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

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Discussion Overview

The discussion revolves around the concept of fields in quantum physics, particularly in relation to forces and the nature of empty space. Participants explore the definition of fields, their existence in empty spaces, and the necessity of fields compared to Newtonian forces.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants define a field as a variable that exists at each point in space, regardless of whether it is empty or not.
  • Others argue that fields are necessary to explain interactions at a distance, as seen in electromagnetism, where charges can affect each other without being in proximity.
  • A participant mentions that fields can exist in "empty space," although they note that true empty space may not exist due to various cosmic phenomena.
  • There is a discussion about the implications of fields in general relativity, particularly how locality is defined by the gravitational field itself.
  • Some participants reference specific experiments, such as those by David Bohm, to illustrate the effects of fields in regions where local field strength appears to be zero.
  • Questions arise regarding the nature of charge, whether it is a property of the field or vice versa, leading to further philosophical inquiries about the essence of charge itself.

Areas of Agreement / Disagreement

Participants express a variety of views on the nature and necessity of fields, with no clear consensus reached. Some agree on the fundamental role of fields in physics, while others challenge specific definitions and implications.

Contextual Notes

The discussion includes references to specific physical theories and phenomena, such as the Aharonov-Bohm effect and Gauss' law, which may introduce complexities and assumptions that are not fully resolved within the conversation.

Who May Find This Useful

This discussion may be of interest to those exploring foundational concepts in quantum physics, particularly students or individuals new to the field seeking to understand the role of fields and forces.

nouveau_riche
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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
 
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hi nouveau_riche! :smile:

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 :wink:

(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)
 


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.
 


tiny-tim said:
hi nouveau_riche! :smile:

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 :wink:

(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)

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

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 [itex]x_\alpha[/itex] 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
[tex] s=\int \sqrt{g_{\alpha\beta}dx^\alpha dx^\beta}[/tex]
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,
 


tiny-tim said:
hi nouveau_riche! :smile:

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 :wink:

(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)

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
 


tenchotomic said:
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.

please elaborate a bit theoretically
 


nouveau_riche said:
please elaborate a bit theoretically

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.
 


nouveau_riche said:
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)

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.
 
hi nouveau_riche! :smile:
tiny-tim said:
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 :wink: on)
nouveau_riche said:
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

yes and no …

by "field" we usually mean the strength of the field, and yes there are rare cases (the aharonov-bohm :wink: 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 :smile:​
 
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  • #10


A. Neumaier said:
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.

do not introduce gravitational fields,they include geometry
 
  • #11


tiny-tim said:
hi nouveau_riche! :smile:



yes and no …

by "field" we usually mean the strength of the field, and yes there are rare cases (the aharonov-bohm :wink: 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 :smile:​

what exactly is charge?
is charge the property of field or field the property of charge?
 
Last edited by a moderator:
  • #12
nouveau_riche said:
what exactly is charge?

uhh? :confused:

charge is charge​
is charge the property of field or field the property of charge?

neither :confused:

(though the divergence of the field strength is equal to the charge density: Gauss' law)
 
  • #13


tiny-tim said:
uhh? :confused:

charge is charge​
why is it charge?
 
  • #14
nouveau_riche said:
why is it charge?

everything has to be something :wink:

everything else is something else :smile:​
 
  • #15


tiny-tim said:
everything has to be something :wink:

everything else is something else :smile:​

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)?
 
  • #17


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

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

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