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In my teacher's class notes, came across this definition of light today:

Light is an excitation of the electromagnetic field, with photons being the lowest energy excitation.

And the way in which it is phrased makes me think that more particles can be seen as excitations of the electromagnetic field, although I really don't know which, and the information I find about the different particles that constitute the standard model is kind of confusing.

Exactly which particles are described as excitations of the electromagnetic field?

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  • $\begingroup$ What's the source of your quote? All quoted material on Stack Exchange sites must have some form of attribution, like a link or the name of the article, and ideally, an author's name. $\endgroup$ – PM 2Ring Feb 16 at 12:05
  • $\begingroup$ It's from a professor's class notes, in an introduction to the subject of light. I wasn't sure I should mention that $\endgroup$ – Rye Feb 16 at 12:13
  • $\begingroup$ Yes, you need to give proper attribution. Please see physics.stackexchange.com/help/referencing That help page focuses on answers, but it applies to questions too. Also see meta.stackexchange.com/q/83955/334566 $\endgroup$ – PM 2Ring Feb 16 at 12:41
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    $\begingroup$ Yes, but I can't provide the "source" of the quote because it's from the private notes, provided to the students. They're not public $\endgroup$ – Rye Feb 17 at 10:35
  • $\begingroup$ Fair enough. The new version of your question is better. And maybe it's better to not name the author of such an odd statement. ;) $\endgroup$ – PM 2Ring Feb 17 at 17:58
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It fits with the long discussion we had on this question :

What is the connection between quantum optical photons and particle physics' photons?

and the answers and comments therein.

People working with quantum optics have a more general view of the term "excitations of the electromagnetic field". In the quantum field theory used in the standard model of particle physics, QED for electromagnetism, there is a single excitation of the electromagnetic field, called the photon field , which is the photon in the table. It is a zero mass, point particle with spin +/-1 and energy=hnu, where nu is the frequency of the classical electromagnetic wave built up by photons of this energy.

Quantum optics physicists use a generalized quantum field theory, which coincides with the particle QED when in vacuum. Classical light in matter has quantum behavior that can be described by a field theory, and they call the excitation of the field they use, photons.

This is what the sentence you quote:

Light is an excitation of the electromagnetic field, with photons being the lowest energy excitation.

implies. I would say the professor is a quantum optics physicist.

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    $\begingroup$ Really nice answer. $\endgroup$ – Árpád Szendrei Feb 22 at 0:20
  • $\begingroup$ Oh, that makes a lot of sense! And the professor is indeed a quantum optics physicist, he in fact teaches quantum optics at a masters' level. $\endgroup$ – Rye Mar 12 at 10:40
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Photons are excitations of electromagnetic field. These are part of the phenomenon that we call "light", so the phrasing in the book is misleading.

Closer to the matter: all particles can be though of as excitations of some field. When the field is electromagnetic, they are called photons.

In other words, the statement should be Photons are the excitations of the electromagnetic field rather than Photons are excitations of the electromagnetic field (I hope my English does not betray me.)

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  • $\begingroup$ Oh, that makes sense. I have a follow up question though. When they are calling photons "the lowest energy excitation", that just means that photons hold less energy then other particles? $\endgroup$ – Rye Feb 16 at 11:38
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    $\begingroup$ @Rye Techncially, this is also incorrect. But the discussion of photons as excitations of EM field requires some background in quantum field theory. Otherwise, one tends to think of photons as particles, which is incorrect, but more or less okay for discussions of low energy excitations. This is an introductory or even popular book, as far as I understand. $\endgroup$ – Roger Vadim Feb 16 at 11:56
  • $\begingroup$ @Rye. One type of field excites ("produce") only one type of particle >> for the EM field, it's photon. Another type of field excite another type of particle. "Lowest energy excitation" does not mean an EM field excites another type of particle. The term "lowest" refers to photon as the "quanta" of light energy; quanta means the smallest possible discrete unit. For example, a 50-watt laser used for 10 sec would have emitted 500 joules of energy. If wavelength is 400 nm, then there were 1,006,834,391,851,891,000,000 no. of photons emitted, each carrying 3.0998159 eV quanta of light energy. $\endgroup$ – Roy Closa Feb 16 at 13:12
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You may be confused by the phrasing of the statement: particles as excitations of a field. This peculiar way of saying it comes from quantum field theory. A 19th-century physicist --before quantum field theory was established-- would probably say "light is the electromagnetic field moving through the vacuum" or "away from sources". It's photons that are excitations of the electromagnetic field. Photons are the so-called quanta of the field. In the same way, electrons are excitations of Dirac's electron field, etc.

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