# Electromagnetic wave vs time?

1. Aug 25, 2014

### jlyu002@ucr.e

Hi PhysicsForums,

I was wondering if this youtube was a correct representation of a photon traveling through space with time.

What I am trying to grasp is that, is the photon creating a trail of static Efields and Bfields as it moves through space as shown in the youtube, or does the photon only have one Efield and Bfield around the photon, in which the Electrif Field and Bfield alternate in their plane, Perpendicular to the propagation of the photon.

Thank you Physicsforums!

Last edited by a moderator: Sep 25, 2014
2. Aug 26, 2014

### Delta²

The photon does not create the EM field because it has zero electric charge.

Photons are created as the energy state of the EM field changes from a ground state with minimal energy to elevated states which have higher energy.

3. Aug 26, 2014

### vanhees71

What's shown in the Youtube movie is a classical electromagnetic plane wave (which doesn't exist in nature but is a very useful mathematical tool since you can describe real electromagnetic waves by superpositions of such plane waves in terms of Fourier series and Fourier integrals).

A photon is a much more abstract concept than these classical waves. It doesn't even make sense to ask about the location of a photon, because it is not possible to define what position means for a massless particle with a spin $\geq 1$. You can only ask with which probability you will detect photon in a detector. The location of this detection is well defined, because the detector is a macroscopic object with mass, where a position observable is well defined.

From the point of view of quantum electrodynamics classical fields are coherent states, which are a superposition of all photon-number (Fock) states. Here the phase of the wave is quite well-defined while the photon number follows a Poisson distribution.

4. Aug 26, 2014

### jlyu002@ucr.e

OO hmm, all these ideas have brought new ideas into my head. I was wondering then, what it meant by Efield and Bfield oscilating. Does that mean as we measure the photon in time, the Efield and Bfield that is associated with the photon would oscillate in their planes? Meaning we measured more Efield in one direction say-y at time t1, and at time t2 we would have an Efield in another direction say-x, but 0 Efield in the y-direction at time 2.

Is this a correct view? It so hard visualizing this. But I think this is wrong bc my description is depicting a Efield and Bfield around a photon that is oscillating.

5. Aug 27, 2014

### Staff: Mentor

What is oscillating is the electric and magnetic field vectors. A field vector represents the direction and magnitude of the force exerted on a charged particle by the field. As an EM wave passes over a charged particle, the particle oscillates back and forth as the field vectors vary in magnitude and direction.

A photon is simply the quantized interaction of the EM wave. This means that when the wave interacts with particles, it does so in finite sized "chunks" of energy which we call photons. The photon is not creating the EM wave, it is inherently part of the EM wave.

6. Aug 27, 2014

### vanhees71

Forget photons! At your level of understanding you have to study classical electrodynamics first. An electromagnetic wave is a space-time dependent electromagnetic field, which you get as solutions of Maxwell's equations.

You cannot follow a photon in time in principle! A photon is created somewhere, then may interact with something (e.g., an atomic nucleus) and then you detect it long after this interaction. You cannot even well define what an interacting photon might be let alone how it behaves in time. Only asymptotically free single-particle Fock states have a proper particle interpretation!

7. Aug 27, 2014

### jlyu002@ucr.e

Ohh, I get it better now. The concept of light is quite intense. Such a weird phenomena in nature. I guess I am a weird phenomena in nature too!