I’d love to know what this dark matter stuff is, and you know that physicists want to know even more.
So, why does dark matter, matter, anyway? Well, this stuff makes up a huge chunk of the Universe, and we want to know what the Universe is made of. If it turns out to be an elementary particle that isn’t part of the Standard Model of particle physics, then that means the Standard Model is wrong, and we’ve got some cool new physics! If, on the other hand, it somehow turns out to be that our understanding of gravity is fundamentally flawed, we still get new physics. And new physics is always exciting! Either way, it’s about learning about our Universe. Think about it: right now, the stuff we know and are familiar with accounts for just 5% of the Universe’s contents. Imagine what ticking off another 25% of the Universe would mean. Don’t know? Neither do I, but that’s what’s so exciting about it! My bet is that it’s a new particle, and the Standard Model is wrong. Maybe it’ll be SIDM, maybe WIMPs, or Kaluza-Klein dark matter. Maybe it’ll be several types of dark matter, with new forces in the dark sector; I mean, why shouldn’t it be, when the visible sector is a particle zoo? Whatever it turns out to be, unraveling this mystery will be a ground-smashing achievement in the world of physics.
That comes from a pretty good summary of many of the current models for dark matter (well, I thought it was an informative summary, but I’m a biologist, so what do I know?). Some day I’m going to need a physicist to sit down with me over a beer and explain this stuff in little words.
The flavor of the month is “Light Dark Matter”.
MAKE UP YOUR MIND!
Little words aren’t that helpful, either.
In biology or mechanical devices or medicine, you can build a simulation in your head to help yourself out. With theoretical physics, I think only mastery of the most esoteric mathematics can let you have an informed opinion.
Dark matter is said to be invisible, odorless and tasteless. It doesn’t interact with itself or other matter or fields except gravity. Many expensive facilities have been constructed to detect dark matter, all to no avail.
In my opinion, dark matter smacks of the so-called luminiferous aether of the late 1900s, which Michelson and Morley famously showed was fictitious. My bet’s on some form of modified Einsteinian gravity, which to date has been perfectly verified in all experiments since Einstein published his theory in 1915. Let’s exhaust that option before we build any more costly experiments.
Physics envy?
Oh, but the acronyms and backronyms. So, so terrible.
I find the whole “dark matter” thing bizarre. The mathematical models don’t add up so the logical conclusion is ghostly, invisible, massless, undetectable particles that are apparently everywhere and make up 75% of the universe.
Sounds a lot like the 21st century luminiferous aether to me.
This is just how almost all physics has worked since James Clerk Maxwell worked out the theory of electromagnetism: you have a bunch of observations that hint that there’s something you don’t understand going on, somebody sits down and does a bunch of horrendously complicated mathematics to come up with some kind of plausible explanation, then the experimentalists go looking for other results that either support or undercut the theory. At the time, electromagnetism, relativity (both special and general), and quantum mechanics were all every bit as speculative and implausible as dark matter, or the luminiferous ether. The luminiferous ether just happens to be one that didn’t pan out.
Actually, I’d argue that relativity and QM were both much more implausible than DM, because they both told us that our fundamental ideas about the nature of reality were wrong. By comparison, DM just hypothesises that there’s another type of particle out there that’s really hard to detect, which is exactly what was said about a great many of the now-known particles in the Standard Model at one time or another. We spent a long time and a hell of a lot of money not detecting the Higgs boson too – until we found it.
Hey, whatever attracts funding and maintains jobs. String theory worked for a while.
@Jon #6:
It’s not just that the models don’t add up, they really don’t add up at all. And the the only real conclusion so far is that the universe behaves as if there is a lot more mass than we can see. Hence Dark Matter. It’s really quite logical.
The whole point of DM is that it’s not massless. Mass is literally the one thing it’s definitely supposed to have.
Other than that, however, this is also an exact description of the theoretical invention of neutrinos.
@#3, weylguy
@#6, jonmelbourne
The problem with that stance is that it actually ends up violating Occam’s Razor. It posits that gravity somehow works differently here on Earth where we can do experiments on it directly (and yes, we can measure gravity directly in the lab) than it does “out there”. If gravity works the same way everywhere, then there has to be more “stuff” out there than we can detect. It has nothing to do with whether gravity is viewed as an exchange of field particles or as warping of spacetime — the stuff we can see is moving as though it is being pulled by gravity by stuff we can’t see, which would have to be distributed in such a way as to be large-scale clumpy like ordinary matter, but not always in the same places as ordinary matter. (And even saying “gravity doesn’t work the same way everywhere” doesn’t really help all that much because it immediately leads to the question “what causes gravity to act differently in our immediate neighborhood than in outer space?”.)
@11:
Well, in Einstein’s day I don’t think we could really measure gravity very well in the lab. IIRC it was an anomaly in the orbit of Mercury that special relativity explained, and general relativity was also first strongly supported by the observation of the bending of light traveling close to our local star. But your main point is well taken.
So when we observe galaxies we find that they don’t behave as a clump of stars and gas should behave if our theories are right. As you say the most parsimonious explanation is that there is stuff out there that we cannot see, but which creates more gravity.
But that might be wrong. If it isn’t wrong, then the Standard Model of particle physics is wrong. If it is wrong, then Einstein was wrong. A century of experimentation and observation has yet to reveal anything that GR does not predict. On the other hand there are I believe some experimental results that at least hint that the standard model might be wrong.
So, at present, dark matter is more likely to be the explanation that G.R. being wrong. Future observation might change that but we can’t, unfortunately, know what future observations will show.
Or to put it more succinctly, Physicists ain’t stupid!
People who really understand this stuff are, of course, welcome to explain to me how wrong I am.
The term ‘dark matter’ is but foo – a place-holder – in lieu of whatever ‘it’ might be. The term has caught on and acquired a certain inertia, but honest physicists are sure of only one thing: its an anomalous gravitational attraction observed only (so far) on the large scale associated with galaxies and galaxy clusters, in the fast motions of individual stars in galaxies and individual galaxies in galaxy clusters. If it is something like neutrinos (an example of a weakly-interacting particle with mass, a so-called ‘WIMP’, although the mass of the neutrino is vanishingly small compared to particles like electrons and nucleons – protons and neutrons) then it could be interspersed with galaxies on that large scale, gravitationally interacting with itself and visible matter, and out-weighing it by a factor of at least 3. Or, it could be something peculiar about how gravitation behaves on that larger scale. Some alternative theories such as MOND (Modified Newtonian Dynamics) and others suggest some kind of departure from Einstein’s description of gravitation as outlined in his theory of General Relativity. What makes the puzzle even more intriguing is that extremely sensitive observations of the motions of massive objects such as binary pulsars, black holes and now even gravitational waves from their mergers have so far demonstrated that GR, which describes gravitation as a curvature of space-time, is faultless. Some think that the long-sought quantum theory of gravity – a linking between GR and the Standard Model of particle physics which is based on Quantum mechanics – may provide an answer. Or, ‘it’ could be something completely different nobody has dreamed up yet. But whatever ‘it’ is, the anomalous motions observed demonstrate beyond all doubt there is something besides the visible matter we see that is responsible for keeping galaxies and galaxy clusters from flying apart.
Lisa Randall, Dark Matter and the Dinosaurs
Lisa Randall studies theoretical particle physics and cosmology at Harvard University, where she is Frank B. Baird, Jr., Professor of Science. A member of the National Academy of Sciences, the American Philosophical Society, and the American Academy
She gives a very reasonable, thoughtful summary of both and possible interaction and how the integration is proceeding.
I recommend this book highly.
dark Matter, because it is matter than does not interact with light in anyway, neither emits nor absorbs. “Black” absorbs all frequencies, while “dark” does nothing (literally) with light and all forms of ElectroMagnetic forces. It currently appears to be a single force form of matter that interacts with gravitational forces only, with the Weak and Strong forces remaining to be tested.
@3 + @6–
Relativity is a very well established theory with many thousands of experiments that concord with its predictions. The problem is that on the cosmological scale, experiments don’t coincide with the predictions of relativity. So the term “dark matter” was created as a conceptual bucket for all the possible explanations of this discrepancy. And the reason this particular term was created is because of the assumption that the effect is most likely undetected mass — which makes a lot of sense when you consider that the vast majority of our observations of mass in the universe are from light-emitting sources and it’s reasonable to assume that there are other sources of mass that we can’t observe with our current telescope technologies because they don’t go advertising their existence with garish displays of photons.
The fact that we can’t directly observe dark matter does NOT make it a failed concept. Most of the Standard Model of particle physics was worked out this way. A particle interaction would be observed, a chunk of the predicted energy was missing, so we postulated a new, unobserved particle and then designed new experiments to make these particles observable. This is the current state with dark matter — we’ve observed a gravitational gap in our predictions and now we’re trying to figure out what the causes could be and how to test for them.
The comparison with the luminiferous æther is wrong in my opinion. The æther was simply an assumption based on the wave properties of light. If it was waves, then surely it must be waves in something?
DM is the complete opposite, it is based on direct observation and makes no assumptions about it’s nature.
So, let’s say that you are working with a #8 Allen wrench at your work bench. You go to lunch, and when you come back, your Allen wrench is missing, but there is an identical wrench on the work bench of your colleague, who is not at lunch, himself. Now, you could think this is indicative of new physics, whereby your wrench teleported to your colleague’s or simply disappeared into another universe while another reappeared on your colleague’s space.
Or, you could presume that the explanation is mundane: Your colleague borrowed the wrench and forgot to bring it back to your space before leaving for lunch. And as alluded to above, there is precedent. In the 1920s, it became apparent that energy and momentum were not conserved in beta decay. Heisenberg and Bohr started investigating the possibility that energy and momentum were only conserved “on average” in the weird quantum world. Pauli introduced the neutrino–nearly massless, no electric charge, and by the physics of the day, undetectable. He said, “I have done a terrible thing, I have postulated a particle that cannot be detected.” Of course, they were eventually detected, and they are now the newest frontier of astronomy. There is no physical law that requires that the Universe make sense to us, but usually, it does, and assuming it does can be a pretty powerful device for advancing that understanding.
PZ–
No need for naming envy. Biology has some great terms: superpredator, mass extinction, endosymbiosis, convergent evolution, futile cycles…
re @15:
Actually, every large scale cosmological measurement has matched Relativity predictions precisely. Like black holes bending light into observable curved paths away from straight lines. The discrepancy you mention is not with the theory itself, it is with our measurement techniques. Assigning mass only to luminous bodies doesn;t provide enough mass to account for the rotational velocities of galaxies. Only by looking more closely at how those visible objects move did we start filling in a “dark” type of matter that provides mass without visibility.
So probably not luminiferous æther but maybe an artefact of Maxwell’s demon?
Ed Seedhouse @12:
No, it was general relativity which corrected the Newtonian explanation of the perihelion precession.
Dark matter isn’t a new luminiferous aether, where people made up a model of physics that needed some extra substance to work. It’s a new wind, where we have a model that works extremely well in some cases, and in others works extremely well but only if you allow the possibility of some additional substance that isn’t immediately visible.
People have worked very hard at alternate theories of gravity. None of them work particularly well, whereas assuming that maybe not everything out there is glowing fits well with several different lines of evidence. http://www.astrokatie.com/faq/#darkmatter has some useful links on the subject.
slither tove@19–
That was what I was getting at. The discrepancy is between what GR says the mass of galaxies should be from the gravitational effects we observe versus how much mass we can directly observe via telescope. Your clarification is an improvement.
Having said that, gravitational lensing is predicted by Newtonian physics as well as GR, so even though it’s regularly used in popular articles as a “proof” of GR, it’s really more of a demonstration that light paths are bent by gravity, a feature that is not exclusive to GR. Mind you, GR and Newtonian theory give different magnitudes of gravitational light bending and we can definitely tell them apart with observations of lensing by our sun — but not, as far as I am aware, from observations of lensing by distant galaxies.
I agree with the idea that dark matter is an artifact of our incomplete understanding of gravity. The theory of entropic gravity is shaping up to be a strong contender; it predicts when gravity is incredibly weak, it begins to decay inversely with distance instead of the inverse squared. It’s only a matter of time and technology before we can test those predictions.
@21:No, it was general relativity which corrected the Newtonian explanation of the perihelion precession.
Yeah, I remembered that after I posted, unfortunately. And thought I’d wait to see if someone else corrected it because, if you make such a mistake, you should wait at least a few days to let someone else correct it. Or you are secretly hoping that no one will notice.
@24: I agree with the idea that dark matter is an artifact of our incomplete understanding of gravity
Well, but as you admit later in the body of your post, there is as yet no evidence for this idea. Whereas there is at least some for the SM not being entirely correct. So I’m betting on Einstein.
Not to say Einstein shouldn’t be tested to see if you are right.
Imagine what ticking off another 25% of the Universe would mean.
Unless we make really good friends with at least 26% of the Universe (and even then), it would mean Big Trouble.
@25:
I was not aware of this internet tradition! ;-)
Personally, I can’t sleep until I’ve corrected errors I notice after I’ve posted.
Ed Seedhouse @12 writes:
I would say you think wrong. More than a century before Einstein’s day, in 1798, Cavandish used a using a torsion balance to measure the attraction between lead weights accurately enough to determine the density of the earth within 1%.
I feel your pain PZ. I while back I had to inspect various pressure vessels that were part of a dark matter detecting rig. I was naturally curious so I asked the guy who was using it a few questions. To be fair he did answer using small words, but to be honest at the end I wasn’t all that much the wiser. I’m just an engineer so what do I know. His pressure vessels were safe to use, which is more than could be said about some of the rigs that other physicists at that particular university dreamt up.
One of the biggest problems with the “modified gravity” models, intended to explain galactic spin patterns without resorting to any “dark matter”, is that in at least one place in the universe, the visible and dark matter portions of a galaxy appear to have been separated. We can see gravitational lensing associated with an invisible mass that does NOT correspond to location of the visible matter:
https://en.wikipedia.org/wiki/Bullet_Cluster
Of course, this is not strong enough evidence to completely settle the matter, at this time. I guess we’ll have to reserve judgement until we have more evidence.
@28:I was not aware of this internet tradition! ;-)
At 74 I feel entitled to invent traditions at whim.
@31, there seems that there might be some interaction with whatever dark matter is and something (either with baryonic matter or gravity. The appearance suggests, to judge from imagery of superimposed x-ray and gravitational lensing, a front much like a compression front (think bow wave) and wake. That’s present in MACS J0025.4-1222 as well.
Perhaps, that gives a modest hint in a direction of inquiry?
@29:
Well, but these days we know the gravitational constant to a margin 4.7 x 10^-5 or 0.00047 % if Wikipedia isn’t lying to me. That seems a tad better but we’ve had 200 years to get there.
“Some day I’m going to need a physicist to sit down with me over a beer and explain this stuff in little words.”
This might help. https://www.youtube.com/watch?v=2rjbtsX7twc
Basically, MOND theories can explain why the galaxies spin faster that they would if there was only the visible stuff. The problem with MOND is that they fail to explain discrepancies between expected and real observations on larger scales. For example, when you look at colliding galaxies, the “dark matter” theory perfectly matches what we see, but MOND utterly fails on that (except by tweaking it still a bit more, which indeed contravenes to occam’ razor principle)
what, a story on creative names and dark matter, and no mention of how in the early days, when observations were showing that something massive yet unobserved was on the outskirts of our galaxy, the two leading contenders were massive compact halo objects, and weakly interacting massive particles? that’s right – it was a battle between the MaCHOs and the WIMPs.
but at least dark matter can be explained as some kind of particle (or particles) that exert mass but don’t otherwise seem to interact. where things really start to get weird is dark energy. observations indicate that it takes up 75% of the universe, and we have no clue what it is.
astro @37:
The simplest explanation is still that it is the vacuum energy of space, with density given by the cosmological constant in the Einstein field equations. It is correct to say we can’t explain the value it has*, but that goes for a whole bunch of stuff. So, is it any more weird than the difference between the masses of the charged leptons (electron, muon, tau)? The SM just tells us that the differences are due to the different strengths of their interactions with the Higgs boson, but that doesn’t really explain anything. Why are the interactions of different strengths?
*Much is made of the huge discrepancy between the observed value and a naive quantum field theory calculation, but the most important word there is “naive”.
The biggest problem I have with Dark Matter is that it increases as the universe expands. That means it is either more influential as the universe expands, or it is multiplying out of nothing (that we know of). I’m not saying that makes any other theory, such as MOND, more likely, just that it makes it sound akin to caloric or luminiferous aether as something which will be superseded by something else when we finally discover it.
No.
1) Dark matter doesn’t do that. (At least not the ordinary candidates you may have heard about … all sorts of wacky things could be invented that wouldn’t even fit the data, but I’m not worried about them.)
2) If this did sound like a good complaint, you’d make it about space: it increases as the universe expands. What’s supposed to be the big problem? I’d be worried if it had contradictory values, such that it’s supposed to both increase/expand and decrease/contract. But generally, I don’t know why I ought to be worried that something (according to theory and observation) simply exhibits consistent behavior of some kind or another. If I’m supposed to have more preconceptions, that tell me which theories to prefer before I even take the time look around at the real world, then there should at least be some good reasons why I should have them.
Don’t know why that should be phrased as an either/or proposition. Why not both or neither, even if you’re talking about Dark Energy, let’s say, instead of Dark Matter?
The funny thing is, to a large number of very smart people at the time, caloric and luminiferous aether didn’t have a discernible smell of “something which will be superseded” eventually. (So, the moral of the story is that whatever you’re doing here is not what seems to have actually worked in those particular cases.)
It’s not like they were logically incoherent or something. Generally, we could see that sort of issue coming a mile away and avoid it without a great deal of trouble. Those featured in respectable scientific theories/models that had been widely accepted for a very long time. Then, to just about everyone’s surprise (and anyone who wasn’t probably should’ve been), they turned out to be incapable of adequately describing/explaining observed phenomena. That’s it. There was no sticker conveniently attached to them the whole time, which read “this looks fishy,” if only somebody like you had bothered to look. They looked and smelled just fine, to most of the people who cared to learn about those subjects, until the whole thing suddenly came crashing down. (Thanks, Einstein.)
methuseus,
Sorry, but you really don’t seem to understand the hypothesis. Look, if I shoot a billiards ball on an unfamiliar table and it curves unexpectedly, which hypothesis shall I entertain first:
1) there is a warp in the table
2) there is some previously undiscovered force that will revolutionize the world as we know it
I’m goin’ with 1), Bob!
The thing is that we have two basic problems–the motions on galactic scales suggest that the matter we are seeing is only a part of what is there. I don’t find that so hard to believe.
Then on very large scales, the force of gravity is being overcome by some other force that is causing the Universe to accelerate away from itself. Since the issue is the scale of space, it makes sense that the force is a property of space-time itself–particularly since we think we know of two other events where the properties of spacetime caused abrupt acceleration: the birth of the Universe and it putative inflationary epoch.
Given the observations and what we know, these are the most parsimonious explanations.
methuseus @39:
I think you’re confusing Dark Matter with Dark Energy. The density of Dark Energy does stay constant as space expands, so it does increase with time. But this is not “something out of nothing” because it’s an intrinsic property of space. Whether the total energy of the universe is conserved is an ongoing debate.
Dunc @10 et al
“The one thing it has is mass”
Not strictly true. It effects regular matter like mass, but that doesn’t necessarily require that it has mass. It might be some funky graviton-affecting-thingy. (GAT is the new Higgs) that has no mass, mass being an attribute of standard matter.