Belated Conference Greetings

I’ve been seemingly running in quicksand ever since returning from the 7th Symposium on Frequency Standards and Metrology, what with the pileup of work while I was away (and everything seemingly breaking during that period of time) and getting ready for our clocks to leave the nest. But now, as I’m burning up my comp time from all that, I’ve had a chance to look back.

The conference was really good, as conferences go. A little over 100 people attended, from labs around the world. I knew perhaps a third of them already (though a few probably did not remember me) and met a few more. I didn’t see any glitches except for one or two instances of technical difficulties, which speaks volumes for the organizers and support staff, because you just know there were issues, and since they didn’t become visible it means they were solved quickly. The accommodations were very nice and the food was decent as far as dining hall food goes. The whole thing came in under the government-rate per diem, and the government is actually pretty stingy about such things.

Many of the talks encompassed the recent push into optical transitions for timekeeping; the microwave transitions used in the established clocks of today run at something a little less than 1010 cycles per second, but an optical transition will be about 4 orders of magnitude higher in frequency. Even if your detection can’t be done to the same level of precision, owing to lower light levels and fewer atoms, the higher frequency represents 2 or 3 orders of magnitude improvement in the overall measurement. The enabling technology for this has been the octave-spanning optical frequency comb, made by pulsed lasers in some nonlinear medium. If you consider the time width of the pulses Fourier-transformed in the frequency domain, you see a whole bunch of laser frequencies separated by the pulse repetition rate, so it looks like a comb. As I’ve mentioned before, you can use these individual frequencies to interrogate atoms, meaning you can measure some narrow clock transition. This becomes really useful when the comb spans an octave, so the low-frequency end can be frequency-doubled and referenced to the high-frequency end, making the comb stable. The repetition rate can be tied into some stable RF or microwave source, and now you know what each frequency is to a very high level of precision. A lot of labs are now doing this.
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It's About Time: More NPR Physics Discussions

A Light Take On The Gravity-Time Relationship

Brian Greene explains the link between gravity and time.

Greene has written a short (less than 40 cardboard pages) new picture book called Icarus at the Edge of Time. It tells the story of a young boy who slips off in a space ship and cruises over to a black hole, only to discover that he’s made a terrible mistake: He forgot one of Einstein’s fundamental observations, which is that time is not the same for everybody everywhere.

[…]

Einstein’s theories posit that as one gets closer to a center of gravity, time will “slow down.” So if you spend the rest of your life closer to the Earth’s center of gravity on 34th Street while I spend the rest of my life at the top of the Empire State Building, time for you will tick a teeny, teeny bit more slowly than time for me.

Einstein meant this not poetically, but literally. If you and I each had a watch, ticking off hundred-billionths of seconds, the watch on your wrist down below on the street would tick fewer times than the watch I was wearing up in the sky. It wouldn’t be a big difference — a few billionths of a second over 20 years — but it would be a real difference. If we decided after several decades to meet and compare watches, we’d see that they would literally differ, that time for the two of us had indeed ticked differently.

via Physics Buzz

Hear Here

Ran across this while Googling for something else. NPR interviewed some of my colleagues about the master clock a while back. This aired in January 2007.

The Atomic Secrets of Accurate Time Keeping

I keep forgetting to use the bell ringing analogy when I explain how clocks work. (oh, and when the interviewer says “and his colleagues” near the end, she’s referring, in part, to me. Better than lackey or minion, I suppose.)

Straightening Out the Tangles in Time

This is timely, as it were. Scientific American has an article on frequency combs that appears to be publicly accessible. Rulers of Light: Using Lasers to Measure Distance and Time

Optical frequency comb applications require exquisite control of light across a broad spectrum of frequencies. This level of control has been available for radio waves for a long time but is only now becoming possible for light. An analogy to music helps in understanding the required level of control. Before the development of combs, lasers could produce a single color, like a single optical tone. They were akin to a violin with only one string and no fingerboard, capable of playing only one note (ignore for the moment that musical notes are much richer than pure tones). To play even a simple piece would require many different instruments, each painstakingly tuned. Each violin would require its own musician, just as every single-frequency laser requires its own operator.

In contrast, one operator can use an optical comb to cover the entire optical spectrum, not merely like a pianist at a piano but like a keyboardist playing an electronic synthesizer that can be programmed to mimic any musical instrument or even an entire orchestra. Comb technology, in effect, enables symphonies of hundreds of thousands of pure optical tones.

As explained in the article, one key for usefulness in timing and frequency is that the comb spans an octave, i.e. a factor of two in frequency, so that a “tine” (spectrum line) of the comb at the low end can be frequency-doubled in a nonlinear crystal, and referenced to a line at the high end, making the comb stable — the frequency of any line is well-known. You can now reference a convenient optical transition to the comb and do clock measurements. Since the frequency is much higher, if a suitable (i.e. narrow) transition can be found the fractional error will be much smaller, and the measurements that much more precise.

One thing that was apparent from the Frequency Standards and Metrology conference is that combs are everywhere. A number of different atoms are being investigated, both neutrals typically trapped in an optical lattice, or ions trapped in, well, ion traps. Once you have a really nice clock, though, you need to have another very nice clock (or clocks) with which you can compare. Multiple clocks can be referenced to a comb, and this is being done in the larger labs. And there are also people investigating better techniques for comparing remote signals using fiber transmission of signals, to overcome the limitations of satellite comparisons.

Is Gravity Ruining Time?

I’ve mentioned I’m at a conference — it’s the 7th Symposium of Frequency Standards and Metrology being held near Monterey. It’s a bunch of scientists getting together every ~7 years to discuss the state-of-the art in frequency standards, clocks, and precision measurements, and float ideas for future experiments. The last one was in St. Andrews, Scotland in 2001 (unfortunately it spanned 9/11/2001, which was a bit of a distraction, to say the least.)

There have been a lot of talks that I couldn’t possibly distill into coherent summaries, but I’ll try to do one or two when I get the chance. I’ve got one for now, though, that doesn’t require as much heavy lifting.

Dan Kleppner gave the first talk (Is Gravity Ruining Time?) as a sort of introduction, and gave some perspective on timekeeping, since he has been doing physics from before the development of the hydrogen maser (making him, as he put it, prehistoric). Two main things came out of this talk: an appreciation of a limitation on how we define the second, and a story about I.I. Rabi.

The second is defined as 9,192,631,770 oscillations between the hyperfine states of an unperturbed cesium-133 atom, but this definition does not explicitly mention anything about relativity, of which gravity is a part. It’s basically taken by convention that we use devices at rest on the geoid (an idealized surface of the earth, basically what it would look like without tides) but devices have reached the point where this may not be good enough. The gravitational redshift is given by gh/c^2 near the earth, and this is about a part in 1016 per meter change in height. Clocks need to be adjusted for their altitude/elevation, and this has been necessary for some time; the effect has been measured in the Pound-Rebka experiment and in the rocket launch of a hydrogen maser by Robert Vessot, and is accounted for in GPS and every other satellite carrying a clock. But ground-based clocks are now getting to be good enough to where sub-meter changes in height will need to be taken into account. And since the geoid can only be determined to several cm and it changes with time (and clocks move with respect to the geoid via earth tides of about 30 cm), this will soon become a significant term in the error budgets of frequency standards. So the point of the talk was that gravity is going to take a more prominent role in frequency and time measurements, and may in fact require a redefinition of the second, though it would not impact “everyday” time.

The story he told about Rabi went something like this: Rabi didn’t like writing articles, so there is no formal writeup of his proposal to use an oscillator tuned to a hydrogen transition as a time measurement device — the idea that would eventually become the hydrogen maser and used in other atomic clocks. But in 1945, after he had the Nobel prize, he gave the Richtmyer lecture to the American Association of Physics Teachers on the topic of using a hydrogen magnetic resonance measurement as a potential timekeeping device, and it was written up by the New York Times science correspondent, William Laurence, in an article called ‘Cosmic Pendulum’ For Clock Planned, in which he gives a very basic summary of the principles Rabi had explained. So the cutting-edge science was “formally” proposed in the Times rather than a science journal. In the AAPT’s list of Richtmyer lectures, Rabi’s is one of the few from that era that were not written up and presented in the American Journal of Physics.

(The Times article is here but the archive is paywalled)

Navigation = Time

I see Chad’s put a brief review of Dava Sobel’s Longitude up over at Uncertain Principles.

I read the book a few years ago and can confirm that it’s a good read. (I missed a chance to hear Sobel give a talk at a conference a few years back — I was sick (>1.0 dogs) and crashed rather than attend the talk.)

The idea behind Harrison’s solution to the longitude problem (knowing the location of a star and what time it is tells you your longitude) is still with us: ‘to know where you are, you need good clocks’ applies to GPS, too.

Crunch Time

Beware the time-eater: Cambridge University’s monstrous new clock

The monster momentarily stops the turning dial with its foot to mark the minutes, shown as blue LED lights shining through slots. It was originally conceived by Taylor as a literal interpretation of the grasshopper escapement invented by his hero, the Georgian clockmaker John Harrison whose fabulously accurate mechanisms solved the problem of establishing longitude at sea.

Another h/t to Caroline

FAIL

The fastest clock in the world, my ass.

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Oooooh. It displays six whole digits past the decimal. Down to the microsecond. (can you sense the sarcasm?) It’s a display. Just because it reads that many digits doesn’t mean the measurement actually has that precision.

I’ve wanted to get a display that went to the picosecond for the lab, but have it flash 12:00:00.000000000000 the whole time. Add it to the list of my unadopted suggestions.

“I see no progress in this industry. These clocks are no faster than the ones they made a hundred years ago.” — Henry Ford

Bringing Home the Gold

From Google Maps to Gold Medal

Kristin Armstrong, who won gold in the Women’s Individual Time Trial in Road Cycling, got a GPS track when she rode the Beijing Olympic course in December of 2007

After returning home to Boise, Idaho, I exported the GPS data to several different formats, one of which I was able to launch with Google Earth. I was then able to trace the entire course from the comfort of my home half a world away and find a similar route to train on back in Boise. This capability along with having the elevation profile proved invaluable in my preparation for my Gold Medal race.

GPS relies on precise time, provided by some colleagues of mine, and knowing where the satellites are relative to the earth, which is aided by some other colleagues of mine. Woohoo! We won gold!