4
$\begingroup$

Sabine Hossenfelder in her column The End of Theoretical Physics As We Know It made the following statement:

For example, we have a perfectly fine theory that describes the elementary particles called quarks and gluons, but no one can calculate how they come together to make a proton.

This is confusing. How can QCD not explain how to "build" a proton or neutron? Specifically what I am thinking of is our ability to calculate the neutron and proton masses with a precision of 0.03%. Can anyone please explain what she may have meant by this statement?


Update

A new result, which makes use of lattice QCD, shows that only 9% of the proton’s mass comes from the mass of the constituent quarks. The remaining ratios are due to the energy of the quark-gluon "soup" inside the proton as well as scale invariance. You can read more at Proton Mass Decomposition from the QCD Energy Momentum Tensor.

I remember Frank Wilczek making the point in his book A Beautiful Question that results of this type seem to suggest that what we think of as solid matter is really vast quantities of condensed energy.

$\endgroup$
  • $\begingroup$ I think she refers to the process of ''Quark Confinment'' which is the reason for why Quarks are bound together. However, there is (if I remember correctly) no theoretical explanation for this process (this problem is, if I remember correctly, closely related to one of the millenium problems in mathematics (Mass Gap in Yang Mills theories)). $\endgroup$ – Creo Sep 5 '18 at 12:04
  • 5
    $\begingroup$ This may be a matter of semantics worth shrugging off, to avoid technicalities. Lattice simulations do calculate hadron masses from QCD, but not arbitrarily accurately and without the intuitive insights that analytical solutions have spoiled practitioners to expect. To give you an analogy, a numerical simulation of an electrostatic system is not quite as good as an analytical image charges solution, but it is still acceptable in engineering applications. Hankering for elegant and precise QCD results should not impede acceptance and appreciation of QCD's completeness, power and correctness. $\endgroup$ – Cosmas Zachos Sep 5 '18 at 13:27
  • $\begingroup$ In Sabine's article, the sentence directly after the quoted sentence is: "The equations just can’t be solved by any known methods". That statement isn't contradicted by a lattice QCD calculation of the proton and neutron masses is it? $\endgroup$ – Hal Hollis Sep 5 '18 at 13:37
6
$\begingroup$

It is a type of nitpicking , imo, and just the title "The End of Theoretical Physics As We Know It " shows a person with a biased view on theoretical physics.

The abstract of the research you quote can be seen here and the accurate numbers have to do with the mass difference between proton and neutron,

The existence and stability of atoms rely on the fact that neutrons are more massive than protons. The measured mass difference is only 0.14% of the average of the two masses. A slightly smaller or larger value would have led to a dramatically different universe. Here, we show that this difference results from the competition between electromagnetic and mass isospin breaking effects. We performed lattice quantum-chromodynamics and quantum-electrodynamics computations with four nondegenerate Wilson fermion flavors and computed the neutron-proton mass-splitting with an accuracy of 300 kilo–electron volts, which is greater than 0 by 5 standard deviations. We also determine the splittings in the Σ, Ξ, D, and Ξcc isospin multiplets, exceeding in some cases the precision of experimental measurements.

unfortunately it is not an open link to the paper.

The calculations of the masses themselves have less accuracy and are works in progress in lattice QCD, here is a review.. My answer here discusses the situation.

So the statement " no one can calculate how they come together to make a proton" is placing very strict mathematical criteria for "calculate", in physics. If it is not a simple formula, it is of less value, denigrating the use of computers in theoretical physics calculations. In this frame of mind, lattice qcd calculations are not theoretical physics, because they use computers and not elbow grease to get at the numbers.

The quote of "shut up and calculate" was a saying long before supercomputers, and certainly needed elbow grease. Ihe statements: "The End of Theoretical Physics As We Know It " and " but no one can calculate how they come together to make a proton" can be paraphrased for novelists:

"The end of novel writing as we know it" and "no one can write longhand a manuscript for a novel". Confusing labor saving devices with creativity.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.