This is compared to the ~1 mile vertical shift resolution of cesium clocks. The fun part of cesium clocks is that you throw three in the back of a minivan and take them camping!

http://leapsecond.com/great2005/

I have had to talk myself out of the buying one of those atomic clocks on a chip. I'm kind of a watch geek, and I find the entire history of timekeeping to be endlessly fascinating, and I think it would be so cool to have atomic clock level of precision just running in my house. I've just never been able to justify actually buying one; outside of looking at a console and saying "how cool it that!?!", I don't know what I'd do with it.

I'm not above buying a toy to look at logs and geek out over it, but I can't justifying spending several grand for it.

This is a great way to compare ultra-precise clocks! Also, I'm looking forward to Einsteinian altimeters everywhere.

> It's an amazing time to be alive. While not this precise, you can have atomic cesium beam clocks of your own for a few thousand dollars each, and some elbow grease.

How hard or expensive would it be for a reasonably equipped lab to build their own optical clock though? I see there are optical clocks the size of few rack units on the market for a rather hefty price, are the materials needed that expensive or is it just the expertise?

> ... vertical shift of a few centimeters could be measured

In what amount of time? Not instantly, right?

Could you detect gravity waves with accurate enough clocks?

Isn’t the second defined as a specific number of cesium transitions?

How can anything … you know what? Never mind. No matter what answer anyone provides, I won’t understand.

This is a great question that always pops up when precision clocks are discussed.

Consider the construction of a precision clock, then build 2 (or more) of them. Take a pair set them to the same initial time, and then let them count time (same height, etc).

Ideally, a pair of perfect clocks would display the same time over time. In practice you see the clocks slowly walk off with respect to each other (a systematic component and a random component), a reasonable first order approximation is to pretend the difference in displayed time shows a random walk behavior. A collection of a large number of clocks would behave like a collection of random walk instances, diffusing in delta-time space.

A poor clock construction would diffuse more quickly, and a better clock construction would diffuse more slowly.

One doesn't need a perfect clock reference to measure the random walk of a clock type / construction. Just compare 2 identically constructed and used ones.

You define it to be accurate, and then measure the precision.

You can build two and see how much they shift relative to each other. That gives you precision.

So what's the point of a clock if you just define it to be correct? Again, having two clocks is what makes it interesting. Some people have commented that according to general relatively there will be measuralbe time dilation, but there are other fun experiments, e.g.

- Measure shift of fundamental "constants": If you have two clocks that use different elements, the frequency ratio can be related to some things we thought were constants in the universe. If they shift, they aren't constant.

- Look for preferred directions in space: does one clock give a different reading if you turn it on its side?

- Some theories predict that dark matter might induce a frequency shift in these clocks. Put the clocks far apart and look for spacial modulations in the dark matter density.

- Measure anything else that had to be tweaked to make the clock stable. This includes the magnetic field, for example, so the clock is also a really sensitive magnetometer.

Since 1967 the second has a physical defintion:

https://en.wikipedia.org/wiki/Second#Atomic_definition

You are not actually measuring the accuracy of the clock, rather you are measuring the magnitude of the noise. The clock source itself is something physically fundamental and unchanging, but it gets mixed with noise.

For example even very small magnetic fields will change the clock speed, thermal changes will as well (so will lots of other things). So you try to shield from that, and keep the temperature stable (and of course you need to figure out every other things that could add noise).

Then you measure all those influences that you just are unable to control, and calculate what affect they have on the clock, and that's your accuracy number.

One way to directly measure that, instead of calculate it, is to have two identical clocks, synchronize them, and let them run. Then compare them, and see if they differ. (Watch out for relativity messing with time.)

You define time based on a physical phenomenon which does not vary.

For example, every electron is exactly the same as every other electron, they do not vary in the slightest. You utilize properties like that to make exact references to time.

it doesn't matter? it's the average of multiple atomic clocks and that's the time we distribute.

it's a human construct so whatever is agreed upon is correct.

Optical atomic clocks based on trapped single ions like this, and also those based on lattices of neutral atoms do not provide a continuous clock signal.

They are used together with a laser (which is a component included in a so-called frequency comb, which acts as a frequency divider between the hundreds of THz of the optical signal and some hundreds of MHz or a few GHz of a clock signal that can be counted with a digital counter; that digital counter could be used as a date and time clock, except that you would need more such optical clocks, to guard against downtime; the present optical clocks do not succeed to operate for very long times before needing a reset because the trapped ion has been lost from the trap or the neutral atoms have been lost from the optical lattice; therefore you need many of them to implement a continuous time scale).

The laser is the one that provides a continuous signal. In this case the laser produces infrared light in the same band as the lasers used for optical fiber communications, and it is based on glass doped with erbium and ytterbium. The frequency of the laser is adjusted to match some resonance frequency of the trapped ion (in this case a submultiple of the frequency, because the frequency of the transition used in the aluminum ion is very high, in ultraviolet). For very short time intervals, when it cannot follow the reference frequency, because that must be filtered of noise, the stability of the laser frequency is determined by a resonant cavity made of silicon (which is transparent for the infrared light of the laser), which is cooled at a very low temperature, in order to improve its quality factor.

So this is similar to the behavior of the clock of a computer, which for long time intervals has the stability of the clocks used by the NTP servers used by it for synchronization, but for short time intervals it has the stability of its internal quartz oscillator.

This new optical atomic clock has the lowest ever uncertainty for the value of its reference frequency, but being a trapped single ion clock it has a higher noise than the clocks based on lattices of neutral atoms (because those can use thousands of atoms instead of one ion), so its output signal must be averaged over long times (e.g. many days) in order to reach the advertised accuracy.

For short averaging times, e.g. of one second, its accuracy is about a thousand times worse than the best attainable (however, its best accuracy is so high that even when averaged for a few seconds it is about as good as the best microwave clocks based on cesium or hydrogen).

Is there really an absolute reference point to measure time other than the big bang or something?

The clock signals can be counted and are accurate over long periods, it is not just a rate that drifts.

Being able to count trillions of ticks is entirely possible in clocks or rotary encoders, just nobody bothers to do so on rotary encoders very often.

I'm not sure using a web of satellite borne clocks to do location to say three inches or 10cm accuracy is a great idea, especially when the "device" is moving at say 70mph. Not only that, you will need some amazingly exact maps that agree with the satellites' idea of "location".

Lane awareness accuracy needs some serious looking at. Lane markings are designed for the human eye for obvious reasons. Those white lines and "cats eyes" are quite literally (lol) visual cues to keep you on track. If we want machines to drive for us then we will need other ways to do lane compliance, rather than just assist.

My Chinese designed MG4 EV has lane assist. It certainly has four optical cameras but beyond that, I don't know - I think it has a fore and aft proximity sensor of some sort. I do know it makes some dreadful errors of judgment with regards lanes. It's normally fine on a motorway or dual carriageway but gets confused on even a major A road (eg A30) when there are striations in the road surface or its wet or the sun is at the right angle to make skid marks look like lines.

If roads had something like a steel wire armoured cable buried underneath each lane and boundary line and there was a suitably cheap detector and sensors that could look far enough ahead and model the road then that might be a reasonably cheap option to augment optical systems.

Now, more sensors cost money, sorry, cost profits. They also add complexity and so on.

Sadly I think we will continue to see daft things like ... I gather that Teslas will only sport optical sensors and no LIDAR.

Autonomous cars are possible but they do need to be given a decent amount of, and variety of, ... sensors.

GPS will get you very well located on a map but you need local sensors to sort out localisation issues.

The main errors in GPS are usually due to variation in the propagation of the signals through the atmosphere, and uncertainty in the exact position of the satellites. (Solutions for these exist, but are generally too expensive and/or impractical for a lot of applications. But e.g. realtime GPS on the order of cm is something you can get now, if you pay for a subscription to the right service or operate your own reference base station)

It could improve accuracy and does help a significant error factor, but the biggest error sources in GNSS aren't clock errors. Even a perfect clock could still have several meters of error.

Jeff Geerling

I wonder if they'd consider implementing NTS.

https://github.com/jauderho/nts-servers/tree/main

Seems pretty tight for those outside of the USA. Is FAX allowed for those who live outside?

Nope, it is not.

A single measurement cannot be precise. Precision is a measure of how close multiple measurements are to one another. Accuracy is how close a single measurement is to its true value.

A clock that can measure a point in time to 19 decimal places with respect to its true value is accurate.