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Living Without Atomic Clocks

cockroachlabs.com

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Re: Living Without Atomic Clocks

#21

Post by andrepd »

bigdubs wrote: post #4
tlarkworthy wrote: post #2

I don't understand why 7ms is considered a good bound for atomic clocks? Hafele and Keating Experiment: "During October, 1971, four cesium atomic beam clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicted that the flying clo…

Something I never understood. If velocity (?) affects time, wouldn't time operate a different speed at different parts of the universe / solar system?

Absolute velocity does not affect time, simply because there is no such thing as absolute velocity. Relative velocity, though, does. For instance, if you look at a clock on a GPS satellite, moving fast relative to you, you can see it run slower than the one on your wrist. Similarly, someone stationary on the surface of the sun would see us here on earth moving in slow motion.

However, someone moving on the satellite would see the clock on the satellite move normally, and us on earth moving in slow motion, because they are on the same frame of reference of the satellite.

So no, time does not operate differently on different parts of the universe per se. It all depends on how it's moving relative to the observer.

Re: Living Without Atomic Clocks

#22

Post by Animats »

Chip-scale atomic clocks are now about $1500.[1] This just gets you a 10.0000000000 MHz oscillator; something else has to count time and provide output.

Passing around the max sequence number as a monotonic sequence indicator has a risk. A bogus sequence number near the max value can cause serious problems.

[1] http://www.microsemi.com/salescontacts?ctype=3

Re: Living Without Atomic Clocks

#23

Post by andrepd »

Retric wrote: post #19
obastani wrote: post #15

Earlier quoted context omitted.

This is based on my limited physics education, but I believe there are two things at play: 1) Velocity doesn't affect time, acceleration does. So if your twin flies to Mars and back, you age more quickly. But if your twin flies to Mars, and you join him after a year (following the same trajectory), you age the same. This is special relativity. 2) Acceleration due to gravity doesn't count. In fact, the opposite is tru…

That's not really correct, if you sit at the center of a large object you don't accelerate, but you experience time dilation. Further, as you dig deeper your acceleration decreases but time dilation continues to increase.

That's incorrect as well. You are mixing two relativistic effects: from the velocity of something relative to an observer, and from the gravity on that spot of the universe (which is what I assume you mean by acceleration).

Also wrong, as you dig deeper into a planet you experience less gravity, not more.

Re: Living Without Atomic Clocks

#24

Post by dingo_bat »

NeutronBoy wrote: post #20
nickpsecurity wrote: post #17

Nice writeup. I disagree that we need chip-scale, atomic clocks. My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter. You can plug them into each machine in the cluster periodically to sync them. Or you can plug it into a master node that connects with low-latency management inter…

> My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter. Google have a variant on this, where they use a GPS receiver in each data centre to provide an accurate time source for local machines.

I've seen this being used extensively in sensor control centers (power grid/power plants), so it's definitely not limited to Google.

Basically by accurate and precise timing, they are able to reconstruct and pinpoint the origin of a failure.

Re: Living Without Atomic Clocks

#25

Post by dingo_bat »

fivesigma wrote: post #5
bigdubs wrote: post #4

Earlier quoted context omitted.

Something I never understood. If velocity (?) affects time, wouldn't time operate a different speed at different parts of the universe / solar system?

Both velocity and gravity (distance from the nearest planet/star) affect time. It does, but the difference is negligible unless we're talking about the surface of a neutron star.

The difference is not negligible if we are talking about applications like GPS etc.

Re: Living Without Atomic Clocks

#26

Post by contravariant »

theophrastus wrote: post #10
bigdubs wrote: post #4

Earlier quoted context omitted.

Something I never understood. If velocity (?) affects time, wouldn't time operate a different speed at different parts of the universe / solar system?

I always thought it would be an interesting physics exam question to ask: "If a clock were landed on Halley's comet, and retrieved on its next orbit, what would be the expected difference in time relative to an earth bound clock?" It's tricky because all orbiting bodies experience a range of velocities as they orbit their star. But I gather Halley's comet has a particularly eccentric orbit (0.9) with a rapid periheli…

That would be one hell of an exam question.

Best method I can think of is to measure the proper time of both the trajectory of the earth and the trajectory of Halley's comet, and compare, but with the combined effects of gravity and a changing speed that could be quite challenging.

It's even worse if you insist on using geodesics instead of elliptical orbits, or if you decide to include the rotation of the earth in your calculations.

Re: Living Without Atomic Clocks

#27

Post by tlipcon »

jasonwatkinspdx wrote: post #16

First, CockroachDB's time api is based on the Hybrid Logical Clocks paper: http://www.cse.buffalo.edu/tech-reports/2014-04.pdf This paper is one of the most interesting published in the last couple of years IMO. I remain surprised at how many people have overlooked it. I'm not sure why this blog post makes no mention of the work, in the past the Cockroach folks have been quite explicit about crediting the research do…

Kudu uses a similar algorithm which we call HybridTime. You can read the tech report here: http://pdsl.ece.utexas.edu/david/hybrid-time-tech-report-01....

and the source here: https://github.com/apache/incubator-kudu/blob/master/src/kud...

Both are probably more readable than patent-ese :)

Re: Living Without Atomic Clocks

#28

Post by nickpsecurity »

NeutronBoy wrote: post #20
nickpsecurity wrote: post #17

Nice writeup. I disagree that we need chip-scale, atomic clocks. My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter. You can plug them into each machine in the cluster periodically to sync them. Or you can plug it into a master node that connects with low-latency management inter…

> My idea was a dedicated, battery-backed piece of hardware that reliably stored time plus could sync other machines. Plugs into an interconnect with ultra-low latency. One for each datacenter. Google have a variant on this, where they use a GPS receiver in each data centre to provide an accurate time source for local machines.

No, they use a GPS with 7ms time spread for servers as you said. What Im describing is a custom device set against an atomic clock to nano or microsecond accuracy which then does the same to the servers via low latency interconnects. Optionally with time server & dedicated networks for reduced admin overhead.

Should do a lot better than 7ms with performance implications.

Re: Living Without Atomic Clocks

#29

Post by [deleted] »

vinkelhake wrote: post #8
tlarkworthy wrote: post #2

I don't understand why 7ms is considered a good bound for atomic clocks? Hafele and Keating Experiment: "During October, 1971, four cesium atomic beam clocks were flown on regularly scheduled commercial jet flights around the world twice, once eastward and once westward, to test Einstein's theory of relativity with macroscopic clocks. From the actual flight paths of each trip, the theory predicted that the flying clo…

The 7ms bound is on the machines that do the actual work, not the ones with access to special clock hardware.

[deleted]

Re: Living Without Atomic Clocks

#30

Post by JabavuAdams »

andrepd wrote: post #21
bigdubs wrote: post #4

Earlier quoted context omitted.

Something I never understood. If velocity (?) affects time, wouldn't time operate a different speed at different parts of the universe / solar system?

Absolute velocity does not affect time, simply because there is no such thing as absolute velocity. Relative velocity, though, does. For instance, if you look at a clock on a GPS satellite, moving fast relative to you, you can see it run slower than the one on your wrist. Similarly, someone stationary on the surface of the sun would see us here on earth moving in slow motion. However, someone moving on the satellite…

The velocity-time relationship is explained by Special Relativity (SR).

The Sun example is a bit more complicated, because there's the velocity-time effect due to SR, but also the fact that you're deep in a big gravity well, which has effects on your measured time due to General Relativity (GR).

SR says that "moving clocks run slow", but deep in the Sun's gravity well, we should also be running slow relative to the Earth. Not sure what the relative size of the two effects is.

EDIT> I assume the SR effect is larger than the GR effect, simply because SR was obvious before GR.

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