by Paul Davies
I heard Dirac lecture on this topic in Trieste in 1971, at a conference held in his honour. He was clearly still very much enamoured of it. Sadly, he died before the matter was resolved. In 1976 two spacecraft landed on Mars. Called Viking, they went there to find life. In that, they were unsuccessful, but whenever they sent signals back to mission control, astronomers were able to compute Mars’s orbit to very great precision by exact timing of the radio traffic. If gravity was slowly weakening over time, the orbit should be getting slightly bigger, and the Viking radio signals would have revealed that after a few years. They didn’t.
If the strength of gravity is fixed, it seems we are living at an atypical cosmic epoch after all, when the two big numbers just happen to coincide. Isn’t that peculiar? The Princeton astronomer Robert Dicke thought there had to be a scientific explanation, and in 1961 he came up with a clever argument. Why are we living now should be interpreted, he said, as why we are living now. Life is now. There could be no life a hundred years after the Big Bang, for example. Quite apart from anything else, it was far too hot for organic chemistry. More seriously, there was almost no carbon around then. Recall that carbon is the key life-giving element – it puts the organic into organic chemistry. It was made by stars and spewed out into space by stellar explosions (see Chapter 10). Thus, reasoned Dicke, life cannot arise in the universe until at least one generation of stars has lived and died, and disseminated its manufactured carbon around the galaxy.
Dicke did a rough calculation to estimate the expected lifetime of a typical star, when expressed in the same atomic units of time that Dirac used. He knew that the answer would depend on both the strength of gravity, which squeezes the star and heats up its core, and the strength of the electromagnetic force, which determines the rate at which heat can flow out of the star. What Dicke found was that the answer hinges only on the ratio of those two forces, i.e. 1040, the same large number that Dirac puzzled over when contemplating the age of the universe. In other words, the lifetime of a star in atomic units will, simply by reason of basic physics, necessarily be the ratio of the two forces. Dicke also pointed out that life would struggle to survive in the very far future, when all the stars had burned out, because complex life needs sunlight as an energy source. In other words, the epoch of the universe in which carbon-based life can exist is bracketed by the lifetime of the first generation of stars (which lived and died rather quickly about 12 billion years ago), and the dwindling population of stars in the far future – maybe in another hundred billion years. Therefore, the habitable epoch of cosmic history will lie between one and a few stellar lifetimes, a duration that in turn is determined by the ratio of the electric to gravitational forces, 1040. Ergo, no coincidence after all! This line of reasoning is a successful example of the use of the anthropic principle (see p. 125).
Dicke’s argument convinces me: I’m not surprised to be living at this cosmic epoch. But what about on the timescale of human history? I might still have found myself living in Shakespeare’s day, or a million years from now in some wonderful utopia-to-be. Why am I around in the twentieth and twenty-first centuries?
There’s a chilling answer to that question. Maybe I am living now because most people live about now. It’s a sobering fact that a fair fraction of all humans who have ever lived are currently alive. That’s because the population has grown so explosively; for tens of thousands of years there were only a few million people on the entire planet. Now we have 8 billion and rising. A person chosen at random from the set of all humans who have ever existed has a reasonable chance of living in the twentieth or early twenty-first century, simply on statistical grounds. However, that doesn’t explain why I am not living in the future. Given exponential population growth, surely the vast majority of humans will find themselves alive in future centuries? These will be the countless billions of our descendants who will populate the planet (maybe even the galaxy) in millennia after millennia to come. Why am I not one among these teeming trillions?
Perhaps because they will never exist. Imagine that we could take an overview of all human history. For millennia, Homo sapiens jogged along, gradually growing in numbers. Then about 500 years ago the numbers began to surge, rising sharply to, say, 10 billion by the mid-twenty-first century. After that there is an equally precipitous plunge towards zero. With such a narrow population spike, most people who ever exist will live about now, give or take a few decades. So, my existence (and yours) at this time is explained by this gloomy analysis, which goes by the name of the Doomsday argument. Its somewhat back-to-front rationale is that, since there is no reason to suppose you or I are special, we are probably living when most people are living, which of course implies that few people will live in our future. It is Doom Soon.
I hate to end on a depressing note, so I should counter that the same reasoning could be applied to any sentient being. True, humans may be in their heyday, but the universe could be seething with life. There may have been all manner of extra-terrestrials around for billions of years, and many more to come in the future – assuming they aren’t all snuffed out by a cosmic-scale catastrophe. And we can add to that all the super-smart post-biological beings mentioned in the previous chapter. If you count ‘me’ as a member of this broader class – a typical sentient observer – then even if the present cosmic epoch represents ‘peak sentience’ there could still be thinking, joyful beings of some sort inhabiting the universe for billions of years to come.
But probably not for ever, as we shall now see.
28. The Fate of Our Universe
On 21 December 2012, I addressed an audience in New Delhi on the subject of the end of the universe. By coincidence, that was the very day that the world was scheduled to end according to the ancient Mayan calendar. In fact, it was just about the right time of day too. I remarked that I wouldn’t waste time discussing the Mayan prediction, on the basis that, if it were correct, we’d all know about it before the end of the lecture.
Humans have a peculiar fascination with eschatology – the study of the final state of things – particularly apocalyptic endings. The Bible prophesies Armageddon and the four horsemen of the Apocalypse. Norse mythology foretells a period of cold, darkness, devastation and despair followed by Ragnarök, the end-of-world battle to decide the fate of the gods. In the Sermon of the Seven Suns, the Buddha describes a period of accumulating chaos culminating in the Earth being engulfed in fire. Cosmologists are not immune from gloomy theorizing either, as I recounted in Chapter 23, but our timescales tend to be longer – mostly billions of years.
Whereas astrologers and mystics might study occult numerology in the Mayan calendar, cosmologists base their predictions on mathematical modelling using gravitational theory. The same equations that describe how the universe evolved into its present state from the big bang can be extrapolated into the far future to determine its likely fate. I’m talking now just about our very own universe – the one we know and love – and not a bigger multiverse, which, if it exists, might contain any number of horrid universes, some grotesquely flawed or, worse still, full of unremitting evil, the demise of which would only be a good thing.
Before the discovery of dark energy, now thought to dominate the mass of our universe, the choice of fate was binary: either the universe would go on expanding, or it would turn around and collapse. In the latter case, nothing could prevent it from imploding to a zero volume and an infinite density, like the big bang in reverse. This ‘big crunch’ wouldn’t happen any time soon, as the turnaround would take many billions of years.
Actually, some cosmologists think the big crunch could really be a big bounce, following which a new phase of expansion and contraction would ensue. Nor does there have to be a single bounce. It’s possible that the universe has been bouncing along for ever in a cyclic manner, each phase of expansion eventually slowing to a halt and then turning into contraction, presaging the next bounce. There are other bouncy cosmological models around, involving higher dimensions of space
and various obscure refinements. All these theories come with their own problems of physics, not to mention plausibility.
Additional destinies are on offer when dark energy is in play, which has the effect of hastening the long-drawn-out death of the universe. If dark energy remains constant, as Einstein originally posited, the end state is just an ever-expanding, featureless vacuum. On the other hand, dark energy might not be simply the energy of empty space but a weird new substance, sometimes called quintessence, with unusual properties we don’t need to get into here.* Suffice it to say that from the point of view of eschatology, the critical feature is only whether the density of dark energy increases or decreases over time, which in turn affects its power to accelerate the expansion of the universe. It’s possible that dark energy will decline and slowly fade to nothing, or even become negative. In the latter case, the universe could eventually collapse to a big crunch. On the other hand, if dark energy grows with time then a very different cosmic fate lies in store. The rate of expansion will get faster and faster, accelerating without limit, culminating in a so-called ‘big rip’, an alarming state of affairs where space, in effect, comes apart at the seams and ceases to exist. All the above predictions describe a lingering demise, but as we saw in Chapter 23, there is always the threat of sudden, unexpected death from the decay of the quantum vacuum or an outbreak of ravaging spacelessness.
Even if the solar system avoids vanishing into a new quantum vacuum or being swallowed up by a marauding bubble of nothing, doom still lurks close to home. When I was a teenager, I told a girl in our road that the sun could explode at any time. It was clear she was somewhat rattled by this dark warning because she often alluded to it. My motivation for raising the subject with her was a pathetic attempt to win her affections with my loveable nerd persona. It failed utterly. She either considered me an incipient sociopath or an insufferable know-all. Even my Buddy Holly glasses failed to impress; a boring owl was the description that got back to me. Anyway, my dire prediction was completely wrong. The sun won’t suddenly blow up. But it won’t stay the same indefinitely either. Over the next few hundred million years it will steadily swell in size and heat output, until the oceans boil. In the fullness of time, Earth may even be engulfed and dragged into a vaporizing death spiral.
Stars can burn for billions of years but sooner or later they either explode, leaving a remnant core as a black hole or a neutron star, or swell up and then contract to form a slowly fading white dwarf (see Chapter 13). But the action doesn’t stop there. We’ve already seen how black holes gorge themselves on anything going, shredding passing stars and guzzling the gas, or swallowing other black holes. The supermassive black holes at the centres of galaxies do it on a grand scale. As a result of the undiscriminating ingestion, black holes just go on getting bigger, while the rest of the universe becomes progressively denuded of matter.
In spite of their voracious appetite, black holes are not, in fact, the final state. Recall Stephen Hawking’s discovery that black holes generate a faint glow of heat. And it really is faint. A stellar mass black hole has a temperature of a hundred-millionth of a degree above absolute zero, while supermassive black holes (like the one at the centre of the galaxy M87) have an even lower temperature. That’s far cooler than the CMB right now, so the net heat flow is from the cosmos into the black holes. But if the universe continues to expand, the cosmic background temperature will eventually fall below any given black hole temperature. At that point, the black hole will start to lose more energy than it gains, as a result of which its mass will go down, and it will shrink in size.
One of the oddities of this process can be seen on the Westminster Abbey headstone (Figure 13) that displays Hawking’s formula for the temperature of a black hole, denoted by the T on the left-hand side of the equation. On the other side, M stands for the black hole’s mass. You can see that if the black hole loses mass, so M gets smaller, T will get bigger. Unlike, say, a mug of coffee that cools as it emits heat, black holes grow hotter as they radiate, making it a runaway process that speeds up over time. The black hole will shrink faster and faster, finally disappearing completely in a flash of gamma rays plus an assortment of subatomic particles. But black hole radiance is so feeble that the timescale for this denouement is mind-stretching. A solar-mass black hole takes 1067 (that’s 10 million trillion trillion trillion trillion trillion) years to evaporate, while the one in M87 would endure for 1097 (10 trillion trillion trillion trillion trillion trillion trillion trillion) years!
Most of the mass released from black hole radiation will end up as photons, some as neutrinos and gravitons, and a tiny fraction in the form of more massive particles like electrons and protons. This wispy soup of particles will become ever more dilute with the continued expansion, as the universe empties, leaving little but a thinning background of photons, neutrinos and gravitons with energies that sink towards zero. Dark energy, assuming it stays positive, accelerates the emptying-out process. The ultimate end state for such a universe is a total void, at 10−28 degrees above absolute zero, that being – according to quantum mechanics – the irreducible temperature of pure dark energy.
Although the idea is currently out of fashion, it remains possible that the universe will eventually collapse to a big crunch. If it does, the consequences for the objects within it are hardly less rosy. When contraction begins, the CMB temperature starts to rise, eventually exceeding that of any stars that may be left, causing them to explode. Near the crunch point it will become too hot for atoms, then nuclei, to exist. Matter will be squeezed to higher and higher densities as the cosmos enters its death throes. The final state is as uncertain as the initial state of the universe. It could spell the end of spacetime in a singularity, or become a gateway to a new phase of cosmic existence we can only guess at. The various scenarios are summarized in Figure 17.
It says something about the heroic nature of the human spirit that, faced with these equally unappealing cosmic destinies, some theoretical physicists have nevertheless tried to put a brave face on it by asking whether life, or intelligence, or sentience – or something – might cling on for ever if the universe is eternal, or at least go out in a blaze of glory when caught in a crunch or rip. Their analyses are quite technical so I won’t dwell on them here, but the issue, at rock bottom, is whether information processing of some sort can continue unabated in the face of a dying universe. Or if it can’t, whether pseudo-resurrection in the guise of Boltzmann brains (p. 120) is a cause for cheer. It all boils down to resources and thermodynamics, and there is no definitive answer. I doubt that this grim prognosis offers much eschatological solace, but we can at least content ourselves that, in the countless aeons left, there is plenty of time for our descendants to come to terms with it.
Figure 17. How will the universe end? These schematic graphs show the alternative fates of crunch, rip or eternal expansion (not to scale).
To finish once again on a more cheerful note, let me point out that there is a possible escape clause from these dismal predictions. Part of the appeal of the steady state theory was its cosmological and biological immortality. The universe could just go on replenishing itself for ever, and life along with it. As ageing galaxies ran out of habitable locales, advanced civilizations could simply decamp to a younger galaxy, given the many billions of years at their disposal to implement the move. Life could pervade the universe in both space and time. Although the steady state theory is now passé, eternal inflation does much the same job, on a grander scale. Now there is the possibility of entire new universes erupting into existence. If ours is heading for Armageddon, could we not take up residence in another? Indeed, could we engineer our own ‘baby universe’ and fix it up with just the right laws to permit life? Or even, as we explored in Chapter 24, make it more congenial?
The idea of baby universes has been taken seriously by many eminent physicists – Stephen Hawking even published a book on the subject. This takes us back to the rarefied field of quantum gravity (see
pp. 109–10), where adventurous theorists are wont to play fast and loose with the laws of physics. One idea that has cropped up is that the formation of a baby universe might be triggered by the gravitational collapse of a star, with the infant cosmos located through a sort of umbilical tube or wormhole that opens up inside the resulting black hole. In that case, if you fell into the black hole, it might not spell doom. Although you couldn’t come back out again in our universe you might end up in a baby one, new-born, raring to go and expanding in big bang-like fashion. Leaping into black hole portals to universes new would surely be the ultimate in emigration – getting out for good, abandoning our entire cosmos to its miserable fate in favour of another with better prospects.
29. Is There a Meaning to It All?
‘The most incomprehensible thing about the universe is that it’s comprehensible.’
Albert Einstein
I once took part in an impassioned television debate about science and religion. At one point, the conversation turned to the philosophy of reductionism, also known as nothing-buttery, which holds that true reality lies with the fundamental physical building blocks of the world, and the great edifice of human achievements and values and culture is, at rock bottom, no more than an illusory embellishment; to maintain otherwise is sentimental twaddle. One of the panellists used a striking illustration to denounce this harsh viewpoint. Am I to suppose, he said, that when I tell my wife I love her, it’s nothing but one meaningless mound of molecules transmitting sound waves to another meaningless mound of molecules? The philosopher A. J. Ayer, an enthusiastic reductionist and prominent atheist, objected strongly to this comment, claiming that he too loved his wife very much, but that the meaning attached to that endearment was entirely a human construct. It is people who create meaning in their lives, he pointed out: it doesn’t descend from on high. ‘But,’ countered another panellist, Hugh Montefiore, the bishop of Birmingham, ‘you’re claiming there is no ultimate meaning.’ At that point Ayer became exasperated. ‘I don’t know what ultimate meaning means!’ he fumed. And there we have it. Meaning is a concept that enriches human lives. A person can lead a meaningful and rewarding life. But does it make any sense to attribute meaning to nature, or the universe?