I was puzzled when I read "Precise measurements find a crack in universal physics by Ingrid Fadelli" (Phys.org, Jan. 15, 2020). The article has some vague statements in the opening paragraphs about "universal physics":

The concept of universal physics is intriguing, as it enables researchers to relate physical phenomena in a variety of systems, irrespective of their varying characteristics and complexities. Ultracold atomic systems are often perceived as ideal platforms for exploring universal physics, owing to the precise control of experimental parameters (such as the interaction strength, temperature, density, quantum states, dimensionality, and the trapping potential) that might be harder to tune in more conventional systems. In fact, ultracold atomic systems have been used to better understand a myriad of complex physical behavior, including those topics in cosmology, particle, nuclear, molecular physics, and most notably, in condensed matter physics, where the complexities of many-body quantum phenomena are more difficult to investigate using more traditional approaches.

Understanding the applicability and the robustness of universal physics is thus of great interest. Researchers at the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder have carried out a study, recently featured in Physical Review Letters, aimed at testing the limits to universality in an ultracold system.

A Web search for the (seemingly redundant) phrase "universal physics" turns up a mix of things. Some stuff appears to be unrelated. Some stuff that turned up was some pseudoscience nonsense rejecting relativity and quantum mechanics in favor of continued belief in Newtonian physics. An occasional result did appear to be related to the Efimov trimer state stuff described in the PRL paper the article discusses.

("Precision Test of the Limits to Universality in Few-Body Physics" Roman Chapurin, Xin Xie, Michael J. Van de Graaff, Jared S. Popowski, José P. D’Incao, Paul S. Julienne, Jun Ye, and Eric A. Cornell Phys. Rev. Lett. 123, 233402 – Published 2 December 2019 -- arXiv preprint here, synopsis here.)

The PRL paper the article is about does not itself include the term "universal physics". However, one of the co-authors of the journal paper is quoted in the phys.org article as saying "Universal behavior is independent of the microscopic details. Understanding the limitations of universal phenomenon is of great interest." And the paper does discuss things like "van der Waals universality".

So what is a decent definition of "universal physics" in this context? (If possible, in terms that might make at least a little sense to a layman.)

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    $\begingroup$ I think you'll have better luck searching for "universality" rather than "universal physics". This refers to systems which may have very different microscopic structure, but exhibit similar global properties. Systems near a second-order phase transition for example exhibit universality, and have the same critical exponents. $\endgroup$ – Clara Diaz Sanchez Jan 15 at 22:01
  • $\begingroup$ @ClaraDiazSanchez Thank you. If that's the more idiomatic wording for the same concept being invoked by the article, I think saying so (along with essentially the description you just gave) should be acceptable as an actual answer. (For the example you gave, it might be helpful to future readers to either expand a bit or link to en.wikipedia.org/wiki/Phase_transition#Modern_classifications and en.wikipedia.org/wiki/Critical_exponent .) $\endgroup$ – Jacob C. says Reinstate Monica Jan 16 at 20:27
  • $\begingroup$ I have had a go @jacob-c-supports-gofundmonica - let me know what you think. $\endgroup$ – Clara Diaz Sanchez Jan 18 at 8:56
  • $\begingroup$ @ClaraDiazSanchez Excellent. $\endgroup$ – Jacob C. says Reinstate Monica Jan 22 at 2:54

Having read the linked article "Precise measurements find a crack in universal physics by Ingrid Fadelli" it does indeed seem to be the case that it concerns the property known generally in physics as “universality”, which has rather unhelpfully been termed “universal physics - as the OP noted “universal physics” gives a fair amount of pseudo-scientific garbage when you google it, but “universality” is much more well-defined and helpful. I can only assume that the article writer/editor thought that “universal physics” sounded more impressive.

The concept of universality comes originally from the study of phase transitions in statistical mechanics. Conventional phase transitions are described by an order parameter. For example, as a ferromagnet is heated, at the Curie temperature its magnetism is lost; the magnetisation is the order parameter in this case, and the Curie temperature is an example of a critical point. Near the critical point the system’s behaviour becomes scale invariant, and can be described by a very small set of parameters. The decay of correlation function, for example, have a power-law form governed by a critical exponent.

It was found that very different systems could have the same critical exponents. So that, in a sense, near the critical point the system’s behaviour does not depend on its microscopic details. This is described in a more rigorous way by the renormalisation group, which can be used to classify systems into various “universality classes". All members of a given universality class have the same critical behavior. A consequence of this is that you can use one system in a given class to simulate another. This is especially useful if one of the systems is relatively easy to control and measure, and the other is less amenable. The example given in the article is of cold atom systems - which are extremely controllable - being used to study Efimov physics.

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