The world's most precise clocks can reveal tiny
time dilations predicted by Einstein's theory of relativity -- but
that's not all. Tom O'Brian of the National Institute for Standards and
Technology talks about using these precise clocks in everything from
cell phones to satellites.
PAUL RAEBURN, host:
This is SCIENCE FRIDAY from NPR. I'm Paul Raeburn, sitting in for Ira Flatow.
Earlier
this year, scientists built the most precise clock on Earth, an
aluminum ion clock. Now, don't try this at home with a roll of aluminum
foil. It's not going to work. These are among the most precise clocks
ever built.
And now researchers have used a
pair of these clocks to test Einstein's theory of relativity at a very,
very tiny scale. They've been able to measure the miniscule changes in
time that occur when you are sitting in a moving car or standing at the
top of a staircase. The research appears this week in the journal
Science.
But what's so important about
measuring time so precisely? Testing Einstein's theory of relativity is
good sport, and it has been for decades now, a century, but is it really
worth all the time and trouble? Let's just give Einstein a break from
all this testing and retesting. Or maybe we can learn something else
from these clocks that even a Rolex would never tell us.
We're
about to find out. Joining me now is Tom O'Brian, chief of the Time
& Frequency Division I love that title - at the National Institute
of Standards and Technology in Boulder, Colorado. Hi, Tom.
Mr. THOMAS O'BRIAN (Chief, Time & Frequency Division, National Institute of Standards and Technology): Hi, Paul.
RAEBURN: How are you? Nice to have you with us.
Mr. O'BRIAN: It's good to be here.
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links.
So Tom, give us a little refresher on the theory of relativity, if you can, and what role time plays in that.
Mr.
O'BRIAN: Well, Einstein developed the theory of relativity, and one way
of looking at it is to say space and time are not really separate but
are intertwined.
And he expressed that in
two different ways. The special theory of relativity, one way of looking
at it is to say when a clock is moving, it appears to tick more slowly
than a clock that's standing still.
And the
general theory of relativity has to do with the effect of gravity on
space and time, and one way of looking at it, it says that the stronger
the gravitational field is, the slower the clock ticks.
So
basically if you're moving really fast, close to the speed of light,
you can see significant changes in the ticking rate of a clock. Or if
you're in a very strong gravitational field, like near a black hole or
something like that, you can see a significant change in the ticking
rate of a clock.
What happened in this
experiment at NIST was that the scientists made a clock that is so
accurate and so precise that instead of having to go at close to warp
speed or something like that or be near a black hole, you can actually
see the ticking, the change in the ticking rate of the clock just by
lifting the clock up about a foot or by making the ion, which is the
ticking part of the clock, just move at even walking or jogging speed.
RAEBURN: So you guys were showing off is what it comes down to.
(Soundbite of laughter)
Mr.
O'BRIAN: That's right. I mean, at NIST, the National Institute of
Standards and Technology, part of our job is to make the best
measurements possible. But it's not just to get the next decimal place
or just for fun.
The measurements that we make affect people's lives every day - and for example, very accurate timing.
I
mean, do you need to be able to measure the time change on your watch
that's going to be caused by you walking? Well, no, your watch isn't
going to measure that, and it's not going to make any practical
difference in your life.
But very accurate
timing and synchronization is a part of our modern technological
infrastructure, and people are using it every day. When you make a
telephone call, when you use a computer network, you're relying on
networks that have to be synchronized to better than a millionth of a
second per day.
Electric power distribution
has to be synchronized to better than a millionth of a second per day,
and the global positioning system, GPS, which allows you to get your
position anywhere on Earth, whether you're driving or walking around
with a handheld receiver or while an airline pilot is flying, that
relies on atomic clocks that are better than a billionth of a second per
day.
RAEBURN: Okay, now, I mentioned in the
introduction, we seem to be hearing about tests of Einstein all over
the place all the time. The poor guy, let's give him a break, and why do
we keep testing Einstein? Is it fun? Is it real? What's going on here?
Mr.
O'BRIAN: We would have been very surprised, perhaps shocked, if we had
found a departure from Einstein's theory of relativity in these
measurements. But you never do know.
I think
it is valuable in and of itself to keep pushing the extremes of
measurement because many of the things that we take for granted today in
science and in technology in fact came about by somebody trying to push
for that one more decimal place and finding something very unusual or
exciting.
While Einstein himself developed a
lot of the theory of relativity just through his mental powers, his
very prodigious mental powers, it was in fact based on precision
experiments, which were looking at things called the ether, which was
some mythical substance through which light was supposed to propagate.
And
when it was discovered by measurements in the late 1800s that, in fact,
this ether did not in fact exist, people had to come up with other
theories to explain what's really going on. And that led, directly and
indirectly, to Einstein developing his theory.
And
things like the discovery of quantum mechanics, which governs
everything from electronics to even the way we're looking at biophysics
nowadays again came about through very precise measurements showing
things that were behaving just a little bit differently than expected
and then pursuing those measurements.
RAEBURN:
Now, are there you talked a little bit about GPS and other sensitive
measurements we need. Tell me a little bit more about that. What are the
potential practical applications from this kind of work?
Mr.
O'BRIAN: Well, for any of those things that I mentioned, such as
telecommunications, distribution of electric power, more precise
positioning with GPS, having better clocks will make all those things
better.
You might be able to pump more calls
onto a limited amount of capacity if you have better timing. The way
you do that is by breaking up the calls into little pieces, sending
different pieces at different times and synchronizing those sending very
accurately so that you can keep the call.
A
global positioning system, of course, basically relies on timing, very
precisely, how long it takes a radio signal from the GPS satellite,
traveling at the speed of light, to reach you from different satellites.
Since that speed is roughly one foot and
one-billionth of a second, the more accurately you can measure time, the
more precisely you can get your location.
But
actually what I think is going to be the major applications for clocks
like this aluminum ion clock, which was part of the relativity
experiment, and similar clocks that NIST and other organizations are
working on, are not necessarily to measure time directly but to measure
other quantities.
So by looking at the
ticking rate changing, by having the clock moved up just one foot,
basically what you're doing is measuring the change in gravity, and
right now, people try to measure changes in gravity for everything from
looking for minerals under the ground, for example oil is less dense
than rock, so there's a very slightly - there's a very slight reduction
in gravity if you happen to be above a source of water or oil or other
minerals, and you can detect that without having to dig holes down deep
into the earth.
Just measuring the very
small changes in the Earth itself that result from everything from
climate change, from changes in the amount of ice that there is, which
pushes down on the Earth's crust, to just how the Earth is changing in
general. Those are important measurements.
And
the ticking rate of atomic clocks can also be affected by things such
as the magnetic fields. Typically, in most atomic clocks, what we try to
do is shield out the magnetic field so it doesn't affect the ticking
rate, so we get the best time measurement possible, but if you let the
magnetic field come in, it becomes a very sensitive magnetometer.
And
in fact not with the aluminum ion clock but with other atomic clocks we
have here at NIST, some of the scientists have even done things like
measured the magnetic fields that are generated by the heart and brain
activity of a mouse.
(Soundbite of laughter)
Now,
again, you might say well, what's the deal with that? This might be a
whole new way of medical imaging, about getting information about normal
physiological processes and disease states.
Right
now, people measure the electrical fields from the heart activity and
brain activity. Those are electrocardiograms and electroencephalograms.
But sometimes, that electrical information has a hard time getting out
of the human body because the human body does have some electrical
conductivity.
