Galactic timekeeping

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sevenperforce
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Galactic timekeeping

Postby sevenperforce » Fri Jun 12, 2015 1:27 pm UTC

Suppose you're a nearly-immortal alien with no home planet who flits around from galaxy to galaxy at will but prefers to kick it in the Milky Way. You've lived for roughly eighteen million years and you're still pretty young.

What timescale would you use to describe, well, time? There are painfully few natural units of time. Here's the sentence I want to change:

"The last time I came this close to death was...err...about eight million years ago."

But you would never say this, because you aren't going to use Earth orbits to define the passing of time. Would you use element half-lives? Would you use some sort of galactic orbit progression...like, "thirteen-degrees-around-the-galaxy ago"? Of course, that doesn't much work either, because that's eight million years ago for the sun, but not for other systems.

Could you use something independent of galaxies, like the redshifting of the CMB? I don't know if the CMB has redshifted significantly in the past 80,000 centuries or not.

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Re: Galactic timekeeping

Postby elasto » Fri Jun 12, 2015 1:37 pm UTC

What about the most accurate way we currently have of measuring the passage of time - the oscillation of atoms?

The most accurate clock ever built only loses one second every 15 billion years

"The last time I came this close to death was...err...about 1029 strontium ticks ago."

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Quizatzhaderac
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Re: Galactic timekeeping

Postby Quizatzhaderac » Fri Jun 12, 2015 3:29 pm UTC

Time is a relationship of events, so I think the question is what events are significant to your characters? With humans, we have enough common ground to make guesses about what happens to one in a year, even regarding things not tied to the Earth's rotation.

For example, if the character spent the last 79,999,999.999 years in cryogenic suspension, the last time they came so close to death might have been eating a microwaved burrito with cold spots in it.

So, supposing your character's deal is that they visit new civilization, spout some gnomic advise and heads off, with the trips between being perfectly boring. They could say: "Last time I was so close to death was 15,000 visitations ago". That gives context of how unusually dangerous this visitation is.
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Re: Galactic timekeeping

Postby Nicias » Fri Jun 12, 2015 3:35 pm UTC

If you want to measure proper time, something like strontium ticks, but probably something about hydrogen, more natural choice.

If you want to measure comoving time, you could use the negative natural log of the CMB in some natural units. (Plank for instance) That puts today's time at about 73.03.

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Re: Galactic timekeeping

Postby sevenperforce » Fri Jun 12, 2015 3:49 pm UTC

elasto wrote:What about the most accurate way we currently have of measuring the passage of time - the oscillation of atoms?

The most accurate clock ever built only loses one second every 15 billion years

"The last time I came this close to death was...err...about 1029 strontium ticks ago."

Yes, that works, but it's not terribly conversational. Especially considering that 1028 strontium ticks is barely over 700,000 years and 1030 strontium ticks is 74 million years; that's a pretty wide range.

Order-of-magnitude numbers don't do well in conversation, both because of a lack of precision and because of a lack of scale. I prefer giving most human-scale distances in feet rather than in yards or inches because I can communicate a very reasonably accurate value using single or double-digit whole natural numbers.

Of course, an alien might have a different approach to communicating quantities, like using nested exponentiation to give a more specific value using only a handful of whole numbers. Eight million years is 4.682825e57 Planck times. Nesting up to four exponentiated single digits of Planck time allows you to represent:

  • 1,340,452 years (8^9^7 tP)
  • 3,069,960 years (9^2^5^6 tP)
  • 10,723,616 years (8^8^8 tP), or
  • 39,257,792 years (6^5^3^5 tP)
That's not as precise as I'd like...it's still a significant jump between each point...but it's better than using orders of magnitude. Ideally, the error would be on the order of 10% of the character's age, or 1-2 million years. I could go up to five exponentiated single digits to improve precision but I dunno how happy Excel would be with me.

Obviously, humans wouldn't have the capacity for the mental math necessary to communicate numbers using this notation in ordinary conversation, but an immensely intelligent alien might find it trivial.

Quizatzhaderac wrote:Time is a relationship of events, so I think the question is what events are significant to your characters? With humans, we have enough common ground to make guesses about what happens to one in a year, even regarding things not tied to the Earth's rotation.

So, supposing your character's deal is that they visit new civilization, spout some gnomic advise and heads off, with the trips between being perfectly boring. They could say: "Last time I was so close to death was 15,000 visitations ago". That gives context of how unusually dangerous this visitation is.

Yeah, this is kind of what I'm getting at. Quoting a time is meaningless unless it has some significance for the person quoting it. Your example is good, except that this being has the capacity for instantaneous intergalactic travel and so the "trips between" don't really exist at all.

When we talk about the events of our life, we're often quoting times in comparison to our current age, whether we realize it or not. When an octogenarian talks about the events "in my twenties" he's implicitly saying, "when I was about quarter of my current age." So for the given example, our intrepid alien is really saying something like, "I haven't come this close to death since I was 56% of my current age, though that one business when I was 72% of my current age was a close call too."

But since quoting "percentage-of-my-age" is awkward, what's a naturally significant time duration which would meaningfully partition the lifespan of an 18 million year old alien? We use Type 1a supernovae as a standard candle to judge distance across galaxies, but I can't think of any intergalactically significant natural processes with a predictably consistent duration of 0.6-2.0 million years.

EDIT: Here are some possible processes...

  • The carbon-burning process in large stars only lasts a short time -- between 400 and 800 years, if I recall correctly.
  • The neon-burning process is more precise, but only lasts a few years.
  • If there are any stellar life stages which predictably take between 500,000 and 2 million years, that could be useful.
  • If there is a readily apparent transition in the afterglow of a Type 1a supernova around a million years after it explodes, that could be equally useful.
  • What's the minimum lifetime of a supermassive star at the Eddington limit? A star of 60 solar masses supposedly has a mean lifetime on the order of 3 million years.
  • Gamma-ray bursts only happen a few times per million years in any given galaxy, and this rate is thought to be roughly consistent in all reasonably-sized spiral galaxies. Maybe something could be made of that.
  • A single step of the proton-proton chain in stellar fusion is reliably around 1 million years, I think, though that might not be consistent for anything other than solar-mass stars.
  • Protostars undergo accretion leading up to the ignition of fusion at their core, and this accretion continues until the heat from fusion makes its way out to the outer envelope, forming a stellar wind. That stellar wind prevents further accretion and the star is "born". I'm guessing the timescale between the conception of a star (fusion ignition) and the star's true birth (a stellar wind) is pretty consistent regardless of the star's mass. Could this be on the order of a million years?

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Re: Galactic timekeeping

Postby Quizatzhaderac » Fri Jun 12, 2015 8:48 pm UTC

When the a train conductor says "The train leaves in two hours forty five minutes", they're making the following statement about the listener:
"I have little idea what the time between now and the train leaving will be like for you, but I can tell you what would happen if you spent it watching a clock."
The listener is left to convert the common reference to one they find meaningful.

The listener's spouse might instead say "Plenty of time for lunch before the train comes.".

I'd argue that the spouse's way is better from a literary perspective; it provides more context, and less irrelevant details.

If the speaker is actually making a statement about itself, there's no need for the listener to be able to convert between events in the speaker's life and the their own. If you feel like the listener or readers of your text would still want to tie it back to their own context, the listener could actually ask for clarification. If the speaker's actually directly defining a term, saying something like "it's three point oh eight one times ten to the sixty-one planck times" sounds less contrived.

sevenperforce wrote:But since quoting "percentage-of-my-age" is awkward
How about "a half lifetime ago"?
Last edited by Quizatzhaderac on Wed Dec 02, 2015 10:37 pm UTC, edited 2 times in total.
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Re: Galactic timekeeping

Postby sevenperforce » Fri Jun 12, 2015 9:02 pm UTC

Quizatzhaderac wrote:If the speakers is actually making a statement about itself, there's no need for the listener to be able to convert between events in the speaker's life and the their own. If you feel like the listener or readers of your text would still want to tie it back to their own context, the listener could actually ask for clarification. If the speaker's actually directly defining a term, saying something like "it's three point oh eight one times ten to the sixty-one planck times" sounds less contrived.

The speaker is talking to himself, but the information is being provided for the reader's benefit. So the value should be something significant to the speaker, but calculable by the reader.

sevenperforce wrote:But since quoting "percentage-of-my-age" is awkward
How about "a half lifetime ago"?

Good, but not great, because this alien doesn't actually know what his lifetime is supposed to be. He knows how old he is, but that's not a "lifetime". If I say "half a lifetime ago" I'm not talking about an event that happened when I was half my current age; I'm talking about an event 40-50 years in the past, because my species has a lifetime of 80-100 years.

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Re: Galactic timekeeping

Postby Wildcard » Sat Jun 13, 2015 5:48 am UTC

I wouldn't bother to measure time directly. Make reference to other things that change, with which the reader has familiarity. More poetical, sure, but plenty scientific if you want to get specific enough, and much less contrived in my opinion.

"The last time I came this near to death, your planet's sun had just formed." Only problem with that is it's several orders of magnitude longer than what you want.

If your alien has any knowledge of earth's history, he could say it way right when XYZ geologic event happened.

...

To hell with it. Write this:
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Re: Galactic timekeeping

Postby Xanthir » Mon Jun 15, 2015 10:37 pm UTC

But make sure to include the hyperlinks in the book, or else the readers will be confused.
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Re: Galactic timekeeping

Postby sevenperforce » Tue Jun 16, 2015 6:41 pm UTC

Is the average random walk time for a stellar photon pretty consistent across star classes?

There's got to be some intergalactic process which consistently takes about a million years.

We use Type 1a supernovae as standard candles, so it's likely that any alien species would do the same. The Chandrasekhar limit is one of only a handful of macroscopic physical values which is going to be identical in every galaxy. Given this consistency, perhaps there's something about the timescale of a Type 1a supernova which would give rise to a long-term intergalactic timekeeping standard.

It takes about 30,000 years for a supernova shockwave to merge with the surrounding galactic medium. Might be a useful value. "I haven't come this close to death for more than 270 supernova-shockwave-deceleration-periods!" Although we'd need a better term, like "flare-lives" or something. "The flare of a Type 1a supernova consistently takes 1.76e55 Planck times to die out, so we measure the passage of time in flare-lives."

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Re: Galactic timekeeping

Postby Tyndmyr » Wed Jul 01, 2015 5:22 pm UTC

Pick an arbitrary point in spacetime, much like GMT is for humans. Pick an arbitrary way of describing time elapsed. Hours, minutes, etc will suffice, but it's mostly arbitrary as well. Consistency is what matters.

Then, adjusting for relative motion w regards to that point, you can have consistent time everywhere. It's not really a big deal. Atomic clocks will suffice for most timekeeping needs.

No doubt larger units than years would enter into common usage if lifespans were sufficiently long. Decade, century are already frequently used. No doubt we'd be casually throwing around slang for billions of years, if that were a routinely experienced length of time.

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Re: Galactic timekeeping

Postby Whizbang » Wed Jul 01, 2015 6:21 pm UTC

The amount of time light takes to go a certain distance.

Since the traveler prefers the Milky Way, he could base his time units on how long it takes for light to travel from one end of the galaxy to the other. The MW galaxy is 100,000 light years across, therefore 8 million years is 80 Galactic Time Units. Modify to the desired level of preciseness.

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Re: Galactic timekeeping

Postby gmalivuk » Thu Jul 02, 2015 2:58 pm UTC

We'd want something more precise than that, though (I mean, it'd still be about 80, but for other purposes we'd need better precision). Which star on one side to which star on the other side? Those stars at what time (since they're all moving)?
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Re: Galactic timekeeping

Postby sevenperforce » Tue Jul 07, 2015 2:23 pm UTC

One possible "boundary" for a galaxy would be the inflection point of its gravitational well with respect to the intergalactic medium...though that's probably a lot farther out than 50,000 ly.

The spiral arms of most galaxies are logarithmic spirals. One could use the best-fit logarithmic spiral to produce an idealized galactic shape with a definable diameter. For a barred spiral galaxy, one could use the axis of the central bar and then define the outer boundary as the point at which the idealized spirals make a full loop around that axis:

fing.png


Works well enough, don't you think?

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Re: Galactic timekeeping

Postby krogoth » Fri Jul 24, 2015 4:39 am UTC

The alternative is to leave it ambiguous in time, but leave a reference to an event, 'the last time I was this close to death I fleeing from Kepler's Supernova' or some war or other large event, maybe "that time i accidentally created a black hole, while delivering pizzas' It really depends a lot on a way the story is progressing,
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Re: Galactic timekeeping

Postby sevenperforce » Fri Jul 24, 2015 1:33 pm UTC

I ended up using "starbirth" with a footnote indicating that a starbirth is the period of time it takes for a new star to reach hydrostatic equilibrium after the initial ignition of fusion in its core. So it's natural enough for the character to say -- "the last time this happened was seven starbirths ago" -- and the enterprising reader will be able to figure it out and thus glean that the character is talking about a duration of time on the order of seven million years.

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Re: Galactic timekeeping

Postby Iv » Thu Aug 20, 2015 5:38 am UTC

Whizbang wrote:The amount of time light takes to go a certain distance.

Since the traveler prefers the Milky Way, he could base his time units on how long it takes for light to travel from one end of the galaxy to the other. The MW galaxy is 100,000 light years across, therefore 8 million years is 80 Galactic Time Units. Modify to the desired level of preciseness.

That feels like the most natural one. I would call that "galactic echoes" or "galactic resonances". Of course the Milky Way is not precisely measured but just assume that the entity chose an arbitrary fixed distance for precision purposes. Or invent the fact that there actually is a peripheral celestial body that echoes events in the central black hole and use this distance to the core as the basis of your echo.

Note as well that galaxies rotate along their axis. The period is around 250 million years. Probably too big for you, but you can divide it in sections to measure time. Just divide it by 24 (cause why not?) and call 10 million years a galactic hour.

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Re: Galactic timekeeping

Postby Beavertails » Wed Dec 02, 2015 8:17 pm UTC

I'd use Kessel Runs (KR) as the unit of measurement.

Each of those is worth <12 parsecs.

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Re: Galactic timekeeping

Postby jewish_scientist » Fri Jan 01, 2016 2:07 am UTC

You could use the time it takes light to travel between the two farthest apart stars in the Milky Way as your standard unit. I cannot imagine that the relative position of them could change that much over the course of a few million years because the Sun rotates around the Milky Way once every 230 million years. No matter what you do, you may want to put a footnote that gives the time in years.

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Re: Galactic timekeeping

Postby ijuin » Mon Jan 04, 2016 5:35 pm UTC

Meh, let's forget trying to use celestial events and instead use the one truly unambiguous measure of time--radioactive decay rates.

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Re: Galactic timekeeping

Postby Sandor » Fri Jan 08, 2016 1:22 pm UTC

ijuin wrote:Meh, let's forget trying to use celestial events and instead use the one truly unambiguous measure of time--radioactive decay rates.

Indeed. Just pick a convenient isotope from here. Or make it that your nearly-immortal alien just assumes that everyone knows the half-lives of every isotope, and just picks the most convenient for whatever time period it happens to be talking about at the time. You would probably have to turn that Wikipedia page into an appendix for the convenience of your human readers.


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