HOW TO BUILD A TIME MACHINE

Not quite a how-to, but a survey of the science behind time travel—if anyone ever invents it.

British pop-science writer Clegg (Inflight Science: A Guide to the World From Your Airplane Window, 2011, etc.) notes that time travel is not just a trope of science fiction any more. Increasingly, serious physicists accept that travel to the future and back, or to the past and back, is theoretically possible. The laws of physics provide a few interesting options, and examining them provides an opportunity to look at some interesting corners of modern science. Einstein’s theories of special and general relativity describe how motion and gravity affect objects and people. Movement at high speeds, or in powerful gravity fields, changes the rate at which time flows. GPS systems have to adjust for discrepancies between clocks in orbit and those on the ground. These effects can slow time down so travelers can visit distant futures without living through all the years between. Of course, there are problems with these theories: Gravitational fields strong enough to slow time also create tidal forces that can rip matter apart. Other theoretically feasible methods of time travel require building structures such as cylinders of infinite length—hardly a practical option. Clegg also dissects schemes such as going into the past and playing the stock market armed with foreknowledge of what will rise or fall. He covers most of the themes advanced by science fiction, plus some of the more adventurous scientific minds, generally avoiding math or tricky technical details.

A solid overview of some of the quirkier corners of physics, with an entertaining connection to pop culture.