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The American relativist,

John Archibald Wheeler,
succinctly explained

Einstein's equations
on relativity this way:

space-time tells matter how to move,

matter tells space-time how to curve.

For theoretical scientists
to move forward,

Einstein's equations now need to be

either proved or disproved.

Two fundamental rules of the universe

as theorized by Einstein,

are that light in any form
moves at the same speed,

meaning that space-time is smooth.

And secondly, that the laws of physics

are the same everywhere in
the universe at any time.

Since the more recent discovery

that the universe is expanding
at an accelerating rate,

other theories counter to
Einstein's have been put forward.

One suggests that space-time is lumpy

and that higher energy light

will feel it more as a hindrance.

To put this theory to the test,
scientists needed to compare

the speed of two particles
of high energy light.

- So you might be thinking,
two tiny particles of light,

why is that important?

Well you need to take the
very large and the very small

together to understand
the universe as a whole.

So far, Einstein's theory of relativity,

which describes space and
time as a smooth fabric

that's distorted or
bent by massive objects,

has been a spectacularly
successful explanation of gravity

and the large scale
behavior of the universe.

Whereas quantum mechanics,

another spectacularly successful model,

describes the workings of
atoms, subatomic particles,

and some of the fundamental
forces of nature.

But scientists have never been
able to reconcile the two.

Both relatively and quantum mechanics

are equally fundamental
in their own regimes.

So scientists want to find
a theory of everything

that describes the universe as a whole.

Several ideas
which attempt to reconcile

relativity and quantum mechanics,

suggest that space and
time are not actually

smooth and uniform, but are
instead a seething froth

when seen at the smallest
scale, like bubble wrap.

A low energy, long wavelength photon

is unaffected by the lumpiness of space,

but a high energy, short wavelength photon

is hindered by the froth.

This makes it move more slowly
than lower energy radiation,

so it breaks Einstein's law
that all light particles

must travel at the same speed.

To test this theory they needed FERMI,

the orbiting gamma ray detector.

- We observed a gamma ray burst.

Gamma ray burst is a huge explosion.

That gamma ray burst produced
a large number of photons,

one of which had enormous
energy, very short wavelengths.

Those photons traveled seven
billion years to reach us,

and yet the highest energy,
the shortest wavelength photon,

arrived within 900 milliseconds
of the lower energy photons.

That's a little bit like
racing two speedboats,

one through water and the
other through molasses,

and having them arrive at the same time,

it just doesn't happen.

Because FERMI saw no
delay in the arrival time

of the two photons, it
confirms that space and time

is smooth and continuous
as Einstein had predicted,

and it shuts the door on
several theories of everything

that had predicted that space and time

might be foamy enough
to interfere with light.

And the observations that we've made

of these two photons with FERMI

takes us one step closer
to achieving the goal

of having a theory of everything

that combines the most successful aspects

of quantum mechanics and
relativity into one unified theory.

Einstein's
theory also predicted

that gravity could bend light,

an astronomical effect as seen
here called an Einstein ring.

Gravitational lensing is
where the light of a galaxy

behind another gravitational
body is bent around it,

forming a ring of distorted light.

But do the numbers add up?

And can the mass of the interfering object

be measured using this theory?

- So we found for a long time,

that on a scale of the solar system,

general relativity is either
the correct theory of gravity

or extremely close to correct.

But we don't actually know
whether general relatively

is the correct description
of how gravity works

on the scales of individual galaxies

or of the universe as a whole.

So we use two phenomena,

one is called gravitational lensing.

So general relativity
says that when you have

a massive object like a galaxy,

that that causes
space-time to be deformed.

And that warping of space-time means that

if you have a second galaxy behind it,

the light coming from that
second galaxy will be deflected.

And if the deforming of
space-time is enough,

you can get multiple energies
of that background galaxy,

warped into what we call an Einstein ring.

And the radius of that
ring, how big that ring is,

tells you how much warping
of space-time is going on.

So we used two telescopes to do this.

We took an image of
the gravitation lensing

using the Hubble Space Telescope,

and that let us measure how
big the Einstein ring is.

And then we used the Very Large Telescope

operated by the European
Southern Observatory in Chile

to measure how fast the stars are moving

in the lensing galaxy.

So we took spectra,

this is measuring how much
energy is emitted per,

basically in each color.

Those spectra tell us how fast

the stars are moving in the galaxy.

Measuring how fast the stars are moving

tells us how much gravity there must be,

holding those stars in their orbits.

And so comparing the amount of
mass that we infer from that

with the amount of warping of space-time

that we see from the lensing,

we're able to test whether
the amount of warping

is consistent with general relativity.

In 1917,
Einstein created the concept

of a cosmological constant
in his calculations

to balance out gravity in his
theory of general relativity.

He later abandoned this concept,
considering it erroneous.

However with the recent discovery

of an accelerating universe,

that constant might help
explain dark energy.

- Dark energy is a hypothetical
substance that explains

why the expansion of the
universe is accelerating.

So the universe has been expanding
every since the Big Bang,

but what's odd, and we've only known this

for the last 15 years or so,

is that that acceleration
is getting faster,

it's accelerating.

Now, naively you would expect

that all of the gravity in
the universe, all of the mass,

would cause things to pull together

and slow down the expansion,

or have it carry on,
going at the same rate.

You can't really come up
easily with a way of explaining

why the expansion is getting
faster and accelerating.

Now, one way of explaining dark energy,

one way of getting rid
of dark energy entirely,

is to say, well all of that interpretation

is based on assuming general relativity

is the correct theory of gravity.

Now if it's not, and a lot
of theoretical cosmologists

have worked on this in the past,

you can come up with ways of
accelerating the expansion

without introducing a dark energy.

Now our work, which we found
that general relativity

is the correct theory on the
scale of individual galaxies,

tells us that if you want
to explain away dark energy,

you have to maintain the
validity of general relativity

on astronomical length scales.

Another factor
of Einstein's relativity

is that gravity and
space-time will function

in a predictable manner, everywhere.

The whole
theories about the formation

of the universe, how the
universe is evolving,

are based on one philosophical
and fundamental assumption,

which is that the law of physics are valid

everywhere in the universe and
at any time in the universe.

While here on Earth, we can
only prove those law of physics,

now and on certain circumstances.

So it's very important in
astronomy to also check

that those law of physics are still valid,

where the gravitational
fields are much stronger.

Recently,
it has become possible

to put that theory to the ultimate test.

The effects of Einstein's
general relativity

will be assessed in the most extreme

gravitational conditions imaginable,

at the heart of our own galaxy.

- We think we know that
this object we have

in the galactic center is a black hole,

but to prove that without any doubt,

we have to come so close that
we actually have to measure

the fabric of space-time
and see that it's that

which the theory of Einstein predicts.

So that's the concept.

Then comes the question of
difficulty, and I have to say,

ooh, that was a long path,
it was very, very difficult.

Because while gravity is
sort of an obvious thing,

you stand here on Earth
and then you sort of

have an idea of what it is,

in reality it's an extremely weak force

and the effects of general
relativity are extremely small,

so you have to measure to a precision

which we are normally
not used to in astronomy.

Well you see the center of Milky Way,

where we suspect there's a black hole,

has stars orbiting this
central black hole.

And these stars are measurement
objects, if you like,

they test the gravity of the object.

And there's one star in particular,

which we've been following
now, believe it or not,

for 25 years, that's more than
half of my scientific career,

with telescopes here at ESO in particular.

And we've charted up
the orbit of this star,

and we know that around this time,

this object will, this star,
will make it as close as

four times the distance
of Neptune to the Sun.

That's 17 light-hours,

so that's very, very close.

And that's the unique
opportunity, to in fact

test out the gravitational theory,

because there, gravity is the strongest.

The star now moves with
about 3% the speed of light,

or several hundred times the speed

of the Earth around the Sun,

and that's when these tiny
little warps in space-time,

which cause general
relativity to be different

from Newtonian theory,
to be most pronounced.

So it took us about a decade
between 1990 and 2000,

to basically come up with
robust evidence for this mass.

And then in 2002,

nature gave us just an
absolutely miraculous star,

which moves so close to this
object that you could see it

zip around the mass in the
matter of only a few years,

and that gave us absolutely
fantastic information.

You could estimate the orbital
parameters, the mass, etc,

and we were fairly sure
it's a massive black hole.

Now the orbital period
of this star is 16 years

and these measurements we took here,

on the then new, Very Large
Telescope, we took in 2002.

Take 16 plus 2002 and that's 2018,

so our star is coming
back to it's original,

very close position near the black hole.

That's the time when we want to be there

and make these measurements.

Preparations for
this breakthrough observation

began in early 2018, at
ESO's Paranal Observatory.

Their cutting edge astronomical equipment

was readied to make measurements

of one of the most extreme
gravitational laboratories,

at the center of the Milky Way.

- For the discovery, a
combination of three instruments

of ESO were used, NACO,
SINFONI, and GRAVITY,

and all of those instruments
are unique in the world.

NACO is adaptive optics in the infrared,

we don't have so many in other telescopes.

SINFONI is a high-resolution spectrograph,

very important to measure at which speed

the star is coming to us or going from us.

And GRAVITY is a
interferometric instrument,

the only one in the world that can combine

four big telescopes,
eight meters telescope,

with a baseline of 130 meters,

so having the same resolution
as a 130 meter telescope.

And it has, in addition, the
capability to do astrometry,

very accurate astrometry,
so it measures movement

that are the equivalent of
an astronaut on the moon

moving a flashlight by
about 10 centimeters.

And the combination of
all these instruments,

interferometric and
classical spectroscopy,

adaptive optics, is what makes ESO unique.

It's having all of them on the same side,

in a position where you can observe

the galactic center in good conditions.

- So by testing, by
measuring these predicted

physical, teeny effects,

they are very, very small
fraction of what we knew so far,

that's why have to make
such precise measurements.

That's how we can test general
relativity in this domain.

- We need to get very sharp images,

and the best way we can get sharp images

is to commit big telescopes.

But since we don't have
this very big telescopes,

we combine telescopes, we
create a super telescope,

in this case, which is
130 meter in diameter.

Even with the impressive size

of the Very Large Telescope,

the only way to precisely
measure the path of the star

around the supermassive black hole

took some innovative telescope teamwork.

- For our work so far, what we have done

is we've taken pictures with
the single big telescopes,

the single eight meter telescopes,

and make them as sharp as you can be.

The problem is, and you
see this in my hair,

there's wind, and the
wind distorts the waves.

And so either you go out in space,

very difficult for an
eight meter telescope,

or you take the eight meter telescope

and you repair the distortions,

which the Earth atmosphere does.

Like on a hot day when
you travel along a road,

and you see the flimmering of
the distant approaching cars.

So that's what we do, that's
called adaptive optics,

and that makes the images
with single telescopes,

already very sharp.

But that's not sharp enough
for what we want to do now,

we really need to make still better,

still 10, 20 times sharper images

to see the tiny effects
of general relativity.

And that we do by taking into account

that ESO not only has one telescope,

but four of these gigantic
eight meter telescopes,

and we can bring them
together, as if it's one.

That's a very challenging experiment,

but we've done this
now, and so we are ready

to make these measurements at
an unprecedented precision.

- So we combine the light
from four telescope,

the very large telescopes,

here in the center of the
mountain, of the observatory.

The four telescope are
separate by 130 meter,

which means that our
super telescope can make

20 times sharper images
than a single telescope.

- We have all four
telescopes, working together,

and in addition to these four telescopes

we have a beam combiner,
and this combines the light

from all the four telescopes
and this is GRAVITY.

GRAVITY is, of course, it's the best,

because we can really trace
the orbit of this star

very, very carefully with
really good accuracy,

so we can now get very nice orbits

and we're trying to test all our theories

with this very nice data we have now.

Two other
state-of-the-art instruments

will reveal an effect called
gravitational redshift.

This is visible when light
from the star is stretched

to a longer wavelength by
the very strong black hole.

- We actually expect that we
can see general relativity,

and how can we see that?

It's actually a slight deviation
of how the star is moving

and this deviation we can
see in the first place,

with the so-called doppler effect.

It's currently approaching us

and it will fly away
and this doppler effect

is actually something we can observe

by the means of spectroscopy,
and spectroscopy in turn means

that you need a spectrograph

and such a spectrograph is SINFONI

and that is the instrument
which will be the one

which actually is observing
the relativistic effects.

Now after an epic
26 year observing campaign,

the effects of Einstein's
general relativity

have been clearly seen for the first time.

- So the experiment we're
doing is extremely simple

in some sense, we are
just measuring the motion

of stars around the black hole.

That is very much like
Earth goes around the Sun

and you can actually
calculate the mass of the Sun

from the knowledge that Earth takes a year

to go around the Sun

and essentially we will
try to do the same.

We try to measure the
mass of the black hole

by seeing how the stars fly around it.

- And right now we are
observing this passage

as the star moves

into this critical curve
around the black hole.

For scientists,
a spectacular show

of orbital mechanics and relativity.

- So what were the most exciting moments

in the observing campaign?

I think actually it
was the very beginning,

about two years ago when
we for the first time,

pointed to the black
hole and to that star,

and actually you see the both of them.

And this was too, a big surprise for us

because we did not expect
that actually inside,

that field of view of what we have.

And so this was very
revealing, to actually see it,

that we can go to very faint,

that we seen the faint black hole

all the time and the star nearby.

And the other most exciting,
probably was that this year,

when the star was moving so fast,

at a fraction of the speed of light,

that you could see it from night to night,

this was very exciting to see.

- And the combination of
all these instruments,

interferometric and
classical spectroscopy,

adaptive optics, is what makes ESO unique.

It's having all of them, on the same side,

in a position where you can observe

the galactic center in good conditions.

- The beauty of it is that
it's a very simple experiment

in the phenomenon.

Yeah, there are some technical challenges

that you have to overcome
to build the instrument

and perform the experiment,
but the concept is very simple.

You can probe the black hole properties

and then you can probe
the gravitational field,

which is a very strong one and up to now,

when whatever tests for
the general relativity

or this geometrical theory
of gravity that we have

has been in the solar system

and also some pulsars,

but in this very strong
regime, it has not been tested.

This is just a starting point for it.

With the developments,

like instrument developments
and the new telescopes,

we will do more and more of this.

More than a
century after he published

the paper setting out the
equations of general relativity,

Einstein has been proven right once more,

with the combined resources of the ESO.

- This is one of the huge benefits of ESO,

and the way ESO works

is that there is always a
very strong collaboration

between ESO and the institutes
in its member states,

which is very unique in the
world because it enables

ESO and the ESO members to
undertake projects like GRAVITY,

which are so complicated
that you need a strong team.

- Well what comes next?

For the next years we
have a pretty good outlook

of what will happen.

So the next effect, which we will see,

is a swatch in position.

This means the orbit of
the star will rotate,

the ellipse will rotate a little bit.

And so this is an effect
which we will see next year,

pretty sure about it.

Then we come to more subtle
effects of general relativity,

but even more exciting;

this is about the space-time
itself around a black hole.

So the space-time around a black hole

will rotate with the black hole,

and so this will move
the orbits of the stars,

yet in other direction.

And this is very exciting
because this property

is very unique to general relativity

and we will only be able,

in that black hole at
the center of Milky Way,

we will have the
precision to measure that.

With the theory
of relativity dusted off

and placed back on its pedestal,

what remains is for scientists to work out

what dark energy is and how
it affects the known universe.