For network stuff, high security and test/measurement networked systems use precision time protocol [1], which adds hardware timestamps as the packets exit the interface. This can resolve down to a couple nanoseconds for 10G [2], but can get down to picosecond. The "Grandmaster" clock uses GPS/atomic clocks.

For test and measurement, it's used for more boring synchronization of processes/whatever. For high security, with minimal length/tight cable runs, you can detect changes in cable length and latency added by MITM equipment, and synch all the security stuffs in your network.

[1] https://en.wikipedia.org/wiki/Precision_Time_Protocol

[2] https://www.arista.com/assets/data/pdf/Whitepapers/Absolute-...

My understanding is that precise measurement of time is the basis of all other measurements: space, mass, etc. They are all defined by some unit of time. So increasing time precision increases potential precision in other measurements.

Including of course information - often defined by the presence or absence of some alterable within a specific time.

We invent new uses for things once we have them.

A fun thought experiment would be what the world would look like if all clocks were perfectly in sync. I think I'll spend the rest of the day coming with imaginary applications.

I use fairly precise time but that's because I control high speed machinery remotely. The synchronization is the important part (the actual time doesn't matter). At 1200 inches per minute, being a millisecond off will put a noticeable notch in a piece.

Ptp and careful hardware configuration keeps things synced to within nanoseconds

For most applications, clock precision of synchronization isn't really necessary. Timestamps may be used to order events, but what is important is that there is a deterministic order of events, not that the timestamps represent the actual order that the events happened.

In such systems, ntp is inexpensive and sufficient. On networks where ntpd's assumptions hold (symetric and consistent delays), sync within a millisecond is acheivable without much work.

If you need better, PTP can get much better results. A local ntpserver following GPS with a PPS signal can get slightly better results (but without PPS it might well be worse)

I guess very few systems have better absolute time than a few microseconds. Those systems are probably exclusively found in HFT and experimental physics.

This past week I tried synchronizing the time of an embedded Linux board with a GPS PPS signal via GPIO. Turns out the kernel interrupt handler already delays the edge by 20 us compared to busy polling the state of the pin. Stuff then gets hard to measure at sub microsecond scales.

Since their precision is essential to measuring relativistic effects, I'm not sure we're near that limit.

For your precise question, it may already be there.

Another commenter mentioned that this is needed for consistently ordering events, to which I'd add:

The consistent ordering of events is important when you're working with more than one system. An un-synchronized clock can handle this fine with a single system, it only matters when you're trying to reconcile events with another system.

This is also a scale problem, when you receive one event per-second a granularity of 1 second may very well be sufficient. If you need to deterministically order 10^9 events across systems consistently you'll want better than nanosecond level precision if you're relying on timestamps for that ordering.

I know that Google's Spanner[0] uses atomic clocks to help with consistency.

[0] https://en.wikipedia.org/wiki/Spanner_(database)

It hit diminishing returns for most things long, long ago, but this physics is directly related to stuff in quantum computing and studying gravity.

> where do we actually hit diminishing returns with clock precision?

ah yes - that would be Planck's second which can be derived from Planck's constant and the speed of light

Yes, there is a limit called "quantum projection noise" that determines how much frequency stability one can achieve with a single-atom clock [1]. With N independent atoms, this limit gets smaller by 1/sqrt(N), but with N entangled atoms one can achieve a 1/N scaling. This is the ultimate limit (Heisenberg limit).

[1] https://journals.aps.org/pra/abstract/10.1103/PhysRevA.47.35...

Not just time dilation, at those scales time apparently can flow backwards for a bit!

https://arxiv.org/abs/2409.03680

> how time was a Western construct

Japan had a whole fancy temporal hour system before Western contact. It was more complicated than our modern framework, as it was based on the time between sunrise and sunset and so the length of the hours had to be adjusted about every two weeks. But they certainly thought quite a bit about it, so I'm not sure how it could be claimed to not be a concept there at the time.

Your professor was just wrong if they claimed time was a Western construct. Calendars, sundials, and other time keeping devices were created independently around the world.

All you need to create a clock is realize that your oil lamp consumes its fuel in a somewhat consistent interval, or a similar observation for the time it takes drips of water to fill a cup. People figured it out.

> time was a Western construct

That really doesn't seem to make sense as written. Even if for "Western" you count all the way to the Middle East (where much of our chronometry originates), there's still a lot found in China and the New World. (From what I can tell, India does not seem to have a strong independent record here? Though they certainly borrowed from the inventors, just like Europe did.)

We've been using sundials since antiquity. What was your professor even talking about?

This topic of time being a western construct, it's impact on society and life is one of the subjects in the excellent book "Borderliners" [1] by Peter Høeg. A favorite of mine, though I never read the English translation. It's a fascinating topic, and impacts our live more than we might be aware of or care to admit. I started thinking about it in a different way after reading this book.

[1] https://en.wikipedia.org/wiki/Borderliners

I don't think its fair to say that atomic clocks represent a western cultural value. After all, they are extremely niche. Physicists care but the average "westerner" does not.

It wasn't even a daily utilized concept in the West until trains were chugging their way to various stations. Farmers didn't need to know the exact minute the cows came home.

And paper money is a Chinese invention. Doesn't mean it's worthwhile to spend two weeks in an anthropology class talking about how much awesomer they are.

straight from the abstract:

> we can demonstrate quantum-amplified time-reversal spectroscopy on an optical clock transition that achieves directly measured 2.4(7) dB metrological gain and 4.0(8) dB improvement in laser noise sensitivity beyond the standard quantum limit.