Events

 I've spend decades as a software/firmware developer of real-time systems, going all the way back to the 1970s when I was writing software in the assembler languages of the IBM 360/370 and the PDP-11. The term "real-time" always seemed kind of ironic, since it is easy, when closely scrutinizing such systems - with their asynchronous, concurrent, and parallel behavior - to come to the conclusion that time doesn't exist. Only ordered events. We don't have a way to measure time, except by counting events produced by some oscillator that ultimately derives its periodicity from nature. We call such devices a "clock", Since the only way to test the accuracy and precision of a clock is with a better clock, it's turtles all the way down.

Turing-award winning computer science Leslie Lamport even wrote what came to be a classic paper on this topic, "Time, Clocks, and the Ordering of Events in a Distributed System" [CACM, 21.7, 1978-07]. He proposed a "logical clock" which was simply a counter that incremented every time it was read, allowing events to be placed in a clear order. I remember reading this paper as a graduate student. And again, later. And again, even later. I may read it again today.

Years ago I mentioned this line of thought to a colleague of mine, who happened to have a Ph.D. in physics and had worked at Fermi Lab. (It's handy to keep such folks around just for this reason.) He immediately brought up the now obvious to me fact that time must exist: Einstein's special and general relativity.

Einstein's theories of SR and GR have been experimentally verified time and again (no pun intended). You can synchronize two atomic clocks side by side, then take one up to the top of mountain (where it experiences less gravity due to being further from the center of the Earth, and hence time runs faster: that's GR) and back down, and find they they now differ by just the predicted amount. This experiment has been done many times.

The U.S. Global Positioning System (and indeed all other Global Navigation Satellite Systems) work by just transmitting the current time to receivers on the Earth. Fundamentally, that's it. All the heavy lifting, computationally, is done by the GPS receiver in your hand. But the atomic clocks inside every GPS satellite have to be carefully adjusted by controllers on the ground to account for GR (because the satellites in their orbits are further from the center of the Earth than you are, and so their clocks run faster), and for SR (because the satellites in their orbits are centripetally accelerated more than you are, and so their clocks run slower). GPS wouldn't give useful results if this correction weren't performed.

The resonant frequency of cesium-133 is the definition of the "second" in the International System (SI) of units. Count off exactly 9,192,631,770 pulses of the microwaves emitted by cesium-133 during the hyperfine transition of their electron in the element's outer electron shell, and that's one second. If cesium is lying to us, we'll never know.

Or maybe we would. Experimental atomic clocks using elements like ytterbium are running in national metrology labs. These are called "optical" atomic clocks because they operate at terahertz frequencies using lasers instead of microwaves at gigahertz frequencies, and their periods are measured in attoseconds instead of nanoseconds. The time is very near in which the definition of the SI second will be changed to use these clocks.

Clocks that are so precise that their position has to be determined by careful surveying because their results are different if the altitude the laboratory optical bench changes by a centimeter, thanks to GR.

Clocks that are still nothing more than oscillators and counters.

(I took the photograph below in 2018: a survey marker embedded in the concrete floor of an optical atomic clock laboratory at NIST's Boulder Colorado facility.)

We Got The Beat

 Time: it's a funny thing.

We can't measure it directly. The best we can do is construct mechanisms that have some kind of periodic behavior and then count the "beats" (as watchmakers call it) that they produce.

There have been all kinds of sources of periodicity used during human history. The heart beat for short periods of time. The movement of the Sun across the sky for the day. The phases of the Moon for periods of a "moonth". The seasons for the year.

There were many attempts to make time keeping devices - candles with marks drawn on them, water clocks that counted drips, sundials, hourglasses. But none of these were accurate enough to measure longitude, the angular east-west distance across the Earth. (Latitude can be determined by the height of the Sun above the horizon, taking the season and the hemisphere into account. There are almanacs still published today with the numbers you need to do this.)

Navigators going all the way back at least to the ancient Greeks and Polynesians had known that timekeeping could be used to determine longitude, by comparing local time (e.g. local noon, determined by the sun) with a clock set to the time at the port from which you departed. But it wasn't until the mid-to-late 1700s that there was a "chronometer" design accurate, precise, stable, and reliable enough to carry on board ship that navigators were able to use it to determine their longitude.

All "modern" clocks, from those initial chronometers until today, consist of an oscillator - a source of stable precise beats - and a counter - the watch face. And all oscillators are made of three basic components: a resonator (a source of periodicity derived from nature), a power source (a falling weight, a spring, a battery, the mains), and a feedback loop (known as an "escapement" in a mechanical clock).

Many things have been used as resonators over the centuries (and all of these are still used today): a pendulum, a balance wheel, a quartz crystal, an atom of cesium, rubidium, aluminum, ytterbium. But no matter how sophisticated clocks become, they still have the three components that can be classified as a resonator, a power source, and a feedback loop.

The clock below - on display at NIST in Boulder Colorado and whose photograph I took in 2018 - was sold by IBM in 1956. It is an electric pendulum "Type 37" clock that set itself from the NIST WWV/WWVH telegraphic time code using a vacuum tube radio receiver. It was typically used in factories as the master clock from which all other clocks were set.

NIST Time and Frequency Seminar 2025

Once again I attended the fire hose of information that is the U.S. National Institute of Standards and Technology (NIST) Time and Frequency Seminar. This three day, typically annual, event, held at their Boulder Colorado laboratories (commuting distance for me), covered such wide ranging topics as optical atomic clocks, practical measurement techniques for time and frequency, how to characterize and analyze frequency and phase errors in data, ways in which television and radio broadcasters might augment GPS for timing and positioning, and much more.

In honor of the event I wore my Rolex Milgauss. Felt cute, might delete later.

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My understanding is that virtually all national time and frequency metrology laboratories, including NIST (civilian) and the USNO (military) in the U.S., use an ensemble of cesium beam atomic clocks and hydrogen maser atomic clocks, the average of which is used to determine their contribution to the measurement of the SI second and the international definition of UTC. These are commercial devices, not laboratory experiments, and aren't astronomically expensive.

They use a combination of both because, even though the cesium resonant frequency is the (current) definition of the second in the international system of units, cesium atomic clocks suffer from jitter (short term variation), while hydrogen masers are more stable. The jitter in commercial atomic clocks is well understood, and the difference between a rack-mounted commercial cesium beam clock and a much larger and far far more expensive cesium fountain atomic clock in labs at places like NIST is all the extra hardware to try to reduce that jitter.

The image below is of the NIST F-3 cesium fountain clock. The collection of commercial cesium beam standards are kept locked up in another room.

Here's the thing: all hydrogen maser clocks suffer from drift (long term variation). And they all drift by a different amount. And it is not understood why. One hypothesis is it's some mechanism of aging of the components. If the manufacturers could eliminate this, they certainly would (and charge more).

The image below is of a decommissioned commercial hydrogen maser clock that I saw at NIST in 2018. You can't typically find one of these at NIST where it can be photographed because the running ones are kept locked in temperature controlled chambers adapted from commercial egg incubators.

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The blinking 1Hz LEDs in this brief video clip literally represent the real-time manufacture of the UTC(NIST) time scale (the U.S. civilian time base) and the U.S. contribution to the international determination of UTC.

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Next to the toe of my shoe is a survey marker embedded in the floor of one of the NIST labs. Atomic clocks are so precise now that centimeter changes in altitude have to be adjusted for, thanks to general relativistic effects.

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This was my fourth (2018, 2023, 2024, and 2025) and probably last time attending the Time and Frequency Seminar. It is so popular that not only does it sell out, but the waiting list is lengthy too. Could be time to let someone else become a certified Time Lord.