# What are electric fields in EMR are and why do they exist?

I am confused about electromagnetic fields. I am fine with the ideas of how photons are emitted from an object (whether through electron excitation or nuclear fusion) and I understand that any electromagnetic radiation contains an electric field as well as a magnetic field, but I am having trouble grasping why these fields exist.

For example, an electric field exists in a thunderstorm because of a charge separation between the clouds and the ground, creating a potential between the two. And as far as I understand, this is how you produce an electric field (have charge separation). What I don't understand is how light that is already moving can create a variable electric field. What exactly sustains this electric field and causes it to oscillate? Is there a voltage difference? Similarly, I am confused with magnetic fields in EMR.

I have a feeling I have a fundamental misunderstanding about light. Could anyone please offer some insight? Thank you!

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#### jtbell

Mentor
What I don't understand is how light that is already moving can create a variable electric field. What exactly sustains this electric field and causes it to oscillate?
According to Maxwell's Equations for electromagnetism, a magnetic field that changes with time is associated with an electric field (more precisely with the spatial variation of the electric field). Similarly an electric field that changes with time is associated with a magnetic field (more precisely with the spatial variation of the magnetic field). The ultimate source of these time-and-space-varying electric and magnetic fields (in classical electrodynamics) is oscillating electric charges and currents somewhere, somewhen.

Loosely speaking, people often say that "a changing magnetic field produces an electric field, and a changing electric field produces a magnetic field." There are people around here who jump on statements like that for implying a cause-and-effect relationship instead of association, so I'm not actually going to say it myself.

Okay, thanks. I guess the esoteric nature of these fields is partly to blame for my dissatisfaction with how I understand the concept. Would it be fair to say that whatever emits the photon in the first place initiates these phenomena?

And would those who are uncomfortable with the idea of a causative relationship between the fields also reprove the notion that these fields are self-propagating? To me (in my sophomoric way of thinking) it's difficult to not think of it this way. Because if they are not, wouldn't these waves eventually fizzle out (in amplitude), being that nothing is in place to sustain them?

#### jtbell

Mentor
Would it be fair to say that whatever emits the photon in the first place initiates these phenomena?
Yes.

And would those who are uncomfortable with the idea of a causative relationship between the fields also reprove the notion that these fields are self-propagating?
The reason why people are uncomfortable with saying that the E and B fields "cause" each other in an electromagnetic wave has to do with the fact that fundamentally, the E and B fields are simply different aspects of a single thing, the "electromagnetic field", which can be described most compactly by the "electromagnetic 4-vector potential" $A_\mu$. It has four components (three of them are the magnetic vector potential, and the fourth is the electric [scalar] potential). From $A_\mu$, you can derive E and B. The components of $A_\mu$ obey a differential wave equation, so fundamentally you have just a single four-component field propagating through space, and the waves of E and B fields that we measure are simply different "aspects" of it, so to speak. This leads directly to the correlations between E and B in an electromagnetic wave.

Nevertheless, we still talk about the E and B fields "causing" each other under various circumstances, as a heuristic device to help us visualize and predict what's happening in practical electromagnetic phenomena. I think such statements are still useful in that sense, so long as we acknowledge at some point that they're not really "fundamental" statements.

jtbell- That makes things much clearer in my mind. Thank you so much for your help.

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