Steve Allen of the Lick Observatory has much more info on this topic. The actual mean solar second is somewhat longer now. However, the mean solar second length used in those definition was calculated using data gathered from 1750 to 1892, and corresponds to the mean solar second from around the middle of that period, i.e., ~1820. The original ephemeris second was defined as 1/86400 of a mean solar day, but due to the variations in the solar day length & the practical difficulties in measuring it, the ephemeris second was redefined in terms of the mean tropical year of 1900, and prior to the adoption of the Cs-133 standard, ephemeris time was determined from observations of the Moon. Together, NIST-F1 and NIST-F2 form the primary time & frequency reference for the USA.įWIW, the SI second was defined in terms of an integral number of Cs-133 hyperfine transitions which approximately matched the current definition of the astronomical ephemeris second. NIST-F1 (also an atomic fountain) has an uncertainty around 5e-16. "Fermi gas" is a 3D quantum gas optical lattice. "Fountain" is an atomic fountain, that value is for NIST-F2. "Beam" refers to a standard off-the-shelf beam maser. Here's a summary of the relative uncertainty of various atomic clocks, from Wikipedia. Such clocks can be used to calibrate a caesium clock, but to use such a clock as a primary time reference it has to reproduce the behaviour of the Cs-133 SI second, warts and all. We now have various clocks that have more precise ticks than the caesium clock. That is, the variation of the Cs-133 hyperfine transition is baked into the definition of the SI second. You count 9192631770 of them and you have an official SI second. So it doesn't actually matter that each of those oscillations have a slightly different duration. An SI second is defined as the time taken for 9192631770 oscillations at the frequency of the caesium-133 hyperfine line at 0 K. There's another issue lurking in your question. But even if we could use a single atom at picokelvin temperatures there would still be some width of the transition frequency, it cannot be a single number, due to time-energy uncertainty. However, even using a small population of such cold atoms, some thermal noise and atomic collisions are inevitable when the cloud is energised by microwaves. Using the optical molasses technique, the atom temperature is reduced to ~40 microkelvin. The modern caesium fountain uses a small "cloud" of laser cooled atoms in freefall. There have been some improvements in the design since then. The caesium-133 clock was developed a few years later, in 1955. The first atomic clock was an ammonia maser built in 1949, but it was less accurate than the best quartz clocks available then. The caesium clock is good, but it's not the best timekeeper that we have. The mirror temperature has to be kept constant, and the space between them has to be an ultrahard vacuum.Īccording to current international standards the second is defined by the Cesium Standard which is the basis of really good atomic clocks. Of course, the bouncing photons will cause the mirrors to vibrate, presumably that's how we detect the reflections. The mirrors need to be positioned precisely, with no outside vibration. It's simply not practically possible to make an Einstein / Langevin-style light clock that could have anywhere near the precision of a good atomic clock. We could use ultraviolet light for a shorter wavelength, and perhaps even x-rays, although you need to use very shallow reflection angles with x-rays. It's much easier to measure time precisely than it is to measure distance, so we'd use an atomic clock to measure & maintain the mirror distance.Īlso, we somehow need to precisely measure the moments when the light bounces off the mirrors, and the precision of that measurement is limited by the wavelength of the light the shortest visible wavelength is around 400 nm, which corresponds to ~ $1.33×10^$ seconds. As John Rennie mentions, the light clock relies on a precise measurement of the distance between the two mirrors, and that distance has to remain stable while the clock is running.
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