# How to define a field? [duplicate]

Possible Duplicate:
What is a field, really?
What are electromagnetic fields made of?

What is a field ? What is magnetic field or other fields made of or what it is, How do u define it (To my knowledge field is thought to exist without a proof and theories are built over it) ?

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## marked as duplicate by Qmechanic♦, David Z♦Jul 6 '12 at 17:21

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

Thanks, The answers in the post tell me that a thing could be defined only if its not fundamental but field is a fundamental so it can't de defined –  Abhay Kumar Jul 6 '12 at 12:31
A field isn't a fundamental object from a mathematical viewpoint. You can define riggidly how fields work/look like –  Michael Jul 6 '12 at 12:42
I didn't get you Micheal –  Abhay Kumar Jul 6 '12 at 12:45
Perhaps you mean something different when talking about fields. But in QFT, a field itself is just some function of space-time (see my answer below). The really interesting part is, how these fields transform. Or more precisely, what type of elements are assigned to each point in space. This is how we define the particle associated to that field –  Michael Jul 6 '12 at 12:50

## 1 Answer

Mathematical Answer:

A field is simple a function of space(-time) that assigns some value, vector or pretty much anything, to a certain point in space(-time).

Nothing fancy really. A normal function like $f(x) = x^2$ can be viewed as a field that assigns a real value to the space $\mathbb R$.

We call the field by the type of element it assigns. So a Vector field assigns a Vector to each point in space.

You can define your fields over whatever elements you wish. Heck, you could create an apple field, by assigning a certain number of apples to each point in space.

Physical Answer:

It's pretty much the same as the mathematical answer. The only difference lies in how you interpret these fields.

So a Vector field, that assigns a vector to each point in space can be viewed as a magnetic field.

For example, think of a ball (the earth). Now, think of a vector field on this ball, i.e. some function that assigns a vector to each point. We say this vector field is smooth, if the vectors of two nearby points only differ by some small $\epsilon$-vector. Think of the vectors as hairs on the ball. Smoothness then just means that the hair looks tidy and two neighbouring hairs have almost the same direction.

You can reconstruct the earth magnetic field by choosing an appropriate function. Interestingly, if you do this, we have a mathematical theorem that says, that such a vector field must essentially have (at least) 2 poles (or a pole of multiplicity 2). In our case, the North and South pole. Or in short "You can't comb the hair on a coconut". This theorem is called the Hairy ball theorem.

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Thanks, I get your point. Can you tell me why does a field exist. What makes magnet have magnetic field around it ? –  Abhay Kumar Jul 6 '12 at 12:47
Well, Ferromagnets have the property that they consist of alot of tiny elementary magnets, all aligned in one direction. Each of these elementary magnets carries a Magnetic Moment: en.wikipedia.org/wiki/Magnetic_moment –  Michael Jul 6 '12 at 13:00