5 ms·

My guess is that you are confusing "antimatter" and "dark matter". If you want some antimmater, you can go to your nearby physics suply store and buy some radi

My guess is that you are confusing "antimatter" and "dark matter". If you want some antimmater, you can go to your nearby physics suply store and buy some radioactive material that produce positrons. It's quite easy. (Radioactive material may be dangerous. Don't fool with that!) If you want antiprotons or antihydrogen, you need a huge particle acelerator. They make plenty of antiprotons in the CERN, to make colisions. They are very difficult to store, so they survive a very short time on Earth. Dark matter is very different. We have some experimental resuls that don't match the current physics theories. The current best guess is that there is some matter that we can't see for some reason. Nobody is sure what it is. Perhaps it's made of very dark big objects or perhaps it's made of tiny particles that don't interact with light. (I'm not sure the current favorite version in the area.) Anyway, some people don't like "dark matter" and prefer to change the theories, but the proposed new theories also don't match the experimental results.

orwin 2y ago
> Anyway, some people don't like "dark matter" and prefer to change the theories It seems a bit more complicated than that, mostly because the vulgarization often available too is quite bad to explain the issue. My understanding: - Our current theories fail to predict/match an array of observations, as if more matter than what we can detect exist. Some scientists called that the "dark matter problem", that's what most physicists working on the subject refer to when they talk about "dark matter". - Every theory you talked about: dark matter big objects, dark matter particles AND the "change the theories" (i guess you talk about the Modified Newtonian dynamics, where you alter Newton's second law at low speed to match some observations) are dark matter theories: theories that tries to explain why the universe act as it is, not matching our current theories, either by adding new things, or by modifying our discovered laws to match our observations. Each of those theories have multiple branch investigated. - the "dark matter particle theory" is sometime vulgarized as "dark matter" on podcasts or in books/articles. This is because more scientists work on particle physics than on gravity or astrophysics (my country present like 3 astrophysics thesis each year, and dozens of particle physics thesis). I think this caused a huge misunderstanding. - Some people with a common understanding (like mine, i meant non-physicists, it's absolutely not derogatory) like MOND because philosophically it is quite nice, and also tend to draw in people with minority/anti-etablishment habitus[0] (cf: most physicists working on those subjects are particle physicists). I'm not saying this theory is worse than the others at all, i'm just saying that the kind of layperson drawn to it can be _really_ sure they're right and profess their beliefs everywhere, and sometime claim that "MOND isn't dark matter", when they really confuse dark matter as a problem to be solved with "dark matter particle theory". Misunderstanding happen to everyone btw, it's really not a big issue. In case you did not talk about MOND but about theories that claim that the issue are with our tools to observe at a distance, some theories include that to explain some of the inconsistencies, never all of them, and those theories seems to really be a minority atm, so hopefull it wasn't about that. [0] Also, those habitus seems to draw in grifters who know they can make quick bucks by selling books/conferences if they look convincing enough, which is why MOND has a weird reputation now, but absolutely serious physicists and mathematicians work on the subject very, very seriously.
- kuschku 2y ago
  > like MOND because philosophically it is quite nice, and also tend to draw in people with minority/anti-etablishment habitus MOND is a non-relativistic theory. It's not even able to explain the orbit of mercury, gravitational lensing or black holes. It's the equivalent of hot gluing jet engines to a roman quadriga, it won't fly.
  - ars 2y ago
    > MOND is a non-relativistic theory. Well obviously, it's literally in the name. A relativistic version of the theory is: https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...
  - GuB-42 2y ago
    Not really a problem, relativistic effects are negligible at the very low accelerations MOND is about. Should MOND work with galaxies, a theory that say "if the force is less than a threshold, use MOND, otherwise, use general relativity" is not pretty, but good enough to match the observations, and that's the important part. There are more elegant ways of doing it (ex: TeVeS), but before looking at the best way to reconcile general relativity and MOND, something we know is possible, it is important to make sure that MOND works at the scales it is supposed to work with. Currently, it doesn't, but dark matter doesn't either. More research is needed, as they say.
    - kuschku 2y ago
      Disclaimer: Personal Opinion: Personally, I don't think MOND will be the solution. But I don't think it's going to be dark matter either. 1. We don't have a a way to unify relativity and quantum mechanics yet 2. Dark matter and MOND effects show up at extremely low accelerations. I have a hunch it's going to need a theory of quantum gravity to properly solve this. A century ago, quantum mechanics was initially formalized to explain the uktraviolet catastrophe of blackbody emissions. The difference between the classical emissions curve and the quantum based emissions curbe is very similar to the correction factor applied by MOND. I don't think that's a coincidence.
pfdietz 2y ago
One theory of dark matter is that it's strange quark antimatter. An asymmetry in the behavior of quarks and antiquarks in the very early universe would have led to antiquarks being preferentially squirreled away in tiny ultradense nuggets of quark matter. While explaining dark matter, this would also explain the observed matter-antimatter asymmetry. Or, rather, it would explain it by saying matter and antimatter are present in equal amounts, but the antimatter is in a different form. If this theory were true, tiny nuggets of this antimatter would be passing through the solar system all the time. Perhaps a future society could detect them and somehow trap some for use as an energy source.
- lazide 2y ago
  For that to be true at the scale required to ‘balance the scales’, we’d have constant annihilation events - which would be impossible to miss.
  - pfdietz 2y ago
    Not true: the surface area of the nuggets could be so small they'd fall below detection threshold.
    - lazide 2y ago
      Any interaction with normal matter (including solar wind or the interstellar medium) would result in extremely energetic annihilation events. these aren’t subtle.
      - pfdietz 2y ago
        And if the nuggets have sufficiently small surface area then the rate of these events would be low enough to be obscured by the background from other processes. It would not be like the annihilation from antihydrogen atoms hitting hydrogen atoms. The density of nuclear matter is some 15 orders of magnitude higher than ordinary matter, so nuggets 1 angstrom in diameter could have 10^15 times the mass for a given upper bound on the interaction rate with ambient gas.
ben_w 2y ago
> The current best guess is that there is some matter that we can't see for some reason. Nobody is sure what it is. My pet conjecture (it's not detailed enough to be a hypothesis) is that this is related to the baryon asymmetry problem. The antimatter symmetry problem is more than just baryons, despite the name, as we also have more electrons than positrons, not just more protons/neutrons than anti-protons/anti-neutrons. There's a few possibilities: 1) the initial value just wasn't zero (an idea I heard from Sabine Hossenfelder) 2) the baryon number is violated in a process that requires conservation of charge This would suggest antiprotons or antineutrons do something which involves the positron at the same time, so perhaps the anti-neutron is weirdly stable or something — neutron decay is a weak force process, and that can slightly violate the charge conjugation parity symmetry, so this isn't a completely arbitrary conjecture. If we've got lots of (for example) surprise-stable anti-neutrons all over the place… it's probably not a perfect solution to the missing mass, but it's the right kind of magnitude to be something interesting to look at more closely. 3) the baryon number (proton/neutron/etc.) and/or lepton number (electron/positron/muon/etc.) is violated in a process that does not require conservation of charge. If you have some combination of processes which don't each conserve charge, you're likely to get some net charge to the universe (unless the antiproton process just happens to occur at the same rate as the positron process); in quantum mechanics I understand such a thing is genuinely meaningless, while in GR this would contribute to the stress energy tensor in a way that looks kinda like dark energy. But like I said, conjecture. I'm not skilled enough to turn this into a measurable hypothesis.
- westurner 2y ago
  What about virtual particles, too? Virtual particles: https://en.wikipedia.org/wiki/Virtual_particle https://en.wikipedia.org/wiki/Virtual_particle : > As a consequence of quantum mechanical uncertainty, any object or process that exists for a limited time or in a limited volume cannot have a precisely defined energy or momentum. For this reason, virtual particles – which exist only temporarily as they are exchanged between ordinary particles – do not typically obey the mass-shell relation; the longer a virtual particle exists, the more the energy and momentum approach the mass-shell relation.
  - 2y ago
ConceptJunkie 2y ago
> If you want some antimmater, you can go to your nearby physics suply store and buy some radioactive material that produce positrons Or buy some bananas. You'll get a positron every once in a while from the occasional Potassium-40 decay.
- gus_massa 2y ago
  I had to look up. Most of the radioctive potassium 40 in bananas decay by electron emmision. A tiny part by positron emision 0.001%. So you will get a lot of noise in the radioacivity detector. A few sources like https://alpha.web.cern.ch/science/positron-source https://alpha.web.cern.ch/science/positron-source and https://ifj.edu.pl/private/jdryzek/page_r12.html https://ifj.edu.pl/private/jdryzek/page_r12.html recomend sodium 22.

orwin 2y ago

> Anyway, some people don't like "dark matter" and prefer to change the theories It seems a bit more complicated than that, mostly because the vulgarization often available too is quite bad to explain the issue. My understanding: - Our current theories fail to predict/match an array of observations, as if more matter than what we can detect exist. Some scientists called that the "dark matter problem", that's what most physicists working on the subject refer to when they talk about "dark matter". - Every theory you talked about: dark matter big objects, dark matter particles AND the "change the theories" (i guess you talk about the Modified Newtonian dynamics, where you alter Newton's second law at low speed to match some observations) are dark matter theories: theories that tries to explain why the universe act as it is, not matching our current theories, either by adding new things, or by modifying our discovered laws to match our observations. Each of those theories have multiple branch investigated. - the "dark matter particle theory" is sometime vulgarized as "dark matter" on podcasts or in books/articles. This is because more scientists work on particle physics than on gravity or astrophysics (my country present like 3 astrophysics thesis each year, and dozens of particle physics thesis). I think this caused a huge misunderstanding. - Some people with a common understanding (like mine, i meant non-physicists, it's absolutely not derogatory) like MOND because philosophically it is quite nice, and also tend to draw in people with minority/anti-etablishment habitus[0] (cf: most physicists working on those subjects are particle physicists). I'm not saying this theory is worse than the others at all, i'm just saying that the kind of layperson drawn to it can be _really_ sure they're right and profess their beliefs everywhere, and sometime claim that "MOND isn't dark matter", when they really confuse dark matter as a problem to be solved with "dark matter particle theory". Misunderstanding happen to everyone btw, it's really not a big issue. In case you did not talk about MOND but about theories that claim that the issue are with our tools to observe at a distance, some theories include that to explain some of the inconsistencies, never all of them, and those theories seems to really be a minority atm, so hopefull it wasn't about that. [0] Also, those habitus seems to draw in grifters who know they can make quick bucks by selling books/conferences if they look convincing enough, which is why MOND has a weird reputation now, but absolutely serious physicists and mathematicians work on the subject very, very seriously.

kuschku 2y ago
> like MOND because philosophically it is quite nice, and also tend to draw in people with minority/anti-etablishment habitus MOND is a non-relativistic theory. It's not even able to explain the orbit of mercury, gravitational lensing or black holes. It's the equivalent of hot gluing jet engines to a roman quadriga, it won't fly.
- ars 2y ago
  > MOND is a non-relativistic theory. Well obviously, it's literally in the name. A relativistic version of the theory is: https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...
- GuB-42 2y ago
  Not really a problem, relativistic effects are negligible at the very low accelerations MOND is about. Should MOND work with galaxies, a theory that say "if the force is less than a threshold, use MOND, otherwise, use general relativity" is not pretty, but good enough to match the observations, and that's the important part. There are more elegant ways of doing it (ex: TeVeS), but before looking at the best way to reconcile general relativity and MOND, something we know is possible, it is important to make sure that MOND works at the scales it is supposed to work with. Currently, it doesn't, but dark matter doesn't either. More research is needed, as they say.
  - kuschku 2y ago
    Disclaimer: Personal Opinion: Personally, I don't think MOND will be the solution. But I don't think it's going to be dark matter either. 1. We don't have a a way to unify relativity and quantum mechanics yet 2. Dark matter and MOND effects show up at extremely low accelerations. I have a hunch it's going to need a theory of quantum gravity to properly solve this. A century ago, quantum mechanics was initially formalized to explain the uktraviolet catastrophe of blackbody emissions. The difference between the classical emissions curve and the quantum based emissions curbe is very similar to the correction factor applied by MOND. I don't think that's a coincidence.

pfdietz 2y ago

One theory of dark matter is that it's strange quark antimatter. An asymmetry in the behavior of quarks and antiquarks in the very early universe would have led to antiquarks being preferentially squirreled away in tiny ultradense nuggets of quark matter. While explaining dark matter, this would also explain the observed matter-antimatter asymmetry. Or, rather, it would explain it by saying matter and antimatter are present in equal amounts, but the antimatter is in a different form. If this theory were true, tiny nuggets of this antimatter would be passing through the solar system all the time. Perhaps a future society could detect them and somehow trap some for use as an energy source.

lazide 2y ago
For that to be true at the scale required to ‘balance the scales’, we’d have constant annihilation events - which would be impossible to miss.
- pfdietz 2y ago
  Not true: the surface area of the nuggets could be so small they'd fall below detection threshold.
  - lazide 2y ago
    Any interaction with normal matter (including solar wind or the interstellar medium) would result in extremely energetic annihilation events. these aren’t subtle.
    - pfdietz 2y ago
      And if the nuggets have sufficiently small surface area then the rate of these events would be low enough to be obscured by the background from other processes. It would not be like the annihilation from antihydrogen atoms hitting hydrogen atoms. The density of nuclear matter is some 15 orders of magnitude higher than ordinary matter, so nuggets 1 angstrom in diameter could have 10^15 times the mass for a given upper bound on the interaction rate with ambient gas.

ben_w 2y ago

> The current best guess is that there is some matter that we can't see for some reason. Nobody is sure what it is. My pet conjecture (it's not detailed enough to be a hypothesis) is that this is related to the baryon asymmetry problem. The antimatter symmetry problem is more than just baryons, despite the name, as we also have more electrons than positrons, not just more protons/neutrons than anti-protons/anti-neutrons. There's a few possibilities: 1) the initial value just wasn't zero (an idea I heard from Sabine Hossenfelder) 2) the baryon number is violated in a process that requires conservation of charge This would suggest antiprotons or antineutrons do something which involves the positron at the same time, so perhaps the anti-neutron is weirdly stable or something — neutron decay is a weak force process, and that can slightly violate the charge conjugation parity symmetry, so this isn't a completely arbitrary conjecture. If we've got lots of (for example) surprise-stable anti-neutrons all over the place… it's probably not a perfect solution to the missing mass, but it's the right kind of magnitude to be something interesting to look at more closely. 3) the baryon number (proton/neutron/etc.) and/or lepton number (electron/positron/muon/etc.) is violated in a process that does not require conservation of charge. If you have some combination of processes which don't each conserve charge, you're likely to get some net charge to the universe (unless the antiproton process just happens to occur at the same rate as the positron process); in quantum mechanics I understand such a thing is genuinely meaningless, while in GR this would contribute to the stress energy tensor in a way that looks kinda like dark energy. But like I said, conjecture. I'm not skilled enough to turn this into a measurable hypothesis.

westurner 2y ago
What about virtual particles, too? Virtual particles: https://en.wikipedia.org/wiki/Virtual_particle https://en.wikipedia.org/wiki/Virtual_particle : > As a consequence of quantum mechanical uncertainty, any object or process that exists for a limited time or in a limited volume cannot have a precisely defined energy or momentum. For this reason, virtual particles – which exist only temporarily as they are exchanged between ordinary particles – do not typically obey the mass-shell relation; the longer a virtual particle exists, the more the energy and momentum approach the mass-shell relation.
- 2y ago

ConceptJunkie 2y ago

> If you want some antimmater, you can go to your nearby physics suply store and buy some radioactive material that produce positrons Or buy some bananas. You'll get a positron every once in a while from the occasional Potassium-40 decay.

gus_massa 2y ago
I had to look up. Most of the radioctive potassium 40 in bananas decay by electron emmision. A tiny part by positron emision 0.001%. So you will get a lot of noise in the radioacivity detector. A few sources like https://alpha.web.cern.ch/science/positron-source https://alpha.web.cern.ch/science/positron-source and https://ifj.edu.pl/private/jdryzek/page_r12.html https://ifj.edu.pl/private/jdryzek/page_r12.html recomend sodium 22.