What happens if speed of light is not constant

If it wasn't zero, the speed of light would not be constant; but from a theoretical point of view we would then take c to be the upper limit of the speed of light in vacuum so that we can continue to ask whether c is constant. Previously the metre and second have been defined in various different ways according to the measurement techniques of the time.

They could change again in the future. We now know that there are variations in the length of a mean solar day as measured by atomic clocks. Standard time is adjusted by adding or subtracting a leap second from time to time.

There may have been even larger variations in the length or the metre standard caused by metal shrinkage. Obviously it would be more natural to attribute those changes to variations in the units of measurement than to changes in the speed of light itself, but by the same token it is nonsense to say that the speed of light is now constant just because the SI definitions of units define its numerical value to be constant.

But the SI definition highlights the point that we need first to be very clear about what we mean by constancy of the speed of light, before we answer our question.

We have to state what we are going to use as our standard ruler and our standard clock when we measure c. In principle, we could get a very different answer using measurements based on laboratory experiments, from the one we get using astronomical observations. One of the first measurements of the speed of light was derived from observed changes in the timing of the eclipses of Jupiter's moons by Olaus Roemer in We could, for example, take the definitions of the units as they stood between and 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. Relativity already tells us what would happen if the speed of light were to change, and the answer is nothing.

Consider a stationary observer on a platform looking at a light clock on a fast-moving train. They would be able to observe that, on the train, the light in the light clock is taking a longer path and hence the clock is taking longer to complete one full cycle than a clock in their frame of reference. The question may be talking about slowing down only the speed of light and not anything else, such as matter, creating a relative difference.

But at its most fundamental, matter is wave functions travelling and collapsing at the speed of light. This means that the speed of matter, and of everything else, is tied to the speed of light. So changing the speed of light would have no effect on anything. Researchers led by optical physicist Miles Padgett at the University of Glasgow demonstrated the effect by racing photons that were identical except for their structure.

The structured light consistently arrived a tad late. Though the effect is not recognizable in everyday life and in most technological applications, the new research highlights a fundamental and previously unappreciated subtlety in the behavior of light.

Generally if light is not traveling at c it is because it is moving through a material. For example, light slows down when passing through glass or water. Padgett and his team wondered if there were fundamental factors that could change the speed of light in a vacuum.

Previous studies had hinted that the structure of light could play a role. Physics textbooks idealize light as plane waves, in which the fronts of each wave move in parallel, much like ocean waves approaching a straight shoreline. But while light can usually be approximated as plane waves, its structure is actually more complicated. For instance, light can converge upon a point after passing through a lens.

The researchers produced pairs of photons and sent them on different paths toward a detector. One photon zipped straight through a fiber.