The twin paradox is a thought experiment involving two identical twins. One of them stays on earth and the other flies into space in a high-speed rocket. When the second twin returns to earth after his journey, he finds, to his surprise, that his twin has aged more than him. This manifestation of time dilation is really intriguing to me and has gotten me excited to learn about Einstein’s theories of special relativity and general relativity.
The first step in learning Einstein’s theories is understanding that the speed of light is a constant. Nowadays, most people know that the speed of light is approximately 3.10^8 m/s but, as this speed used to be very hard to measure due to lack of technology, people thought that the speed of light wasn’t constant and even invented a theory involving a medium called “luminiferous ether”.
In the late 19th century, people knew that light behaved like a wave. They therefore believed that light, just like other waves, required a medium to propagate. They thought that this was the case because they had never studied waves that didn’t require a medium to propagate. For example, they knew that sound waves need air/water/solid to propagate. Naturally, they believed that light also travelled through a medium and they decided to call it “luminiferous ether”. Nowadays, we know that waves do not always require a medium to propagate (mechanical waves, ie. sound, do need a medium while electromechanical waves, ie. light, don’t need a medium).
But, if “luminiferous ether” would be the medium by which the light propagates, it would implicate that the speed of light would be dependent on the light’s direction. Light would move faster when emitted in the same direction as Ether and slower when emitted in the opposite direction (think about trying to talk to someone while standing against the wind compared to with the wind – one is a lot less effective than the other).
Michelson and Morley designed a device that could test if the speed of light was dependent on the direction in which it was emitted. This experiment would in that way prove the existence or non-existence of ether.
The device split a beam of light and then bounced it off mirrors (so that it moved in different directions). At the end of the experiment, the light eventually reunited and finally hit the same target. If the speed of light was dependent on its’ direction, the light in the experiment would move at different speeds and the recombined light beams would have to be slightly out of phase with each other (which would implicate that the light wouldn’t have arrived at the same time).
Michelson and Morley found that the interference pattern measured by the detector never changed. They could rotate the device left, right, up, down (every direction!), it never made a difference. They concluded, with the help of numerous other experiments, that “luminiferous ether” couldn’t exist.
This experiment is actually known to be the most famous failed experiment in physics. The unexpected result made it easier for other scientists to solve problems involving the speed of light (by supposing that it is constant).
It was later shown that the speed of light is always the same – it isn’t affected by a hypothetical “luminiferous ether” – and, that time and space will even bend themselves to fit (so that ‘light can stay a constant’).