MOST people take the existence of life for granted, but to a physicist like me it seems astounding. How do stupid atoms do such clever things? Physicists normally think of matter in terms of inert, clod-like particles jostling each other, so the elaborate organisation of the living cell appears little short of miraculous. Evidently, living organisms represent a state of matter in a class apart from the rest.
Sixty years ago, physicists thought they were on the verge of solving the riddle of life. Quantum theory, which had been developed in the late 1920s, seemed weird enough to account for the bizarre properties of living things, and physicists were flush with its success in explaining matter. Why shouldn’t it – or some extension of it – also explain life?
It was an effort that didn’t get very far, and for nearly half a century the idea remained on the sidelines. But now a handful of scientists are urging another, more careful, look at quantum phenomena in biology. Last year, NASA was sufficiently intrigued to convene a workshop on quantum life at the Ames Laboratory in California, its premier astrobiology centre. The workshop was attended by physicists, biologists and information scientists from several countries. Spurred by the popularity of quantum technology, which is dominated by the burgeoning fields of nanotechnology and quantum computation, these scientists are suggesting that perhaps nature got there first. The living cell is full of nanomachines designed and refined by biological evolution. Could it be that some of them acquired their amazing properties by deploying fancy quantum tricks?
Advertisement
At first sight, quantum mechanics seems to be an unpromising basis for life. It is, after all, rooted in Heisenberg’s uncertainty principle, which acknowledges the inherent fuzziness of atoms and molecules. Since life is in the business of accurately replicating information, and requires breathtakingly elaborate molecular choreography, such uncertainty appears more a threat to life than a possible foundation.
Take, for example, the way a quantum particle’s energy is linked to time. Measure one precisely and you’ll lose precision in the other. And that poses a problem for biology, as it compromises the accuracy of the timing processes vital to life’s molecular dance. If a complex process requires several components to conspire with a certain accuracy – that is, to be at the right place at the right time – then quantum time uncertainty will threaten the successful execution of the process.
This issue was investigated in the 1950s by the quantum pioneer Eugene Wigner, who applied the uncertainty principle to the operation of quantum clocks. Wigner derived a formula which related the uncertainty in a time interval measured by the clock to the clock’s size and mass. Using this formula it is possible to calculate the smallest possible size that a clock of a given accuracy can have. If Wigner’s analysis is applied to the operation of molecular timekeeping in the living cell, it seems inevitable that a very small organism’s internal timekeeping will suffer from quantum clumsiness.
Mycoplasmas, the smallest known autonomous organisms, can measure as little as 300 nanometres across and weigh in at less than 0.1 picograms. If the entire mycoplasma cell is regarded as a clock, Wigner’s formula says its timekeeping will cease to be reliable over timescales of an hour or more – or over even shorter periods if the cell’s internal clock is smaller. This implies that any molecular process that takes longer than this will become error-prone and may break down altogether, unless the clock is continually reset by some external factor.
Curiously, an hour turns out to be typical of the reproduction cycle of these organisms, a concordance first remarked upon by Peter PĂ©si
of St John’s College in Santa Fe, New Mexico. Is this just a coincidence, or do mycoplasmas operate on the edge of quantum uncertainty? It is tempting to believe that these cells have evolved to the quantum limit, attaining maximum reproductive efficiency consistent with the uncertainty principle.
Careful timekeeping
Applying Wigner’s formula to the internal components of a cell, which are obviously much smaller than the cell itself, leads to more stringent timekeeping limits that may have a bearing on the minimum speed at which it can reliably operate. One vital part of a cell’s reproductive machinery is a little motor, called a polymerase enzyme, which crawls along unzipped strands of DNA and forges the links that match up the unpaired nucleotide bases with complementary bases floating through its environment. The physics of this motor has been studied experimentally by Anita Goel and her colleagues at Harvard University. Putting in the numbers for the mass and size of the motor produces the result that the minimum speed allowed by quantum uncertainty is somewhat over 100 base pairs per second, which again turns out to be the going rate, according to Goel’s experimental work. Any slower, and quantum errors would cause the motor to become “all fingers and thumbs,” messing up the job by performing its tasks in the wrong order, failing to close the bonds, or lining up molecules in slightly the wrong place or at the wrong angle.
“Is this coincidence? It is tempting to believe these cells evolved to the quantum limit”
Proteins will also suffer from quantum quivers. A protein starts life as a long chain of amino acids, but it can’t get to work until it has folded itself into a ball of a very specific shape, a process that also requires some fancy choreography that is still far from understood. According to Wigner’s formula, the uncertainty of a clock is proportional to both its mass and the square of its size. Applying this to an isolated unravelled protein suggests a quantum-imposed upper limit on the folding time that is proportional to the cube of the number of amino acids making up the chain. Experiments show that there is indeed a scaling law for protein folding times with a power approaching 3.
The examples I have given so far imply that any role quantum mechanics might play in life is a negative one, limiting what organisms can and can’t do. Might there be a positive role too? One area where this is possible is in a cell’s information processing, a task at the heart of life. Apoorva Patel of the Indian Institute of Science in Bangalore thinks living cells may use quantum mechanics to boost their information-processing efficiency. This could explain why the genetic code is the way it is, and why it is universal among Earth’s organisms. Patel considered what happens in the vicinity of the polymerase enzyme as it builds up a new DNA strand during replication, using the original strand as a template (żěè¶ĚĘÓƵ, 15 April 2000, p 20). DNA stores genetic information as a sequence of nucleotide bases. There are four types: A, C, G and T. For raw material, the enzyme must scavenge nucleotide bases from its environment to rivet to the growing strand. But the order in which it adds them must complement the sequence of bases in the template strand. If the bases just blunder onto the scene at random, there is only a 1 in 4 chance the right one will present itself for each successive link. But quantum mechanics could improve those odds.
Quantum theory describes atoms and molecules as waves, which can overlap and combine coherently – a phenomenon known as superposition. For example, an atom can exist in a superposition of excited and unexcited states, or of states corresponding to several spatial locations at once. Superpositions are exploited in quantum computation. For example, they are the basis of a mathematical procedure called Grover’s algorithm that a future quantum computer could use to hunt for a target among an unsorted jumble of data – the equivalent of finding a name in a telephone directory when you know only the phone number. For a database consisting of N objects, Grover’s algorithm uses superposition on a quantum computer to slash the search time to the equivalent of sifting only 
N objects by normal trial and error.
Could it be that living cells make use of Grover’s algorithm in the genetic code to improve the odds of finding the right nucleotide to 1 in 2? This is not a huge gain, but it may be useful enough to have evolved by natural selection over billions of years.
The genetic code is crucial to protein production, the mechanism at the heart of all biological activity. Life is based on the interplay between two classes of molecules that are, chemically speaking, very different: nucleic acids (DNA, for example), and proteins. To pass information between them, the cell’s molecular machinery reads the DNA letters in groups or “words” of three letters which code for the 20 standard amino acids.
Biologists have long wondered about these key numbers: why four bases, 20 amino acids and three-letter groups? Are they arbitrary, or do they possess some deep significance? Patel has an explanation: the numbers 3, 4 and 20 emerge naturally from an application of Grover’s algorithm. To suggest that this means the living cell is a functioning quantum computer would be stretching things a bit, because Grover’s algorithm needs only the phenomenon of wave superposition, not the entire paraphernalia of a quantum computer. But you might call it “quantum enhanced” information processing. It could work by exploiting the wave nature of molecules, although Patel admits that the details are hazy.
Patel is not the first person to suggest that biological systems exploit quantum effects to their advantage. In the 1960s, Herbert Frohlich pointed out that the vibrations of biological membranes can act like the state of matter known as a Bose condensate. This is the strange state in which quantum particles lose their individuality and lock together to form a single super-particle with astonishing properties. Superconductors, in which electrons pair up in a condensate, provide one example. The cooperative motion of the condensate would enable the membranes to store and transmit energy in a coherent mechanical manner, thus achieving a measure of long-range order unavailable by purely chemical processes.
Good vibrations
There have been several other speculative suggestions about how the quantum world may impinge on biological systems. Ever since Francis Crick and James Watson worked out the structure of DNA, people have suggested that mutations might be the result of quantum fluctuations. There is experimental evidence that quantum tunnelling – in which a particle spontaneously penetrates a barrier it lacks the energy to surmount – is responsible for some mutations and for the extraordinary catalytic properties of enzymes. In the 1990s Johnjoe McFadden and Jim Al-Khalili at the University of Surrey in the UK went further by claiming that the uncanny ability of certain bacteria to mutate their way out of trouble when faced with a shortage of nutrients might be traced to a more elaborate type of quantum process. Most controversially, the University of Oxford mathematician Roger Penrose and Stuart Hameroff of the Center for Consciousness Studies at the University of Arizona set out a detailed theory in which information is quantum mechanically processed in the brain at the sub-neuronal level, inside microtubules made of the protein tubulin.
“There is evidence that quantum tunnelling is responsible for some mutations”
These are all, as yet, largely untested speculations. But there is one obvious place to look for quantum effects in biology: the origin of life. Somehow, life emerged from the inchoate ferment of a molecular soup. Nobody knows what pathway led from non-living chemicals to simple organisms, or even whether some such transition was an odds-on favourite or exceedingly unlikely. But whatever the chances, quantum mechanics serves to improve them. Superpositions allow many possibilities to be explored at once. If the state of matter we call “living” enjoys certain distinctive physical qualities, it might be formed from the superposition with unusual efficiency. McFadden has suggested, for instance, that if a particular state enabled a molecule to self-replicate, this might halt the process of molecular exploration.
One way to test this would be to create an “almost-living” soup from the detritus of once-living cells, and then measure the ability of key biological molecules to self-assemble or to arrive at autocatalytic cycles, self-reinforcing chemical loops that many researchers believe are the first step on the road to life. If some sort of quantum magic is at work, it could show up in an enhanced probability for these life-encouraging transitions to occur, relative to other transitions that do not lead to life.
Although far from definitive, these various considerations suggest that perhaps molecular biology shouldn’t completely shrug quantum mechanics aside. Set against this, however, is a major objection. It goes by the name of “decoherence”. To produce its remarkable effects, quantum physics relies on the wave nature of matter. All waves are characterised by an amplitude and a phase; the phase of the wave is a measure of where it is in its cycle of oscillation.
A completely isolated quantum system, such as an atom in an impenetrable box, will always have a well-defined phase for its wave. Such a wave is said to be “coherent”. But in the real world, where atoms are continually assailed by influences from their environment, the phases of their waves tend to get mangled in pretty short order: they “decohere”. Once the phases of quantum waves get all scrambled up, the atoms associated with them lose most of their weird quantum properties, including the ability to form quantum superpositions, and behave in most respects simply as smaller versions of everyday objects. So the extent to which quantum mechanics is likely to play a significant role in biological organisms depends on how fast the relevant molecule’s environment decoheres its associated waves.
Although there is general agreement that this is a problem, opinions differ sharply over just how rapidly decoherence will occur in a real living system. Simple mathematical models suggest that a large molecule like a protein, immersed in a biological cell, will decohere in much less than a trillionth of a second – too fast for anything very biologically interesting to happen. But simple, general models often fail to account for the peculiar features of real systems.
One of the most spectacular quantum phenomena, high-temperature superconductivity, was completely unexpected. Its explanation lies with the specific and complicated structure of the superconducting substance, which in some way prevents decoherence. Might certain bio-systems also possess special configurations that bestow decoherence-evading qualities on their components?
All this is still little more than a vague “perhaps”. It may be that some enzymes exert a screening effect, shielding other molecules from decohering noise. Moreover, the complicated nature of bio-systems, involving phenomena such as feedback and the collective coordinated motions of many components, render simple models of decoherence unreliable. The speed of decoherence depends on exactly how the quantum system couples to its environment. In simple systems, this boils down to straightforward factors such as the mass of the molecule and the ambient temperature. But more elaborate mathematical models show that not all parts of a quantum system decohere equally fast. Sometimes certain parts of the quantum character decohere rapidly, but in such a way as to protect others, creating exceptional decoherence-free modes of vibration amid the general phase scrambling. As so often in physics, the devil is in the detail, and it is too soon to rule out the possibility that in special cases quantum coherence might endure long enough for biologically significant things to happen.
Living computers
The truth about this could prove important: scientists working on the tough problem of building a functional quantum computer face an identical challenge – how to keep decoherence at bay long enough to perform some useful information processing. Many imaginative suggestions are being tried out, so far with limited success. But given the huge investment of time and money in this project, it would seem like a good idea to check out whether living cells have already come up with a way to do it. Even if cells are not fully fledged quantum computers, they may still have found interesting ways to deploy quantum effects here and there.
“Life somehow emerged from the ferment of the quantum molecular world”
Again, however, it’s a “perhaps”. The role of quantum processes in living matter is still unclear. It is entirely possible that quantum mechanics was the midwife of life, but has played an insignificant role since. Equally, it could be that biogenesis was a purely classical process, but that as life evolved it learned to manipulate quantum short cuts to improve its efficiency.
Erwin Schrödinger, one of the founders of quantum mechanics, correctly surmised that the key to life lay with its molecular structure, and he deduced – also correctly – that the information content of genes was encoded in the complex atomic arrangement of large molecules. It was Schrödinger’s 1944 book What Is Life? that inspired physicist Francis Crick to study the properties of DNA. The subsequent discovery of the DNA double helix and the unravelling of the genetic code confirmed Schrödinger’s hunch.
But the pioneers of quantum mechanics intended more than this. They expected that life’s peculiar qualities would turn out to be fundamentally quantum in nature. Decades later, those expectations have not been confirmed. Quantum mechanics is indispensable for explaining the shapes and sizes of molecules and their chemical properties, but once that is taken on board, chemists are generally still happy to settle for a “ball-and-stick” view of biological molecules. Explicit quantum effects rarely enter into their considerations.
All scientists agree that life somehow emerged from the ferment of the quantum molecular world. The key issue is on which side of the quantum-classical divide the transition to life occurred. Niels Bohr once said that anyone who is not shocked by quantum mechanics hasn’t understood it. I believe that anyone who is not shocked by life hasn’t understood it. The question before us is whether quantum mechanics is shocking enough to explain life.