Talk:Plasma (physics)

Early History New Version

The first two paragraphs are still misleading, and unsupported by references. The Bence-Jones biography nowhere documents that Faraday referred to "fourth state of matter". Not does a search find a reference to "radiant matter". Ihhutch (talk) 15:30, 28 November 2025 (UTC)

Did you actually read the Crookes' paper? He admits all this himself. But of course it's in the Faraday's biography by Bence Jones. E.g., on p.269, referring to a lecture from 1819:

` Radiant state. —Purely hypothetical.
` Reasons for belief in existence. Distinctions. Experimental evidence. Kinds of radiant matter admitted.
` Such are the four states of matter most generally admitted.

Evgeny (talk) 09:38, 30 November 2025 (UTC)

Ok. I think I accept your first paragraph. I was wrong about "radiant matter". I don't know why my text search did not find it.

It seems to be true that Bence-Jones does not use the phrase "fourth state of matter", but he does refer to Faraday's remarks about four states of matter. So I withdraw my criticism of the first paragraph. Sorry.

I remain critical of your statement "Plasma was first identified in laboratory by Sir William Crookes". Since Faraday definitely studied it in the lab, and (we now agree) called it a state of matter, why would one give priority to Crookes by saying he "first identified" it. There's no reason to say he did anything _first_ with plasma other that to observe and study it at lower pressures.

Likewise the third paragraph "Systematic studies of plasma began with the research of Irving Langmuir" is unnecessarily phrased in a way that is arguably false. Both Crookes and Faraday had studied plasma systematically. So it would be more factual to say something like what I proposed before

"Irving Langmuir and his colleagues in the 1920s conducted extensive experimental and theoretical studies of low pressure ionized gases in the 1920s of abiding importance; and introduced the term "plasma" in 1928:[14]" Ihhutch (talk) 14:40, 30 November 2025 (UTC)

Well, we're discussing subtle details. Sorry, I have no time for a deep research on the subject (the history of science is neither my specialty nor hobby); Wikipedia isn't supposed to contain original research anyway. Not that a textbook is guaranteed to get everything right. For reference, let me quote a paragraph from Sec. 1.5 of

To summarize the highlights of this chapter, we can say that the understanding of matter, the development of electricity, and the unveiling of the structure of the atom have led to the discovery of a new state of matter. It is more than 100 years since Crookes experimented with electrical discharges in vacuum tubes and suggested the existence of a new kind of gas: a gas composed of charged particles, such as a mixture of an electron gas and a proton gas. In 1923, the American chemist Irving Langmuir investigated the electrical discharges in gases and in 1929, while experimenting together with another American scientist, Levy Tonks, used the term plasma to describe the oscillations of the electron cloud during the discharge. This electron cloud was shining and wiggling, similarly to a jelly-like substance which reminded Langmuir of a blood plasma.

As you see, they don't mention Faraday at all - and later, in Sec. 6.13 "Landmarks in the Development of Plasma Physics", wrongly attribute to Crookes the term "the fourth State of matter".

If you have time and inspiration, please pen a separate article on the history of plasma research (based on credible sources, of course), then this short section will point to it. Evgeny (talk) 17:31, 30 November 2025 (UTC)

References

Specific term used early, only defined later

Section1  Early History, 3rd paragraph, offset quote:  "Except near the electrodes, where there are sheaths containing very few electrons".

The term SHEATHS appears in italics, emphasizing its importance.

Typically a pop-up definition or link is offered for the reader unfamiliar with a term, especially one that has a more specific meaning within a specific page (here, relating to electrostatics) compared to the word in general use.

I wondered what I was missing here, given the italicized word ... was this some sort of physical, electron-shedding sheath located near the electrode?

Reading further down this page I discovered in Section 3, subsection 3.3 Plasma Potential, the term "Debye Sheath". Along with a popup and link attached to this term. The text here provides a nice, clear explanation of exactly what these sheaths are, and why they contain few electrons (as stated in the section 1 quote).

I can't help but wonder if it isn't possible to immediately relieve the mystery of what precisely these sheaths mentioned in the first section are, given the clear and concise material provided further down in section 3? PeacefulPlanet3 (talk) 19:43, 18 December 2025 (UTC)

@PeacefulPlanet3 It is possible to relieve the mystery by continuing to read the article up to the end of the "Properties and parameters" section as you describe having done yourself. Delaying discussion of the physical properties of sheaths to this later section seems perfectly reasonable to me.

Additionally, the word is italicized in the original text, and italics are generally maintained in quotations to conform with MOS:CONFORM. The original italicization was likely used to introduce the new terminology, not for emphasis. The quoted passage is the only place where the term is used in the entire paper, but earlier papers from Langmuir (e.g., from 1923) clearly use italics to introduce the term. CoronalMassAffection (talk) 13:00, 28 December 2025 (UTC)

Density or concentration?

Throughout the article, term density is employed to mean "number of particles per volume unit", those particles being ions, electrons, and others. In my opinion, what is being dealt with is not a density (which is mass/volume), but a concentration. Concentration is a physical quantity that can be expressed under several forms, among which "number of particles per volume unit". Ekisbares (talk) 08:25, 28 December 2025 (UTC)

A quantity with units of "number of particles per volume unit" is referred to as a number density of particles. Number concentration seems to be the equivalent term favored in chemistry. Referring to a number density as simply a "density" and providing the units to specify that it is a number density rather than a mass density, as is done in Plasma (physics)#Density and ionization degree, seems fine to me. CoronalMassAffection (talk) 13:23, 28 December 2025 (UTC)

Expand definition of plasma at start of article to encompass collective behavior?

The introductory sentences currently refer to plasma as "having undergone an appreciable degree of ionization" and consisting "of a significant portion of charged particles (ions and/or electrons)." While this definition works for the vast majority of plasma currently in the universe, it does not encompass phenomena like quark-gluon plasmas (important in cosmology) and positron-electron plasmas (in which the primary positively charged particles are positrons rather than ions, and which have been investigated in the context of relativistic astrophysical jets and laboratory experiments). There are also non-neutral plasmas.

Definitions of plasma often focus around collective behavior. Particles in a gas interact solely through binary collisions, and do not exert long-range forces on each other. In contrast, enough particles in plasmas are able to exert long-range forces on each other to result in phenomena like plasma oscillations and Debye shielding. For example, if there's a local buildup of electric charge in an electron-ion plasma, the electrons will act to rearrange themselves to get the plasma closer to quasineutrality.

Chen (2016) uses the definition: A plasma is a quasineutral gas of charged and neutral particles which exhibits collective behavior. This is probably the most commonly used definition among the plasma physicists I know, though it doesn't include non-neutral plasmas (because of "quasineutral") and I don't think it includes quark-gluon plasmas either (though I don't know that much about the topic). It does include electron-positron plasmas, however!

A complication of using the term collective behavior is that, at least in my own experience, it's generally confusing and tricky to explain. 😅 Is there a way we could reasonably include collective behavior in the introductory sentences when defining plasma? I'm wondering if it would be worth creating an article on collective behavior (plasma). 🤔 Gzymqlxwj (talk) 02:43, 28 April 2026 (UTC)

Well Chen starts with Plasma in physics, not to be confused with blood plasma, is an “ionized” gas in which at least one of the electrons in an atom has been stripped free, leaving a positively charged nucleus, called an ion. And then Any ionized gas cannot be called a plasma, of course; there is always some small degree of ionization in any gas. So its my sense that Chen is thinking about conventional electromagnetic plasma, which is what I think this article should be about. Quark-gluon plasma and positron-electron plasma are their own things which got named and analyzed by comparison; I don't think we should try to generalize and cover "all things plasma". We should mention these and summarize their articles. Similarly we should leave these more technical terms like quasineutral/collective behavior for the article. Chen say By “collective behavior” we mean motions that depend not only on local conditions but on the state of the plasma in remote regions as well. and we can summarize that in the intro without using the jargon. Johnjbarton (talk) 03:08, 28 April 2026 (UTC)

Quark-gluon and positron-electron plasmas shouldn't be covered in this article. These (especially the quark-gluon plasma) are completely different things unrelated to "the fourth state of matter". There is a WL to quark-gluon plasma under "See also". When one writes an article about the positron-electron plasma, it can be put there as well. BTW, I think "dusty plasma" should also be mentioned only under "See also". Evgeny (talk) 06:53, 28 April 2026 (UTC)

Thank you for sharing your insights and perspectives!

A separate article on electron-positron plasma (pair plasma) would be fantastic! The equations describing the fluid dynamics of pair plasmas are very similar to electron-ion plasmas, except with $m_{e}=m_{i}$ so effects like the Hall effect are absent. Pair plasmas are host to numerous physical processes that occur in electron-ion plasmas (Fiuza & Chen 2023). "Anti-matter plasmas" are included in the conference abstract sorting categories used by the APS Division of Plasma Physics, and a SciX search for Physics of Plasmas articles with "electron-positron" or "pair plasma" in the title currently yields 432 articles, so there's at least some evidence that the broader plasma physics community categorizes pair plasmas as plasma. So, I do think there is a strong case to expand the initial definition of plasma to encompass pair plasmas (while keeping the focus of the article on electron-ion plasmas).

I'll agree that quark-gluon plasma only warrants a brief mention in this article since they're extremely different from classical plasmas, though I'd find a sentence or two that puts them in context to be helpful.

Dust is often a major component of plasmas in both the laboratory and nature, so having a section summarizing dusty plasmas in this article is important to keep. The ISM is a very dusty place, and dust plays important roles in chemistry and heating in the ISM. Dust is often present in magnetic fusion devices (Krasheninnikov et al. 2011). Parker Solar Probe has also frequently observed dust in the inner heliosphere. I'd treat dusty plasma similarly to a section on pollution in an article on air. Gzymqlxwj (talk) 22:20, 28 April 2026 (UTC)

At some point we could change the name of this article to something like "Electromagnetic plasma" and use "Plasma (physics)" as an overview with common properties and short summaries of the detailed articles. But we definitely need an article that focuses on the electromagnetic case rather than adding other cases here. Johnjbarton (talk) 23:55, 28 April 2026 (UTC)

Just created draft:electron-positron plasma! 🌌 Gzymqlxwj (talk) 00:44, 29 April 2026 (UTC)