After the lights had gone out, Prabir knelt at the edge of the deck and ejaculated into the water. He rested his head on the guard rail, suddenly cold in the breeze coming in off the sea. The images of her body faded instantly; it was obvious now that he’d never really wanted her. It had been nothing but a temporary confusion between the friendship she’d shown him in the kampung, and the fact that he hadn’t touched Felix for what seemed like a lifetime. It had never occurred to him that he might have lost the knack for celibacy, that after nine years it could take any effort at all to get through a mere three or four weeks.

When he returned to his sleeping bag and closed his eyes, he saw Felix lying beside him, smiling and sated, dark stubble on the golden skin of his throat. When had it become conceivable to betray him?But instead of agonising over one stupid, aberrant attempt at infidelity, better to think of the changes he could make back in Toronto to put an end to all the far greater risks he’d been courting ever since they’d met. Felix had been patient beyond belief, but that couldn’t last forever. The simplest thing would be to let Madhusree have the apartment to herself; he’d keep paying the rent until she graduated. He’d move in with Felix, they’d have a life of their own, a mutual commitment without reservations.

It was not unimaginable any more. Even if he’d had the power to imitate his father in every respect, it would not have brought Radha and Rajendra back to life. And he no longer cared that he couldn’t read between the lines and extract some kind of unspoken blessing from his parents. There had to be an end to what they would have wantedand what they would have done.

He had to take what he believed was good, and run.

An hour after they’d left Teranesia behind, Grant emerged from the cabin looking bemused.

She said, ‘Strange news from São Paulo.’

Prabir grimaced; it sounded like the title of one of Keith’s Country Dada albums. ‘Please tell me we’re not turning back.’

‘We’re not.’ Grant ran her hand through her hair distractedly. ‘I’d say the last thing they need is more data. We seem to have given them rather more than they can cope with.’

‘What do you mean?’

She handed him her notepad. ‘Joaquim Furtado, one of the physicists on the modelling team, has just posted a theory about the protein’s function. The rest of the team have refused to endorse it. I’d be interested to hear what you think.’

Prabir suspected that she was merely being polite, but he skimmed down the page. Furtado’s analysis began with a statement no one could dispute: the discrepancies between the computer model and the test tube experiments proved that there were crucial aspects of the molecule’s behaviour that the simulation was failing to capture. Various refinements to the model had been tried, but so far they’d all failed to improve the situation.

One of the many approximations made by the modellers involved the quantum state of the protein, which was described mathematically in terms of eigenstates for the bonds between atoms: quantum states that possessed definite values for such things as the position of the bond and its vibrational energy. A completely accurate description of the protein would have allowed each of its bonds to exist in a complex superposition of several different eigenstates at once, a state that possessed no definite angles and energies, but only probabilities for a spectrum of different values. Ultimately, the protein as a whole would be seen as a superposition of many possible versions, each with a different shape and a different set of vibrational modes. However, to do this for a molecule with more than ten thousand atoms would have meant keeping track of an astronomical number of combinations of eigenstates, far beyond the capacity of any existing hardware to store, let alone manipulate. So it was routine practice for the most probable eigenstate for each bond to be computed, and from then on taken to be the only one worth considering.

The trouble was, when the São Paulo protein was bound to DNA, many of its bonds had two main eigenstates that were equally probable. This left no choice but to select the state of each bond at random: the software tossed several thousand dice, and singled out a particular conformation of the molecule to analyse. And in the first test tube experiments, nature had appeared to be doing virtually the same thing: when the strands of DNA had been copied with random errors, SPP had seemed to be merely amplifying quantum noise when it chose a different base to add to the new strand. But the near-perfect copying of the fruit pigeon chromosome, and the successive intergenerational changes in the DNA from the Suresh butterfly specimens, showed that something far subtler was going on.

The crucial subtlety, Furtado claimed, was that none of the probabilities that controlled the shape of the protein really were precisely equal. One or the other would always be favoured, though the balance was so fine that the choice would depend, with exquisite precision, on the entire quantum state of the strand of DNA to which the protein was bound. Furtado conjectured that SPP was exploiting this sensitivity to count the numbers of various ‘counterfactual cousins’ of the DNA: similar, but non-identical sequences that might have beenproduced in its place, if only its recent history of random mutations had been different. If the most numerous cousins dictated the sequence of the new copy of the DNA, that explained why the mutations weren’t random, why they never killed or disadvantaged the organism. They’d been tested, and found to be successful: not in the past, as Grant had hypothesised, but in different quantum histories.

Prabir looked up from the notepad. ‘I don’t know what to say. Nobel prize-winning physicists have been throwing rotten fruit at each other for a hundred years over interpretations of quantum mechanics, and as far as I know they’re still at it. Nobody has ever resolved the issues. If Furtado thinks the Many Worlds Interpretation is right, there’s a long list of famous physicists who’d back him up, so who am I to argue? But drawing information from other histories is something different. Even most believers would tell you it could never be done.’

Grant said, ‘That’s pretty much my own feeling.’ She leant over to see how far he’d read. ‘There’s some interesting speculation later on, suggesting the kind of data analysis the protein could be performing to extract the interference patterns between the DNA and its cousins from all the noise produced by thermal effects. If any of it’s true, though, SPP must have evolved into a veritable quantum supercomputer.’

Prabir scrolled down and glanced over the section she’d described; most of the equations were completely over his head, but there were passages of text he could follow.

Could a very bad quantum supercomputer have found a gene for a slightly better one? And so on? Furtado was claiming as much, but in the final section of the article he admitted that it was impossible to prove this directly; modelling any version of the São Paulo protein to the necessary level of precision was out of the question. He was, however, planning an experiment that could falsify his hypothesis: he was synthesising a copy of one of the fruit pigeon chromosomes, right down to the methylation tags. This molecule would be identical to the biological chromosome in both its raw sequence of bases and every known ‘epigenetic’ chemical subtlety, but its quantum state would not be correlated with that of the DNA in any living bird, real or counterfactual. If SPP copied this with the same low error rate as the natural version, Furtado’s flamboyant theory would go down in flames.