In 1895, HG Wells published ‘ The Time Machine’. This extraordinary novel of anticipation appears a few years before the end of the 63-year reign of the prudish Queen Victoria, which precedes by only a few years the end of the unchallenged reign of… Newtonian physics. In fact, Albert Einstein published in 1905 his special theory of relativity, which undermines the concept of space-time inherited from Galileo. This theory also has the strangeness of predicting a form of travel to the future … to Wells’ delight, no doubt! In Galilean space-time, time is the same for everyone, everywhere. In Einstein’s theory, on the other hand, the durations measured by observers moving relative to each other no longer coincide: time is relative.
Since 1905, a real journey through time has been made, but not with the machine Wells jokingly imagined. The human who traveled the farthest in time to date is Sergei Krikalev: it jumped to the future of … 1/48 This lead over other humans is the result of the 803 days this cosmonaut spent aboard Mir and the ISS (the International Space Station). Onboard these space stations, it was moving at some 27,000 kilometers per hour around the planet. Compared to our clocks on the ground, its own clock, that is to say, its own time, has progressed less: it has aged less than us. In other words, Sergei Krikalev has advanced to the future of 1/48 of a second.
Is it possible to travel to the past?
The effects of such a trip to the future would be much more noticeable if great distances and speeds were involved. If Sergei Krikaliov left in 2015 for a star located 520 light-years (Betelgeuse) at 99.995% of the speed of the light, on his return he would have aged only ten years. We would then be in 3015, that is to say, that 1000 years would have passed. “We can travel to the future. It’s just a matter of time and money, “explains astrophysicist Richard Gott of Princeton University.
On the other hand, the idea of traveling to the past is problematic. In the Spatio-temporal framework of special relativity, it is impossible: time can be contracted or lengthened there, but never reversed. On the other hand, this would be possible within the framework of general relativity, a theory published in 1915 (ten years after special relativity), which shows gravity as the consequence of the deformation of space-time.
How would one travel there to the past? The answer is not obvious, because the equations of general relativity admit many solutions corresponding to very different situations from each other. Only some of these solutions describe universes where trips to the past are possible. Answering the question of the journey to the past therefore amounts to proving that our world is one of these universes.
Small forays into the future
We do not know if this is the case. The question is open and actively studied. What is the theoretical tinkering that might suggest going into the past is possible? Could it be that the Universe is banning it for some unknown reason? Physicists speculate on these questions not to travel to the past, but because thinking about them opens up perspectives …