Category Archives: Time

Dropping the Minus Sign

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Or, in this case, the “not”

Why aluminum should replace cesium as the standard of time

The technique involved is neat: for some atoms you can find wavelengths where the AC Stark shift is the same for the two levels in the clock transition, so the atom is unperturbed by the presence of the trapping light. So you trap them in an optical lattice, with confinement like a far off-resonance dipole force trap (FORT). This means you can continue to confine the atoms while it is in the superposition where it is oscillating between the two clock states.

The big advantage of this method is that you can trap millions of atoms easily in an optical lattice and that should make such a clock much more robust than a fountain, while achieving at least the same kind of accuracy.

Actually, that’s not the big advantage. Fountains trap millions of atoms (even billions, depending on your collection technique). The advantages are that you’d keep that many atoms (fountains lose signal from the original collection because the cloud spreads out, so the number you toss is an order of magnitude bigger than the number that return), you can interrogate the atoms for a longer period of time (an advantage shared by ion trap clocks/frequency standards) and avoiding cold-collision frequency shifts (atoms in close proximity tend to interact strongly, as they can interact for a relatively long time, and this changes the state of the atom, introducing an error in the signal)

However, “at least the same kind of accuracy” isn’t enough. I’ve noted before that international standards are a political issue. Cesium beam standards are commercially available. Furthermore, dozens of labs have or are building fountains, at some investment of time and money to gain the expertise in doing so (because atomic fountains are not, nor are they likely to become, a commercially available item). The countries doing this will likely be reluctant to switch to a standard that requires even more money and acquired expertise in a new technique for marginal gain in accuracy and precision. Especially in light of how many new options for secondary standards have emerged in just the last decade — an even better candidate may emerge as technology advances.

Can You Spare a Second?

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Why, yes, I can. I just happened to find an extra one floating around here.

The International Earth Orientation and Reference Systems Service (IERS) has announced a leap second.

A positive leap second will be introduced at the end of December 2008.
The sequence of dates of the UTC second markers will be:

2008 December 31, 23h 59m 59s
2008 December 31, 23h 59m 60s
2009 January 1, 0h 0m 0s

The difference between UTC and the International Atomic Time TAI is:

from 2006 January 1, 0h UTC, to 2009 January 1 0h UTC : UTC-TAI = – 33s
from 2009 January 1, 0h UTC, until further notice : UTC-TAI = – 34s

The proud parents had previously stated they didn’t care whether the leap second were added or removed, just that it had all of its fingers and toes.

"Classic" Timekeeping, Part III

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Part I, Part II

Timekeeping measurements always rely on the comparison of two oscillators; when you check to see if your clock or watch is running fast or slow, you do this by comparing it to another clock. Finding disagreement between two clocks won’t tell you a priori which one is the culprit, just as in the adage that a man with two clocks is never sure what time it is. But comparing three clocks allow pair-wise comparisons, and begin to allow one to assign a stability to the individual clocks.

Comparisons are what the scientists did in the second paper in my review, “Time, Analysis of records made on the Loomis chronograph by three Shortt clocks and a crystal oscillator.” The quartz crystal oscillator gave the input to the Loomis chronograph, and the three Shortt clocks were compared to crystal, and could then be compared with each other by differencing the data, which removes the crystal from the measurement.

The interesting (to me) part of the paper begins a few pages in, where they begin discussing the influence of the moon. The moon should give rise to a change in amplitude that would occur over an interval of 24h 50m, and should be distinguishable from diurnal terms present in the pendulum clocks. Two different time series were analyzed, one having a duration of 54 days, and the other having a duration of 146 days. This was long enough to average out noise terms, since the preliminary estimate of the effect was 153 microseconds per half-period of oscillation (i.e. one second)

The theory of the effect of direct attraction is presented in terms of tidal potentials, and it, of course, ends up depending on the angular position of the moon and the latitude of the observer. There are secondary effects as well. The tidal effect of the moon is not only on the water, but on the solid earth as well, though because it is not particularly elastic, the earth’s deflection is smaller, and this changes the radius by a small amount. There is a redistribution of mass when this occurs. Further, there are the local effects of the depression of the ocean bed and coast at high tide (as this was fairly near new York City), and the change in mass that occurs because of the water. It turns out that these indirect effects very nearly cancel, and the results should be close to the 153 microseconds predicted by direct attraction.
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I Come to a Different Conclusion

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There’s a video out there in the ether that purports to measure time dilation in a car. I’ve already shown that this can, and does, happen, but you need to have some pretty expensive toys at your disposal to make the measurement.

For a good experiment (I’m perhaps charitably assuming this wasn’t just an out-and-out fraud), one would also want to measure the stopwatches against each other to make sure they were running at the same rate, and calibrate them if they weren’t. Ideally you’d want several clocks, but that’s a little advanced for this level of execution. Then, you’d want to make sure that you weren’t perturbing the clocks with different environments, like temperature differences, so make sure you aren’t blasting the AC on the stopwatch. Finally, you’d want to predict the difference to compare it to the measured difference. Ignoring effects from any elevation changes, a half-hour trip at 60 mph is going to give you a dilation of around 7 picoseconds.

My conclusion is that your stopwatches suck.

"Classic" Timekeeping, Part II

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(Part I)

The state-of-the-art timekeeping technology a century ago was comprised of pendulum clocks. Refinements were made in the areas of obvious problems, such as the mechanical escapement which robs the system of energy, the vulnerability to changes in length from temperature and humidity, and vibrations. The culmination of this was the clock of W. H. Shortt, which had two pendulums, a master and a slave. The master oscillator was a free pendulum, and as it did no work to drive any mechanism, it was able to keep very precise time. The pendulum was made of invar, a material that had a very low thermal coefficient of expansion, and was encased in a chamber that was evacuated to several millitorr of pressure. The chamber was bolted to a wall that typically rested on a massive platform of the type used for telescopes, which minimized effects from vibrations. The pendulum was given an occasional boost to keep its amplitude roughly constant. The slave pendulum, which did the mechanical work of the system, received periodic electronic impulses from the master clock to correct its motion. This type of clock could keep time to better than a millisecond a day. A shortcoming (as it were) was in the measurement of the time; as Loomis notes

This remarkable result is accomplished through the possibility of averaging a large number of observations. A single impulse from a master Shortt clock has an uncertainty of 1 or 2 milli-seeonds. The master pendulum carries a small wheel. The impulse arm rests on this wheel, and as the pendulum swings out the pallet on this arm travels down the edge of the wheel, finally falling clear . It then trips an arm which falls, making the electric contact . If the small wheel is not exactly circular the arm will fall at slightly different times as the wheel is given a small turn with each fall. These variations are entirely smoothed out when a series of sparks are averaged.

So while the clock is precise in the long-term, the system of measuring it (described below) is limited at shorter durations.

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"Classic" Timekeeping, Part I: Introduction

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Following the suggestion and subsequent reminder (nothing like a deadline to get the creative juices flowing) from gg at Skulls in the Stars, I’ve got two “old” papers that I’m going to summarize.

I recommend choosing something pre- World War II, as that was the era of hand-crafted, “in your basement”-style science. There’s a lot to learn not only about the ingenuity of researchers in an era when materials were not readily available, but also about the problems and concerns of scientists of that era, often things we take for granted now!

These are from 1931, fulfilling the pre-WWII criterion, when you still had individuals engaging in research that were self-financed or supported by a patron and much of the equipment was self-manufactured. The science in this case was largely self-funded, and as for the “basement,” well, it’s a pretty fancy basement as you’ll see, as one might suspect of someone who can fund his own science. But classic nonetheless. There’s a bit to do, and I’m going to break it up into more manageable chunks.

The papers in question are from the Monthly Notices of the Royal Astronomical Society, Vol. 91, published in 1931, and are “The Precise Measurement of Time” by Alfred L. Loomis (p. 569-575) and “Time, Analysis of records made on the Loomis chronograph by three Shortt clocks and a crystal oscillator” by Brown, E. W. & Brouwer, D. (p.575-591). (I, know, I know. They sound like tabloid headlines, don’t they?) The first paper describes various apparati used, and the second describes a particular measurement that was of interest to me.

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Clocks!

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Over at Skulls in the Stars

We want to put a sundial up on the new building on which I’ve been working. (sooooo close to being done, too). I had joked at one point that we would mount a light that would move around the gnomon, so you could read it at night. Just like that first clock.

But, take any of these with a 5 MHz or even 1 pps input, and we could do something with it …

UPDATE: an animated version of the “word” clock

Giove-B Launched

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Second European Positioning Satellite Launched

Galileo, which should be operational by 2013, will be both an alternative and a complement to the U.S. Global Positioning System (GPS) and the Russian GLONASS.

There are some articles which call Galileo a competitor to GPS, which really isn’t the case; this article does a better job. I expect a PND purchased a few years from now, once there are many Galileo and GLONASS satellites in operation, will be configured to accept and process data from all of them.