The Arrows of Time

‘The cosmos is what it is,’ Tarquinia replied. ‘The laws of optics and mechanics and gravity are simple and elegant and universal… but a detailed description of all the things on which those laws play out seems to be nothing but a set of brute facts that need to be discovered individually. I mean, a “typical” cosmos, in statistical terms, would be a gas in thermal equilibrium filling the void, with no solid objects at all. There certainly wouldn’t be steep entropy gradients. We’ve only been treating the existence of one such gradient as a “law” because it was the most prominent fact in our lives: time came with an arrow distinguishing the past from the future.’

Ramiro said, ‘But isn’t there still a question of how brutish the brute facts are? We know that the home cluster’s entropy was much lower in its distant past, and the same was true of the orthogonal cluster. The most economical explanation is that both clusters shared a common past.’

Tarquinia said, ‘So you want to cling to the notion of parsimony? A single region of low entropy is already stupendously unlikely, but even if we have no choice about that, you want to hold the line and refuse to allow two?’

‘You don’t think that’s reasonable?’

Tarquinia thought it over. ‘I don’t know what’s reasonable any more,’ she said.

Ramiro closed his eyes for a moment, raising some crude scrawls on his chest based on Tarquinia’s diagram, but keeping them private. ‘Forget about whether or not the clusters have a common past; forget about the orthogonal cluster entirely. Suppose the only thing we rely on is the fact that the home cluster had vastly lower entropy in the past.’

‘All right.’

‘So the state of the home cluster long ago is already “special”,’ he said, ‘compared to a random gas made of the same constituents. But now if we take it for granted that this state could, potentially, give rise to all kinds of situations analogous to the experiment we just did with the engine, which result would require the most “unlikely” original state? The result where none of those situations ever actually arise: no fast-moving object ever emits a burst of light in such a way that the light would need to be emitted, as well, by other objects scattered around the cluster? Or the result where such events do occur, with the original state guaranteeing coordinated action by all the different emitters? And you can’t say “neither”; it has to be one or the other.’

Tarquinia buzzed wryly. ‘When you put it that way, I’d bet on a lack of restrictions, not a lack of conspiracies. I mean, consistency comes for free; we’re not entitled to say that it’s freakishly unlikely when the cosmos does whatever needs to be done to avoid contradicting itself. What’s unlikely would be for the requirements of consistency to go out of their way to avoid offending our usual notions of cause and effect.’

‘Yeah.’ Ramiro buzzed. Though they’d reached a consensus of sorts, his own argument didn’t really silence his disquiet. A part of him would never be able to accept that distant dust particles were creating the engine’s exhaust as much as the engine itself was.

‘I’d better spread the good news,’ Tarquinia said.

‘Not so good for Pio’s gang.’

‘Don’t be so cynical,’ Tarquinia chided him. ‘They have one less thing to fear now, like all of us. Why shouldn’t they be happy?’

Ramiro said, ‘Wait and see. By the time they’re out of prison I’m sure they’ll have thought of some new reason to give up on the home world.’

Tarquinia wasn’t in the mood for an argument about the migrationists. ‘Thanks for talking this through with me. I’ll feel a lot less rattled now when I report to the Councillors.’

‘Any time.’

Her feed vanished from the console, replaced by the status display for the main engines.

As he contemplated the results of the test, Ramiro couldn’t help feeling a twinge of disappointment. If every engine working in the void had been selectively inhibited, excursions by the gnats away from the Peerless would have become extraordinarily challenging. He’d seen some elaborate proposals from the instrument builders that aimed to get around the problem – most of them requiring high-powered beams passing between the Peerless and the gnat, though a couple had sought to exploit the mountain’s weak gravity as the controlling force, and one had involved extremely long ropes.

Automating any of those outlandish schemes would have made an exciting project after the turnaround. But now gnats would just be gnats, and it was beginning to look as if he’d soon have nothing new to work on at all.

‘Corrado?’ Ramiro swayed sideways so he could open the door fully without banging his head. ‘You should have warned me, I would have prepared a meal—’

‘I’m not here for your cooking,’ his uncle replied brusquely. He glared at Ramiro impatiently, waiting for him to move down the ladder so he could pass through the hatch.