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Relative Masses of 7-Billion-Year-Old Protons and Electrons Confirmed to Match Those of Today’s Particles

The views expressed are those of the author and are not necessarily those of Scientific American.

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100-m dish of Effelsberg radio telescope

Courtesy MPI Bonn

The mass of the proton in relation to its much lighter counterpart, the electron, is known to great precision: the proton has 1836.152672 times the mass of the electron. But has it always been so?

Quite possibly, according to new research which taps the cosmos as a vast fundamental-physics laboratory. A study of a distant galaxy strongly suggests that the proton-to-electron mass ratio, denoted by the Greek letter mu (µ), has remained essentially constant for at least half the age of the universe. The findings appeared online December 13 in Science.

Julija Bagdonaite of VU University Amsterdam and her colleagues used the 100-meter Radio Telescope Effelsberg [above left] to measure the absorption of radiation by methanol, a form of alcohol, in the ancient universe. Methanol (CH3OH) imprints multiple absorption lines on a spectrum of light, owing to the molecule’s various rotational states, and the interplay among those states depends on the relative masses of the constituent electrons and protons.

The researchers detected methanol absorption lines in a so-called gravitational lens system called PKS1830-211—a chance alignment of a faraway galaxy backlit by an even more distant source of radiation. The foreground galaxy lies at redshift 0.89, meaning that its light has traversed the cosmos for roughly half of the 13.7 billion years that have elapsed since the big bang. (Redshift is an empirical measure used to gauge vast cosmic distances.) The galaxy’s gravity bends radio waves from the background object—a flaring supermassive black hole called a blazar—and imprints its own spectral signature in the process, due to the presence of methanol and other molecules in the galaxy. “The absorptions of the radio waves have occurred seven billion years ago,” Bagdonaite said in a prepared statement. “And the radio waves traveling to Earth carry the fingerprint of the methanol molecules in the distant past.”

She and her colleagues determined that the methanol in the PKS1830-211 system behaves just as predicted—the electrons and protons in the ancient molecules had relative masses indistinguishable from those measured in laboratories on Earth today. The value of mu seven billion years ago, they concluded, could not have differed from the present-day value by more than 0.00001 percent.

Any divergence from a stable proton–electron relation would invalidate a core assumption of physics—that the laws of nature remain the same wherever (or whenever) you look. So it’s reassuring to know that—in one key respect, at least—the more the universe changes, the more it remains the same.

About the Author: John Matson is an associate editor at Scientific American focusing on space, physics and mathematics. Follow on Twitter @jmtsn.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. rloldershaw 5:28 pm 12/13/2012

    Into the rubbish bin go all the speculative nonsense about variable constants and particle masses.

    Now would you like to see the proton, neutron and (for the first time ever) the electron masses retrodicted using basic General Relativity and Electromagnetism (i.e., the Einstein-Maxwell equations in Wheeler’s Geometrodynamics form)?

    Just go to and see the latest two “New Developments”.

    Why are journalists always talking about dubious model-building (at best) like string theory, supersymmetry and the egregious quantum chromodynamics, when a far more unified and exquisite paradigm for all of nature is left ignored for 35 years?

    Let’s get real for a change and leave the Platonic pseudo-science to the fashionistas who that have hyped it for 40 years.

    Robert L. Oldershaw
    Discrete Scale Relativity

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  2. 2. jtdwyer 7:16 pm 12/13/2012

    SA Editors – will someone stop the ‘lamar-bot’ from eating your comments? BTW – ‘lamar’ is not a person…

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  3. 3. jtdwyer 7:48 pm 12/13/2012

    Very interesting findings regarding a critical fundamental question. Nice report, also. Searching for a freely available preprint version of the referenced research report (thanks), I was unsuccessful but did find another good report with additional details at:

    I did find an earlier free research report from this principal author, Julija Bagdonaite, that reports on the same findings for a directly viewed (much nearer) quasar whose light was absorbed by molecular hydrogen clouds. I expect the methodology is essentially the same. Please see:
    “Constraint on a variation of the proton-to-electron mass ratio from [Molecular hydrogen] absorption towards quasar Q2348-011″,

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  4. 4. europamoon100 3:56 pm 12/14/2012

    Now, someone (rloldershaw) wants to object to quantum chromdynamics, a theory on the microscopic scale that has been verified over and over again by experiments in nuclear physics laboratories? What a bunch of rot his comments are.

    Quaantum chromodynamics is the theory of up quarks, down quarks, strange quarks, charmed quarks, top quarks, bottom quarks, and gluons, and these have been observed over and over again in particle accelerators at the Fermilab, Stanford University, and CERN. I don’t see how anyone can object to quantum chromodynamics except for someone who is somewhat odd.

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  5. 5. europamoon100 4:44 pm 12/14/2012

    Quoting: “Methanol (CH3OH) imprints multiple absorption lines on a spectrum of light”
    This is not entirely incorrect or entirely correct.
    Since the observations in question were made by a radio telescope, the absorbtion lines that were observed are in the radio spectrum (microwaves) and not in the spectrum of visible light. That should have been emphasized.
    In reality, methanol and similar compounds also have many absorbtion lines in the infrared spectrum, but none in visible light. This is the reason why alcohol vapor looks transparent.
    Another way of stating the sentence is “Methanol (CH3OH) imprints multiple absorption lines on the electromagnetic spectrum.”
    Then, if you want to call all kinds of electromagnetic radiation “light”, that is roughly correct, but precison would call for “electromagnetic radiation” since radio waves (microwaves) were involved.

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  6. 6. jtdwyer 6:06 pm 12/14/2012

    europamoon100 – Well put. However, natural light emission sources most often include both visible and non-visible spectra – we might only see the narrow range of visible spectra, but we can certainly feel the heat of infrared spectra and suffer burns from UV and higher wavelengths.
    “In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.”

    While in some instances precision might lead one to refer to light that includes non-visible spectra as EMR, the greater clarity is gained by referring to visible light spectra as ‘visible-light’ and other spectra as their common classifications of radio, microwave, infrared, ultraviolet, X-rays and gamma rays.

    In common practice, I think the term ‘light’ includes broad spectrum EMR emissions. Usually only in photographic imaging is it critical to distinguish the commonly recorded spectra of ‘visible’ light from broad spectrum EMR emissions.
    Excellent point that transparent elements do not produce visible light spectral lines – thanks!

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  7. 7. jgrosay 7:18 pm 12/14/2012

    A question of an illiterate in physics: if the ratio of mass between an Electron and a Proton is a “fundamental constant” that can’t be deducted but must be discovered by observation, are we sure that the e=mc2 is not a fundamental constant, we are now in the situation of checking this and other postulates of General Relativity
    by experiments, but did A Einstein arrived to the equation that states the relationship between matter and energy by intuition or deduction? Is e=mc2 a “fundamental constant” or is not? If it is, how did Albert E obtain this equation?

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  8. 8. J Hanford 9:18 pm 12/14/2012

    “Excellent point that transparent elements do not produce visible light spectral lines”

    Um, oxygen, nitrogen, hydrogen and helium all have lines that occur in the visible-light portion of the electromagnetic spectrum (the hydrogen-alpha line at 656.3nm is quite prominent in many emission nebulae, for example).

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  9. 9. jtdwyer 10:02 pm 12/14/2012

    J Hanford – Well, that’s a good point, but doesn’t that make them, at least to some extent, non-transparent?

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  10. 10. American Muse 5:21 am 12/15/2012

    If the proton/electron mass ratio is shown to remain constant through time, that is all it shows. It does not mean that matter remains the same throughout time. If matter evolves (like everything else does), then the findings simply mean that proton mass changes by the same proportion as electron mass.

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  11. 11. jtdwyer 7:31 pm 12/15/2012

    But if there are unvarying universal constants, which this result implies, then how does matter fundamentally evolve while maintaining a constant relation between protons and electrons? Certainly elements are recombined to form more massive atoms through progressive fusion, but these findings may be evidence that spacetime expansion and contraction do not in any way affect matter – only spacetime. That’s just one example – there may be others you are unaware of.

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  12. 12. Damir Ibrisimovic 4:50 pm 12/16/2012

    Dear rloldershaw,

    I would not jump to the conclusion about constancies of constants. Have a look at in which direction PKS 1830-211 is placed.

    Our universe is full of surprises:

    Now, as fine structure constant (α) changes, a lot of other constants change. Ratios between them might be “constant”, but this waits to be confirmed. At the moment, the gravitational lens PKS 1830-211 does not allow for sufficient precision in measurements. (Have a look at what gravitational lenses actually do.)

    Have a nice day,
    Damir Ibrisimovic

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  13. 13. Daniel35 8:29 pm 12/31/2012

    But aren’t these the same kind of wavelengths that tell us how fast the light source is moving away, therefore how far in time and space? Does this not lead to confusion? How can we tell whether red shift is dependent on speed away from us or on changes of relative masses over time, or both?

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