# Time Travel 1

In 1895, H.G. Wells published The Time Machine, in which he spoke in detail about time travel. This extraordinary foresight appears several years before the end of the undivided rule of Newtonian physics. Indeed, Albert Einstein published in 1905 his special theory of relativity (today it has become obsolete), which threatens the concept of space-time inherited from Galileo. This theory also has the weirdness of anticipating travel to the future or the past.

In Galileo’s space-time, time is the same for everyone, in all places. In Einstein’s theory, on the contrary, the durations measured by moving observers relative to each other no longer coincide: time is relative.

But humanity was able to start time travel.

## Travel to the future

Now about the journey to the future. The record holder for time travel today is Sergey Krikalev: he jumped into the future by 1/48 of a second. This advantage over other people is the result of 803 days that the cosmonaut spent aboard the MIR Station and the ISS (International Space Station)

On board these space stations, it did indeed move at a speed of about 27,000 kilometers per hour around the planet. Compared to our personal watch, his own watch, that is, his own time (?), Has advanced less: he has aged less than we. In other words, Sergey Krikalev has moved into the future by 1/48 of a second.

But was it really a journey through human time, or was it a journey of a clock measuring time? After all, the clock raised on the roof of the skyscraper will show a different time than the clock on the first floor of this building. Although the events in this building occur at the same speed in one time passed. Or at different speeds and different elapsed time? But! We are not yet able to take the necessary measurements.

## Can you travel back in time?

The implications of such a journey into the future would be much more visible if great distances and speeds were at stake. If Sergei Krikalev had traveled in 2015 to a star located at a distance of 520 light years at 99.995% of the speed of light, on his return he would have aged only ten years. Then we would be in 3015, that is, 1000 years would have passed. “We can travel to the future. It’s just a matter of time and money, ”says astrophysicist Richard Gott of Princeton University.

On the other hand, the idea of ​​traveling back in time is problematic. In the space-time framework of the special theory of relativity, this is impossible: time can contract in it or lengthen, but never turn back. On the other hand, this would be possible within the framework of General Relativity, published in 1915 (ten years after special relativity), which makes gravity a consequence of the deformation of space-time.

How would we travel back in time? The answer is not obvious, because the equations of general relativity admit many solutions corresponding to situations that are very different from each other. Only a few of these solutions describe Universes where travel to the past is possible. There is a high probability that in different Universes all the laws of physics, time and gravity are DIFFERENT.

Time Travel, laboratory of honest physics