What is the shape of a photon's electric and magnetic fields?

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Discussion Overview

The discussion revolves around the shape and characteristics of a photon's electric and magnetic fields, exploring whether these fields are confined to planes or extend in three dimensions. Participants examine the implications of classical and quantum descriptions of electromagnetic fields, and how these relate to the understanding of photons.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions the shape of a photon's fields, asking if they are confined to planes or extend in three dimensions.
  • Another participant states that the expectation values of the magnetic and electric fields of a single photon are zero, suggesting that fluctuations are the primary characteristic.
  • Several participants request examples or illustrations to clarify the discussion about the fields of a photon.
  • One participant argues that a photon is not a distinct entity but rather the quantum representation of an electromagnetic field, which can be a plane wave or a finite wave packet, affecting only the amplitude.
  • Another participant clarifies that classical electromagnetic plane waves do not confine fields to the illustrated planes and that the diagram represents the net effect of many photons rather than a single photon.
  • A later reply expresses surprise at the misunderstanding of the illustration, emphasizing that it does not represent a photon traveling through space and questioning what an electron emits instead.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of diagrams representing electromagnetic fields and the nature of photons. There is no consensus on the shape of a photon's fields or the implications of classical versus quantum descriptions.

Contextual Notes

Participants highlight the potential for misunderstanding in educational materials regarding the representation of photons and electromagnetic fields, indicating a need for clearer explanations.

CosmicVoyager
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Greetings,

I see graphs of the amplitude of a photon's fields over time, but no illustrations of the shape of the field at any instant in time.

Are the fields confined to planes like they look in the graphs? Or do they extend in three dimensions like the field around a magnet?

Thanks
 
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The expectation values of the magnetic and electric fields of a single photon are 0. So basically all is fluctuation.
 
CosmicVoyager said:
Greetings,

I see graphs of the amplitude of a photon's fields over time,

Can you show us an example?
 
A photon is not a special little bullet, with a unique and special shape. It is the quantum representation of an electromagnetic field. Any electromagnetic field. It may be the quantization of an infinitely long plane wave, in which case it has a well-defined energy and momentum. Or it may be the quantization of a finite wave packet, in which it has a spread in energy and momentum. Quantization only affects the amplitude.
 
jtbell said:
Can you show us an example?

[URL]http://upload.wikimedia.org/wikipedia/commons/3/35/Onde_electromagnetique.svg[/URL]

The graph is of the amplitude of the electric and magnetic fields over time.

Are the fields confined to the planes the arrows are on? Or are the three dimensional such as a field around a magnet?
 
Last edited by a moderator:
Where does it say that this is the field of a single photon?

For a classical electromagnetic plane wave, the fields are not confined to the planes shown in this diagram. The diagram shows the magnitude and direction of each field at various points along the horizontal line that runs from left to right. In a plane wave, the fields are the same along all horizontal lines that run parallel to the one shown in the diagram.

For another way of looking at it, see the following post and the attached diagram:

https://www.physicsforums.com/showpost.php?p=533190&postcount=6

A classical electromagnetic wave is an approximate, alternate description of the net effect of bazillions of photons. If you're in a situation where there are only a few photons, there is no classical electromagnetic wave. There is a quantum field with wavelike properties, that is associated with photons, but this field is not the classical E and B fields. It's more closely related to the classical electric scalar potential \phi and magnetic vector potential \vec A, which together form a relativistic four-vector field that gets quantized in QED.
 
jtbell said:
Where does it say that this is the field of a single photon?

For a classical electromagnetic plane wave, the fields are not confined to the planes shown in this diagram. The diagram shows the magnitude and direction of each field at various points along the horizontal line that runs from left to right. In a plane wave, the fields are the same along all horizontal lines that run parallel to the one shown in the diagram.

For another way of looking at it, see the following post and the attached diagram:

https://www.physicsforums.com/showpost.php?p=533190&postcount=6

A classical electromagnetic wave is an approximate, alternate description of the net effect of bazillions of photons. If you're in a situation where there are only a few photons, there is no classical electromagnetic wave. There is a quantum field with wavelike properties, that is associated with photons, but this field is not the classical E and B fields.

Well this is surprising. Shocking. I have thought for decades that this illustration was of light, of a photon. That a photon was as oscillating electromagnetic field. I am certain that most people, if shown that illustration would think that was a photon. This is another case of school and books doing terrible job and not making things clear. They should be aware that people frequently misunderstand what the illustration is and clarify it.

I can't believe that is not what I thought it was all this time. It is not an illustration of a photon traveling through space. Damn.

I thought that illustration was of what is emitted by an electron. I thought that traveled through space until it was absorbed by another electron. Now I don't know what the heck an electron emits. What *does* it emit? What is the shape of it?
 
Last edited:

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