4 ms·

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).

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 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?"

tlb 8y ago
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.
teraflop 8y ago
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-dilation-between-mars-and-earth-due-to-different-mass https://space.stackexchange.com/questions/33590/time-dilatio...
- wlesieutre 8y ago
  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 8y ago
    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 8y ago
>"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#Precise_clocks https://en.wikipedia.org/wiki/Pulsar_clock https://en.wikipedia.org/wiki/Pulsar_clock
CydeWeys 8y ago
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 8y ago
  Consistently defined time is less useful when events at time 12:04 cause events at time 12:02
  - CydeWeys 8y ago
    Can you give a concrete example of how that might happen?
    - CydeWeys 8y ago
      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.
blattimwind 8y ago
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.

tlb 8y ago

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.

teraflop 8y ago

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-dilation-between-mars-and-earth-due-to-different-mass https://space.stackexchange.com/questions/33590/time-dilatio...

wlesieutre 8y ago
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 8y ago
  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 8y ago

>"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#Precise_clocks https://en.wikipedia.org/wiki/Pulsar_clock https://en.wikipedia.org/wiki/Pulsar_clock

CydeWeys 8y ago

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 8y ago
Consistently defined time is less useful when events at time 12:04 cause events at time 12:02
- CydeWeys 8y ago
  Can you give a concrete example of how that might happen?
  - CydeWeys 8y ago
    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.

blattimwind 8y ago

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.