Does wall clock time matter for anything but logging? For everything else one could just create any form „time“ to keep stuff in sync, no?
Clock synchronization is a nightmare
41–50 of 165 posts
Re: Clock synchronization is a nightmare
#42Clock sync is such a nightmare in robotics. Most OSes happily will skew/jump to get the time correct. Time jumps (especially backwards) will crash most robotics stacks. You might decide to ensure that you have synced time before starting the stack. Great, now your timestamps are mostly accurate, except what happens when you've used GPS as your time source, and you start indoors? Robot hangs forever. Hot take: I've se…
THIS is what will save us from the robot uprising!
Re: Clock synchronization is a nightmare
#43Earlier quoted context omitted.
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.
There is distinction between seeing when events happened, and when they really happened. The latter can be reconstructed by an observer. In special relativity, time is relative and when things actually happened can be different in different frames. Casually linked events are always really in the same order. But disconnected events can be seen in different orders depending on speed of observer.
> But disconnected events can be seen in different orders depending on speed of observer.
What are "disconnected events"? In a subtle but still real sense, are not all events causally linked? e.g. gravitationally, magnetically, subatomically or quantumly?
I can understand that our simple minds and computational abilities lead us to consider events "far away" from each other as "disconnected" for practical reasons. But are they really not causally connected in a subtle way?
There are pieces of space time that are clearly, obviously causally connected to each other. And there are far away regions of the universe that are, practically speaking, causally disconnected from things "around here". But wouldn't these causally disjoint regions overlap with each other, stringing together a chain of causality from anywhere to anywhere?
Or is there a complete vacuum of insulation between some truly disconnected events that don't overlap with any other observational light cone or frame of reference at all?
Re: Clock synchronization is a nightmare
#44Clock sync is such a nightmare in robotics. Most OSes happily will skew/jump to get the time correct. Time jumps (especially backwards) will crash most robotics stacks. You might decide to ensure that you have synced time before starting the stack. Great, now your timestamps are mostly accurate, except what happens when you've used GPS as your time source, and you start indoors? Robot hangs forever. Hot take: I've se…
If your requirements are “must have accurate time, must start with an inaccurate time, must not step time during operation, no atomic clocks, must not require a network connection, or a WWVB signal, must work without a GPS signal” then yes, you need to relax your requirements.
But it doesn’t have to be the first requirement you relax.
Re: Clock synchronization is a nightmare
#45Earlier quoted context omitted.
There is distinction between seeing when events happened, and when they really happened. The latter can be reconstructed by an observer. In special relativity, time is relative and when things actually happened can be different in different frames. Casually linked events are always really in the same order. But disconnected events can be seen in different orders depending on speed of observer.
> But disconnected events can be seen in different orders depending on speed of observer. What are "disconnected events"? In a subtle but still real sense, are not all events causally linked? e.g. gravitationally, magnetically, subatomically or quantumly? I can understand that our simple minds and computational abilities lead us to consider events "far away" from each other as "disconnected" for practical reasons. Bu…
We now know that gravity moves at the speed of light. Imagie that you aretwo supernovas that for some unknown reason, explode at essentially the same time. Just before you die from radiation exposure, you will see the light pulse from each supernova before each supernova can 'see' the gravitational disruption caused by the other. Maybe a gravity wave can push a chain reaction on the verge of happening into either a) happening or b) being delayed for a brief time, but the second explosion happened before the pulse from the first could have arrived. So you're pretty sure they aren't causally linked.
However if they were both triggered by a binary black hole merger, then they're dependent events but not on each other.
But I think the general discussion is more of a 'Han shot first' sort. One intelligent system reacting to an action of another intelligent system, and not being able to discern as a person from a different reference frame as to who started it and who reacted. So I suppose when we have relativistic duels we will have to preserve the role of the 'second' as a witness to the events. Or we will have to just shrug and find something else to worry about.
Re: Clock synchronization is a nightmare
#46Does wall clock time matter for anything but logging? For everything else one could just create any form „time“ to keep stuff in sync, no?
Isn't it also useful for checking validity periods for stuff like TLS certs or JWTs or Kerberos tickets?
Re: Clock synchronization is a nightmare
#47Earlier quoted context omitted.
There is distinction between seeing when events happened, and when they really happened. The latter can be reconstructed by an observer. In special relativity, time is relative and when things actually happened can be different in different frames. Casually linked events are always really in the same order. But disconnected events can be seen in different orders depending on speed of observer.
> But disconnected events can be seen in different orders depending on speed of observer. What are "disconnected events"? In a subtle but still real sense, are not all events causally linked? e.g. gravitationally, magnetically, subatomically or quantumly? I can understand that our simple minds and computational abilities lead us to consider events "far away" from each other as "disconnected" for practical reasons. Bu…
Causality moves at the speed of light. Events that are farther apart are called spacelike and aren't causally connected.
I think you might be confusing events that have some history between them, and those are influence each other. Like say right now, Martian rover sends message to Earth and Earth sends message to them, those aren't causally connected cause don't know about the other message until light speed delay has passed.
Re: Clock synchronization is a nightmare
#48On the flipside, clock sync for civilians has never been easier. Thanks to NTP any device with an Internet connection can pretty easily get time accurate to 1 second, often as little as 10 ms. All major consumer computers are preconfigured to sync time to one of several reliable NTP pools.
This post is about more complicated synchronization for more demanding applications. And it's very good. I'm just marveling at how in my lifetime I from "no clock is ever set right" to assuming most anything was within a second of true time.
Re: Clock synchronization is a nightmare
#49Does wall clock time matter for anything but logging? For everything else one could just create any form „time“ to keep stuff in sync, no?
Isn't it also useful for checking validity periods for stuff like TLS certs or JWTs or Kerberos tickets?
We could use some made up „time“ for that since it’s not made for human consumption just sync between different systems.
Re: Clock synchronization is a nightmare
#50Absolute synchronization impossible?? Challenge accepted.