# Re: TimeTravel

Dan Fabulich (daniel.fabulich@yale.edu)
Sun, 25 Oct 1998 14:37:01 -0500

Two responses:

Hal Finney wrote:
>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.

You know what? You're absolutely right. :)

This would probably have some really awful effects on whomever is within that light cone! Anyone who could have been reached by light emitted by the inventor could travel instantly to any time since the gateway's invention, as well as any other time/place within the light cone. Eep!

Brian D Williams wrote:
> I wouldn't be easy to convince, but we were arguing something
>different. Dan's postulate (if I understood him) was that since no
>visitor has returned from the future this proves time travel is
>impossible. I guess to make it clearer there are at least two
>possibilities. 1) time travel (backward) is impossible 2) The
>future isn't here yet. So the lack of the visitor doesn't prove 1
>because 2 is also possible.

At first I thought I understood you, but now I can't make sense of what you might mean by "the future isn't here yet."

If we find that no time travellers have travelled back to any time within the last week (time period A), then no time traveller will travel back into A. We know this because if the traveller did travel into A, then they would appear in A, and this would contradict our earlier finding that no traveller appeared in A. This isn't faulty logic here as far as I can tell.

Another way of putting it: Suppose I experience time period A. At the very instant time period A ends, (call it instant B,) I either write a Y on a certain piece of paper if a visitor showed up, or I write the letter N if there wasn't. Now imagine that I live through A and at instant B I write an N. Many years later, a time traveller decides to go back to time period A. Assume that I can find and identify time travellers perfectly. (This is a false assumption, obviously, but that isn't what we're debating here.)

Here's the question: Did write Y or N at instant B?

First we note that it is not sensical to ask the question before instant B. We also note that after that instant, the question has a definite yes/no answer. It's not possible for me to have put N on the page and then, later, for me to have put a Y on the page at instant B. Instant B is an instant. The events that take place during an instant in the past do not change in the future.

I would answer the question thus: when the time traveller came back, and I identified the traveller, it would revise all of the history to follow. I *would* have noticed a time traveller in time period A, and at instant B I would have put a Y instead of an N.

However, in this case, at NO time in the past did the page have an N. It might have an N in some alternate universe/history/world, but not this one. In this universe, there would never have been a time in which the page said N.

In other words: if arbitrary time travel is possible in the future, and the time traveller returns to my past/present, and I can identify time travellers, I would always have remembered seeing one. (I have an N on my page.) It has nothing to do with whether the future is "here" or not. When the future happens, the past would be altered. We can look at the past, which will not change in the future, to see what effects if any the future may have on the past.

-Dan

"Decay is inherent in all compounded things. Strive unceasingly."

• The last words of Buddhadasa Bhikkhu