Selected 2019 Particle Data Group Physical Constant Values

There are 30 experimentally determined fundamental physical constants in the core theory made up of the Standard Model and General Relativity. This is close to, but not quite, a minimal set of physical constants for these theories, as a small number of them can be derived in principle from some of the others (e.g. the Z boson mass could be, in principle derived from the W boson mass and two of the three coupling constants).  As the data below indicate, these are known to varying degrees of precision. Lighter particle masses and smaller quantities are known to greater absolute precision, while relative precision varies considerably. 

One of the great unsolved problems of physics is how to derive more of these experimentally measured constants from a smaller number of fundamental quantities through a "deeper" or "within the Standard Model" theory, as they naively appear to have some sort of pattern and functional relationship to each other, but no widely accepted physics theories has done so successfully in the half a century since the Standard Model was devised.

There are also integer or simple fraction constants in the Standard Model that are set by theory like the quantum numbers of the various standard model fundamental particles, the masses of the photon and the gluon (zero), and the CP violation parameter of the strong force (zero).

The global average of the best available measurements of these measurements, compiled by the Particle Data Group (link in sidebar) which is the "default" source for these values by physicists, with a handful of additional notable data points, are set forth below. These are updated at least once a year, and sometimes more often (for physical constants for which there have been new experimental measurements) which is why I am posting the current values for easy reference.

In addition to the Standard Model constants, the core equations of the Standard Model are summed up in a Standard Model Lagrangian and some related renormalization beta functions, along with definitions of what the terms in it mean and conventions regarding how they are operationalized, together with some foundational concepts of quantum mechanics generally. The Lagrangian take a full t-shirt to set forth in full detail in fairly small print, and each of the beta functions (one for each of the 26 constants particular to the Standard Model) is similar in size.

The electromagnetic and weak force components of the Standard Model can be applied in a practical manner basically directly from the Lagrangian with high precision. 

The QCD component is, in practice, to difficult to calculate with analytically from first principles in all but the most simplified, stylized and symmetric circumstances, and so a variety of approximations with limited ranges of applicability including Lattice QCD for most non-perturbative QCD calculations, and various perturbative QCD methods for higher energy scale applications, are used rather than the exact QCD Lagrangian terms, in most cases. There are roughly half a dozen to a dozen perturbative QCD approximations used to do actual calculations in wide use.

Core theory is known to be an incomplete description of the fundamental laws of nature because it does not explain the phenomena attributed to dark matter (although it does explain "dark energy").

Standard Model fundamental boson masses (3)

Higgs boson mass 125.10 ± 0.14 GeV

W boson mass 80.379 ± 0.012 GeV (the W boson mass/Z boson mass ratio is 0.088147 ± 0.00013)

Z boson mass 91.1876 ± 0.0021 GeV

Standard Model fundamental charged fermion masses (9)

The heavy quarks (c, b ad t) and the charged lepton masses are pole masses (i.e. evaluated at the energy scale where the rest mass of the particle and the energy scale are the same) and at a 2 GeV energy scale for the light quarks (u, d and s).

Electron mass 0.5109989461 ± 0.0000000031 MeV

Muon mass 105.6583745 ± 0.0000024 MeV

Tau lepton mass 1,776.86 ± 0.12 MeV

Up quark mass 2.16 + 0.49 - 0.26 MeV

Down quark mass 4.57 + 0.48 - 0.17 MeV

Strange quark mass 93 + 11 -5 MeV

Charm quark mass 1.27 ± 0.02 GeV

Bottom quark mass 4.18 + 0.03 - 0.02 GeV

Top quark mass 172.9 ± 0.4 GeV (direct measurements)

The charged lepton masses and top quark masses are direct measurements. The up, down, strange, charm and bottom quarks are only observed indirectly since they are always confined in hadrons, and in practice, are determined via Lattice QCD methods (which sometimes purport to measure these quantities with relative precisions greater than shown above from the Particle Data Group).

It is not feasible to measure the pole masses of the up, down and strange quarks, which are ill defined, because these quarks are always confined in hadrons (i.e. composite particles with components bound by the strong force) which characteristic mass-energies far in excess of their masses a regime in which perturbative QCD in which pole mass is well defined, is not applicable. The lightest hadrons containing only up and down valence quarks are the proton, the neutron and the pions. The lightest hadrons containing only strange quarks are kaons.

Strictly speaking, the "Yukawas" of these particles, rather than their masses, are the fundamental quantities in the Standard Model. A Yukawa is basically the "Higgs field charge" of a particle, while the mass of each of these particles is a function of the Higgs vev (discussed below) and its Yukawa.

Standard Model Coupling Constants (3)

Strong force, aka SU(3), coupling constant 0.1179 ± 0.0010 (this is a downward revision as of 2019) evaluated at the Z boson mass-energy scale.

Fermi coupling constant 1.166 378 7 ± 0.000 0006 × 10^−5 GeV^−2  (functionally related to the weak force, aka SU(2), coupling constant)

Fine structure constant 7.297 352 5693 ± 0.000 000 0011 ×10^−3 = 1/137.035 999 084 ± 0.000 000 021 (functionally related to the electromagnetic force, aka U(1), coupling constant)

Standard Model CKM Matrix Physical Constants (4)

The CKM matrix in the Standard Model sets forth the probability of each kind of quark producing a different flavor of quark via a W boson mediated interaction (a flavor changing charged current). This has nine components, the square of which is a probability, and can be summarized with four parameters that can be defined in multiple ways, all of which produce the same matrix elements.

Standard Model Neutrino Physical Constants (7)


Neutrino masses and mixings (7)

In principle, the Standard Model has seven parameters related to neutrinos: three neutrino mass eigenvalues and four parameters that describe the PMNS matrix which describes the probability of a neutrino of a particular flavor oscillating into a neutrino of a different flavor in a manner closely analogous to that of the CKM matrix (as in the case of the CKM matrix there are multiple ways to parameterize the PMNS matrix which produce identical values for its nine entires).

In practice, some of the PMNS matrix parameters (particularly the charge parity violation phase) are known only very imprecisely, and we know the magnitude of the differences between the mass eigenvalues and have certain floors and ceilings to the sum of the three neutrino eigenvalues (from very different methodologies) and to the maximum masses of each of the three neutrino mass eigenvalues, but have only very imprecise ranges of values for the absolute masses of the neutrino mass eigenstate. In particular, it is not yet certain if the neutrino masses observe the same lightest, medium and heaviest mass hierarchy with generation seen in the other fundamental fermions of the Standard Model or not, although cosmology and mixing parameter values taken together increasingly favor a normal mass hierarchy.

The Standard Model and the PMNS matrix, implicitly assume that neutrinos have what is known as "Dirac mass" rather than Majorana mass (in which case there would be additional CP violating phases), but without associating that with the Higgs mechanism use for the fundamental charged particles of the Standard Model and without truly explaining this with the neat consistency of the electroweak model explanation of other fundamental particle masses in the Standard Model at all.

The data below are in the form in which the actual values are directly measured (sine squared values of real valued mixing angles and squared values of mass differences) rather than the underlying parameter values which are easily derived from them with a scientific calculator.

in^2(theta23):

The full data for the parameters shown as ". . . " in the chart above are as follows:

sin^2(theta23) theta23 could be either side of a 45 degree angle based upon existing measurements and assuming a "normal" mass hierarchy for the neutrino masses. But existing experiments, while capable of determining that theta23 is not 45 degrees, but can't determine if it is greater or smaller than those values, which is why there is both an octant I and an octant II value.

$\mathbf{0.512}{}_{\mathbf{-}\mathbf{0.022}}^{\mathbf{+}\mathbf{0.019}}$	OUR FIT  Normal ordering, octant I
$\mathbf{0.542}{}_{\mathbf{-}\mathbf{0.022}}^{\mathbf{+}\mathbf{0.019}}$	OUR FIT  Normal ordering, octant II
−0.028$\mathbf{0.536}{}_{\mathbf{-}\mathbf{0.028}}^{\mathbf{+}\mathbf{0.023}}$	OUR FIT  Inverted ordering

Delta m32^2 with normal ordering is 2.444 ± 0.034 (the number shown in chart is inverse mass hierarchy).

Sum of neutrino masses Σmν < 0.12 eV (95%, CMB + BAO); ≥ 0.06 eV (mixing).

Directly measured neutrino mass limits:

Neutrino Flavors

The number of neutrino flavors in the Standard Model is a theoretically determined, rather than experimentally measured value, but the experimental measurements are consistent at the two sigma level with the Standard Model value of 3:

Effective number of neutrino flavors Neff 2.99 ± 0.17 (cosmology measurements) (the expected value of this measured physical constant with exactly three types of neutrinos is 3.045 rather than zero for technical reasons related to the way that radiation impacts the relevant observables). This measurement includes all light neutrinos (up to the order of roughly 1-10 eV in mass) that oscillate with each other, and is independent of whether or not the interact via the weak force.

Number of light (i.e. less than 45 GeV) neutrino flavors from Z boson decays Nν = 2.984 ± 0.008. The Standard Model theoretical value is 3.

General Relativity Physical Constants (2)

Newton's constant G 6.708 83 ± 0.000 15 × 10^−39 hc (GeV/c^2)^−2

Cosmological constant Λ 1.088 ± 0.030 × 10^−56 cm^−2

General relativity can be boiled down to a single tensor valued equation with definitions of the symbols and terms used in it and the conventions about how this equation is applied. It is difficult, although not entirely intractable to compute predictions with analytically outside highly stylized and symmetric circumstances, and in other circumstances, approximations motivated by the core equation are used.

The cosmological constant is small enough that it can be ignored even in quite large scale general relativity calculations, such as those on the scale of whole galaxies. The cosmological constant is one of multiple ways of describing what is also known as "dark energy".

Note also that the Standard Model and General Relativity are inconsistent at a fundamental level. The Standard Model is a quantum field theory, while General Relativity is a deterministic classical physics theory. Efforts to formulate a theory of quantum gravity to bridge this gap have been elusive. The Standard Model is fully consistent, however, with Special Relativity. Fortunately, because there are few overlaps between the domains of applicability of the Standard Model an those of General Relativity, and there are simplifications and approximations that can be used in the places where there are overlaps (like the internal properties of neutron stars), the inconsistencies between the Standard Model and General Relativity doesn't greatly impair practical applications of these core theories. Usually, general relativity can be safely ignored in Standard Model applications, and the Standard Model's subatomic scale laws of nature are irrelevant to applications of General Relativity.

Other fundamental constants and unit conversions (2)

In the SI system (the most up to date version of the metric system) to physical constants, the speed of light and Planck's constant, have been used to define the meter per second, and the Joule*second, respectively, at values as close as experimental measurements as of 2019 allowed to the values of the more arbitrary values of the meter, second and Joule prior to being defined in terms of fundamental constants, in order to insure back compatibility.

Both of these constants are used in the Standard Model. The speed of light is a physical constant also used in General Relativity.

Speed of light in vacuum c 299 792 458 m s−1  (exact due to revised definition of units used)

Planck constant h 6.626 070 15×10−34 J s (exact due to revised definition of units used)

The reduced Planck constant a.k.a. "h bar" h (h divided by two pi) is 6.582 119 569. . . × 10−22 MeV s. This value is exact, but it a transcendental number since it includes a factor of pi (i.e. it has an infinite number of base ten digits).

The units of electromagnetic charge in the SI system, in contrast, is not defined directly in terms of fundamental electron charge. One electron charge in SI units is 1.602 176 634×10^−19 C

Footnote Regarding Derived Physical Constants

Many other physical constants, in principle, are possible to derive from the fundamental constants in the Standard Model. In practice, however, direct measurements of these quantities are actually used. Among them are:

* The decay width of every fundamental particle and hadron (this is inversely proportional to the half-life and mean lifetime of a an unstable particle). In theory, electrons, photons and gluons are stable and do not decay or oscillate in the Standard Model, as are protons and as are neutrons that are bound in stable atomic nuclei. The decay width of a free quark other than a top quark, however, isn't terribly meaningful or well defined in the Standard Model, as other flavors of quarks are always confined outside quark-gluon plasma.

* The electric and magnetic dipole moments of every particle.

* The decay width and branching fraction of every possible decay path of fundamental particles and hadrons that are not stable. The hypothetical lepton number violating processes like neutrinoless double beta decay and flavor changing neutral currents are not possible at the tree level in the Standard Model and are profoundly suppressed beyond the tree level in the Standard Model.

* The charge radius of every fundamental particle and hadron.

* The nuclear binding force between protons and neutrons in an atom.

* The half-life of every unstable atomic isotype.

* The rest mass of every possible hadron.

* The chemical properties of every atomic isotype.

* All of the Standard Model constants other than c and Planck's constant, at energy scales other than those at which they are conventionally quoted per renormalization beta functions, all coefficients of which can be derived from theoretical first principles. This may be ill defined at extremely high Planck energy scales that have never existed anywhere in the universe at any time more than tiny factions of a nanosecond after the Big Bang.

* The characteristic QCD energy scale called Λ.

* The Higgs vacuum expectation value (vev), which is also derived from the Fermi coupling constant, per a non-PDG source using PDG values, is 246,227.9579 MeV with an uncertainty on the order of 0.0010 MeV.

Some of the derived physical constants, such as the masses of the proton and neutron, are known to far greater precision than the fundamental constants from which they are derived. 

This is particularly common for physical constants that require QCD to determine. This is because doing calculations with QCD is computationally extremely difficult to do to any given level of precision, and for the closely related reason that as a result of this computational difficulty, it is very difficult to measure the quark masses and the strong force coupling constant to high relative precision. The roughly 1% relative precision in the strong force coupling constant places a ceiling on the precision of all QCD calculations to the extent that they depend upon this fundamental physical constant as all QCD calculations do to some degree or another.

Footnote regarding non-fundamental, non-derived physical constants

Fundamental physical constants are those necessary to apply the "laws of nature" set forth in the Standard Model and General Relativity and any beyond the Standard Model theory that adds additional such constants. Derived physical constants are those that can, in principle, be determined from the fundamental physical constants.

There are also a variety of observationally measured physical constants which are neither fundamental, nor capable of being derived from fundamental physical constants. These are mostly descriptive constants of particular physical systems including the universe.

These include: the mass of the Earth, the Sun, the Milky Way galaxies, the universe and various other physical objections or systems in space; the age of the universe; Hubble's constant (although this is closely related to the cosmological constant); the aggregate baryon number of the universe, the aggregate lepton number of the universe; the number of bosons in existence at any one time in the universe; the mix of atomic elements in the universe or other smaller physical systems (e.g. the planet Earth or the Sun); the distances between particular places on Earth and/or in the Universe; the proportionate mix of ordinary matter other than neutrinos, neutrinos, radiation, dark matter,  and dark energy in the total mass-energy of the universe.

Bell Beaker Family Structure

This gem of a paper escaped my notice when it came out.

We present a high-resolution cross-disciplinary analysis of kinship structure and social institutions in two Late Copper Age Bell Beaker culture cemeteries of South Germany containing 24 and 18 burials, of which 34 provided genetic information. By combining archaeological, anthropological, genetic and isotopic evidence we are able to document the internal kinship and residency structure of the cemeteries and the socially organizing principles of these local communities. The buried individuals represent four to six generations of two family groups, one nuclear family at the Alburg cemetery, and one seemingly more extended at Irlbach. While likely monogamous, they practiced exogamy, as six out of eight non-locals are women. Maternal genetic diversity is high with 23 different mitochondrial haplotypes from 34 individuals, whereas all males belong to one single Y-chromosome haplogroup without any detectable contribution from Y-chromosomes typical of the farmers who had been the sole inhabitants of the region hundreds of years before. This provides evidence for the society being patrilocal, perhaps as a way of protecting property among the male line, while in-marriage from many different places secured social and political networks and prevented inbreeding. We also find evidence that the communities practiced selection for which of their children (aged 0-14 years) received a proper burial, as buried juveniles were in all but one case boys, suggesting the priority of young males in the cemeteries. This is plausibly linked to the exchange of foster children as part of an expansionist kinship system which is well attested from later Indo-European-speaking cultural groups.

Karl-Göran Sjögren, et al., "Kinship and social organization in Copper Age Europe. A cross-disciplinary analysis of archaeology, DNA, isotopes, and anthropology from two Bell Beaker cemeteries" bioRxiv (December 11, 2019) https://doi.org/10.1101/863944

Combined Reactor Data Basically Rules Out Sterile Neutrinos

One of the ways we study neutrinos, and in particular, their oscillations between neutrino masses and flavors described in the Standard Model by the PMNS matrix, is to compare the mix of neutrino types produced at a reactor source with the mix of neutrino types observed at a distant neutrino flavor detector.

Some of these experiments have shown results that individually seem inconsistent with the Standard Model scenario in which there are three neutrinos and three neutrino mass eigenstates that oscillate between each other. 

W and Z boson decays strongly confirm that only there are only three neutrino flavors with masses of 45 GeV (i.e. 1,000,000,000 eV) or less that interact via the weak force that also drives nuclear beta decay (constraints on the largest of the neutrino masses from astronomy observations are on the order of 0.1 eV at most for the heaviest neutrino mass, with direct observations limiting these masses to on the order of 1 eV). But, it is harder to rule out the possibility that there might be a fourth "sterile neutrino" (or possibly additional sterile neutrinos as well) that oscillates with other flavors of neutrinos in an extended PMNS matrix, but do not interact via electromagnetism, the strong force, or the weak force.[1]

Individually, reactor neutrino oscillation measurements show deviations from the expected values that can be interpreted as a eV mass scale sterile neutrino that oscillates via with the three active neutrinos, but does not interact via the weak force. 

But, the experimental evidence from various reactor experiments for a sterile neutrino reactor anomaly compared across all of the experiments simultaneously is mutually inconsistent, essentially ruling out the existence of sterile neutrinos at a statistically significant level over essentially all of the plausible parameter space for sterile neutrinos.

These results compliment astronomy measurements interpreted in light cosmology models regarding of the number of effective neutrino flavors (Neff), which also strongly favor a three flavor possibility over a four plus flavor possibility.

Previous papers have suggested that apparent sterile neutrino reactor anomalies may actually represent a failure to model the neutrino production of nuclear reactor sources with varied proportions of different nuclear fuels, or other methodological errors.

The new paper and its abstract are as follows:

Searches for electron antineutrino, muon neutrino, and muon antineutrino disappearance driven by sterile neutrino mixing have been carried out by the Daya Bay and MINOS+ collaborations. This Letter presents the combined results of these searches, along with exclusion results from the Bugey-3 reactor experiment, framed in a minimally extended four-neutrino scenario. Significantly improved constraints on the θμe mixing angle are derived that constitute the most stringent limits to date over five orders of magnitude in the sterile mass-squared splitting Δm241, excluding the 90% C.L. sterile-neutrino parameter space allowed by the LSND and MiniBooNE observations at 90% CLs for Δm241<5eV2.Furthermore, the LSND and MiniBooNE 99% C.L. allowed regions are excluded at 99% CLs for Δm241 < 1.2 eV2.

Daya Bay, MINOS+ Collaborations, "Improved Constraints on Sterile Neutrino Mixing from Disappearance Searches in the MINOS, MINOS+, Daya Bay, and Bugey-3 Experiments" arXiv 2002.00301 (February 2, 2020).

[1] Fundamental particles with spin 1/2 are called "leptons" and leptons with no electromagnetic charge are called neutrinos. They were originally hypothesized to exist to maintain conservation of mass-energy, linear and angular momentum and lepton number in nuclear beta decay interactions.

Charged fundamental particles of a given flavor with spin 1/2 all have four subtypes (left parity particles, right parity particles, left partity antiparticles, and right parity antiparticles) (with three color variants of each of these four subtypes for quarks), that are otherwise identical.

But, neutrinos in the Standard Model come in only two types (left parity particles and right parity antiparticles).

This is because, in the Standard Model, the weak force interacts only with left particle particles and right parity antiparticles. The parity counterparts of these particles do not interact via the weak force. Further, leptons, in general do not interact via the strong force of the Standard Model, and neutrinos have no electromagnetic charge.

Thus, right parity neutrinos and left parity antineutrinos, if they existed in a beyond the Standard Model extension, would not interact via any of the three Standard Model forces. So, another name for sterile neutrinos is "right handed neutrinos", even though, in theory, a neutrino could lack all three Standard Model force interactions for reasons than their parity.

The interaction with the Higgs field that gives rise to fundamental particle mass in the equations of the Standard Model presumes the existence of a particle that has both left parity and right parity versions of every fundamental particle and antiparticle. But, these equations break down in the case of neutrinos which have only one parity of particle and one partity of antiparticle for each of the three active flavors.

For this reason, for a long time, the Standard Model assumed that neutrinos, which are by far the lightest fundamental fermions, had no mass rest mass at all, and hence didn't need to derive mass from the Higgs field, something that was experimentally disproven a couple of decades later.

As a result neutrino masses are a little corner of the modern Standard Model that isn't completely worked out yet, and there are competing explanations for their masses.

As I explained in comments to another recent post at this blog, I don't think any of the leading explanations of neutrinos mass (either a see-saw mechanism with right handed neutrinos, or "Majorana mass" associated with a particle being its own anti-particle) are very plausible, although I don't have another alternative to offer.

Interactions Between Pre-Columbian South American Populations

The Andes highlands started to homogenize around Y1K. 

There is lots of differentiation between populations at the geographic boundary between the Andres highlands and the Amazonian region. Unsurprisingly, people in the highlands have high altitude adaptation genes, while people in the Amazon has infectious disease resistance genes, that are absent in their respective counterparts.

But, there is evidence of admixture between these populations in the northern coastal part of the South American continent where geographic barriers were not so great and there is archaeologically attested interactions between the people of the Pacific coast of South America and Amazonian peoples.

The sample size is 363 modern genomes from diverse populations with about a fifth of them from a prior study. 

It is also worth noting as an aside that European admixture is quite thin relative to what one might naively expect in the studied populations, in both regions, except for a small subset of the Fertile Andes, and that African admixture is thin across the board in this region.

Figure 1  

Genetic structure and gene flow of Western South American natives. 

Eighteen native groups along the Coast, Andes, and Amazon regions were sampled. A) Grey dashed line in the center shows the division between Fertile Andes and Arid Andes. We showed the geographical distribution coupled with ADMIXTURE patterns for the lowest cross-validation value (K=5) using the dataset of 1.9M unlinked SNPs (including Native Americans, Europeans, and Africans). Blue and green dashed lines delimited the groups that showed highly significant value for the gene flow test (|Z score| > 4). Matsiguenka 1= Matsiguenkas-Sepahua, Matsiguenkas 2= Matsiguenkas-Shimaa B) Haplotype based inference of ancestry profile for each Native American population, each bar corresponds to the ancestry composition for a native population. For this analysis, Matsiguenka samples were merged into one. Colors for the ancestry profile correspond to the proportion of DNA shared between the population and donor groups detailed in the legend of section A.

Western South America was one of the worldwide cradles of civilization. The well known Inca Empire was the tip of the iceberg of a cultural and biological evolutionary process that started 14-11 thousand years ago. Genetic data from 18 Peruvian populations reveal that: (1) The between-population homogenization of the central-southern Andes and its differentiation with respect to Amazonian populations of similar latitudes do not extend northward. Instead, longitudinal gene flow between the northern coast of Peru, Andes and Amazonia accompanied cultural and socioeconomic interactions revealed by archeological studies. This pattern recapitulates the environmental and cultural differentiation between the fertile north, where altitudes are lower; and the arid south, where the Andes are higher, acting as a genetic barrier between the sharply different environments of the Andes and Amazonia (2). The genetic homogenization between the populations of the arid Andes is not only due to migration during the Inca Empire or the subsequent colonial period. It started at least during the earlier expansion of the pre-Inca Wari Empire (600-1000 YBP) (3) This demographic history allowed for cases of positive natural selection in the high and arid Andes vs. the low Amazon tropical forest: in the Andes, HAND2-AS1 (heart and neural crest derivatives expressed 2 antisense RNA1, related with cardiovascular function) and DUOX2 (dual oxidase 2, related to thyroid function and innate immunity) genes; in the Amazon, the gene encoding for the CD45 protein, essential for antigen recognition by T/B lymphocytes in viral-host interaction, consistent with the host-virus arms race hypothesis.

Victor Borda, et al., "The genetic structure and adaptation of Andean highlanders and Amazonian dwellers is influenced by the interplay between geography and culture" bioRxiv (January 31, 2020) doi: https://doi.org/10.1101/2020.01.30.916270

The body text introduction from the paper provides good historical context for the findings (citations omitted):

Western South America was one of the cradles of civilization in the Americas and the world. When the Spaniard conqueror Francisco Pizarro arrived in 1532, the pan-Andean Inca Empire ruled in the Andean region and had achieved levels of socioeconomic development and population density unmatched in other parts of South America. However, the Inca Empire, which lasted for around 200 years, with its emblematic architecture such as Machu Picchu and the city of Cuzco, was just the tip of the iceberg of a millenary cultural and biological evolutionary process. This process started with the peopling of the region (hereafter called western South America), that occurred 14–11 thousand years ago, involving the entire Andean region and its adjacent and narrow Pacific Coast. 

Tarazona-Santos et al. proposed that cultural exchanges and gene flow along time have led to a relative genetic, cultural, and linguistic homogeneity between the populations of western South America compared with those of eastern South America (a term that hereafter refers to the region adjacent to the eastern slope of the Andes and eastward, including the Amazonia), where populations remained more isolated from each other. For instance, only two languages (Quechua and Aymara) of the Quechumaram linguistic stock predominate in the entire Andean region, whereas in eastern South America natives speak a different and broader spectrum of languages classified into at least four linguistic families. This spatial pattern of genetic diversity and its correlation with geography, environmental, linguistic and cultural diversity was confirmed, enriched and rediscussed by us and others. 

There are pending issues: First, whether the dichotomic organization of genetic variation characterized by the between-population homogeneous Southern Andes vs. between-population heterogeneous Central Amazon, extends northward. This is important because scholars from different disciplines emphasize that western South America is not latitudinally homogeneous, differentiating a northern and in general lower and wetter fertile Andes and a southern, higher and more arid Andes. These environmental and latitudinal differences are correlated with demography and culture, including different spectra of domesticated plants and animals. Indeed, the development of agriculture, of the first urban centers such as Caral and its associated demographic growth, occurred earlier in the northern Fertile Andes (around 5ky ago) than in the southern arid Andes (and their associated Coast), with products such as cotton, beans, and corn domesticated in the fertile north and the potato and South American camelids in the arid south. In human population genetics studies, the region where the between-population homogeneity was ascertained by Tarazona-Santos et al. was the arid Andes. 

Not All Experiments Are Created Equal

4gravitons make an interesting analysis of four different kinds of experiments and their respective likelihoods of discovering something interesting.

Some experiments are almost sure to discover something new because we know nothing about something new except that it is out there to measure. Some experiments are strongly expected to reproduce the status quo and are just dotting i's and crossing t's to more fully establish the range of the status quo with the off chance that an unexpected anomaly is discovered. 

In between are theory testing experiments. These can be powerful or merely paradigm confirming, unless a theory can be easily modified to adapt to any set of results, or there are many similar theories, not all of which can be falsified simultaneously.