Ok, but is it made up of red components and green components in the sense that there is a combined wave that would look like the two frequencies added together, or is it simply that photons don't interfere with each other as they propagate through the em field?

Signals added together (of two frequencies or just one) is exactly what we mean when we say interference in physics. What they don't do is interact. They don't create other frequencies, the presence of a second signal doesn't cause the first signal to propagate differently, etc.

I don't have the necessary intuitions in this area to understand your answer. I thought you had ruled out interference earlier because of the linear medium. It seems like I could reword my original question as, "do photons interfere with each other as they propagate through the em field?" Could you give me a yes/ no on that, or is it a bad question?

I posted before I saw your edit. I get what you're saying now. So basically, every photon propagates through the em field as a pure sine wave, it sounds like. Could you confirm? Also, the two-slit experiment: is the interference pattern also due to there being a screen? Or is it because the wave in the two-slit experiment is actually a probability wave rather than an em wave?

In a linear medium, they interfere but they do not interact. If you look at what I said earlier I said, "The different components don't interact so long as they are propagating in a linear medium." Interference is just two signals being in the same place so that their fields add up. Interaction is one signal affecting the behavior of the other.

The two slit experiment demonstrates interference between two signals of the same frequency. The interference occurs wherever the two signals overlap. We can observe it because the detection process is nonlinear --- the intensity of light we (or a camera or a photodetector) see is proportional to the square of the field strength.

Two signals? Wouldn't that mean two different photons? That would confuse me a lot, unless there's some reason for two photons of the same frequency to combine and two of different frequencies to remain independent

The two-slit experiment does continue to work when you have only one photon in the system at a time...but discussing that is going to open up a whole lot more questions. Try to just understand the classical understanding of the experiment first.

I said two signals, because the two slits are really just a way to break up one source into two signals that are coherent with each other.

You could do the same experiment with two radio towers broadcasting the same frequency.

(And that's actually why you often see two or three radio towers together --- they use interference to produce a directional signal covering some regions (say north and south of the antenna) better than others (east and west))

Ok, so why do the signals from different towers interfere with each other?

Ah, sorry, interfere/interact

They interfere because they are in the same place.

They don't interact because air is a linear medium (unless you have a very very strong signal).

So any photon is an independent sine wave traveling through the em field, and they just don't see each other at all?

Hmm, I guess they see each other, to make interference happen, but the field doesn't, like, sum them as they pass through each other?

It's a bit more complicated than that --- except in very special circumstances photons are generally not monochromatic. They actually have a waveform that has a range of frequencies (i.e. a wave packet)

Their fields sum. (the E field and the H field). That's what interference is.

They don't generate other photons with different wavelengths. They don't cause each other to change their paths. Because they don't interact.

They don't have to "see each other" for interference to happen.

"other photons with different wavelengths" -- that helps a lot

Us "seeing" two photons at the same place doesn't mean the two photons "see each other".

So the sum in the em field only happens when the photons pass through the same point in space?

...

Again, try to get your head around the classical understanding first. Thinking about photons instead of waves just raises a lot of extra complications.

I thought photons are waves

You have to be very careful about what you mean when you say "photons pass through [a] point in space" for example.

It's not necessarily a good description of what's going on.

But I didn't realize that a quantum of light could cover a range of frequencies

A photon is the explanation for the phenomenon that you can only add or subtract energy from the EM field in discrete amounts (quanta).

It's a pheonomenon related to generating or absorbing light.

But it's not very helpful for understanding how light energy gets from one place to another.

But a photon travels as a wave, I thought?

Maybe I need help formulating the question(s) better

Understanding how the photon "travels" gets into deep juju.

As a starting point it's better to think that EM waves travel according to the classical description of Maxwell's equations, but when we exchange energy between the EM field and other forms of energy, it always happens in discrete "lumps" and we call those lumps photons.

I thought "photon" was just a way of referring to the wave

Let me see how much of this I'm understanding

1. A quantum of em can cover a range of frequencies

I don't think that's a good description. I'd say a photon refers to what happens when we exchange energy between the EM field (or wave) and some other form of energy.

Some people say "light travels as a wave and interacts with matter as a particle". But that's probably too glib for Physics.SE.

2. Two quanta of em can interfere with each other in a certain way that does not involve a new quantum being created with the two waveforms being added together

Are 1 & 2 fairly accurate?

Just so you're aware: I'm an engineer who works with optoelectronics. The physicists on the main site might have a more nuanced view of things than I do.

I'm not ready for nuance anyway, heh

As the single-photon limit of the 2-slit experiment shows, one photon can even interfere with itself.

Ok, and that really is em-wave interference, not quantum probability wave interference?

4. The em field doesn't support compression waves; em radiation doesn't propagate as a compression wave in the em field

@SaganRitual I don't know where compression waves come into it. What would be compressing?

That thing about compression was my attempt at summarizing what I've learned from you, in the hope that you could tell me whether I'm understanding. I think basically, the em field doesn't behave like air, for example, with compression waves that totally combine with each other

How can the wave function be the same as the em wave? Doesn't the wave function say something about position and momentum (of the particle)? That can't be the same as the frequency of the em radiation? Thanks for spending the time with me. I got a downvote on my question, which is why I try to avoid stackexchange--people are too critical of naive questions, like they think the asker should get a PhD in physics before asking