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?Unfortunately it doesn't mention anything about inertial frames, but you can consider a measurement in an inertial frame to be implied. At the moment you can measure macroscopic distances most accurately by sending out laser light pulses and timing how long they take to travel using a very accurate atomic clock.Tags: Scholarly Research PaperEssay On Hamlet ConflictMedia Dissertation IdeasEssays Term Paper Bicentennial ManThe Assignment ImdbBeginning An Essay With A QuoteEssay Honor SocietyEssay About Global WarmingSocial Business Plan Competition
(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 1676.) We could, for example, take the definitions of the units as they stood between 19.
Then, the metre was defined as 1,650,763.73 wavelengths of the reddish-orange light from a krypton-86 source, and the second was defined (then as now) as 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of caesium-133.
Experiments have shown that the mass of the photon must be very small if it is not zero (see the FAQ entry What is the mass of the photon? Any such possible photon rest mass is certainly too small to have any practical significance for the definition of the metre in the foreseeable future, but it cannot be shown to be exactly zero—even though currently accepted theories indicate that it is.
If the mass weren't zero, the speed of light would not be constant; but from a theoretical point of view we would then take is constant.
See the FAQ article Have physical constants changed with time?
(Note that the fine-structure constant does change with energy scale, but I am referring to the constancy of its low-energy limit.) Another assumption on the laws of physics made by the SI definition of the metre is that the theory of relativity is correct.There may have been even larger variations in the length or the metre standard caused by metal shrinkage.The net result is that the value of the speed of light as measured in m/s was slowly changing at that time.All of chemistry depends 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 was used.The SI definition also assumes that measurements taken in different inertial frames will give the same results for light's speed.This is actually a postulate of special relativity, discussed below.They also usually mean the speed as measured in an inertial frame.This vacuum-inertial speed is denoted 299,792,458 m/s. Definitions are adopted according to the most accurately known measurement techniques of the day, and are constantly revised.We have to state what we are going to use as our standard ruler and our standard clock when we measure .In principle, we could get a very different answer using measurements based on laboratory experiments, from the one we get using astronomical observations.