I love telling the stories of Neptune and Vulcan. Not the Roman gods, the planets that were originally hypothesized to explain the mysterious motions of other planets. Neptune was propsed by Urbain Le Verrier in order to account for deviations from the predicted orbit of Uranus. After it was discovered, he tried to repeat the trick, suggesting a new inner planet, Vulcan, to account for the deviations of the orbit of Mercury. It didn’t work the second time; Einstein’s general relativity, not a new celestial body, was the ultimate explanation.
In other words, Neptune was dark matter, and it was eventually discovered. But for Mercury, the correct explanation was modified gravity.
We’re faced with the same choices today, with galaxies and clusters playing the role of the Solar System. Except that the question has basically been answered, by observations such as the Bullet Cluster. If you modify gravity, it’s fairly straightforward (although harder than you might guess, if you’re careful about it) to change the strength of gravity as a function of distance. So you can mock up “dark matter” by imagining that gravity at very large distances is just a bit stronger than Newton (or Einstein) would have predicted — as long as the hypothetical dark matter is in the same place as the ordinary matter is.
But it’s enormously more difficult to invent a theory of modified gravity in which the direction of the gravitational force points toward some place other than where the ordinary matter is. So the way to rule out the modified-gravity hypothesis is to find a system in which the dark matter and ordinary matter are located in separate places. If you see a gravitational force pointing at something other than the ordinary matter, dark matter remains the only reasonable explanation.
And that’s precisely what the Bullet Cluster gives you. Dark matter that has been dynamically separated from the ordinary matter, and indeed you measure the gravitational force (using weak lensing) and find that it points toward the dark matter, not toward the ordinary matter. So, we had an interesting question — dark matter or modified gravity? — and now we know the answer: dark matter. You might also have modified gravity, but one’s interest begins to wane, and we move on to trying to figure out what the dark matter actually is.
But some people don’t want to give up. A recent paper by Brownstein and Moffat claims to fit the Bullet Cluster using modified gravity rather than dark matter. If that were right, and the theory were in some sense reasonable, it would be an interesting and newsworthy result. So, you might think, the job of any self-respecting cosmologist should be to work carefully through this paper (it’s full of equations) and figure out what’s going on. Right?
I’m not going to bother. The dark matter hypothesis provides a simple and elegant fit to the Bullet Cluster, and for that matter fits a huge variety of other data. That doesn’t mean that it’s been proven within metaphysical certainty; but it does mean that there is a tremendous presumption that it is on the right track. The Bullet Cluster (and for that matter the microwave background) behave just as they should if there is dark matter, and not at all as you would expect if gravity were modified. Any theory of modified gravity must have the feature that essentially all of its predictions are exactly what dark matter would predict. So if you want to convince anyone to read your long and complicated paper arguing in favor of modified gravity, you have a barrier to overcome. These folks aren’t crackpots, but they still face the challenge laid out in the alternative science respectability checklist: “Understand, and make a good-faith effort to confront, the fundamental objections to your claims within established science.” Tell me right up front exactly how your theory explains how a force can point somewhere other than in the direction of its source, and why your theory miraculously reproduces all of the predictions of the dark matter idea (which is, at heart, extraordinarily simple: there is some collisionless non-relativistic particle with a certain density).
And people just don’t do that. They want to believe in modified gravity, and are willing to jump through all sorts of hoops and bend into uncomfortable contortions to make it work. You might say that more mainstream people want to believe in dark matter, and are therefore just as prejudiced. But you’d be laboring under the handicap of being incorrect. Any of us would love to discover a modification of Einstein’s equations, and we talk about it all the time. As a personal preference, I think it would be immeasurably more interesting if cosmological dynamics could be explained by modifying gravity rather than inventing some dumb old particle.
But the data say otherwise. So most of us suck it up and get on with our lives. Don’t get me wrong: I’m happy that some people are continuing to work on a long-shot possibility such as replacing dark matter with modified gravity. But it’s really a long shot at this point. There is a tremendous presumption against it, and you would have to have a correspondingly tremendous theory to get people interested in the possibility. I don’t think it’s worth writing news stories about, in particular: it gives people who don’t have the background to know any better the idea that more or less everything is still up for grabs. But we do learn things and make progress, and at this point it’s completely respectable to say that we’ve learned that dark matter exists. Not what all of us were rooting for, but the universe is notoriously uninterested in adapting its behavior to conform to our wishes.
Just because they can both be used to describe motion does not mean that one is analogous to the other. After all, energy and momentum can both be used to describe motion, but completely different aspects of it.
Days are not the dimension of time. They are a unit of time. The rotation of the Earth no more causes time than the ticking of a clock. And, as I’ve said, the physical processes of which we are aware act within a framework of background space-time. The Earth rotates because the physical laws that govern the universe obey an attractive law of gravity that obeys general covariance in 3+1 dimensional space-time. The reason this is so is that the attractive law of gravity causes, on small enough scales, for matter to become clumped into nearly spherically-symmetric potentials, potentials which force there to be conservation of angular momentum. And so a body like the Earth spins because it came originally from a large, diffuse cloud in orbit around our Sun, the collapse of which is unlikely to result in zero spin.
So yes, as far as the known physical laws are concerned, time is a cause, but not the only cause.
Jason,
Exactly. One scalar, one vector. Just like temperature and time.
If time is a line, then units of time would be line segments. As such, the point is still the same, this dimension goes from future to past, as energy goes past to future. The rotation of the earth is process and the units defined by it are relative to context, not some underlaying dimension.
No. It doesn’t result in zero spin. It eventually radiates back out. Matter contracts, energy expands.
It is a description.
Temperature and time are both scalars.
This statement is nonsense. Energy is a property of matter. Now, matter which has energy may expand, depending upon the properties of said matter and the initial conditions. But the statement “energy expands” is itself devoid of any meaning.
Jason,
Than why propose it’s a dimension at all? I’m the one arguing it lacks dimension because content and context go in opposite directions. Can you describe any concept of time that isn’t a vector, such as the motion of the earth(momentum), or a vector of a scalar, as in thermodynamics?
More often I’ve had others argue the opposite, that matter is a property of energy. What is matter? Strings? Personally I’m a dualist on this, as with most things, that they are opposite sides of the same phenomena. Energy is the content of matter and matter is the consolidation of energy. E=mc2
Jason,
Think about what mathematics is for a moment. Each term is essentially shorthand for some previously agreed upon concept and just like any other language, it is an attempt to approximate reality. It is not the foundation of reality. It is a map, not the territory. Like any map, it must trade off between efficiency and accuracy because the reductionism required for efficiency edits detail. As Stephen Wolfram pointed out, it would take a computer the size of the universe to compute the universe. The result is that no matter how effective your mathematical description of reality is, it is still limited, so that when you use it to make projections about reality, these limits will become flaws in your model. Now the possibilty exists that there might be other descriptions of reality that may make just as much sense in the context used, but when projected, also develop errors. When these two systems come in contact, the political tendency is to smooth over the differences, rather then go back and re-examine the foundational assumptions. It’s worked this way throughout history. That’s why we have any number of competing religions and philosophies, with any number of compromises creating more confusion and conflicts based on frictions no one has any grasp of how they arose. I think it may be a factor in the conflict of QM and Relativity and that the cure will not come from some grand compromise, but from going back and examining every detail of the original assumptions as objectively as possible.
Because that’s the way the math works out. And whether or not time acts as a dimension is, fundamentally, a mathematical question. The only meaningful objection to this statement would be a mathematical objection, and you have presented none.
So a logical objection is meaningless? What is this? Mathematical infallibility?
Do you remember how the story ended? The emperor really was naked.
Past and future don’t exist outside the information and potential contained by the present.
It’s meaningless because the objection is not framed in the same language in which the thing objected to is defined.
That’s a clear retreat into formalism, not an exploration of logic.
Your ivory tower is a sand castle.
I refer you to the alternative science respectability checklist:
http://blogs.discovermagazine.com/cosmicvariance/2007/06/19/the-alternative-science-respectability-checklist
The problem here is that you’re not even bothering to present a theory: you’re only attempting to present a philosophical description of events that are perfectly explained by current theories. This is meaningless. If you want to present anything meaningful, you need to deal with the math that is the true description of the theory.
You’re right. It isn’t a theory. It’s a fact. Content goes past to future. Context goes future to past. Time isn’t a dimension. It is process.
Math isn’t an ideal, it’s a model. Time can be modeled as a dimension. History does it all the time. It’s called narrative. Beginning to end. As you read it past to future, it reels out from your future into your past.
Believe it or not, but there is a world outside that classroom and what you learn of it is a model, not an ideal.
The reason that world out there has no meaning is because meaning is static and reductionistic, while the world is dynamic(like time) and wholistic(like space).
Models are static and reductionistic. Ideals are religious illusion. You may believe mathematics is god, but that doesn’t make it so.
Look, John, all of this is just meaningless waffle to excuse why you think that you have deduced something about how the world works, all the while not having put in the time and effort required to actually understand the relevant science that has already gone there before you. Believe it or not, there are people that have thought on these issues far longer and far harder than you or I have. Until you are willing to take the time to really understand the science you are talking about, you are going to be unable to produce anything that is meaningful.
Who knows? Maybe your idea has some merit. It is possible. But, as described, it is too ill-formed to have any meaning. If you learned more about the mathematics and physics of relevance (differential geometry, general relativity, and quantum mechanics), you might actually be able to take your vague notions and turn them into a concrete idea. But, chances are, you’ll either find that your idea was meaningless or somebody else had that exact same idea before you and fleshed it out much more fully.
We would all be speechless and thoughtless, if we were only allowed to discuss and contemplate ideas that are both true and original.
“.. to magically reproduce its predictions in case after case.”
There is the other case, the colliding
galaxies:
So, how can the Dark Matter Theory explain this observation ?
As concerns the Bullet cluster, it would be interesting to map
also the distribution of cold hydrogen (before starting a
respectable hype about the First Direct Observation of the Dark
Matter (at the risk to mould a bullet:)); one might be even not
convinced that we observe the end of cluster collision,
not the beginning.
I am a layman with three questions provoked by your article:
answers could be very simple; (1( Nonsense; (2) interesting but irrelevent; (3)
worth a reply.
(1) as the speed of light is a constant, could there possibly be a comparable or analogic constant for gravity?
(2) Is it possibe for two massive black holes to collide and or merge with each other and if so is there a mathetical or theoritical model for this or, indeed, an astronomical or empirical observation which can only be explained by postulatiing such phenomena?
(3) Could these speculations possibly having any bearing on dark mater?
Robert,
1) The speed of gravity is also the speed of light, at least as far as General Relativity is concerned. This speed pops right out of the Einstein equations (scientists are testing this right now, though it turns out to be a bit difficult to even write down a theory of gravity where the speed of gravity is different).
2) Black holes can most definitely merge. This is one of the events that people hope to be able to detect with the next generation of gravitational wave detectors, as it is expected to occur frequently as a result of galaxy mergers, where the supermassive black holes at their centers merge shortly afterward. And if it turns out that there is a visible component of the merger (which is possible, from matter that surrounds the two black holes), then we will be able to use these merger events to constrain cosmology. Here’s a video that shows a simulation of such an event:
http://www.youtube.com/watch?v=sG8_uexPhaI
Hopefully we’ll be able to say we’ve detected such a merger within a decade or so.
3) Doubt it.
Pingback: The Big Picture » Blog Archive » Dark matter… whatever that is
Jason, you should get a medal for the patience and thoughtfulness you’ve exhibited on this thread! Bravo.
Cosmologists seem very confident that dark matter exists, but how confident are they that it will eventually be unambiguously detected? If DM detection experiments keep coming out with negative results, it will always be possible for clever theorists to come up with a theory that is empirically equivalent to CDM.
Professor Moffat adds, ‘If the multi-billion dollar laboratory experiments now underway succeed in directly detecting dark matter, then I will be happy to see Einsteinian and Newtonian gravity retained. However, if dark matter is not detected and we have to conclude that it does not exist, then Einstein and Newtonian gravity must be modified to fit the extensive amount of astronomical and cosmological data, such as the bullet cluster, that cannot otherwise be explained.’
I’m dreadfully sorry to perform necromancy on an old thread, but I am interested in one thing. I’m going to eat my own words from before and admit that I well could well have been wrong about naysaying this dark matter/dark energy stuff. In particular, I’m interested in the answer to the following question. Are sources of the posited dark matter defined to include black holes, neutron stars, and non radiating “cold” matter, like interstellar dust/asteroids/comets? Or are these astrophysical objects regarded as conventional matter (my earlier and possibly erroneous assumption)? There certainly is probably quite a bit of matter that is not burning in stellar furnaces, for instance, or even orbiting such, that is not immediately visible. I kind of assumed that this was taken into account in the original (and current!) calculations of the distribution of matter in the Milky Way, but I’ve been wrong before about the state of knowledge in an area.
Tumbledried, all of those things count as “ordinary” matter, not dark matter. We can measure the total amount of ordinary matter whether or not it is directly visible, using primordial nucleosynthesis and temperature anisotropies in the cosmic microwave background. The total amount of gravitating matter is substantially more than the total amount of ordinary matter, which implies that there must be dark matter.
Ah I see, thank you.