Until a few centuries ago, it was painful that time was something constant for everyone, that it flowed in the same way in any situation, in any circumstance and that nothing could be done to "Control it". However, thanks to Einstein's theory of relativity and thanks to the studies of other famous scientists of the twentieth century, a revolutionary discovery has been reached: time does not flow in the same way for everyone, but varies according to the reference system from which observe an event. Let's take a simple example. Let's suppose we travel at half the speed of light towards Proxima Centauri, the star closest to the Sun, and we start the chronometer as soon as we start from the Earth. Proxima Centauri is located at a distance of 4.2 light years from our planet; this means that it would take 4.2 years to reach this distance if we traveled at the speed of light (about 300000 km/s). Now, let's imagine that a human friend of ours, who remained on Earth, started another chronometer at the same moment in which the one we started on the spacecraft and that he blocks it upon our arrival on Proxima Centauri. Contrary to what you think, incredibly we and our friend will be recording two different time frames. For our friend on Earth, 8.4 years will have passed, as can be easily understood from the above (This is because we on the spacecraft travel at half the speed of light, so to cover a distance of 4.2 light years it would take 8.4 years) but for us it will have been 7.3 years. How is such a thing possible? It has been shown that when traveling at a speed close to the speed of light, time slows down, flows more slowly than the time of a man who remains "still" or moves at negligible speeds compared to that of light. Theoretically, when we travel on a plane (about 1000 km/h), time flows slower for us than for someone who is not traveling on it. However, since we are at traceable speeds compared to those of light, time slows down by a very small, practically insignificant amount. Time, according to the theory of general relativity, also slows down near masses that deform space-time, but we will talk about this in the next article.