Scientists have mapped the distribution of galaxies and suspect that particles called "axions" make up dark matter.
  •  Lvxferre   ( @lvxferre@lemmy.ml ) English
    37•1 year ago

    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.

    •  Vincent-Van-Vega   ( @Vincent-Van-Vega@kbin.social ) 
      9•1 year ago

      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?

      •  Lvxferre   ( @lvxferre@lemmy.ml ) 
        7•1 year ago

        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.)

    •  hikaru755   ( @hikaru755@feddit.de ) English
      4•1 year ago

      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

      •  krzyz   ( @krzyz@szmer.info ) English
        8•1 year ago

        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.

        •  hikaru755   ( @hikaru755@feddit.de ) English
          2•1 year ago

          Thanks for the reply! I’m not sure I fully got that, though. So it seems to be that it’s not actually about position (the absolute coordinates), but about the velocity of the particle? So, you could just always use a coordinate system that has the particle at its origin so that its position doesn’t need to change, and just invert the vector of its velocity to get the same result?

          Edit: Went over the Wikipedia article, I think that cleared it up a bit - it’s not actually about a single particle being inverted in an otherwise unchanged system, but the whole system that you’re observing being inverted, is that correct? In that case, it would actually not matter what point is chosen as the origin, as the relative positions of everything would work out to be the same no matter the origin of the inversion. That makes a bit more sense then.

    •  KNova   ( @knova@links.dartboard.social ) English
      2•1 year ago

      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)

      •  tate   ( @tate@lemmy.sdf.org ) 
        3•1 year ago

        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