‘You get the point? Globally speaking there is no past and future. There are only events - like points on a huge graph, with axes marked space and time. That’s the way to think of it. The events swim around, like fish; and the further away they are the more they swim, from your point of view. So there is no one event on the Base, or on Earth, or anywhere else which can be mapped uniquely to your now. In fact there is a whole range of such events at distant places, moving at different speeds.

‘Because of that looseness, histories are ambiguous. A single location on Earth itself has a definite history, of course, and so does the Base. But Earth is maybe ten thousand light years from here. It’s pointless to map dates of specific events on Earth against Base dates; they can vary across a span of millennia. You can even have a history on Earth that runs backwards as seen from a moving ship.

‘Now do you see how faster-than-light screws things up? Causality is controlled by the speed of light. As long as light has time to travel from one event to another they can’t get out of order, from wherever they are viewed, and causality is preserved. But in a ship moving faster than light, you can hop around the spacetime graph at will. I took a FTL jaunt to the Fog. When I was there, from my point of view the history of the Base here was ambiguous over a scale of decades . . . When I came home I simply hopped back to an event before my departure.’

I nodded. ‘But it was just an accident. Right? This doesn’t always happen.’

‘It depends on the geometry. Fleeing the Xeelee, we happened to be travelling at a large fraction of lightspeed towards the Base when we initiated the hyperdrive. So, yes, it was an accident. But you can make Tolman manoeuvres deliberately. And during every operation we always drop Tolman probes: records, log copies, heading for the past.’

I did a double-take. ‘You’re telling me it’s a deliberate tactic of this war to send information to the past?’

‘Of course. If such a possibility’s there you have to take the opportunity. What better intelligence can there be? The Navy has always cooperated with this fully. In war you seek every advantage.’

‘But don’t the Xeelee do the same?’

‘Sure. But the trick is to try to stop them. The intermingling of past and future depends on relative velocities. We try to choreograph engagements so that we, not they, get the benefit. And of course they reciprocate.’ Dakk grinned wolfishly. ‘It’s a contest in clairvoyance. But we punch our weight.’

I tried to focus on what was important. ‘OK,’ I said. ‘Then give me a message from the future. Tell me how you crossed the chop line.’