‘What? Oh yes, Tom, yes of course.’ Gibson sat down again. ‘But before we go any further I need one thing more from both of you.’
‘And if you don’t get it?’ Petrie wanted to know.
‘Your air tickets are open returns.’
Freya gave an apprehensive little laugh. ‘I can’t wait to hear this.’
‘For the next four days I want you to remain within the grounds of the castle. You are to have no communication with the outside world without my authorisation. After that, we review the situation.’
Freya and Petrie exchanged glances. Petrie said, ‘Tell us more.’
Gibson looked worried. ‘I can’t. Not without your promise of confidentiality. If you knew what this was about you’d understand.’
Petrie turned to Freya. ‘What do you think?’
She looked doubtful. ‘It sounds military. Maybe a Star Wars thing that I wouldn’t want to touch.’
Gibson’s eyeballs rolled with alarm. ‘I need you. There’s no time to get anyone else.’
Freya smiled happily. ‘You need us. We don’t need you. It looks as if we’re in a strong bargaining position.’
Gibson sat down across from Freya, and sighed. ‘I hate all women.’
6
Patterns
Having just sat down, Gibson jumped up again. ‘Follow me.’
He led the way round the bend of the left-hand corridor. At its end was an ornate door. He waved them dramatically through it.
The room was about fifty metres long. On its high barrelled roof was a fresco of cherubim. The little creatures were on a hillside, reading maps or turning hour-glasses. One, its wings an aerodynamic impossibility, was flying through a star-spangled sky, holding a pennant bearing the words Sapientissimi Opus. The room was lined with books on either side and antique globes were scattered around.
‘This is the theological library, folks. Take a look at this.’ A single strip of perforated paper was laid along the full length of its polished floor and back up the other side. The numbers were upside down and the scientists bent double over them, walking slowly backwards.
‘There’s this big cave complex a few hours north of here, see? It has a deep lake half a mile across and we’ve built an aluminium scaffolding under it to hold light detectors. We have fifty thousand photocells, laid out in a cubic lattice. The numbers in the left column go from one to fifty thousand. They’re just labels. The next column records light intensity picked up from each detector. As you see, there’s nothing but zeros.’
‘I’m not surprised,’ Petrie said, moving backwards. ‘How can you expect to record underwater light in a pitch black cave?’
Freya said, ‘GUTS decay has been ruled out for ten years now. You have to be talking Çerenkov.’
Gibson gave her a look of open admiration. ‘Ten out of ten. Funny things happen when you go faster than light. Çerenkov radiation is one of them.’
‘I thought you couldn’t travel faster than light,’ Petrie said.
‘Only schoolboys think that,’ Gibson said smugly.
Petrie bristled, then decided the man was too absurd to be taken seriously. ‘Thank you, Charlie, for treating me like an idiot.’
‘Light moves slower in water,’ Freya explained. ‘In principle you could swim through water faster than the local light speed. If you did, you’d leave a trail like a sonic boom, only with light rather than sound. That’s Çerenkov radiation.’
Gibson said, ‘Fortunately you’re not here for your physics.’
Petrie resisted the urge to punch the arrogant toad on the nose. ‘Why am I here?’
‘Patience.’
‘I’ve always thought of Çerenkov radiation as faint,’ Freya said, scanning the numbers.
‘To the eye, yes. The retina needs sustained light for about a fiftieth of a second before it records anything. But our detectors have quantum efficiencies pushing a hundred per cent; they can track a single photon. Which is one good reason, incidentally, for being deep under the ground.’
‘Okay.’ Petrie was scanning the figures impatiently. ‘So a particle tracks through the water and you pick up its trajectory from the trail of light.’
‘We time it to a ten-millionth of a second — about the time it takes a particle to cross the lake top to bottom. That’s the numbers in the third column. Now come over here.’ A table, which in the eighteenth and nineteenth centuries had presumably hosted dinners for thirty or forty, was now covered by more computer printout, three inches deep, laid out along its forty feet. Petrie saw an incomprehensible mass of numbers. He moved along the table, flicking through the lists. ‘Hey!’
Gibson nodded. ‘Yes. No more lists of zeros. This is a single particle track, a cosmic ray. It’s moving through the water at superluminal speed, so it leaves a Çerenkov trail.’
‘It penetrated how far?’ Freya’s voice registered incredulity.
‘One point seven kilometres of limestone karst.’
‘Hold on. You’re under a mountain.’
‘Yes. Most cosmic rays are stopped by a metre or two of ground. Watch that chair.’
‘That’s awesome,’ she said lamely.
‘And it had the kinetic energy of a fast cricket ball. What sort of hell’s kitchen it must have escaped from I can’t begin to imagine. But that’s not why you’re here. No, folks, that’s not even remotely why you’re here. We can backtrack the trajectory, sort of. This particular particle seems to have come in a straight line from a galaxy called M104, about fifty million light years away.’
‘The Sombrero, I know it,’ Freya said, as if she’d vacationed there.
Petrie was still scanning the columns. ‘So this particle had been travelling for fifty million years before it zipped through your lake?’
‘Yes, but like I say that’s not why you’re here. By no means.’
Some Austrian prince, all haughtiness and whiskers, was glaring down at them from the panelled wall. Petrie continued, ‘No doubt you’ll get round to telling us.’
‘We’re trying to solve the dark matter problem.’
‘What’s that?’
‘Groan,’ Gibson said. ‘Nothing special, just the biggest mystery in the Universe, that’s all. Okay, let me do this in words of one syllable. We can weigh galaxies from their mutual orbits, knowing the strength of gravity. They turn out to be ten times more massive than we’d expect from their luminosities. That is, a typical galaxy has ten times more mass than the sum of all its stars. That’s a big discrepancy.’
‘You mean, out there, gravity’s stronger than you think?’
Gibson looked as if he was fighting a sudden pain.
Freya laughed. ‘You shouldn’t say things like that, Tom.’
‘Why not?’
‘It’s heresy. The law of gravity is sacred.’
‘So what’s the party line?’
Gibson said, ‘The solution is obvious. There must be a lot of dark matter inside galaxies. It has gravity but it doesn’t shine.’
‘The problem’s much worse than Charlie is telling you,’ Freya volunteered. ‘When you get to big clusters of galaxies, say hundreds or thousands of them, you find that they’re flying around at speeds vastly in excess of what’s expected from their visible mass. Something invisible is stopping these clusters from flying apart.’
‘How do you measure their speeds? You don’t see them moving?’
‘No, but their light is shifted to the red or blue pro rata with their velocities, like the change in pitch of a train whistle when it passes. You find that the dark stuff in clusters has to be about ninety-five per cent of the total mass. What we’re saying here is that the whole of astronomical science is devoted to only five per cent of what’s out there.’
‘Right. So rather than change the law of gravity, which is sacred, you assume that there’s some exotic new type of particle. Invisible but with mass. If you find it, you have a handle on the missing ninety-five per cent of the Universe.’