# I am new to this quantum world,just read about fields in this

1. Jul 19, 2011

### nouveau_riche

i am new to this quantum world,just read about fields in this domain
the question is-what is field?,is it there in empty spaces?
why was the need of fields when we have forces as were in newton model?(i guess this could go philosophical but i'l like to have the scientific one)

any help would be grateful

2. Jul 19, 2011

### tiny-tim

hi nouveau_riche!

newton had forces acting at a distance

eg one electric charge can be affected by another charge even though they are nowhere near each other

fields mean that the charge isn't affected by other (distant) charges, it's only affected by the field where it is

(and yes, fields would exist in "empty space" if there was any, but there isn't because of all those neutrinos, the cosmic microwave background radiation, and so on)

3. Jul 19, 2011

### tenchotomic

Re: fields

Field is a variable that is defined at each point in space(whether it is empty or not)

Even before in quantum theory,the need for field arises in electrodynamics where they are required to carry energy and momentum,so as to save energy conservation.

4. Jul 19, 2011

### jfy4

Re: fields

I don't mean to hijack here, so if the OP is still confused I'll take a break, but I think my question is somewhat relevant. In another thread awhile back I brought up this idea with the gravitational field. Normally, a field interacts locally with matter (like what was written up above). And in GR we have that the gravitational field intereacts with its corresponding matter locally also. But locality is defined by the gravitational field. That is, distance is defined on the spacetime manifold. Then how is the gravitational field local to anything if the gravitational field is necessary a priori to define locality?

Let's say I am an electron. I can interact with the electromagnetic field at some coordinate $x_\alpha$ which are coordinates cast across the spacetime manifold (the gravitational field). Now, while in the grand scheme coordinates might be unimportant, locality isn't. But close-ness'' is defined by
$$s=\int \sqrt{g_{\alpha\beta}dx^\alpha dx^\beta}$$
which depends on the gravitational field. How can this same equation be applied to (what I have been referring to as) the distance between the gravitational field and matter? Surely the gravitational field must also be local to matter in order to interact, but how is locality defined without the gravitational field before hand?

Sorry to re-phrase so many times, I just want to try and be clear.

Thanks,

5. Jul 20, 2011

### nouveau_riche

Re: fields

there was an experiment taken by david bohm in which an electron could feel the effect of magnetic field of a system in a region where the magnetic field due to that system was zero
,so i guess your definition is not supported there

6. Jul 20, 2011

### nouveau_riche

Re: fields

7. Jul 20, 2011

### tenchotomic

Re: fields

Consider interaction between two accelerating point charges.The effect of motion of one charge will arrive on another charge only after some delay(since no information can travel faster than c).This delay can be explained only by postulating that an electromagnetic field propagate at speed of c from one charge and produces an effect on another charge.

8. Jul 20, 2011

### A. Neumaier

Re: fields

A field is what occupies space and makes it nonempty. For example, the gravitational field has three components and tells the direction of the gravitational force at each point in space.

9. Jul 20, 2011

### tiny-tim

hi nouveau_riche!
yes and no …

by "field" we usually mean the strength of the field, and yes there are rare cases (the aharonov-bohm effect … see http://en.wikipedia.org/wiki/Bohm-Aharonov_effect#Potentials_vs._fields") in which the local strength is irrelevant, and we must instead allow the use of strength at a distance

but "field" can also mean the potential of the field (or both the potential and the strength), and in that case what i said is correct …

fields mean that the charge isn't affected by other (distant) charges, it's only affected by the field where it is

Last edited by a moderator: Apr 26, 2017
10. Jul 21, 2011

### nouveau_riche

Re: fields

do not introduce gravitational fields,they include geometry

11. Jul 21, 2011

### nouveau_riche

Re: fields

what exactly is charge?
is charge the property of field or field the property of charge?

Last edited by a moderator: Apr 26, 2017
12. Jul 21, 2011

### tiny-tim

uhh?

charge is charge​
neither

(though the divergence of the field strength is equal to the charge density: Gauss' law)

13. Jul 22, 2011

### nouveau_riche

Re: fields

why is it charge?

14. Jul 22, 2011

### tiny-tim

everything has to be something

everything else is something else

15. Jul 22, 2011

### nouveau_riche

Re: fields

you really are misinterpreting my statements

firstly,suppose that charge is not yet being discovered,how did the physicist got into that(physical property of charge)?

just by seeing the behaviour of particles i n presence of other one,in that case they still don't know the charge of other,and when they found the repulsion ,how can be they so sure of their similar nature of something (as you say as charge)?

16. Jul 22, 2011

### tiny-tim

Last edited by a moderator: May 5, 2017
17. Jul 22, 2011

### A. Neumaier

Re: fields

Therefore charge is a property of the electromagnetic field in a given volume -- namely the integral over the charge density in that volume.