# Theory of relativity for dummies

The special theory of relativity, which at the beginning of the last century overturned generally accepted ideas about the world, still continues to excite the minds and hearts of people. Today we will try to figure out together what it is.

In 1905, Albert Einstein published Special Theory of Relativity (SRT), which explained how to interpret motion between different inertial frames of reference - simply put, objects that move at a constant speed in relation to each other.

Einstein explained that when two objects move at a constant speed, one should consider their motion relative to each other, instead of accepting one of them as an absolute frame of reference.

So if two astronauts, you and, say, Herman, are flying on two spaceships and want to compare your observations, the only thing you need to know is your speed relative to each other.

Special theory of relativity considers only one special case (hence the name), when the motion is straight and uniform. If a material body accelerates or turns to the side, the SRT laws no longer apply. Then the general theory of relativity (GTR) comes into force, which explains the movements of material bodies in the general case.

Einstein's theory is based on two basic principles:

1. The principle of relativity: physical laws are preserved even for bodies that are inertial reference frames, that is, moving at a constant speed relative to each other.

2. The principle of the speed of light: the speed of light remains unchanged for all observers, regardless of their speed in relation to the light source. (Physicists denote the speed of light with the letter c).

One of the reasons for Albert Einstein's success is that he put experimental data above theoretical ones. When a number of experiments revealed results that contradicted the generally accepted theory, many physicists decided that these experiments were wrong.

Albert Einstein was one of the first who decided to build a new theory based on new experimental data.

At the end of the 19th century, physicists were in search of a mysterious ether - a medium in which, according to generally accepted assumptions, light waves should propagate, like acoustic waves, for which air is needed to propagate, or another medium - solid, liquid or gaseous. The belief in the existence of the ether led to the belief that the speed of light should change depending on the speed of the observer in relation to the ether.

Albert Einstein abandoned the concept of ether and suggested that all physical laws, including the speed of light, remain unchanged regardless of the speed of the observer - as experiments have shown.

## Homogeneity of space and time

Einstein's Special Theory of Relativity postulates a fundamental relationship between space and time. The Material Universe, as you know, has three spatial dimensions: up and down, right and left, and forward and backward. One more dimension is added to it - the temporal one. Together, these four dimensions make up the space-time continuum.

If you are moving at a high speed, your observations in relation to space and time will differ from those of other people moving at a slower speed.

The picture below shows a thought experiment to help you understand this idea. Imagine that you are on a spaceship, holding a laser in your hands, with the help of which you send beams of light to the ceiling on which a mirror is fixed. The light, reflected, falls on the detector, which registers them.

Let's say your ship is moving at a constant speed equal to half the speed of light (0.5c). According to Einstein's SRT, it doesn't matter to you, you don't even notice your movement.

However, Herman, watching you from a resting starship, will see a completely different picture. From his point of view, the beam of light will travel diagonally to the mirror on the ceiling, reflect from it and fall diagonally onto the detector.

In other words, the trajectory of the light beam will look different for you and for Herman, and its length will be different. Therefore, the length of time it takes for the laser beam to travel the distance to the mirror and to the detector will seem different to you.

This phenomenon is called time dilation: time on a starship moving at high speed, from the point of view of an observer on Earth, flows much slower.

This example, like many others, clearly demonstrates the inextricable link between space and time. This connection is clearly manifested for the observer only when it comes to high speeds close to the speed of light.

Experiments since Einstein published his great theory have confirmed that space and time are actually perceived differently depending on the speed of movement of objects.

## Combining mass and energy

In his famous article published in 1905, Einstein combined mass and energy in a simple formula that has been known to every student ever since: E = mc ^ 2.

According to the theory of the great physicist, when the speed of a material body increases, approaching the speed of light, its mass also increases. Those. the faster the object moves, the heavier it becomes. In the case of reaching the speed of light, the mass of the body, as well as its energy, become infinite. The heavier the body, the more difficult it is to increase its speed; to accelerate a body with infinite mass, an infinite amount of energy is required, therefore it is impossible for material objects to reach the speed of light.

Before Einstein, the concepts of mass and energy in physics were considered separately. The brilliant scientist proved that the law of conservation of mass, like the law of conservation of energy, are parts of a more general law of mass-energy.

Due to the fundamental connection between these two concepts, matter can be turned into energy, and vice versa - energy into matter.