I've heard before that everything in physics can be thought of as either a field, or its excitation. Is there some intuitive explanation of how I can look at gravity, light, electromagnetism, etc as a field/excitation?
Answer
A field is just something that has a value at every spatial coordinate (or close to that). It is easy to think of gravity and electric/magnetic fields. Consider gravity, no matter where you go, you can always state the value of gravity at that position (and it's direction but that is not a necessary condition). For example, at sea level, we can say the acceleration due to gravity is $9.81m/s^2$ and at other places, like far out in intergalactic space, we can pick any location and say the gravity is (somewhere around) $0m/s^2$ (it's not, but that's pretty close). Similarly, we can do the same thing with electromagnetism; we can point to any spacial coordinate and give a value for the electric and magnetic fields. Barring some radical physics, usually you will find the values of a field to be continuous across space. Thus, you "could" think of a field like a body of water; for a vector field (like the ones you mentioned), each point in space would give the speed and direction of flow of the water. For a scalar field (no direction), each point would give the temperature of the water. Note while each point in the water may have different values, there will still be a smoothness/evenness to its flow.
Light is slightly different. As you pointed out, it is an excitation of the EM fields. An excitation of a field is when the values of the field in some localized area are fluctuating/changing relatively rapidly. In my water analogy, it would be like waves/ripples in the water (depending on the amplitude of the excitation). The overall body is still flowing more or less the same, but in a localized area, there are relatively rapidly fluctuating values.
Hope that helps.
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