Albert Einstein is one of the most famous physicists of the 20th century. However, in addition to amazing theories that describe the large-scale world with incredible accuracy, he revealed one curious phenomenon: the stronger the gravity, the slower time passes.

Einstein called his first theory known to the whole world as the Special Theory of Relativity. It was special because it dealt with constant speeds. To reconcile it with the real world, in which objects are constantly accelerating and decelerating, he needed to investigate the implications of his theory when it came to acceleration. This attempt to generalize and take into account all the general phenomena led to the discovery of the relationship between time and gravity. Einstein called his new theory General Relativity.

Newton believed that the flow of time is like an arrow. It moves steadily only in one direction - forward. Einstein suggested that time changes in inverse proportion to speed. And due to its fluidity, like space, it "deserved" its own measurement. Moreover, Einstein argued that space and time are a single whole - a flexible four-dimensional fabric on which all events in the Universe take place. That's what he called it - the fabric of space-time. When the physicist published his work with all its conclusions, she was greeted with disbelief.

According to General Relativity, matter stretches and contracts the fabric of spacetime. It turns out that objects are not attracted to the center of the Earth in some mysterious way, but rather, on the contrary, are pushed by the curved space around them. Like a slope, the curvature of space-time accelerates objects moving downward, although the degree of this acceleration is not always the same. The force of gravity increases with the approach to the surface of the Earth, where the curvature is more intense.

If the force of gravity increases as it moves down, the object will freely fall to point B on the surface faster than to point A at a higher altitude. According to the Special Theory of Relativity, the time for a freely falling object at point B should go slower relative to the object at point A due to the fact that the speed of the object at point B is higher.

## What is time

What is the correct time? Einstein postulated that there is no absolute time. Time is relative depending on the system of forces to which it is subjected. This is formally called a frame of reference. The time that passes within your system is called your own time. If the laws of motion are to be the same for all observers, regardless of their motion, then time must slow down. That is, the faster you move, the slower your clock ticks relative to other clocks. This is exactly what the heroine Anne Hathaway told the character Matthew McConaughey in "Interstellar" after the descent to a distant planet: "One hour on this planet is equal to seven Earth years."

So, is observing slowed down time a limitation of our primitive neurological makeup, or is time really slowing down? And what does time dilation mean in general? In the end, this brings us to the question: what is time? This is not just a question that philosophy students ask each other over a glass of beer. The concept of time has puzzled natural philosophers and physicists since time immemorial.

The main function of time is to keep track of the chronology of events. However, until the last 400 years, people have determined time based on the assumption that stars move around the Earth, and not vice versa. Regardless, everything worked reasonably well to a certain extent - due to the fact that the days and seasons repeated predictably, and when you have something that repeats predictably, then there is a mechanism for keeping track of time.

Galileo used the recursive nature of such a mechanism to compute motion. The description of the movement would be impossible without some indication of time. But this time has never been absolute. Even when Newton formulated his laws of motion, he used the concept of time, in which two pairs of clocks tick synchronously not with absolute, independent time, but with each other. Synchronization is the reason why humanity has built such a sophisticated and accurate atomic clock.

The concept of time is built on the simultaneity or decisive coincidence of two events - like the arrival of a train and the unique coincidence of the clock hands at that moment. Einstein's theory states that this must be influenced by movement. If the two observers on the platform and the train cannot agree on what is at the same time, they cannot agree on how time itself flows.

## Movement distorts time

To understand the effect of motion on predictability, consider a simple timing mechanism. Imagine a time tracking apparatus consisting of a photon bouncing between two mirrors spaced a finite distance apart. Let one second pass during the reflection period of the photon. Now we will place two such devices at points A and B above the surface of the Earth and right on it (as in the example described above) and see how they count the time when a freely falling object sweeps past them. In turn, this object measures its own time using the same clock. What will they show?

Seeing the reflection of a photon between two moving mirrors is like watching a tennis ball bouncing on a moving train. Even if the ball bounces perpendicularly to someone on the train, it describes triangles to a stationary observer outside.

When the apparatus moves forward, it seems that the photon, like a ball, travels a greater distance after being reflected. It turns out that one result of our experiment is distorted! Moreover, the faster the apparatus moves, the more time it takes for the photon to reflect, thereby extending the duration of a second. That is why the course of time at point B turns out to be slower than at point A (remember: due to gravity, the object falls at point B faster than at point A).

Of course, this difference is negligible. The difference between the time measured by clocks on the tops of the mountains and on the surface of the Earth is only a few nanoseconds. Nevertheless, Einstein's discovery was a real breakthrough. Gravity really interferes with the passage of time, which means that the more massive an object, the slower time flows near it. Some physicists even make a reservation that all objects in the Universe seem to feel it and try to fall where time goes slower, from places where time goes faster.

## Legs are younger than the head

Today, gravitational time dilation is not only a well-known phenomenon from the field of theoretical physics, but also a practical tool. Thanks to the discovery of Einstein and his equations, we have such a wonderful thing as GPS navigation, which could not work so accurately if the difference between the course of time on the Earth's surface and the course of time in near-earth orbit was not taken into account. Gravitational time dilation also helps theoretical physicists and astrophysicists to build accurate theories about what is happening in distant space near objects that we cannot physically get close to (for example, black holes and neutron stars).And yes, given this phenomenon, it turns out that your legs - albeit infinitely insignificantly - are younger than your head.