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Clock synchronization is a nightmare

arpitbhayani.me

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Re: Clock synchronization is a nightmare

#11

Post by hinkley »

emptybits wrote: post #6

Normally I would nod at the title. Having lived it. But I just watched/listened to a Richard Feynmann talk on the nature of time and clocks and the futility of "synchronizing" clocks. So I'm chuckling a bit. In the general sense, I mean. Yes yes, for practical purposes in the same reference frame on earth, it's difficult but there's hope. Now, in general ... synchronizing two clocks is ... meaningless? https://www.yo…

Einstein was worried about whether people in two different relativistic frames would see cause and effect reversed.

Re: Clock synchronization is a nightmare

#12

Post by georgelyon »

Unfortunate that the author doesn’t bring up FoundationDB version stamps, which to me feel like the right solution to the problem. Essentially, you can write a value you can’t read until after the transaction is committed and the synchronization infrastructure guarantees that value ends up being monotonically increasing per transaction. They use similar “write only” operations for atomic operations like increment.

Re: Clock synchronization is a nightmare

#13

Post by m463 »

emptybits wrote: post #6

Normally I would nod at the title. Having lived it. But I just watched/listened to a Richard Feynmann talk on the nature of time and clocks and the futility of "synchronizing" clocks. So I'm chuckling a bit. In the general sense, I mean. Yes yes, for practical purposes in the same reference frame on earth, it's difficult but there's hope. Now, in general ... synchronizing two clocks is ... meaningless? https://www.yo…

it might be meaningless, but in practical terms just don't check util.c from the gravity well into the git repo in orbit.

Re: Clock synchronization is a nightmare

#14

Post by blibble »

maximinus_thrax wrote: post #8

I wouldn't say it's a 'nightmare'. It's just more complicated than what regular folk think computers work when it comes to time sync. There's nothing nightmareish or scary about this, it's just using the best solution for your scenario, understanding limitations and adjusting expectations/requirements accordingly, perhaps relaxing consistency requirements. I worked on the NTP infra for a very large organization some…

PTP isn't even that much more difficult, as long as you planned for it form the start

you buy the hardware, plug it all in, and it works

Re: Clock synchronization is a nightmare

#15

Post by emptybits »

hinkley wrote: post #11
emptybits wrote: post #6

Normally I would nod at the title. Having lived it. But I just watched/listened to a Richard Feynmann talk on the nature of time and clocks and the futility of "synchronizing" clocks. So I'm chuckling a bit. In the general sense, I mean. Yes yes, for practical purposes in the same reference frame on earth, it's difficult but there's hope. Now, in general ... synchronizing two clocks is ... meaningless? https://www.yo…

Einstein was worried about whether people in two different relativistic frames would see cause and effect reversed.

Wild. My layperson mind goes to a simple example, which may or may not be possible, but please tell me if this is the gist:

Alice and Bob, in different reference frames, both witness events C and D occurring. Alice says C happened before D. Bob says D happened before C. They're both correct. (And good luck synchronizing your watches, Alice and Bob!)

Re: Clock synchronization is a nightmare

#17

Post by mrkeen »

emptybits wrote: post #15
hinkley wrote: post #11

Earlier quoted context omitted.

Einstein was worried about whether people in two different relativistic frames would see cause and effect reversed.

Wild. My layperson mind goes to a simple example, which may or may not be possible, but please tell me if this is the gist: Alice and Bob, in different reference frames, both witness events C and D occurring. Alice says C happened before D. Bob says D happened before C. They're both correct. (And good luck synchronizing your watches, Alice and Bob!)

That will be the case when Alice stands close to where C happens, and Bob stands close to where D happens.

It's a little trickier to imagine introducing cause-and-effect though. (Alice sees that C caused D to happen, Bob sees that D caused C to happen).

I think a "light cone" is the thought-experiment to look up here.

Re: Clock synchronization is a nightmare

#18

Post by [deleted] »

emptybits wrote: post #6

Normally I would nod at the title. Having lived it. But I just watched/listened to a Richard Feynmann talk on the nature of time and clocks and the futility of "synchronizing" clocks. So I'm chuckling a bit. In the general sense, I mean. Yes yes, for practical purposes in the same reference frame on earth, it's difficult but there's hope. Now, in general ... synchronizing two clocks is ... meaningless? https://www.yo…

[deleted]

Re: Clock synchronization is a nightmare

#19

Post by hinkley »

emptybits wrote: post #15
hinkley wrote: post #11

Earlier quoted context omitted.

Einstein was worried about whether people in two different relativistic frames would see cause and effect reversed.

Wild. My layperson mind goes to a simple example, which may or may not be possible, but please tell me if this is the gist: Alice and Bob, in different reference frames, both witness events C and D occurring. Alice says C happened before D. Bob says D happened before C. They're both correct. (And good luck synchronizing your watches, Alice and Bob!)

Yes that definitely happens. People orbiting Polaris would be seeing two supernovas explode at different times than us due to the speed of light. Polaris is 400 light years away so the gap could be large.

But when you are moving you may see very closely spaced events in different order, because you’re moving toward Carol but at an angle to Doug. Versus someone else moving toward Doug at an angle to Carol.

Re: Clock synchronization is a nightmare

#20

Post by hinkley »

mrkeen wrote: post #17
emptybits wrote: post #15

Earlier quoted context omitted.

Wild. My layperson mind goes to a simple example, which may or may not be possible, but please tell me if this is the gist: Alice and Bob, in different reference frames, both witness events C and D occurring. Alice says C happened before D. Bob says D happened before C. They're both correct. (And good luck synchronizing your watches, Alice and Bob!)

That will be the case when Alice stands close to where C happens, and Bob stands close to where D happens. It's a little trickier to imagine introducing cause-and-effect though. (Alice sees that C caused D to happen, Bob sees that D caused C to happen). I think a "light cone" is the thought-experiment to look up here.

If Bob and Alice are moving at half the speed of light in opposite directions.
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