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Quantum field theory questions?

  1. Jan 26, 2015 #1
    In QFM, what does it mean to say that an electron is just an excitation of the electron field? Does this apply to all particles? Does it mean to say that an electron is the quanta of the electron field?
     
    Last edited: Jan 26, 2015
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  3. Jan 26, 2015 #2

    naima

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    I think that there is a vacuum ##|0\rangle## for the electrons, another for photons and so on. There is creation operator ##a^\dagger## for each field. The vacuum has a null occupation number(##a^\dagger a |0\rangle = zero##. You can call quantum the result of a creation operator on a vacuum (as the result has a number of 1)
     
  4. Jan 26, 2015 #3
    I don't really know what your saying. When Quantum field theory says that an electron is a excitation in the electron field, what does that mean? Does that mean that it is a quanta of the field or a particle of the field... what does excitation of the field mean?
     
  5. Jan 26, 2015 #4

    naima

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    You know that in an hydrogen atom an electron may be on his ground state (lower energy) he may be excited when a photon has been annihilated by the electron.
    When we speak of fields we say that a field is in is ground state ##|0 \rangle## when its energy is minimum. its particle number is null. This field can interact with another field so that it is excited. it is no more on the ground state but in a state with a particle occupation number equal
    to one. That is why we say the electron is an excitation of the electronic field
    the word quanta is not important.
     
  6. Jan 26, 2015 #5

    bhobba

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    You need to become familiar with the quantisation of the Harmonic oscillator:
    http://en.wikipedia.org/wiki/Quantum_harmonic_oscillator

    It turns out when you quatitsise a field its mathematically similar with the creation and annihilation operators defined for the harmonic oscillator behaving the same as creating and destroying particles in a quantum field.

    To fully understand it though you need to study the theory:
    https://www.amazon.com/Quantum-Field-Theory-Gifted-Amateur/dp/019969933X

    That text explains very carefully exactly what's going on.

    Thanks
    Bill
     
    Last edited by a moderator: May 7, 2017
  7. Jan 26, 2015 #6
    So electrons in atoms can be modelled as excitations of the electron field.. but why do we still discuss about whether electrons are particle or wave in the atoms.. why not just mention them as field excitations?

    But then, is it possible this model of electron as field excitations can be replaced with other models.. meaning qft is just a temporary or effective field theory?
     
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  8. Jan 26, 2015 #7

    bhobba

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    Advanced texts do not discuss if electrons are particles or waves - that's the domain of popularisations and beginning texts. It well known to be neither and that was decided ages ago when Dirac came up with his transformation theory at the end of 1926.

    Due to mathematical difficulty you do not use QFT to solve the usual problems of QM. QM is in fact a limit of QFT so while its correct to say electrons are excitations of an electron field a hammer is not usually chosen to crack a nut and for ordinary QM its a bit more vague - but mathematically easier.

    Indeed the Effective Field Theory approach to renormaliation thinks of QFT as basically just effective.

    I think you might find the following book interesting:
    https://www.amazon.com/Fields-Color-theory-escaped-Einstein/dp/0473179768

    Thanks
    Bill
     
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  9. Jan 27, 2015 #8
    I bought the book you suggested above and reading it since yesterday. But it says qft didn't solve the mystery of the double slit too. Quoting in the book:

    "Field collapse: Field collapse is not an easy concept to accept- perhaps more difficult than the concept of a field. Here I have been working hard, trying to convince you that fields are a real property of space - indeed, the only reality - and now I am asking you to believe that this field quantum, spread out as it may be, suddenly disappears into a tiny absorbing atom. Yet it is the process that can be visualized without inconsistency. In fact, if a photon is an entity that lives and dies as a unit, field collapse must occur. A quantum cannot split and put half its energy in one place and half in another, or live here and die there. That would violate the basic quantum principle. While QFT does not provide an explanation for when or why collapse occurs, somne day we may have a theory that does. In any case, field collapse is necessary and has been demonstrated experimentally."

    Bill, Is this why the double slit experimental are not explained using QFT.. because field collapse is just as inexplanable as particles passing both slits? Also I think most physicists don't attribute any physical existence of the quantum fields.. and field collapse if real needs instantaneously faster than light signaling.. this is almost like saying the wave function was real.. only this time the quantum fields are real but fields collapse superluminally. What is your thoughts of this?
     
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  10. Jan 28, 2015 #9

    bhobba

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    Well I think we need to be clear what explain means. With the assumption the field changes on observation then the double slit is explained. Did you expect the explanation not to assume some things? Any explanation does that.

    It doesn't explain the measurement 'problem' (postulate is probably a better description than problem - but its pretty standard so I will stick with it) which any observation in QM has - but in general QM merely accepts it as a primitive of the theory.

    In modern times decoherence is used to elucidate better the measurement postulate which has a number of parts. It explains a number of those parts such as a preferred basis - it doesn't explain the so called problem of outcomes which colloquially is why do we get any outcomes at all, or more technically how an improper mixed state becomes a proper one. Exactly what that issue is, is explained here (sorry that its rather technical but we are getting into deep waters here):
    http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

    If I remember correctly that book you got also mentions decoherence - but doesn't give any details. This may help at your level:
    http://www.ipod.org.uk/reality/reality_decoherence.asp [Broken]

    Thanks
    Bill
     
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  11. Jan 30, 2015 #10
    Bill. Have you actually read the book you recommended days ago (the "Fields color theory escaped Einstein"? I finished reading it and it's a very unusual book with a different interpretation of QFT and even special relativity. For example. There is a passage inside that says:

    "While I hope you can accept, as did FitzGerald and Lorentz, that length contraction happens because the field equations require it, it would be nice to have some intuitive insight into the phenomenon. We must recognize that even if the molecular configuration of an object appears to be static, the component fields are always interacting with each other. The EM field interacts with the matter fields and vice versa, the strong field interacts with with the nucleon fields, etc. These interactions are what holds the object together. Now if the object is moving very fast, this communication among fields will become more difficult because the fields, on the average, will have to interact through greater distances. Thus the object in motion must somehow adjust itself so that the same interaction among fields can occur. How can it do this? The only way is by reducing the distance the component fields must travel. Since the spacing between atoms and molecules, and hence the dimensions of an object, are determined by the nature and configuration of the force fields that bind them together, the dimensions of an object must therefore be affected by motion"

    He also explains why time dilation occurs because the fields need finite time to travel in between atoms. And the reason c can't be exceeded is because the fields need finite speed. And mass increase because of more inertia the fields have to resist. Ok. Why don't I see these explanations given by the average physicists? And how many subscribe to this view?

    About General Relativity. It says something like "What Schwinger is saying is that four dimensional geometry does not exist in the real world - it is "impossible", but matheticians "who think algebraically" are free to construct equations that would describe it if is did exist. By stretching our minds, some of us can even form a vague mental image of what four-dimensional curvature would be like if it did not exist. Nevertheless, saying that the gravitational field equations are equivalent to curvature is not the same as saying there is curvature. In QFT, the gravitational field is just another force fields, like the EM, strong and weak fields, albeit with a greater complexity that is reflected in its higher spin value of 2."

    Read the rest in the book.

    For others besides Bill. Can you share or comment what could possible be wrong with it?
     
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  12. Jan 30, 2015 #11

    bhobba

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    I have read it.

    There are a few areas of disagreement, but it's overall OK.

    Please understand this is written at the populist level. For example why C cant be exceeded is the reason he states - but what he didn't point out is the laws of physics must be the same in all inertial frames, nor does he give an exact definition of inertial frames, which are in fact the most important things - the speed of light thing simply fixes a constant that follows from those. It must be said neither do other books at the lay level either - the full truth requires a reasonable grasp of calculus and linear algebra:
    http://www2.physics.umd.edu/~yakovenk/teaching/Lorentz.pdf

    So you are left with analogies and picturesque language.

    Its different - and it's intended to be because its presenting a very advanced topic, QFT, to a lay audience.

    Also note Dr Brooks is no dummy - he studied QFT under an acknowledged master - Julian Schwinger.

    Thanks
    Bill
     
    Last edited: Jan 30, 2015
  13. Jan 30, 2015 #12
    QFT is about operators acting on the state vectors and these are not supposed to be physical.. but why did the author kept emphasizing the field is physical and in fact literally pass thru both slits and then the fields vanish when one part gets "detected" by the detector atom? Who are other physicists who hold this view?
     
  14. Jan 30, 2015 #13

    bhobba

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    Why do you think that? IMHO its about putting time and position on equal footing as required by relativity and making position a parameter rather than an observable. This leads to quantum fields which means operators are assigned to points in space - the state space becomes a lot trickier being a Fock space. There are deeper analysis possible as detailed in Wienberg's texts but challenging would be a very mild way of describing it.

    Because it is physical in exactly the same way as any other observable in QM is physical eg in ordinary QM momentum and position are physical.

    He isn't talking about ordinary QM here - he is talking about its most fundamental and advanced incarnation - QFT.

    Thanks
    Bill
     
  15. Jan 30, 2015 #14
    I read in sci am that "A classical field is like a weather map that shows the temperature in various cities. The quantum version like a weather map that does not show you "40 degree," but "sqrt ()". To obtain an actual temperature value, you would need to go through an extra step of applying the operator to another mathematical entity, known as a state vector, which represents the configuration of the system in question"

    How could these operator fields be physical like in qm position or momentum? And our instrument just detects particles.. we can't detect the raw field.. in the same manner we can't detect wave function because these are just mathematical devices to produce detections. Do you attribute the quantum fields as more real than wave function? But function fields are composed of configuration of wave functions too, and they collapse similarly to QM wave function so how can the quantum fields wave function be more real?

    Also what is the theoretical reason why we can't apply QFT to a piece of wood such that the atoms can be annihilated or be created making the wood indeterminate.. I read something like QFT can be applied to free fields (?) only and not to bound states like atoms. Why?
     
  16. Jan 31, 2015 #15

    bhobba

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    A field is just as real as momentum etc - you can measure it - eg you can measure EM fields.

    Why don't you think we cant? It's just mathematically much more difficult than QM which is so difficult we cant even apply that to atoms more complex than hydrogen and have to use computer approximations. For wood - that's way beyond our present ability in any kind of exact sense - but qualitatively we know what's going on as biology texts explain - from dim memories of biology at school.

    QFT can be applied to free fields.

    Thanks
    Bill
     
  17. Jan 31, 2015 #16
    But how do you detect electron fields? We detect electron particle instead of the electron field.

    But QFT deals with annihilation and creation operators.. if you apply this to wood.. won't this just destroy it since you are annihilating the wood atoms making it vanish?
     
  18. Jan 31, 2015 #17

    naima

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    The problem is that there is no apple tree fields interacting with EM field. If so we could easily get fruits from a laser.
     
  19. Jan 31, 2015 #18

    bhobba

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    What makes you say that?

    It's not true - but I am curios why you would think that.

    The reason fields exist is its required to ensure conservation laws are respected as required by Noethers theorem. If you have access to it the following book explains it:
    https://www.amazon.com/Gravitation-Spacetime-Hans-C-Ohanian/dp/1107012945

    Specifically it has to do with certain no-go theorems worked out by Wigner.

    Thanks
    Bill
     
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  20. Jan 31, 2015 #19

    bhobba

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    You reasoning for such a statement escapes me. What QFT shows is that a quantum field is mathematically equivalent to creation and annihilation operators but the QM of everyday objects is the dilute limit of that where you deal with one or no particles. If you want to delve into that see the following reference:
    https://www.amazon.com/An-Introduction-Realistic-Quantum-Physics/dp/9812381767

    Be aware however you are delving into mathematically advanced waters.

    The book I recommended takes you about as far into QFT without delving into such detail.

    Thanks
    Bill
     
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  21. Feb 3, 2015 #20
    In QM, time is a parameter in that all system have similar time. But Einstein says time varies in different locations, so in QFT, we have time as coordinate.. but a fixed coordinate. .

    General Relativity teaches us that spacetime is curved in the presence of matter.. so QFT can be further improved to have fields that can interact with the geometry. I'd like to know if general relativitisic QFT would be simply about planck scale physics (or quantum gravity) or it would also predict or model other above planck scale effects like information dynamics (for example from the holographic principle) akin to how QFT can give us interactions and particles that can annihilate or create (larger than planck scale) versus that of plain QM?
     
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