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Comment by ketanmaheshwari | Hacker News Reader

Would time even make any sense given that we use the very system to measure time.

In the sense that one second is 9,192,631,770 transitions between two energy levels of the caesium-133 atom, no, there's no change. 50 billion years is just shorthand for a multiple of this number of transitions. International Atomic Time, or TAI, is precisely defined for this kind of measurement.

Whether TAI is a fixed number of leap seconds away - potentially billions - from UTC, is another question entirely. If 'one day' is one rotation of the planet Earth around its axis, and 'one year' is one rotation around the Sun, and hours, minutes, and seconds are fractions of these rotations, yeah, those cease to have much meaning when the planet is swallowed up by and dissolved into the Red Giant Sun's expanding chromosphere.

Yes:

> The second is defined as being equal to the time duration of 9,19,26,31,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the fundamental unperturbed ground-state of the caesium-133 atom.

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

Unless the laws of physics aren't constant over very long time periods for some reason, we could (if we were around to do it) measure time 10^18 years from now just the same as we do today.

In Oklo, Gabon there was a natural nuclear fission reactor about 1.7 billion years ago. The concentration of uranium and groundwater was just right that nuclear fission occurred naturally.

We can measure the elements left behind and we have determined that the fine structure constant was the same then as it is now.

If the laws of physics have remained constant for that long then most scientists think they will remain constant in the future.

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

Oh, sure, I wasn't saying the laws of physics were likely to change, simply that if they did, the SI definition of time might not be well posed.

That said, although I agree the laws of physics are likely to remain constant, 1.7 billion years is only just about 12% of the age of the Universe. And, that time period only reflects the most recent 1.7 billion years. So, there's still some theoretical wiggle room there for the fine structure constant to vary over time. After all, heat death is not expected to occur for much more 100 trillion years (if that is our ultimate fate)! [0]

Edit: Come to think of it, if the laws of physics aren't constant, how would it be possible to pose a coherent definition of a unit of time at all?

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[0]: https://en.wikipedia.org/wiki/Future_of_an_expanding_univers...

We stopped using the solar system to measure time a while ago, and now use atomic clocks. Civil time is periodically adjusted to match the rotation of the Earth, but the amount of adjustment necessary is measured using atomic clocks, and atomic clocks are still the basis of civil time.

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