RE: O Dark Matter, Where Art Thou?
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COVID in the house? Oh no. You just slipped that in. I hope everyone is OK.
You and I are apparently fans of the same movies. Previously you referred to a Hitchhiker's Guide to the Galaxy. This week it's O Brother Where Art Thou. If you tell me you also are a fan of the The Blues Brothers (original, probably before your time), that's the trifecta of movie fandom.
Of all your blogs this one for me was the most completely and easily (there was nothing easy about it😇) understood. Knowledge is cumulative and your writing is great (and funny).
I believe I understand the basics, almost intuitively. There must be dark matter because otherwise the standard model doesn't work and the standard model has to work or the universe as we know it doesn't make sense.
I have a couple of question that arise not so much about your search for the elusive dark matter. One is about Newtonian physics. I thought Einstein's Theory of Relatively disproved Newtonian principles. And yet you say here that
In order to provide an explanation without enforcing Newton’s theory of gravitation to be incorrect, we need to assume that some invisible matter is present in our universe.
The second question has to do with black holes: I always associated dark matter (perhaps incorrectly) with black holes. Black holes (in the popular consciousness) are supposed to consume energy. However, in your blog you explain that
Two dark matter particles can annihilate into two Standard Model particles. That gives rise to cosmic ray and gamma ray signals that can be observed in satellite experiments orbiting around Earth.
So from the dark matter we can get energy? (Cosmic and gamma rays)
Now I recognize that I may be mixing apples and oranges. But, I'm at the stage of learning where I am associating ideas. In other words, your blog and your information is beginning to have real-world context for me, even if I'm on the wrong track 😆.
This is a great blog, @lemouth, and the summaries really do help.
Hope everyone in your house gets well soon, if they are not well now. Take care of yourself.
Regards,
Thanks for passing by my weekly post ;)
The little one brought it back from school. As he is tested every other day (that's the burden on the shoulders of the youngsters here, the virus spreading so much at primary schools), we discovered on Friday morning that he was positive. We were all shocked as he had no symptom at all. On Friday night, one good third of his class was positive, and half of it at the end of the week-end...
We thus isolated ourselves and implemented some social distancing at home. Except that the little one was living in a constant excited state (which is very exhausting for the parents), everyone was fine and without any symptom. Yesterday (Tuesday) evening, all our self-tests were negative. Today we will go for antigen tests that are more solid, and hopefully go back to a normal life from tomorrow onward.
In fact, it turns out that I really like to use references to movies in my titles. I have selected the topic of my next blog, but unfortunately the only movie I can think about are not so great (the invisible man, chameleon, etc.). I however still have one week to come up with a good idea.
Maybe here, I would rephrase as [...] and the Standard Model has to be extended for the universe as we see it to make sense. This is however debatable as there are still viable ways to live without dark matter (even if not so appealing).
I would phrase it differently. Newtonian gravity works well, but only within its domain of application. When we enter the area of very massive objects like stars or galaxies and the physics close to them, then we cannot use Newtonian gravity as such but need to rely on its relativistic generalisation. The same holds for special relativity when objects move at high speeds.
This being said, for the rotational motion of stars in galaxies, Kepler's laws (or Newtonian gravity) work well and there is no need to use any relativistic generalisation. The reason is that the objects considered lie sufficiently far from each other.
I guess this answers the first question.
You may be interested in knowing that there was a reference to black holes in one of the very first versions of this post (the one that was more than 4,000 words and that I had to butcher a bit... ;) ). Usually, black holes and dark matter are different objects, although we have setups in which dark matter can be made of (primordial) black holes (see here for instance). Such a construction is however not a very common one.
In the present blog, I focus on scenarios in which dark matter has a particle physics explanation (i.e. it is not a black hole). The rest follows from this assumption (and the second one in which we assume dark matter to be connected to the Standard Model). If those assumptions do not hold, the signals of dark matter should be searched for differently.
Yes. In microscopic physics, we have three forms of energy: mass energy, kinetic energy and potential energy. Only the total amount of energy is conserved, so that we can convert one form of energy onto another without any problem. This is what is happening for cosmic/gamma ray production from dark matter annihilation.
I hope all of this clarifies (the comment starts to be as long as the post :D ). Thanks again for passing by and the very good questions!
I love the way you describe the little one as being in a 'constant excited state'. It might be challenging to deal with but it sounds very sweet.
As for your response: another lesson in itself. My basic physics literacy has grown so much since I started reading your blogs. Learning about the basic building blocks of the universe, of matter, of physical existence--it's exciting.
I can read news reports now about physics and they make more sense to me. I'm not inclined to skip over them anymore :) Thank you!
Thanks to you for being one of my most constant reader (and commenter) :)