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A magnetic compass needle blows out near a wire through which current flows, the discovery had been made by Hans Christian Oersted in 1820. So the magnetic and electric fields had to be somehow connected. Ten years later, Michael Faraday discovered that the movement of a magnet in the vicinity of a wire loop generates an electric current in it. And in 1855 Maxwell finally realized that a variable electric field also produces a magnetic field. He combined the two fields into an electromagnetic field and predicted the existence of electromagnetic waves that would move at the speed of light.

All these discoveries led to the most diverse inventions, inventions that still determine our daily lives today. But they could not solve the mystery of gravitation. One thing was clear, gravitation was the elemental force of the universe. If you wanted to find out how it works, you had to understand the universe, the space in which gravity takes place.

Until the beginning of the twentieth century, the scholars agreed with Newton that space was unchangeable. Its extension is measured in the three directions, length, width and height and is therefore three-dimensional. Any point in space can be specified by these coordinates. The space itself is not visible but only what is in it. But would space still exist if there were no things in it? Or was the space always filled with an immeasurably fine substance, the so-called ether, an idea that goes back to René Descartes in the seventeenth century?

The ether could transmit forces, just like air transmits sound. It could be the medium for the transmission of the gravitational force. When Descartes contemporary, the Dutch physicist Christiaan Huygens demonstrated the wave nature of light, he believed that the ether was the medium that made the propagation of light waves possible. For where there are waves, there must be a medium that vibrates, the world ether. That was the general conviction at that time.

However, this ether would have to have strange properties, on the one hand it would have to behave like solid or liquid bodies in which transverse waves can spread like that of light, on the other hand, it should not resist the movement of a mass, otherwise planets on their orbit would be slowed down so much that they would have to fall into the sun.

In 1887, physicists Albert A. Michelson and Edward W. Morley attempted to prove the existence of the world ether with an extraordinarily sensitive optical instrument, the Michelson Interferometer, to measure the speed of light in different directions. They argued that if the ether was the medium in which the light waves propagated, so then the measured speed of light had to be different with the ether wind than it had to be.

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