Re: TimeTravel

Hal Finney (
Thu, 22 Oct 1998 09:46:32 -0700

Dan Fabulich, <>, writes:
> FTL movement => arbitrary backwards time travel.
> Now suppose, for example, that your flavor of time machine uses special
> limiting "gateways." You build the gateway today, wait a week, walk into
> the gateway, and you've just travelled one week back in time.
> It would appear that the inventor's ability to time travel is limited: If I
> build the gateway tomorrow, I can't travel back to today, though I can
> travel back to tomorrow from any arbitrary point in the future.
> Unfortunately for this argument, I can couple this simple device with an
> engine that moves me around at relativistic speeds (subluminally) and get a
> faster-than-light engine. Indeed, any trip can be made in an arbitrarily
> short amount of time simply by travelling back in time during the trip.
> So, for example, if I wanted to get somewhere a light-week away, I could
> set up my gateway, fly there in a week and a day, then gate back to a week
> ago. I find myself having arrived at my destination only one day after
> departure. A photon I might have emitted from my origin would arrive six
> days later. Since FTL travel allows arbitrary time travel, a limitation of
> this kind is no real limitation at all.

This is an interesting point, but I believe it is false. I should mention that I do understand special relatively and the way in which FTL travel can lead to time travel.

The gateway has two ends, which are linked together. If you restrict the motion of each end to be subliminal, then each end must stay forever within the forward light cone of its creation. There is no way either end can move into the past light cone of its creation if it only moves subliminally. Therefore you can never come out of a gateway mouth anywhere but in the forward light cone of its creation.

It may be true that you can get the effect of FTL travel by moving through the gateways, but there are restrictions on the parts of spacetime you can visit because the gateways themselves don't move FTL. Therefore you don't have "arbitrary" FTL travel and the premise fails.