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B What Is an elementary particle?

  1. Mar 3, 2016 #1
    Hi all,

    When we read this article http://www.slac.stanford.edu/pubs/beamline/27/1/27-1-weinberg.pdf it seem that there are not single answer to this question.

    Does the answer depends on what physical theory we use ? If we use string theory we don't get the same answer as if we use quantum field theory ?

    Best regards
     
  2. jcsd
  3. Mar 3, 2016 #2
    Physics doesn't answer such a question, but the question is nevertheless a good one.

    Our models generally describe observed phenomena and should be useful in making predictions. They rarely tell us "what something is."

    The 'particles don't care how we model them, but I 'd agree with your statement: when you ask different scientists, even ask different questions, you'll get different answers.

    All known particles are modeled in the Standard Model of particle physics, which is actually a hodge podge of quantum field theories in flat spacetime coupled with observed quantities for which we have no theory, like the mass of an electron. Flat space time means no gravity, no 'graviton' particles.

    Rovelli: “…we observe that if the mathematical definition of a particle appears somewhat problematic, its operational definition is clear: particles are the objects revealed by detectors, tracks in bubble chambers, or discharges of a photomultiplier” ......

    A particle is in some sense the smallest volume/unit in which the field or action of interest can operate….Most discussions regarding particles are contaminated with classical ideas of particles and how to rescue these ideas on the quantum level. "

    As presented in Weinberg's "The quantum theory of fields" vol.1: The primary objects are particles described by irreducible unitary representations of the Poincare group. For realistic systems with varying numbers of particles we build the Fock space as a direct sum of products of irreducible representations spaces. Then the sole purpose of quantum fields (=certain linear combinations of particle creation and annihilation operators) is to provide "building blocks" for interacting generators of the Poincare group in the Fock space. In this logic quantum fields are no more than mathematical tools.

    And the deeper one goes, the 'crazier' it gets:

    It seems that expansion of geometry itself, especially inflation, can produce matter [particles]. Gravitational perturbations [wave inhomogeneaties] in an expanding space produces observable [point] particles. Mathematical transformations between inertial and accelerated frames also seems to produce particles: such different observers see different vacuum energies...and such energy differences result in particle production.

    A related phenomena is the "Unruh effect." A vacuum state is observer dependent! The uncertainty principle requires every physical system to have a zero-point energy….http://en.wikipedia.org/wiki/Zero-point_energy “...Vacuum energy is the zero-point energy of all the fields in space.....the energy of the vacuum, which in quantum field theory is defined not as empty space but as the ground state of the fields......which leads to virtual particles.

    If you are accelerating and I am inertial, we do NOT measure the same vacuum energy! This means if you are accelerating, you detect heat, particles, I do not.

    "There is not a definite line differentiating virtual particles from real particles — the equations of physics just describe particles (which includes both equally). .... In the quantum field theory view, "real particles" are viewed as being detectable excitations of underlying quantum fields. As such, virtual particles are also excitations of the underlying fields, but are detectable only as forces but not particles. They are "temporary" in the sense that they appear in calculations, but are not detected as single particles.
    http://en.wikipedia.org/wiki/Vacuum_...g_vacuum_state

    String theory is unproven and posits particles are actually one dimensional objects, like a string. The Stand Model of particle physics models them as the point like interactions of extended quantum fields.
     
  4. Mar 4, 2016 #3

    vanhees71

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    Argh! This Wikipedia article about "the vacuum" is very misleading. What's true in it is for sure the first sentence. The rest is too weird to be commented in detail. Particularly the claim that the notion of virtual particles in the form given in this article is accepted is simply ridiculous. What's accepted are the very successful results of perturbation theory in QFT, i.e., a formal expansion of the scattering amplitudes in powers of the coupling or (essentially equivalently) in powers of ##\hbar##, leading to the Dyson-Wick series, which is an asymptotic series. These calculations can be tremendously simplified by using Feynman diagrams, which appear to look like scattering processes in a very intuitive manner, but what's behind them really is just a shortcut to write down the expressions for the perturbative S-matrix elements, which can be compared to experimental results concerning scattering processes of real particles.

    The particle interpretation in relativistic QT is pretty narrow in the sense that it is possible only for socalled "asymptotic free states". The picture is the following: If you have a short-ranged interaction you can describe particles that are much farther away from each other than the range of the interaction as non-interacting ("free") particles. Mathematically they are free when the distance between becomes "asymptotically large", that's why one talks about asymptotic free states. What's happening when the particles come close to each other and the interaction becomes relevant, is solely describable by quantum-field theoretical abstract quantities, and it is not generally clear, how to interpret this "transient states" in terms of particles. Usually it is save to say that you better don't attempt to interpret them as particles, and usually you cannot resolve the time evolution of a transition. So it is save to say not to talk about "virtual particles" but only about observable facts.

    Indeed there are a lot of manifestation of socalled "radiative corrections" (which is a terminus from the time where relativistic QFT was merely restricted to QED, i.e., the description of the electromagnetic interaction, but it applies as well to the entire standard model of elementary particle physics). The paradigmatic once in QED are the Lambshift of the hydrogen-atom levels and the anomalous magnetic moment of the electron. They are due to the corrections of perturbation theory beyond the leading order, where loops occur in the Feynman diagrams. They describe the fluctuations of the quantum fields that can indeed be qualitatively understood by the energy-time uncertainty relation, but the measurable consequences have little to do with particle-like notions. In the case of the Lamb shift you measure a feeble splitting between hyperfine transition lines which are degenerate in the leading-order (tree-level) approximation. For the anomalous magnetic moment of the electron it's the deviation of the socalled Lande-gyro factor relating the electron spin with its intrinsic magnetic moment from its tree-level value 2. In both cases the calculations have been driven to high orders in perturbation theory and the agreement between the QED prediction (including also corrections from the strong interaction, which introduces the largest uncertainty in the theoretical calculation) and measurement is overwhelming, but it has nothing to do with "virtual particles", which you cannot sensibly define at all.
     
  5. Mar 4, 2016 #4
    To clarify a bit,Micro, your original post is on target. Explaining particles, really the models we use to describe their behavior, gets complicated very quickly. Are quantum fields 'real' or just tools? Different people may emphasize one or the other.

    [Vanhees knows way more about all this than I. I have to keep it basic for myself as well as for you.]

    Here is another source, but it too has some statements experts here will find extremely objectionable.
    http://www.physics.ucdavis.edu/Text/Carlip.html#Hawkrad Parts I like, which may be somewhat less than perfectly technically correct, follow:

    So not only do different models, different physical theories yield different answers, people often don't agree on the explanations, that is, the interpretation of the math.

    A classical configuration of a field typically does not have a single frequency, but it can be Fourier decomposed into modes [broken down into components] with fixed frequencies. In quantum field theory, modes with positive frequencies correspond to particles, and those with negative frequencies correspond to antiparticles. {and complex numbers correspond to virtual particles}…..

    So note that people look at math and conclude: That seems to look like this, or that, experimental observation, a measurement. Is it 'real' or just a fantasy? It's 'real' if that interpretation turns out to make valid predictions and matches observational measurements; if not, the interpretation has to be tossed out for,say, different math or a different interpretation.

    [If you have had trig already, you might think of relationships like Sin2X = 2SinXCosXas a type of 'decomposition']. So a sine wave of one frequency can be considered identical to the product of two other trig functions as shown of different frequency....and you can do this endlessly for the myriad of trig relationships.

    But those frequencies may not be so simple to interpret: We know from special relativity that frequency depends on time, and in particular on the choice of a time coordinate; two observers in relative motion will see different frequencies [!!] for the same source. In special relativity, though, while Lorentz transformations [used in Einstein's special relativity] can change the magnitude of frequency, they can't change the sign, so observers moving relative to each other with constant velocities will at least agree on the difference between particles and antiparticles.

    For accelerated motion this is no longer true[!!!] , even in a flat spacetime. A state that looks like a vacuum to an unaccelerated observer will be seen by an accelerated observer as a thermal bath of particle-antiparticle pairs. This predicted effect [is the] Unruh effect….

    The above is a decent intuitive description of how difficult it is to define a 'particle', even to determine how we objectively detect such a particle, in a manner that all theorists would agree is 'accurate'.

    So the lesson for me is that physical models describe interactions, not the object particles themselves.

    If all this is not enough, search these forums for "What is a particle." and settle in for a lot of reading!
     
  6. Mar 4, 2016 #5

    vanhees71

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    Well, there are some links to excellent websites about the standard model on the popular-science level, e.g.,

    http://www.particleadventure.org/
     
  7. Mar 4, 2016 #6
    Hi alw34,
    Thank for your answer.

    Is it the same mathematical representation for elementary and composed particle ? In fact my question is about elementary relate to particle.


    Thank,
    Patrick
     
  8. Mar 4, 2016 #7
    Thank i am going to read it

    Patrick
     
  9. Mar 4, 2016 #8

    mfb

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    You can use the same representation if the internal details do not matter, e.g. for a proton in a hydrogen atom. That gives a very good approximation.
     
  10. Mar 5, 2016 #9
    Hi all

    Does we can say that only elementary particle acquire mass with Higgs mechanism and therefore this is not the case for proton in a hydrogen atom ?

    Is there any fundamental physical properties that characterize unambiguously the elementary particles ?

    Patrick
     
  11. Mar 5, 2016 #10

    mfb

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    The quark masses contribute to the proton mass (although that contribution is small for a proton). Most of the mass is independent of the Higgs mechanism, however.
    They are not made out of other particles?
     
  12. Mar 5, 2016 #11
    Composite particles being made of elementary particles, thus all physical properties that characterised elementary particles also characterised composite particles and reciprocally ?

    For example, there are different types of quarks described with properties as flavor, generation, color. Each type of quark has properties that allows it to bind together with other quarks. Does this properties make physical sense also for composite particles ?

    Thanks
    Patrick
     
  13. Mar 5, 2016 #12

    mfb

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    Some properties of composite particles don't make sense for elementary particles and vice versa. Some properties are relevant for both.

    As an example, composite particles have a non-zero size. Elementary particles do not (according to current knowledge).
    Quarks are sorted by generations, sorting hadrons in generations in the same way doesn't work. As a classical analogy: people have an age, but groups of people do not have a single value "age".
     
  14. Mar 5, 2016 #13

    Garlic

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    I want to add something.
    Why are electrons considered to be elementary particles despite the existence of quasiparticles?
     
  15. Mar 5, 2016 #14

    mfb

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    Quasiparticles are quantum states in many-body systems.
     
  16. Mar 8, 2016 #15
    Physics does not answer such a question, in fact physicists stare at you funny for even asking such a question which is telling.

    It suggests the state of the theory is woefully incomplete and they get defensive. It's shocking that after a century of particle physics they have no real idea of what a particle is.
     
  17. Mar 8, 2016 #16

    vanhees71

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    I don't understand this statement.

    By definition an elementary particle is one that can be described by a quantum field in the formalism of relativistic quantum field theory. Empirically it depends on the energy scale you look at it, whether this holds true to a good accuracy or not. E.g., at low scattering energies you can treat protons as elementary particles described by a Dirac field with good accuracy. This holds no longer true for higher scattering energies. In fact with high-energy electron scattering on protons it was revealed that the proton seems to be a bound state of three partons. That lead to the discovery that Gell-Mann and Zweig's "mathematical constructs" they called "quarks" are in fact reall things rather than mathematical constructs. Nowadays we consider the quarks as elementary particles described by Dirac fields and the proton as a complicated bound state of quarks and gluons within the theory of the strong interaction, called Quantum Chromodynamics (QCD), which has the funny property (indeed not yet fully understood) that the elementary quanta described by them (quarks and gluons) never occur as free particles, which is called confinement.
     
  18. Mar 8, 2016 #17
    "I don't understand this statement" typifies the modern physics mind well. "By definition...." Fundamental particles are not defined, they are measured. Your math to describe them is defined and thus is only a model, not the thing itself. Reality is forced to conform to the theory rather than the other way around. What if field theory is not the best way to describe particles? But you have defined particles as only that which can fit into field theory.
     
  19. Mar 8, 2016 #18

    vanhees71

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    Well, of course, it's to be measured whether the particle behaves like an elementary one. You need, however, to define what an elementary particle is, before you can compare the definition with your measurements. There's no measurement possible without theory. Even as apparently simple a quantity as the length of my table needs an assumption about geometry before you can measure it!
     
  20. Mar 8, 2016 #19

    mfb

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    It is the best way we found so far. And it works incredibly well. It is a very good model, and physics is all about models.
     
  21. Mar 8, 2016 #20
    There is some merit to this, to be sure. People who have spent careers studying such things are apt to defend the vast wealth of knowledge that has been accumulated. Someone said something to the effect "We know so much, we understand so little." And that can thinking can infuriate people in almost any field. So better to focus on open questions and future opportunities, perhaps?

    For example, we don't know how to economically develop fusion power,anti gravity flight vehicles or transport between dimensions, maybe because we don't even know if such dimensions [parallel universes, for example] exist.

    I like "...is the best way we have found so far", but I don't like to say our physics 'works incredibly well' so much.
    For me, 'incredibly well' would answer what particles are, where they come from, and the Standard model would include gravity and be based on fundamental principles. We don't have a physical 'theory of everything" yet.

    Physics is great within its confines and limitations, but vast new new horizons remain. On the other hand, compared to our understanding of the human body and, say, the brain, or human behavior, I'd be willing to argue no branch of science surpasses that of the achievements of physics.

    Hmmm, do the vaccines of medicine and perhaps genetics give physics a 'run for its money'? Maybe, but I'd say viruses are still smarter and faster than we.
     
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