Future Of Spacetime [Paperback]

Physicist Richard Price leads off with a concise refresher-essay, "Welcome to Spacetime." Danish physicist Igor Novikov explores classic time-travel paradoxes, with some cool diagrams and novel results: in essence, "closed timelike curves" [note 2] are theoretically possible, but paradoxes aren't allowed -- with a time-machine, you could visit your grandfather, but you couldn't kill him. The universe wouldn't permit it -- which in essence is Hawking's Chronology Protection conjecture. Hawking speculates that the unfortunate time-traveler would be incinerated by (literally) a bolt from the blue. Well, what he actually says is, "one would expect the energy-momentum tensor to be infinite on the Cauchy horizon" [note 3], which (sigh) is a pretty typical Hawking attempt at "popular" science.

Fortunately, Thorne himself is a master popularizer, and he ends up explaining Hawking's ideas as well as his own. His essay amounts to an update chapter for his wonderful 1994 book, Black Holes & Time Warps: Einstein's Outrageous Legacy, which I enthusiastically recommend: <http://www.sfsite.com/10b/bh67.htm>. Thorne reluctantly concludes that things really don't look very good for wormholes, especially for time travel -- though he does leave a tiny ray of hope for some super-advanced future civilization to make wormholes for space travel [note 4]. Thorne notes that our grasp of basic physics is so crude that we can really only understand maybe 5% of the stuff that fills our universe -- the "normal" baryonic matter that makes up people, planets and stars. Thorne guesses that 35% of the universes's mass is in some unknown form of "cold dark matter", and the remaining 60% is some even more mysterious form of "dark energy" -- so there's certainly plenty of room left for discovery!

The book concludes with a nice explanation of why good popular-science books are needed, by noted pop-science writer Timothy Ferris, and with Alan Lightman's essay on "The Physicist as Novelist". Lightman, a former student of Thorne's, went on to write Einstein's Dreams and other well-regarded novels.

The Future of Spacetime is written for a general audience -- aside from Hawking's essay, everything should be understandable to any science-literate reader. I particularly recommend it to readers who've liked Thorne's earlier pop-science works.
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Note 1). a clever play on festschrift, the traditional name for such a tribute volume.

Note 2). As Hawking cheerfully points out, "closed timelike curve" is just physics-speak for time travel, because you can't admit you're studying that sci-fi stuff in a grant proposal...

Note 3). Arthur C. Clarke notes that "the most convincing argument against time travel is the remarkable scarcity of time travellers..."

Note 4). As you may know, a faster-than-light spaceship could also be used as a time-machine, another reason why most physicists think FTL travel is very unlikely. I'd love to see a theoretical treatment of FTL travel that wouldn't violate Hawking's "Chronology Protection Clause"... Note also that there's no theoretical barrier to wormhole spaceships travelling a bit *slower* than light.

Review first published at SF Site:
http://www.sfsite.com/11a/fs139.htm

Hawking and Thorne, grasp it: Time-travel is physically IMPOSSIBLE.