7 ms·
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 w
by tlarkworthy 11y ago
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 clocks, compared with reference clocks at the U.S. Naval Observatory, should have lost 40+/-23 nanoseconds during the eastward trip and should have gained 275+/-21 nanoseconds during the westward trip ... Relative to the atomic time scale of the U.S. Naval Observatory, the flying clocks lost 59+/-10 nanoseconds during the eastward trip and gained 273+/-7 nanosecond during the westward trip, where the errors are the corresponding standard deviations. These results provide an unambiguous empirical resolution of the famous clock "paradox" with macroscopic clocks."
So if we are loosing nano seconds per day, couldn't we fly clocks around the datacenters and resync every month. 7ms seems beatable and not a terrible operational overhead for a fast globally consistent database.
- duskwuff 11y agoMaybe they've mixed up ms and µs? Their claim of "between 100ms and 250ms" for NTP also seems way too high.
- jschwartzi 11y agoIt likely has more to do with network latency than with any fundamental physical limitation of the system.
- hnkimb3558 11y agoThese are conservative upper bounds and are meant to encompass long tail offsets. You can usually get NTP down to < 10ms offsets, but if you rely on that, you'll likely run into problems which end up violating your database guarantees.
- bigdubs 11y agoSomething I never understood. If velocity (?) affects time, wouldn't time operate a different speed at different parts of the universe / solar system?
- fivesigma 11y agoBoth 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.
- dingo_bat 11y agoThe difference is not negligible if we are talking about applications like GPS etc.
- yeureka 11y agoI am not a physicist but I believe the answer is yes as time is affected by acceleration.
- fossuser 11y agoThe movie Interstellar does a decent job demonstrating this (well the gravity piece anyway with the proximity to black holes). I also think satellite clocks have to take time relativity it into account (both due to distance from earth's gravity and their speed). Randall Munroe has a pretty entertaining write up that does a decent job giving a high level overview in pretty easy to understand terms: http://www.newyorker.com/tech/elements/the-space-doctors-big-idea-einstein-general-relativity http://www.newyorker.com/tech/elements/the-space-doctors-big...
- theophrastus 11y agoI 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 perihelion of around 70km/sec, an aphelion of about 1km/sec, and about 75 years for a single orbit (ΔT' = ΔT / sqrt(1 - (v²/c²)))
- vinkelhake 11y agoThe 7ms bound is on the machines that do the actual work, not the ones with access to special clock hardware.
- lifty 11y agoIndeed. The machines that do the actual work will still use a an inaccurate clock and get periodic corrective updates. That being said, you should be able to get a much higher precision using PTP and some decent network cards. There are plenty of factors that come into play (network latency, congestion, etc) but 7 ms sets the bar low.
- easytiger 11y ago7ms is even achievable with a well done local NTP setup
- deleted 11y ago[deleted]
- jasonwatkinspdx 11y agoIn this context, 7ms refers to the size of the TrueTime uncertainty interval, which is different from the typical instantaneous clock error. TrueTime returns an interval that is guaranteed to contain the absolute idealized physical "now." So this interval represents a worst case boundary, and if a system using TrueTime is able to separate events by the current interval, for example by forcing a committer to wait it out, then it can provide a temporal order that reflects the absolute real world physical order, despite any clock uncertainty. It's a bit different to think about than how we typically work with time, but it's a powerful primitive in a globally distributed system.
- easytiger 11y ago> I don't understand why 7ms is considered a good bound for atomic clocks? It isn't, GPS + ptp can give slaves on the same lan +/- 700ns very cheaply