During the quarter-finals match Argentina vs. England of the 1986 FIFA World Cup, Diego Armando Maradona scored a famous goal that opened the way to the victory of the match, and to Argentina’s final winning of the football championship. Famously left-handed, he was facing the English goalkeeper Peter Shilton (20 cm taller than Maradona), when he jumped to head-hit a ball that was bounced back from an English defender. While Shilton rose his hand to catch the ball, Maradona jumped higher and hit it first into England’s goal, very clearly using his left-hand fist hold close to his head. Asked about the illegal scoring, after the match, Maradona stated the goal was “a little from the head of Maradona, and a little from the hand of God”.
I have been left-handed for most of my life. When I was in the first year of primary school, my teacher, a big blond woman wearing thick pink lipstick for as much as I can remember, used to bind my left hand behind my back to force me to properly write with the right hand. Today such things would be ground for throwing the teacher out of school, but in the Italy of the 60s they were sort-of acceptable. Not to my father, who came to the school one day and sang a warning to my teacher in such notes that she immediately dropped her peculiar pedagogic treatments. Anyway, being left-handed was some kind of humiliation back then, until I realized that the thing could make me also somewhat special, as I learned of Einstein, Shakespeare, Napoleon or Mozart. Around age 8 or 9, I discovered the peculiar writing habits of Leonardo da Vinci, who was left-handed too, and immediately picked up his idea of writing mirror-backwards, starting at the right side of the page and pulling to the left. More or less at the same time I was forcing myself to learn to write with both hands; however, I always used my left hand to manage tools and playing sports, the only exceptions being playing guitar and hitting golf balls, for which I still follow the standard convention.
Humans are not the only living beings to show handedness, that is a preference for one hand over the other. Other primates [W. D. Hopkins, Psych. Bull. 132, 538 (2006)] exhibit right-handed or left-handed propensity, as do animals that technically do not have hands. For instance, research has shown that some mice are righties while others are lefties, and some frogs preferentially jump away from predators in one direction over another. Most snail species grow their shells right-handed, with very rare exceptions of evenly right-left populated species. Climbing plants conserve their handedness within each species: all the Loniceras always climb by winding left, all the Convolvolus climb by winding right. The bacterium Bacillus subtilis constantly forms right-handed spiral colonies, which reverse into left-handed when the temperature is raised.
Handedness in animals is related to brain asymmetry, that is, differences in function and anatomy between the two brain hemispheres. For long time it was thought that only humans had a split in brain functions, or lateralization, with some tasks specialized to the left side and others to the right. More recently, however, studies in primates, birds, fish and reptiles, have suggested that asymmetric brains are indeed common in many vertebrates. Clear signs of lateralization have been identified even in extinct species [R. R. Reisz et al., Curr. Biol. 30, 2374 (2020)]. Bill Hopkins, who studies primates at Georgia State University, found that chimpanzee populations are about 65-70 percent right-handed, gorillas are about 75 percent righties, and about 66 percent of orangutans are lefties. However, the underlying evolutionary mechanisms that lead to lateralization are still shrouded in mystery.
By direct experience, I can tell that undergrad students always find it difficult to understand the weak nuclear force. Maybe because we physicists in the first place do not have yet a complete understanding of the story. Of the four (actually three) fundamental forces, the electroweak force is known to violate parity conservation, meaning that it is an interaction capable of distinguishing between left and right. The important physical quantity in this case is helicity, the dot product of a particle’s momentum vector and its spin. In radioactive decays, electron-antineutrino, or positron-neutrino pairs are emitted: while electrons and positrons can have either positive or negative helicity, neutrinos always have h=–1 and are called “left-handed”, and antineutrinos always have h=+1 and are called “right-handed”. Right-handed neutrinos and left-handed antineutrinos do not seem to have a place in our universe, and we still don’t understand why. Our current explanation of the electroweak force, which unites weak nuclear interactions and the electromagnetic interaction, is based on two weak charged (W±) and one weak neutral (Z) currents, which act in a rather different way from the electromagnetic current. As we know well, the electromagnetic force between two electrons is repulsive, since they have the same charge. Instead, the weak W force is felt only by left-handed electrons. As a result, the beta decay produces mostly left-handed electrons, although in principle the two variants should be equally probable. The weak Z force is even more cunning: it makes right-handed electrons to feel more attracted, and left-handed electrons to be slightly repelled, by positive charges such as the atomic nuclei.
Although this asymmetry is observable only at very high energies, it suggests interesting consequences also at low energies. Because of the Z force, when an electron is near the nucleus, its direction of motion tends to align with its spin. The electron density distribution around an atom should therefore incorporate a “helical” motion component that, although extremely weak, can be observed in the absorption and emission of circularly polarized photons. Delicately precise experiments have proved that this is indeed the case, as reviewed by the famous (at least in France) Bouchiat family [“Parity violation in atoms”, Rep. Prog. Phys. 60, 1351 (1997)]. Then, going from atoms to molecules is an obvious next step.
Stereochemistry teaches us that molecule enantiomers must have exactly the same energy. Enantiomers are specular forms of the same molecule that cannot be superimposed after mirror transformation, for example lactic acid or glucose. Such molecules clearly distinguish between left and right, and are therefore dubbed chiral, like any other object having this property. However, our friends chemists include only electromagnetic forces in their highly-refined ab-initio calculations of molecular energies. In this framework, the spin-orbit (electromagnetic) interaction makes electron spin to align against the electron’s direction of motion, such that in right-handed molecules left-handed electrons predominate, and vice versa in left-handed molecules. However, the effect being perfectly symmetrical, the energy of the two enantiomers must be exactly the same. Enter the weak Z force. Because the Z force interacts differently with right- and left-handed electrons, it produces an energy shift in the molecules, which splits the energy of the two enantiomers. This subtle effect was first theoretically predicted in 1985 by S. Mason and G. Tranter, then at the King’s College in London [“The electroweak origin of biomolecular handedness”, Proc. R. Soc. London A397, 45 (1985)], but experiments require a degree of precision that is only now coming within reach [see e.g. Satterthwaite et al., Symmetry 14, 28 (2022)]. Such extra stabilization energy coming from the weak Z force could be at the origin of the dominance of one family of enantiomers over the other, often observed in nature.
The absolute dominance of left-handed amino acids in proteins, and right-handed sugars in nucleic acids, or homochirality, is one of the biggest mysteries in biology. One day in year 1857, while observing under the microscope tiny crystals of tartaric acid produced from wine lees, Louis Pasteur noticed that some molds had grown on the sample. Instead of just trashing the bad sample and moving on, he shone polarized light on it, and surprisingly observed that those crystals changed the angle of light polarization. The big deal was that this change was not happening in clean tartaric acid samples. He reasoned that the clean samples contained statistically a same amount of left-handed and right-handed tartaric acid enantiomers; but, for some mysterious reason, the sample contaminated by molds contained instead only one enantiomer of the kind. Pasteur realized that the chemistry of molds, and by extension of all living cells, has a preferred handedness. He came to view handedness as one of the clearest distinctions between living and non-living. “Life is a function of the asymmetry of the universe – he wrote to the French Academy of Sciences – and of the consequences of this fact: l’Univers est dissymétrique.”
All amino acids but glycine have a L- and D-form (coming from “levo” and “dextro” original Pasteur’s nomenclature). However, when looking at eukaryote proteins we only find the L-forms. The occasional presence of certain D-amino acids in the human body has been linked to several diseases including schizophrenia, amyotrophic lateral sclerosis, and age-related disorders such as cataract and atherosclerosis. [J. Bastings et al., Nutrients 11, 2205 (2019)] How could biomolecules with such a definite chiral preference arise in evolution, when from the chemical point of view the reactions lead with equal probability to L- and D-forms? Was the chiral asymmetry a precondition for the emergence of life, or did it arise later as a consequence of biological constraints? Back in 1953, F. C. Frank developed a simple model illustrating how spontaneous symmetry breaking in a chemical system with two competing species can lead one type of molecules to quickly take over the other, if by some mechanism the enrichment in the one induces the decrease of the other. Which microscopic mechanism could be doing this? In 1957 Vester and Ulbricht, at Yale, proposed that since electrons from beta decay are left-handed, they would emit left-polarized photons when interacting with atoms; such radiation would decompose preferentially one enantiomer, thus increasing the opposite chirality population. Over time, different autocatalytic mechanisms, such as the Soai reaction, have been also proposed to explain the homochiral amplification. The weak Z force could be another obvious candidate, but its strength is so weak that in order to produce the snowball effect of one chiral enantiomer taking over the other, it should be greatly amplified. [See a recent review by N. Hawbaker and D. Blackmond, “Energy thresholds for chiral symmetry breaking in molecular self-replication”, Nature Chem. 11, 957 (2019)]
The handedness of the universe may turn into an obsession. After spending many years of his career at Fermilab searching for charge-parity violation in heavy particles (hyperons) radioactive decay, Michael Longo of the University of Michigan turned to astrophysics. He looked at a few thousand spiral galaxy images provided by the robotic Sloan Digital Sky Survey (SDSS) telescope in New Mexico, and claimed that there was a preferred handedness to the rotation of the spiral galaxies in the SDSS. The galaxies apparently tended to rotate around a common axis, which happened to align with the same direction as the so called Axis-of-Evil anomaly, an intriguing non-random pattern of hot and cold spots in the cosmic microwave background that had been discovered in 2006.
In 2007, BBC Radio 4’s Today programme listeners were asked to go online in their spare time and help catalogue nearly a million galaxies according to their shape and, if possible, the direction of their rotation. As it turns out, such tasks are much simpler for a human to do than using computerized pattern recognition. One year later, almost 150,000 amateur cyber-astronomers had submitted more than 50 million classifications, enabling to carry out scientific research that would have otherwise been impossible: every galaxy had been observed by 40 people on average, reducing the statistical error to a dreamy low limit. Among the spiral galaxies, about 36% had their handedness identified, resulting in a catalogue of 35,000 galaxies with which to explore the supposed handedness anomaly. The initial results of this study showed significantly more anticlockwise than clockwise rotating galaxies. After correction for a number of bias factors, eventually, a more comforting rotation symmetry was obtained, the sense of rotation of the analyzed galaxies appearing rather randomly distributed.
More recent studies, also including data from the SDSS, the Planck orbital observatory and the Pan-STARRS survey network, seem to tip over again the needle towards an intrinsic handedness of the universe. Lion Shamir, of Kansas State University, has conducted a detailed analysis of data from over 200,000 galaxies and found that, although the distribution of clockwise and anti-clockwise rotating bodies is almost even (51 to 49), the spinning distribution forms a distinctly non-random pattern in space, which incidentally seems again to broadly coincide with the Axis-of-Evil asymmetry. Could the early universe have been less random than it appears today, and be spinning about some axis right after the Big Bang?
To end on a more personal note, being left-handed in a right-handed world presents its risks. According to various studies, we lefties have reduced survival statistics in old age and increased risk of accident-related injuries [see e.g. S. Coren, Am. J. Public Health 79, 8 (1989)], likely because so many things in our daily life (e.g. scissors, gearshifts, can openers, configuration of power tools) are designed for right-handed use. But, as Victor Hugo said, The left-handed are precious: they take places which are inconvenient for the rest.