Original subtitles

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- [Narrator] 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.

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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.

- [Narrator] 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.

(trilling)

- [Narrator] 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.

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- [Narrator] 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.

- [Narrator] Another factor of Einstein's relativity

is that gravity and space-time will function

in a predictable manner, everywhere.

- [Francoise] 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.

- [Narrator] 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.

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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.

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- [Narrator] 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.

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- 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.

- [Narrator] 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.

- [Narrator] 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.

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- [Narrator] 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.

- [Narrator] 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.

(slow desert music)

- 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.

- [Narrator] 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.

(slow triumphant music)

- [Narrator] 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.