5 ms·

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

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.

tomp 8y ago
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 8y ago
  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-time-element https://www.w3.org/TR/html52/textlevel-semantics.html#the-ti...
atoav 8y ago
Relaticistic effects mean you have to resync clocks anyways, just like satelites in orbit need to do
hermitdev 8y ago
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 8y ago
  Really? Can't you just anchor to e.g. one revolution of the planet? That should be synchronizable...
  - gmueckl 8y ago
    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 8y ago
      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 8y ago
        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 8y ago
    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 8y ago
wlesieutre 8y ago
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

tomp 8y ago

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 8y ago
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-time-element https://www.w3.org/TR/html52/textlevel-semantics.html#the-ti...

atoav 8y ago

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

hermitdev 8y ago

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 8y ago
Really? Can't you just anchor to e.g. one revolution of the planet? That should be synchronizable...
- gmueckl 8y ago
  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 8y ago
    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 8y ago
      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 8y ago
  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 8y ago

wlesieutre 8y ago

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