Relative time travel

Toss a ball straight up in the air while walking, and it will fall back into your hand just as it would when standing. The ball has only up and down motion for you. For a stationary onlooker, however, your ball tracks more quickly through a longer parabolic arc, with up, down and forward motion. The frame of reference thus determines the dynamics of motion. Galileo perfectly understood this basic principle of relativity1; Newtonian physics entirely accommodates it2.

If you travel fast enough and for long enough, on a long-haul flight for example, then you might detect different physics, due to special relativity3. Bring along an atomic clock on an eastward flight around the world, outpacing Earth’s own eastward rotation, and you will find at journey’s end that time itself has passed detectably more slowly for you than for those you left behind, and yet more slowly than for those flown westwards4. You will have aged a tiny bit less than them, even though you won’t feel it.

Board a rocket to space, and you might detect another effect, due to general relativity5. As the rocket lifts up and away from Earth’s gravitational pull, your onboard atomic clock will tick detectably faster with each centimetre of altitude6. Satellite communications and GPS only work by accounting for dependencies of time itself on the frame of reference7.

Suppose that you wish for your space odyssey to carry you ever faster from Earth8. As your relative speed increases, so also does your relativistic mass3, ever raising the energy requirement to go faster. You approach only asymptotically towards the speed of light, meaning that you never quite attain its 299,792 km per second9. Special relativity reserves this ‘universal’ speed – constant across all frames of reference10 – for the photon, the massless quantum of all electromagnetic radiation11,12. You can always radio home to Mother Earth, then. But beware the far greater passage of time down there, for you will be reaching out to the future.

Milestones in history

• Galileo Galilei’s understanding of basic relativity, 1632.
• Isaac Newton’s classical mechanics, 1687.
• Albert Einstein’s theory of special relativity, 1905.
• Hafele-Keating experiment, 1972.
• Albert Einstein’s theory of general relativity, 1915.
• Optical lattice clock, 2015.
• GPS, 1973
• Only slightly more wishful than the Starshot lightsail, 2018.
• Ole Rømer’s first determination of the speed of light, 1676.
• Michelson-Morley experiment, 1887.
• James Maxwell’s theory of light as electromagnetic radiation, 1865.
• Albert Einstein’s photon hypothesis, 1905.

C.P. Doncaster, Timeline of the Human Condition