Of Particular Significance

Chapter 4, Endnote 1

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.)


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A decay of a Higgs boson, as reconstructed by the CMS experiment at the LHC