- 354 years ago, John Wilkins,
a bishop and founder of the Royal Society
published a 638 page essay
laying out a scheme for a universal global language,
a common tongue to be used among scholars, philosophers,
and governments for the sharing of knowledge.
Buried among a handful of pages in that essay
is another idea,
one that would end up changing the world forever.
A standard and universal system of measurement
based on the natural world.
And at the base of that system of measurement
would not be a rod of particular length, or a mass of metal,
instead it'd be based on time.
Today, other than a handful of notable exceptions,
(clears throat)
the modern world relies on a shared system of measurement
much like what Wilkins first proposed, the metric system.
But at its root there's always been a problem,
a measurement system tied to physical objects
will only ever be as precise as those physical objects.
So, in 2019, the base units
that make up the foundation of the metric or SI system
were redefined to no longer rely
on imperfect human artifacts.
Now, they're defined by natural mathematical constants
that never change.
This standard system of measurement
is the foundation of modern science and society,
but for any of it to work,
there's still one unit we must physically measure,
one unit to rule them all, the second.
So, who decides what a second is?
These are the things that keep me up at night.
(upbeat music)
Hey smart people, Joe here.
And this is the US Naval Observatory in Washington DC.
This is one of the best rooms I've ever been in in my life.
(inhaling) I wish you could smell this,
it just smells like knowledge and important stuff.
- Yeah, yeah.
- That is Geoff, and inside of the building there,
you will find, I kid you not,
what are probably the most precise measuring devices
ever built by human hands, and they are clocks,
very, very fancy clocks.
It's also where the Vice President lives,
that's her helicopter taking off while we were filming.
You're right, I do take that for granted.
- Yeah.
- Helicopter time.
(claps)
- [Geoff] I'd love to wait for that.
I always tell people that of all the things in nature
that we can measure, the one that we can measure
with the highest precision
is the one that we know intrinsically the least about.
- Time. - Time.
I cannot tell you what time is,
but I can tell you exactly what time it is.
This is where time more or less begins.
So, let's look over here.
- We're at the beginning of time, right here,
is that what you're saying?
- We're at the beginning of time, right.
- Cell phone networks, the internet, power grids,
financial transactions, air travel,
the very fact that some server somewhere
is letting you load this video right now,
none of it would be possible without machines like these.
And what those machines do
is figure out exactly how long a second is.
So, how do we do that?
Okay, so let's lay this out for a second.
See what I did there?
Prepare for a bit of massively oversimplified history.
Since humans became humans,
our species has marked the passage of time
using various cycles,
the Earth's orbit and the seasons,
the rising and setting sun, even tides.
For most of our history these worked well enough,
but as everyday life changed
we needed to divide and measure time
in smaller and smaller slices.
The ancient Egyptians were the first
to slice up the night and day,
into smaller parts called hours,
And the Greeks were like "Hey, can we borrow that?"
Then, bam, there's 24 hours in a day.
Fast-forward to around 1,000 years ago,
and since people were using round things
to keep track of time,
astronomers were all about this weird base-60 number system
from the Babylonians.
(dramatical music)
It's actually pretty genius,
because you can easily divide anything with 60 units
into one, two, three, four, five, or six parts.
Anyway, the first recorded person
to subdivide time into 60 parts
was an Iranian astronomer named al-Biruni,
and later European astronomers
put some Latin on those base-60 slices.
The first small part they called the minute,
and the second small part they called the second.
That's literally where the word comes from.
But normal everyday people didn't have any use
for minutes or seconds until 100s of years later.
The first clock with good enough mechanics
to even count seconds was made by Christiaan Huygens,
using a pendulum just under a meter long
that swung once a second.
Apologies to the Dutch for my pronunciation of that name.
- Christiaan Huygens.
- This evolution from planetary cycles to ticking machines
established the divisions of time we still use today,
24 equal hours divided into 60 equal minutes,
sliced into 60 equal seconds, the fundamental unit of time,
1/86,400th of an Earth rotation long.
This clean and simple mathematical definition
has just one problem,
Earth is a terrible thing to base time on.
Over long time scales,
Earth's rotation is actually slowing down.
A year in the Devonian period was 400 days long,
a day only lasted 22 hours,
and even on a day-to-day basis
Earth slows down and speeds up because of other planets,
the moon, how mass moves around inside the planet,
all kinds of things.
So, history had invented this basic unit of time
based on a fraction of a day,
and then scientists came along
with all this new, precise mathematical astronomy,
and realized that definition stunk.
So, by the early 20th century,
science decided to find a more precise way
to measure seconds,
they tried using the stars,
they tried basing it on the year,
but what they eventually settled on was atoms.
Okay, atomic clocks sound really complicated.
- Caesium beam frequency standard,
external oscillator generates a microwave frequency
and induce those atoms to go into resonance,
oscillating at that frequency for equal flux of atoms
in the two hyperfine states.
- And they kinda are, actually,
but here's the most important thing
you need to understand about clocks,
except for sundials and hourglasses,
every single clock does the same thing.
It has something inside that wobbles at a very precise rate,
all any clock does is count up those wobbles
and convert them into a unit that we actually understand.
A pendulum makes one beat per second,
a quartz crystal and a $5 digital watch
beats 32,768 times per second,
atomic clocks wobble billions of times per second.
The most widely used atomic clocks use cesium atoms.
When microwave radiation is humming
at the exact right frequency,
it does something special to cesium atoms.
So, you turn the dial on your little radiation machine
until you hit the sweet spot
that excites the cesium atoms,
then you count 9,192,631,770 wobbles of that radiation.
They took this old unit of time,
based on something that might change every day
and changed its definition
using something that never changes, physics.
- Since 1967, the second has been defined
as the interval of 9,192,631,770 hyperfine transitions
of the valence electron in a undisturbed cesium-133 atom.
Now, I will be brutally honest
and tell you when I started working here,
I thought a second was one Mississippi,
and for the most part it still works that way.
- The most precise atomic clocks today
can accurately measure a second
to more than 15 decimal places,
and what makes them so precise
is they tick 9 billion-something times per second.
Today's best atomic clocks won't loose a second
in something like 300 Million years,
which means if aliens had come down
and dropped one on Pangaea,
it would still be accurate to within a second today,
as long as some clumsy T-Rex
didn't trip over the power cord or something.
The second as currently defined
is the most precise measurement of the universe
we've ever accomplished,
but as science continues to advance
the definition of a second
may have to become even more precise to keep up.
- The technology is getting to the point now
where that microwave frequency isn't precise enough
for the applications that require precise time,
so we are now working to develop
optical frequency standards.
- [Joe] It would be-- - Optical frequency is
five orders of magnitude higher, so--
- Five more decimal places of precision
on what a second is.
- Right. - Where modern atomic clocks
use long-wavelength microwave radiation,
the next generation of atomic clocks
will measure atomic changes
based on visible light frequencies,
filling in even more decimal places of precision
for what a second is.
But even though cesium atoms do the same physics
everywhere in the universe,
atomic clocks are imperfect things built by people,
environmental conditions, relativity, magnetic fields,
a bunch of things can make two atomic clocks disagree
by minuscule fractions of a second.
- What we do here at the Naval Observatory,
there's an old saying that a person with one clock
knows what time it is, a person with two is never sure.
Here at this facility,
we operate about a 100 atomic frequency standards.
All the information of each of those clocks
comes up here where we have a computer system
that analyzes the whole ensemble of clocks,
about every 100 seconds.
The output of this then goes into
this rack of equipment over here,
where it generates a one pulse per second tick, if you will.
- In fact, because we can measure seconds
more precisely than anything else in the universe,
since 2019, the basic units for length,
mass, electrical current, temperature, light intensity,
are all now fundamentally based on the second.
Atomic time almost works too well
because it keeps time better than the planet itself,
so we have to constantly correct atomic time
to stay in line with messy old Earth.
That basically happens like this,
at midnight on January one, 1958
two clocks were both set to 00:00:00.
One clock marks each day by measuring Earth's rotation
with respect to the distant stars,
and counts off 86,400 Earth seconds between each rotation.
The other clock ticks off
86,400 precise atomic seconds every day.
But because Earth's rotation speeds up and slows down,
the first clock's seconds aren't always the same,
and these two clocks eventually get out of sync,
so every so often atomic time
counts off one extra second at the end of a day
to bring Earth rotation time and atomic time back in sync.
That's a leap second.
Since 1972, 27 leap seconds have been added
to keep Earth in sync with the more precise atomic wobbles.
(upbeat music)
It isn't the numbers on a clock
that keep our society functioning on time,
measuring the basic unit of time is far more critical,
the hidden anchor at the foundation of modern life.
Every financial transaction, every cellphone call,
every Netflix show or YouTube video you watch,
scientific observations, an Uber driver picking you up,
the power grid staying on.
There are billions of things on this planet, maybe more,
that have to know exactly what a second is in order to work.
And that comes from rooms full of servers like this.
- Network time protocol
is the time backbone for the internet,
and that's distributed by this rack of equipment
that's in here, that says network time protocol.
- So, the internet--
- The internet-- - ... gets its time--
- Gets its time from here. - ... from there.
There are other servers like these
at the National Institute of Standards and Technology
in Colorado.
Altogether these respond to millions of requests
every second, sending back tiny packets of data
containing official certified time.
And if you watched my previous video
you Know that every time you use GPS,
you're actually measuring time.
One of the primary missions of the Naval Observatory
is to tell GPS satellites exactly what time it is,
and if those time signals are off
by even a millionth of a second,
your GPS position could be off by hundreds of meters.
The second is not a fundamental bit of the universe,
a second is a long and pretty random number.
That's because every method we've used to define a second
has been based on the definition that came before it,
it's something that we made up
because some people a long time ago
divided up the day in this weird way.
We've been stuck with seconds for 700-something years,
and we've just figured out
new and better ways to measure it,
and now we orient our whole lives around this.
From the moment we wake up to when we fall asleep,
everything we do revolves around what time it is,
school, work, meetings, our social lives.
We just take knowing what time it is for granted,
but there's an astonishing amount of technology and science,
and lots of very smart people,
working every second to give us seconds.
Stay curious.
Thank you so much for watching our latest episode
and, while you're here,
I wanna tell you about a show over on PBS Food
called "Serving up Science".
They are back for an all new season.
The PBS family is a big family,
there's so many great shows to check out
and we know you're gonna love this one.
It's hosted by a dear old friend of mine,
Sheril Kirshenbaum.
They're newest episode is all about junk food,
why some of the food that makes us happiest
might not be the best for our biology.
Find the link down in the description
and make sure and tell them
that that "Be Smart" sent you.
Thank you so much for spending some time with us
and watching this video,
now is the time of the video where I remind you
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I also wanna tell you about our Patreon page
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your support helps us make videos like this
and visit amazingly cool places
like the clock of all the clocks.
There's info down in the description
where you can learn more,
and we'll see you in the next video.
Does this cost more than a Rolex?
- Yes.
- We're gonna have no excuse for being late anywhere,
ever again for the rest of our life.
Future time travelers, come visit us right now.
(laughs)