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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