Personally, I think that popular science books ought to devote more pages to the issue of how language is used in science. The words scientists choose are central to communication and miscommunication both among researchers and between scientists and non-scientists. The problem is that all language is full of misnomers and contradictory definitions, and scientific language is no exception.
One especially problematic scientific word is “matter.” It has multiple and partly contradictory meanings within particle physics, astronomy and cosmology. For instance,
- (Quote) It’s not even clear that “dark matter,” a term used widely by astronomers and particle physicists alike, is actually matter.
- (Endnote) Among possible dark matter particles are axions and dark photons, neither of which would obviously qualify as “matter.”*
Why might one not view them as matter?
Dark Matter NEED not be “Matter”
Let’s assume for a moment that dark matter exists (remembering that the evidence for it, though strong, is circumstantial, and is still disputed by some scientists.) If it exists, all we know about it from observations of the cosmos is that
- it is a substance that causes and responds to gravity, and that
- it is slow-moving (or “cold”, in astronomer-speak), allowing it clump inside galaxies and clusters of galaxies.
In the scientific dialect of cosmologists, who study the history of the universe, such a substance is automatically referred to as “matter”. For them, “matter” refers not to the details of the substance, only to some of the things that it does.
What about dark matter’s substance? It is possible that it consists of large numbers of subatomic particles, and let’s assume that for the moment. But there are many candidates for what those particles could be. Only some of them are what particle physicists would call “matter,” since their definitions of the term “matter” (there’s more than one) are different from the definition typically used in cosmology.
Among practicing scientists, a popular candidate (not necessarily a likely one) for dark matter’s particles is called a “WIMP” — a Weakly-Interacting Massive Particle — which would interact with ordinary atoms via a combination of the weak-nuclear and Higgs forces. Such particles — somewhat like neutrinos, only with much larger mass — would be called “matter” by most particle physicists, even though they do not in any sense participate in ordinary matter made from atoms.
But another possibility is that dark matter is made of a hypothetical particle called a “dark photon.” A dark photon resembles a photon, but differs from it two ways:
- it has extremely weak interactions with all known particles;
- it has a mass, just as a Z boson does, except that its mass is much, much smaller.
Since photons are universally viewed as “not matter”, many particle physicists would feel a bit ambivalent about calling dark photons “matter”. (We don’t typically call Z bosons “matter”, either… both photons and Z bosons emerge from the same two photon-like parent particles, so it’s not really obvious we should say that one is matter and the other is not.) At best, the terminology would be debatable.
Yet another possible source of dark matter is known as axions… but in this case, dark matter starts off its life as an effect of the whole axion field waving back and forth. Such a wave, now made more complicated by gravitational interactions, was initially much more similar to a laser (though a standing wave rather than a traveling wave) than it is to a horde of independent particles. This makes axions a lot more like photons than like ordinary matter, and again raises the question of what terminology to use. Experiments that search for axion dark matter specifically make use of the fact that the axions in dark matter can be viewed as more like a wave (“coherent”) than a crowd of randomly moving particles (“incoherent”.) Would particle physicists always want to refer to this substance as “matter”? I think it would depend on whom you asked.
More generally, there are many books and lectures which refer to the two classes of particles known as “bosons” and “fermions” as “force particles” and “matter particles”. (Admittedly, I personally never use this terminology, as I find it misleading.) Examples of bosons are photons and Z bosons; examples of fermions are electrons and quarks, ingredients for atoms. Since dark photons and axions are bosons, they would be categorized by this definition as “force particles”, not as “matter particles.” Obviously this makes a linguistic mess.
Such issues aren’t limited to “matter”. Scientific terms are often used in multiple ways in different contexts; their meanings differ not only between different research fields but even within a single research field. Sometimes a term will be used in contradictory ways by the same person within a single one-hour seminar!
My point is not that we should try to make our language super-precise. That would be hopeless; our terminology changes over periods of a few years, and we can’t fix it in stone. Instead, I would suggest that we scientists need to be more careful about our use of language when speaking to non-experts. Our wording is full of internal inconsistencies that hold huge potential for misinterpretations, and we have to be very clear and consistent if we want to avoid befuddling interested non-experts.
(Note: I have also written about the ambiguities and confusions surrounding “matter” and “energy” here.)
10 Responses
I think what we are hoping for is something that doesn’t wreck general relativity. i.e., it’s “dark matter (or gravitates like matter)” or everything we know is wrong. Take the second choice only in an emergency – rebuild if abolutely necessary.
The perturbative linear wave is accelerated by the non-perturbative gravitational wave (fluid dynamics), in coherence (VEV), as an almost non-repeating pattern. ?
Matter is ‘Non-perturbative Vacuum expectation value’?
It includes the gravity from the ‘flip (acceleration)’?
Puts a new spin on the phrase, “What’s the matter with you?”
Dark matter may not be matter by any definition (it isn’t in MOND type theories) and, if it is matter, it may not be dark (macrophysical dark matter, such condensed quark matter, might be quite visible if you were up close). It’s better to view “dark matter” as just a label for various physical effects we can’t explain than an actual descriptor.
You and I both know that. But many non-scientists, reading the words used by scientists in books and by journalists in their articles, have no way to know that. (And “dark energy” is worse.)
Recently, have often heard the question,”Do we need a new cosmology?” A typical description of the Big Bang states all space/time, matter and energy came into being in an instant. There may have been many bangs and ours is just one amongst many. Sticking to our universe, our space/time frame, the distribution of dark energy and dark matter may be explained because we banged into something rather than the current explanation that nothing came before and suddenly here we are. Inflation once exceeded light speed, slowed and is gaining momentum again. Dark energy distribution noted between streams of galaxies. We either have to invent a new kind of matter or a new cosmology.
The answer to the question is “probably not”; the discrepancies in cosmology are real, but only at the 10% level, and the press is probably over-blowing them. We’re only discovering them because our understanding of the universe’s history has now reached the stage where precision measurements are possible, and little details start to matter.
The press is surely over-blowing them. Hey, it’s a living.
I would like to add the meaning of the verb matter, as in Matt matters.