I read this story fairly recently, but I read a lot of time travel SF last year so it may be an older story. (I've searched my ebooks and can't find it.)

An odd telescope, or perhaps a normal telescope looking at an odd star, finds that the result of viewing the star depends on the future somehow. E.g., if there were to be an occultation of the star tomorrow, then when you look today at the same time your view will be blank.

Therefore by preparing occultations today, we can affect what was viewed yesterday, and thus send binary messages into the past.

This was a "description only" sort of story -- there was no dialogue (or very little), just exposition. Most of the story dwelled on how the society was affected by knowing what was going to happen. And whether you could trust the messages you got from the future.

I thought it was Greg Egan but I've searched his bibliography and can't find it there.


Pretty sure this is Greg Egan's "The Hundred Light-Year Diary" which you can find in the collection Axiomatic. Summary (with some spoilers) here:

Another story from early in Egan’s career, “The Hundred Light-Year Diary” (Interzone, 1992), hints at the tack taken almost twenty years later in the Orthogonal trilogy. In this rather depressing short, a time-reversed star has been found—the light we receive from it comes from the future, not the past as with other stars. By setting up a shutter system between the Earth and the star, messages can be conveyed from the future to the past (the present of the story). Due to limited bandwidth, every citizen is allocated a certain number of bytes per day of messaging that they can send back in time to themselves. The narrator is a political operative who marries his wife based on both their messages about each other but then embarks on an affair that he had not written to himself about. His lover picks him up by telling him that she knows that he’ll come with her and of course he believes her, because everyone has knowledge about the critical days of their lives. But it turns out that she’s been ignoring the messaging system and living her life spontaneously, something he finds exotically attractive. In the end, his marriage dissolves into bitterness, none of which is reflected in his cheerful, positive messages to his past self. He finds that he can’t change what he’s writing—he can’t deviate from the script of the messages he’s already received. But the messages increasingly bear no resemblance to the life he’s actually living.

Note that the premise of the story seems to be based on an actual (but very speculative) theoretical physics model proposed by astrophysicist Thomas Gold in which entropy will switch from increasing to decreasing in the future as the universe switches from expanding to contracting--philosopher of science Huw Price discusses the possibility that the changes in the direction of entropy change might be happen at different moments for different galaxies, and that this would allow for a backwards-in-time communication system similar to the one described by Egan, on pages 18-22 of this paper, and at greater length in his book Time's Arrow and Archimedes' Point.

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  • So I was right, it was Egan. Thank you very much, especially for the Thomas Gold reference! – Ross Presser Feb 25 '16 at 17:01
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    The same plot device is reused in Egan's Orthogonal trilogy, which is set in a universe whose exotic physics make it easier to arrange for this kind of weird entropy-reversal effect. – David Feb 25 '16 at 20:27

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