Classical Field Theory for a system of particles

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Discussion Overview

The discussion revolves around the physical representation of fields in classical field theory, particularly in the context of a system of particles such as electrons. Participants explore whether a classical field can adequately represent a small number of particles or if it is more suited for a large number, and the implications of quantization on these fields.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that the field, φ, in classical field theory is typically constructed from a large number of coupled harmonic oscillators, but question whether it can represent just a few electrons.
  • Others argue that in quantum field theory (QFT), fields are position-dependent operators that create or annihilate particles, suggesting a distinction between classical and quantum treatments.
  • It is noted that a quantized field can have states that do not correspond to a specific number of particles, indicating a complexity in the relationship between fields and particles.
  • Some participants mention that an unquantized classical field generally does not represent particles unless it is a classical probability density field, emphasizing the quantum nature of particle-wave duality.
  • One participant references Leonard Susskind's course on classical field theory, questioning the physical purpose of classical fields if they are primarily intended for quantization.
  • Another participant discusses hydrodynamical models of charged gases, noting that these models assume a continuous matter representation and express skepticism about the utility of certain fields without quantization.

Areas of Agreement / Disagreement

Participants express differing views on the physical representation of classical fields and their relation to particles, with no consensus reached on whether classical fields can effectively represent a small number of particles or if they are only meaningful in a quantized context.

Contextual Notes

There are limitations regarding the assumptions made about the nature of classical versus quantized fields, and the discussion does not resolve the implications of these distinctions on the physical interpretation of the fields.

LarryS
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In classical field theory, the field, φ, is usually constructed from a very large number of coupled harmonic oscillators. Let's say our φ consists of just electrons.

What does φ best represent physically, a very large number of electrons or can it represent just a few electrons? Which is the best fit, or does it matter?

As always, thanks in advance.
 
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referframe said:
In classical field theory, the field, φ, is usually constructed from a very large number of coupled harmonic oscillators. Let's say our φ consists of just electrons.

What does φ best represent physically, a very large number of electrons or can it represent just a few electrons? Which is the best fit, or does it matter?

As always, thanks in advance.

In QFT the fields are position-dependent operators that create/annihilate particles at different points in space. They're formally quite analogous to the creation and annihilation operators that one encounters in the quantum mechanical treatment of the harmonic oscillator.
 
A quantized field can also have states that are not eigenstates of the particle number operator, e.g. don't have an exactly specified number of electrons or other particles.

An unquantized classical field doesn't usually represent any kind of particles (unless it's some kind of classical probability density field), the particle-wave duality is a completely quantum mechanical thing.

Only fields that have a linear field equation can be represented as a system of harmonic oscillators.
 
hilbert2 said:
A quantized field can also have states that are not eigenstates of the particle number operator, e.g. don't have an exactly specified number of electrons or other particles.

An unquantized classical field doesn't usually represent any kind of particles (unless it's some kind of classical probability density field), the particle-wave duality is a completely quantum mechanical thing.

Only fields that have a linear field equation can be represented as a system of harmonic oscillators.

Leonard Susskind has an internet video course on "Classical Field Theory" in which he discusses how a classical field of charged particles becomes gauge invariant by coupling the field (covariant derivative) to the EM four-vector. But he never really goes into any detail regarding the physical nature of the "system of charged particles". So, my question is kind of within that context.

Are you saying that the classical field that he describes has no physical purpose other than a field that is to eventually be quantised?
 
I've seen some kind of hydrodynamical models where an electrically charged gas that has a density field ##\rho## and a current density field ##\mathbf{j}## is coupled with electric and magnetic fields, but those models assume that matter is continuous and doesn't consist of particles. I can't see how anything useful could be done with fields like Klein-Gordon or Dirac fields without quantizing them, but it's possible that I'm wrong.
 

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