The SF Site Featured Review: The Future of Spacetime

The Future of Spacetime

Stephen Hawking, Kip Thorne et.al.

Norton, 220 pages

Stephen Hawking
From PBS:
Stephen Hawking (b. 1942) has devoted much of his life to probing the space-time described by general relativity and the singularities where it breaks down. And he's done most of this work while confined to a wheelchair, brought on by the progressive neurological disease amyotrophic lateral sclerosis, or Lou Gehrig's Disease. Hawking is the Lucasian Professor of Mathematics at Cambridge, a post once held by Isaac Newton.

In the late 1960s, Hawking proved that if general relativity is true and the universe is expanding, a singularity must have occurred at the birth of the universe. In 1974 he first recognized a truly remarkable property of black holes, objects from which nothing was supposed to be able to escape. By taking into account quantum mechanics, he was able to show that black holes can radiate energy as particles are created in their vicinity. But perhaps his most impressive feat was writing the international bestseller A Brief History of Time. The book spent more than four years on the London Sunday Times bestseller list—the longest run for any book in history.

A review by Peter D. Tillman

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This slim volume consists of six essays, based on talks presented at the Kipfest ¹ on the occasion of Kip Thorne's sixtieth birthday. Thorne, the Feynman Professor of Physics at Caltech is best known to the general public for his 1988 wormhole "time machine" proposal, and indeed much of the book is taken up exploring the question, "is time travel possible?"

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" 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" 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. 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 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.

¹ a clever play on festschrift, the traditional name for such a tribute volume.

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...

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

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.

Pete Tillman has been reading SF for better than 40 years now. He reviews SF -- and other books -- for Usenet, "Under the Covers", Infinity-Plus, Dark Planet, and SF Site. He's a mineral exploration geologist based in Arizona. More of his reviews are posted at www.silcom.com/~manatee/reviewer.html#tillman .