How Does Jackson's Use of Manifolds Enhance Understanding of Electrodynamics?

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Discussion Overview

The discussion revolves around the use of manifolds in understanding electrodynamics as presented in Jackson's "Electrodynamics." Participants explore the necessity and implications of defining fields as functions between manifolds, particularly in relation to different topologies and practical applications in physics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the necessity of using manifolds to explain fields, suggesting that the mapping from R^4 to R^3 might suffice.
  • Another participant emphasizes that the definition of field X is crucial for understanding the context, indicating that it could represent arbitrary electromagnetic fields.
  • A later reply argues that using manifolds is necessary for considering spaces with topologies other than R^n, such as fields defined on an n-dimensional sphere.
  • Another participant points out that practical applications often involve boundaries that are better described as two-dimensional manifolds, rather than subsets of R^3.
  • One participant mentions that thinking in terms of manifolds can facilitate simplifications and generalizations, such as applying the generalized Stokes theorem in various contexts.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and implications of using manifolds in electrodynamics. While some argue for their importance in certain contexts, others question their necessity, indicating that multiple competing views remain in the discussion.

Contextual Notes

Participants note that the discussion hinges on the specific definition of field X and the topological properties of the spaces involved, which may not be universally agreed upon.

kidsasd987
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This is from Jackson, "Electrodynamics"
a field is a fuction mapping phi: M -> T, x -> phi(x) from a base manifold M into a target manifold T.

field X: R3 * R1 -> R3
x(r,t) ->X(x)I think this is eucledian R4 to R3 so I wonder why Jackson explained this with the concept of manifolds?
Is it necessary?
 
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Impossible to say unless you specify what X is.
 
Orodruin said:
Impossible to say unless you specify what X is.
http://www.thp.uni-koeln.de/alexal/pdf/electrodynamics.pdf

its on section 1.2.1 and field X is any arbitary EM fields (E,D,B,H)
 
kidsasd987 said:
This is from Jackson, "Electrodynamics"
a field is a fuction mappingphi: M -> T, x -> phi(x) from a base manifold M into a target manifold T.

field X: R3 * R1 -> R3
x(r,t) ->X(x)I think this is eucledian R4 to R3 so I wonder why Jackson explained this with the concept of manifolds?
Is it necessary?

It is necessary if you want to consider spaces which have topologies other than that of R^n. For example a field on an n-dimensional sphere S^n.
If you restrict yourself to defining a field as a function from R^m to R^n, then you could not define a field on S^n as a global function because S^n is not homeomorphic to R^m for any n, m >= 0. In the example you mention here, the domain space is just R^4 with the usual topology and diferentiable structure, but this space is of course a manifold so it fits in the definition the book gives.
 
Consider also that in practical applications, the boundaries of a particular region might not be made up of chunks of planes isometric to R2. Often the boundary of a cavity or waveguide, or the surfaces of antennas and reflectors are best described as two dimensional manifolds rather than as subsets of R3 or R3,1.
Also, thinking in terms of manifolds makes some nice simplifications and generalizations easier later on, like using the generalized Stokes theorem for all integration over bounded regions, curves, or surfaces.
 

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