Proton masses don’t change with time

A new paper has looked into the question of whether the masses of particles change with time. That this may be an issue may have not occurred to people (at least it had not occurred to me) but when you think about it, it is a valid concern: Are the masses of particles the same now as they were at the beginning of the universe?

While the masses of elementary particles like the electrons, quarks, and neutrinos seem unlikely to change with time, it is not so obvious for composite particles like the proton. The masses of these composites are largely made up of the energies of interactions of their constituent quarks and gluons and if the strengths of those forces change with time, then couldn’t the masses also change? This is a question with considerable import since the models of nucleosynthesis following he Big Bang depend on the masses of protons and neutrons.

As with many empirical questions in science, ingenious experimentalists have found a way to get at the answer. A team of physicists have analyzed 17 data points from ten different absorption spectral lines of the methanol molecule CH3OH from a distant galaxy PKS1830–211 and used that to deduce the ratio of the proton mass to the electron mass of those molecules. The galaxy has a redshift of z=0.89 which means that the light was emitted about 7.5 billion years ago. What the researchers find is that mass ratio has changed by less than one part in 10 million compared to the present time.

For those who have access to the journal Physics Review Letters, the paper appeared in the December 6, 2013 issue PRL 111, 231101 (2013) and can be seen online here.


  1. oualawouzou says

    There are days I wonder why more people do not realize how ridiculously cool science can be.

  2. Alex says

    That’s really reassuring, and what a wonderful new confirmation that we really do understand with great precision the laws of nature which hold at the opposite end of the observable universe. I have often heard Astronomers and Astrophysicists being accused of arrogance because of allegedly presumptuous statements about the laws of nature at remote places, and the fact that we can do this kind of consistency tests at all is really a powerful rebuttal of this accusation.

    Although I don’t share your judgement that change in proton mass is a more plausible candidate than elementary particle masses. The former is a function of the strength of the QCD interaction (albeit a complicated one) and the latter of the strength of the Yukawa Interaction. That’s not a big qualitative difference for me. Also, since all these quantities are not independent, and the observed value of the Yukawa interaction becomes a function of the strong coupling because of renormalization, and vice versa, I would be additionally surprised if one changed, but not the other.

  3. says

    The mass of a proton does not change… as long as it stays a proton.. Protons can become neutrons and vice versa, and neutrons are slightly more massive than protons. I believe it would be more correct to say that the mass of a proton does change (by way of electron capture) but it ceases to be a proton when this happens.

  4. Pierce R. Butler says

    … mass ratio has changed by less than one part in 10 million compared to the present time.

    That seems roughly in line with the proton decay hypothesis:

    One of the implications of the grand unification theories is that the proton should decay with a half-life on the order of 10^32 years.

    These, as cosmologist Carly Simon has advised us (confirmed by Dr. Pink et al.), are the good old days.

  5. Pierce R. Butler says

    If my protons weren’t decaying, I wouldn’t’ve screwed up the html in # 5: only the half-life statement belongs in the blockquote.

  6. says

    It’s because they’re not educated enough to understand and unwilling to put in the work to learn. Claiming “godidit” is so much easier than reading a bunch of science books.

  7. colnago80 says

    The decay of a proton was mentioned in Phil Plait’s second book, Death from the Skies but it wasn’t clear what a proton would decay into. Since there is no lighter baryon AFAWK, a proton decay would violate baryon conservation.

  8. Alex says

    That’s correct. Also, since the stuff it decays into must contain leptons or neutrinos, you also have to violate lepton conservation (unless there are other new particles that we don’t know about). Grand unified theories usually violate both because of how the unified force carriers interact (see e.g.,), the question usually is only how strongly. What is usually conserved is the difference between baryon and lepton number, but this is not enough to protect the proton.

  9. Alex says

    Concerning the decay products –
    In most Grand unified theories, Protons are predicted to decay mostly into either a neutral meson (Pion, Kaon…) and a positron, or a positive meson (Pion, Kaon…) and a neutrino. You can surely imagine more complicated decays, anything that you can dream up which preserves energy, momentum, spin, electrical charge and color charge, could in principle happen.

  10. oualawouzou says

    I don’t pretend to understand it all myself (though I have *some* science education), but the simple fact that by looking at something extraordinarily big, we can better undestand something that is extraordinarily small feels me with more awe than a wafer purported to both be and not be flesh at the same time.

  11. Jared A says

    I’m not a cosmologist, so I could be misunderstanding, but it seemed like the conclusion was that they did not measure any significant change in proton mass, which gives an upper limit of mass change of 1 part per 10 million based on their estimation of their systematic errors.

    So it doesn’t disprove the proton decay hypothesis, but neither does it support it.

  12. Rob Grigjanis says

    If there’s a link between proton decay and (very slightly) varying proton mass, I’m not seeing it.

  13. lpetrich says

    A big problem: what spectral lines did they use as a reference? I read the paper at arxiv, and I couldn’t find out. Here’s how they vary as a function of particle masses and other parameters:

    Electronic (electron orbital): (me*α^2)
    Fine-structure (electron spin-orbit and spin-spin): (me*α^2) * α^2
    Hyperfine (electron-nucleus spin-spin): (me*α^2) * α^2 * (me/mN)
    Molecular vibration: (me*α^2) * (me/mN)^(1/2)
    Molecular rotation: (me*α^2) * (me/mN)

    α = fine structure constant
    me = electron mass
    mN = nucleon average mass

    These are, in turn, dependent on various other Standard-Model parameters:
    α — electroweak gauge couplings g1, g2
    me — Higgs vacuum energy * Higgs coupling to electron
    mN — QCD energy scale

  14. Jared A says

    The use of radio telescopes and the following phrase suggest rotational transitions:

    “It was recently pointed out that the interplay between the internal and overall rotation in the methanol molecule (CH3OH) results in speci c transitions having an enhanced sensitivity for a possible drift in mu [9, 10]. Some of these transitions involve low lying rotational energy levels populated at the low temperatures characterizing the bulk of the interstellar molecular gas.”

  15. colnago80 says

    Well, in order to conserve angular momentum, one of the decay products of a proton must be a spin 1/2 particle. Obviously positrons and neutrinos qualify.

  16. Rob Grigjanis says

    The lines are in Table 1 of the paper (click on PDF in the upper right corner of Jared’s link).


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