World's Most Precise Clocks Test Relativity

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.


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.

RAEBURN: If you have questions, give us a call, questions for me or for Tom. Our number is 1-800-989-8255. That's 1-800-989-TALK. If you're on Twitter, you can tweet us your questions. Our account is @scifri. If you want to know more about what we're discussing this hour, go to our website,, where you will find all kinds of 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.

RAEBURN: All right. Tom, we're going to have to run. We're just about out of time.

Mr. O'BRIAN: Sure.

RAEBURN: But fascinating stuff. Congratulations on the new achievements. And the lesson I take from this, I think, is that those people who live in the penthouse at the top of my building are going to get older faster than I am. So there's some satisfaction in that. How am I doing? Is that right?

(Soundbite of laughter)

Mr. O'BRIAN: Well, that's basically right.

RAEBURN: Okay. Our guest has been Tom O'Brian, chief of the Time & Frequency Division at the National Institute of Standards and Technology in Boulder, Colorado. Thanks a lot, Tom.

Mr. O'BRIAN: Good to talk to you.

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