New Atomic Clock Reaches the Boundaries of Timekeeping 249
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."
Old saying (Score:5, Insightful)
A man with one watch always knows what time it is.
A man with two watches is never sure.
Re:Old saying (Score:5, Funny)
A man with an atomic watch won't shut up about it.
Re:Old saying (Score:5, Funny)
A man with an atomic watch better not keep it in his pocket.
Re:Old saying (Score:4, Funny)
Re:Old saying (Score:5, Funny)
Re:Old saying (Score:5, Interesting)
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 (Score:5, Informative)
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3 satellites for a basic fix. The 4th element is a ground station that corrects for most error. Further satellites can add a bit to accuracy but only with diminishing returns.
Re:Old saying (Score:4, Informative)
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 (Score:4, Informative)
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 (Score:4, Informative)
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 [wikipedia.org].
Re:Old saying (Score:5, Informative)
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).
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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.
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Umm, no.
Selective Availability (the coarser signal that used to be available to civilians) was turned off a long time ago. Now everyone gets the military grade signal.
Which doesn't mean that there's no point to that "fourth signal" you refer to. It's called differential GPS, and uses a GPS receiver at a surveyed point to broadcast corrections to (relatively nearby) receivers. However, t
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Re:Old saying (Score:4, Insightful)
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 (Score:5, Informative)
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 (Score:3, Insightful)
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.
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GPS doesn't use triangulation. It uses trilateration.
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If you are in a boat you probably have a good idea about your altitude, so a 2d fix is all you really need.
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You need a four dimensional fix. You must solve for X, Y, Z, and t. This requires a fix on four satellites. You can work with three if you assume you are on the surface of the earth and know your elevation.
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You need a four dimensional fix.
Mr. B. Banzai would like a little word with you.
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yeah and GPS absolutely doesn't rely on clocks...
Kinda does as it based entirely on synced time transmissions and triangulations. I don't think the coverage was that great in the 1800's though so they had to fall back on hand written directions from ye olde google maps I guess.
Re:Old saying (Score:5, Funny)
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 (Score:5, Insightful)
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.
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Re: Old saying (Score:5, Interesting)
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Only on that level. For most everything else, it's fine. So not so much "fundamentally", you just need to apply it appropriately, much like Newtonian physics. You don't need to factor in QM to build a plane.
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Because it would be meaningless to "compensate" for the time difference between clocks moving and accelerating differently.
It isn't "meaningless", it's just not very useful.
The real problem is that with "clocks" this precise, there must be an acceleration standard in addition to the other standards.
I suggest that 3 be placed on the same fixed level surface, at some kind of "elevation" standard based on effective gravity, so that tectonic changes. Because this is an obvious source of potential error. Just for one example, a recent media article was all about "sea level rise" on the East U.S. coast, when in fact most if not
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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.
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I don't think so. When it comes to physics, I'm an informed layman at best, but I don't think that the concept itself is flawed. The flaw comes in when people try to apply it in a situation where it just doesn't apply. As an example, it certainly doesn't apply at astronomical distances; we can't know exactly where Alpha Centauri is and what's happening there right this minute; we can only know what was going on 4.366 years ago.
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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.
I'll admit, I don't understand why the arbitrary reference time we use currently is any less valuable now that we have surface clocks whose real time is measurably changing due to relativistic effects.
Re: Old saying (Score:2)
I think it is wrong to say simultaneity is wrong. The fact that the two clocks are correct does not mean simultaneity does not exist, it only means that we cannot measure simultaneity with clocks.
The fact is thinga happen in parallel in the universe, and so simultaneity exists, because for any event another event would have to have happened at the same time.
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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 choice
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In the larger universe, it's more obvious that there is no single frame of reference to which you can pin "simultaneity". "Same time" only makes sense in a single frame of refernce.
Comment removed (Score:5, Informative)
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Yes but you can perform a transformation to translate from one reference frame to another, and while the universe may not give special meaning to any particular reference frame, there's nothing stopping us from doing so. So in theory you can define a "universal" time if you want.
Similar to choosing where to place the origin on a sheet of graph paper. It doesn't really matter where you put it (mathematically at least) but once its there you have a perfectly meaningful "universal" point of reference to use
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Because you'd have to do it in realtime, or whatever passes for it when dealing with relativistic effects on this scale. Otherwise, microscopic shifts in the Earth's crust will soon change the slew rate, and then you're back to square one. But calculating the slew rate will inevitably take longer than one tick of the clock, so you can't do it in realtime either.
Though this property isn't entirely useless. For example, you could use two of these clocks to build extremely sensitive seismometers: first you wou
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In general relativity, you can paint a coherent picture. Both the observer deep in a gravity well and a distant observer agree that higher gravity makes clocks run slower. But that's not true of special relativity at all. Given two clocks moving relative to one another, each observer thinks the other's clock is running slower and they're equally right. There's simply no universal clock, no standard that's more right than any other clock.
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Should a second in China be minusculy shorter than one in America?
No. It should be slightly longer.
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This is not a new problem, suddenly created by this clock. It already exists with our standard definition of time.
TAI as a time scale is a weighted average of the time kept by over 200 atomic clocks in over 50 national laboratories worldwide [wikipedia.org]
So now we'll be averaging due to relativity differences as well as precision errors.
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I actually addressed this just yesterday on Stack Exchange:
http://astronomy.stackexchange... [stackexchange.com]
Thanks!
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If your watch is broken and runs at 99% speed then I think you only get the right time every 49.5 days or so...
Re: Old saying (Score:5, Funny)
I have a broken watch, can't quite work out when 88:88 is going to roll past though.
Old saying (Score:4, Funny)
That feeds into best practice for configuring NTP clients - configure one upstream source, or at least three. Never two.
Re:Old saying (Score:5, Informative)
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.
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Hey, I have a dozen watches (you insensitive clod) -- and I know how to calculate mean, median, and standard deviations!
So there.
They finally invented a clock so accurate... (Score:5, Insightful)
...that it can't be used to tell time reliably.
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...that it can't be used to tell time reliably.
Hilarious, You win the internet. Thanks @unitron.
Thanks, one more and I'll have the complete set!
Problem... (Score:5, Funny)
That sounds like a 0th world problem...
That's quite a warranty! (Score:2)
Re:That's quite a warranty! (Score:4, Interesting)
Not warranted for war, plagues, pestilence, or the power going out for an extended period.
Re:That's quite a warranty! (Score:5, Funny)
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!
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But for one simple payment or $100,000, you can pre-purchase your batteries for the next 500 billion years!
...and also not true (Score:3, Informative)
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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.
Time is relative to the imperfect Universe in which it and the clock exist.
Re:...and also not true (Score:5, Insightful)
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Dammit where are my mod points? You just shot down a whole lot of pedants with that comment.
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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 pract
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...with respect to the current standard margin of error
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"Perfect means zero error"
Years ago, I used to think like you, but now I disagree with this statement. For me, perfect means meeting the requirements. Zero error is a requirement but not a reasonable one. My life got much more pleasant when I realized I should aim for meeting reasonable requirements instead of an elusive zero error target.
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So perfect really means an error bound by the limits of physics.
But since those limits are energy dependent a high energy process could actually have a better time resolution than this clock can provide...hence it is not perfect.
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Five Billion Years? (Score:2)
"...able to keep perfect time for five billion years."
If they were able to create a device that could actually keep the time for five billion years, perfect or not, I would be pretty damn impressed.
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"...able to keep perfect time for five billion years."
If they were able to create a device that could actually keep the time for five billion years, perfect or not, I would be pretty damn impressed.
My first thought as well, would take a hell of a Radioisotope to generate power for that long.
Wars and such would disrupt it's power after some time if not on an independent power supply.
But in the end due to the relativistic differences (Earth quakes and such); of anyone looking at the clock for the time it was started or past would make it useless - unless you did the math.
Misleading Title (Score:4, Interesting)
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.
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Yeah, but, ... if the clocks have to be up in space to be fully accurate, etc., What is the meaning of the word "today" in the sentence "You will not go to space today" ?
[joke, dammit!!!]
But... (Score:2)
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Where in the universe is it the "correct" time?
Wherever you decide it should be. The universe doesn't have any special reference frame so you just make one up. Similar to how somebody just arbitrarily picked a line of longitude to be GMT+0 back when they were figuring out time zones.
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what's the up-time? (Score:2)
This Clock is Definitive! (Score:2)
Better than cesium (Score:2)
Setting the Clock? (Score:2)
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But (Score:2)
They still have to change it every 6 months to account for daylight saving or the reverse.
Really? (Score:2)
The world's current time is coordinated between atomic clocks all over the planet. But that can't happen with the new one.
However precise the clock is, we can still use it to coordinate time all over the earth. Just use the same number of significant digits we use today. Nobody is forcing you to consider ALL the available digits of precision.
Discworld (Score:2)
http://discworld.wikia.com/wik... [wikia.com]
Answer and a quote. (Score:2)
Easy answer, build N+1 of them and use the 'average' value of time that they generate.
One of my favourite quotes applies here:
"When you have a clock you always know what time it is. When you have two you are never quite sure." - Mark Twain.
Gravity (Score:4, Interesting)
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.
Incredibly precise time (Score:2)
The Doctor should be arriving any time, so to speak.
Re:Can it be used as an accelerometer? (Score:4, Interesting)
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 :)
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affordable? sure. and the new model comes in a room just under the size of your house.. great for taking on camping trips .. as long as you have a big ass trailer.
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If time is a human construct...
It's not.
...[if] two identical clocks can't remain synchronized with each other, can they really be said to be precise at all?
Yes, because the fact that they can't remain synchronized is a result of the actual behavior of time and space. Both clocks are being perfectly accurate (and precise) -- so accurate and precise that they are measuring effects that would make *any* two clocks drift apart.
Re:Hmm, says here: (Score:5, Informative)
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 [wikipedia.org] (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).
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"slow down" time. Now if the earth has "Lumpy" GRAVY [youtube.com],
FTFY.. just needed some Zappa magic :P