[MUSIC]
Time is the most measured quantity in the world.
We use it to plan our lives, find our way, and make our fortunes.
Because so much depends on accurate timekeeping, the National Institute of Standards and Technology,
or NIST, is constantly working to build better clocks.
NIST'S new atomic time standard, the F-2 fountain clock, is the most accurate clock
of its kind in the world. If NIST-F2 ran continuously for 300 million years, it would not stray
from perfect time by one second.
"In some very real sense, the job of NIST is to recognize and bring forward the best
standards you can have for certainly the basic quantities of length and mass and time and
things like that. And presumably you end up pushing as hard as you can to build the best
standard you can. And when you can do something better than the Earth gives you naturally
and its rotation, well, you go ahead and you start down that road. And then people keep
making it better, and when you make it better, somebody starts having a use for it, and so
you have to make a better one, and this process continues to this day."
The new clock is three times more accurate than the previous NIST standard, NIST-F1.
Several improvements were made in NIST-F2, the most important being to nearly eliminate
small errors caused by background radiation.
First, lasers slow and cool the atoms to just a fraction of a millionth of a degree above
absolute zero.
A laser then lofts the ultracold atoms into a tube inside a liquid nitrogen-cooled chamber.
The cold chamber reduces errors caused by thermal radiation, which slightly alters the
"ticking rate" of the atoms.
Within the clock, the cesium atoms make two passes through a microwave chamber. If the
frequency of the microwaves matches the atoms' ticking rate perfectly, their outermost electrons
flip over and enter an excited state.
When a laser beam hits the atoms, only those in the excited state respond by absorbing
and re-emitting the light. Scientists know they have found the right microwave frequency
when they see the atoms re-emit the most light.
"That frequency is defined, it's the only defined frequency we have, okay? So all other
frequencies you measure with respect to that frequency, and that frequency is, as I said,
it's 9 billion times a second. Well, if I count 9,192,631,770 cycles of the radiation
that makes the electron flip over, then that's one second, and now I count another 9 billion
I've got another second and so on, and so you simply keep counting."
NIST scientists use that second to recalibrate their commercial atomic clocks, which run
more reliably, but aren't nearly as accurate as a cesium fountain.
We need ultra-accurate time for a host of applications, including the global positioning
system, telecommunications, and the internet.
But do we really need time accurate to within 1/300,000,000th of a second per year?
"So the short answer is that there is probably no relevance to the best clock that we can
build today in everybody's life tomorrow, but that clock is going to be the progenitor
for something that really is important 10 years from now, I would predict. The fact
of the matter is the commercial atomic clocks you go buy today, and, look, there are companies
in business selling these things, and if there wasn't a market there wouldn't be. The
clocks they sell are as good as the clocks we could possibly build in the laboratory
20 years ago and they're being used all over the place. And so we have to stay out
in front of that if for no other reason than we have to be able to calibrate them. So we
have to be that much better than they are in order to measure them."