All Ends That Wells Starts

P.D. Smith, in the TLS for May 18, reviews Build Your Own Time Machine: The Real Science of Time Travel ("Much too difficult for us") by Brian Clegg:

Clegg ... makes the intriguing point that time travel mechanisms based on relativity cannot send anything back before the moment at which they were switched on. Therefore, as no one has yet invented a time travel machine, we would not expect to see visitors from the future. But he qualifies this, saying that it does not preclude the possibility that unknown alien civilizations may have already invented time travel technology. And, he adds, if they are advanced enough to voyage into the fourth dimension, it is possible that they may also have mastered the art and science of remaining invisible.

Fantastic though these suggestions sound, Clegg never fails to highlight the practical difficulties of time travel. He uses the famous twins paradox, a thought experiment derived from relativity, to illustrate some of these. According to this, if one of the twins remains on Earth while the other travels away in a spaceship at extremely high speeds, the effects of both time dilation due to the high speeds and the acceleration cause them to age differently. As Van Veen [Nabokov's "epicure of duration" in Ada, or Ardor (1969)] puts it in his study of The Texture of Time, "the galactonaut and his domestic animals, after touring the speed spas of Space, would return younger than if they had stayed at home all the time". But although the scientific concepts sound (relatively) simple, the application of the science to achieve time travel is challenging, to say the least. Clegg calculates that you would need 10 billion times the amount of energy produced by every power station in the United States running for 250 years in order to push a spaceshuttle-sized craft up to 90 per cent of the speed of light. And, of course, according to relativity, the faster something goes the more its mass increases. So, in fact, you would need vastly more power - perhaps the equivalent of 800 or so years of power generation. And even then it would take eight years to travel a mere eleven years into the future.

Despite the title, the lesson of Build Your Own Time Machine seems to be that no one will be doing so any time soon. Clegg sets up elegant and fascinating theoretical possibilities for time travel only to show that the practical difficulties are virtually insurmountable. The options for potential time travellers include cutting up a neutron star to build a vehicle made of neutron star material whose gravitational field slows time; quantum entanglement (described by Einstein as "spooky action at a distance") which enables small-scale teleportation; and Gödel's proposal of a rotating universe in which spacetime is curved, making it theoretically possible to loop back in time. The longest chapter in the book ("Alice through the wormhole") considers the truly mind-boggling physics of black and even white holes, the latter described as "a black hole that runs backward in time ... a singularity of creation rather than of destruction". Theoretically, two white holes back to back in space might create a wormhole, a shortcut through space and, indeed, through time, because you would travel through it at speeds faster than light. The renowned physicist and futurologist Michio Kaku has even designed a time machine based on this idea. Clegg remains distinctly sceptical, however, and even Kaku admits: "This is for a very advanced civilization, not for us"...

In his WSJ review of Clegg's book, Sean Carroll, author of From Eternity to Here: The Quest for the Ultimate Theory of Time, notes that among the popularizing book's many science-lite anecdotes we learn

that Einstein was ordered by his doctor not to buy tobacco; interpreting this literally, Einstein simply filched the tobacco from his friend Niels Bohr.

112 days ago [or from now?]
 Daniel Lee Wrote [will have written?]:
I'd explicate all this but I don't have time right now. Maybe I'll get to it earlier.

And in other dispatches from the ineffable in the new TLS, in "This Week's Contributors", we learn that religion-book reviewer 

Mark Vernon's new books, The Big Questions: God and God: All that matters are forthcoming.*

*DSL.'s projected rejoinder to Mark Vernon, All That Matters Is Not God may, timespace permitting, be reviewed in a future issue.