# Universe-wide time system

Is there a time system that two spacecraft traveling in different galaxies could share?

For example, a spacecraft jumps to an unknown location in the universe, far away. How could they synchronize? Or know the “universal time”.

For time measure there is something perfectly accurate such as a photon clock.

• A "perfectly" accurate reading of the CBR temperature might help imply what "time" it was. But... pretty tricky, eh? Commented Dec 28, 2014 at 1:45
• I assume you are imagining a setting with some kind of FTL travel? What is the mechanism for this? Commented Dec 29, 2014 at 10:44

First of all, simultaneity is an ill-defined concept in the modern understanding of the universe, so realistically, the question can be answered with a simple no.

Second of all, I'll assume we're talking different (and arbitrarily distant) galaxies in the same universe. When you bring in multiple universes, there's no good reason to assume the time dimension's arrow would be parallel and pointing in the same direction, so comparing times across universes wouldn't make much sense either (imagine one universe's time arrow as perpendicular to the other --- this actually happens inside event horizons of black holes.)

But let's try to avoid answering no, because that's boring, right? Let's imagine some partial workarounds. On earth in the distant past, sailors would determine their position with reference to the stars. In the universe, you can use the temperature of the Cosmic Microwave Background to know how long it's been since the Big Bang, since it will decrease as time passes and the universe expands. With sufficiently accurate measurement, you can get arbitrarily precise results (down to a certain limit).

Under a Newtonian framework, that would be a universal clock. However, in the real world, two observers using the CMB clock will assign perfectly reasonable times to a distant event (say observer A 'waving' from his galaxy to observer B on Earth), with the slight snafu that the times won't actually match.

• As long as there are no significant relativistic effects going on (high velocity or gravity), isn't it true that there is (practically) simultaneity? In your example of observer A 'waving' to observer B, let's say 2.3 million light years away, observer A records it at U (universal epoch)+0 years. Observer B sees it at what he locally records as U+2.3 million years, and so knows that it occurred at U+0 years. (you could use each galaxy's velocity relative to the CMB to account for the small relativistic effects) Commented Dec 28, 2014 at 3:46
• @TimS. The answer is 'yes (sort of), but you can't do anything interesting with it'. For anything at interstellar/intergalactic distances either it's A) so far away and so long ago that you can't meaningfully interact with it, or B) something's moving at relativistic speeds and 'simultaneous' is no longer a particularly useful concept. (And quite possibly both at once...) Commented Dec 28, 2014 at 4:56
• Additionally, A and B may experience different gravitational fields. Given that even the difference between the gravitation on the surface of the earth and at the height the GPS satellites makes clock adjustments essential, that will likely introduce some noticeable skew over 2.3 million years. (Plus, how does B know she’s 2.3 million light years from A in the first place?) Commented Dec 28, 2014 at 10:49
• @TimS. No, it's not exactly true. The Lorenz transform has an x term for distance in it, and when distances scale up to intergalactic distances, that becomes significant even at everyday speeds. See the famous Andromeda Paradox Commented Dec 28, 2014 at 12:40

With the theory of relativity currently being much depended on and not being proofed wrong, time is very subjective and depends on the condition of the observer. I suppose what you means is the approximate time that every stationary or slow-moving object tells you it is, perhaps a clock back on Earth. I believe your problem in the question is that photons travel at a fixed speed which seems insufficient for intergalactic synchronization purposes.

This superficially seems analogous to the time delay caused by low network speed when sending or synchronizing data(time, real-time computer game data, video calls, etc), called latency. As implied by the time precision needed by modern activity, time synchronization network protocols have been developed to synchronize time while minimizing latency issues, including the NTP and PTP. However, intergalactic round-trip time is pretty much unacceptable and impractical.

If light cannot solve the problem, it seems like the only way to solve it is the superluminal "spooky action at a distance" as described by Einstein -- quantum entanglement. Quantum teleportation has recently seen some success. Yet, this is just another disappointment: relativity doesn't say nothing can move faster than light. It says no mass or information can move faster than light. We can instantaneously teleport quantum states, but real communication would have to wait, for as least the duration light needs to reach the target.

The final answer, there is still no means of communication faster than light. Maybe warping spacetime might help? Anyway, if your story is Sci-Fi, you could just say that they can create ultra-small wormholes and send information over that. It would certainly drain lots of power though.

Well, still using the clock. I know this answer problably isn't good, but if Team 1 go to an galaxy and Team 2 go to another, is better they start to count the time of the trip, for example: "About an hour ago that we took off from Earth."

• Unless they go to the same speeds, clocks won't be in synch for much time. Commented Dec 27, 2014 at 17:34
• yeah, now you said everything :) Commented Dec 27, 2014 at 18:40

I might suggest something in the nature of quantum entanglement where in a central clock defines an arbitrary time that all dependencies then rely on with perhaps an offset like timezones based on distance from that clock at that time. Depending on the society at play it's not unrealistic to imagine a series of relays that are used to create a distant communication network that could also relay such information.

• How does entanglement provide anything to allow you to synchronize time? Commented Aug 9, 2016 at 19:04
• entanglement if it doesn't break in the process of FTL would remain synchronized across any distance and would do so both forward and backward through time, though it does hinge on whether or not it can withstand such travel. It's also a premise for intergalactic communication. Commented Aug 9, 2016 at 19:14
• Two observers check their particles and get a corelated result. It does not matter whether A or B looks first, or that whichbis first depends on the choice of reference frame. It doesn't matter if A waits 50 years, he'll get the same result. So how does that have anything to do with synchronizing? The concept is exactly the opposite, being outside of time. Commented Aug 9, 2016 at 19:18
• «It's also a premise for intergalactic communication.» no, entanglement does not allow for communication at all. You might look at some of the questions on Physics. Commented Aug 9, 2016 at 19:20
• imagine an array of entangled particles, one side is read only the other is write only. On the write side it's constantly changing in such a manner as to indicate time and the other understands how to interpret the array seeker.com/… Commented Aug 9, 2016 at 19:33