How to check general relativity?


General relativity, proposed by Einstein at the beginning of the 20th century, revolutionized our understanding of space and time and became a new theory of gravity, which is widely used today and has no full-fledged alternatives.

Like all new ideas in science, Einstein’s theory had to be tested by comparing its predictions with experimental data to verify its correctness. In this article, we will look at the effects that prove the truth of general relativity.

General relativity explains the force of gravity in terms of how spacetime is “curved,” or more precisely, the theory relates the force of gravity to a change in the geometry of spacetime.

That is, general relativity presupposes the existence of another dimension that we cannot directly see, and any object with mass, for example, a planet, will distort space-time beneath it in that dimension invisible to us.

According to this theory, all planets move in circular orbits not under the influence of gravity. In fact, it seems to us that they move in curved orbits, in fact, in the theory of relativity, their orbits are straight, but only in 4-dimensional space-time. In fact, the orbits of the planets in general relativity are geodesic — the shortest path between two points in space-time.

Mercury is the closest planet to the Sun, therefore it feels better the curvature of space-time caused by the large mass of the Sun and the various effects of general relativity for it are the most significant among all the planets. Einstein also wondered whether the curvature of space-time could lead to noticeable features in the motion of Mercury that are not provided for by Newton’s law. It turned out that it can.

The orbit of Mercury has a very large eccentricity. Its perihelion is one third less than the aphelion, and interestingly, the position of the perihelion is always changing. The perihelion shift of Mercury is about 43 arc seconds per 100 years. Scientists for a long time could not explain the reasons for this effect within the framework of Newton’s gravity, so they put forward hypotheses about the gravitational effect of other, so far unknown, planets near Mercury.

And then general relativity appeared, according to which, in 1915, Einstein calculated this shift and obtained an almost exact coincidence with the observed 43 arc seconds per century. The result turned out to be extremely close to the observed, and this gave Einstein even more confidence in the correctness of his theory. Later, the relativistic displacement of perihelion was also observed for the orbits of several asteroids approaching the Sun and other planets, however, for the planets this effect was barely perceptible due to their distance from the Sun.

Newton’s theory of gravity assumes that gravity does not affect light in any way, but according to the prediction of general relativity, light will also move along a geodesic in space-time and near massive objects the trajectory of photons will also seem to us a curve. And according to the results of numerous experiments, light does move along a curved path near the Sun, we see this as a displacement of the position of the star when it is next to the disk of the Sun in the celestial sphere. And this shift ideally coincides with the predictions of general relativity.

Likewise, light from distant stars or galaxies can be distorted when passing near a massive body. For example, a galaxy will act like a gravitational lens, bending light and creating multiple images of objects behind it.

Thus, the mathematical equations of Einstein’s general theory of relativity are verified many times. And today the use of this theory is the most accurate way to predict the gravitational interaction.

Author: Alexey Nimchuk. Edited by Fedor Karasenko.

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