Every once and a while it is appropriate to step back and look at the big picture.
Where are there prospects for new discoveries in physics, in my lifetime and perhaps the lifetime of my children (i.e. 40-60 years)?
There are several areas where I think the likelihood of progress is good.
Dark Matter, Dark Energy and Quantum Gravity
I am optimistic that science will solve the problems of dark matter, dark energy and quantum gravity in my lifetime, or baring that, within the next 40-60 years.
My biggest reason for optimism is that the quality, diversity and sheer quantity of our astronomy data collection in the last few decades is incredible and continues to improve. There are obvious ways, with improved gravitational wave detection devices and space telescopes, for example, that we can continuously continue to improve the quality of our astronomy data without major technological breakthroughs on a 40-60 year time horizon.
We also have lots of scientific resources devoted to the effort to understand these issues, a widely shared belief within the scientific community that they are among the most important unsolved scientific questions, a healthy diversity within the scientific community of approaches to answering these questions, and considerable sophisticated computational and analytical resources to address it.
I strongly suspect that the ultimate conclusion will be that dark matter and dark energy phenomena arise from flaws in either how we operationalize general relativity, or from quantum gravity effects of some kind. This will almost exclusively be in very weak gravitational fields as second or lower order effects. I do not think it is likely that the strong field predictions of general relativity will require any modification.
These considerations may or may not resolve the Hubble constant measurement tension (which could also simply be a matter of experimental methodology - this isn't the first time, historically that there has been a Hubble constant measurement tension and previous tensions have been resolved with improved measurements).
Once we figure this out, questions like galaxy and galactic cluster formation, cosmological inflation, and matter creation will have to be reconsidered.
New astronomy observations will also highly constrain these theories and rule of many, if not most, of the theories that are popular right now in these areas of cosmology.
I strongly suspect that a correct solution of the dark matter and dark energy questions will resolve almost all of the anomalies evaluated relative to the lambdaCDM model of cosmology that have been accumulating with respect to galaxy and galactic cluster formation and large scale structure.
I strongly suspect that continued progress in ruling out beyond the Standard Model particle physics and in astronomy will lead to a theory of matter creation in which the period before the Big Bang is antimatter dominated and balances out our matter dominated universe becoming the conventional wisdom.
I suspect that progress in quantum gravity and our understanding of dark matter and dark energy, along with new astronomy data, will highly constrain theories of cosmological inflation, but I am agnostic over whether this will render that part of cosmology obsolete, or will simply clarify its nature. I don't think that the question of cosmological inflation will have a consensus final resolution half a century from now.
We are well on our way to filling out the last not well-explored corner of Standard Model physics, which is the physics of neutrinos.
The Standard Model has seven experimentally physical constants pertaining to neutrinos - three neutrino masses and four PMNS matrix parameters. We have precisely measured the two differences between the neutrino masses and two of the four PMNS matrix parameters.
We have reasonably precisely measured a third PMNS matrix parameter and are close to being able to discriminate between two alternative possible values for it (a bit more than 45º or a bit less).
We are close to determining the hierarchy of the neutrino masses and to better constraining the absolute values of the three neutrino masses (which is a single measurement once the mass hierarchy is determined).
The fourth PMNS matrix parameter (the CP violating phase) is the least precisely measured experimentally determined constant in the Standard Model, but we've pretty much ruled out the possibility that it is zero and are close to having a much better measurement of it.
We are making good progress at constraining the possibility that neutrinos have beyond the Standard Model properties as well. Experimental progress in constraining "non-Standard neutrino interactions" (NSI), in confirming or ruling out the existence of "sterile neutrinos" that oscillate with ordinary neutrinos in an extension of the PMNS matrix but don't interact via the weak force, the search for right handed neutrinos and left handed antineutrinos, and in constraining the extent to which neutrinoless double beta decay occurs if it occurs at all.
The last point is also pertinent to determining the mechanism by which neutrinos acquire mass. I strongly suspect that naive Majorana mass theories will be ruled out within the next decade or two. But I am less optimistic that an affirmative solid theoretical explanation for how neutrino mass arises will be found.
We are also gaining a more precise descriptive understanding of the "neutrino background" from various sources, the sources and nature of high energy neutrinos in cosmic rays, the details of how neutrinos are produced in events like supernovas and mergers of neutron stars, and the mix of different types of neutrinos in the universe as a whole which is important to understanding big picture cosmology issues. For example, we are likely to gain a much more precise determination of the ratio of neutrinos to antineutrinos in the universe within the next half century which is very pertinent to understanding the process by which matter was created in the wake of the Big Bang.
There are a number of anomalies that are in tension with the Standard Model of Particle physics, many of which will probably be resolved in the next decade or two.
One of the most prominent is the muon g-2 anomaly which two high precision measurements of the quantity in the next few years should greatly help clear up up, either by resolving the tension between experiment and theory in this high precision measurement which is sensitive globally to most significant differences that could exist between the Standard Model and reality at energy scales low enough to be experimentally probed someday. I strongly suspect that the new experiments will eliminate or reduce the tension between the current state of the art experimental measurement and the current state of the art theoretically calculated value.
Another of the most prominent anomalies is the apparent existence of charged lepton universality violations great than those expected at a higher loop level due to interactions with neutrinos, whose different flavors do not have identical properties apart from their masses (as the PMNS matrix quantifies). I strongly suspect that these anomalies will also disappear as experiments get better.
A third anomaly is the inferred X17 boson which I strongly suspect will eventually be ruled out or explained with conventional means through further analysis.
A fourth anomaly is the difference between the measurements of the half-life of free neutrons by two different methods which I suspect will be resolved with better experimental setups.
A fifth anomaly, in cosmology and astronomy, is the Lithium anomaly in Big Bang nucleosynthesis, which I suspect is due to a mix of observational shortcomings and a failure to consider some subtle aspect of a very complex analysis.
All of these anomalies (and also the sterile neutrino anomalies seen in reactor neutrino measurements) are in my view very likely to disappear within my lifetime.
We've made great progress already in more or less completely understanding baryons (composite particles with three valence quarks or three valence antiquarks bound by gluons), pseudoscalar mesons (composite particles with a valence quark and a valence antiquark of a different flavor bound by gluons that have spin-0 and odd parity), and vector mesons (composite particles with a valence quark and a valence antiquark of a different flavor bound by gluons that have spin-1 and odd parity).
We have now crossed the threshold into understanding excited hadrons (i.e. higher energy states of other hadrons), quarkonia (composite particles with a valence quark and a valence antiquark of the same type bound by gluons, which are mesons), "molecules" of two mesons, a meson and a baryon, or two baryons that are bound by residual strong force interactions (much like protons and neutrons bound in an atomic nucleus), true tetraquarks, pentequarks, and hexasquarks (systems of four, five or six valence quarks bound directly to each other by gluons rather than in a hadron "molecule"), and blended states of hadrons that are bosons (composite particles with even numbers of valence quarks) including blended states that include glueballs (composite particles made up of gluons without valence quarks).
This broader understanding is leading to progress in understanding the true structure of scalar mesons (i.e. spin-0, even parity) and axial vector mesons (i.e. mesons with spin-1 and even parity) mesons, and other exotic resonances.
In the "medium-long" term time frame (i.e. 40-60 years), I'm reasonably comfortable that all of these issues will be more or less completely worked out in principle. We will have determined every theoretically possible hadron resonance, described its structure, and experimentally confirmed its properties. Even 40-60 years from now, however, I suspect that the experimental measurements of these properties will be more precise than those calculated from first principles.
I'm also cautiously optimistic that great progress will be made in calculating parton distribution functions (i.e. a probabilistic description of which fundamental particles with which momentums are found within a hadron) from first principles rather than merely through empirical trial and error, and in improving the precision with which we know the Standard Model physics constants pertinent to quantum chromodynamics (i.e. the Standard Model physics of the strong force) such as the strong force coupling constant and the quark masses to at least one or two orders of magnitude more precision. We will also make progress in doing calculations with QCD with more precision and less of a computational burden.
The existing Standard Model laws of physics applicable to hadron physics are already known, in principle at least, exactly and completely, although operationalizing these laws of physics in a manner that we can do meaningful calculations with is an ongoing process.
For the most part, we don't need any huge break throughs, although I wouldn't rule out the possibility that the existing QCD laws of physics are supplemented in the next half century with one or two or a few subtle additional rules or principles (e.g. a discovery that there is some principled reason that there can be no pure, free glueballs, only of the earliest predicted hadrons which is never been observed).
As I noted in my previous post, the heaviest observed hadron resonance that is well characterized to date is just about 6 GeV and the theoretical maximum mass of a heavy hadron resonance is in the low tens of GeVs. So, we don't need particle accelerators vastly more powerful than those we already have to make great progress in this area.
The Physics Desert, Grand Unification, Theories Of Everything And Within The Standard Model Physics
I strongly suspect that we will see a "desert" or "nightmare scenario" over the next 40-60 years in which we see no experimental evidence of beyond the Standard Model physics (apart from whatever resolution is found of the dark matter and dark energy question and of the nature of neutrino mass), and as noted above, that the anomalies that we do see will disappear.
Quantum gravity, when it is figured out, will tweak the beta functions of all other renormalizable Standard Model physical constants somewhat, which may or may not shed new insights on our theoretical expectations regarding the extremely high energy behavior of the Standard Model at energy scales which will not be reached anytime in the next 40-60 years by experiments or astronomy observations.
I think that it is increasingly clear that the masses of the fundamental particles, the CKM matrix, and the PMNS matrix, which account for 23 of the experimentally measured constants in the Standard Model are fundamentally electroweak phenomena, as are, by definition, the electromagnetic and weak force coupling constant. Thus, 25 of the experimentally measured constants of the Standard Model are electroweak phenomena, while only one of the Standard Model physical constants (the strong force coupling constant) is not an electroweak phenomenon (excluding Planck's constant and the speed of light in a vacuum).
It is also almost inevitable that the measurements of every single one of those physical constants (and also of the three Standard Model coupling constants and the gravitational coupling constant) will improve significantly in the next 40--60 years.
I am cautiously optimistic that the electroweak sector of the Standard Model, informed by increasingly precise measurements of its experimentally measured physical constants, will be explained from far fewer true physical constants with a further elaboration of unified electroweak phenomena.
Hints like (1) Koide's rule, (2) the approximate accuracy of extension of Koide's rule to quarks and patterns in this extension's errors, (3) the possible accuracy of neutrino sector extension of Koide's rule, (4) the fact that all particles that interact via the weak interaction have mass while all particles that don't interact via the weak interaction do not, (5) the fact that the sum of the square of the masses of the fundamental particles is equal to the Higgs vacuum expectation value squared (which in turn is a function of the electroweak coupling constant and the W boson mass), and (6) the patterns in the numbers we see more generally in the physical constants that we have measured (e.g. in the CKM matrix and in its correlations with the quark masses and parallel trends with neutrino masses and the PMNS matrix), all point to the possibility that the answer is out there and might be possible for a single genius to solve in one fell swoop and reduct the number of experimentally measured Standard Model physical constants from 26 to perhaps as few as 6 to 14.
Beyond development of a theory of quantum gravity and a reduction in the number of Standard Model physical constants with an expansion of electroweak theory, however, I am not very hopeful that there will be any progress towards Grand Unification of the Standard Model forces, or a "Theory of Everything" in the next half century.
We may rule out a lot of potential solutions with new experimental data and the quantum gravity progress described above. But I don't think that we will affirmatively find one.
On the other hand, I do think that there is an outside chance that once our scientific community accepts that there is almost nothing beyond the Standard Model to discover, and finds a gravitational explanation of dark matter and dark energy, thereby eliminating the incentive to create beyond the Standard Model physics to explain these phenomena, that progress could be made with a community that has spent the last half century going down the wrong fork in the road, that would now be refocused in the right direction in a much more highly constrained fashion.
I am optimistic that science will solve the problems of dark matter, dark energy and quantum gravity in my lifetime, or baring that, within the next 40-60 years.
lqg or string ?
If I knew, I'd have a Nobel Prize.
Approaches that use a graviton, and approaches that use a semi-classical or discrete spacetime (or quantize inertia) both seem viable and might even turn out to be equivalent at some deep level in a manner similar to how different versions of String Theory turned out to all be branches of M-Theory.
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