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Is there a universal time (duration) reference, an intuitive focal point that works for long periods (compared to the Planck time), and can be used as a signal of intelligence?

Assuming a Kardashev Type III-level civilization can construct its own pulsar, is there a rotation period with which it could signal that it was artificially constructed?

While the second is now defined as the universally "reconstructible":

The second, symbol s, is the SI unit of time. It 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 9 192 631 770 when expressed in the unit Hz, which is equal to s−1.

it depends on knowledge that is extremely difficult to come up with independently given only observations.

Self-synchronizing codes and many kinds of nonlinear modulation schemes exist, but these are likely pretty hard to accomplish on a star-scale.

What I've seen being proposed so far:

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    $\begingroup$ I believe this question is valid on the Astronomy SE forum as it could be used to inform SETI investigation strategies. $\endgroup$
    – Connor Garcia
    Commented Nov 2, 2022 at 16:37
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    $\begingroup$ How about Pi MHz? $\endgroup$
    – uhoh
    Commented Nov 2, 2022 at 21:58
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    $\begingroup$ @uhoh Pi is good, but MegaHertz depends on the definition of the second $\endgroup$
    – 2080
    Commented Nov 2, 2022 at 21:59
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    $\begingroup$ @2080 it's retro humor :-) $\endgroup$
    – uhoh
    Commented Nov 2, 2022 at 22:00
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    $\begingroup$ SETI searches could settle for a unit characteristic of this galaxy rather than the Universe, but even then it's not obvious there's one other civilizations would recognize. For example, we can't use "one galactic year", as it only works for stars on our galactic gravity equipotential. (It's probably too long for your purposes, too.) $\endgroup$
    – J.G.
    Commented Nov 2, 2022 at 22:39

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If you can construct one, you can construct two. Make the period of one π times the period of the other. That could never happen naturally.

This neglects the fact that pulsar periods change with time.

It also neglects the fact that this question is less about astronomy and more about worldbuilding.

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    $\begingroup$ A similar approach has been discussed in the paper: Mathematical encoding within multi-resonant planetary systems as SETI beacons arxiv.org/pdf/2204.14259.pdf $\endgroup$
    – 2080
    Commented Nov 2, 2022 at 17:39
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    $\begingroup$ More about both literary and physical world building. $\endgroup$ Commented Nov 3, 2022 at 20:29
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I don't think we would want to derive time units from a standard that changes over time. So, the rotation rate of unusual objects would not be an ideal choice. Nor would the peak power frequency of the CMB.

Recognizing this, we (humans) use half-life times or transition frequencies to tie our time standards to consistent physical phenomena.

Which transition frequency would it make sense to derive a standard time unit from? The most prevalent element in the Universe is hydrogen. It makes sense to tie a time standard to the frequency of an atomic hydrogen state transition like the 21cm hydrogen line or the $\text{Lyman}-\alpha$ transition.

Our number system is base $10$, but I think a base $2$ would be more easily universally recognized. So, if a civilization wanted to choose a pulsar rotation based on the 21cm hydrogen line, they would choose it to be some $2^n \times 1420.40575177 \text{MHz}$. Something similar was done on the Voyager mission plates to express a universal time constant.

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    $\begingroup$ I would recommend something other than a integer multiple harmonic. Instead of 2, "How about pi?" (from here) $\endgroup$
    – uhoh
    Commented Nov 2, 2022 at 23:31
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    $\begingroup$ @uhoh So, a nanosecond is $10^{-9}$ seconds. I propose using similar multipliers to differentiate large values of universal time units, but with $2^{-30}$ (for example) for the same order of magnitude differences. If we were broadcasting a signal to alert other civilizations of our existence, and that signal was close to actual hydrogen emissions, I agree it might be good to multiply by $\pi$ so our signal couldn't be mistaken as natural. However, in general I think it would be fine to use the unit in some factor of 2^n. For example: It's my birthday, I am 35 mega-uhohs old today! $\endgroup$
    – Connor Garcia
    Commented Nov 3, 2022 at 0:28
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A pulsar will normally slow down as it ages.

A sure sign of an artificial pulsar would be one that doesn't slow down.

Now, you said Kardashev type III, that is a civilization capable of capturing all the energy from a galaxy.

With that kind of power, you don't need to fool around with pulsars. Just blink the entire galaxy.

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