Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

No mass *trons and early mass?

  1. Jun 16, 2011 #1
    I've been reading about the early universe (The book: The First Three Minutes) and it prompted a few questions:

    Is it right that a dominant part of the early universe was electrons and positrons?

    Given that electrons and positrons were a big part of the early universe, was there some early process whereby they ended up becoming masses (or just the positrons did)? I'm trying to understand how the balance in numbers of electrons, protons and neutrons came about.

    I find it difficult to get my head around a particle having no mass (photons). Is it that their mass is so insignificantly small, compared to protons and neutrons, that their mass has no measurable effect? Or is it that they actually do have zero mass?

    Edited to correct something I misread. Apologies.
    Last edited: Jun 16, 2011
  2. jcsd
  3. Jun 16, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I think that's right: http://en.wikipedia.org/wiki/Lepton_epoch

    Electrons and positrons do have mass. Particles with mass can be created out of pure energy: E=mc2.

    The universe has zero total charge, and charge is conserved. One example of a mixture of particles that has zero total charge is a mixture of electrons, positrons, neutrinos, and photons (as in the lepton epoch). Another example is a mixture of protons, neutrons, electrons, neutrinos, and photons (the part of today's mix that we understand well). Mix #1 existed at early times because the temperature was high enough to produce electron-positron pairs spontaneously.

    Empirically, we can set a very low limit on their mass:
    R.S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", Physical Review Letters 80 (1998) 1826, http://silver.neep.wisc.edu/~lakes/mu.html
    Note that massless particles still gravitate, basically because their energy is equivalent to some mass by E=mc2. There was an era when gravity in our universe was mainly due to photons: http://en.wikipedia.org/wiki/Radiation-Dominated_Era

  4. Jun 18, 2011 #3
    Thanks Ben, thanks for your comprehensive reply. :)

    Do we know this for certain? My memory of chemistry is that unstable chemicals change often because of movement of uneven charges. (I recall some man made chemicals were difficult to maintain for this reason.) Could it be that the BB had some provocation from such an imbalance?

    I've often wondered about this process. Is it quantum machinery that does this? (Trying to form a picture in my mind of how it might happen - energy pushing quantum particles together or some such.)

    I noted from the link in that article to Leptons, this quote: "Charged leptons can combine with other particles to form various composite particles such as atoms and positronium."

    Which made me wonder. If Leptons were the dominant species, then there were fewer "other particles" at that time. Which means more "other particles" must have been created by the process you mentioned, mass out of energy. Is that right? Was that the annihilation process?
  5. Jun 18, 2011 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member


    No, it's a purely classical (i.e., non-quantum mechanical) fact.

    The main change has not been that leptons and antileptons have been annihilated with each other.

    The questions you're asking make me think you could get a lot out of reading The First Three Minutes.

  6. Jun 18, 2011 #5
    hehe.. I'm reading it, a dozen pages at a time.. but you're right
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook