Causality loops and 35-year struggle to make time travel convincing

Ronald Mallett, professor of physics at University of Connecticut, explains: ‘The astronauts thought they had landed on another planet that was ruled by apes. But what they found out was they had travelled so fast that they had arrived into Earth’s future. That accurately depicts Einstein’s special theory of relativity.’ In short, if we’re going to time travel, it would most likely be into the future. Travelling into the past? That would require phenomena such as wormholes – stuff that is yet to be observed in space.

Time travel already happens. Kind of. The Large Hadron Collider can send subatomic particles at 99.9999 per cent the speed of light (just below 299 792 km/second). At that speed, the particles experience time 6 900 times as slowly as the scientists working the thing. According to general relativity, by travelling at that speed, those particles are going into the future. The same principle applies to astronauts, just on a much smaller scale. In September 2015, cosmonaut Gennady Padalka returned to

Earth after 879 days in orbit, where he travelled around at 27 400 km/h. ‘When Mr Padalka came back, he found the Earth to be 1/44th of a second into the future of where he expected it to be,’ says J Richard Gott, professor of astrophysical sciences at Princeton. What would it take to go bigger, into tangible Terminator-style human time travel? ‘If you want to visit Earth in the year 3000,’ Gott says,

‘All you have to do is get on a spaceship and go 99.995 per cent of the speed of light.’ At that speed, if a crew travelled 1 000 light years round trip to exoplanet Kepler186f, the Earth would be in the year 3019 upon return. But their internal clocks would have slowed relative to the Earth’s chronology. ‘They are only going to age about 10 years,’ says Gott. While a millennium would pass for us, for them it would be a decade. But reaching that speed is a problem. The fastest spacecraft yet, the Parker Solar Probe, only hits 0.00067 per cent of the speed of light. The most promising solution? Antimatter fuel. Let’s get going. – Matt Blitz

18 SEPTEMBER / OCTOBER 2020