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Comment by aerophilic | Hacker News Reader

This is interesting… and I wonder how this plays out across different planetary bodies. It effectively means how we define a second will be different across the board.

Can anyone weigh in on whether the MKS standard of a second is based on something in free space, or here on earth?

If universal (based on free space) Would this mean we should all convert to/from this second, and treat our own second as relative?

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It doesn't really work like that: a second is a second anywhere in the universe, but time at different places in the universe doesn't progress at the same rate.

ie. the issue is not with how we define the second. The issue is expecting that if two people in different frames of reference wait for one second, that they will continue to be "synchronised".

In fact it's worse than that, it's impossible to even define the concept of "synchronisation" for entities in different locations. You can only know that two things are synchronised if they are in the same location.

If the two entities are in the same frame of reference (ie. not moving or accelerating relative to each other) then you could argue that you just send a beam of light there and back, and assume the light takes the same time in each direction in order to synchronise... But we don't even know that light travels at the same speed in all directions! We've never been able to measure the "one-directional" speed of light, we can only measure the round-trip, so for all we know light could have a preferred direction in the universe and travel faster in that direction, as long as it is the same amount slower in the other direction. We would never be able to tell.

"The second [...] is defined by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.[1]"

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

so unless caesium behaves differently at different locations in the universe, this definition should be pretty uniform.

Cesium on the moon would continue to have the same frequency on the moon, but a cesium clock on the moon would run 56 microseconds fast per day relative to an identical clock on earth, because time itself moves differently.

So I'm not an expert, but this behavior actually would behave differently in different gravity wells. Due to relativity, while you would observe a cesium clock working normally on the moon, your Earth buddy would disagree with your clock because the buddy would be down in a larger gravity well.

Sortof. It would behave the same to the observer sitting next to the atomic clock no matter their local frame of reference. The point though is that we have atomic clocks on both Earth and (soon) the Moon, so we need some sort of standard for coordinating between them.

That's the whole point, it does. Or, said differently, it behaves the same in whatever reference frame it's in, but observers in other reference frames can observe it being slower or faster.

Here's an example with very elementary particles, muons:

https://phys.libretexts.org/Bookshelves/University_Physics/B...

Not big deal, we already correct for this problem in GPS

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