Speed of Light
The constancy of the vacuum speed of light has tremendous implications: the flow of time is personal, and when we move around, our clocks - no matter how precise - are not synchronised. It is now time to look at why we think that the vacuum speed of light is a fundamental constant of nature. Before we start, two comments:
Cherenkov Radiation:
The speed of light propagating through a medium such as water is smaller than the vacuum speed of light. I just wanted to mention it to avoid confusion when you find different claims. This decrease of speed has some beautiful consequences: in a reactor, the electrons can move with velocities lower than the vacuum speed of light but faster than its speed in the medium. This mechanism creates a bluish light (Cherenkov radiation), the analogue of a sonic boom with light rather than sound waves.
Secondly, frequently people believe that they do not understand Einstein’s theory of Relativity, such as the flow of time, as discussed in my previous blog. They are wrong! I grew up in the mountains and was seriously impressed as a kid when I saw the mythical object ‘horizon’ and how flat an ocean can be. But would I say that I do not understand oceans?
To avoid feeling that you don’t understand something, this is what you have to do: Be aware of your assumptions. It is humankind’s recipe for success to develop a concept for one setting and then apply it to another without any justification whatsoever. Be not surprised if the idea is not valid in the ‘other’ setting. Why should it be? We tend to react by claiming: I do not understand. For example, a tennis ball moves with 5 m/s in a train and the train moves with 30 m/s in relation to the landscape. Everyday life experience tells us: the tennis ball flies through the landscape with 35 m/s. We have an excellent concept for the setting - which we call everyday life - when the speeds involved are much smaller than the vacuum speed of light. The ‘other’ setting is then a spacecraft moving with 80% speed of light, and the speeds do not add up at all. Do not say that you do not understand it: your assumption just did not come true. There is nothing to understand: observe! Take a good look at the ocean and be amazed.
Sun - bringer of life
Light is weird.
Every other wave has a medium to propagate: water waves use water - like ripples on a pond. Sound is slight pressure variations in the air. Light does not need a medium to propagate. If this weren't true, the universe would look so different. The space between the sun and Earth is largely empty if we disregard the odd particles flying around now and then. How would the sun send its warmth to Earth if light needs a medium? Planets would be deeply frozen, lightless worlds, and life as we know it would not exist. If we look at a sunset, we see the light that has left the sun's surface about nine minutes ago. It has traversed the emptiness between the sun and Earth just to be seen by our eyes.
Given that every other wave moves in a medium, it was pretty natural in the 19th century to assume there is a medium for light, just one we can not see (yet). The medium was called luminiferous aether. Like the wind, it would blow in a particular direction through our solar system. We then would expect that speed of light changes depending on whether we travel with the flow of the aether or against it. This was the effect that Albert Michelson and Edward Morley tried to measure in 1887. It became known as the most important failed experiment in Physics. Light seems to propagate in all directions with the same speed (isotropy).
The Michelson-Morley experiment has been repeated many times with ever better instrumentation and methods. A recent example is from Herrmann and collaborators in 2009: the variations in the speed of light over the speed itself is smaller than 0.000 000 000 000 000 010.
From Maxwell’s equation to the theory of special relativity.
We could have known….
James Clerk Maxwell has unified the theory of electric and magnetic fields, combining the equations to the set, which we now know as Maxwell’s equation. In 1865, Maxwell found wave type solutions of these equations, which can propagate in the absence of any medium. He, therefore, predicted the existence of radio waves. He also found that they can only propagate with one speed. The latter feature raised some unpleasant questions to reconcile this property of light with everyday experience. It took until 1905 when Albert Einstein was the first to think this through and offer redemption in his theory of Relativity.