Dark matter: Science Friday with Weinberg, Hooper, Cooley

The background is temporarily "nearly white" today because I celebrate the Kilowatt Hour, also known as the Electricity Thanksgiving Day. Between 8:30 and 9:30 pm local time, turn all your electric appliances on and try to surpass one kilowatt. By this $0.20 sacrifice, you will fight those who want to return us to the Middle Ages and who organize the so-called Earth Hour.

Ira Flatow's Science Friday belongs among the better or best science shows. Yesterday, he hosted some very interesting guests and the topic was interesting, too:

Understanding the Dark Side of Physics

The guests were Steven Weinberg, famous theorist and Nobel prize winner from Austin; Dan Hooper, a top Fermilab phenomenologist; and Judi Cooley, a senior experimental particle physicist from Dallas



And if you have 30 spare minutes, you should click the orange-white "play" button above and listen to this segment.




It doesn't just repeat some well-known old or medium-age things about dark matter. They start the whole conversation by discussing a very new story so that even listeners who are physicists may learn something new.




An observation was announced that imposes new upper limits on the self-interaction of dark matter. If it interacts with itself at all (it of course interacts gravitationally but if there is another contribution to its self-interaction), the strength of this force is smaller than an upper bound that is more constraining than those we knew before.

See e.g.

Hubble and Chandra discover dark matter is not as sticky as once thought
Dark matter does not slow down when colliding with each other, which means that it interacts with itself even less than previously thought.

---

The nongravitational interactions of dark matter in colliding galaxy clusters (by David Harvey+3, Science)

If you remember the "bullet cluster" that showed the existence of dark matter – and its separation from visible matter, they found 72 similar "clusters" and just like the 72 virgins waiting to rape a Muslim terrorist, all of them make the same suggestion: some dark matter is out there. They say that the certainty is now 7.6 sigma when these 72 observations are combined.

However, the dark matter location remains close enough to the associated visible stars which allows them to deduce, at 95% confidence level, that the cross section per unit mass isn't too high:\[

\frac{\sigma_{DM}}{m} \leq 0.47\,{\rm cm}^2 / {\rm g}

\] The dark matter just doesn't seem more excited by itself than it is by the visible matter. Theories with "dark photons" are the first ones that are heavily constrained and many natural ones are killed. But maybe even some more conventional WIMP theories may be punished.

I think that if you have worked on my proposed far-fetched idea of holographic MOND, you have one more reason to increase your activity. And I guess that all axions are just fine with the new finding.

Weinberg clarifies the situation – why dark matter isn't understood too well (it's dark!) etc. – very nicely but many other things are said in the show, too. When the two other guests join, they also discuss other dark matter experiments, dark energy, gravitational waves, string theory etc.

Funnily enough, a layman listener wanted the guests to describe the cataclysms that would occur if the dark matter hit the Earth. The response is, of course, that dark matter hits our bodies all the time and nothing at all happens most of the time. I can't resist to ask: Why would a layperson assume that dark matter must be associated with a "cataclysm"?

People have simply liked to think about cataclysms from the very beginning of primitive religions, and the would-be modern era encourages people to unscientifically attribute cataclysms to many things – carbon dioxide was the most popular "culprit" in the recent decade (and of course, there are many retarded people around us who still believe that CO2 emissions are dangerous). People just can't get interested in something if it is not hyped by a talk about catastrophes.

At one moment, Weinberg (who also promoted his new book about the history of physics, To Explain the World) wisely says that dark matter is preferred because it's also supported by some precision measurements of the CMB – and because it's much easier and more conservative to introduce a new particle species than to rewrite the laws of gravity. Flatow is laughing but it is a serious matter. Flatow is a victim of the populist delusion that there are so many particles which must mean that they were introduced because they don't have any natural enemies. But particles are introduced when they are seen or at least glimpsed.

Lots of particles are used by theoretical physicists because they are being seen experimentally every day and even the new particles that are not sharply seen yet are being introduced because they explain some observations or patterns in them – in this sense, the particles are being seen fuzzily or indirectly (at least when the theorist behind them has any quality). And all theories involving new particles compete with other theories involving other new particles so it's no "unrestricted proliferation of new concepts without standards". Instead, it's the business-as-usual science.

The real question is whether a rather conservative theory with new particle species is more likely – and ultimately more true – than some totally new radical theory that denies that physics may be described in terms of particles and fields. Of course that a true paradigm shift may be needed. But the evidence that it is so – or the ability of the existing, radically new frameworks to convince that they are on the right track – isn't strong enough (yet?) which is why it seems OK to assume that the discrepancies may be fixed with some new particle species.

Also, Flatow is laughing when Weinberg calls the visible matter a contamination – because it's significantly smaller than dark matter which is still smaller than dark energy (by the magnitude of the energy density). Most laymen would find this laughable, too, and it's because the anthropocentrism continues to be believed by most laymen:

We are at the center of the Universe and everything we know from the everyday life must play an essential role in the most profound structure of the Universe. But as science has been showing for 500 years or so, this simply ain't so. If I ignore the fact that the Czechs are the ultimate average nation in the world, we the humans are a random update to one of many long branches of the evolution tree that arose from some rather random complex molecules revolving around an element that is not the most fundamental one, and the whole visible matter around us is a contamination and the clump of matter where we live is a mediocre rock orbiting a rank-and-file star in an unspectacular galaxy – and the Universe itself may be (but doesn't have to be!) a rather random and "not special" solution of string theory within the landscape.

Hooper mentions the 1960s and 1970s as the golden era of classical physics – and the recent years were slower.

At the end, Cooley and Weinberg discuss string theory – experimenters can't test it so the theory isn't useful for them but it's right that people work on it, and it has never been the case that all predictions of theories had to (or could) be tested. Weinberg wraps the discussion with some historical examples – especially one involving Newton – proving that the principle that all interesting predictions must be testable in the near future is misguided.

The short discussion on sciencefriday.com is full of crackpots irritated by the very concept of "dark matter" and the research of dark matter.



Off-topic: One of the good 2015 Czech songs, "[I Am Not a] Robotic Kid" (lyrics preaches against parents' planning their kids' lives and against conformism). Well, I should say "Czech-Japanese songs" because the leader of Mirai, the band, is Mirai Navrátil – as the name shows, a textbook example of a Czech-Japanese hybrid. He actually plans to sing in Japanese as well. It's their first song.