as the wave function of a photon" and "A photon is a plane wave" are different statements. If you completely agree with the first, you're rejecting the second.

 

[Giulio Prisco]

Hi Guys and Gal's

In answering a question in general physics I came across the following which explains at a reasonably basic level what a photon is, spontaneous emission etc at the level of basic QM with a bit of math:
http://www.physics.usu.edu/torre/3700_Spring_2015/What_is_a_photon.pdf

IMHO its much better than the usual misleading hand-wavey stuff and even if you don't follow the math would allow a general gist to be had.

Thanks
Bill

What is a Photon? Introduction to Quantum Field Theory
https://works.bepress.com/charles_torre/92/
Now this course is published in book form, open access CC.
It seems a good first intro, with all and only the detail that the newcomer really needs, I wish I had this book when I was studying these things the first time.

 

[vanhees71]

Looks indeed great!