Tuesday, September 20, 2016

Reframing Fundamental Physics

Even though we have the Standard Model of physics, a lot of it basically isn't used for any applied purpose other than analyzing the debris created when we hurl protons or electrons or atoms at each other at very high speeds in particle accelerators.

The Limited Practical Applications Of QCD

In particular, while will have a full set of equations for quantum chromodynamics (the physics of the strong force), outside the collider context, almost no applications actually use QCD calculations. Instead, nuclear engineers and physicists other than high energy collider physicists use tables that contain the experimentally measured properties of various hadrons and atomic nuclei, because experimentally measurements of all commonly encountered hadrons and atomic nuclei are profoundly more precise than calculations from first principles using QCD in the comparatively low energy circumstances in which we usually encounter them in nature and in practical applications of nuclear physics.

QCD does provide us with a list of essentially all possible pseudo-scalar mesons, vector mesons, baryons, tetraquarks and pentaquarks, but even in helping us to determine all possible hadrons, fails to straightforwardly explain the observed spectrum of scalar mesons and axial vector mesons, and likewise fails to straightforwardly explain why it is that we don't observe pure glueballs with the properties that one calculates from QCD. QCD certainly helps us distinguish between plausible ways of explaining what we see, and implausible ones.  But, it doesn't even definitively provide us with a complete list of all possible hadrons.  Ultimately, in the year 2016, we rely on experimental results and not QCD theoretical considerations to determine that.

The Limited Practical Application of Standard Model Weak Force Calculations And Antimatter

Similarly, tables are generally used, rather than first principles calculations, to predict the weak force decays of hadrons that appear outside colliders (top quarks don't hadronize, but pretty much only appear in the collider context). Indeed, for the most part, the only weak force interactions that ever matters for practical purposes are the weak force decay of down quarks to up quarks in neutrons, also known as "beta decay", and the weak force decay of muons to electrons.

Likewise, while we have an excellent experimental and theoretical understanding of antimatter, in practice, pretty much the only kinds of antimatter encountered outside the collider context, are the electron antineutrino produced in beta decay, the muon antineutrino produced in the decay of muons to electrons, and the tau antineutrino produced when an electron or muon antineutrino oscillates into a tau antineutrino. And, it takes truly extraordinary instrumentation to detect any kind of neutrino (apart from what can be inferred from the missing energy and momentum in an interaction that produces them). To the best of my knowledge, currently existing instrumentation that cannot even distinguish between neutrinos and antineutrinos when a neutrino is directly detected.

Furthermore, while scientists occasionally encounter muons (and even more rarely positrons in cosmic rays) in nature, weak force decays of other fundamental or composite particles such as hadrons other than neutrons, are quite rare outside the collider context. Probably the only one encountered with any frequency are pions and kaons which are again well described based upon experimental evidence.

Indeed, in most contexts, experimental data collapses the weak and strong force to simply describe the decays of various kinds of fundamental and composite particles.

The Limited Practical Application Of Higgs Physics

And, while the Higgs boson is important from a fundamental theoretical perspective for explaining the nature of the mass of the fundamental particles of the Standard Model, in practical applications, the mass arising from the Higgs mechanism in a hadron is so muddied by the mass arising from the gluons in a hadron, that it is irrelevant for hadrons, and in the case of charged leptons, experimentally measured charged lepton masses are known precisely and there is no need to know how these masses actually arise.

Quantum Electrodynamics (QED) Is Used Every Day

Of course, this doesn't address photons (which are described by a part of the Standard Model called QED) which are understood for all practical purposes exactly, or the quantum mechanical motion of individual particles like an electron in the absence of interactions with force carrying bosons.


In summary, for the most part, high energy physics and a large share of the Standard Model, while providing a deeper understanding of the universe and providing the means, in principle, to explain a book full for experimentally measured physical constants, is basically used in no context other than explaining phenomena that pretty much never happen outside of colliders.


While the high energies seen in colliders are seen in only very isolated circumstances in nature today, or non-collider circumstances, this doesn't necessarily mean that they have never been relevant are never relevant.

First, learning the greater complexity provides hints to understanding the aspects of fundamental physics we still do not understand.

Second, in the very early universe, shortly after the Big Bang, there were energies as high as, and indeed, well in excess of, those found in colliders. Understanding high energy physics may shed light on cosmology and astronomy as a result.

Third, there may be high energy physics in isolated circumstances in nature today, such as places near black hole event horizons and supernovas, for example.

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