For millennia, people have gazed in wonder at the world about them and asked the big questions: how did the universe come to exist? What is it made of? Where do human beings fit into the picture? Is there some sort of meaning behind the great cosmic scheme of things?
Until recently, such questions have been restricted to religion and philosophy. Now scientists are starting to weigh into the discussion too, propelled by spectacular discoveries in astronomy and subatomic physics. And one puzzle above all has grabbed their attention: the universe looks suspiciously like a fix.
The issue concerns the existence of life, which balances on a knife-edge, delicately dependent on a number of special features obligingly built into the structure of the universe.
IMAGINE PLAYING GOD, and tinkering with the properties of the universe. Before you is a machine crammed with knobs and dials — call it a 'Designer Machine'. One knob controls the strength of gravity, a lever varies the masses of all electrons, yet another dial fixes the number of spatial dimensions, and so on. It turns out that to set the variables for the universe we see today, you need to adjust the position of about 30-something knobs, and everything else follows from them. And here's the rub.
Change just a few of the settings (or 'parameter values', to use the jargon) even an infinitesimal amount, and there would be nobody around to witness the result. Unless the settings are unerringly close to their present values, we'd have no universe, no life and certainly no humans.
One of the best-known examples of this life-friendly 'fine-tuning' of the laws of physics concerns carbon, the element on which all known life is based. The Big Bang that kicked off the universe coughed out plenty of hydrogen and helium, but no carbon. So where did the carbon in our bodies come from? The answer was worked out in the 1950s: most of the chemical elements heavier than helium were manufactured in the cores of stars, as the product of nuclear fusion reactions. It is the energy released by these reactions that makes the Sun and stars shine.
However, while the details of stellar nuclear reactions are fairly straightforward, there is a notable exception: carbon. Most nuclear reactions in stars occur when two atomic nuclei, rushing around at tremendous speed care of the searing temperatures, collide and fuse, forming a heavier element. But carbon cannot be made this way because all the intermediate steps from helium to carbon involve highly unstable nuclei. The solution, spotted by University of Cambridge astronomer Fred Hoyle, is for carbon to form from the simultaneous collision of three helium nuclei.
THERE IS, HOWEVER, a snag. The chances that three helium nuclei will come together at the same moment are tiny. So Hoyle reasoned that a special factor must be at work to boost the rare reaction and lead to our abundance of carbon. If not, then life in general, and Fred Hoyle in particular, would not exist!
Hoyle knew that nuclear reactions can sometimes be greatly amplified by the phenomenon of resonance, similar to the way that an opera singer can shatter a glass by hitting a certain pitch. Carbon nuclei can resonate too, if the masses and energies of the colliding particles that go to form it are just right. Hoyle worked backwards — he knew the particle masses and energies, and he used them to predict the existence of a carbon resonance.
He then pestered Willy Fowler, a nuclear physicist at the California Institute of Technology, to do an experiment to test the prediction. And sure enough, Hoyle was right. Carbon has a resonant state at exactly the right energy to enable stars to manufacture abundant carbon, and thereby seed the universe with this life-encouraging substance.
Hoyle immediately realised just what a close-run thing this mechanism is. Like Baby Bear's porridge in the story of Goldilocks, the energy of the carbon resonance has to be "just right". Too high or too low, and the consequences for life would be catastrophic.
So what determines the carbon resonance? Ultimately it depends on the strength of the force that binds protons and neutrons together in the nucleus. That force is one of the unexplained parameters of basic physics — one of the knobs on the Designer Machine if you like. If the strength of the force that determined the carbon resonance was only a fraction stronger or weaker, it is doubtful there would be observers in the universe to worry about the distinct absence of carbon.
Hoyle himself was deeply impressed by this discovery. "It looks like a put-up job," he quipped. "A commonsense interpretation of the facts suggests that a superintellect has monkeyed with physics," he later wrote. A similar conclusion was reached by the Princeton physicist Freeman Dyson: "In some sense, the universe knew we were coming."
IN OTHER WORDS, it smacks of 'Intelligent Design', a modern re-working of creationist arguments aimed at getting it taught in U.S. schools. Leonard Susskind, a Stanford University theoretical physicist and one of the fathers of string theory, agrees. Pointing to a catalogue of all-too-convenient "coincidences" in the laws of physics, he thinks we can no longer adopt a blinkered attitude to the weird bio-friendliness of the universe. "There is an elephant at the back of the room," he says.
So what is going on? Some religiously-inclined individuals have seized on the enigma of cosmic bio-friendliness as evidence that a benevolent God has 'fine-tuned' the laws of physics with the deliberate intention of making conscious beings like us. Needless to say, most scientists take a dim view of that explanation.
Unfortunately, orthodox science doesn't have a ready response, apart from the evasive answer that if the universe didn't have the properties it does, there would be nobody around to bemoan the fact. This popular rejoinder carries the unstated implication that the problem of why the universe is so peculiarly suited to life can be shrugged aside as an inconvenient truth.
Recently, however, some world-renowned cosmologists claim to have solved the enigma at a stroke. Led by Martin Rees, president of Britain's Royal Society, their theory goes like this.
Suppose our universe isn't all that there is, but is merely an infinitesimal component of a vast and elaborate patchwork quilt of different universes — a 'multiverse'. The laws of physics, rather than being universal, are more like local by-laws, each universe having its own distinctive set, perhaps allocated randomly in a gigantic cosmic lottery. In that case, the vast majority of universes would lack the delicate fine-tuning that biology demands, and so would go unobserved.
But by chance, a tiny fraction of universes would possess just the right laws — with just the right values of the biologically critical quantities — for life to emerge. It would then be no surprise that we find ourselves in a life-encouraging universe because we could hardly find ourselves in one that forbids living organisms. What at first seems like a fix is in fact nothing of the kind; we have simply hit the cosmic jackpot.
SO IS THAT IT? Mystery solved? Well, not quite. This 'multiverse' explanation is just a little too glib. For example, what about the multiplicity of other universes? Not only is it extravagant to invoke all that cosmic real estate, there has to be some sort of universe-generating mechanism to create the varied patches in the cosmic quilt.
Several mathematical theories exist that describe the creation of universes. In fact, one of these, called eternal inflation, is the currently favoured explanation for the Big Bang. And a by-product of the inflation theory is the prediction of countless other Big Bangs, in vastly distant regions of space and time. However, the need to assume — without any explanation whatsoever — a universe-generating mechanism, an inflating space, and a host of other advanced physical ideas, leaves the multiverse theory falling far short of a complete explanation for why the universe is as it is. So while a multiverse is a distinct improvement on Intelligent Design, it is by no means a panacea.
I suspect we can do better than this. Previous attempts to explain why the universe is fit for life all suffer from a fundamental shortcoming: they appeal to something outside the universe: an unexplained god, physical laws that exist reasonlessly, or a vast ensemble of other universes.
It would surely be better to try and explain as much as possible about the universe by appealing to processes within it. But here we hit a major snag. Scientists have always assumed that the basic laws of physics are immutable mathematical relationships, imprinted on the universe at the moment of the Big Bang. It is a viewpoint often called Platonism, after the famous Greek philosopher. According to Plato, mathematics isn't an invention of the human mind; rather, it exists independently in a non-material realm of perfect, idealised forms that lie outside the physical universe. Mathematics is discovered, not created.
Many theoretical physicists follow in the Platonic tradition, and envisage the laws of physics as infinitely precise eternal mathematical relationships that are simply 'there', transcending physical reality. Note the curious asymmetry involved here: the universe depends for its properties on the laws, but the laws are in no way affected by the universe. I believe we will never achieve a satisfactory and complete scientific account for why the universe is as it is so long as we cling to Platonism — to externally-imposed, immutable mathematical laws.
BUT WHAT'S THE ALTERNATIVE? In recent years, a radically new view of physical laws has emerged, prompted by the growth of the science of computation. At rock bottom, a law of physics is simply an algorithm that takes input data and returns output data. The motion of the planets round the Sun is an easy example — it can be calculated using Newton's laws of motion and gravitation. Knowing the positions and motions of the planets today, we can work out where they will be, say, this time next year. So today's information about the planets is 'processed' by the laws, and next year's information is delivered as output. Looking at it this way, the laws of physics are akin to computer software. And the hardware? Well, the universe itself, of course.
Regarding the universe as a gigantic computer on which the laws
of physics are 'run' prompts us to ask a curious question. Any computer is fundamentally limited in its performance by two factors: speed and resources. So is there an analogous limit to the power of the Great Cosmic Computer?
The answer is yes. The universe may be vast, but it is finite in both age and size. The reason for the latter concerns the finite speed of light. Since the Big Bang that gave birth to the cosmos 13.7 billion years ago, light can have travelled at most 13.7 billion light-years. So there is a 'horizon' in space beyond which we cannot see, however good our instruments may be. As nothing can travel faster than light, the horizon represents a basic limit to communication. Expressed simply, two regions of the universe beyond each other's horizon cannot combine the results of their 'computation'.
It is fairly straightforward to work out the theoretical maximum number of bits of information contained within a volume of space encompassed by the horizon. For example, our volume contains about 1080 atoms, each capable of representing a few bits of information. Taking into account all the known particles, including photons, neutrinos and gravitons, the total information content of the universe is no more than about 10120 — that is, one followed by 120 zeros.
What is the significance of the finite informational capacity of the universe? In the Platonic view, there is no significance, because Mother Nature computes in the Platonic heaven of infinitely precise mathematical relationships and infinite resources. But according to the software view of the laws, it is meaningless to invoke any mathematical procedure or relationship that exceeds the theoretical information limit.
For example, a well-known law of physics states that electric charge cannot be created or destroyed. The orthodox interpretation of this statement is that the charge carried by, say, an electron, cannot vary one iota: the charge is fixed to infinite precision. But the software view of the laws denies that any physical quantity can be specified with a precision better than one part in 10120. If the laws of physics are the cosmic program, then there will be irreducible sloppiness, or wiggle room, in their operation.
The number 10120 is so huge that for almost all practical purposes it might as well be infinite. Electric charge, for example, cannot be measured to better than one part in a trillion. So we are unlikely to notice any 'flaws' of nature arising from the inherent lack of precision in the cosmic computation.
HOWEVER, THE INFORMATION LIMIT on the universe, though enormous today, was smaller in the past. That is because the universe was younger and light had not travelled as far. Most cosmologists believe the basic structure of the universe was forged in the first split second after the Big Bang, at roughly 10-32 seconds. At that time, the universe contained maybe only 1020 bits of information, implying a serious uncertainty or imprecision in the results of the operation of the laws of physics. So we can think of the universe starting out with very ill-defined or approximate laws. Over time, the laws sharpened and focussed into the form we observe today. What we want to explain is why, from this higgledy-piggledy start, the laws zeroed in on such an uncannily bio-friendly form.
Could the universe have fine-tuned itself in order to bring about life and consciousness? At first sight, the idea seems preposterous. How could the universe in the first split second, when its laws were still malleable, 'know about' the emergence of life billions of years later?
A possible answer to this question lies with quantum physics. At the atomic level, the orderly process of cause and effect — seen in macroscopic systems like planets in their orbits or billiard balls on a table — dissolves into ghostly patterns of vibrating energy. This indeterminism is captured by the famous uncertainty principle of Werner Heisenberg: it is impossible to know all the variables describing a quantum particle at the same time.
The way quantum uncertainty is usually confronted is as follows: a system such as an atom can be prepared to be in a certain definite state, such as in a particular position or moving at a particular velocity. At a later time the experimenter measures some property of the atom, such as its position. In general, the outcome of the measurement cannot be reliably predicted from the initial state: there will be a spectrum of possible outcomes, each with a certain probability.
Compare this with billiard balls. If you know where one is and you roll it in a certain direction at a certain speed, you'll be able to predict where it will be at a later time. However, with atoms, even if you know precisely what state an atom is in at one moment, you generally cannot predict what will be found at a later moment when the measurement takes place.
That much is agreed, and very well confirmed by experiment. But a problem arises when the quantum system is the entire universe. The initial state of the universe is not something any experimenter can prepare, so we need to think about quantum uncertainty differently when it comes to cosmology. We make observations of the universe today, and can use them to infer something about the past. The laws of physics are symmetric in time, so quantum uncertainty works both forwards into the future and backwards into the past. Therefore, any given observation of the universe made today is consistent with a large number of possible histories, stretching all the way back to the Big Bang. However, we can reject any histories that are incompatible with the emergence of life and observers, otherwise we would not be here to make the observations in the first place. So the very fact that an observation gets made today in some sense helps shape the reality of the past — even the far past.
BIZARRE IT MAY SEEM, but linking the present with the past via quantum measurement is part and parcel of standard quantum mechanics, and can even be demonstrated experimentally, although so far only over very short time scales. This backwards-in-time effect cannot be used to send information into the past, or to change the past, but it does constrain the past to conform to the present.
So can the 'retro-causation' aspect of quantum physics explain why the laws of physics are fine-tuned for life? Not in the usual formulation, no. Although quantum mechanics requires the presence of many alternative pasts, every allowed history develops over time in conformity with the same laws of physics. The differences come about purely from inherent quantum uncertainty, not from any variations in the laws of physics as such. What we would like to explain is why the laws themselves are bio-friendly, thus permitting at least some quantum histories containing observers.
To do so, we would need to find some way of applying the general principle linking future to past through quantum observations to the laws of physics themselves. Until now, such an application would have been meaningless, because the laws were regarded as fixed and infinitely precise. But treating the laws as cosmic software, with an inherent flexibility, neatly lends itself to the task.
Observations made throughout the entire duration of the universe can contribute to fashioning the form of the laws in the first split second after the Big Bang, when they were still significantly malleable. Thus the potential for future life acts like an attractor, drawing the emerging laws towards a bio-friendly region of the available parameter space. In this way, life, mind and cosmos form a self-consistent explanatory loop.
For four hundred years, science has been based on the implicit belief that the laws of nature are themselves supernatural, and thus off limits to scientific inquiry. The time has come to challenge this fundamental assumption and seek a natural physical mechanism that enables the universe to generate its own bio-friendly laws.
In the eternal quest to explain life, the universe and everything, it could be that life explains the universe even as the universe explains life. Which pretty much covers everything.
Paul Davies is the director of Beyond: the Centre for Fundamental Concepts in Science at Arizona State University, a member of the Cosmos Editorial Advisory Board and author of The Goldilocks Enigma.
