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Photons and neutrinos

  1. Apr 22, 2009 #1
    Neutrinos are almost massless, travel close to the speed of light, and pass through matter almost undisturbed. Photons, on the other hand, have no mass, travel at the speed of light, yet are absorbed and/or reflected even by fairly sparse matter such as gasses. Why?
     
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  3. Apr 22, 2009 #2

    mgb_phys

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    A photon has an electromagnetic field that can interact with charged particles - a neutrino has no charge and so unless it hits something head-on doesn't interact
     
  4. Apr 22, 2009 #3
    But a photon has no charge either, right? So both particles have little or no mass, travel close to or at the speed of light, have no charge, but the photon has an electromagnetic field whereas the neutrino doesn't. I understand the particle-wave duality but, from the particle perspective, how does a particle without a charge have an electromagnetic field? I mean, other than by definition, or perhaps that's what it is -- a defined particle to make the mathematics work.
     
  5. Apr 22, 2009 #4

    Yoo

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    Photons are electromagnetic fields, or rather waves in the electromagnetic fields ...
     
  6. Apr 22, 2009 #5

    jtbell

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    Neutrinos interact only via the weak interaction. Photons interact via the electromagnetic interaction. The weak interaction is much weaker than the electromagnetic interaction.
     
  7. Apr 23, 2009 #6

    malawi_glenn

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    It is a difference between waves and waves, all waves do not carry same information.

    How a particle without charge has an electromagnetic field you will probably learn in a class in electrodynamics.
     
  8. Apr 23, 2009 #7
    OK, so how do you distinguish between waves?
    so if I give you a wave function (as in a map from R^4 to C) how can you tell
    if this corresponds to a photon, or a neutrino...or the entire universe for that matter.
     
  9. Apr 23, 2009 #8

    malawi_glenn

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    mathematically one can not, but physics also deal with quantities and units.

    The photon wave carries the density of E and B, and the Neutrino the corresponding density for the weak force (roughly speaking).

    A photon, in quantum field language, is the quanta of the electromagnetic field - a vector field (spin 1). A Neutrino is a quanta of a fermion field (spin 1/2). So the photon and the neutrino has many differences mathematically, but this you will learn later in school.
     
  10. Apr 23, 2009 #9
    what! what calculations can you do in "physics" that you can't do mathematically.

    you are really mixing all sorts of objects here. Let's restrict to the "scalar" case
    (R^4 to C). We can deal wil spin 1/2, spin 1,... independantly. For example for
    spin 1/2 : how do you distinguish between any two maps R^4 to C^2. That aside,
    I think I know what you're trying to say as far the photon "carrying em field" and
    a neutrino "corresponding to weak force density" so let's fix the gauge group to
    be the electroweak SU(2)xU and let's look at the right handed neutrino which is
    a scalar...how do you distinguish between two scalar functions in this setting?...

    followed by many more years of unlearning
     
  11. Apr 23, 2009 #10

    malawi_glenn

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    In physics we assign units and dimensions to what we measure and describe.

    You are trying to describe particle physics with inappropriate language, you must to it in quantum field theory. You are trying to describe it with wave-equations.

    Let us instead do the comparison between ultra relativistic electrons and neutrinos, both which are fermions. Electrons have interactions which are both weak and EM, neutrinos only weak - hence electrons which moves at same speed as neutrinos will be absorbed more easily than the neutrinos. Just have a look at the Standard Model Lagrangian and derive all possible interactions.

    It is not true that neutrinos travels close to light speed, that is a reference dependent statement! You can find a frame of reference where the neutrino is non-relativistic, so you can't logically say that since it has no charge and travel close to light speed it should interact similar as the photon does. So logically your argument fails there, since the neutrino speed is a reference dependent statement and interactions are frame independent.

    You derive what interactions particle has by inspecting the SM - Lagrangian interaction terms, not by looking at particle speeds in some reference frame.
     
  12. Apr 23, 2009 #11

    malawi_glenn

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    As an example, consider the mathematical equation:

    a + b = c

    where a,b and c are real numbers.

    in physics, a, b and c must have the same units:

    1kg + 5m = what?
     
  13. Apr 23, 2009 #12
    Malawi is right. This is not even a "high-energy physics" question. Take the x-component of the magnetic as a function of y, and take the temperature as a function of time. Those are two real function of a single real variable. How do you distinguish ?
     
  14. Apr 23, 2009 #13
    Thanks for all the replies. I understand this a lot better now.
     
  15. Apr 23, 2009 #14
    Actually both of you are way off...we probably do
    speak different languages and a lot is being lost in the translation...
    let's leave it at that.
     
  16. Apr 23, 2009 #15

    malawi_glenn

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    the wavefunction just tells you the probability density to detect a particle at a certain position with a certain momentum - it has nothing to do with interactions. Quantum Mechanics is not enough to describe particle physics, you need quantum field theory.
     
  17. Apr 23, 2009 #16
    Actually, this raises another question I’ve had, related to time dependence of the wave function.

    Say I generate a single photon from a laser with the laser pointed along the x direction at a known time. I realize QM says that there is uncertainty on the photon’s position over time, as described by the probability density of the wave function. Does the wave function itself evolve over time, or does it remain the same as it was when the photon was generated?

    Not sure if this is the correct way of thinking about it, but I’m thinking of the photon’s wavefunction traveling in the x direction and its probability density being spread along the y-z plane centered along the x axis with a certain variance. With this thought, my question is, does this density function continue along the x axis but with increasing variance along the y-z plane? Or does its variance remain the same as it was at the beginning?

    The reason for my question is that if the wave function expands with time, it seems that the photon would have a higher probability of being absorbed by nearby matter over time or, if not absorbed, in the end it could be just about anywhere in the universe.
     
  18. Apr 24, 2009 #17

    malawi_glenn

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    The wave function evolves with time according to the time dependent Schrödinger equation. How the initial wave package will look after at time T, depends on it's initial shape and the form of the Hamiltonian.
     
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