The so-far unsuccessful search to find direct evidence for the existence of dark matter is raising an issue in science that is often misunderstood and rarely gets the attention it deserves. And that issue is how we know in science that something does not exist. I discuss that in some detail in my book The Great Paradox of Science (yes, yet another plug for those who have not read it to buy it!) because it is hard to understand the logic of scientific progress without it. The history of science is replete with things that were once thought to exist but are no longer so. The aether and phlogiston are two famous example and another is N-rays. Trying to understand why we think those entities no longer exist will enable us to better understand when it might happen that dark matter is also thought to not exist.
It has never been proven that aether and phlogiston do not exist because such proofs of nonexistence are impossible. Instead, a scientific consensus arises that decides that things do not exist when two conditions are met. One is that no corroborated direct affirmative evidence is found for its existence and the other is that the item ceases to be necessary as an explanatory concept because an alternative explanation for the phenomena it purported to explain is found. This process can take a long time.
The search for dark matter is currently at the first stage of a failure to detect a direct signal, despite the current belief that it constitutes about 85% of all the matter in the universe. Despite major efforts at building larger and larger detectors to find a signal, it is proving elusive.
How can something be so abundant and yet so hard to see? Part of the problem is that we do not know what dark matter consists of. The detectors of dark matter are based on theories of what it might be but if we are wrong, we will not detect it however sensitive or large we make the detectors. It would be like trying to detect radio waves with a detector sensitive to just visible light, like our eyes. Sensitivity does not matter if you are looking for the wrong thing.
Current searches have focused on things called ‘wimps’ which stand for ‘weakly interacting massive particles’.
These efforts have involved building detectors deep underground where they are shielded from subatomic particles – triggered by cosmic rays hitting the upper atmosphere that constantly shower down on Earth and which would trigger streams of false positive readings on their instruments.
“The expectation has been that a wimp will strike a xenon nucleus and the resulting flash of light will be spotted by a detector and so reveal the presence of a dark matter wimp,” said Ghag. “Despite years of effort, we have yet to see a single flash like that, however. We need greater sensitivity.”
Now researchers are pinning their hopes on the two most sensitive wimp-hunters ever designed. One, built below Italy’s Gran Sasso mountains, is known as XENONnT. The other, Lux-Zeplin, has been constructed in an old South Dakota gold mine. Both devices have been filled with several tonnes of xenon – much more than has been put in any previous device – and that should increase chances of a nucleus being struck by a wimp.
Ghag, a member of the Lux-Zeplin team, said: “Both devices are now being put through operational tests, and in a few months those trials will be completed. We may find we have detected dark matter over that period – which would be very good news. If not, both devices will be run without interruption for several years. Essentially, the more xenon we have in our machines and the longer we run our detectors, the better our prospects of collisions occurring and dark matter revealing its presence.”
However, it is now accepted there is a prospect that this will not happen and dark matter could remain elusive. As Mariangela Lisanti, a physicist at Princeton University in New Jersey, stated in the journal Science recently: “The wimp hypothesis will face its real reckoning after these next-generation detectors run.”
If Lux-Zeplin and XENONnT fail to find Wimps, the two teams of scientists will have one final chance to use current technology to find them – by joining forces to create one final super-large detector that would contain tens of tonnes of xenon, a rare and expensive gas to isolate, and which would be run for several years.
If the next generation of searches prove to also be a bust, there are two options: one is to abandon wimps as the candidate particle and postulate a different one. Some are already exploring that option. That would require the building of different types of detectors, and would be long and expensive. The cheaper alternative would be to abandon the dark matter hypothesis and use another explanation for the anomalous behavior of galaxies for which dark matter was created as an explanatory concept. Modified Newtonian Dynamics (or MOND) is one such candidate. If such an alternative theory gains ground and makes predictions that are sustained, especially surprising ones, then we may reach the stage where we can decide that dark matter does not exist. And that is how it may join the aether, phlogiston, and N-rays as things that were once thought to exist but are no longer.
But getting there is a long and arduous process. At this stage, the alternative theories have not as yet reached that stage.
We also need dark matter to explain the gravitational lensing of the bullet cluster. MOND won’t do it. You would need a theory of gravity where the gravity comes from where the matter would have been if the matter as collision less even though it’s not, and that seems to me to be rather perverse. MOND is not going to replace dark matter.
If the existence of something would give some predicted measurement that we then do not measure, one could say it does not exist.
There is no supermassive black hole orbiting the earth closer than the moon. It does not exist, because if it did we would have noticed its effects.
GOTS @1: For the bullet cluster, the situation is more complicated than that. Here is Sabine Hossenfelder.
A more recent video she made, which includes problems with dark matter, and the radical notion that DM versus MOND might be a false dichotomy.
Ok. I stand corrected. Looks like there’s disagreement between experts. Thanks Mano.
Rob, I know you’re an expert. I’d love for you to weigh in with your expertise, because this is way beyond my level. I can just parrot arguments with a minimal amount of understanding.
http://backreaction.blogspot.com/2017/01/the-bullet-cluster-as-evidence-against.html
I’d like to offer a response from one of the comments from her site which is a response to her attempt to explain how modified gravity might cause gravity to originate from somewhere other than where the traditional matter is (and without creating new particles).
My comment is that this isn’t really a modification of gravity, but rather it’s another kind of “dark matter”. A very unusual kind of particle-less dark matter. It wouldn’t be a modification of Einstein’s field equations. It would simply be the addition of additional (quantum) fields whose energy that would warp spacetime according to Einstein’s field equations.
So, I don’t see the appeal of this model in terms of elegance, simplicity, or parsimony. I mean, it might be right, but I don’t see how one can really call this a compelling alternative to dark matter, and further I don’t see how one can really say that this isn’t just another form of dark matter.
I recall Sean Carroll making a similar argument, albeit IIRC without finessing the difference between particle quantized dark matter field vs particle-less dark matter field.
GOTS @5: I’m not an expert in this field. But I do have a handle on the basic arguments.
I’ll keep this short, because I’m tired, but I’ll probably come back tomorrow to elaborate.
It comes down to this; sometimes a field description works better than a particle description. MOND works better at the scale of galaxies. DM works better at cosmological scales. At the scale of galaxy clusters, both have problems. The appeal of a model lies in the domain within which it works.
Fields and particles are not mutually exclusive, but sometimes a field description works better. For example, a particle of mass m and charge q and velocity v in a magnetic field B experiences a force F and acceleration a given by
F = ma = qv x B
where ‘x’ is the vector cross product. You could try to express this in terms of the excitations of the quantized field (i.e. photons), but that would be rather unwieldy to say the least.
Rob,
No problem, and thank you very much kind sir.
GOTS @5: I think you and the commenter you quoted are labouring under a misapprehension. Adding one or more fields that couple to gravity is not equivalent to just adding another source of matter/energy. It’s understandable that you might think so, since you’ve no doubt heard of electron fields, and quark fields, etc. But…
First off, quantum physics plays no role here; it’s all classical (as in non-quantum) physics. Classical fields don’t correspond to particles, although they do interact with matter, or charge, etc. The extra fields in modified gravity theories change the structure of spacetime, such that the non-relativistic low acceleration limit is MOND. There is no extra matter or energy. If you could show that they are equivalent to adding matter/energy in some way, you could collect your Nobel Prize.
So, no, MOND is not about another kind of ‘dark matter’. There is either (in principle detectable) matter, or there isn’t.
Thanks!
This is the post that is a transcript to the second video:
https://backreaction.blogspot.com/2021/05/dark-matter-situation-has-changed.html
(I have no comments; I just like having a text record)
(I note that there are 159 comments to the post, FWIW)
“…we may reach the stage where we can decide that dark matter does not exist.”
“We decide” is an ideological act, a political act.
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“We conclude” follows scientific method, mathematical method.
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Theoretical scientists -- unfortunately -- pursue ideology more than science.