Slashdot Mirror
Atomic Clock Turns 50
karvind writes "BBC has an interesting story on the 50th birthday of atomic clocks. The first accurate caesium atomic clock was developed at the NPL in 1955 by Dr Louis Essen. And after 5 decades In September the US National Institute of Standards and Technology (NIST) used computer chip fabrication techniques to make a small atomic clock. The final development should see a battery-operated system about the size of a sugar lump. NIST also has a page on history of atomic clocks"
It seems that more and more of everything is sync'd with this. My clock radio at home auto-updates, clock on the wall, the cellphones, my Linux and Mac PC's and cable box.
Only thing left are the clocks with a single AA battery on the wall, and at some point they are going to use the pervasive WWVB time signal that is broadcast from Colorado and operated by the National Institute of Standards and Technology
This technology has really come a long way and is deeply embedded within our lives. Especially if you consider that before the atomic clock, time varied considerably between different locales.
Newsfollow.com
In Latin, the AE is pronouned like an "ai" dipthong and c's are always hard consonants. So Caesar would be pronounced like "Kaiser" and by analogy Caesium would be pronounced "Kaisium."
After all, I am strangely colored.
That's like asking how does the "clock" in your computer get set to the right time. (Not the system clock, the crystal that generates the clock frequency the electronics operate off of).
Atomic clocks just "tick", not display an actual time. They provide an extremely reliable and high frequency tick which makes them so valuable.
=Smidge=
Atomic clocks count the number of vibrations within an atom, so know how much time has passed to a high degree of accuracy. Absolute time however, cannot really be known, as we have no reference point to measure it from (unless we find someone who has been counting since the big bang happened!).
The standard day-to-day time system is UTC (rather mysteriously standing for Coordinated Universal Time) and it is based on the rotation of the earth. This is decided by the BIPM. As the length of a day is not precisely divisible by a second, leap seconds occasioanlly have to be added.
Atomic clocks were primarily developed to deal with former, measuring the passage of time. This turns out to be a very important and difficult problem in all fields of science and engineering. The reason is that any error is measureing the passing of time will be amplified and make all other things very wrong.
So, after using the sun, water, gears, and springs, someone finally figured out that if certain atoms were excited, they would vibrate very regularly. By counting the vibrations, we could measure the passage of time.
Now, we don't measure time in vibrations of an atom. We measure the passage of time in seconds. So how long is a second? The hand waving definition is the second is 1/60 of a hour, and the hour is 1/24 of a day, so we count the vibrations over a day, then divide by 24, and divide again by 60, that is the vibrations in a second. Of course we have to decide how to measure a day without using a clock! This can be done, and after much argument, the scientist just give up and agree on thier best guess. The key thing is that everyone agrees on how many vibrations are in a second, so we are now able to say difinitively that something takes a second, or 10 seonds, or 100 seconds, or 1.2352 seconds.
So, the measurement of the passing of time is important to science, and even important to the making sure that you don't spend an extra second in class or at work, but what does this have to do with your question, which is getting to work or class on time.
Well, ultimately that is just a decision we make. There are standard clocks that measure time in universal time(UT), which used to called greenwich mean time(GMT). This time is adjusted geographically so that 8:00 am does occur in the middle of the night in the US. And that is they key. We set the time so that 8:00 am is in the morning, and 8:00 pm is always when most good children are in bed.
How we set time, so to speak, was done by looking a the sun and the moon. You can in fact look up the data for sunrise and sunset in your area and set your clocks by this phenomemom. This is the same thing we do with calendars and seasons. Much is set around the longest and shortest days of the year. For instance, the day in which there is an equal amount of sunlight and darkness is called the equinox, and this day that occurs in the northern hemisphere, when the amount of daylight is icreasing, is curently called March 20 or 21, and is the first day of spring.
It is interesting that that while we have had accurate clocks for a long time, that is we can accurately measure a second, syncronization has a been a problem. For years we have radio signals to synchronize enabled clocks, but now with the internet we can all be on the same time. Although for some reason some clocks are still set a few minutes off in either direction.
"She's a scientist and a lesbian. She's not going to let it slide." Orphan Black
The definition of the second is now based on the physical quantity that atomic clocks measure, so the clocks are, when functioning properly, correct by definition. They can therefore just look at how much agreement a bunch of clocks have with each other. That is, they don't have to worry about the issue of all of the clocks being systematically fast or slow, like if they were mechanical watches which could all be consistant, but all tick at a rate different from a second, because the second is defined such that this is not the case.
The Big International Scientific Conference that got together to define a new time scale to replace GMT had no difficulty coming up with the name "Coordinated Universal Time", but deadlocked when it came time to decide between the English acronym (CUT) or the French one (TUC). So they decided to use the symbol UTC, which doesn't stand for anything.
Leap seconds are used to keep UTC in sync with the Earth's rotation. Since the Earth's rotation is steadily slowing down, UTC would drift away from any sensible time if it wasn't adjusted every now and then. So they add the occasional extra second to keep them in sync.
GPS time runs at the same rate as UTC, but has no leap seconds, and is currently 13 seconds different. People who navigate by the stars use UT1. Then there is the Terrestrial Dynamical Time that astronomers use, which is another matter entirely.
...laura