Basic arithmetic also corroborated

November 22, 2008

I am never surprised to see poor science journalism. As a Socratic friend likes to say, there is no craft of writing about everything, and many journalists are soon out of their depth when they wade into scientific waters.  Nonetheless, some articles are worse than others.  This is pretty bad:

PARIS (AFP) – It’s taken more than a century, but Einstein’s celebrated formula E=mc² has finally been corroborated, thanks to a heroic computational effort by French, German and Hungarian physicists.

A brainpower consortium led by Laurent Lellouch of France’s Centre for Theoretical Physics, using some of the world’s mightiest supercomputers, have set down the calculations for estimating the mass of protons and neutrons, the particles at the nucleus of atoms.

According to the conventional model of particle physics, protons and neutrons comprise smaller particles known as quarks, which in turn are bound by gluons.

The odd thing is this: the mass of gluons is zero and the mass of quarks is only five percent. Where, therefore, is the missing 95 percent?

The answer, according to the study published in the US journal Science on Thursday, comes from the energy from the movements and interactions of quarks and gluons.

In other words, energy and mass are equivalent, as Einstein proposed in his Special Theory of Relativity in 1905.

This is like claiming that the success of the moon-landing finally corroborated the 1/r² nature of the gravitational force. It is true in a sense: if we had been wrong about gravity the mission would have failed, and it didn’t fail.  But it’s not as though there were any doubts about gravity.

In the same way, the massive calculation carried out by these physicists takes it for granted that E=mc².  If it were false they wouldn’t get the right answer, but this is hardly the long-awaited evidence that finally allows us to put our doubts about old Einstein to rest.  Radiation therapies and atomic bombs wouldn’t exist if E=mc² were wrong. The whole history of particle physics is one long affirmation of the equivalence of mass and energy.

It’s too bad that this article so badly misses the point, because the science here is really very interesting.  Quantum chromodynamics (QCD), the theory that describes the strong nuclear force (and therefore the physics of atomic nuclei), is largely intractable using traditional analytic methods, and for decades physicists have thought about simulating physical systems dominated by this force, such as protons, on a computer.  This approach, called Lattice QCD (because the modeling process discretizes, or “puts on the lattice”, space-time coordinates), has been gradually improving over the years.   The basic idea goes like this: A proton can be thought of as a set of three quarks exchanging gluons.  The gluons (which also interact with one another to create a terrible mess) behave something like springs: they bind the three quarks into a ball that we call “proton”. It should be possible to start with the masses of the constituent quarks, model the dynamics of the gluons exchanged between them, and show that the energy of the whole system adds up to the measured energy (or mass) of the proton, or the neutron, or whatever particle is under consideration.  Actually, if I read the paper’s abstract correctly, they have not started directly with quark masses (which for interesting reasons are tricky to define), but have used pion masses to calibrate their calculation.  Still, if the approach is working it is certainly a triumph, but the laurel rightly goes to quantum chromodynamics, not to mass-energy equivalence.


Abstract: Ab Initio Determination of Light Hadron Masses

4 Responses to “Basic arithmetic also corroborated”

  1. Adam Hincks Says:

    The article isn’t on the archive, so I couldn’t take a look beyond the abstract. But doesn’t starting with the pion mass make this less remarkable? Or is the pion mass used as a measure of the quark mass after removing the gluon interaction energy with the lattice calcuation? (Perhaps that’s what they mean by extrapolating to a physical point?)

  2. cburrell Says:

    I wasn’t able to see the full article either; I agree that the abstract alone is too vague. I don’t see how they could have started from the pion and worked up to the proton, so I took the reference to the pion to mean that they used its mass to calibrate their calculation, and then showed that they could calculate the masses of other particles without introducing any new parameters. I’d like to see the full paper.

    Some interesting things seem to be happening in physics these days. You would probably know more than me, Adam. Yesterday I heard about this interesting anomaly in the cosmic ray energy spectrum, which is consistent with decays of Kaluza-Klein particles. We’ll see.

  3. Adam Hincks Says:

    Again, too bad there isn’t more than the abstract (I’m away from the University at the moment where there’s a subscription). I’d first of all be interested in the statistics. Both senarios — a nearby pulsar or dark matter collision — would be neat, but probably hard to corroborate in the near future.

  4. cburrell Says:

    Yes, there are probably a variety of possible explanations for the anomaly. The paper picks on those two. The anomalous bump in the energy spectrum is obvious — it looks like four or five standard deviations at its peak.

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