What Are Photons? Effects of Electric Charges

[DrewD]

The EM field has to travel through space.

The disturbance in the EM field travels. There is always a field, but the equipotentials "move" at the speed of light. The way that you are thinking about it may be conceptually useful to you, but I don't think there is any real benefit and it doesn't play out well in the math.

 

[Drakkith]

Two electrons in a vacuum, electron 1 and electron 2. They are repelling each other. Electron 1 is moved to a new position. Is electron 2 instantly repelled from the new position of electron 1? Or does it take some time before electron 2 is repelled from the new position?

This example is unclear and you're skipping what happens to the electron during the time that it's moved from point A to point B. Both electrons are constantly under an electric force and are accelerating, so it's not easy (for me) to say what happens.

 

[123987]

The way that you are thinking about it may be conceptually useful to you, but I don't think there is any real benefit and it doesn't play out well in the math.

So how do you think about the EM field? In your mind is it everywhere, exists independent of charge and is warped by charge, like how space-time is warped by mass to cause gravity?

 

[Joel A. Levitt]

Are photons the things which are constantly emitted from electric charges in all directions, which then interact with other electric charges, or are photons something else?

If photons are constantly emitted from electric charges in all directions, and photons are particles, as the ring of photons spreads out, wouldn't there be gaps in the ring of photons?

All of Drakkith's replies are correct. You can trust him.

Physics theories are successful to the extent that they accurately predict a diversity of observed phenomena, and if they are not falsified by other observations. Classical E&M was mostly successful in the 19th century, predicting what had to that time been observed. E&M fields and waves were invented to enable the application of certain mathematics, e.g., the divergence theorem, that made prediction easier.

Theory is convenient, but data is sacred.

The photoelectric effect was studied subsequently. This effect is the emission of electrons (i.e., negative charges) from surfaces of solids when they are illuminated by light. If light behaved like an E&M wave (i.e., carrying energy continuously along its path), then electrons would be emitted only after enough time had passed after the start of illumination for the light to deliver enough energy to the solid's surface. But, what was observed was that electrons were emitted immediately. And, it was inferred that light energy propagated in globs. These globs were named photons.

Thinking about photons, forget the concepts: particles; E&M fields, and E&M waves. From the point of view of contemporary physics these concepts are in a sense archaic. Photons are best described as waves, but not E&M waves. They are best described by the wave solutions of the Schrödinger wave equation.

 

[sophiecentaur]

@Drakkith

All of Drakkith's replies are correct. You can trust him.

Print that out and stick it on your shaving mirror. Read it every morning and 'preen yourself'. :)

 

[haruspex]

Then why are the electrons not instantly repelled?

I feel that underlying the OP question (at least in part) is the philosopher's discomfort with action at a distance.
The physicists caught up with that when they invented 'gauge' theories. In the modern view, space is not empty, but filled with a seething collection of virtual particles that continuously pop in and out of existence. In this view, forces that seem to act at a distance are mediated by interactions among virtual particles.
Virtual photons are the particles (yes, particles, but of no rest mass) which mediate the electric and magnetic fields. Hence, any alteration at the source of a field propagates at light speed.
Fields mediated by particles with nonzero rest mass propagate more slowly.

 

[Mark Harder]

Right now, this is what I'm thinking. First, what do we mean by 'traveling' and 'accelerating'? This brings us back to the concept of inertial reference frames and the relativity of motion. (Wasn't Einstein's first paper on special relativity titled "On the electrodynamics of moving bodies", or some such? ) Suppose you and an electric charge are in a reference frame within which the charge is not moving. If you are close enough to the charge, you could measure the strength of an electric field around the particle with an electrometer or some such. You could measure the distance dependence of the strength and direction of the force exerted by that field on a test charge. As expected, the force will fall off as the inverse square of the distance, and the direction of the force will be either towards or away from the charge, i.e. the force behaves like a vector, with magnitude and direction. Mathematically speaking, there is what's called a 'vector potential' - a mathematical function that assigns a vector to every point in space. But that field can't extend to the edge of the universe unless it sat there immobile in your frame since the beginning of time. If it hasn't, then there will be an edge to that field, a moving boundary (again in your reference frame). Another measuring instrument located beyond this boundary will detect something like a tsunami, an abrupt increase in the electric field that remains constant after the event. In other words, it will have detected a wave (sort of).

Now let the charge in your laboratory begin moving. Both the distance and direction of the charge w.r.t. (with respect to) your lab instruments will change and eventually the same thing will be detected in any reference frame anywhere ( as long as it isn't traveling together with the charge, the case if the charge is constant w.r.t. that other frame.) as a wave-front passing over the instruments. Your moving charge has just created a wave in your lab. That wave will then propagate throughout space. Until it reaches other detectors, however, the 'old' field will be measured there, with the same vector field as before. But the charge that created it is no longer at its center, i.e. the field out there is no longer in step with the charge in your lab and in this sense, it exists without the charge that created it, since that charge is no longer 'there'! Suppose now the charge in question moves sinusoidally. The perturbation in the electric field too will oscillate sinusoidally. Your instruments will have detected an EM wave, and as such this oscillating field will spread throughout space and eventually be detected in all reference frames out there as a classical EM wave.

What I have just said is wrong. Why? I called a purely electric field perturbation an EM wave. What about the 'M' in EM? I have omitted a very important pair of concepts here: acceleration and magnetism. Maxwell's field equations show that when electric charges move they create magnetic fields, and vice versa. Furthermore, when charges accelerate, perturbations of their associated magnetic fields propagate in tandem with the perturbations of their electric fields. Hence, we have a true EM wave. I never studied Maxwell's equations and special relativity, so I'm slipping into waters over my head, so I'll stop except to say that this has been a purely classical explanation. It doesn't take into account quantum effects, i.e. those photons, or relativity which shows that EM radiation doesn't travel as a nice centrosymmetric wave. This is important in understanding synchrotron particle accelerators, where particles traveling in circles close to the speed of light constantly emit x rays concentrated in the instantaneous direction of the particles. The accelerator must constantly replenish the lost energy and add even more if it wants to accelerate the particles even further. Furthermore, the intense, focused beam of x-rays is useful in its own right, for experiments involving materials structure analysis, and so forth.

There's also an important difference between EM and gravity fields. Gravity fields and waves carry only one force, gravity, not 2 as in EM waves. I imagine this was a bete noir in Einstein's later days, when he was trying to create a field theory that would unify EM and gravity.

 

[Mark Harder]

EM waves are disturbances in electric and magnetic fields. Those fields exist in a vacuum. They need no 'medium' to exist, so why would changes in the strength and direction of those fields require a 'medium' like the aether? BTW, I don't like the positing of unseen undetectable entities of any kind in order to explain something. Besides the aether, there was the 'phlogiston' theory of heat 'flow' - an invisible, weightless fluid that flows from one body to another and causes the receptacle of this mysterious substance flow to increase in temperature. Shown to be BS a long time ago, but the idea still hangs on in our lazy habit of talking about heat 'flow'. The most recent version, according to my amateur opinion, is dark matter. There it is, it's invisible, it barely interacts with anything, so we can't exactly expect to verify its existence by direct observation. It's woo woo and don't like it.

 

[Dale]

Moderators note: several posts with misinformation and some clutter have been removed. The participants are reminded to review the forum rules prohibiting the promotion of Lorentz aether theory and other such discredited theories: https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/

 

[lightgrav]

The basis of the OP's error was thinking of an Electric Field vector as "moving" along the vector direction, rather than "pointing" in that direction.
This is perhaps due to ambiguous phrases such as "Electric Field goes from positive to negative", which can still be found even in textbooks.
This common mis-conception starts because the gravity field "g" is introduced as an acceleration (of what?), rather than a vector Field intensity
(it will be prevalent until we purge textbooks of that treatment ... good luck with that - I've been trying for 25 years!)
The "light model" which treats it as a bunch of "photon particles" whose Energies depend on wavelength, is much less satisfying (in many reader's minds) because it seems to ignore the Electric and Magnetic aspects of the light, which had been used all thru optics to derive Snell's Law (for example) and the Poynting vector's Energy Intensity, and to explain refractive index. If (point-to-point) "least time" is used to derive Snell, there is no inference that the Huygens (Displacement current?) wavefront has ANY transverse extent!
Interesting that nobody has mentioned that in this thread yet, since the OP's original question was whether gaps occur between photons at large distances from the source. A more complete presentation of a photon model would include calculating the number of wavelengths in a photon
(several thousand, using dE/dt or dB/dt estimates for simple natural transitions),
and explaining how each photon's self-trapping will limit its transverse spread (thereby setting a minimum E-field intensity even from distant stars).

 

[Mark Harder]

I feel that underlying the OP question (at least in part) is the philosopher's discomfort with action at a distance.
The physicists caught up with that when they invented 'gauge' theories. In the modern view, space is not empty, but filled with a seething collection of virtual particles that continuously pop in and out of existence. In this view, forces that seem to act at a distance are mediated by interactions among virtual particles.
Virtual photons are the particles (yes, particles, but of no rest mass) which mediate the electric and magnetic fields. Hence, any alteration at the source of a field propagates at light speed.
Fields mediated by particles with nonzero rest mass propagate more slowly.

What does 'OP' mean? 'Optical Physics'?

 

[Drakkith]

What does 'OP' mean? 'Optical Physics'?

OP means Original Poster. AKA the person who started the thread.