# ELI5 Can you explain the general theory of relativity

## theory of relativity

The theory of relativity is a theory of physics developed by Albert Einstein. It made him famous all over the world. The theory of relativity is very difficult to understand. It is so incredible that even many great physicists from Einstein's time initially thought it was nonsense.

One of the two most important ideas of Einstein was that time does not always pass quickly. For example, a second can sometimes take longer and sometimes shorter. The second idea was that lengths are not always the same. A meter is therefore sometimes longer and sometimes shorter. The time and the lengths are "relative", hence the name of the theory.

Einstein built on this “special theory of relativity” and later developed the “general theory of relativity”. This also brings gravity into play and the so-called "curvature of space and time". But that is even more complicated.

### Is this theory just an idea or a reality?

Einstein claimed that how quickly a second passes or how long a meter is, depends on the speed of a body. That still seems strange to us today, as we experience something different every day. But Einstein's assertions and with them his theory could be confirmed by measurements.

How quickly time goes by actually depends on the speed at which you are moving. The faster you move, the slower time goes by. That seems wrong, but with two extremely accurate clocks this could be confirmed in an experiment: One clock traveled around the world in an airplane and the other clock stayed in place. For the traveling watch, the time actually passed more slowly, so that it lagged somewhat after its round the world compared to the resting watch. That was very little, but clearly measurable.

How long a stick is, for example, also depends on the speed at which the stick moves. The faster it moves, the shorter it becomes.

And both, time and length, also change depending on how far you are from Earth. A clock in a satellite that flies high above the earth in weightlessness moves more slowly than a clock on earth.

### What is the meaning of the famous Einstein formula?

Mathematical formulas play a major role in the theory of relativity. Many of these formulas are very complicated. The most famous one is very simple, it is:

*E.*=*m***·***c*^{2}

One speaks that "*E.* equal *m* times *c*Square". What does this formula mean? It says that energy can arise from mass and vice versa. The great "*E.*"Stands for energy, the small"*m*"Stands for mass and the small"*c*“Stands for the speed of light. "*c*^{2}“ („*c* Square “) means that you have to measure the speed of light with yourself. That makes an unimaginably large number. If you now look at the formula, you can see that a huge amount of energy can arise from very little mass. Because you have to add the number for the mass (e.g. 1 kilogram) with the huge number *c*^{2} multiply to calculate the energy.

### Does the theory of relativity make itself felt in our everyday life?

Effects of the theory of relativity only become clear at speeds approaching the speed of light. We move very slowly compared to light. Therefore, the changes in length and time are extremely small for people, they cannot be noticed. But at very high speeds and very long distances they have a noticeable effect.

The navigation system in the car could never find an address exactly if one were to calculate without the theory of relativity. The speeds of the satellites and their distance from the earth are very great. There are clearly measurable differences here. Without Einstein's theory you might be able to find a city with the help of the navigation system, but never a specific house. So we use results of this strange theory in everyday life.

The famous Einstein formula also has a meaning for our lives. For example, the sun constantly loses a tiny part of its mass and turns it into a huge amount of energy: the light and heat that make life on earth possible in the first place. Atomic energy, which is used to generate electricity in nuclear power plants, is based on the conversion of little mass into a lot of energy.

Albert Einstein, the founder of the theory of relativity in 1905

With such an extremely precise clock, an atomic clock, the relativity of time could be proven.

The navigation device in the car is only so accurate thanks to the theory of relativity.

There is also an article on “Theory of Relativity” for beginners on MiniKlexikon.de and other search results from Blinde Kuh and Ask Finn.

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