What's Eating the Universe?

by Paul Davies

  Although this line of inquiry seems on the right track, nobody has yet pulled it all together to account for the actual degree of matter–antimatter asymmetry. It’s possible, however, that we are on the verge of a breakthrough on this puzzle. A new clue was announced in April 2020 from an experiment in Japan, in which beams of neutrinos and anti-neutrinos were shot through the Earth from a particle accelerator in Tokai, on the east coast, to an underground neutrino detector located at Kamioka, near the west coast. During the brief duration that the beam traversed 295 kilometres of solid rock, the particles underwent subtle changes. The degree of change differed slightly between neutrinos and anti-neutrinos, hinting at a symmetry violation that could be big enough to explain the mystery of the missing cosmic antimatter.

  All of which leaves us with an interesting philosophical question. Why are there any asymmetries in the laws of nature? The universe would be altogether simpler if everything was balanced, although there would be no stars or planets, or beings to admire the simplicity. But it’s possible one could have one’s cake (or rather one’s universe) and eat it too, if the observed imbalance in our universe is compensated for elsewhere. For example, maybe there’s a mirror universe where all things are transposed, a universe made of antimatter with the arrow of time reversed, relative to us. An Anti-World. Where might this Anti-World be? There are many possibilities: through a black hole or wormhole in space, interpenetrating our world but invisible to us, a parallel world . . . Or perhaps the big bang is a symmetry point, with time’s arrow pointing away from it in both directions – an idea suggested by Sakharov himself. There’s no evidence for any of these speculations, but they do have a certain attractiveness, especially for those scientists who think that nature must be both simple and elegant. Symmetry is undeniably a factor in beauty, as any cosmetician will tell you. But in cosmology too? Well, beauty is in the eye of the beholder, so perhaps a little asymmetry also has aesthetic appeal.

  Spin

  Now here’s something to make your head spin. Dirac’s equation predicts that electrons spin like tiny tops. All electrons do it at exactly the same rate, unceasingly: it’s intrinsic to their nature. But the equation also says there’s something really weird about intrinsic spin compared to, say, a gyroscope or the spinning Earth. If you hover over the North Pole and look down, you see the Earth spinning anticlockwise. If a cosmic giant tipped the planet upside down before your eyes, you would now find yourself looking down on the South Pole and Earth would be turning clockwise. Roll the planet round another half-turn and all would be restored. Nothing confusing about that. Electrons, however, behave quite differently when rotated through one complete revolution (360o): they don’t come back to where they started. You have to roll them right around twice (i.e. through two complete revolutions, or 720o) for them to return to their starting state. Physicists can actually test this property using magnetic fields to rotate electrons. It’s as if these point-like particles possess a double view of the world. Just as some people are colour-blind and miss the full splendour of the world about us, so humans are ‘geometry-blind’ and can see only half of reality.

  13. Gravity Conquers All

  In 1930, a twenty-year-old Indian student named Subrahmanyan Chandrasekhar was sailing from what was then Madras (now Chennai) to England to study astrophysics at Cambridge University. To occupy his time during the long voyage, he began toying with the equations describing the stability of stars. From those untutored jottings came a discovery so shocking that it almost wrecked the young student’s career.

  Astronomers of the day had only a sketchy understanding of what makes stars tick. They knew that a star is a ball of hot gas engaging in a mammoth balancing act. The gas tries to expand into the vacuum of the surrounding space, but gravity holds it back. In stars like our sun, an equilibrium is achieved, so long as the gas stays hot, which requires it to consume fuel. But what happens when the fuel runs out? It seems that gravity would inevitably prevail, making the star contract; the smaller the radius, the fiercer the gravitational grip would become. Astronomers had long been familiar with tiny stars known as white dwarfs, which contain a mass comparable to the sun but squashed into a volume the size of a planet. These burned-out stars are so compressed that their atoms are squeezed cheek by jowl.

  It was once thought that the laws of quantum physics would prevent any further compression, but from his shipboard deliberations Chandrasekhar concluded otherwise. His equations suggested that if a star has a big enough mass, the crushing effect of its immense gravity would cause the atomic electrons to rattle around so fast they would approach the speed of light. Far from the star stuff getting more rigid, the effect would be to make it even more compressible, presaging a cataclysmic implosion. In the absence of any other factor, the ball of matter would plunge down its own gravitational well and disappear into what mathematicians call a singularity – a point of infinite density and geometrical curvature representing a boundary or edge of space and time.

  Chandrasekhar was able to calculate the critical mass above which this instability would set in. The answer he obtained was 1.44 solar masses, now known as the Chandrasekhar limit. On reaching England, he proudly announced his result, only to find he was dismissed as a young upstart peddling frivolous nonsense. In the 1930s, the gravitational collapse of a star was considered too outlandish to take seriously. The most distinguished astronomer of the day, Sir Arthur Eddington (he of the bent light beams), publicly ridiculed Chandrasekhar during an infamous encounter at the Royal Astronomical Society, declaring that there should be a law of nature ‘to prevent a star from behaving in this absurd way!’ It was a devastating put-down for a brilliant aspiring scientist. Chandrasekhar was so stung by Eddington’s derision, he decided to leave Britain and settle in the United States, where he followed a highly distinguished career until his death in 1995.

  Eddington was no fool, yet on this subject he was wrong. If a burned-out star has a mass exceeding Chandrasekhar’s limit, it does indeed undergo a spectacular convulsion, collapsing and exploding at the same time: the core implodes in a tiny fraction of a second, while the rest of the star gets blasted into space in what is known as a supernova. An explosion of this type was witnessed by Chinese astronomers in 1054. One possible fate of the core is to form a neutron star, in which the very atoms are crushed by gravity into a ball of neutrons about the size of a city. Neutron stars were discovered in the late 1960s (see the next chapter) and today many are known and intensively studied. There is one where the 1054 supernova was located, and the shattered remains of the rest of the stricken star now form a cloud of gas known as the Crab nebula (see Figure 11).

  Figure 11. The Crab nebula, showing the detritus from a star seen to explode in 1054. A neutron star (not visible in the picture) lies near the centre, a relic of the original star’s core that imploded under its own weight.

  More massive stars end their days by totally collapsing to black holes. Although it took decades for the concept of a black hole to be fully understood and accepted, the basic idea was implicit in general relativity almost from the start. A paper published by Karl Schwarzschild in 1916 contained the fundamental concept. But for decades it was dismissed as a mathematical artefact with no physical meaning. Even Einstein would have none of it. It took the youthful genius of Chandrasekhar to prove that such an object could exist and form from the transformation of a dying star. And in 1972, the first black hole – a stellar remnant in the constellation of Cygnus – was discovered by astronomers.

  The subject of black holes is now an entire branch of astronomy in its own right, and we have to take seriously what Eddington and Einstein refused to confront: that some stars can undergo total gravitational collapse. When this happens, the effect on the imploding stellar material is violent in the extreme. But as we shall see, the effect on time is even more violent.

  14. Warped Time and Black Holes

  Many is the occasion that my wife has been so engrossed in shopping that she returns, l
ate and flustered, with a throwaway remark like ‘Sorry, the time absolutely flew by!’ Whereas I, hanging about waiting for her with nothing to do, think she has been gone ages. Yet the actual interval of time was exactly the same for both of us: the perceived discrepancy is all in the mind. But what if the duration between the same two events – ‘wife enters shop, wife emerges from shop’ – really was different for the two of us, not just psychologically but as measured by an accurate clock? The idea may seem bizarre, but that is exactly what is implied by the theory of relativity. Not only can space be warped, but time can too.

  Warped time is one of those dazzling topics that makes physics such a thrill to study. As a teenager, I liked to bewilder my friends and family with fun facts about clocks getting out of kilter and twins ending up with different ages. Mostly they didn’t believe a word of it. And in fact, I still get contacted by sceptics who flatly refuse to accept that time can be warped. They tie themselves up in all sorts of logical knots attempting to refute it. Time is one of those things we feel inside ourselves in a visceral way, and people can react strongly when told that this most primitive of experiences isn’t solidly rooted in reality.

  When I was a young lecturer at King’s College London in the 1970s, I worked close to the offices of the journal Nature. They got so many papers submitted by lay scientists trying to pick holes in the time-warping predictions of the theory of relativity that they had a special section to deal with them, and I used to go along every couple of weeks to sift through the submissions and frame polite responses. Mostly this did the trick, but there were a few tiresomely persistent correspondents, and some who even threatened legal action if their arguments were rejected.

  Warped time was already a decades-old idea when I dealt with these submissions. As long ago as 1915 Einstein predicted that gravity slows time, and he gave a formula for how much. For example, a clock at the summit of Mount Everest should tick 0.0000000001 per cent faster than Big Ben, meaning it would gain about a second every 33,000 years. It happens because the top of Everest is farther from the centre of the Earth than London, so gravity is slightly weaker there. Though extremely small, the effect is real enough: it shows up clearly in the Global Positioning System, which uses highly accurate timing on board orbiting satellites.

  For decades, gravitational time warps were largely ignored by astrophysicists as being too small to be of interest. But all that changed dramatically in 1967 with the discovery of the first neutron star by a Cambridge postgraduate student, Jocelyn Bell. In the glorious tradition of impoverished British science, Bell had hooked up a radio receiver to a lot of, in effect, chicken wire, strung out between poles in a field, forming a cheap and cheerful radio telescope. One day she noticed what she described as ‘a little bit of fuzz’ in the printed trace of the signal. It turned out to be a spinning neutron star emitting regular radio pulses. At the time, Bell and her PhD supervisor, Antony Hewish, wondered whether the pulses might be a signal from an alien civilization, so they dubbed the source LGM, for Little Green Men, and kept a lid on it pending further evaluation. Had it been an alien message, Bell would have been immortalized for an entirely different reason. In the event, she and Hewish soon found another pulsing radio source, and it became clear they were dealing not with ET, but a new class of astronomical object.

  On the surface of a neutron star, gravity is so fierce that a hypothetical clock would be slowed by about 30 per cent relative to Big Ben. Because neutron stars are held together so tightly by gravity, they can spin up to a thousand times a second without coming apart. As they rotate, they emit slender beams of radio waves that paint the galaxy like a lighthouse beam. From Earth, the beams are detected as brief repeating radio pulses, arriving with extraordinary regularity; pulsars are actually the most accurate clocks in the universe. It was these staccato pulses that Bell first noticed. When, a few years later, a pair of neutron stars was found locked in close orbit, the system became a physicist’s dream for testing general relativity. As the stars cavort around each other, spacetime buckles and writhes, and the pulsar clock faithfully tracks the shifting time warps.

  For a ball of matter with a given mass of material, the strength of gravity at the surface rises as the radius is reduced. If, by some magic, Earth were squeezed to half its radius, clocks at the surface would be slowed by an additional 0.0000001 per cent. Go on shrinking and the warp factor would rise faster and faster until, if Earth were squashed to the size of a large pea, it would become infinite. At this point the escape velocity from the Earth’s surface would reach the speed of light, so no light could escape. The pea-sized Earth would now be a black hole. As long ago as 1783, an English clergyman named John Michell deduced there could be objects in space with gravity strong enough to prevent the outflow of light. That was long before general relativity, but it turns out that Newton’s theory of gravity yields the same formula: light can’t escape when a ball of matter is confined to less than a certain specific tiny radius. And black holes certainly are tiny by astronomical standards: a 10 solar mass one, for example, has a radius of only about 30 kilometres.

  Because light provides an absolute speed limit, no information about the interior of a black hole can get out. It is surrounded by a one-way imaginary membrane called an event horizon, so named because we cannot know from outside what events happen inside. Black holes swallow everything and give nothing back. That means the identity of anything that falls in is lost. Whether formed from matter or antimatter or green cheese, black holes all look the same from the outside.

  So much for ‘black’; what about ‘hole’? Inside the event horizon, even outwardly directed light gets pulled inwards, towards the centre. Light tries to get away but is dragged back by the enormous gravity. Matter can’t go faster than light, so no material in the universe is stiff enough to remain at a fixed radius when light itself is being hauled inwards. Therefore, the material of the object that imploded also has to go on shrinking, perhaps to a point of infinite density – a singularity – leaving behind empty space. The surroundings are thus both black (when viewed from afar) and empty; hence a ‘black hole’ in space.

  People often wonder what it would be like to fall into a black hole. Would time freeze? The answer is no. Time is relative. It stands still only in relation to a distant clock. For you, the in-falling observer, nothing weird happens to time, or to a travelling clock you take along for the ride. But if you compare your clock readings with, say, those back on Earth, you would certainly notice a huge and escalating discrepancy as you plunge to oblivion. Actually, the inferred time warp would be the least of your problems. The intense and varying gravity would rip you to shreds before you even entered the black hole, an effect often described as ‘spaghettification’.

  Many black holes are the collapsed remnants of stellar cores, as Chandrasekhar foretold, but there are also supermassive black holes at the centres of galaxies; nobody is sure how these formed. The Milky Way, for example, contains a black hole with a mass equivalent to about 2 million solar masses, while the galaxy M87 has a monster equivalent to 6.4 billion suns. These awesome objects have notoriously voracious appetites and will vacuum up anything – stars, gas, dust, other black holes – that venture too close. But they are messy eaters. The ingestion events are so turbulent and violent that a huge amount of energy gets released as the tormented material churns about and swirls down the hole. Some of this energy is swallowed by the black hole itself, but a large fraction spews out into space, often blasting surrounding material into narrow high-energy jets. In March 2020, the largest such explosion ever recorded was detected coming from a giant cluster of galaxies about 390 million light years away in the constellation of Ophiuchus. The energy of that single momentary burst was equivalent to a hundred billion times the energy output of the sun over its entire lifetime. Violent outbursts like these make black holes look like explosive sources of matter rather than sinks, which explains why Fred Hoyle thought they represented the creation of new matter (see ‘Did the b
ig bang really happen?’, pp. 21–2). But far from facilitating the birth of matter, they spell its death.

  15. Is Time Travel Possible?

  It’s not often you come across yourself in a novel, but that’s what happened to me when I read Gregory Benford’s award-winning book Timescape. Greg, whose day job is astrophysics, came to visit me at King’s College London in 1976 while researching the novel, to talk about the nature of time and what distinguishes past from future. The story describes a world in 1998 sliding towards eco-doom, and a band of heroic scientists (including one Paul Davies, still labouring away at King’s College) cook up a scheme to send messages back in time to 1962, warning of the coming catastrophe and urging humanity to take steps to avert it.

  Signalling through time is challenging enough, but what about actually travelling through time? If you don’t like living now, is it possible to go to somewhen else? It’s a tantalizing idea. What child hasn’t longed for their next birthday, or Christmas, or a summer holiday? When you are young and bored it seems like ages to wait for future treats. In my own childhood, I used to fantasize about speeding up the flow of time or, even better, leaping over the intervening weeks. I did this too when something unpleasant loomed, like a visit to the dentist. Wouldn’t it be great, I mused, to press a button and hop to the day after the appointment?