Hey Crazies.
Cosmic time is measured in billions of years.
That’s thousands of millions.
What? Huh? What?!
When we hear words like million or billion, all we really hear is big.
So let’s get a better perspective with a different unit of time:
the galactic year.
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What’s a galactic year?
The time it takes the Sun to orbit the galaxy.
It’s about 230 million years.
Can we just make up a unit like that?
Well yeah. All units are made up.
Alright, let’s start with some basics.
Any measurement has two required parts:
A number and a unit.
You need both or it’s not a measurement.
And don’t give me any of that “the unit is implied” non-sense.
Anyway, a measurement is whatever a measurement is.
The distance between the Sun and Earth is an average of about 93 million miles.
Miles is the unit and 93 million is the number,
but it doesn’t matter what number and unit we assign to this distance.
I could just as easily say this is 150 million kilometers
or 150 gigameters.
None of that actually changes the distance between the Earth and the Sun.
The point is the unit choice is up to us, so we might as well chose one that’s convenient.
Since the measurement is whatever it is, using a larger unit means we get a smaller number.
So let’s make the unit here really big.
We can call this distance 1 astronomical unit or A.U.
We can just do that?
Oh yeah, for sure.
All units are made up.
Calling the Earth-Sun distance 1 A.U. is convenient for distances within our solar system.
Mars is 1.5 A.U. from the Sun.
Jupiter? 5.2 A.U.
Neptune? 30 A.U.
Check out this video, if you want to learn more about the scale of the solar system.
Ok, we just chose a convenient unit for distance.
We can do the same with time.
There are a lot of different units for time.
The standard base unit is seconds, but that’s way too small for measuring cosmic time.
We’ve also got minutes, hours, days, months, and even years,
but they’re still not big enough.
Remember, cosmic time is measured in thousands of millions of years.
A million years, or what we’d call a mega-year, is still a little too small.
Some of our numbers will be uncomfortably large.
A billion years, or what we’d call a giga-year, has the opposite problem.
Some of our numbers will end up a little too small.
We need a unit that’s in-between the two and that's where the galactic year come in.
The time it takes for the Earth to travel around the Sun once is defined as 1 year.
For the rest of the video, we’ll call that one Earth-year.
The time it takes for the Sun to travel around the galaxy once is about 230 million Earth-years.
We call that one galactic year.
It’s like a sweet spot between a million and a billion.
It’s exactly what we need.
The beginning of the known universe was 13.8 billion Earth-years ago,
which is equivalent to 60 galactic years ago.
That’s it. That’s the number. Not 60 million. Not 60 thousand. Just 60.
Is that cool or what?!
We’ve made our unit just the right size so that our numbers are ordinary everyday numbers.
So let’s see what happens with the rest of cosmic time.
Galaxies begin to form about 2 galactic years after the big bang.
That’s about 58 galactic years ago.
Our own Sun is a third generation star,
so it didn’t form for another 36.3 galactic years,
about 21.7 galactic years ago.
The Earth formed shortly after, about 20 galactic years ago.
Some things still happen quickly though. I don’t think you can keep using years.
Ok, then we’ll subdivide it.
Fine!
If we’re calling this a year,
then we can easily divide it the same way we divide a normal Earth-year.
Months vary a bit too much, so we’ll skip those and jump right to days.
Let’s say there are 365 galactic days in a galactic year,
and 24 galactic hours in a galactic day,
and 60 galactic minutes in a galactic hour, and
and so on.
That way we can maintain the same perspective we had with galactic years.
It might have only taken 2 galactic years for galaxies to form,
but it only took about 14 galactic hours for the universe to go transparent.
That’s how early the cosmic microwave background was released.
Way before galaxies!
The universe was dark for a long time.
Can we do geologic time too?
Sure. Why not?
The Earth formed 20 galactic years ago.
The Moon formed about 160 galactic days later, so not long after.
The Earth spent a few galactic years being continuously bombarded with space rocks.
Around 15 galactic years ago, that bombardment slowed just enough for life to form.
Seriously. Life formed the moment it was possible.
By 14 galactic years ago, photosynthesis had evolved in cyanobacteria.
By 11 galactic years ago, some living cells had formed a cell nucleus.
At this point, Earth still only has single-celled life.
Multicellular organisms don’t appear until three galactic years ago.
Apparently, that was a rough transition.
After that though, life got pretty good at this whole evolution thing.
By two galactic years ago, there were plants on land.
By one and half galactic years ago, there were animals on land.
The dinosaurs went extinct less than a galactic year ago, about 103 galactic days ago.
Modern humans evolved about 7.5 galactic hours ago,
human civilization began just 27 galactic minutes ago,
and a single human life is only about 12 galactic seconds long.
Whoa.
We are such a tiny fraction of cosmic time
and that’s only the time that’s happened so far.
Four galactic years from now, the Sun will be so hot that the Earth’s oceans will boil away.
In about 22 galactic years, the Sun will expand into a red giant engulfing the Earth.
Any record we ever existed here will be erased from the universe.
That got depressing real fast.
As you can see though, galactic years is an extremely convenient unit for cosmic time.
For quicker jumps, we can easily divide it up into smaller units to get bigger numbers.
But, galactic years works quite well most of the time
and gives us a really good perspective on cosmic and geologic time scales.
It keeps the numbers for all these events as ordinary everyday numbers.
They’re numbers we have a good handle on.
So, hopefully that helped with your perspective a little bit.
If you have any questions, please ask in the comments.
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Don’t forget to subscribe if you’d like to keep up with us.
And until next time, remember, it’s ok to be a little crazy.
Why did you only use straight paths for the photon if all paths are possible?
What about the curved ones?
Ok, yes, curved paths are possible.
It’s just that they’re difficult to animate,
especially with the phasor arrows.
I only needed a few paths to make my point, so I chose the easy ones.
Anyway, thanks for watching.