Can someone explain what this means please?

  1. I read this but dont know what it means

    Particles in a carbon atom interact with each other and their environment all the time, quantum field theory describes those interaction with creation and annihilation operators.

    I thought creation and annihilation operators created and destroyed particles(or field quanta), does this mean when particles in a carbon atom interact with eachother or the environment, some of the particles in the atom are created or destroyed?
  2. jcsd
  3. K^2

    K^2 2,470
    Science Advisor

    Yes, it does. But that's not really a problem, because what you think of as particles are actually entire messes of things.

    Consider a neutron. It is often said that neutron consists of three quarks. One up quark and two down quarks. That is incorrect. Neutron contains a huge number of quarks, antiquarks, leptons, and all kinds of bosons. They are constantly being created and destroyed.

    However, for every fermion in a neutron, you will be able to find an anti-particle, with exception of three quarks you'll have left over without a pair. These are the valence quarks. The choice of which one they are is not unique, but there will always be three valence quarks in a neutron, and none of the creation or annihilation processes within the neutron can change that count.
  4. so the 3 valence quarks in the middle will never be annihilated then and the electrons orbiting the nucleus will never be annihilated by these operators?
  5. K^2

    K^2 2,470
    Science Advisor

    Three valence quarks aren't the specific ones. Any quark you pick will get annihilated. It's just that any way you assign pairs, you'll always be left with 3 quarks that have no pair. Which 3 depends on how you chose pairs.

    Same deal with electron. It's not quite as messy, and thinking of electron as a single particle isn't as bad a mistake as thinking of neutron as just three quarks, but everything in field theory ends up being a quasiparticle.
  6. so none of the quarks in the proton have been there for any considerable time frame? they all just keep getting created and annihilated?

    is there anything in a proton that has been there for a considerable time frame?
  7. K^2

    K^2 2,470
    Science Advisor

    Electric charge stays the same. A few other quantum numbers. But there isn't a specific particle that persists. These constantly change.
  8. Cthugha

    Cthugha 1,717
    Science Advisor

    To add to what has been said: Creation and annihilation operators typically mean that particles in a certain state are created or destroyed. Consider a problem with a large number of particles, maybe the extreme problem of a solid with its roughly 10^22 particles. Lots of scattering events and other interactions happen all the time. What now happens when two particles collide, is fo example that both start with a certain momentum and end up with a different one. One could now keep track of all 10^22 particles individually (bad idea and almost impossible to handle computationally) or one just keeps track of the occupations of all possible states. Then you can describe such a scattering event by annihilation of two particles in the initial momentum states and creation of two particles in the final momentum states. So particles in the states of interest are created and destroyed using this formalism, but the total particle number is conserved.
  9. if we formulate the above situation in terms of string theory though, are any of the strings the same?
  10. K^2

    K^2 2,470
    Science Advisor

    That's kind of like asking if any of locations are the same.
  11. what does all this mean then for identification? including ourselves? if the fundamentle bits of us are quarks and electrons and these bits are always be created and destroyed, how can we be the same people?
  12. K^2

    K^2 2,470
    Science Advisor

    That's not really a physics question. Ship of Theseus will get you started, but you'll need to talk to some philosophers for better explanation.
  13. so you would say we are not then. we cannot be if everything is replaced.

  14. some please explain to me what this means...

    electrons are quasiparticles? but quasiparticles are fictious, does that mean electrons dont exist? what are they then?
  15. Nugatory

    Staff: Mentor

    The problem is that we're trying to use English to describe concepts that can only be precisely expressed in mathematical terms. All this talk of "quasiparticles" and "annihilation" and such like... and the idea that the electrons, protons, neutrons, quarks that we're talking about here are particles like little grains of sand except even smaller... Those are all analogies. They're helpful for getting an intuitive feeling for how things work, but they get vague when you push on them for exact meanings.

    To get the solid answer that I think you're looking for, you'll have to dig down into quantum electrodynamics and quantum field theory, take on the underlying math.
  16. No, he called them quasiparticles, quasiparticles are fictionous particles accrording to wikipedia. Is he saying that single electrons dont exist at all?
  17. thats all I want to know. wikipedia says:

    In summary, quasiparticles are a mathematical tool for simplifying the description of solids. They are not "real" particles inside the solid. Instead, saying "A quasiparticle is present" or "A quasiparticle is moving" is shorthand for saying "A large number of electrons and nuclei are moving in a specific coordinated way.

    how does this relate at all to electrons? electrons or electrons quanta in a field dont exist??
    Last edited: Dec 14, 2012
  18. Nugatory

    Staff: Mentor

    No, he's not saying that. Maybe the best way of saying it in English is that the thing that we call an "electron" behaves mostly like a little teeny charged grain of sand if you don't get too close; but if you could look at it closely enough you'd see a continuously churning maelstrom of N electrons and N-1 anti-electrons with N constantly changing as creation and annihilation is happening.

    But remember what I said about analogies, not accurate, getting all vague when you push on them, no substitute for the math, all that stuff. What I said above is not a really accurate explanation, just the best that I can do with English instead of math.
  19. but electrons are fundamental, not like looking inside a proton. all the wild activity occurs in the proton

    if you look at an electron then you will see numerous electrons creating and annihilating? not just one electron.

    ive never heard of that. these electrons creating and annihilate would have to be smaller then, if as you say when you look closer you only see it...
    Last edited: Dec 14, 2012
  20. Nugatory

    Staff: Mentor


    Well, you cannot possibly "look at" an electron, so when I said "see" I was speaking metaphorically. But that's still the best English-language description of what quantum field theory says that I can come up with. Maybe someone else can come up with a better description... But the real answer is in the math of QFT, and unfortunately there's no quick easy way through that.
  21. K^2

    K^2 2,470
    Science Advisor

    Precisely. The term quasiparticle is more commonly used in condensed matter, where implications are different, but principle is much the same. Except, we are talking about interaction of particle with vacuum rather than with some sort of medium.

    Vacuum is very far from being empty, and you cannot describe track of a "single electron". You are describing dynamic of electromagnetic field with net quantum numbers corresponding to one electron, but which is far more complicated otherwise. Hence, what you observe as an electron "from a distance" is a quadiparticle, same as these you'd read about in the Wikipedia article. That article simply happens to lean heavily towards condensed matter, so this might not be entirely clear.

    Perhaps, I should have called it a dressed particle. But that doesn't convey the point I was trying to make. "Dressed particle," makes it sound like you can distinguish the bare particle from the dressing. You really cannot.
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