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"Light in a vacuum moves at the speed of propagation of causality," he said.

I looked back at him with a blank expression. I had no idea what that meant.

We were walking past the accident site towards the tents that had been housing the investigation.

The engine had been built to accelerate everything up to twice the speed of light, and preliminary experiments had seemed promising. Thanks to the new antigravity struts, it could accelerate in next to no-time without causing any damaging amount of g-forces for the crew.

The science had been extremely promising. So had the tests.

"It's a misnomer to think of 'the speed of light' as the limitation. The speed of light, you see, is just a product of the constraints of causality. It's really the speed of propagation of causality."

"What do you mean?" I asked.

"A photon can not exist at rest; it would have no mass at rest."

I nodded. That was pretty elementary.

The ship had been tested at .99c without any problems, after ramping up gradually.

"But the 'actual' speed of light is irrelevant. We perceive it as the fastest possible because it is at the limit of propagation of causality. A photon travels at the speed of propagation of causality as long as it does not interact with anything. When it interacts, it does not slow down, but it gets absorbed and re-emitted and seems to slow down."

I nodded again. This was high-school physics, that I had admittedly forgotten most of, except for his talk about "propagation of causality".

"It's moving at the maximal possible speed because it is the simplest - it doesn't really interact, but gets absorbed and re-emitted; there's no, for lack of a better explanation, 'computation' needed each step."

I recalled he had written a paper I had not understood about causality as a simulated computation.

"So you're saying a photon can move fastests because it can't really do anything?" I asked.

He nodded. "Yes, you're starting to see it."

I was not as confident as he seemed to be.

"If anything moved 'faster than light', or more accurately, faster than the propagation of causality, the effects would trail it. You'd get a 'causal boom' the same way going faster than sound causes a sonic boom."

He held his hand together like a ball, and moved them outwards and opened them to signify an explosion.

"But if the speed of propagation of causality was half, we wouldn't perceive the difference. What we perceive is clocked to the propagation of causality."

He stopped to think for a moment.

"Take a movie. If you slow the movie down to half speed, the 'in-movie' time does not change. A day is still a day to the characters, because everything they measure time against is in the movie-world and slowed down the same amount."

"Or consider a computer simulation. The propagation of causality in the simulation is clocked to the speed of operations in the simulation. That can be clocked to real-time, but it also could run arbitrarily slowly. Entities in the simulation could not measure how long something takes 'outside' the simulation without an external reference."

It was mostly confusing me.

"What we didn't realise is that this is what time is. Causal propagation."

He went quiet and looked out over the horizon for a moment, contemplating something, or seeking words. He turned back to me.

"And if you cross the causal barrier, and trigger a causal boom, then a wave of effects follows in its wake like a shockwave. Like the waves trailing a speedboat. You see?"

He pointed at a speedboat out in the bay, before looking at me for a moment, awaiting a reaction. I was not yet sure what he meant.

"Or like the sonic boom I mentioned."

I nodded.

"Or more aptly, perhaps, like a laggy computer game struggling to catch up to a burst of activity faster than what your processor can handle."

That I could picture.

"To an observer, everything would still appear to happen no faster than the speed of light. Even gravity only acts at the speed of light. And that would be true for most intents and purposes because the actual effects would trail. Without a way to measure, to say otherwise, it makes sense to simplify and say the events happened when their effects propagate."

"Now imagine what happens once we cross that threshold. It's so obvious in retrospect, we should have tested for it."

It was not obvious to me, but I tried to picture that laggy computer game, and how chaotic it can be.

He took off his glasses and wiped them absentmindedly while shaking his head.

"Suddenly you have a build-up of changes, and the effects need to happen, but the effects are constrained to the speed of light."

"Do you see the problem now?"

It was starting to dawn on me. I recalled calling people "hackers" as a child when their characters suddenly moved erratically all over the place, or my location in-game changed jerkily as the game was abruptly trying to reconcile the outcome of an overloaded CPU or a slow network connection. I was still not sure how to reconcile that with what had happened here.

The ship had been nudged up to 1.01c and seemed to work. 1.5c. 2c.

Finally, the speed of light was no longer a barrier.

All the unmanned tests had worked.

"The flaw, of course, was that the engine we use has no moving parts past the initial burst of acceleration. Nothing that we could detect problems with. We didn't see the shear in the reactions in the engine, because, well, those reactions were always chaotic anyway, and shear is not a meaningful concept in super-heated plasma."

He closed his eyes, as if to unsee what we'd both witnessed when we arrived on-site after the accident.

"But people move."

"People move. People cause effects. Motions that seem perfectly innocent, but moves matter into space that in one reference frame is empty, but in another is still full of matter."

He looked down. It looked like he was trying not to cry.

"The shockwave of even someone moving an arm..."

His voice trailed off.

We'd both seen the mangled heaps that were what remained of the crew.

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