In the movie "Interstellar,"
a landing craft explores a planet near a black hole
that has a mass 100 million times that of the Sun.
The crew spend just a few minutes on the planet,
but when they return to the main ship,
23 years have elapsed.
I've waited years.
It's 23 years, four months, eight days.
The idea of time dilation
by going into deeper gravitational fields
is actually pretty accurate.
First theorized by Einstein
in his theory of relativity, it's known as time dilation.
The very bizarre thing
is that when you're in a place where gravity is stronger,
time moves more slowly.
But that's not all.
Another way time dilation occurs
is through your relative movement through space.
If you have two observers that have clocks
that tick at exactly the same rate,
and then one of those observers
moves away at a very high speed,
the moving observer's clock will tick more slowly.
Their time will literally pass slower.
And a lot of people are confused by this
'cause they think it might be a mechanical thing.
It's the clock. But no, it is in fact, literally time.
It doesn't matter what the clock is.
It can be a clock. It could be your heartbeat.
It's time.
This is even true for someone who is walking
compared with someone sitting at a table.
Time moves slower for the person who is walking.
Similarly, for gravitational time dilation,
someone who is at sea level
compared with someone on a mountain
will also experience time at a slower rate.
That's because gravity is stronger
when you're closer to the Earth's center of mass.
But in these examples,
the amount of time dilation is so minuscule
that it's totally unnoticeable.
Apart from a few examples,
humans don't deal with fast enough speeds or enough gravity
to experience a significant amount of time dilation.
But that may not matter.
Because atomic clocks are reaching a sensitivity
that can measure even the tiniest amount of time dilation.
I think that's the place where relativity
will start to become far more important.
It's not because we can go so fast.
I mean, I wish we could go fast,
but I think the real place
will be because we can measure things
with an incredible precision.
That's amazing.
That level of precision
will enable us to use time
in ways we never thought possible.
Like monitoring tectonic plates,
large movements deep below Earth's surface,
and climate change.
Atomic clocks could even lead to new discoveries
regarding the nature of time itself.
I think that's one of the really exciting things
about this, is that we're really exploring the unknown
or able to sense effects
that no one has been able to see before
using the precision of the clocks.
In 2010, two ultra-precise,
synchronized atomic clocks were put next to each other.
These clocks were so precise that they would lose
about one second over the history of the entire Earth.
Then one of them was raised
by just 33 centimeters.
The result?
The clock that was higher up began to tick faster.
But nothing was wrong with the clocks.
It was actually time that sped up for the higher clock,
relative to the lower clock
due to the tiniest decrease in gravity
as the clock moved higher up.
Ultra-precise clocks measured the effects of relativity
side by side, being changed by that much.
Absolutely amazing, fantastic experiment.
Atomic clocks were used in the '70s
for that very purpose,
testing relativity and other scientific experiments.
They flew some atomic clocks on airplanes
around the world,
compared the time before and after doing that.
What they found confirmed their predictions:
Time slowed down for the moving clocks.
The technology ended up being integral to devices
that we depend on every day, GPS.
Global navigation satellite systems
depend on atomic clocks to help determine where you are.
The time it takes for a signal
traveling at the speed of light to reach your phone
will give you the distance.
Using four or more of them,
you can triangulate your position.
I really think of the atomic clocks
as constantly working in the background, you don't see it,
but atomic clocks are really important
for having the whole thing run.
The invention of GPS meant that time dilation
suddenly went from being a cool theory
to something that needed to be dealt with on a daily basis.
Due to the speed at which the satellites
are orbiting the Earth, 14,000 kilometers an hour,
and the lower gravity in space,
they are actually experiencing time differently
relative to us on Earth.
If you add everything up,
the clocks change by about 40 microseconds,
40 millionths of a second of a day,
which sounds like nothing.
But if you did not take that relativity into account,
every day, your position on your receiver
would say that you were 6 miles off
from where you actually are.
So it would very quickly be completely useless.
These dramatic effects of relativity
are central to the work done here
at the National Institute of Standards and Technology
or NIST in Boulder, Colorado.
NIST is a leader in the development
of optical atomic clocks.
These clocks use lasers to control atoms
and measure their oscillations.
Atomic clocks are currently
the most stable and accurate way
of measuring time intervals.
And like any other clock, at the heart of an atomic clock
is a very stable oscillator.
So you think about a grandfather clock.
It has a pendulum that serves as the reference oscillator.
It oscillates at one cycle per second.
An atomic clock is very similar.
So at the heart of an atomic clock is an oscillator,
which is the atom itself.
Instead of having a bunch of gears and levers
and springs and things, it's a series of lasers.
Atoms trapped in an electric field
receive a series of pulses,
which aid in controlling and measuring their oscillations.
Instead of one tick per second, these clocks oscillate
or tick a million billion times per second,
allowing for much more precision.
So it's a really small inaccuracy.
It's hard for anyone to conceptualize
that level of performance.
These clocks are so reliable
that it would take more than double the age of the universe,
so around 30 billion years, to drift by even a second.
That precision means relativity
can also be measured with incredible accuracy.
So if you move a clock up by 1 centimeter of height,
it will tick faster by this one part in 10 to the 18.
And now we can detect that kind of change.
Although this kind of precision
could be problematic
since atomic clocks keep the official time
that synchronize devices all over the globe.
That seems like a huge problem,
I mean how do you decide where to place the clocks
if the tiniest change in gravity
causes time to slow down or speed up?
That is a good point
and it's a problem that we're sort of anticipating.
Now the clocks are getting so good
that we don't know their gravitational potential well enough
to define time globally at that same level.
It does seem like a simpler environment
somewhere out in space,
farther away from these local gravitational variations.
The problem of determining standard time aside,
this 1-centimeter level precision
has the potential to revolutionize
the way things are measured,
using clocks to calculate height and gravity
based on the amount of time dilation.
This is particularly useful in geodesy,
the study of the Earth's shape, orientation in space
and its gravity field.
We're studying the motion of the crust of the Earth,
the plate tectonics, the tides, the polar motion of Earth.
There are many, many effects that observe
and that can be explained.
For instance, in Sweden and Canada,
it's rising on the order of 1 to 2 centimeters per year.
That's quite a lot.
And on another part of our planet,
there's actually a downward land movement.
So somewhere it's going up and somewhere it's going down.
And such effects, of course, change everything
like the global sea level, earthquakes.
They changed even the rotation of the Earth and much more.
Understanding these movements
can help aid in understanding climate change,
natural hazards and construction of major structures
like bridges and roads.
If atomic clocks were used in this field,
they could in theory give you height measurements
that are sensitive to just 1 centimeter.
A level of accuracy GPS is not capable of.
And because current methods utilize GPS and a gravimeter
for determining gravity at a single point,
it doesn't always give you a clear picture
of what's going on.
Well, I'm from Germany, so let's give you an example.
In Germany, we're proud to have our highest mountain.
It's called Zugspitze. It's almost 3,000 meters high.
But also this mountain has permafrost underneath,
which is now melting.
And that actually gives rise to a shrinking of the mountain.
So the top of the mountain is probably decreasing
by also 1 or 2 centimeters per year.
But we have not observed or measured it yet
because at that place if you would put a gravimeter,
you would not see the signal
because on a mountain you have varying snow cover
from summer and winter.
The snow is changing, the groundwater level is changing,
and all these local changes and mass
are disguising the actual effect.
So if you would combine now your gravimeter
with the clock on top of such a mountain,
then you really could resolve what
the melting of the permafrost is doing to the Alps
because the clock would not be sensitive
to a very local mass change like snow cover.
But it would really measure an averaged mass
and gravitational potential
and really show you the physical height of the mountain.
That's why geodesy missions using atomic clocks
are expected to take place in the next few years.
So there's actually a mission in Germany,
which geodesists want to make,
for the first time, a clock measurement.
They want to bring a clock to Heligoland -
this is the offshore island in the Northern Sea -
to compare and get the data
because at the moment this island could not be leveled.
It's too far out in the ocean
and satellites would not resolve it.
It's too small.
And that's why there's right now a geodetic mission
to bring a portable atomic clock to this island
and compare it to a continental clock.
Heligoland's location and size
make getting precise height readouts difficult.
Using GPS with gravimeter data requires multiple readouts
every 2 kilometers,
so offshore areas or areas near a shore
technically require you to place a gravimeter in the ocean,
which isn't possible.
That's why the island's elevation
hasn't been accurately measured to this day.
Clocks can bridge the gaps to islands,
deserts or mountainous areas,
which are hard to access with classical leveling methods.
In turn, they can also act as height reference points
for understanding ocean dynamics and global sea levels.
But it's not all about geodesy.
We may even learn things about gravitational effects
that we don't entirely understand.
It's hard to predict what the measurements will tell you,
but certainly, we're up to many surprises.
And this is actually the boundary that we physicists
are trying to push now,
to measure more and more precise time, frequency,
space-time,
as we know it's connected in general relativity.
Atomic clocks could be the key
to answering some of the biggest mysteries in physics today
that shape the universe as we know it, dark matter.
We know that there's dark matter
that's making up most of the matter in the universe,
and there are also models of dark matter
that can be searched for
and constrained using atomic clocks.
So there are a lot of scientific applications.
As clocks get more precise, we could find
that constants used in everyday physics equations are wrong.
And that could completely change
how we understand the universe.
Atomic clocks that we're building
are the most sensitive way
to look for this present-day variation
in the fundamental constants.
It really has the potential to revolutionize physics.
So do you think atomic clocks could lead
to an Einstein-esque discovery about time?
I think it's certainly possible.
If we did detect that there was a drift
in the fundamental constants over time,
that would be a revolution
similar to the discovery of relativity
or the discovery of quantum mechanics.
Basically, we're doing a lot of tests
on the predictions of Albert Einstein
because can we really assume
that general relativity is the sole theory
explaining everything or is there maybe a deviation
from the predictions that general relativity makes?
The fact is, I don't think
we do understand time very well.
It's always possible that there is going to be
some bright young person out there
who's really terribly disturbed by the conundrum of time
and thinking it over
and might come up with something brilliant.
But it's going to require a different way of thinking.
It's going to require that flash a-ha moment
that happens once per century or even once per millennia.