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

[DigiShaman]

And the man with a broken watch can still say the right time twice a day.

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

[tgeller]

A man with an atomic watch is better than two in the bush.

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

[__aaltlg1547]

No, that's wrong. Four satellites are needed to get a unique position solution. The ground stations only broadcast correction data that can move your position solution by a few meters at most.

Re:Old saying

[Jane Q. Public]

Should a second in China be minusculy shorter than one in America?

No. It should be slightly longer.

Re:Old saying

[Jane Q. Public]

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

Just as basic geometry would normally dictate, 3 satellites are sufficient to find your basic location and elevation. (There are actually 2 solutions to the equation, but one of them makes no sense because it's at some point out in space.)

The 4th element is what is known as a "ground segment", which is used to increase the accuracy of the 3-satellite triangulation. Any further satellite signals are used only to further increase accuracy.

The 4th element is gradually being moved into orbit, which WILL make it a 4-satellite lock for accuracy boost. But the fact remains that your basic geolocation including elevation is still fundamentally based on 3 satellites. The 4th signal is only to improve accuracy for civilians, as they do not have access to the military-accurate signals.

Re: Old saying

[Anonymous Coward]

Sorry, that doesn't make sense. It's a 4D problem because you don't know what time it is. Three spheres intersect at a point, but there are still multiple possibilities because you don't know how big the spheres are.

Re:Old saying

[smallfries]

Actually basic geometry does not say that at all. The receiver does not get given an accurate distance to each satellite, instead it is an inacurrate relative difference in distance. The intersection between the three spheres is a 3d region rather than a point. The extra fix is required to constrain the equations to a single point. There is more info here.

Re: Old saying

[Hadlock]

As of about 2010 we already had clocks so accurate you could demonstrate relativistic effects by separating them by just a few feet. It sounds like the vibration from walking in the same room as these is enough to knock them out of sync.

Re: Old saying

[lgw]

Simultaneity doesn't mean what you want it to mean. There's no arbitrary universal time in which things can be said to happen simultaneously. Even the order of events can be different depending on the frame of reference of the observer (but causality is always protected). You can pick an arbitrary frame, of course, but you can only observe distant events after the speed-of-light delay, which itself depends on relative velocity. Since we're constantly accelerating, even that comes down to arbitrary choices.

Or in other words, if you and I are moving relative to one another, each of us can have a definition of simultaneity, but we can't agree on what events are simultaneous. Two atomic clocks at different places on Earth won't have any reference frame in which they keep exactly the same time (though they'll agree to quite a few significant digits).

Re:Old saying

[itzly]

Not quite. The altitude depends on the local sea level, which not only depends on the tide, but also on the local gravity field and shape of the Earth. So, yes, you can get a 2D fix, but it won't be as accurate as a full 3D fix.

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

[nabsltd]

You need a 4th one for the time. Without an accurate time reference, you can't determine distance to satellites.

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

The signal from different satellites (which includes the time, the satellite ID, and the satellite position) is enough by itself to give you everything you need, and by determining how long each signal took to reach the receiver, the position can be fixed.

You only need 3 satellites if your position is already generally known (i.e., what hemisphere), or if the receiver assumes you are reasonably close to sea level. With 4 satellites, you can get a fix with no previous knowledge of where you were. Four will also give you accurate altitude after a few iterations.

Re: Old saying

[hawkinspeter]

That might work if we knew that the Earth was perfectly spherical and uniformly dense. However, it's going to be almost impossible to keep track of all the differences in velocity and gravity at different points on the earth in order to compare the measurement of time at one clock with the measurement of time at another clock. The clocks would be correctly measuring time, they'd just never agree with one another due to their frame of reference being different.

Re:Old saying

[possiblywrong]

This is incorrect. (I assure you: I *use* this technology in depth. :) ) The confusion arises from thinking about just the spatial component of the problem you are trying to solve, and not also the temporal. A GPS receiver has a generally *inaccurate* internal clock (which means it can be cheap, one of the brilliant parts of the design, IMO). Think of the "pseudorange" from a single satellite as providing a single equation with *four* unknowns: the three dimensions of position, and the *error* in your internal clock. To solve for all four unknowns, four equations (and thus four measurements) are needed.

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:Old saying

[cellocgw]

You need a four dimensional fix.

Mr. B. Banzai would like a little word with you.

Re:Old saying

[jfengel]

Yep. Effectively, it's a four-variable problem: x, y, z, and t. If you had a synchronized atomic clock with you, you might be able to do it with just three satellites, but that would be pretty bulky and delicate.

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:"More precise than any clock before"

[NoNonAlphaCharsHere]

Shit, you're right. We should just forget this whole physics thing and go back to building henges.

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!

...and also not true

[Roger W Moore]

It can't keep 'perfect time' for any length of time at all. Perfect means zero error. This might be an astoundingly accurate clock but that does not make it perfect.

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.

Re:...and also not true

[BlackPignouf]

A mathematician and an engineer are sitting at a table drinking when a very beautiful woman walks in and sits down at the bar.

The mathematician sighs. "I'd like to talk to her, but first I have to cover half the distance between where we are and where she is, then half of the distance that remains, then half of that distance, and so on. The series is infinite. There'll always be some finite distance between us."

The engineer gets up and starts walking. "Ah, well, I figure I can get close enough for all practical purposes."

Misleading Title

[arobatino]

New Atomic Clock Reaches the Boundaries of Timekeeping

On Earth, maybe. It's not a theoretical limit - the article itself points out that you can put the clocks in space.

Ye suspects the only way we will be able to keep time in the future is to send these new clocks into space. Far from the earth's surface, the clocks would be better able to stay in synch, and perhaps our unified sense of time could be preserved.

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:"More precise than any clock before"

[khellendros1984]

Precision is basically the number of significant values that taking a measurement yields. Accuracy is how close to the true value the measurement is. The clocks are both precise and accurate, to the degree that things that we don't normally need to consider (velocity that the clock is moving, the strength of gravity acting on the clock, etc) can be measured. The "problem" is that time flows at different speeds under different conditions, and the clocks can't remain synchronized with each other because reality doesn't actually remain in synchronicity.

Also, time isn't a human construct any more than the other dimensions are. Measurement of time is a human construct, but it's also designed to reflect reality.

Old saying

[rekoil]

That feeds into best practice for configuring NTP clients - configure one upstream source, or at least three. Never two.

Re:Setting the Clock?

[jpellino]

Simple. Call (860) JAckson-48123. You even get the temperature.

Gravity

[Triklyn]

I find immense beauty in the fact that they set out to make as perfect a tracker of time they could. And end up creating an improved gravity detector when they ran into a wall. :) tell me again, that basic science doesn't deserve funds.