Originally shared by John Baez
The crisis in theoretical physics
The Italian physicist Gian Francesco Giudice has come out with a great article about the crisis in theoretical physics. Here are some quotes, with comments to help nonexperts:
There are many indications that, following the recursive pattern of scientific revolutions, we are now witnessing the beginning of the phase of crisis. The lack of new physics in the initial stages of the LHC project is putting into question the logic of naturalness when applied to the Higgs; the absence of a positive detection in dark matter experiments is casting doubts about nature taking advantage of the WIMP miracle. We are not simply confronted with experimental data excluding a model or a class of models. We are confronted with the need to reconsider the guiding principles that have been used for decades to address the most fundamental questions about the physical world. These are symptoms of a phase of crisis.
The LHC is the Large Hadron Collider, which so far has not discovered any physics beyond the Standard Model. Our current thinking says it takes some ‘fine-tuning’ of parameters to make the Higgs boson as light as it is. The philosophy of naturalness seeks to avoid such fine-tuning, and pushes physicists toward theories that explain the lightness of the Higgs in some more systematic way.
One way is called supersymmetry, and supersymmetric theories generally predict the existence of weakly interacting massive particles, or WIMPs, that could explain dark matter. The WIMP miracle is that theese theories can explain the light mass of the Higgs and dark matter in one blow! Physicists been excited about this for several decades. Unfortunately we have not detected any WIMPs… so nature may not be as fond of this miracle as physicists have been!
The Standard Model of particle physics is a superb monument attesting to the inner beauty of nature and the power of human logical deduction. It is astounding how natural phenomena, in all their complexity, can be summarised by a single principle – the gauge principle – and described by a compact set of equations. And it is equally astounding how humans have been able to crack this secret. Along this path, the synthesis of general relativity with the physical laws derived in the microworld has led to the ΛCDM model, which can successfully describe a huge array of cosmological observations, the present large-scale structure of the universe and its early history, in terms of a handful of parameters. This is today’s consolidated normal science.
The gauge principle says, in simple terms, that you can only tell if two particles are in the same state if you move them next to each other so you can compare them. Working out the mathematical consequences of this principle leads to gauge theories which explain the forces we see in nature.
The ΛCDM model is the current theory of the Big Bang and the expanding universe. To fit the data, this theory requires cold dark matter (that’s the CDM) and also dark energy (whose density is described by a constant called the cosmological constant or Λ).
And yet, this superb monument of knowledge is insufficient to address some fundamental questions. The Standard Model is incapable of shedding light on the dynamics underlying electroweak symmetry breaking or explaining the structure of quarks, leptons, and their mass pattern at a fundamental level. The theory of inflation, in spite of its stunning conceptual successes, could not be linked univocally with a unified theory of particle physics. Moreover, the ubiquitous phenomenon of eternal inflation has changed the perspective on the outcome of an inflationary universe and its properties. We have plausible explanations for the cosmic baryon asymmetry, but we lack any conclusive empirical confirmation. The nature of dark matter is still unknown. The observed value of the cosmological constant is hard to reconcile with the rules of effective field theory, and quantum gravity is still beyond our grasp.
This is a laundry list of famous open problems.
We don’t know why there are as many elementary particles called quarks and leptons as we see. We don’t know why they have the masses they do.
The inflationary cosmology, which predicts very rapid expansion of the very early universe, has some evidence in favor of it – but to work it requires new particles we haven’t seen. Thus, it can’t be ” linked univocally with a unified theory of particle physics”. In many theories of inflation, patches of the universe expand rapidly, like bubbles of boiling water, while other patches do not… and in many such theories this bubbling goes on forever: that’s called eternal inflation.
Cosmic baryon asymmetry is a fancy way of saying there’s a lot of matter in the universe but almost no antimatter. We have plausible theories for why this is true, but we don’t know if they’re right – there’s no “conclusive empirical confirmation”.
The cosmological constant is very small, and like the light mass of the Higgs boson this is unexplained, and seems to require fine-tuning.
Finally, we don’t know how to combine gravity and quantum mechanics. String theory tries to do this, but we don’t know if it’s right, and it has problems.
None of these problems are new, and theoreticians have been tackling them for decades. What is changing is the feeling that the paradigm that so successfully led to the Standard Model may not be the right tool to make further progress. There is a widespread sensation that the organising principles based on symmetry and separation of scales, which follow from an effective quantum field theory approach, in spite of their triumphs, must be superseded by new organising principles. Physicists are in search for new conceptual paradigms, which is another symptom of a phase of crisis.
Very briefly, effective quantum field theory says how the laws of physics in the micro-microworld can look different from the laws in the microworld or the macroworld. This is the separation of scales.
The author goes into much more detail, and for me that’s the good part. Skipping ahead many pages… he ends on a note of stirring optimism. We are lucky to be alive at a time when there are so many mysteries to solve!
When I started studying physics, I fell in love with the era when quantum mechanics was born. An era of confusion, inexplicable experimental results and crazy theoretical ideas, rebellion against the sacred laws of physics and heated debates at the Solvay conferences. How much I wished I could have been there: I missed my chance, I thought.
As I advanced in my studies, I was captivated by the excitement of the post-war period, when physicists had to deal with untamable infinities and a zoo of particles, produced by cosmic rays or popping out from pioneering accelerators, and whose meaning was obscure. The era of Shelter Island and the birth of QED, the attempts with Regge trajectories, S-matrix, bootstrap to end up with gauge quantum field theory. How much I wished I could have been there: I missed my chance, I thought.
Well, now I realise that my chance is today. As a scientist, I have the privilege to live in a new era of crisis. Ideas thrive in the periods of crisis dominated by uncertainty and confusion, when physicists are in search of a paradigm change able to deal with the puzzles they are confronted with. There is no lack of open fundamental questions we must tackle today: the nature of the Higgs boson, the structure of quarks and leptons, inflation, the cosmic baryon asymmetry, dark matter, dark energy, quantum gravity, and more. But there is also a widespread feeling that our theoretical tools – which have been so successful in bringing particle physics to its present stage of maturity – are becoming inadequate to address the next layer of open questions. A new paradigm change seems to be necessary.
Experimental physics is reacting to the present status of crisis with a broad and ambitious program that will enable humanity to cross the borders of knowledge on many fronts. Theoretical physics is exploring new directions and looking beyond the boundaries of traditional particle physics, across different disciplines. Revolutions in science don’t happen overnight: it took thirty years for quantum mechanics to develop from Planck’s black-body radiation to Dirac’s equation; twenty-five years for the Standard Model to go from QED to the asymptotic freedom of QCD. We can’t expect to find all answers today. But we are experiencing all the right symptoms – unresolved fundamental questions, an old paradigm that seems to run out of mileage, bold experimental projects, revived theoretical curiosity – that indicate we are living in the dawn of a new era.
Read the whole thing here:
• Gian Francesco Giudice, The dawn of the post-naturalness era, https://arxiv.org/abs/1710.07663
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