Tags: time | space | physics | ligo

A Clock So Accurate, It Could Change How We Do Physics

A Clock So Accurate, It Could Change How We Do Physics
(Alessandra Rc/Dreamstime.com)

Friday, 04 January 2019 04:51 PM Current | Bio | Archive

The New Year seems an appropriate time to talk about time.

Over the centuries, the need for accurate timekeeping has been driven by demand. The electrical grid needs a way to precisely break a second up into 60 smaller slices. There’d be no bobsled event in the Olympics without a way to time rides to a thousandth of a second.

John Harrison’s 1735 marine chronometer gave sailors a way to determine their longitude by using clocks, thereby stemming the loss of thousands of ships. There’d be no Internet without highly accurate timing circuitry to coordinate the flow of data packets. And GPS would be useless without atomic clocks to measure the time it takes for radio signals from satellites to reach your car.

It would seem that we reached the end of the line in both our need for, and ability to achieve, precision and accuracy when atomic clocks appeared in the 1960s.

“Precision” refers to how finely a clock can subdivide a second. “Accuracy” is about how consistent a device is. A mechanical pocket watch typically ticks about four times a second. Every fourth beat, the second hand moves to the next number. The original Bulova Acutron used 360 vibrations of a tuning fork to define a second, which, considering that the display was only capable of displaying whole seconds, seems like overkill. But the key for that watch wasn’t precision, it was accuracy. Where a pocket watch might drift as many as 30 seconds a day, the Acutron was good to two seconds a day. (Harrison’s marine chronometer was almost as good, but it weighed about 30 pounds, including its glass case mounted on gimbals.) This is far better than the clock in your computer, which can go several seconds out of whack in an hour. The reason it seems so accurate is that, several times a day, it sends a message to a central atomic clock asking, in essence, “Hey, what time is it now?” and resets itself.

Oscillating electrical circuits, which are used in computers, determine how fast the processors can execute instructions. When you see “3.2 gHz” on the box your PC came in, that means that there’s an electronic clock inside ticking at that speed.

Every clock depends on counting something that’s happening with regularity. The faster it’s happening, whether it’s mechanical ticking or a crystal vibrating or a circuit oscillating, the more precise the clock is going to be. And the more consistent the something is, the more useful and reliable the clock.

Which brings us to the atomic clock, which is based on microwave vibrations in a cesium atom. At more than 9 billion vibrations per second, the atomic clock is incredibly precise and also incredibly accurate, drifting by no more than a second over the course of several hundred million years. This is why the internationally accepted definition of the second was recently changed to make it based on the vibrational frequency of cesium.

The atomic clocks in the GPS system are on the ground. Just like your computer, several times a day the satellites check in to see how far their internal clocks have drifted and reset them. Why do GPS clocks need to be so accurate? Because an error of one millionth of a second when timing the arrival of the signal from a satellite would throw the location off by a thousand feet. And since GPS is used for such demanding tasks as land surveying, which needs accuracy of under an inch, the clocks have to be reliable to within a ten-billionth of a second.

Atomic clocks can do that, and you might be wondering why anybody would ever need anything better. Well, for practical purposes you really don’t. But for certain kinds of research, you really do.

Which brings us to the new “optical lattice” clock, which is so accurate it makes the conventional atomic clock look like a sundial in comparison. It’s based on ytterbium atoms, rather than cesium, and by holding those atoms in a “lattice” created by laser beams, mindboggling precision and accuracy are achieved.

Let’s start with accuracy. If you’d started one of these babies ticking back when the universe began some 14 billion years ago, it would have drifted less than a tenth of a second by now.

And as for precision: The clock divides a second into 35 million trillion pieces.

Impressive. But so what?

“So what” is that this machine will allow us to do experimental physics that wasn’t possible before, because there were no devices capable of measuring things precisely enough.

A quick refresher on relativity. (If you’ve seen the movie “Interstellar” this will be old news to you.) Time is not constant. It changes depending how fast something is moving and how close to a gravitational field it is. Thus, astronauts orbing the earth age about a millionth of a second a day slower than we on earth do. “Time dilation” is not speculation; it’s fact. An atomic clock carried on board an Apollo capsule came back lagging its earthbound twin by precisely the amount predicted by Einstein’s theory.

There are practical considerations as well as academic ones. GPS wouldn’t work without Einstein’s time dilation equations built right into the circuitry, making relativistic adjustments for the fact that the satellites are moving very fast and are experiencing less gravity than GPS units on earth. The effects are small, on the order of millionths of a second, but as we’ve seen, the resulting differences would make for whopping errors in position.

So the guys who built the optical lattice clock had an idea: Was it possible to build a portable clock so accurate it could detect differences in the passage of time caused by how high above the earth it was?

An atomic clock can do that, so long as the difference in altitude is hundreds of miles, the speed is about 17,000 mph, and you’re not looking for a lot of precision.

But an optical lattice clock, because of its blisteringly high “tick” rate, should be able to detect a slowing in the passage of time if it’s raised up off a table by less than half an inch. It should also be able to detect changes in gravity on the earth’s surface as you move it around the globe. We can do that now, by measuring aberrations in the orbits of satellites, but the results are nowhere near as precise as the ones you could get from the new clock.

Something it should also be able to do is detect gravitational waves. This is another of Einstein’s predictions: In the same way that a magnet moving in space generates electromagnetic waves, a massive object moving in space should generate gravitational waves. It makes sense if you looked at Einstein’s equations, but no one had any idea of how to detect one of these waves. Gravity is by far the weakest of the four known forces (you can defy the gravity of the entire planet Earth just by lifting your hand in the air), and a gravitational wave is about as close to nothing as you can get and still be something. Even two planets colliding would barely create a ripple.

About forty years ago someone had in interesting idea. The one event in the universe that might create a sizable enough gravitational wave to measure would be two black holes, each with the mass of a few dozen suns, whirling round each other and then colliding. Such a cataclysm would send a tremor through the very fabric of space time. The effect would be tiny, making things contract about one ten thousandth the width of a proton, but build a clever enough machine and you might see it.

So a machine called LIGO was built — two actually, a few thousand miles apart, because unless both of them felt the same ripple, all you had was noise. Incredibly, a few days after turning them on, the first verifiable signal of a black hole collision was received. And at least ten more arrived in the months following.

It was a monumental technical achievement, and it’s a good thing, too: LIGO cost over a billion dollars. But (and this is just me speaking) as a scientific breakthrough it was of dubious merit: The Theory of Relativity has been verified thousands of times and it’s not clear that experimental proof of this one predicted phenomenon advanced the cause of physics very much.

But imagine being able to do the same thing with two pairs of clocks costing a cheap couple of million. Because once those clocks have been synchronized, they should be able to detect ripples in space time caused by black hole collisions and other events with even more precision than LIGO. And while we might not need any more proof of relativity, these clocks might be able to give us the first direct evidence of the existence of dark matter, a theoretical construct that might explain why the universe didn’t fly apart billions of years ago.

The various things that could be done comprise a long list, and I’ll write about them sometime soon. But for right now, that these new clocks even exist is mighty cool enough.

Lee Gruenfeld is a managing partner of Cholawsky and Gruenfeld Advisory, as well as a principal with the TechPar Group in New York, a boutique consulting firm consisting exclusively of former C-level executives and "Big Four" partners. He was vice president of strategic initiatives for Support.com, senior vice president and general manager of a SaaS division he created for a technology company in Las Vegas, national head of professional services for computing pioneer Tymshare, and a partner in the management consulting practice of Deloitte in New York and Los Angeles. Lee is also the award-winning author of fourteen critically-acclaimed, best-selling works of fiction and non-fiction. For more of his reports — Click Here Now.

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The New Year seems an appropriate time to talk about time.
time, space, physics, ligo
Friday, 04 January 2019 04:51 PM
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