5 ms·

You are mixing two things up - figuring out the time an event happened and trusting a timestamp that someone attached to an event. Within a single inertial refe

You are mixing two things up - figuring out the time an event happened and trusting a timestamp that someone attached to an event. Within a single inertial reference frame there is a canonical way to order events, namely by the time coordinate in that reference frame which is the same everywhere. And for all practical purposes of building a distributed system on Earth, Earth is an internal reference frame. Ensuring that nobody cheats when assigning time stamps to events is an entirely different problem. As said, hand an atomic clock to everyone and you can know the order of any two events, relativity plays no role. Making sure that nobody can or will cheat will require some more effort, but again relativity plays no role.

EGreg 8y ago
You didn’t explain how a distributed byzantine system can synchronize a global time that’s continuous. I am saying that you won’t be able to — local time is continuous and you should use that for comparisons. It’s nowhere near as straightforward as you think to make global time work. You will find that “making sure nobody can or will cheat” will require exactly the kind of effort that will make the concept of a global timestamp meaningless. Also, why do you need a global timestamp in the first place? Upon further reflection you will probably realize that all your concepts of time actually require exactly what I said: two paths to the same local reference point, where a comparison is made. Your ideas about global time are an illusion based on simplifying assumptions that are not true.
- danbruc 8y ago
  I only object your claim that relativity - as in special or general theory of relativity - has any bearing on the design of distributed systems on Earth. In particular, you can order events occurring at different places if you have properly synchronized clocks. The fact that a common solution is to use logical clocks like Lamport clocks is because it is easier to do so than to maintain a network of precisely synchronized clocks. But that does not mean it can not be done with synchronized clocks, see for example Google's Spanner [1] database. I never addressed what else is necessary to make this work in the presence of Byzantine faults. But I am also pretty sure that just sending information about events from A and B to C and asking C about the ordering is not sufficient, after all C may be faulty and give a wrong answer, change its mind after every query, or just not respond at all. Admittedly you did not provide many details, so I may very well totally misunderstand what you had in mind. But, just to repeat, I am not really addressing the requirements of a Byzantine fault tolerant system, I am just saying that the special or general theory of relativity is not relevant for distributed systems on Earth. [1] https://en.wikipedia.org/wiki/Spanner_(database) https://en.wikipedia.org/wiki/Spanner_(database)
  - EGreg 8y ago
    I think you misunderstood my point. I said that relativity is a special case of this principle that time is a local phenomenon.
    - danbruc 8y ago
      Which is not really true, time is frame depended, not location depended. Just take the twin paradox, when they start and later meet again, they are in the same place but different amounts of time have passed for each of them. The closest thing I can think of is that the ordering of two events is not ambiguous if they are timelike separated while for spacelike separated events you can find different reference frames which disagree on the order of the events.
      - EGreg 8y ago
        What you said doesn’t disprove what I said :)
        danbruc 8y ago
        Given that we have never actually defined what »time is a local phenomenon« is supposed to mean, that may or may not be true. But given that relativity puts time and space on a more equal footing than Newtonian mechanics, where you now can turn time into space and space into time, I remain pretty doubtful that something that can be summarized as »time is a local phenomenon« is likely to be interesting and correct because it seems to point in the direction of not treating time and space in a unified manner.
        EGreg 8y ago
        Listen, space is a local phenomenon too. It is just about correlations. A lot of the nonlocal effects in De Broglie Bohm theory become classical once you consider close-by objects and neighborhoods. They are just super correlated. Why? We don’t know. But with so many correlations, you can compare way more easily between them. https://www.wired.com/2016/01/quantum-links-in-time-and-space-may-form-the-universes-foundation https://www.wired.com/2016/01/quantum-links-in-time-and-spac...

EGreg 8y ago

You didn’t explain how a distributed byzantine system can synchronize a global time that’s continuous. I am saying that you won’t be able to — local time is continuous and you should use that for comparisons. It’s nowhere near as straightforward as you think to make global time work. You will find that “making sure nobody can or will cheat” will require exactly the kind of effort that will make the concept of a global timestamp meaningless. Also, why do you need a global timestamp in the first place? Upon further reflection you will probably realize that all your concepts of time actually require exactly what I said: two paths to the same local reference point, where a comparison is made. Your ideas about global time are an illusion based on simplifying assumptions that are not true.

danbruc 8y ago
I only object your claim that relativity - as in special or general theory of relativity - has any bearing on the design of distributed systems on Earth. In particular, you can order events occurring at different places if you have properly synchronized clocks. The fact that a common solution is to use logical clocks like Lamport clocks is because it is easier to do so than to maintain a network of precisely synchronized clocks. But that does not mean it can not be done with synchronized clocks, see for example Google's Spanner [1] database. I never addressed what else is necessary to make this work in the presence of Byzantine faults. But I am also pretty sure that just sending information about events from A and B to C and asking C about the ordering is not sufficient, after all C may be faulty and give a wrong answer, change its mind after every query, or just not respond at all. Admittedly you did not provide many details, so I may very well totally misunderstand what you had in mind. But, just to repeat, I am not really addressing the requirements of a Byzantine fault tolerant system, I am just saying that the special or general theory of relativity is not relevant for distributed systems on Earth. [1] https://en.wikipedia.org/wiki/Spanner_(database) https://en.wikipedia.org/wiki/Spanner_(database)
- EGreg 8y ago
  I think you misunderstood my point. I said that relativity is a special case of this principle that time is a local phenomenon.
  - danbruc 8y ago
    Which is not really true, time is frame depended, not location depended. Just take the twin paradox, when they start and later meet again, they are in the same place but different amounts of time have passed for each of them. The closest thing I can think of is that the ordering of two events is not ambiguous if they are timelike separated while for spacelike separated events you can find different reference frames which disagree on the order of the events.
    - EGreg 8y ago
      What you said doesn’t disprove what I said :)
      - danbruc 8y ago
        Given that we have never actually defined what »time is a local phenomenon« is supposed to mean, that may or may not be true. But given that relativity puts time and space on a more equal footing than Newtonian mechanics, where you now can turn time into space and space into time, I remain pretty doubtful that something that can be summarized as »time is a local phenomenon« is likely to be interesting and correct because it seems to point in the direction of not treating time and space in a unified manner.
        EGreg 8y ago
        Listen, space is a local phenomenon too. It is just about correlations. A lot of the nonlocal effects in De Broglie Bohm theory become classical once you consider close-by objects and neighborhoods. They are just super correlated. Why? We don’t know. But with so many correlations, you can compare way more easily between them. https://www.wired.com/2016/01/quantum-links-in-time-and-space-may-form-the-universes-foundation https://www.wired.com/2016/01/quantum-links-in-time-and-spac...