[dahfizz]

I don't really see the problem with standardizing on 'seconds since the epoch' for any interplanetary communication and having clients convert to something more meaningful to humans. Any other "universal" time format will be just as meaningless when days and seasons are all different lengths.

[wlesieutre]

At a certain point of accuracy you run into issues with this - seconds don't pass at the same rate due to relativistic effects (both speed and gravitational).

An example we already deal with is GPS satellites orbiting Earth. Their clocks tick a little bit slower on account of how fast they're orbiting, and tick a bit faster on account of being further up Earth's gravity well. The gravitational effect is stronger, and the net effect is that a GPS clock advances an extra 38 microseconds over the course of a day (as measured by a clock on the ground).

http://physicscentral.com/explore/writers/will.cfm

So when you try to standardize on "seconds since the epoch," the inevitable question is "Seconds according to who?"

[teraflop]

I don't think the relativistic effects would be a problem in practice. You can synchronize clocks on other planets to the UTC or TAI reference time, which are already specified in terms of the Earth's reference frame.

Because of relativity, clocks on other planets would very slowly drift relative to UTC on Earth. But the drift is on the order of a few parts per billion (see e.g. [1]) which is comparable to what you'd expect anyway, even from a very high-quality temperature-controlled crystal oscillator. So it doesn't add any new clock synchronization difficulties that you wouldn't have anyway.

For the most demanding applications -- the ones that require atomic clocks -- you would still need to take relativity into account. You can either measure time passing at the local rate (in cases where you need to know locally elapsed time to high precision) or you can measure UTC, which allows you to assign a consistent ordering to events on different planets. But for most ordinary purposes, the distinction is irrelevant.

[1]: https://space.stackexchange.com/questions/33590/time-dilatio...

[wlesieutre]

If I did my math right that's 0.178 seconds per year. As long as everyone's handling time the same way you shouldn't have a problem, but it's enough to be noticeable if someone forgets about it.

I suppose the more noticeable oddness for most people would be that the speed-of-light delay in communications from Earth to Mars varies so much. Around 3 light minutes up to 22 depending on where they are in their orbits.

[Rebelgecko]

That's around what I got from my back of the envelope Math too (well, I got around 300ms/year). Small enough where you can adjust a leap second everyone few years to take care of the drift. Kind of like converting between UTC, TAI, and UT1.

It may need to be compensated for when receiving transmissions between the planets, but I don't know enough about RF to judge how meaningful the difference is. There was a spacecraft (I think Huygens?) that actually had problems with Doppler shift. I think in that case they forgot to take it into account entirely, so it wasn't that they just forgot the relativistic component.

[swebs]

>"Seconds according to who?"

According to a very accurate pulsar, of course.

https://en.wikipedia.org/wiki/Pulsar#Precise_clocks

https://en.wikipedia.org/wiki/Pulsar_clock

[CydeWeys]

GPS satellites already control for relativistic effects. Their required accuracy is so high that they wouldn't work without doing so.

You would simply also correct for relativistic effects caused by the orbits of different planets, and boom, you have a pretty consistently defined time that is valid across the entire solar system.

[hopler]

Consistently defined time is less useful when events at time 12:04 cause events at time 12:02

[CydeWeys]

Can you give a concrete example of how that might happen?

[CydeWeys]

The only thing I can think of involves truly relativistic speeds, which just isn't going to happen for human travel anytime soon, if ever, and certainly not within the solar system.

[tlb]

Seconds according to the atomic clocks at NIST. People operating clocks moving at relativistic speeds will apply the necessary corrections, as GPS satellites already do.

[blattimwind]

Note that GPS compensates for relativistic effects to allow GPS receivers to act as very accurate clocks (GPS time receivers are considered stratum 0 clocks). If you didn't care about using GPS time receivers you wouldn't have to care about relativistic effects with GPS at all (because all satellites are more-or-less exposed to the same relativistic effects), because a 3D/4D fix already synchronizes the receiver to the satellite's clocks.

Relativity compensation in GPS satellites does not increase spatial accuracy.

[hermitdev]

The problem comes in due to relativity. Mars and Earth (and other planetoids) travel at different speeds. The very length of a second will drift over time.

[tomp]

Really? Can't you just anchor to e.g. one revolution of the planet? That should be synchronizable...

[gmueckl]

No. You would have to observe the planet from your position and that observation changes based on how you move relative to the planet. The whole of SRT and GRT work in such a way that no general clock synchronization can ever exist for observers in different frames of reference.

If you take a reference with you on a journey through the solar system, that reference will stay accurate for an observer travelling with it and drift as seen from an observer that stayed behind. Both views are accurate. Locally, the amount of time that has passed for each observer was different. If you would correct for any observed drift, you would mess with your fundamental unit of time in your local frame of reference and get incorrect measures of time passed in your own frame.

[tomp]

What I had in mind is, you can observe your planet's position and velocity (relative e.g. to center/rotation of the galaxy), and the target planet's relative position and velocity, and use both to translate the observations into virtual "stationary" observations... And "unit of time" would be defined as 1/n of one planetary/galactic revolution, so every observer would be able to calculate their own "galactic time" independently. Well, maybe GPS already does something similar.

[gmueckl]

This would not work as what you have then is not a fixed unit, but something that keeps forever changing. And that is useless in metrology.

[hermitdev]

No, you can't. Atomic clocks measure the oscillation of atoms (I think Cesium is currently used), and though constant at a fixed point, vary when theyre moving at different speeds. GPS wouldn't be possible without taking this into account, for example. A second on earth will never be the same as a second on Mars.

[tomp]

A second is duration, but we're really interested in instants. Why can't you can define an instant independently of the observers, e.g. in reference to the rotation of the galaxy (i.e. "this happened at angle 37 from Milky Way - Andromeda line, at distance 403 parsecs from Milky Way center, at 692.3 Milky Way rotations since epoch (Big Bang)").

[hopler]

Distance depends on how fast you are moving. As do the endpoints of a rotation.

[tomp]

Yeah I don't entirely understand why we don't that already... Like, websites should be sending UNIX time to clients, and then the browser should display it in users local timezone/format.

[icebraining]

HTML already has a <time> tag, which one can use to send a machine readable representation of the textual content; it would be easy for browsers to display it in local tz/format. Of course, almost nobody uses it, as usual.

https://www.w3.org/TR/html52/textlevel-semantics.html#the-ti...

[atoav]

Relaticistic effects mean you have to resync clocks anyways, just like satelites in orbit need to do