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B What causes the physicality of matter?

  1. Aug 5, 2016 #1
    I don't know if I've asked the right question, but here's what I want to know:

    If I plunge my hand into a pool of water, which of the fundamental interactions is keeping the atoms in my hand separate from the atoms in the water?

    I always thought it was the strong interaction, but then I learned electrons are leptons and don't have it. I hope you can see what I'm after here.

    Also, what force keeps the electrons in their orbit?
     
  2. jcsd
  3. Aug 5, 2016 #2

    mfb

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    The electromagnetic interaction, sort of. Ultimately it is a purely quantum-mechanical effect: the pauli exclusion principle prevents your electron clouds of atoms from overlapping with electron clouds of atoms in the water.

    The strong interaction is only relevant inside nuclei.

    The electromagnetic interaction.
     
  4. Aug 5, 2016 #3
    Okay, good, that's what I thought. Does the pauli exclusion principle explain why electrons don't crash into the nucleus?

    PS-I'm trying to understand how the elements were formed during the big bang. I understand how the strong interaction formed protons and neutrons, then bound them into nuclei, from the quark-gluon plasma, but what I don't comprehend is how electrons came into existence, or how they ended up orbiting around the nuclei.
     
  5. Aug 5, 2016 #4

    ohwilleke

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    1. Short answer to "how electrons came into existence": Nobody really knows.

    The term for how electrons came into existence is "leptogenesis". And, the real question the plagues leptogensis, and also the question of how quarks came into existence, which is called "baryogenesis" is how there came to be so many more quarks than anti-quarks, and so many more electrons than positrons in the universe, when all process that we observe today produce new quarks only in quark-antiquark pairs, and new leptons only in lepton-antilepton pairs. (Creating new quark-antiquark pairs, and new lepton-antilepton pairs happens all of the time according to more or less perfectly understood processes in the Standard Model such as W and Z boson decay.)

    These processes are also limited by conservation of electric charge, but that is not so problematic, because the net electric charge in the universe is zero or very nearly so.

    But, we don't know why there is this matter-antimatter asymmetry. One way to achieve this is to have a process that creates more matter than antimatter, and then you need all the antimatter to annihilate in collisions with matter, so that what is left over being almost all matter. But, no such process has been identified. It also doesn't help that most of the particles in the universe are neutrinos and we don't know the ratio of neutrinos to antineutrinos in the universe because its really hard to tell the difference when measuring them when you don't know their source.

    A process that does not conserve the net number of quarks minus antiquarks in the interaction violates conservation of the quantum number B (baryon number). A process that does not conserve the net number of leptons minus antileptons in the interaction violates conservation of the quantum number L (lepton number).

    The only violation of independent B and L conservation in the Standard Model involves a hypothetically possible but never observed high energy process called a sphaeleron process in which B-L rather than B and L separately is conserved. But, models of the Big Bang have shown that this process combined with CP violation that is found in the Standard Model (since any process that treats matter and antimatter differently must by CP violating), cannot account for the matter-antimatter asymmetry in the universe if it started at B=0 and L=0 as pure energy. An alternative is that at time zero, both B and L had their current values or something close modified only by sphaeleron processes since then, and that we are simply presumptuous in assuming B=0 and L=0 to be the initial conditions of the universe.

    Other processes beyond the Standard Model (because they are prohibited by the Standard Model) that would violate independent conservation of baryon number and lepton number (usually while preserving B-L or B+L) are flavor changing neutral currents and neutrinoless double beta decay. Neither of these phenomena have ever been observed either and have been rigorously ruled out to high precision (although the effort to push that precision greater is ongoing and an important area of investigation). At a minimum, these processes are rare enough that they don't contribute meaningfully to changing matter-antimatter symmetry today and not enough to account for what we observe earlier in the universe.

    Presumably, any process that gives rise to baryogenesis or leptogenesis with the observed matter-antimatter asymmetry occurs only in very early parts of the Big Bang-like conditions which is why we don't observe it in nature or in high energy physics experiments today.

    2. Electrons end up orbiting around nuclei because nuclei are positively charged and charged leptons that are not antimatter are negatively charged, so they are attracted to each other until they are paired up one proton to one electron. Electromagnetic forces are powerful.

    3. I leave to someone else an explanation of why electrons don't crash into the nucleus which I basically understand but I am not very good at explaining succinctly. It is more to do with conservation of momentum and less to do with the Pauli exclusion principle.
     
  6. Aug 5, 2016 #5
    The whole idea of baryon asymmetry is pretty intimidating. Is it possible that the antimatter just went somewhere else?
     
  7. Aug 6, 2016 #6

    mfb

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    No, that is a different effect. There is no possible particle state where an electron is fully in the nucleus.
    There is no realistic model that would lead to such a separation.

    If B+L is violated and baryons can be converted to antileptons, then the electrons we see today can be this antimatter, together with neutrinos.
     
  8. Aug 6, 2016 #7
    Hey, ich sehe, dass Sie in Deutschland sind. Ihr Englisch ist echt super! An welcher Universitaet sind Sie?

    Okay, two more questions.

    1. Are covalent and ionic bonds both due to electromagnetism?

    2. Does the nucleus of an atom have any influence on electrons? For example, do the electrons in one atom behave any differently than electrons in other types of atoms?
     
  9. Aug 6, 2016 #8

    DrClaude

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    Only English is allowed at PF :smile:

    Only the electrostatic interaction comes into play in chemical bonding, but quantum mechanics is often necessary to explain it (except maybe for a purely ionic bond).

    Necessarily, otherwise all atoms would be identical. The main role is that of the total nuclear charge. The spin of the nucleus can also play a role on some electronic energy levels.
     
  10. Aug 6, 2016 #9
    Sorry, didn't know.

    So does the strong force play a role in making each atom unique? Is the spin of the nucleus caused by the strong force, or does it just hold everything in the nucleus together? If so, what causes the spin?
     
  11. Aug 9, 2016 #10

    ChrisVer

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    What would you mean by behaviour?
    If you have more protons in a nucleus, you will have more electrons too; so the way the atom forms its chemical bonds is going to change.
    If you have more neutrons nothing is going to change much (except maybe some splitings due to the nuclear spin and angular momentum).
     
  12. Aug 9, 2016 #11

    ChrisVer

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    yes, there are such models (like https://en.wikipedia.org/wiki/Plasma_cosmology) although I find it an unlike possibility which lacks any kind of observational backup... in general even if they have evolved through time, they state that somehow matter and antimatter are pretty much separated and so it does not annihilate (such as theories concerning "domains").
     
    Last edited: Aug 9, 2016
  13. Aug 9, 2016 #12

    ohwilleke

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    To be clear, all electrostatic interactions are consequences of the fundamentally quantum mechanical nature of electromagnetism called "quantum electrodynamics" or QED for short. Quantum mechanics is not something different from electrostatic interactions. Maxwell's equations are a classical effective theory that can be derived from QED but which does not capture of of the quantum aspects of QED.

    To be clear, electrons are all identical, and likewise, all protons are identical, and all neutrons are identical. The difference in behavior between atoms is caused by the different electromagnetic interactions that arise with different numbers of protons and different numbers of electrons in the nucleus.
     
  14. Aug 9, 2016 #13

    ohwilleke

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    The strong force's main role in chemistry is the role is plays in determining which isotypes are and are not stable, and the half-life and decay modes of isotypes which are not stable. In practice, this is determined experimentally rather than from first principles using quantum chromodynamics (QCD) which are the equations governing the strong force, in all but the simplest scenarios. But, in principle, all of the radioactive properties of atoms can be derived from the strong force and other parts of the standard model (the weak force and electromagnetism need to be considered as well).

    The unique chemical properties (except atomic mass) are in turn a consequence of the number of electrons attracted to the nucleus due to the electric charge of the nucleus due to the number of protons in the nucleus and their spatial distribution.

    The intrinsic spin of each proton and neutron happens to equal the sum of the intrinsic spin of its quarks, but for reasons that are far less direct than this simple and perfect mathematical relationship would suggest, because the experimentally, there are other sources of spin in a proton or neutron that somehow miraculously cancel out. I'm honestly not entirely certain how spin is defined at the scale of an entire nucleus.
     
  15. Aug 10, 2016 #14

    DrClaude

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    But you don't need QED to explain chemical bonding. My point, which was not clear, is that the only interaction you need in the Schrödinger equation is the Coulomb interaction in order to explain chemical bonding (adding spin-orbit coupling can also help in some cases).
     
  16. Aug 10, 2016 #15

    ohwilleke

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    Fair enough. I just wanted to be clear not to leave the impression that electromagnetism and quantum mechanics are different forces.
     
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