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In case anyone is wondering about what antimatter actually is, antimatter is basically like normal matter, but the electrical charges are reverse. Take hydrogen

by bmh100 10y ago

In case anyone is wondering about what antimatter actually is, antimatter is basically like normal matter, but the electrical charges are reverse. Take hydrogen for example. It is a massive, positive particle (the proton), orbited by a light, negative particle (the electron). Anti-hydrogen is a massive, negative particle (the antiproton) orbited by a light, positive particle (the positron) [1]. Why do matter and antimatter annihilate each other? Matter and antimatter are very similar, except for opposite electrical charges. Opposite charges attract. If antihydrogen meets normal matter, the positrons quickly find and combine with the electrons, and the antiprotons do the same with protons. These combinations all release radiation. Therefore, one of the main challenge with studying antimatter is making sure that it doesn't touch normal matter, which is basically everything in the lab. Why don't electrons and protons normally combine, being oppositely charged? Electron sit in stable orbitals, volumes of space with associated energies. These orbitals are like valleys. While an electron nearing the nucleus will fall into an orbital, it would take extra energy to get the electron out of the orbital and into the nucleus [2]. [1]: https://en.wikipedia.org/wiki/Antihydrogen https://en.wikipedia.org/wiki/Antihydrogen [2]: http://physics.stackexchange.com/questions/30939/what-keeps-electrons-in-an-atom-from-flying-away-or-falling-into-the-nucleus#30943 http://physics.stackexchange.com/questions/30939/what-keeps-...

euyyn 10y ago
"Falling into the nucleus", as stated in that Stack Exchange question, is a classical (as in "wrong" :) ) way to look at it. The 1s orbital of the electron is spherically symmetric, with non-zero probability of the electron being right in the center of the atom (so within the nucleus), and decreasing probability of it being farther away. So e.g. a good approximation of the He atom is of two neutrons, two protons, and two electrons, all in exactly the same position (with the electrons having "more spread"). At the end of the day, there are interactions that are seen in Nature and interactions that aren't. We codify the patterns as conservation of lepton number, baryon number, etc. If the numbers of what you have add up to zero, they can annihilate. These conservation laws also accommodate other weird things, like beta decay, in which a neutron decays ("splitting") into a proton, an electron, and an anti-neutrino. It's not that the neutron is "composed" of those three things. But its total energy is higher than the sum of the energy at rest of those three particles: Whenever that's the case and the conservation laws allow it, we see it happen naturally.
- paulmd 10y ago
  > At the end of the day, there are interactions that are seen in Nature and interactions that aren't. That's provably false. Every reaction occurs in nature, even the vanishingly improbable ones, just at an extremely low reaction coefficient. They're still there, just at a probability of 10^-14 or whatever.
  - pranjalv123 10y ago
    What? There are interactions that can never occur because they're forbidden by the laws of physics - you'll never violate conservation of [energy, baryon number, momentum, etc.] even with probability 10^-14.
    - paulmd 10y ago
      Sure, technically correct is the best kind of correct I suppose. But if the reaction is the least bit reversible (and most are) then the reverse reaction is still proceeding even though the forward reaction is stronger. It's just doing so at an incredibly low rate, and the products are likely to be immediately converted by the forward reaction. You are looking at averages and claiming they hold for every single event in a stochastic simulation. Taking the example of entropy - it's not impossible that entropy decreases in a system, it's just less likely than it increasing. On a stochastic level, entropy decreases all the time, it's just that on average it increases more than it decreases. Reactions work the same way. You're not actually making chemicals react as a singular act, you're creating a forward reaction that occurs more rapidly than the reverse reaction. If we had a hypothetical chemical Maxwell's Demon - you could "bottle" up the tiny bits of those outputs from the reverse reaction before they underwent the forward reaction again. https://en.wikipedia.org/wiki/Maxwell's_demon https://en.wikipedia.org/wiki/Maxwell's_demon
      - euyyn 10y ago
        You're assuming one can expect the knowledge of chemical reactions to apply to particle interactions, and that is not the case. For example particle accelerators can shoot particles against some target one by one, and so there's no need to look at averages. Some interactions never occur, to the best of our measuring ability (i.e. highest energies and number of repetitions). Some others are right away forbidden by laws much stronger than entropy in thermodynamics, in the sense that they're not averages, but mathematical derivations off the symmetries of the universe.

by bmh100 10y ago

euyyn 10y ago

"Falling into the nucleus", as stated in that Stack Exchange question, is a classical (as in "wrong" :) ) way to look at it. The 1s orbital of the electron is spherically symmetric, with non-zero probability of the electron being right in the center of the atom (so within the nucleus), and decreasing probability of it being farther away. So e.g. a good approximation of the He atom is of two neutrons, two protons, and two electrons, all in exactly the same position (with the electrons having "more spread"). At the end of the day, there are interactions that are seen in Nature and interactions that aren't. We codify the patterns as conservation of lepton number, baryon number, etc. If the numbers of what you have add up to zero, they can annihilate. These conservation laws also accommodate other weird things, like beta decay, in which a neutron decays ("splitting") into a proton, an electron, and an anti-neutrino. It's not that the neutron is "composed" of those three things. But its total energy is higher than the sum of the energy at rest of those three particles: Whenever that's the case and the conservation laws allow it, we see it happen naturally.

paulmd 10y ago
> At the end of the day, there are interactions that are seen in Nature and interactions that aren't. That's provably false. Every reaction occurs in nature, even the vanishingly improbable ones, just at an extremely low reaction coefficient. They're still there, just at a probability of 10^-14 or whatever.
- pranjalv123 10y ago
  What? There are interactions that can never occur because they're forbidden by the laws of physics - you'll never violate conservation of [energy, baryon number, momentum, etc.] even with probability 10^-14.
  - paulmd 10y ago
    Sure, technically correct is the best kind of correct I suppose. But if the reaction is the least bit reversible (and most are) then the reverse reaction is still proceeding even though the forward reaction is stronger. It's just doing so at an incredibly low rate, and the products are likely to be immediately converted by the forward reaction. You are looking at averages and claiming they hold for every single event in a stochastic simulation. Taking the example of entropy - it's not impossible that entropy decreases in a system, it's just less likely than it increasing. On a stochastic level, entropy decreases all the time, it's just that on average it increases more than it decreases. Reactions work the same way. You're not actually making chemicals react as a singular act, you're creating a forward reaction that occurs more rapidly than the reverse reaction. If we had a hypothetical chemical Maxwell's Demon - you could "bottle" up the tiny bits of those outputs from the reverse reaction before they underwent the forward reaction again. https://en.wikipedia.org/wiki/Maxwell's_demon https://en.wikipedia.org/wiki/Maxwell's_demon
    - euyyn 10y ago
      You're assuming one can expect the knowledge of chemical reactions to apply to particle interactions, and that is not the case. For example particle accelerators can shoot particles against some target one by one, and so there's no need to look at averages. Some interactions never occur, to the best of our measuring ability (i.e. highest energies and number of repetitions). Some others are right away forbidden by laws much stronger than entropy in thermodynamics, in the sense that they're not averages, but mathematical derivations off the symmetries of the universe.