In a science brief, The Economist covers the mystery of time.

In 1887, Albert Michelson and Edward Morley found to their surprise that the speed of light traveling in different directions relative to the movement of the earth’s surface is constant. In 1905, Albert Einstein provided an explanation—his special theory of relativity—for the constancy of the speed of light. Time is “malleable, passing differently in different places, depending on how those places are moving with respect to one another. Indeed, at the speed of light, it stops altogether.” In 1915, Einstein argued in his general theory of relativity that space and time are connected and that they interact with mass.

As a consequence of the second law of thermodynamics (temperature differences tend to vanish, Ludwig Boltzmann, 1877) “any system will become more disordered as time passes. That applies as much to two gases mixing as it does to a teenager’s bedroom.” In 1927, Arthur Eddington drew the conclusion that time is unidirectional: There is a fundamental asymmetry between a system moving towards the future (increasing disorder) or the past (decreasing disorder).

Time travel is possible—in particular if one has easy access to “wormholes”—or so it appears. But the grandfather paradox lurks: Can one travel back in time and kill one’s ancestor in order to render one’s one birth impossible …?