New Atomic Clock Reaches the Boundaries of Timekeeping

SonicSpike sends an article from NPR about a high-tech clock being built at the University of Colorado Boulder. It's more precise than any clock before, able to keep perfect time for five billion years. "At the heart of this new clock is the element strontium. Inside a small chamber, the strontium atoms are suspended in a lattice of crisscrossing laser beams. Researchers then give them a little ping, like ringing a bell. The strontium vibrates at an incredibly fast frequency. It's a natural atomic metronome ticking out teeny, teeny fractions of a second." But this precision leads to a problem: the relativistic differences between keeping the clock on the floor versus hanging it on the wall now introduce more significant fluctuations than the clock itself. "Tiny shifts in the earth's crust can throw it off, even when it's sitting still. Even if two of them are synchronized, their different rates of ticking mean they will soon be out of synch. They will never agree. The world's current time is coordinated between atomic clocks all over the planet. But that can't happen with the new one."

Comments

Old saying

[plover]

A man with one watch always knows what time it is.
A man with two watches is never sure.

Re:Old saying

[SJHillman]

A man with an atomic watch won't shut up about it.

Re:Old saying

[fustakrakich]

A man with an atomic watch better not keep it in his pocket.

Re:Old saying

[Anonymous Coward]

Another old saying I like, from sailors:

When going to sea, take one clock or three, but never two.

(Knowing the time was essential for navigation, to figure out longitude, back in the days before GPS navigation.)

Re:Old saying

[lgw]

A further problem with hyper-accurate clocks is relativity. TFS mentions the issues with general relativity - strength of gravity affects timekeeping. But there's a more profound issue once you get crazy-precise: only co-moving clocks can be synchronized in the first place. The concept of synchronization simply doesn't apply to clocks moving at different velocities - and two clocks at different positions on the rotating, orbiting Earth will never quite be moving with the same velocity. That relativistic effect is tiny, but it's not even hypothetically reconcilable: there are only so many significant digits of time possible to share between clocks in different locations.

Re:Old saying

[Sarten-X]

Thanks to GPS, the accuracy has improved, but now you need four clocks to get a 3-dimensional fix, and more to improve accuracy. Fortunately, on the open sea there isn't much blocking your view of the sky.

Re: Old saying

[mpoulton]

Because it would be meaningless to "compensate" for the time difference between clocks moving and accelerating differently. Time literally moves at different rates in different reference frames. The clocks are correct; the problem is that the concept of similtaneity is fundamentally flawed.

Re:Old saying

[Jane Q. Public]

"Whoever said a bird in the hand is worth two in the bush hasn't been putting his bird in the right bushes." -- Gallagher

Re: Old saying

[jouassou]

wouldn't that make the concept of time fundamentally flawed?

In any given reference frame, time is a well-defined quantity. The fundamentally flawed concept here is the idea of some kind of universal time that passes at the same rate everywhere in the universe, because relativity tells us that the observed passage of time is affected by things like velocity, acceleration, and gravitation, and therefore varies between different reference frames -- and we have no objective reason to say that any particular reference frame in the universe is inherently superior.

So while the atomic clock might measure the local passage of time with near perfect accuracy in the reference frame where we place it, the results will just be approximate in any other reference frame.

Re:Old saying

[WuphonsReach]

Best practice in the real world is four reference clocks or only one. With just three configured you run into the problem of ending up in the "just two clocks situation" more often then not. At which point, NTP is likely to oscillate between the two remaining good candidates (without the "prefer" keyword).

How you choose to configure NTP is a tricky art depending on how resilient you want to be and whether you have a local time source or need less then 5ms accuracy. For most situations (99% of servers), being within 500ms of the "internet time" is enough. Your goal is mostly to avoid the issue where the clock is off by tens of seconds or worse.

Re: Old saying

[bloodhawk]

I have a broken watch, can't quite work out when 88:88 is going to roll past though.

Re:Old saying

[psmears]

No, it is NOT wrong. Your assertion is a common misconception. I assure you: I looked into this technology in depth.

Then perhaps you can provide a reference?

Just as basic geometry would normally dictate, 3 satellites are sufficient to find your basic location and elevation.

No: basic geometry dictates that, to find a position in three dimensions, you need three measurements of distance. The trouble is that the signal from one satellite gives you no information about your position: the signal (roughly speaking) tells you where the satellite is, and the time by that satellite's clock - but since you have nothing to compare it against, you have no idea how long that signal took to reach you, so you get no information about your position.

It's similar to if you asked me my height, and I said "I'm a foot taller than Fred". If you don't know how tall Fred is, you're no nearer to knowing how tall I am, even though you've been given one measurement. Sure, if you look at probability distributions of height you can have a good guess at how tall I might be, and this is similar to getting a 2D fix (where you assume that your elevation is "at or near the surface of the Earth"), but you can't know for certain that one of both of us don't have unusual heights.

Once you have a signal from two satellites, you can subtract the timestamps, which doesn't directly tell you position, but tells you which satellite is closer to your position, and by how much. This allows you to constrain your position in one dimension (i.e. you still have two degrees of freedom - a surface rather than a solid), and another satellite's signal will give you another constraint (pinning you down to a line); only with a fourth satellite can you determine your position precisely (well, actually the solution can give more than one point but generally only one is realistic).

Re:Old saying

[Chrisq]

yeah and GPS absolutely doesn't rely on clocks...

GPS doesn't have much to do with a saying from the 1800's.

If you thought you were raising a valid point, YOU FAIL IT.

If you thought you were being funny, YOU FAIL IT.

HTH, HAND

You would think so until you hear the full saying:

When going to sea, take one clock or three, but never two
Or a GPS receiver will do

Re:Old saying

[itzly]

Every GPS signal is the time...that's how it works.

Obviously yes. But the signal you hear from each satellite is offset by an unknown amount (assuming unknown time/position). So, you need to solve for 4 variables (time, x, y, z), so you require 4 satellites, as you said later on. If you know 1 of the variables (for instance, because you have an accurate local clock, or you guesstimate the altitude), you can survive on 3 satellites, but it will be less accurate.

Note that you need to know local time to nanosecond accuracy, so a regular quartz oscillator is only useful for a short time after synchronisation, and will drift away fairly quickly.

Re:Can it be used as an accelerometer?

[Em Adespoton]

that was my first thought -- these things, if they could be manufactured to be affordable, would be great for relative positioning -- although I was thinking seismometers, not GPS. If you had a network of them, you could instantly (well,at the speed of light) map out any changes in their positioning.

Which reminds me; as my head is moving faster than my feet relative to the centre of the earth, they age at different rates. Same principle at work here. But it means I should spend more time standing on my head :)

They finally invented a clock so accurate...

[unitron]

...that it can't be used to tell time reliably.

Problem...

[Lab Rat Jason]

That sounds like a 0th world problem...

Re:That's quite a warranty!

[Pope Hagbard]

Not warranted for war, plagues, pestilence, or the power going out for an extended period.

Re:That's quite a warranty!

[PolygamousRanchKid]

Yes, but you probably need to change the battery every few million years or so. That's where they will make their money . . . kinda sorta like printer cartridges or iPhone batteries.

You get the atomic clock cheap, but those extras cost you!

Re:Hmm, says here:

[marcansoft]

Moving faster causes time to slow down (special relativity), but so does beeing in a deeper gravitational well (general relativity). As you move away from the Earth, both effects have opposite (but not equal) magnitude. I'm too lazy to do the math right now, but here's a walkthrough (for the case of GPS satellites, but the same equations hold; you just need to know the distance from Earth's center to Death Valley and to Mount Everest, and work out their linear velocity from that).

Re:...and also not true

[Bengie]

Since "perfect" is impossible, making it a useless word, lets just redefine it to something useful, like the colloquial usage of "close enough with respect to the current standard margin of error.