Dark Matter, on the surface, appears to be a wild idea... Some invisible thing that we can't see that controls the movement of most of the universe.

Yet, we are searching for it. Why? What evidence is so strong that we believe it exists, rather than modifying our basic equations in physics for the macro-scale?

We spend large amounts of money on the search for Dark Matter and so far haven't found a single thread of evidence it's real (yet). The same is true with string theory, as far as I understand. At what point do researchers in physics make the leap from wild theoretical ideas to physical experiments?

I understand the need to "rule out" ideas, but where is the line drawn?

Dark Matter is an area that I'd like an explanation on personally. From a laymen's perspective, it seems like we're searching for the end of the rainbow.

  • $\begingroup$ I was just writing another answer about the evidence for non-baryonic dark matter. The possibility that gravity just works differently on galaxy-sized length scales is also under consideration (usually called MOND), but the fact is that the dark matter scenario is much better supported by observational evidence than it was twenty years ago, even though no one has yet said "here in this jar is some dark matter." $\endgroup$ – rob Dec 17 '14 at 7:18
  • $\begingroup$ I don't think this can be usefully answered here. The process by which funding committees decide an experimental programme is worth funding does not obey any physical laws that I'm aware of. $\endgroup$ – John Rennie Dec 17 '14 at 7:19
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    $\begingroup$ To be blunt, dark matter only appears wild if you don't know the history of what was carefully considered and ruled out before this hypothesis came to the fore nor what evidence accumulated in favor of the idea while other--less unusual--ideas were considered more plausible. In other words dark matter as a serious candidate hypothesis didn't spring fully formed from anyones forehead, it was reluctantly admitted through the servants entrance when nothing more respectable could be found to do the job. $\endgroup$ – dmckee --- ex-moderator kitten Dec 17 '14 at 8:24

First, it's important to realize that all proposed changes to physics need to be tested, whether they amount to adding new stuff to the universe or modifying equations that have worked fine thus far. Suppose someone comes along and says, "I can explain this supposed dark matter by modifying gravity," and lots of theorists agree. Great. Now observers will go out and make observations that distinguish between the old theory and the new theory. Remember, observational astronomy : theoretical astrophysics :: experimental physics : theoretical physics. If nature didn't push us in the direction of looking for dark matter particles in lab experiments, we'd be spending more money building telescopes or doing other things to test gravity. If there is a finish line past which we cannot think of any science to do, it certainly isn't in sight.

rather than modifying our basic equations in physics for the macro-scale

There are people who think dark matter is just a modification of how gravity works. The primary such theory is MOND. And indeed tests of such theories are proposed. Alternate theories such as MOND have been steadily falling out of favor, however, due to the overwhelming evidence in favor of a new matter component.

We spend large amounts of money on the search for Dark Matter and so far haven't found a single thread of evidence it's real

There is an overwhelming amount of evidence pointing toward some missing matter. The Wikipedia article nicely lists the main observations contributing to this conclusion, going all the way back to the early 1930s:

  • Originally astronomers noticed the motions of galaxies in clusters, assuming the systems were in virial equilibrium, were too fast unless there was unaccounted-for gravitational mass.
  • Around the same time, a similar discovery was made for stars orbiting galaxies.
  • Gravitational lensing measurements confirm these masses, even if you thought the clusters weren't in equilibrium.
  • In one striking case, the gravitational mass is clearly offset from the visible mass, and models that try to adjust the large-scale behavior of gravity cannot simply displace the center of gravity from the center of mass.
  • The large-scale structure of the universe -- as seen in the distribution of galaxies and intergalactic material -- looks as we would expect from having a large amount of non-interacting mass.
  • In fact, the evolution of the universe as a whole only makes sense with a non-interacting matter content, as otherwise the cosmic microwave background and supernova distance measurements would be very different from what we observe.

All this evidence has been checked over and over again, and most agree that it all checks out. In fact, all these different lines of reasoning lead to the same estimate for how much dark matter there is. Also note that scientists didn't just jump on the bandwagon 80 years ago -- it took decades of collecting evidence for the majority to become convinced that there really is some form of dark matter.

At what point do researchers in physics make the leap from wild theoretical ideas to physical experiments?

This is a valid question. The rough answer is when the theory is important enough that verification or falsification is worth the resources required to test it, taking into consideration what else can be accomplished with those same resources. If no one to speak of believes the theory, testing it is a low priority. If everyone is so convinced of the theory they would doubt the experiment before they doubted the theory, then testing is also a low priority. If there are simply more interesting things (or better government-funded things) for experimentalists and observers to do with their time, they probably will spend less time on the theory in question.

It's also worth highlighting that a theory must be testable in order to be tested. The common gripe from experimentalists about string theory is that it does not lend itself to many feasible experiments. Direct detection of dark matter, on the other hand, is not beyond the realm of possibility: There exist masses and cross sections for hypothetical weakly interacting particles such that

  • such particles would explain indirect observations to date,
  • such particles would not have been directly observed with any previous experiments, and
  • such particles would be detected with experiments we can build today.

Such experiments should either verify the whole theory or falsify part of it. Whatever the outcome, they are intended to produce results.


Chris White's answer covers the content of the question . I will reply to the title

At what point do researchers in physics make the leap from wild theoretical ideas to physical experiments?

It is very very seldom that wild theoretical ideas lead to physical experiments. And when they do, as with special relativity, which the Michelson Morley experiment supported, if one looks how the theory appeared, one discovers that it is an outgrowth of classical electromagnetic theory. Theories usually appear as modifications of existing theories because there has been experimental evidence that needed new theoretical explanations, as with the photoelectric effect.

I would put general relativity as the only clear example which displayed thinking by theorists out of the box, and was taken up and studied with the astronomical observations. In my opinion it was the weight of Einstein who already was well known when he proposed it that made it acceptable so easily, and of course that it did not contradict any theories current at the time.

If one looks in the internet wild theoretical ideas are a dime a dozen, but they will never be tested in the way you think, because the tests are not done by the persons proposing the theory but by a different set of experimental physicists or observational astronomers, and then a number of things enter as described by Chris, including whether the experimenters believe there is any sense in the predictions of an out of the box theory.

And two words about string theory. It is an extension with new mathematical tools of the existing theories of particle physics. The theorists working on it do so because it is at the moment the only direction which has quantization of gravity and the possibility to embed the standard model of particle physics. Those are two strong hints of being on the right track. At the moment there is not standard string theoretic model, if it is proposed then I am sure tests will come of various forms. As I am fond of saying, maxwell's equations when they appeared just bound up together disparate laws from electricity and magnetism, but look what developed afterwards. I expect once a string theory model is finalized new phenomena will be predicted to test it.


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