Is Dark Matter the new Phlogiston?
June 26, 2006 6:33 PM   Subscribe

Is Dark Matter the new Phlogiston?

Phlogiston is an obsolete 17th Century scientific theory which holds that all flammable materials contain a colourless, weightless matter which is freed in the process of burning.

Dark Matter, and it's sister force Dark Energy, are invisible, omnipresent entities which theory suggests accounts for well over 95% of all the density of the universe.

Will the Dark forces turn out to be nothing more than a 20th century scientific folley? My favourite refutation of these Dark forces conerns the existence of higher dimensions of space-time in which alternate universes hang mere nano metres away, excerting a gravitational force which registers on our instruments, but can never be directly accessed.

Which alternate hypothesis draws you? and, In the annals of history, what other 'laughing stock theories' could Dark Matter be better compared to?
posted by 0bvious to Science & Nature (35 answers total) 9 users marked this as a favorite
 
Either dark matter exists, or our understanding of gravitation is wrong. Its gravitational effects are consistently and universally observed. So absent a completely new theory of gravitation, dark matter is here to stay.

But who knows: maybe gravity is the phlogiston of the 17th-21st centuries.
posted by mr_roboto at 6:44 PM on June 26, 2006 [1 favorite]


I don't think dark matter will eventually be considered a laughing stock theory. Our understanding of gravity predicts how massive celestial systems should behave. If we see a star wobbling, we assume that there's something nearby like a planet which we can't see because it's too faint.

Likewise, the behavior of galaxies differs from what theories predict, assuming we can see all matter. The simplest explanation is that there's matter we can't see.

Is it possible that there's no dark matter and that the right explanation turns out to be something exotic like higher dimensions? Absolutely, but going to the more complex explanation seems the greater folly. See Occam's razor.
posted by justkevin at 6:49 PM on June 26, 2006


Yeah, that's what I thought and then I watched a few documentaries on it and saw how it fits into larger theories and is predicted by the speed at which things orbit other things in space.
posted by 517 at 6:56 PM on June 26, 2006


You think that Dark Matter is a simpler explanation than higher dimensions? I agree, the numbers involved are less, but an invisible, ever pervasive matter which has no other purpose than to balance the universe perfectly? Appearances often show simple explanations are also naive explanations...
posted by 0bvious at 6:57 PM on June 26, 2006


Why would you expect most matter to be visible from Earth? Most of it is very far away, and it only emits light when it's involved in a high-energy nuclear reaction.
posted by mr_roboto at 7:03 PM on June 26, 2006


22% of the universe is supposed to be made of this stuff. Why haven't we found one chunk of it nearby?
posted by 0bvious at 7:04 PM on June 26, 2006


Jupiter?
posted by mr_roboto at 7:14 PM on June 26, 2006


Seriously, you realize you and I are both made out of dark matter, right?
posted by mr_roboto at 7:18 PM on June 26, 2006


Because we haven't been very far out into the universe, and our sun illuminates everything nearby?
posted by equalpants at 7:18 PM on June 26, 2006


Actually, in all fairness, baryonic dark matter--like you, me, and Jupiter--only accounts for a small fraction of the total.

Know one knows for sure what makes up the rest, but that doesn't mean we haven't observed it: its gravitational effects are unambiguous.
posted by mr_roboto at 7:24 PM on June 26, 2006


I think the analogy between dark matter and phlogiston is a good one. Both are substances that are not directly observable, but whose presence is strongly implied by indirect observation combined with current theories. And future theoretical advances may mean we no longer need to posit dark matter, just as the discovery that combustion is oxidation made phlogiston obsolete.

But I wouldn't describe phlogiston as a laughing-stock theory. It made plenty of sense at the time.
posted by beniamino at 7:26 PM on June 26, 2006


the existence of higher dimensions of space-time in which alternate universes hang mere nano metres away, excerting a gravitational force which registers on our instruments, but can never be directly accessed

first of all, i have no idea what the hell this means.

dark matter has an unfortunate name that makes it sound mysterious and spooky, which is somehow appealing to people in the same way that the many-universes interpretation of quantum mechanics is.

it's a lot simpler than a lot of people would have you believe. basically, our view of the universe outside our immediate vicinity comes almost-entirely through light, which we detect with various types of telescopes. therefore what we know of galaxies, stars and so on is necessarily restricted to what we can see, which is called luminous matter, since it emits light of some sort.

various measurements on the velocities of distant stars and so on don't jibe with what would be predicted by our current understanding of gravitation. so, there are two simple ways to resolve this apparent conundrum:

- our understanding of gravitation may be incomplete.
- there may be stuff out there that we can't see.

there's also, i guess, the possibility that we are dealing with a combination of the above two things.

to my mind, the notion that there is mass in the universe that we can't see for whatever reason is a lot simpler explanation than the notion that general relativity is wrong and newtonian gravitation breaks down over long distances. either is certainly possible, but i gotta go with occam's razor on this one.
posted by sergeant sandwich at 7:31 PM on June 26, 2006


Why haven't we found one chunk of it nearby?

Most theories on what dark matter consists of propose that it's either small particles that are very difficult to detect like nutrinos or large dark objects that aren't very common like brown dwarfs, or both.
posted by justkevin at 7:32 PM on June 26, 2006


Sergeant Sandwich said:

"the existence of higher dimensions of space-time in which alternate universes hang mere nano metres away, excerting a gravitational force which registers on our instruments, but can never be directly accessed

first of all, i have no idea what the hell this means."

Try this google search for related material
posted by 0bvious at 9:03 PM on June 26, 2006


There's one theory that states that dark matter only shows up in the equations are done using Newtonian physics. When you calculate everything using General Relativity, everything works out fine.

So it's a bit strong to say that dark matter is "needed" in order to explain the universe.
posted by Paris Hilton at 9:11 PM on June 26, 2006


Although yeah 0bvious' theory about extra dimensions hiding things doesn't really make sense.
posted by Paris Hilton at 9:12 PM on June 26, 2006


sargeant sandwich seems to have the best answer so far.

Dark matter is a useful way to quantify what we don't know. I think everyone hopes to dispense with it soon, and replace it with a more specific understanding of the missing stuff. Either that, or fix our understanding of gravitation.
posted by b1tr0t at 9:14 PM on June 26, 2006


As a physics grad student, but one whose studies are far, far removed from dark matter, I have to say that I think it's a decent theory. All it effectively posits is that there is some form of matter that interacts very weakly with the stuff out of which we (and everything else we see) are made. That doesn't strike me as ridiculous, especially when it's all that's needed to explain well what would otherwise be serious problems with our observations of galactic motion. Considering that there are some reasonably well-thought-out candidate particles, I'm willing to go with it. Especially considering that the tweaking to gravity at large distances would need to be quite exotic, it feels like a more likely theory.

Dark energy, on the other hand, feels like a fudge to me. Despite the name, it has nothing to do with dark matter, and is much stranger and less understood. I wouldn't be surprised if new ideas about the nature of the universe radically change our notion of dark energy, even to the point of making it unnecessary. That said, as best as I can tell, it is a catchall term for the cause of the acceleration we are seeing in the expansion of the universe, so it'll probably wind up being something, but the name is very much a misnomer.
posted by Schismatic at 9:40 PM on June 26, 2006


Most theories on what dark matter consists of propose that it's either small particles that are very difficult to detect like nutrinos or large dark objects that aren't very common like brown dwarfs, or both.

This debate had perhaps the best pair of acronyms in the history of physics: MAssive Compact Halo Objects (MACHOs) versus Weakly Interacting Massive Particles (WIMPs). Unfortunately, the debate is pretty much settled in favour of the WIMPs these days — if there were MACHOs out there in the galactic halo, we'd be able to see their gravitational lensing effects, and we haven't thus far. It's too bad, because I'd love to be able to bandy those acronyms about some more.
posted by Johnny Assay at 10:55 PM on June 26, 2006


Paris Hilton said:

"Although yeah 0bvious' theory about extra dimensions hiding things doesn't really make sense."

Not my theory.

Try M-theory and its many off-shoots for a glimpse into the world of higher dimensional branes and gravity being shared across universes mere nano metres away from our own.
posted by 0bvious at 10:57 PM on June 26, 2006


In regard to Paris Hilton's link about using full general relativity instead of Newtonian physics: although that kind of explanation is very satisfying, there are a number of problems with it. There have been a few responses published suggesting that the authors from Paris' link actually did their math incorrectly. An even bigger problem is that galactic rotation curves are not the only means of calculating the mass of a galaxy. Other means include gravitational lensing and analysis of multiple-galaxy systems. Also, the average density of the universe can be computed from a variety of cosmological sources, and compared to the amount of matter we see in traditional forms. The results from these different estimation methods all indicate that there is dramatically more matter then we would otherwise expect ---or our understanding of gravity is flawed. Since the foundations of general relativity rest on very few, seemingly simple assumptions, most physicists regard the existence of a large amount of difficult-to-detect matter as the simplest explanation.
posted by Humanzee at 12:04 AM on June 27, 2006


Hello. Cosmologist here.

Before I get going I will link to this FAQ which will probably be useless to the questioner, but will help for anyone who doesn't have a clue what this question is about.

Mostly, what has been said so far is right. The state of play on the dark matter front is that dark matter is the leading contender for explaining why we are short of luminous matter. It's not by any means considered a done deal though, and MOND, most specifically in the relativistic formulation that Bekenstein came up with called TeVeS is an alternative that receives plenty of interest. It's a great theory scientifically as it's dead easy to invalidate, at least in principle(unlike some that have been getting a lot of press *cough*ID*cough*). It's not been yet, to my knowledge, but it'd only take, say, one rotation curve that doesn't fit TeVeS to do it, or there are other routes to invalidating it. A paper I read a few days ago suggests that gravitational lensing tends to happen more often and more extremely in TeVeS for instance, and if this turns out to be true and we don't see enough such lensing events it could be a problem for TeVeS.

So, alternatives to dark matter are coming in second, but they're by no means laughing stock theories or anything. They get serious consideration, at least in part because they're often able to make strong predictions we might feasibly invalidate.

One last point about dark matter is that we know one kind of dark matter definitely exists - neutrinos. We detect these all the time in neutrino observatories, but it's very hard to do, and low energy neutrinos are pretty much impossible to detect but would have a gravitational effect. These particles are ghostly in the extreme, and stand a 50% chance of making it through a light year's worth of solid lead without being stopped, and were proposed to deal with the problem that energy and angular momentum seemed to go missing in certain nuclear decays. You could claim that the energy and angular momentum wasn't being conserved, was going off somewhere altogether different or whatever, but ultimately we started detecting these particles and now they're detected routinely.

What we do know is that this particular brand of dark matter isn't any good. We've already ruled it out for helping with our gravitational problems because neutrinos are 'hot' - they move too fast. This means you can't get them to clump up in one place and help to hold a galaxy together. It's not that implausible to propose something that acts a bit like a neutrino but is much heavier.

Also particle physicists have plenty of candidates for particles it might be which are motivated for one reason or another such as supersymmetry.

On to dark energy. What do we know about dark energy? Well, we know that our ordinary general relativity models without it don't match what we see. Something is missing from them. Noone is committing themselves to what is missing at this stage, and a lot of money is going in to figuring out some more about it. You'll hear lots about the cosmological constant, and the cosmological constant is just the simplest form of dark energy that matches the data, but we don't have any really strong reasons for thinking it is a constant right now. We have a bunch of theories competing right now, with names like phantom energy and quintessence models, but we just don't know right now which is right. We're working hard on ruling out as many as we can though.

Phlogiston's an easy comparison to make, or you can compare things to the heliocentric model which suddenly did away with the bells and whistles of epicyclic orbits, but science doesn't always go that way. Sometimes you really do need extra bells and whistles. The neutrino has shown us that.
posted by edd at 12:55 AM on June 27, 2006 [14 favorites]


Oh I'll add something about M-theory and branes. It's not my field at all, but as I understand it gravity tends to leak out of our brane and this is the suggested explanation for why it's weak. It's supposed to make gravity stronger on very small scales as a result. I'm not sure it's able to make gravity stronger on very large scales, which is what would be needed to explain galactic rotation curves.
posted by edd at 1:13 AM on June 27, 2006


I don't have anything to add to the topic but I just wanted to say that edd's comment was fantastic and that I got a real chuckle out of the phrase "In regard to Paris Hilton's link about using full general relativity instead of Newtonian physics."
posted by Rhomboid at 6:15 AM on June 27, 2006


Why I love ask.me
posted by dmd at 6:26 AM on June 27, 2006


If M-Theory is based on string theory, which that wikipedia article indicates, and string theory is completely untestable, doesn't that leave M-Theory as yet another mathematical flight of fancy?
posted by phrontist at 6:30 AM on June 27, 2006


M-theory and string theory are generally pretty hard to test. I'm not so familiar with the particle physics side of it all, but on the cosmology side when you start talking about branes you can come up with models where there are some definite testable predictions. You can attempt to perform the necessary experiments and see if you can rule some specific cases out, but it might not be possible to rule out M-theory in general.

String theory might generally make predictions about things happening at certain very high energy scales we can't achieve, but a specific braneworld cosmology could predict that something funny is going on on a small (but not subatomic) scale to gravity. You can literally do a tabletop experiment to say that if this is happening the length scale involved must be smaller than say 0.1mm, or, if in the future we're very lucky we might be able to say that on a certain length scale something funny is going on with gravity, and then that's evidence in favour of branes.

Relatively old but good article on the subject.

Again, this is stuff I'm less familiar with, but I can drop people more involved an email if any questions come up. No promises they will answer though.
posted by edd at 6:59 AM on June 27, 2006


As far as I understand it, String Theory and M-theory, might be tested by a new piece of scientific equipment which as just gone online. From BBC news:

One of the great scientific experiments of our age is now fully underway.

A German/UK team has put the giant GEO 600 gravitational wave detector in a continuous observational mode.

The Hanover lab is trying to detect the ripples created in the fabric of space-time when black holes fall onto each other or massive stars explode.

Success would confirm fundamental physical theories and open a new window on the Universe, enabling scientists to probe the moment of creation itself.
- link
posted by 0bvious at 9:19 AM on June 27, 2006


Metafilter: gravity tends to leak out of our brane
posted by Rumple at 12:22 PM on June 27, 2006


I've written a book about the recent revolution in cosmology. To clarify a bit...

First, what theorists believe:
As has been said above, there's two types of dark matter. Baryonic, or "ordinary" dark matter is regular atoms (mostly hydrogen) in various forms that doesn't shine. This type of dark matter has been observed in the form of gas clouds (which have been seen) and MACHOS (which have also been seen, but don't seem to make up the bulk of the baryonic dark matter halo around galaxies). Baryonic dark matter makes up about 4-5% of the stuff in the universe; about 10% of that is luminous, and 90% of it is dark.

Non-baryonic, or "exotic" dark matter, makes up about 22% of the stuff in the universe. This is matter that isn't made up of atoms. What could exotic dark matter be? Well, neutrinos make up part of it -- about 0.5% or so, but as our resident cosmologist says, neutrinos can't make up the bulk of exotic dark matter, because they're fast-moving. Too much of this "hot" matter, and galaxies wouldn't form. Nobody really knows what the exotic dark matter is made of, but WIMPs (weakly-interacting massive particles) are a good candidate; many particle physicists believe (for other reasons) that there are as-yet undetected particles known as neutralinos that would fit the bill quite nicely. There are other candidates, though -- and physicists are busy searching for them.

The rest of the stuff in the universe -- more than 70% -- is dark energy, a mysterious force that opposes the force of gravity. Scientists are almost completely clueless about what is causing this force; there are some theoretical possibilities having to do with the nature of the quantum vacuum (which, in itself, is a very strange subject), but that's another matter.

Now *why* do cosmologists believe this very, very strange picture? The short version of the story is that there are several different lines of evidence that are pointing in precisely the same bizarre picture. Observations of distant supernovae, of galaxies and galaxy clusters, of abundances of light elements in the universe, and of the remnant radiation left over from the era about 400,000 years after the big bang are all giving surprisingly consistent answers -- everything's pointing to this model of the universe. This picture, about 4 percent baryonic matter, 22% exotic matter, and 74% dark energy is known as the concordance model. The *long* version of why cosmologists believe this is in the book.
posted by cgs06 at 2:10 PM on June 27, 2006


One other thing... the Geo600 story overreaches a bit. The instrument won't be able to see any gravity waves that cosmologists are interested in; even the space-based version on the drawing board (known as LISA) would have a hard time detecting cosmological gravity waves. Geo600 (and LIGO) are interesting to astronomers but not really to cosmologists.
posted by cgs06 at 2:15 PM on June 27, 2006


In answer to the 'Laughing Stock' question, the story of Van Helmont's Tree is of interest.

A young Willow tree was placed in a sealed pot, weighed, then fed nothing but water. After some months it was weighed again. The measurements were precise, The flourishing tree - hugely increased in weight - was made of Water and nothing else.

What was wrong with the method and the conclusions? They ignored the air, the experimenters just didn't consider it for a moment. Perhaps Cosmologists are missing something just as important.
posted by grahamwell at 2:22 PM on June 27, 2006


Is Heim theory something that bears on this? According to the article:
* H1 predicts gluons, carriers of the strong nuclear force.

* H3 and H4 predicts the W bosons and Z boson, carriers of the weak nuclear force.

* H5 predicts photons, carriers of the electromagnetic force.

* H10 predicts quintessence, a weak gravitational-like repulsive force that would cause the expansion of universe.

* H11 predicts gravito-photons, as yet theoretical unobserved particles that would, theoretically, allow the conversion of an electromagnetic field into a gravitational-like field.

* H12 predicts gravitons, carriers of gravity.
(emphasis mine)

Is it considered among cosmologists/physicists to be something worth investigating, or has it been discredited in any way? Is this the same thing as the "quintessence model" that you mention above, edd?

What does Loop Quantum theory say about it, if anything?

Not advancing it as a contender, just curious what the actual scientists think about it. I think this stuff is tremendously interesting.
posted by zoogleplex at 2:37 PM on June 27, 2006


I've only heard of Heim theory from the web (that is, non-physics pages). It's not clear to me if I don't hear about it professionally because it's wrong or because it's not been peer-reviewed like the Wikipedia article said.

What it says about H10 - it is talking about the same quintessence I was, yes.

I will note that the theoretical values might look close to the real numbers, but they're actually a long way from the experimental bounds. The theory as it stands is, from what I see in that Wikipedia article, already ruled out*. It also mentions that it's predicting hadron masses as if they're elementary particles, which they're not. Whether they can fix that, or whether they can attract enough interest to get people working on fixing it I couldn't say.

I'm also an experimental person, not a theoretical person, so I would be disinclined to talk about theories of everything.

* a few σ is normally considered enough to rule something out. Physicists might argue about whether to draw the line at 3, 4, 5σ or whatever. Heim theory is often out by 100σ from what I see there.
posted by edd at 11:14 PM on June 27, 2006


Interesting, thanks!
posted by zoogleplex at 4:54 PM on June 28, 2006


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