Sunday, September 15, 2019

Why Is The Universe So Complicated In Ways That Don't Matter?

The Standard Model of Particle physics sets forth a huge range of possible phenomena and interactions. But, most of them are observable only in high energy collider experiments recreating circumstances that have not existed naturally in the near vicinity of Earth or the Sun for many billions of years, certainly, long before life as we know it came into being.

Why do we have such a sophisticated set of parts and rules for such a simple universe that makes so little use of so many of those parts and rules?

There are six kinds of quarks: top, bottom, charm, strange, down and up (in order of mass). But, all ordinary matter made up of quarks is made up of up and down quarks, with an occasional strange quark flitting into and out of existence in a kaon within an atomic nucleus. Gluons and quarks are also always confined within hadrons at temperatures cool enough to come into being on Earth or in the Sun, so we never see them in isolation. There are hundreds of possible mesons and hadrons (even before venturing into tetraquarks, pentaquarks, and hexaquarks), but protons and neutrons and several mesons with no significant strange quark or heavy quark components (i.e. the pion, the omega meson, the rho meson and the sigma meson) that are involved in the nuclear force, suffice to describe almost everything in the observable world to a high degree of accuracy.

Apart from nuclear physics, which only has a narrow range of engineering and scientific applications (apart from understanding fundamental physics for fundamental physics' sake), we don't need to know anything about the strong and weak nuclear forces at all, beyond the fact that nuclei hold together in the absence nuclear fission, nuclear fusion and radioactive decay which could be described with far simpler phenomenological models (and as a practical matter are dealt with that way, even today, in most engineering applications). Nuclear weapons, nuclear fission reactors and the early forms of nuclear medicine were invented as practical applications of nuclear physics before QCD or electroweak theory had reached their modern Standard Model form.

We know that there are three kinds of neutrinos, and three kinds of anti-neutrinos, and some of the rather mysterious properties like neutrino oscillation, but they interact so weakly that there aren't many applications in which knowledge of them is helpful, and there are even fewer applications in which it is necessary, possible and helpful to distinguish between neutrino flavors.

We know that there are two kinds of particles (muons and tau leptons) that are just like electrons but more massive, and how they behave, and we even use muons in a number of practical applications.

But, in a universe with twelve kinds of fundamental fermions, most of what we observe can be understood with just three of them (up quarks, down quarks and electrons), throwing in the electron neutrinos, muon neutrinos and muons and bringing the total to six, for the truly sophisticated. A world without second and third generation fermions at all would be almost impossible for a casual observer to distinguish from our own.

We live in a universe with twelve or thirteen kinds of fundamental fermions, but the eight kinds of gluons are always confined, the Z boson has negligible relevance practically, the W+ and W- boson can be summed up in a black box theory of weak force decay for most practical purposes, and the photon and possibly the hypothetical graviton, are all that we need to deal with for most purposes.

One needs to understand special relativity for many practical purposes, but the far more mathematically and conceptually difficult general relativity for far fewer. We need to understand quantum electrodynamics for many practical applications, but electroweak theory and quantum chromodynamics, for only a very few applications.

We know all of the fundamental physics that will ever be necessary to understand chemistry and biology and geology from first principles.

We understand the least about gravity, but fortunately, while knowing more about it is important in terms of cosmology, and explaining what astronomers see in the very distant depths of the universe, none of the mysteries of dark matter and dark energy have any practical relevance to a species that may never settle more than a handful of nearby star systems, a scale too small for either of those phenomena to have any real relevance. For all but the highest precision applications, even in the solar system and its nearby neighboring star systems, plain old Newtonian gravity and special relativity are quite sufficient to meet our needs.

And, I think that we will probably master the main problems of quantum gravity, dark matter and dark energy, if not in my lifetime (not an unlikely possibility if I live to a ripe old age), in the lifetime of my children or grandchildren (assuming that I will have any). Knowing this won't have many applications, but it will be satisfying and I suspect it will overhaul a lot of mainstream astrophysics related to conventional wisdom about cosmological inflation, the early universe, dark matter and dark energy, in a way that may leave some philosophical ripples that escape into the larger culture.

Cracking the unsolved problems of high energy physics is something we crave, and we might someday discover a layer beneath what we know now that explains Standard Model physics in terms of something simpler at a more microscopic level that unifies it and provides a means from which its many arbitrary constants can be derived from first principles. But, it increasingly looks as if there is no beyond the Standard Model physics that would make differing predictions from the Standard Model in any way that matters.

I am skeptical that we will penetrate that deeper level any time in the next several centuries, no matter how many billions of dollars of resources we throw at it, and I am even more skeptical that we will be able to find any technological applications for it if we do. Understanding the deeper underpinnings of the Standard Model, or at least some of them, will probably only satisfy our intellectual curiosity and make our knowledge of a few physical constants that could then be calculated from first principles. a few orders of magnitude more precise that what we measured experimentally something that is already quite precise in most cases.

Really the only bright spots in terms of progress for the next few centuries in Standard Model physics are an improved understanding of neutrinos, increasingly accurately measured fundamental physical constants, and an increased ability to apply QCD to high energy physics experiments, neutron star properties and the Big Bang.

Much of what we know already is only applicable to the early moments of the universe right after the Big Bang and has little application once nucleosynthesis has run its course.

While we have not yet reached the point of complete scientific knowledge, our understanding of fundamental physics is such that we already know almost everything that could have a technological application that would be economically useful in any way.

We know such more beyond what is economically or technologically useful already, and yet, this knowledge is already esoteric to a great extent.

Now, this doesn't mean that just because we know all of the fundamental rules of physics that we need to use, all of the laws of nature that matter to us, that there isn't lots of critical and economically valuable science to be done, explaining all of the implications of those fundamental rules that are relevant to our complicated world. In areas like condensed matter physics, nuclear engineering, genetics and medicine, there is much to be learned. But, you can go a long way towards doing that with QED and practical simplifications of those fundamental rules for circumstances were they need to be applied and experimental measurements more precise than those that could be derived from first principles.

13 comments:

DDeden said...

Gravity: mass attraction.

andrew said...

mass-energy attraction, not just mass. And the cosmological constant at face value, is energy creation.

neo said...

I've asked both on PF and quora,

an electron and position are identical except electric charge, so in what ways do the 2 differ that causes this difference?

similarly, electron and muon and tau, in a higgless universe are almost identical, though they do differ in flavor, which only shows up in the higgs field.

in what ways do these charged leptons differ that accounts for flavor?

and what causes flavor to determine higgs coupling?

andrew said...

@neo

Those are questions (like the title question of this post) that physics doesn't answer with the kind of answer you are fishing for.

The SM decrees that every fundamental particle has an anti-particle of opposite charge (if applicable and parity) - except that neutrinos different from anti-neutrinos only in parity (and gluons are a bit more muddled because group theory with eight rather than nine of them as you would naively expect with three kinds of color charge). The only SM particles without anti-particles are photons and Z bosons.

The SM decrees that there are three generations of fermions with one up quark, down quark, one charged lepton and one neutrino weak force state each, that differ from each other only in mass (in every case), in CKM matrix interactions (for quarks) and in PMNS matrix interactions for neutrinos. In the case of quarks and charged leptons in the SM, the mass differences (but not the CKM matrix entries) arise from the Higgs Yukawa of the particle. We're not sure where neutrino mass comes from (although it is increasingly clear that neutrinos, like quarks and charged leptons, have an ordinary mass hierarchy).

We know how weak force interactions can give rise to new particles that conserve various quantum numbers and that W+ and W- bosons can change fermion flavors (in the case of quarks from up type to down type or from down type to up type, in the case of charged leptons from one charged lepton flavor to another plus neutrinos).

We know that each fundamental fermion of the same mass, charge and parity is indistinguishable from every other fundamental fermion of that type except for +/- spin and except for momentum and location and obeys fermi statistics.

We know that each fundamental boson has the same properties as every other fundamental boson of that type apart from frequency and/or momentum, polarity, helicity, and location, and obeys Bose statistics.

These assumptions, along with unifying concepts like couplings and the nature of a propagator and creation/annihilation function for each, which differ mostly in parameters, gets us everything but gravity.

We know how particles can transform into other particles and interact with other particles, but we don't have any deeper "why" than that. None of the fundamental particles are "composite" in the ordinary sense and they behave like point particles to the limits of experimental measurement.

We have hints at deeper structures and rules. The sum of the fundamental masses squared equals the Higgs vev squared. Koide's rule for charged leptons works. The CKM matrix and PMNS matrix don't look random in relation to generations of fermions or masses, even if the relationship isn't exactly elementary. There are plausible reasons that might be causes for there being exactly three generations of fermions rather than more or less (e.g. the mean lifetime of the weak force bosons is only slightly shorter than that of the top quark and that may serve to prevent higher generation particles from forming).

andrew said...

continued . . .

My personal conjecture is that the masses of the fundamental fermions are a product of a dynamic equilibrium of all possible W boson interactions and that the overall mass scale of the fundamental fermions is set by the Higgs vev and allocated among them from a default of equality, via W boson interactions (reinforced by the fact that all fundamental particles that don't interact with W or Z bosons are massless and that all massive fundamental particles interact with W and Z bosons).

I also conjecture that the CKM matrix and PMNS matrix are logically prior to the fundamental fermion masses and influence how that dynamic equilibrium is struck, and that the CP violating phase in each of these matrixes may actually have a distinct cause from the other three parameters of those matrixes.

In part for these reasons, my own private conjecture, FWIW, is that the W bosons plays a larger part, and the Higgs boson and Higgs field plays a smaller part, in establishing the masses of the fundamental particles and their transition probabilities than is conventionally assumed, and the the Higgs and weak force bosons are more intimately intertwined that is commonly understood.

Thomas Andersen said...

Unlike Andrew, I view the Standard Model as teetering.

It won't be some theoretical wave that topples it.

It will be a useful device that clearly violates the Standard Model
Or a dark matter that is not a particle,
Or an explanation for dark energy that does not fit.

But it's not easy to topple something like the Standard Model. The Standard Model is at this point almost unfalsifiable, as many paradigms get as they become more complicated and powerful. Massive neutrinos were not part of the Standard Model, but now they are (to many people, not me) EG: https://home.cern/science/physics/standard-model

neo said...

"My personal conjecture is that the masses of the fundamental fermions are a product of a dynamic equilibrium of all possible W boson interactions and that the overall mass scale of the fundamental fermions is set by the Higgs vev and allocated among them from a default of equality, via W boson interactions (reinforced by the fact that all fundamental particles that don't interact with W or Z bosons are massless and that all massive fundamental particles interact with W and Z bosons)."

how does this accord with the mainstream view that it is particle's yukawa coupling to the higgs field, though then the question is what determines yukawa couplings.


how does string theory answer these various issues, for example, how does a string that vibrates as an electron differ from a positron, or an electron from a tau or electron from neutrino.

and there was bilson-thompson braiding and wen's string net approach, both seemingly died.

Mitchell said...

Despite the naysayers, string theory is alive and well, and provides a totally plausible example of "a layer beneath what we know now that explains Standard Model physics". But as string-watchers know, there are a lot of possibilities and the theory is very incompletely understood or usable. The phenomenological surprise that standard model is still valid at LHC scales, the criticality of the Higgs mass, numerology like Koide's, these all simply change the target that string phenomenologists need to aim at.

I believe I have previously answered @neo's question about how string theory could differentiate between one fermion species and another, at Physics Forums; and in a thread by @Spinnor on charge, I probably said something towards the question of how matter vs antimatter is represented in string theory.

neo said...

@mitchell can u provide the link again, must have been buried from all the other posts

neo said...

string theory is only plausible if we live in an 10 or 11 dimensional universe with SUSY, and that the landscape includes deSitter spacetime.

these claims remain unverified to date, and occam's razor would suggest a simpler theory that is consistent with current known experimental results.

andrew said...

@ThomasAnderson

"It will be a useful device that clearly violates the Standard Model
Or a dark matter that is not a particle,
Or an explanation for dark energy that does not fit."

Dark matter that is not a particle, and dark energy are not part of the Standard Model, would not topple the Standard Model, since the Standard Model doesn't include gravity and neither dark matter nor dark energy are part of the Standard Model. Those could topple GR or at least the cosmological constant, but not the SM.

It would take far less than a "useful deice that clearly violates the Standard Model" to topple it. Five sigma experimental evidence of a new non-gravitational force, and/or new particles that are not just four generation fermions or perhaps a boson that is involved in neutrino oscillation and/or mass generation, would be more than sufficient.

andrew said...

@Mitchell

I don't disagree that a deeper layer might look something like String Theory (although a deep layer might be an intermediate one that doesn't go that deep as well).

But, string theory is in a pretty sorry state. And, I think that it is unlikely the its 10-11 dimensional aspects, brane theory, the notion that it has supersymmetry as a low energy approximation, its preference for Majorana neutrino masses, its desire to find lepton number and baryon number violating phenomena, and its tendency to focus on anti-de Sitter space even thought we don't live in it, will make it into any realistic theory. Likewise, its myriad vacua defeat the naive expectation that it would be a single theory with one or two fundamental constants from which everything else could be derived uniquely from first principles.

It isn't implausible to me that a string theory that looks more like reality and rejects a lot of the theoretical biases like those that I list in this comment that gave us what we have today could someday be extracted from the mess of current M-theory research. But, I don't think we are anywhere close to it yet, and questions that everyone expected that string theory would soon answer at its outset have proved to be intractable.

In particular, string theory is a promising solution to the point particle problem, to the infinities in quantum calculations, and to a mathematically tractable graviton. The framework of one or two kinds of fundamental strings or ribbons or knots whose oscillation modes correspond to fundamental particles remains seductive and is potentially consistent with the SM in some form. But, I don't expect to see any of that worked out in my lifetime, that of my children, or that of my grandchildren, despite the immense legion of theoretical physicists who are working in that area.

Thomas Andersen said...

Andrew,

Thanks for the reply.

This is not a rebuttal of what you wrote, just a different (perhaps more cynical) way of looking at physics.

As you say, the SM does not today have DM particles, but if one was found that looked like a WIMP - it would quickly be assimilated into the SM, like massive neutrinos, etc. I guess its more just words at this point, but the SM is complex and ill-defined.

The SM does not today include graviton, but finding one would merely redefine the Standard Model to include it, as 'everyone knows' that the graviton is a spin 2 boson. Indeed the graviton is shown on many Standard Model posters, etc.

Any particle that could be described as a fermion or boson would just be an addition to the SM. It needs to be something out of left field to actually get physics rolling again.

RE:
Andrew wrote:
"
Dark matter that is not a particle, and dark energy are not part of the Standard Model, would not topple the Standard Model, since the Standard Model doesn't include gravity and neither dark matter nor dark energy are part of the Standard Model. Those could topple GR or at least the cosmological constant, but not the SM.


It would take far less than a "useful deice that clearly violates the Standard Model" to topple it. Five sigma experimental evidence of a new non-gravitational force, and/or new particles that are not just four generation fermions or perhaps a boson that is involved in neutrino oscillation and/or mass generation, would be more than sufficient."