Category Archives: Time

Time Has Come Today, Part II

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Building a Better Clock

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

I really hope he was kidding, but assuming he was it’s pretty funny to a timing geek like me. The way you make clocks better is by making them more precise and accurate, and the levers for this are hidden in the equation for “counting the ticks”. If our ability to count precisely is somehow limited, e.g. if we had an oscillator — like a wheel — and we could measure its angle to a precision of 3.6º, then letting it go for one oscillation represents a 1% measurement, but that same absolute error for 100 oscillations is 0.01%, and we can get there either by integrating longer or by having an oscillator with a higher frequency. So “more ticks” is better … if we don’t have a noisy oscillator. Certain noise processes don’t integrate down, so another lever is to improve the noise, or possibly the noise characterization, of our clock.

Better clocks came in the form of Harrison’s chronometer which could be put to sea, and which included advances like using multiple kinds of metals to reduce temperature effects, and a spring which maintained constant tension. On land, improvements came in the form of better pendulum clocks, culminating in Riefler and Shortt clocks in the early 1900’s, with temperature compensated pendula (to inhibit the length from changing), kept under moderate vacuum to reduce drag and possible humidity effects, and were capable of performing at a precision of around a millisecond per day, and are examples of going to a higher frequency (a period of a second rather than a day) and minimizing the noise effects. Going into the 1930’s-1940’s, quartz oscillators, using much higher frequencies (many kHz rather than 1 Hz) became the best clocks.

Up to this point, the length of the second was defined in terms of a fraction of the tropical year in 1900, which was close but not identical to 86,400 seconds per day (being off by a few milliseconds), but atomic standards were investigated and in 1967 the definition of 9,192,631,770 oscillations of Cs-133 hyperfine transition was adopted, and atomic timekeeping defined Coordinated Universal Time (UTC) starting in 1972. This also marked the start of inserting whole leap seconds to match atomic time with earth rotation time; prior to that it was done by adjusting clock frequencies or inserting fraction-of-a-second steps in time.
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Time Has Come Today, Part I

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Last week I gave a seminar at Augusta State University called “It’s About Time” and promised to write up a summary of the talk, so here it is (sans a few cartoons and some data I don’t have permission to show). Some of the material I have discussed before, and some has been covered recently at the Virtuosi and, previously, at Uncertain Principles. Both discussions are good, but as I had noted for the former post, there are some subtleties to the discussion that one might not be expected to know if one isn’t exposed to timekeeping on a semi-regular basis.

The Chicago Way

I raised the questions asked in Chicago’s 1969/1970 song “Does Anybody Really Know What Time It Is?” the lyrics to which includes the followup question, “Does Anybody Really Care?”

Does Anybody Really Know What Time It Is? No.
Does Anybody Really Care? Yes.

(at which point I paused for comedic effect, as if this were the end of the talk. I crack myself up sometimes)

The basic point of the first answer is that there is no predefined “truth” for what time it is. There are choices/decisions that go into that determination, so the time is a voted quantity in addition to being a measured quantity — measurement limitations are not the only reason the answer is “no”.

For the second question, which is the whole motivation for precision timekeeping, the answer had better be “yes” or else there is no justification for performing the task. The motivation for the navy (both here and abroad) for timekeeping is navigation, and this dates back to Harrison and the “longitude problem”. To know your latitude it’s fairly straightforward — the north star is almost due north, so finding its angle in the sky relative to the horizon gives you that information, or you can get the information from the declination of the sun at noon. But the longitude isn’t so easy; for a long time navigation was done by dead reckoning, but with increased ocean travel and the reach of the British Empire there was too much “dead” in dead reckoning, and so the British navy sought a way to improve navigation.
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Is There a Sand Shortage?

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Time is running out – literally, says scientist

[I]f time gradually slows “but we naively kept using our equations to derive the changes of the expansion with respect of ‘a standard flow of time’, then the simple models that we have constructed in our paper show that an “effective accelerated rate of the expansion” takes place.”
While the change would be infinitesimally slow from an ordinary human perspective, from the grand perspective of cosmology – in which scientists study ancient light from suns that shone billions of years ago – this temporal slowing could be easily measured.

Interesting hypothesis. Though the article mentions this as a “radical suggestion” the headline is much more certain, which has become a peeve of mine. Conjecture is one of the things that scientists do. We play a lot of “what if” games, and most of this gets shot down when we realize a conflict exists with existing observations, or someone points them out when you compare notes with a colleague. This happens a lot in the lab when faced with a novel set of data — what was going on here? Is this a problem with the equipment or some new effect? The former is much, much more likely than the latter, but until you run tests and replicate the results reliably, you are faced with a mystery, and chasing the solution is both frustrating and somewhat intoxicating. (The scariest scenario is when the anomalous signal just disappears and you can’t replicate it). But you can also do it with the models you build. What happens if a particular term in an equation is or isn’t small, when the opposite usually holds? What if there is some additional effect? You play with the equations and see where it goes.

If you don’t get tripped up by these “slain by an ugly fact” obstacles, you can formulate a model that could possibly be tested. Eventually, you present it for others so that they, too can comment on and think about it. Many of these ideas never pan out, at each stage of this evolution and distillation. These authors have an idea that has gotten to this point. It doesn’t appear it has yet been rigorously tested to see of it holds up — one needs to know what specific predictions it makes that distinguish it from the current models. It’s just not obviously wrong after having had some level of scrutiny.

I think that the headline editors and journalists do a disservice when they attach much more certainty to (in this case) as-yet untested ideas that show up in the journals, or any single peculiar experimental result that pops up.

Fly Like an Eagle

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I intend to write up parts of my recent presentation into a blog post, but here is something on timekeeping from The Virtuosi: Time Keeps On Slippin’

However, there is a subtlety to one of their arguments that requires more detail

The second measure of “good-ness” is precision or, in watch parlance, stability. This is essentially a measure of the consistency of the watch. If I have a watch that is consistently off by 5 minutes from the official time, then it is not accurate but it is still stable.

This is true but does not extend far enough. If a watch is consistently off by 5 minutes (or constant amount) from another source, then both must be running at the same frequency — they are accumulating phase at the same rate. But stability goes one step further. Even if the clocks were running at different rates, and phase was accumulating between them (i.e. one is running fast), you can make the same statement. The fractional frequency stability — given by the Allan deviation — depends on how the difference of the two frequencies, as measured over different intervals, changes. But the difference doesn’t have to be zero. A clock that consistently gains e.g. a second per day is also a very stable device: the frequency difference is always ~1/86400, but it’s constant, and the Allan deviation looks at the difference between subsequent frequency comparisons.

The danger here is in the assumption that two stable clocks will give you the same time readings. That’s not what we mean by stability. Stability is a measure of how the frequency is changing. An analogue would be a common conceptual mistake in using Newton’s second law: an object at rest feels zero force, but a force of zero does not mean the object is at rest — it simply means an object has a constant velocity.

Another Leap of Timing Faith

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Wait just a (leap) second

A nice little summary of leap seconds and the current state of affairs. But there’s a comment at the end that I think has dropped a minus sign.

Why not just decouple the two clocks, and let them go their separate ways?
A lot of scientists do in fact feel this way. But it turns out to be really, really complicated to do that. A lot of computer systems (including satellite navigation systems) have software written a while ago, and changing that would be difficult and have unforeseen consequences. Fiddling with that may be dangerous.

Decoupling atomic time from earth rotation time requires no fiddling — you just stop inserting leap seconds into UTC. Clocks generally don’t get their cues from earth rotation, they get them from synchronization to official time, which is atomic time (in the US). It’s the fiddling — the insertion of the leap seconds into the atomic time signals — that contains the potential pitfalls.

Having countries change their official time from GMT (which is mean solar time) to UTC would be technologically trivial. It turns out that in the US this happened just a few years ago; the wording describing our time zones was changed from GMT to UTC in the America Competes Act in 2007. Even though the basis for time had been atomic time anyway, it wasn’t official until then, but nothing really changed (as far as I can tell) when the law took effect.

Taking a Non-Leap of Faith?

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Time running out for ‘leap second’ that has kept us in step with our slowing planet

[T]hat [next] change could be the last of its kind for the leap second and for our fiddling with time. Telecommunications organisations and financial groups say the continual adding of leap seconds to computers increases the chances of errors being made. Precisely timed money transactions could go astray or vehicles could be sent tens of metres out of position if they are a second out in their measurement of time. Hence the bid to ban the leap second.

But there’s this – a nit at which I must pick

“However, these new, highly accurate atomic clocks also revealed that the Earth’s rotation is slowing down because of movements within the core of the Earth.

“The rate of change is not constant, however; it fluctuates over the years. Indeed, sometimes it does not slow down at all.”

Any change in mass distribution will contribute to a change in rotation rate because angular momentum will be conserved, but a really big term in all of this is the tidal braking from our interaction with the moon. The other contributions add noise to this, which is why the rotation speed can level off or even increase temporarily.

But since moment of inertia depends on R^2, changes in the core must involve a lot more mass relative to changes on the surface of the earth (from weather patterns and water location, for example) to contribute.

 

I had linked to an article about the problems with leap seconds some months ago, and the author came and gave a talk at the Observatory this past fall. Hearing details of some of the potential problems was interesting — the issue is that programmers generally don’t think about leap seconds, so how a system will respond is dicey, and as more and more systems rely on automation the odds of a dangerous failure increases.

There are systems that simply shut down at leap-second insertion time rather than deal with the unknown response of the computer code. But leap seconds are inserted at midnight UTC. Most of Europe is partying, without much business going on, or planes in the air. But it’s 4 PM in California, and in the morning in Asia. There’s potential for some serious complications.

The bottom line is that most people don’t care about leap seconds. Dropping them will impact astronomers, and mildly offend our sensibilities when noon on the solstices does not have the sun line up overhead — assuming you are at a longitude where this currently happens. But that’s just it: most of us aren’t. We accept time zones as a compromise between precise astronomical time and coordination and scheduling of our lives. It won’t surprise me if leap seconds are deemed more trouble than they are worth.

The Shroud of Duration

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Skulls in the Stars: So, what is a “temporal cloak”, anyway?

Loosely speaking, this has also been referred to as a “history editor”. Naturally, the discussion of “cloaking” has again brought out references to “Harry Potter cloaks” and other dramatic imagery; the reality is much more mundane, but still fascinating — and an amazing achievement. Let’s take a look at what was done, what was not done — and why it’s quite cool!

First, let’s get rid of some misconceptions that the terminology naturally brings to mind. The terms “space-time cloak” and “history editor” make it sound like the device is ripping a hole in the fabric of space-time itself — like a time machine equipped with a big eraser! This is definitely not what is happening here! There is no manipulation of time itself, but rather a manipulation of a beam of light to hide something that the light would otherwise detect.

An analogy for what this isn’t is a strobe light. A strobe can freeze our perception of motion, but not the motion itself — you just aren’t getting that information. This system is hiding information in a different way.

Lights! Camera! Action! Mostly Lights, Though

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We had a film crew from the History Channel at the Observatory a few days ago, filming a segment for an upcoming special on inventions that changed the world. One of these is the clock, so naturally they wanted to speak to some people who could tell them about clocks. I showed them around the lab and they liked the setting a lot more than any of our operational clocks; they’re all nicely packaged up and quite boring. (One type — the hydrogen maser — is literally a black box, and the fountain physics package looks like a water heater.)

So they filmed a segment in our lab, and since I was there making sure they didn’t touch anything they shouldn’t be touching (they didn’t — they were quite well-behaved), they had me and one of my lab-mates stand in the background, pretending to work at one of the optical tables. We might end up on screen for ten seconds or so in the final cut.

Being geeks meant that we drooled a bit over the equipment that they brought. This is part of their lighting system, a bank of LEDs, which has the advantage over traditional equipment that it draws much less power since LEDs are much more efficient. This means they can run it off of a battery and not have to worry about whether there is an outlet nearby, and it also doesn’t heat up very much.

 

With the lights at full power it saturates the camera.

 

Turned down a bit you can see the LEDs a little more clearly.