[mikewarot]

If you have a server in Chicago, and another in Denver, news of any event takes about 0.005 seconds to reach the other. Thus causality between the nodes depends on the location of the observer. If an observer half way between sees simultaneous events, both ends will see the other event happening after the local event.

Thus, physical clocks, no matter how well synchronized, will never agree everywhere.

[judofyr]

The happens-before relation that they use in this article (and in the whole field) does take this into account. It’s only defined in terms of local events (i.e on the same node) and the time when messages are sent/received between nodes.

A perfectly synchronized physical clock totally works here.

[mikewarot]

In physics, perfect clocks, can only perfectly synchronized for a moment of time, from an arbitrary observation point. They will always drift apart, because no two points in the universe have the exact same stress energy tensor across time.

Clocks have improved to the point where a 2 centimeter difference in height in the same room results in an observable difference in time. If you take into effect the gravity of the Sun and Moon, no two points on earth experience the same amount of gravity over time.

Now, if you have a logical clock, you might be able to pull consistency out of this, but the real world doesn't work that way, unless you lower the observation quanta to hours, minutes, seconds, on Earth only, and use UTC which isn't consistent across time.