![]() 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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