Article published at aeon.

“We have now fixed the speed of light in a vacuum at exactly 299,792.458 kilometres per second.

Why this particular speed and not something else? Or, to put it another way, where does the speed of light come from?

(…) thanks to Maxwell and Einstein, we know that the speed of light is connected with a number of other (on the face of it, quite distinct) phenomena in surprising ways.

But neither theory fully explains what determines that speed. What might? According to new research, the secret of c can be found in the nature of empty space.

(…) But if we’re talking about empty space, there shouldn’t be any particles in there, should there?

(…) In the advanced version called quantum field theory, a vacuum is never really empty. It is the ‘vacuum state’, the lowest energy of a quantum system. It is an arena in which quantum fluctuations produce evanescent energies and elementary particles.

(…) in a quantum vacuum (…) elementary particles (…) rapidly pop in and out of existence.

These short-lived phenomena might seem to be a ghostly form of reality. (…) An electric field applied to the vacuum distorts these pairs to produce an electric response, and a magnetic field affects them to create a magnetic response. This behaviour gives us a way to calculate, not just measure, the electromagnetic properties of the quantum vacuum and, from them, to derive the value of c.

(…) So it is reasonable to hope that c will at last be grounded in a more fundamental theory. And then – mystery solved?

Well, that depends on your point of view.

The speed of light is, of course, just one of several ‘fundamental’ or ‘universal’ physical constants. (…) The gravitational constant G, for example, defines the strength of gravity throughout the Universe. At small scales, Planck’s constant h sets the size of quantum effects and the tiny charge on the electron e is the basic unit of electricity.

(…) Sets of two to 10 constants have been proposed, but one useful choice has been just three: h, c and G,collectively representing relativity and quantum theory.

(…) But some constants involve no dimensions at all. These are so-called dimensionless constants – pure numbers, such as the ratio of the proton mass to the electron mass. That is simply the number 1836.2 (which is thought to be a little peculiar because we do not know why it is so large). According to the physicist Michael Duff of Imperial College London, only the dimensionless constants are really ‘fundamental’, because they are independent of any system of measurement. Dimensional constants, on the other hand, ‘are merely human constructs whose number and values differ from one choice of units to the next’.

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