Any big brains out there know of a universal way to measure time?

Like let’s say we’ve made contact with an alien race and have figured out communications. We want to tell them a thing will happen at a specific time, but they have a different means of measurement that we can’t understand. Is there a universal constant way to measure time that doesn’t assume a standard planetary rotation, or the reactions of a specific atom to specific conditions? A constant regardless of environmental conditions that with the basic equipment of a reasonably techno-savvy civilization could easily be measured

*edit* Could the hydrogen line frequency be used as constance reference point perhaps?

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ElwoodBlues · M

The measured length of an interval of time, even using the same clocks, depends on the relative velocity of reference frames; that's part of what Einstein's special relativity is all about (it also depends on how deep you are in a gravity well, or how much acceleration you are undergoing; that's general relativity).

In short, time is not invariant, nor is length. However, there is a relativistic invariant, usually represented with an s:

s² = x² + y² + z² - C²t²

So if you and your aliens can both observe two events out there in the universe, and you can measure the time and distance between them, you have a universal ruler. Maybe there are two pulsars not too far apart you both can observe.

You can also both build identical clocks, say hydrogen masers, and measure the pulsar timings in your local frames of reference, and this will give you time correction factors. But be careful, as the Earth orbits the sun it's velocity relative to your aliens will change and thus the time correction factors will change.

Zaphod42 · 46-50, M

@ElwoodBlues All of those factors are the reason I’m trying to find a universal constant to gauge time with. But after looking through a few articles on Einstein and Hawking and their search for a universal constant of any kind I’m realizing a strong pulsar visible for several hundred light years is probably the best I’m going to get.
In my story the alien empire has endured for better than 1.3 million years. I’m assuming in that time they’ve advanced the science of physics a bit further than we have, and I was hoping to find a hypothetical universal constant that they could have based their own universal version of the metric system off of, including the passage of time.

ElwoodBlues · M

@Zaphod42 In SF you can invent whatever you like: extra dimensions in the universe; wormholes; a successor to quantum mechanics without the fundamental randomness; etc, and give that tech to your aliens.

But the apex of [b][i]human[/i][/b] knowledge about space & time is general relativity, and has been so for the past 100+ years. If you want to stay within those limits it's up to you.

BTW, I'm not sure the s measure is invariant under general relativity (my undergrad education only extends to special rel) but it shouldn't matter much if the pulsars are both observable.

Zaphod42 · 46-50, M

@ElwoodBlues Yeah, but I’m trying to take the approach of Jules Verne and the like…base the sci-fi on actual science rather than making up preposterousness that flies in the face of what we know.

ElwoodBlues · M

@Zaphod42 BTW, most pulsars are radio sources in the 300–2000 MHz frequency bands. Another interesting frequency band to know about is the "water hole,' 1420 to1662 MHz.

[quote] The frequency band between about 21 cm and 18 cm is known as the “Water hole”. It is an observation frequency widely used by radio telescopes in radio astronomy. Being included between the emission lines (absorption) of the gas HI and the OH group (abundant in the interstellar medium), the spectrum between these frequencies forms a sort of “silent” channel in the background of interstellar radio noise.
It has been speculated that these frequencies could be the most logical and natural choice for interstellar communications with extraterrestrial civilizations. [/quote]
[b]https://physicsopenlab.org/2020/09/08/milky-way-structure-detected-with-the-21-cm-neutral-hydrogen-emission/[/b]

Tastyfrzz · 61-69, M

@ElwoodBlues now I'm curious. Might there be more factors to that equation for other dimensions that we don't account for?

ElwoodBlues · M

@Tastyfrzz It's possible. There are "superstring" theories that posit 10 or 11 dimensions, but the others outside the familiar four are tightly curled, so not a possible direction of travel. General relativity says that gravity fields curve our four dimensional space-time continuum; do we need to posit another dimension in which to embed the curved 4D?? It's all above my pay grade!!

Tastyfrzz · 61-69, M

@ElwoodBlues Back when I was at cambridge in 1980. (Sounds impressive but we were there for a day for a math seminar) and there was this one physicist in the cafeteria under a table working out equations for 50 dimensions.