In the early 17th century, Johannes Kepler discovered that the paths of the planets around the Sun are ellipses and not the circles beloved of the Greeks. In the late 17th century, Isaac Newton explained this. An ellipse is the path of a body experiencing a force of gravity that weakens according to an inverse square law—one that is four times as weak at twice the distance, nine times as weak at three times the distance, and so on.
Newton’s law of gravity is “universal”, which means that every mass pulls on every other mass. Consequently, each planet is tugged not only by the gravity of the Sun but by the gravity of every other planet. This causes a planet’s elliptical path to gradually change its orientation in space, or “precess”. In the 19th century, however, astronomers discovered something odd about Mercury, the closest planet to the Sun. Even after its dance to the tune of the other planets had been taken into account, there was a tiny bit of precession left unaccounted for.
To explain the “anomalous procession of the perihelion of Mercury”, the French astronomer Urbain Le Verrier predicted the existence of a blisteringly hot world closer to the Sun that was tugging on Mercury. For 50 years, there were intermittent claims to have seen “Vulcan”. But no such planet existed (though the name Vulcan survives today as the home world of Star Trek’s Mr Spock). On 18th November 1915, however, Albert Einstein gave a lecture to the Prussian Academy of Sciences in Berlin in which he explained the anomalous motion of Mercury with a revolutionary new theory of gravity.
Newton, a titanic figure in science, was wrong. Planets do not orbit because of an invisible tether connecting them to the Sun, but because a mass like the Sun creates a valley in the surrounding space-time and a planet rolls around that valley like a ball in roulette wheel. The reason Mercury’s orbit departs noticeably from the Newtonian prediction is that, being closest to the Sun, it must navigate the steepest part of the valley of space-time.
Einstein’s theory of gravity—the general theory of relativity—not only predicted the existence of black holes, but also the idea that the universe cannot have existed forever but began in a “big bang”. And it was a minuscule discrepancy in the motion of the planet Mercury that provided the first evidence of the theory.
The anomalous case of Mercury—and its significance—is described by Cambridge particle physicist Harry Cliff in his new book, Space Oddities. Anomalies in physics are gold dust. And today, new ones are desperately being sought in the hope that they might unlock the door to further levels of understanding of nature. It’s this hunt that is the general subject of Cliff’s book.
We need to find anomalies because our best theories of the microscopic and macroscopic worlds—the “Standard Models” of particle physics and of cosmology—are incomplete. They fail to explain important features of the world, so are considered to be approximations of deeper, more fundamental theories.
Take the Standard Model of particle physics, which describes how the fundamental particles of matter—the quarks and leptons—are glued together by three fundamental forces. It provides no explanation for why the particles have the masses they have; why forces have the strengths they have; and why, bafflingly, nature has chosen to triplicate its basic building blocks. If this is not bad enough, it ignores the mysterious “dark matter” and “dark energy” that add up to 95 per cent of the mass-energy of the universe.
Unfortunately for physicists, points out Cliff, the Standard Model, despite being incomplete, is stunningly good at predicting what is observed in experiments. Consequently, physicists scrabble around for anomalies in the nth decimal place of their measurements.
Cliff recounts in much detail his own search for anomalies. He works on the “LHCb” experiment at the 27km subterranean Large Hadron Collider, near Geneva, where protons are collided head-on at stupendous energies and the physicists sift through the subatomic debris in the hope of finding their speck of gold dust. The “b” stands for “beauty”, one of nature’s six “flavours” of quark. It is a super-heavy version of the down-quark, which, along with the up-quark and electron, helps to make up all atoms.
A beauty quark is interesting, says Cliff, because it is unstable and 10,000 times heavier than an electron, which means there are a host of lighter particles for it to disintegrate into and hundreds of ways it can do that. “Some of the decays are particularly sensitive to the hidden influence of new forces or particles, perhaps even particles of dark matter,” says Cliff. “So, if we make precise measurements of the properties of these decays and they come out different from theoretical predictions made using the Standard Model, then that can provide indirect evidence of the existence of something beyond our current understanding.”
Cliff and his colleagues ride a rollercoaster from hope to despair and back again as anomalies pop up, disappear when theory and experiment converge, only to be replaced by new putative anomalies. With any anomaly, says Cliff, there are four possible explanations: “1. The result is a statistical fluke, 2. The experimenters made a mistake, 3. The theorists got their sums wrong, 4. You’ve discovered something! Have a biscuit.” Sadly, for Cliff and his colleagues, the Rich Teas have stayed in the jar.
As one tutor wrote above his desk: ‘I’ve learned so much from mistakes, I think I’ll make another’
But Cliff is optimistic. You have to be, as an anomaly hunter. Even when glitches turn out to be mirages, he says, it is still possible to learn new things about how to avoid fooling yourself. As one of Cliff’s undergraduate tutors wrote above his desk: “I’ve learned so much from mistakes, I think I’ll make another.”
Unusually for a scientist, Cliff writes not only about his own field but, commendably, acts as a journalist and goes and talks to other physicists about their own hunts for anomalies. One such anomaly was found by the Antarctic Impulsive Transient Antenna, or Anita. “The instrument, whose 48 gleaming microwave horns make it look rather like a giant sound system, floats suspended from a NASA balloon as big as football stadium, slowly circling the continent on the circumpolar winds,” writes Cliff.
Cliff gets the inside track from Anita team member Linda Cremonesi, of University College London, who describes the process of the antenna being assembled near McMurdo Station, a US research station on Ross Island. If you think this sounds somehow romantic—scientists and technology competing against the Antarctic ice—think again. Water must be conserved, so showers are restricted to two a week and laundry to once a week. “Everyone ends up a little bit smelly,” says Cremonesi. When a snowstorm makes it impossible for the team to return to their base from the hangar where Anita is being put together, they have to sleep on the floor with—perhaps this is too much information—“buckets strategically placed in the corners to serve as makeshift toilets.”
Launched into the Antarctic sky, Anita detects the radio waves from subatomic debris created as high-energy neutrinos slam into the ice from space. Paradoxically, however, the experiment spots radio waves from an impossibly energetic particle coming not from the sky above but from the ice below. The exciting speculation is that this may be evidence of an “axion”, a hypothetical particle that might belong to the universe’s dark matter, which is known to outweigh the visible stuff by a factor of six.
Disappointingly, the result is not corroborated by the IceCube neutrino detector, near the Amundsen-Scott South Pole Station. And this highlights that Cliff’s account is a survey of odd results that, by their nature, are unlikely to stand the test of time. It can make the book feel a little bit of a tease—even if Cliff is an engaging writer who communicates the excitement of doing science and weaves in lots of interesting physics.
Not surprisingly, Cliff is good on that physics. However, in moving from the microscopic to the macroscopic world, he does let through the following: “Around 13.8 billion years ago, the universe burst into being from a tiny point, far smaller than an atom”—which, unfortunately, reinforces the common misperception of non-scientists that the Big Bang happened at a point. In fact, Einstein’s theory merely describes the separation between masses growing—though it is true that the region around each point reachable by an influence travelling at the speed of light was indeed once smaller than an atom, which I think is what Cliff means.
Arguably, it is the macroscopic (rather than the microscopic) realm that features the anomaly with the best chance of being real and heralding new and exciting science. It concerns what is known as the “Hubble constant”—the cosmic expansion rate. Astronomers estimate it in two distinct ways. First, they observe objects receding in the relatively local universe. Second, they observe the “afterglow” of the Big Bang fireball. This “cosmic background radiation” tells them the expansion rate when light first broke free from matter, and, from this, they extrapolate to the present day. The two expansion rates should be the same. But they aren’t. Local observations indicate an expansion rate greater than that of the cosmic background radiation. This is the “Hubble tension”.
The best bet is that astronomers have made a mistake. To measure how fast objects such as galaxies are fleeing, it is necessary to know their distance. But, since we cannot go out in the universe with a tape measure, this is difficult. Most likely, the Hubble tension, like all the other anomalies, will go away. But maybe it won’t. Perhaps it is telling us that we have overlooked a major cosmic ingredient that has boosted the expansion rate of the universe.
Given the incomplete nature of our best theories about physics and cosmology, it is a dead cert that, sooner or later, one of the anomalies will turn out to be real. Cliff clutches onto this hope. Somewhere, he is sure, is an anomaly as important as the motion of Mercury—and it will open up a new world to physicists, just as revolutionary and profound as Einstein’s general theory of relativity.
