# What does it mean to have a function of a field?

There was a Leonard Susskind lecture on the Higgs Boson I watched the other day, and he talked about graphing the field where the domain was some sort of field space. The lecture is here at about 6 min: https://www.youtube.com/watch?v=JqNg819PiZY&t=350s

But this idea seems bizarre to me. I have some sort of conceptual difficulty imagining functions of fields. The way I've dealt with it is by imagining that the domains of functions of fields are defined over all possible field configurations. So, as if there was a space that contained the classical gravitational, E&M fields and all other variants, all "side-by-side" in this domain. Is that a good way to visualize these objects? It does seem to raise the question; Susskind treated his domain like it was continuous: really it seemed like a manifold. What does it mean for fields to be "next" to each other? How can you continuously vary all fields so that they could all live inside the same manifold?

And, what is the domain space of say the lagrangian density $$\mathcal{L}(\phi, \partial\phi)$$? I can treat $$\mathcal{L}$$ just like any other function, but I still can't see what it means physically to have a function of a field, and so my conceptual trick sort of breaks down when I have a multivariate function where one variable is the derivative of another. I'm perfectly fine treating these things as mathematical objects, and I can, say, derive the Euler-Lagrange equation using this function. But this doesn't feel great, it kind of feels like I'm cheating. I know this is a really basic and likely fundamental question, but I couldn't find anything online.

Does anyone have any good way to visualize what such functions mean?

• Without a link to Susskind’s lecture, it is unclear what you are asking. Commented May 16, 2020 at 5:10
• 1. The 'distance' between fields can be defined rigorously (for a change) but I would not bother with that. Just think of is as an inner product $<\phi_2|\phi_1>$ 2. The field and its derivative (their variations) are regarded as independent in the Lagrangian formalism. This is the interesting part you might look into. 3. Visualization is hard. Maybe visualizing the variation makes sense.
– user257090
Commented May 16, 2020 at 6:58