[DannyB2]

According to relativity, you cannot get two different observers to agree on any kind of universal time, or even the order in which two events occurred.

Observer A says that event X happened before Y.

Observer B says that event Y happened before X.

And they're both right from their POV.

[duckerude]

My understanding is that it's just that the order can depend on the frame of reference. If you agree on a frame of reference then you can agree on the time. If you both try to use your own frame of reference then you run into trouble.

There's no preferred frame of reference, meaning that any frame of reference could work, but you can derive one from the cosmic microwave background if you don't like picking something too arbitrary.

[AngryData]

Couldn't you just pick a pulsar or something, all agree to take their measurements from X distance from pulsar, and use that as the standard? It would be time + location, making your local time depending on the distance and number of pulses from the 0 starting pointing. As long as you kept account of the number of pulses as you moved through space, you could use that as a standard, and whenever you met up somewhere else in the universe, you could base your time based on their observed number of pulses.

Im not sure pulsars are actually that precise and predictable to not require regular adjustments to equate local times, but it would provide a standard anybody could utilize as long as they could keep an eye on it.

Of course it would break down if someone broke light speed, either with wormholes or some unknown technology, but as long as you are below lightspeed then there shouldn't be any problems.

[Varcht]

Pulsar seemed like a good idea but seems it has some difficulties.

Due to their small size, pulsars are relatively weak radio sources. Therefore, the largest radio telescopes in the world are usually needed to observe them. As we have seen, pulsars emit their largest intensity at low radio frequencies around 400 MHz. In particular at such frequencies, however, the pulses suffer from propagation effects when they travel to Earth through the interstellar medium. [0]

[0http://www.jb.man.ac.uk/distance/frontiers/pulsars/section4....

[greiskul]

You are correct, but thankfully relativity tells us that in this case both events are independent, so neither can be the cause of the other, so it's not that bad of a disagreement.

It's even useful as a consistency model for some distributed systems.

[walls]

Would it be theoretically possible to have a time based on the universes rate of expansion?

[yingw787]

Hmm...would this only be relevant on the scale of nano/picoseconds, or seconds or higher granularity, given a frequent synchronization schedule?

[Symmetry]

The further apart the observers are and the larger their relative velocities the larger the effect is. There's nothing to stop the differential from being years in extreme cases.

[simcop2387]

For any speeds we'd be likely to be traveling, in system, it'd likely only be a problem for milliseconds if that. We'd still be able to correct for where things are as we approach them or plan to approach them.

[yingw787]

Yeah, I was thinking only of in the Solar System. Beyond that, the primary bound on commercializeable space is probably spacecraft propulsion, which will take a while.

[z3t4]

Can't it just be solved with a equation ? If you know how fast time runs relative to yours !?