# Difference between baryonic, inertial and gravitational mass? (and relation to dark matter)

Inertial mass: determined from Newton's f = ma

Gravitational mass: determined via Newton's law of gravity

Baryonic mass: sum of mass of all baryons.

1. Which ones will include mass of dark matter?
2. Since dark matter does not interact with matter, therefore it should not have "normal" inertial mass.
3. Does dark matter interact with itself and have its own "dark matter" inertial mass?
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I believe the current hypothesis is that all matter, dark or otherwise, has "mass" which is affected in the same way by gravity. Dark matter does not interact with normal matter via electromagnetism or other forces, but gravity closes this gap. – Florin Andrei Nov 8 '11 at 21:44
@FlorinAndrei: Isn't that pretty much what "matter" means? – Keith Thompson Nov 9 '11 at 8:09

Indeed, as Florin said, dark matter interacts with normal matter via gravity, so it has the first two for sure. It is one of Einstein's core axioms that those two are identical, universe-wide. So numbered question 2 is based on incorrect premises, and not really answerable at all. Question 3's premise is semi-incorrect, insofar as it implies some special kind of inertial mass for dark matter, when it's really just regular old inertial mass.

The most popular theory of dark matter currently is "WIMP"s- weakly interacting, massive particle. That "weak" doesn't just mean weak in the everyday, non-technical sense. It means that WIMPs interact not just via gravity, but also via the weak force, i.e. the one responsible for radioactivity and whatnot. There is also some theoretical work done on "Super-WIMPs", which is an awful, awful, misleading name. They got the name in analogy to WIMPs, but in fact Super-WIMPs are postulated to interact only via gravity, so the WI in their acronym is totally inaccurate.

I think the state of the art on WIMPs and Super-WIMPs is that they are not composed of baryons, and so would not fall under the third category of mass. (By the way, nobody has detected any WIMP or Super WIMP directly, or even given a cogent statement of WTF they actually are. All theories related to dark matter are at this stage prima facie TOTALLY ABSURD AND PREPOSTEROUS, and their only selling point is that in the eye of most beholders, the alternate explanations are even worse. Physics is really in trouble over this. Everyone is praying for deliverance from the LHC.)

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This answer was apparently robot-like enough to have to fill out a captcha. I think I'm a little insulted. – Andrew Nov 9 '11 at 0:48
Come on, to claim that all proposed dark matter candidates are TOTALLY ABSURD AND PERPOSTEROUS is not reasonable. If the universe really does have a broken supersymmetry then the least massive supersymetric particle is a very reasonable hypothesis for a dark matter candidate. I agree there is 0 evidence right now that the hypothesis is correct but the LHC or the various dark matter searches could soon provide the evidence needed. Are you claiming that any hypothesis with no evidence is TOTALLY ABSURD AND PERPOSTEROUS? – FrankH Nov 10 '11 at 20:55
Yeah, if the universe has a broken supersymmetry. The theory on this has galloped sooooo far out ahead of experiment that, yeah, it's absurd and preposterous that anyone gives any kind of credence to it, or that there is anything like consensus. IMHO. I have never been a fan of any kind of mystery matter. If the LHC does deliver its much anticipated deliverance, I'll say "Well there you go," and buy all the scientists a beer. Or I would if there weren't thousands of people working there. – Andrew Nov 11 '11 at 2:34
The suggestion that "physics is really in trouble" when confronted with the unknown belies your understanding of the scientific method. This is simply not how science works. – Trixie Wolf May 1 at 14:19