Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. - Edit 1
Before modification by Joel at 02/05/2011 01:40:05 AM
The more exotic a position is and the less direct evidence it has the more skeptical I am, but as it becomes more familiar, particular if that's due to mounting evidence, my skepticism proportionately decreases. If dark matter is "simply" matter affected by gravity as well as the weak and strong nuclear forces but not electromagnetism that's not so controversial since, as you note, neutrinos are already known to do that, and black holes effectively ignore everything but gravity as well. The idea of some radically new kind of matter bound by few if any of the known rules apply (which is the impression I, rightly or wrongly, often get in discussions of dark matter) is more of a hard sell for me.
Neutrinos don't interact via the strong force, actually.
My mistake; just trying not to be too restrictive (it makes sense, I just wasn't aware it was definitively proven).
Most proposals for new dark matter particles extend the Standard Model in one way or another, rather than simply ignoring it. The fact is that no particle we understand today has the necessary properties to produce the evidence that we have. Skepticism about any specific proposed particle is still valid, because there haven't been direct detections yet, but skepticism about the necessity of some new particle is no longer warranted.
I'm not convinced of that, though it's quite possible that's just because I'm not well enough informed about the subject.
Not ENTIRELY different; they're both cosmological issues. Large undetected mass might explain apparent acceleration in much the way it's thought to explain the Bullet Cluster--but doesn't. Dark energy doesn't seem to have much definition beyond being a placeholder for the actual explanation, and the characteristics required of it to provide that explanation. While I certainly support hypothesizing I also think it important to guard against fitting data to the curve rather than the reverse. Again, I don't claim to be an expert on cosmology or particle physics, but the little my amateur interest has shown me suggests that may be more common than anyone would like to admit.
No, large undetected mass would not explain acceleration, because the gravitation of matter is attractive. Dark energy causes a repulsive gravity-like force, which leads to acceleration. At this point, the leading idea is that dark energy is due to the cosmological constant, a perfectly allowed term in Einstein's field equations. The main question is why the cosmological constant has the value that we measure experimentally.
Large undetected masses wouldn't explain truly accelerated expansion, but could easily explain APPARENT acceleration (which is why I attached that all important qualifier). Large, distant and undetected masses exerting increasingly strong gravitation on intervening objects could easily make it appear the rate of expansion was increasing, particularly if the amount of such large undetected masses was, say, five times greater than that of observed matter. Even exotic dark matter operating in that way is less radical than dark energy, and it's not fair to suggest dark energy is coëval with the cosmological constant, that the former is a perfectly natural and inevitable consequence of the latter. Dark energy is no more a slight modification of the well established cosmological constant than exotic dark matter is a slight modification of well established normal matter.
Sometimes it feels like physics and the profession of physics are two different things, or at least only somewhat related. For example, the flip side of what you just said is that if neutrinos are dark matter then dark matter hasn't been a new concept needing evidence amounting to proof since the first neutrino was detected. Of course, looking for more relatives of the neutrino isn't quite as sexy (and grantworthy) as looking for mysterious theoretical dark matter that literally changes the whole universe. Doesn't encourage international treaties and funding for supercolliders, y'know?
Neutrinos are dark matter, but that doesn't mean that they come anywhere close to accounting for all of the dark matter that we observe. I don't understand this distinction you make between "relatives of the neutrino" and "mysterious theoretical dark matter." A new particle is a new particle.
Yes, but the distinction is that a single new particle will almost by definition have some unique properties (if a neutron had a positive charge equal to the proton, would it be a "neutron", or just a "heavy proton"?) It does not follow from the existence of one such particle that there is a whole family of particles with all or most of the same properties comprising entire systems or galaxies representing 80% of matter in the universe. That's a BIG leap from the existence of neutrinos, which probably has something to do with why neutrinos aren't cited as proof of exotic dark matter, nor the leading cause for believing it exists on astronomic scales.
Also, you might note the history of neutrinos themselves: the neutrino was first posited to explain particle decays which seemed to be missing some energy in the decay products. It wasn't detected directly until a decade afterward. Sounds like a familiar scenario, doesn't it?
Two and a half, but who's counting?
I fully support experimentation and observation to TEST unproven theories, but not with the overt goal of PROVING them. Once upon a time (and since neutrinos were detected) many fundamental particles were thought to exist, yet ultimately proven mere composites, to say nothing of theoretical particles that have been as conclusively DISPROVEN as anything can be.If the evidence is there, fine, and yes, we should look for it if the best theories say it's likely to be there. But the reference to particle physics set off the same alarm bells for me that the history of theoretical particle physics has: I'm not interested in finding new (and expensive and time consuming) ways to search for evidence of something simply because the old ways didn't find any. Sometimes when you dig for gold and don't find any it doesn't mean you should dig deeper, it means you should dig elsewhere, and the fact proving an unconventional theory is a great way to get your name in the history books is just one more reason to be skeptical of those making the attempt. Everyone wants to win a Nobel Prize like UTs Stephen Weinberg but no one wants to win an Ig Nobel Prize like John Bockris down the road at Texas A&M.
Why exactly do references to particle physics set off alarm bells for you? Particle physics is a huge success story of going from prediction to detection; it happens over and over again in the history of the field.
It's also a huge story of going from prediction to rejection, and I think it's very important to keep that in mind. Failure can and ideally should be as instructive as success; that's why earth, air, fire and water are no longer considered the four fundamental particles of a geocentric universe. I'm a lot less worried about the LHC spawning a planet swallowing black hole than the attitude I often sense amounting to "The great thing about the HLC is that it will answer the most important question of the day: How big must a collider be to find the Higgs boson?" If it doesn't exist the answer is "pretty freaking big.... "
If your prediction is that the gold is two miles underground, and you don't see any after digging only one mile down, that's no reason to give up. You knew that you probably wouldn't see any after only a mile, but you still had to dig that first mile in order to be able to dig the second mile.
Sure, but if you don't find it after that second mile you shouldn't conclude "our theory was wrong--it's actually THREE miles down. *dig, dig, dig*"