BeHind tHe sCenes: Einstein TheOry of relaRivity

In 1905 Einstein formulated the theory of special relativity, which solves the contradictions between Galilean relativity and electromagnetism. Ten years later, in 1915, the Einstein field equation - the heart of general relativity theory - solves the conflict between special relativity and Newton's theory of gravitation. A new physics and a new way of looking at the universe is born. Here is the story (and especially the scope) of that discovery.

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At the last minute, Einstein had a rethink. Something was missing. He resumed the article he had prepared for the prestigious scientific journal Annalen der Physik , and added a post-scriptum , three pages written with crisp and neat spelling, to illustrate a final, inevitable consequence of his theory: energy is equivalent to the subject, E = mc 2 .
Thus, the most famous formula of the entire history of science appeared for the first time in the post scriptum of an article signed by Albert Einstein, an obscure clerk of the Berne patent office.
This formula states that the impalpable energy can be transformed into concrete material, and vice versa ... an almost magical event, but (perhaps for this reason) understandable to everyone. The rest of the theory of relativity, on the other hand, is more difficult to digest: to understand it, we need to overturn what the senses, experience and even the old physics books tell us.

THE THEORY OF RESTRICTED RELATIVITY

But let's go in order. When we talk about relativity, in general, we put together two different writings by Einstein, one from 1905 (special relativity) and one from 1915 (general relativity) , then published at the beginning of 1916). How can they be distinguished?
It is simple: general relativity deals with the force of gravity, the narrow one does not. Therefore, all phenomena involving gravitational attraction, such as black holes, concern general relativity.
Here is what the theory of special relativity says:
Everyone knows that the senses can trick us. When we look at a long straight road, for example, we have the impression that it is narrowing in the distance, but we do not at all dream of confusing this feeling with reality. Relativity does the same thing: it discards everything that depends on the point of view, and preserves what remains constant in any condition.

TIME SLOWS DOWN, THE MASS GROWS, OBJECTS BECOME SHORTER

Finding out what does not vary, however, is not easy. The weather? Common sense tells us that if a bell tolls in New York and after a moment another bell tolls in Rome, the order of the two events is unquestionable. The theory of relativity instead affirms that the speed of the observer also influences the perception of the before and after, and therefore that the passage of time is not universal.
How did Einstein get to such a conclusion?
The German scientist started from the fact, well known even in his time, that the light propagates with very high speed but not infinite, exactly 299.792 kilometers per second. But the speeds we measure, depend on our own speed: the car that surpasses us, for example, sometimes seems to be exasperatingly slow. If this also applies to light, the rays emitted by a star should seem to us faster or slower depending on whether the Earth approaches or moves away from the star. But this does not happen, the speed of light does not change, and this strangeness was first demonstrated by two American physicists, Michaelson and Morley.

Einstein drew the consequences. If a speed remains constant even when, according to logic, it should change, then there is only one explanation: it is the tachometer that does not work as usual. And not because of him, explains Einstein, but because they change the objects that the poor speedometer has to measure: space and time are no longer the same. And the instrument faithfully records the result: a speed that never changes.
But how do space and time change? Here is an example. If an astronaut on the moon looks into the cabin of a passing rocket, he would see his colleagues on the rocket moving in slow motion, and the objects on the spaceship "shorten" along the direction of the motion.
But even the astronauts in transit would see the colleague on the Moon moving in slow motion. Why? If on the one hand time slows down, on the other hand it should not accelerate? Not at all. Think of two men a hundred meters away: the first sees the other shrunk from the distance, but not for this the second sees the first enlarged. The theory of relativity then introduces the concept of a temporal perspective caused by speed.

All the oddities of relativity derive from this one concept, also the equation E = mc 2 . According to the old theories, in fact, continuing to push a body its speed should increase to infinity, and this is impossible: nothing can go faster than light. What happens then? Simple: the energy supplied does not increase the speed of the body, but its mass: the body becomes increasingly "heavy". In this sense, mass is only a form of energy. And on August 6, 1945, with the launch of the atomic bomb on Hiroshima, the world had the most convincing demonstration of this principle.

A letter by Einstein to the president before the atomic bomb lunch

Not all speeds are relative …

Speed ​​is a matter of points of view

In fact, when measuring the speed of an object (for example a projectile) it is impossible to take into account its own speed. Just think of a bullet fired from a moving train: if the measuring instrument is on the train, it detects a certain value, say 200 kilometers per hour; if instead it is outside, it detects the sum of two speeds, that of the train and that of the projectile. One can think that this last value is the right one, because it is detected as "still". But this is not true, because even the ground moves (with the planet) around the Sun.

Snail world

So, is speed always relative? No, says Einstein: that of light is absolute, that is, it does not change if it is measured by the train or if it is measured from the ground. On the other hand, with the snail speeds typical of the world in which we live, for the daily life the phenomenon is irrelevant.

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GENERAL RELATIVITY

General relativity, a complex mathematical construction that required ten years of study. With it, Einstein intended to construct a mathematical model of the laws that govern the universe: in fact, special relativity works well only in areas of space-time in which gravity is irrelevant, that is, where there is little matter. The results that Einstein obtained constitute a complex of equations that, just like a computer program, gives different results depending on the data that are inserted.
That is why general relativity has never ceased to provide new information: its equations can analyze any cosmic situation being conceived, or identified. For example, equations can tell if and under what conditions a black hole can form in the cosmos, and what would happen in its surroundings.

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The key concept of the theory, however, can easily be expressed in words: gravitation alters space-time. In other words, a concentration of matter bends space (and time), like a bowling ball would bend a trampoline. The most obvious consequences? When space is deformed by the presence of a star, the light rays follow the deformation and describe a curve. Time, on the other hand, flows more slowly in the vicinity of the great masses.
But why should we believe it? Even if Einstein started from the facts, his mathematical construction has come to risky conclusions. Could he have made a mistake? Yes, but so far no one has found it. On the contrary, the experiments have always confirmed the theory. Beginning with the one made by the British astronomer Arthur Eddington, who in 1919 organized an expedition to the island of the Prince, off the coast of Africa, to verify (during an eclipse) if the mass of the Sun really curved the rays coming from the stars.