Asymmetries exist in the Universe, which is a wonderful thing. Imperfections are necessary for the existence of stars and, therefore, life.
Humans that are left-handed are in the minority, with a 1:10 ratio. But make no mistake: the Universe, from subatomic particles to life itself, favors left-handedness. Indeed, the Universe would be a very different world without this basic imbalance in Nature — bland, primarily filled with radiation, and devoid of stars, planets, or life. Even still, in the physical sciences, there is a pervasive aesthetic that promotes mathematical precision — articulated as symmetry — as the blueprint for Nature. And, as is so often the case, we get caught up in a false dichotomy of needing to pick sides: are you for “all is symmetry” or are you an iconoclast of imperfection?
Why scientists like symmetry is due to antimatter
“Beauty is truth, truth beauty,” as Keats famously said. However, if you insist on equating Keats’ beauty with mathematical symmetry as a means of discovering the “truth” about natural laws — as is common in theoretical physics — the danger is that you will associate symmetry with “truth” in such a way that the mathematics we use to represent the Universe through physics will reflect mathematical symmetry: the Universe is beautifully symmetric, and the equations we use to describe it must reveal this symmetry. Then and only then will we be able to approach the truth.
“It is more vital to have beauty in one’s equation than to have them suit experiment,” said the eminent physicist Paul Dirac. If any other less well-known physicist stated anything like that, they’d be laughed at by their peers and labeled a crypto-religious Platonist or a quack. But it was Dirac, and his elegant equation, based on symmetry ideas, did predict the presence of anti-matter, or the notion that every particle of matter (such as electrons and quarks) has an anti-particle counterpart. That’s a very remarkable achievement: using symmetry mathematics to solve an equation, humanity were able to uncover a whole new domain of matter. It’s no surprise that Dirac was dedicated to the deity of symmetry. It inspired him to make an incredible discovery.
It’s important to note that antimatter doesn’t have the strange connotation that it does. In a gravitational field, anti-particles do not rise. A number of their physical features, most notably electric charge, have been reversed. As a result, the positron, the anti-particle of the negatively charged electron, has a positive electric charge.
Asymmetry is responsible for our existence
But there’s a difficulty that Dirac wasn’t aware of. According to the principles that govern the behavior of Nature’s basic particles, matter and anti-matter should be equally numerous, i.e., they should occur in a 1:1 ratio. One positron for every electron. However, if perfect symmetry had prevailed, matter and antimatter should have annihilated into radiation fractions of a second after the Big Bang (mostly photons). That, however, was not the case. As an excess, around one in a billion (approximately) particles of matter survived. And that’s a wonderful thing, since everything we see in the Universe — galaxies and stars, planets and moons, life on Earth, every form of matter clump, alive and nonliving — emerged from this little surplus, this basic imbalance between matter and antimatter.
Our research over the last several decades has shown that the rules of nature do not apply equally to matter and antimatter, despite the universe’ supposed symmetry and beauty. One of the most intriguing unanswered puzzles in particle physics and cosmology is what process might have caused this little excess, this imperfection that is ultimately accountable for our existence.
There is an internal symmetry operation that converts a particle of matter into one of antimatter in the language of internal (“internal” as in altering a particle’s attribute) and external (“external” as in rotating an item) symmetries. The procedure is known as “charge conjugation,” and it is denoted by the capital letter C. Because of the observed matter-antimatter imbalance, Nature does not exhibit charge-conjugation symmetry: particles and their antiparticles cannot be transformed into one another in certain instances. The weak interactions, which are responsible for radioactive decay, specifically contradict C-symmetry. The culprits are neutrinos, the oddest of all known particles, commonly referred to as “ghost particles” because of their ability to pass through matter almost undetected. (Right now, there are around one trillion neutrinos per second arriving from the Sun and passing through you.)
We need one additional internal symmetry termed parity, which is symbolized by the letter P, to understand why neutrinos break C-symmetry. A “parity operation” transforms a thing into its mirrored counterpart. You are not, for example, parity-invariant. The heart is on the right side of your mirror picture. Parity, like tops, is connected to how particles spin. Particles, on the other hand, are quantum things. This implies they can’t merely spin around in any direction. Their spin is “quantized,” which means they can only spin in a few different ways, similar to how old-school vinyl records could only be played at three different speeds: 33, 45, and 78 rpm. One rotation “speed” is the least quantity of spin a particle may have. (It’s essentially equivalent to a top spinning straight up.) It may rotate either clockwise or counterclockwise when seen from above.) That’s how electrons, quarks, and neutrinos work. They have spin 1/2, which may be either +1/2 or -1/2, corresponding to the two rotating directions. Curling your right palm around with your thumb pointing up is a good way to notice this. Positive spin is counterclockwise, whereas negative spin is clockwise.
We should receive a left-handed anti-neutrino if we apply the C operation to a left-handed neutrino. (Yes, even if the neutrino is electrically neutral, it has an electrically neutral anti-particle.) The issue is that in nature, there are no left-handed anti-neutrinos. Only left-handed neutrinos exist. Charge conjugation symmetry is violated by neutrinos’ weak interactions, which are the only interactions they have (apart from gravity). For those who like symmetry, this is a problem.
Asymmetry triumphs in the case of a CP violation
Let’s take it a step farther. We should receive a right-handed anti-neutrino if we apply both C and P (parity) to a left-handed neutrino: the C flips the neutrino into an anti-neutrino, and the P flips left-handed into right-handed. Anti-neutrinos are, in fact, right-handed! We seem to be in good shape. The weak interactions violate C and P independently, but the overall CP symmetry operation seems to be satisfied. In practice, this implies that left-handed particle reactions should occur at the same rate as right-handed anti-particle reactions. Everyone felt a sense of relief. In all known interactions, there was optimism that Nature was CP-symmetric. Beauty had returned.
The thrill did not last long. In the decays of a particle termed neutral kaon, denoted as K0, James Cronin and Val Fitch found a tiny violation of the combined CP-symmetry in 1964. In other words, K0 and their anti-particles do not decay at the rate predicted by a CP-symmetric theory. The community of physicists was stunned. Beauty had vanished. Again. It has never fully recovered. CP violation is a natural occurrence.
There are so many inconsistencies
Particles choose a preferred direction of time as a result of CP violation, which has a deeper and more puzzling significance. At the microscopic level, the asymmetry of time, which is a hallmark of an expanding Universe, occurs! This is enormous. It’s so big, in fact, that it’ll soon be deserving of its own article.
And there’s one more shocking truth regarding imperfection that we’ll discuss. Life is also “handed”: all living species, from amoebae to grapes to crocodiles to humans, have left- and right-handed amino acids and sugars. We generate 50:50 combinations of left-handed and right-handed molecules in the lab, but this isn’t how nature works. Left-handed amino acids and right-handed carbohydrates are nearly entirely preferred by life. Again, this is a large open scientific subject on which I’ve spent a lot of effort. Next time, let’s go there.