- cross-posted to:
- astronomy@mander.xyz
- cross-posted to:
- astronomy@mander.xyz
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Some context on axions.
For decades scientists noticed that quantum phenomena respect something called CP-symmetry, that is a really posh way to say “if you sub your particle with an antiparticle and invert its spatial coordinates, the laws of physics will stay the same”. So for example it doesn’t really matter if you have an electron at (1,1,1) or a positron at (-1,-1,-1), the effect is the same.
There are some exceptions (called CP violations) but for most part CP-symmetry is a real phenomenon. And one of those violations that could happen is when you’re dealing with quarks (the junk that protons and neutrons are composed of). And if something can happen, in quantum mechanics, it will happen. So why don’t CP violations happen?
Based on that, in 1977 Roberto Peccei and Helen Quinn predicted a new particle, that they named “axion” after some laundry detergent, as “it washed off the problem” (yup - scientists being silly with names, nothing new). That axion particle would be a really small particle, but with mass (unlike photons), and it compensates the predicted CP violation.
Now, to the link: apparently axions are a real thing, and they explain (at least partially) what dark matter is supposed to be.
Thank you for the thoughtful response, as I was genuinely interested to know what axion was.
As a side note, I remember reading about CP violations a while back (scientists love to use the metaphor of “how to distinguish left from right” to explaining those) and they were saying that CPT symmetry was inviolable, but they did also thought the same about Charge ©, Parity (P) and Time (T) before. Do you think we will ever see a CPT violation?
Do you think we will ever see a CPT violation?
I don’t think that we will, but people are still looking for it, albeit indirectly, and focusing on special relativity. Stuff like “are the photons still obeying Lorenz symmetry here? What about there? And there?”. (Note: Lorenz symmetry violations would imply CPT violations.)
We don’t know if axions are a real thing. This is still highly speculative.
That’s why I said “apparently” - there’s still some reasonable doubt on their actual existence. But the fact that they fit like a glove in situations that they were not intended for (dark matter) gives them far more credibility IMO.
Can you expand on what “invert its spatial coordinates” means? From your example it seems you’re just flipping around the origin of your coordinate system, but since there is no fixed, “natural” reference frame that would provide a “true” origin, isn’t that origin completely arbitrary and the math should then work out with any origin you use? I feel like I’m missing something here
Not OP, but: it works similarly to looking at the system in a mirror. The clock’s hands turn, well, clockwise, but if you look at the mirror their movement is anticlockwise. Importantly, if you look at that mirror in another mirror, it will be clockwise again. Add yet another mirror and it’s anticlockwise.
With a single mirror at position x=0 (and YZ plane), you invert “x” position, so (1, 1, 1) becomes (-1, 1, 1). “Inverting” the spatial coordinates ((x,y,z) -> (-x, -y, -z)) is effectively the same as looking at system through 3 mirrors, located at x = 0 (YZ plane), y = 0 (XZ plane) and z = 0 (XY plane), but that is a bit hard to visualize/arrange in practice so usually you would think of it as an equivalent operation of a point reflection around (0, 0, 0). You are right that the point is arbitrary: the important thing is, among others, that clockwise movement becomes anticlockwise.
As the other poster said, it’s like a mirror. If you mirror the system, and swap the particle with its antiparticle counterpart, you’ll get the same effect on the point that you decided “this is my origin”. Like this:
Here the origin is the tip of the pencil. It’s completely arbitrary, but it should still hold true in most cases. The exceptions are the CP-violations.
Thanks for the explanation. Very interesting. Is CP-symmetry the same thing as “time based symmetry”? (Not sure if that’s what it is called. I’ve heard there is some symmetry that means certain physical phenomenon behave the same even if time runs backwards, or something. I’m probably not doing a great job explaining)
No, time-based symmetry is the T-symmetry. It’s violated all the time because some events only run one way, never to the other; for example, if you burn sugar into carbon dioxide and water, you can’t simply “unburn” it by gathering the carbon dioxide and water together.
Then there’s the CPT-symmetry, that says that those violations of the T-symmetry should have a correspondent CP-symmetry violation, cancelling each other out exactly.
Quantum mechanics explains these ultra-light particles as “fuzzy” because they exhibit wave-like behavior
How’s that different from any other particle? Aren’t they all waves in quantum fields?
I don’t know if this helps but starting at roughly 39:00 of this Fermilabs lecture the lecturer mentions that lighter the dark matter particle is more wave like it behaves and so you have to imagine like a “fluid” of waves that can be macroscopic.
Ok, I just gotta say that watching this type of talk here, puts my mind at rest that the Fediverse is going to be alright. That we are in the right place.
We got the smart people from Reddit
We are the smart people from Reddit. Even if we don’t know everything, we know which way the wind is blowing.
Speak for yourselves, I’m an idiot
You still successfully figured out Lemmy which some of friends are either incapable of or unmotivated enough to do
Checking in.
Thanks for linking that, i really enjoyed watching all of it!
It isn’t different and you are rightfully confused about the way they phrased that.
I like how scientists couldn’t fathom that their math was wrong so they made up a story to cover for it and now they are in way over their heads.
Echoes of Einstein and his Cosmological Constant, aka Lambda. “The biggest blunder of my life” he called it… then it turned out to be correct, although it’s now known as the Hubble Constant.
The cosmological constant and Hubble constant are related, but not the same thing (I’m guessing that’s what you meant). Einstein felt there had to be something preventing the universe from collapsing. When he learned that the universe was expanding, he realized that it could just be momentum preventing the collapse, so lambda was a “mistake”.
It was possible that the universe could be expanding, but with no dark energy. Then it would have a Hubble constant but no lambda
So long as it’s not resource harvesting robots, ever expanding outward from their host planet…
