You’ve probably heard of the twin paradox. What is it about ? Imagine two twins. One of them is sent into space at a speed close to that of light while the other remains quietly waiting on Earth. On his return, a year will have passed for the brother who left for space, compared to two for the one who remained on Earth. How is it possible ? Quite simply because with speed, time contracts. In the absence of a suitable ship and adequate technologies, this experiment is not about to be carried out. However, there is no need to go into space to verify its veracity.

Indeed, even on Earth, time does not flow uniformly. Thus, as with the twin paradox, time passes more slowly for a moving object than for a stationary object. Another example: modern precision clocks make it possible to assert that time passes more slowly in the plains than in the mountains. The reason ? Time is slowed down by the masses; and the Earth is a gigantic mass. Unlike the twin paradox, the observed time lag is minimal and is calculated in nanoseconds. This is the reason why no one realized it until scientific advances proved it.

“The idea of ​​a single universal time comes from Galileo and especially from Newton. Although very intuitive, this idea was called into question by Einstein. For the latter, it is the existence of a maximum speed of propagation information in the universe, or the speed of light which is about 300,000 kilometers per second, which leads to space-time. Clearly, space and time are inseparable for Einstein, this which leads to the fact that time can no longer be unique even for supposedly perfect clocks. In reality, there are a multitude of proper times”, explains Pierre Spagnou, engineer, professor of physics at ISEP and author of Les mysteries of time – From Galileo to Einstein.

If the variations in the passage of time are of a very small amplitude on Earth, the fact remains that they can have a concrete impact. “GPS location is calculated by estimating the distance between a receiver and a satellite revolving around the Earth. The signal propagates between these two points at the speed of light, describes Pierre Spagnou. take into account the lag between the two clocks because a clock placed on board a satellite advances after a day of 40 microseconds compared to a terrestrial clock. However, a lag of 40 microseconds amounts to a lag of 12 km on the position”.

Even more, the progress of science opens the way to chronometric geodesy, i.e. the fact of measuring a difference in gravitational potential by comparing the frequencies of two clocks placed at different altitudes on Earth. “If the altitude of one of the clocks fluctuates, we should soon be able to observe a fluctuation in the frequency of that clock from a distance. This means that we could measure small variations on the Earth’s surface. For example, earthquakes. In the near future, we will therefore, in a way, be able to visualize space-time, it’s just incredible”, rejoices the specialist. For the most impatient, a solution: stay as still as possible and avoid the mountain.