In , they reported that their approach gave the correct numerical value. This result is satisfying. But it is not definitive. For one thing, Urban and colleagues had to make some unsupported assumptions. It will take a full analysis and some experiments to prove that c can really be derived from the quantum vacuum. Nevertheless, Leuchs tells me that he continues to be fascinated by the connection between classical electromagnetism and quantum fluctuations, and is working on a rigorous analysis under full quantum field theory.
At the same time, Urban and colleagues suggest new experiments to test the connection. So it is reasonable to hope that c will at last be grounded in a more fundamental theory. And then — mystery solved? These are believed to apply to the entire universe and to remain fixed over time. The gravitational constant G, for example, defines the strength of gravity throughout the Universe.
The numerical values of these and other constants are known to excruciating precision. For instance, h is measured as 6. But all these quantities raise a host of unsettling questions. Are they truly constant? Why do they have those particular values?
What do they really tell us about the physical reality around us? Aristotle believed that the Earth was differently constituted from the heavens. Copernicus held that our local piece of the Universe is just like any other part of it. Today, science follows the modern Copernican view, assuming that the laws of physics are the same everywhere in spacetime.
But an assumption is all this is. It needs to be tested, especially for G and c, to make sure we are not misinterpreting what we observe in the distant universe. In , cosmological considerations led him to suggest that it decreases by about one part in 10 billion per year.
Was he right? Probably not. Observations of astronomical bodies under gravity do not show this decrease, and so far there is no sign that G varies in space. Its measured value accurately describes planetary orbits and spacecraft trajectories throughout the solar system, and distant cosmic events, too. Radio astronomers recently confirmed that G as we know it correctly describes the behaviour of a pulsar the rapidly rotating remnant of a supernova 3, light years away.
Similarly, there seems to be no credible evidence that c varies in space or time. Are they fundamental? Are some more fundamental than others? Then, the metre was defined as 1,, Unlike the previous definitions, these depend on absolute physical quantities which apply everywhere and at any time. Can we tell if the speed of light is constant in those units?
The quantum theory of atoms tells us that these frequencies and wavelengths depend chiefly on the values of Planck's constant, the electronic charge, and the masses of the electron and nucleons, as well as on the speed of light. By eliminating the dimensions of units from the parameters we can derive a few dimensionless quantities, such as the fine structure constant and the electron to proton mass ratio.
These values are independent of the definition of the units, so it makes much more sense to ask whether these values change. If they did change, it would not just be the speed of light which was affected. The whole of chemistry is dependent on their values, and significant changes would alter the chemical and mechanical properties of all substances. Furthermore, the speed of light itself would change by different amounts according to which definition of units you used.
In that case, it would make more sense to attribute the changes to variations in the charge on the electron or the particle masses than to changes in the speed of light. In any case, there is good observational evidence to indicate that those parameters have not changed over most of the lifetime of the universe. See the FAQ article Have physical constants changed with time? Another assumption on the laws of physics made by the SI definition of the metre is that the theory of relativity is correct.
It is a basic postulate of the theory of relativity that the speed of light is constant. This can be broken down into two parts:. To state that the speed of light is independent of the velocity of the observer is very counterintuitive.
Some people even refuse to accept this as a logically consistent possibility, but in Einstein was able to show that it is perfectly consistent if you are prepared to give up assumptions about the absolute nature of space and time. In it was thought that light must propagate through a medium in space just as sound propagates through the air and other substances. The two scientists Michelson and Morley set up an experiment to attempt to detect the ether, by observing relative changes in the speed of light as the Earth changed its direction of travel relative to the sun during the year.
To their surprise, they failed to detect any change in the speed of light. Fitzgerald then suggested that this might be because the experimental apparatus contracted as it passed through the ether, in such a way as to countermand the attempt to detect the change in velocity. Lorentz extended this idea to changes in the rates of clocks to ensure complete undetectability of the ether. During the s this idea was seriously challenged.
First, by Dutch scientist Isaac Beeckman in , who set up a series of mirrors around a gunpowder explosions to see if observers noticed any difference in the when the flashes of light appeared. Could this be because light took a longer time to travel from Jupiter when Earth was further away?
Further experiments with beams of light on our own planet edged scientists closer to the right number, and then in the mids physicist James Clerk Maxwell introduced his Maxwell's equations — ways of measuring electric and magnetic fields in a vacuum.
Maxwell's equations fixed the electric and magnetic properties of empty space, and after noting that the speed of a massless electromagnetic radiation wave was very close to the supposed speed of light, Maxwell suggested they might match exactly.
It has units after all: meters per second. And in physics any number that has units attached to it can have any old value it wants, because it means you have to define what the units are. For example, in order to express the speed of light in meters per second, first you need to decide what the heck a meter is and what the heck a second is.
And so the definition of the speed of light is tied up with the definitions of length and time. In physics, we're more concerned with constants that have no units or dimensions — in other words, constants that appear in our physical theories that are just plain numbers.
These appear much more fundamental, because they don't depend on any other definition. Another way of saying it is that, if we were to meet some alien civilization , we would have no way of understanding their measurement of the speed of light, but when it comes to dimensionless constants, we can all agree. They're just numbers. One such number is known as the fine structure constant, which is a combination of the speed of light, Planck's constant , and something known as the permittivity of free space.
Its value is approximately 0. Just 0. Like I said, it's just a number. So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.
That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know. Learn more by listening to the episode "Why is the speed of light the way it is? Thanks to Robert H, Michael E.
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