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  • Writer's pictureSven Piper

Wave-particle duality - The ether and the special theory of relativity

Updated: 7 days ago


Wave-particle duality created with Dall-E
Wave-particle duality created with Dall-E

The wave-particle duality of light is one of the great mysteries of modern physics, because sometimes light appears to consist of particles and other times it behaves like a wave. In this special you will learn more about this abstract phenomenon, which even the greatest physicists have so far failed to solve.


The concept of the ether was introduced because it was believed that light propagates from the Sun to the Earth in waves and, like every type of wave except electromagnetic waves, needs a medium to travel. This becomes particularly clear with sound waves, which propagate through the air by vibrating. In a vacuum, however, sound waves cannot propagate as they lack the necessary medium, hence the well-known saying: 'In space, no one can hear you scream'.


It was also thought that the ether is absolutely still and that all celestial bodies move in it. From this, scientists concluded that it should be possible to prove the existence of the ether by finding out how fast the earth moves through the stationary ether. From this it should be possible to calculate how fast it moves relative to the rest of the universe. It should therefore be possible to prove the existence of the ether by determining this so-called 'absolute motion'.


One of the most famous experiments was carried out at the end of the 19th century. The young naval officer Albert A. Michelson and the well-known chemist Edward W. Morley carried out an experiment at Case-Western Reserve University in Cleveland, Ohio, in 1887.


The experiment went down in history as the Michelson-Morley experiment. Light from a light source was split by a splitter and each beam was reflected by a mirror and passed back via the beam splitter to a detector, in this case a telescope.


Michelson-Morley experiment created with Dall-E
Michelson-Morley experiment created with Dall-E

The idea behind it


If two beams of light that are perpendicular to each other are generated in the beam guide, they need different times to return to the beam guide. The light beam that first travels against the ether flow and then with it takes a tiny fraction of a second longer than the light beam that travels perpendicular to the flow and back.


The whole apparatus was mounted on a sandstone plate, which in turn floated on mercury. This construction was intended to dampen vibrations and, as the whole thing could be rotated, the investigation could be carried out in different directions.


The major breakthrough was to come between July 8 and 12, 1887, but to the great surprise of Michelson and Morley and the rest of the scientific elite, the experiment failed.


It was not until 1905 that a 26-year-old third-class patent clerk at the Swiss Patent Office found the solution. His name was Albert Einstein.


In the special theory of relativity he postulated, he stated that it is impossible to determine the motion of the earth or any other celestial body relative to an ether, as there is no absolute motion, only relative motion between objects. As a result, it is also impossible to know whether a celestial body is really at rest or moving in the universe.


Einstein's theory also showed that light can propagate through a vacuum without the need for a medium such as the ether.


Another key statement of the special theory of relativity is that the speed of light remains the same regardless of whether the light source is moving or not and also whether the observer is moving or not. This means that the concept of the ether as a stationary, absolute frame of reference is no longer necessary.


These decisive statements, which turned the entire physical world view on its head, made the aether superfluous, as we cannot determine the absolute movement of the earth and we will therefore never know whether the aether exists or not.

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