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Particles or fields?

  1. Nov 21, 2006 #1

    Demystifier

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    In your opinion, what are more fundamental objects: particles or fields?
    In other words, is QFT just a convenient mathematical way to calculate the properties of particles,
    or
    are particles just specific states of quantum fields?
     
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  3. Nov 21, 2006 #2

    Demystifier

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  4. Nov 21, 2006 #3

    CarlB

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    I refuse to vote. I think that there exists a "now", and that when an event from the past is examined, one will find that a particle description fits it well. When one examines an event from the future, I think that this event also exists in spacetime, but must be described by a field.

    The field is a sort of stress of spacetime, and as the moving hand of time passes through the event, the stresses become extreme, and eventually spacetime breaks down. Particles are the points at which spacetime has dislocated. The probability interpretation simply says that the dislocations are random, but are more likely in places where the stress is higher.

    This is like Bohmian mechanics, but with the wave and particle not being operative, at the same time, for any single snapshot of the event.
     
  5. Nov 21, 2006 #4

    arivero

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    pleres kai stereon to on, to de kenon to me on...
     

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  6. Nov 21, 2006 #5
    I prefer fields.

    In a very simple sense they are more general. For example, it could be said that (infintely dense point) particles are a subset of fields, i.e. a field with a value defined for just one position and zero elsewhere. For quantum particles we have a probability distribution which is also a specific type of field... etc..

    This imo makes them more fundamental, since you can model a particle as a field without any tricks involving infinity, but the reverse is not true.
     
  7. Nov 21, 2006 #6

    arivero

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    It seems that force carriers, bosons, are more properly treated as fields, while fermions, in some sense matter fields, are more properly to be thought as particles. There is a subtle point here, that there is no such thing as a classical fermionic field: this is seen by explicitly putting the Planck constant in the Lagrangian, instead of setting it to 1 as usual.

    One could think that an object whose main properties are spatial is a field, and an object whose main properties are kinematical/positional is a particle. But everything gets mixed in the quantum world.

    It also happened to Democritus, that after clearly dividing spatial properties (no-thing) from positional ones (thing), got to the problem of how forces were communicated, and need to create a new concept, eidola, to understand our actual bosonic carriers. So at the end one has four categories: spatial boson, positional boson, spatial fermion, positional fermion.

    For the mathematically minded, it could be better to think on de Rham duality instead of the wave/corpuscle issue. What is more fundamental, the form or the cycle? The density or the volume where we integrate this density? The volume or the density to be integrated in this volume?
     
    Last edited: Nov 21, 2006
  8. Nov 21, 2006 #7

    vanesch

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    I voted particles, because I think fields are more fundamental :tongue2:
    (and if I would have voted fields, I would then be in a state where particles are more important).

    Honestly, the two descriptions are entirely equivalent. So what...
     
  9. Nov 21, 2006 #8

    robphy

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    "QFT in curved spacetime" suggests that the notion of particle is dependent upon some structures that are not available in a generally curved spacetime.

    See, e.g., http://arxiv.org/abs/gr-qc/0608018 "The History and Present Status of Quantum Field Theory in Curved Spacetime" by Robert M. Wald
     
  10. Nov 21, 2006 #9

    selfAdjoint

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    This expresses my notions too. I voted "fields" but I actually believe that whatever comes prior to spacetime is the source of all, and its geometrical/topological/combinatiorial properties determine what we experience at low energies as the quantum universe.
     
  11. Nov 21, 2006 #10

    turbo

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    My view is that particles are condensations of fields, and if so, fields are more fundamental. Of course, this wanders into chicken or egg territory since it is difficult to conceive of one without the other, so I may have wandered down a path that is insupportable. Condensations allowing the existence of particles have spacial and temporal aspects - I'm not sure if these can constrain the existence of underlying fields.
     
  12. Nov 21, 2006 #11

    CarlB

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    If by "condensations", you mean places where fields become infinite (while becoming zero everywhere else, then I agree.
     
  13. Nov 21, 2006 #12

    turbo

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    I think that I can agree to this concept. Nature appears to accord to quantum weirdness in this regard.
     
  14. Nov 21, 2006 #13

    CarlB

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    I'm now voted for fields. A dislocation eliminates the stress in the field except at a point (particle), where it becomes infinite (as in a delta function), by instead of defining an infinitesimal change to the coordinates (the derivative of which is the field), it defines a step function in the coordinates, a place where the old spacetime is no longer continuous, but instead suddenly jumps. The derivative is then a delta function and defines a point particle position.
     
  15. Nov 21, 2006 #14

    selfAdjoint

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    CarlB, it seems to me that by setting up local infinities as a criterion of reality, you have captured the true spirit of particle physics!:devil:
     
  16. Nov 22, 2006 #15

    Demystifier

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    Until recently, I was also thinking that fields are more fundamental. However, one of the reasons I have changed my mind is the (old) cosmological-constant problem. The point is that it is the problem only if you take the vacuum energy seriously, i.e., if you assume that there is something even in the absence of particles. (This something is a vacuum expectation value of the field energy-momentum.) On the other hand, if you assume that only particles have physical reality, the old cosmological-constant problem simply does not appear. (For an attempt to exploit this idea in more details, see
    http://arxiv.org/abs/gr-qc/0611037 )
     
  17. Nov 22, 2006 #16

    Demystifier

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    Interesting view, but the field-or-particle dilemma is more subtle than this.
    See e.g. the post of robphy above.
     
  18. Nov 22, 2006 #17

    hellfire

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    I have voted for fields. Quantum field theory on curved backgrounds shows that the field is the fundamental entity. What are the reasons to think that it is wrong? Isn't the CMB power spectrum produced during inflation an empirical (indirect) proof that the theory is correct?
     
    Last edited: Nov 22, 2006
  19. Nov 22, 2006 #18

    Demystifier

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    I do not see how CMB confirms the observer dependence of particles suggested by QFT in curved backgrounds. Perhaps you had something else in mind?
     
  20. Nov 22, 2006 #19

    turbo

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    Let us suppose that the vacuum actually is a sea of virtual-particle pairs popping into existence and annihilating in accordance with the HUP. Is there a fundamental quantum law that could prevent this vacuum field from self-gravitating into collapse? What if, like real particles, virtual particles obey the Pauli exclusion principle and resist being packed into proximity with similar same-spin particles. This effect would dynamically balance the the vacuum's tendency to collapse and result in the fine-tuned CC that we observe, regardless of the density and energy of the field at any location.
     
  21. Nov 22, 2006 #20

    Demystifier

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    There are no virtual particles. (They are nothing but a verbalization of certain Feynman diagrams.) But if you rephrase all this in terms of fields, then it certainly makes sense qualitatively. But the problem is to achieve this quantitatively.
     
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