The interfermoter that can weigh black holes

The new near-infrared interfermoter GRAVITY, at the ESO/VLTI Observatory is enabling physic breakthroughs, by testing General Relativity and weighing supermassive black holes.

The new near-infrared interfermoter GRAVITY, at the ESO/VLTI Observatory is enabling physic breakthroughs, by testing General Relativity and weighing supermassive black holes.

The new near-infrared interfermoter GRAVITY, at the ESO/VLTI Observatory is enabling physic breakthroughs, by testing General Relativity and weighing supermassive black holes.

One hundred years after Albert Einstein published his Theory of General Relativity, a new instrument called GRAVITY has begun to test the predictions of this fundamental theory in the most extreme environments of the Universe: Black Holes.

These behemoths reside in the centres of our Milky Way and other galaxies, weighing many millions or even billions of solar masses.

Combining the four Very Large Telescopes of the European Southern Observatory (ESO) in Chile, GRAVITY provides astronomers with a unique image quality, spatial resolution and sensitivity in the near-Infrared wavelength range – it is going to revolutionise astronomy.

An idea come true

In 1916, Einstein’s theory of gravitation, known as Theory of General Relativity, entirely changed physics and our view of the world.

According to this theory, the effects of gravity are often perplexing: time is lagging, space is curved, and there are even parts of space – black holes – that are entirely decoupled from the rest of the Universe.

Many predictions of General Relativity have been successfully tested under widely different conditions, and applications based on this theory (such as worldwide navigation systems) have changed our everyday lives.

However, our understanding of the Universe is based on the fundamental principle that all laws of nature shall be valid everywhere in the Cosmos, even in the most extreme corners of the parameter space.

We would expect the deviations from the theory to be strongest – and therefore ‘easiest’ to detect – under the most extreme conditions. It is therefore of fundamental importance to test the Theory of General Relativity in the extremely strong gravitational fields around supermassive black holes; and it is a beautiful coincidence that the existence of such black holes is itself a prediction of the theory.

With this in mind, in 2006 a European team of astronomers and engineers embarked on an innovative and technologically extremely challenging project: they started to develop the GRAVITY instrument, which would allow for the first time to perform such measurements near the supermassive black hole at the centre of our home galaxy.

This project was led by the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, in conjunction with the Paris Observatory–PSL, the Université Grenoble Alpes, CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Portuguese CENTRA – Centro de Astrofisica e Gravitação, and ESO.

Just 10 years later, GRAVITY was finalised and could be deployed to the ESO telescopes in Chile, where it passed its first tests with flying colours.

Zooming in on black holes with GRAVITY

The supermassive black hole at the centre of our Milky Way (referred to as Sagittarius A*) is the laboratory of choice for testing general relativistic effects in strong gravitational fields.

No other massive black hole is closer to Earth. Still, its angular size as seen from Earth is comparable to the size of a €1 coin on the moon.

In order to achieve such an incredible angular resolution, GRAVITY takes advantage of a technology called interferometry – a trick that was first described and applied by Albert Michelson in the 1880s.

By combining the light from all four telescopes of the VLT (each with a mirror of eight metres in diameter), a much higher resolution can be achieved, equivalent to one gigantic telescope mirror of 130m in diameter.

However, technologically this is extremely challenging. While interferometry has been implemented for decades in radio astronomy, previous attempts for optical or near-infrared astronomy were unsuccessful.

Now, GRAVITY has managed to overcome these difficulties, a technological breakthrough that allows astronomers to explore the secrets of black holes.

GRAVITY’s precision and sensitivity surpasses its predecessors by a factor of 100 to 1,000. Its ‘€1-coin-on-the-Moon’ resolution, phrased the other way round, would mean that an astronaut on the Moon could read the headlines of newspapers on Earth – or look closely at the black hole in the centre of our home galaxy.

Time passes more slowly

The black hole in the centre of our Milky Way reveals itself first and foremost by its enormous gravitation. Just like planets in the gravitational field of the Sun, stars orbit the black hole in the Galactic Centre.

For more than 25 years, the Galactic Centre group at MPE measures the changing positions and velocities of these stars. In particular, one star – called ‘S2’ – orbits the black hole with a period of 16 years in a very eccentric orbit (analogous to a comet orbiting the Sun), coming very close to the black hole at its closest approach.

The latest of these close approaches happened in 2018, and GRAVITY was commissioned at the VLTI just in time to observe this milestone.

At its closest point, S2 is at a distance of less than 20 billion km from the black hole (120 times the distance between the Sun and the Earth) and moving at a speed in excess of 25 million km per hour (8,000 km/s, i.e. 2.5% of the speed of light).

GRAVITY creates such sharp images that it even revealed the motion of the star from one night to the next as it passed close to the black hole.

However, the astronomers were not only interested in the star’s changing position, but also in its appearance. General Relativity predicts that clocks are slower in a strong gravitational field (i.e. time passes more slowly).

This time dilution results in an effect that is called relativistic redshift: light from the star is stretched to longer wavelengths by the very strong gravitational field of the black hole.

Combining the extraordinary precision and sensitivity of GRAVITY with the spectral resolution of the SINFONI instrument (another instrument developed at MPE), the astronomers were able to measure this gravitational redshift: the change in the wavelength of light from S2 agrees precisely with that predicted by Einstein’s theory of General Relativity.

For the first time, this deviation from predictions of the simpler Newtonian gravity has been observed in the motion of a star around a supermassive black hole. More than 100 years after he published his paper setting out the equations of General Relativity, Einstein has been proven right once more — in a much more extreme laboratory than he could have possibly imagined.

With 30% of the speed of light

Stars are not the only objects moving around the black hole; another tracer revealing the presence of such an extreme object, despite the fact that the black hole itself is invisible, are in-falling gas clouds.

These heat up to more than a billion degrees and shine brightly for several tens of minutes as so-called ‘flares’. Astronomers assume that this emission comes from compact ‘magnetic thunderstorms’ in this very hot gas, in analogy to the radiation bursts of solar flares.

In the summer of 2018, during the measurements described above, GRAVITY serendipitously caught strong infrared emission in three prominent bright flares from the hot gas orbiting around Sagittarius A*.

The analysis of this data revealed a spectacular result: in all three cases hot gas was circulating the black hole at 30% of the speed of light.

The radius of the clouds were very close to the event horizon, the point of no return, beyond which nothing can escape the gravitational potential – not even light.

These observations confirmed exactly the theoretical predictions for such hot spots orbiting at the innermost edge of stable orbits for a black hole with four million solar masses.

Taken together, these results are a resounding confirmation of the existence of massive black holes predicted by the Theory of General Relativity.

However, while the black hole at the centre of the Milky Way is the closest massive black hole, it is not the most massive or the most spectacular of these extreme objects.

Whirlpools around supermassive black holes

More than 50 years ago, the astronomer Maarten Schmidt identified the first ‘quasi-stellar object’, or quasar, named ‘3C 273’, as an extremely bright but distant object.

The energy emitted by such a quasar is much greater than in a normal galaxy such as our Milky Way and cannot be produced by regular fusion processes in stars.Instead, astronomers assume that gravitational energy is converted into heat as material is being swallowed by an extremely massive black hole.

Today, supermassive black holes are thought to exist in the centres of basically all big galaxies. They can reach several billion solar masses. When matter is accredited into these black holes, it heats up and outshines the entire rest of the galaxy.

This extreme luminosity allows them to be observed at vast distances; quasars are among the most distant astronomical objects that can be observed. However, this luminosity is also what makes the physical state in the centres of quasars so hard to measure quantitatively.

For instance, the mass of a black hole could be inferred from the orbits of stars that circulate around it – the method of choice for the black hole at the centre of the Milky Way. But in a quasar, such stars cannot be observed as their light is buried under the bright light from the in-falling gas.

Until now, almost all known black hole masses have therefore been measured by a different method called ‘reverberation mapping’. Brightness variations of the quasar’s central engine cause a light echo once the radiation hits clouds further out – the larger the size of the system, the later the echo.

The time delay between both observations serves as a characteristic length scale – on the order of one light month, or the size of our Solar System. The length scale provides information on the geometry around the black hole and can be used to estimate its mass.

However, this method is based on assumptions on the distribution and kinematics of the gas swirling around the black hole – in a region called the ‘broad line region’, as the emission lines are broadened due to their fast rotation.

Due to the small angular size of this inner region (one light month at a distance of billions of light years), so far it had not been possible to observe its structure and gas movements directly.

GRAVITY resolved the broad line region for the first time ever, allowing astronomers to observe the actual motion of gas clouds around the central black hole in the quasar 3C 273.

The observations reveal that the gas clouds whirl around the central black hole in a disk with a radius of 150 light days and with a speed of up to 4000km/s. Thus geometry, structure, and the kinematics of the gas clouds were now known, and the mass of the central black hole in 3C 273 was calculated to amount to 300 million solar masses.

These values are consistent with previous results derived from reverberation mapping in this object. Thus, GRAVITY provides both a confirmation of the main previous method to determine black hole masses in quasars and a new and highly accurate, independent method to measure such masses. It thereby promises to provide a benchmark for measuring black hole masses in thousands of other quasars.

Quasars play a fundamental role in the history of the Universe, as their evolution is intricately tied to galaxy growth. While astronomers assume that basically all large galaxies harbour a massive black hole at their centre, so far only the one in our Milky Way has been accessible for detailed studies.

This is the first time that astronomers were able to spatially resolve and study the immediate environs of a massive black hole outside our home galaxy, the Milky Way. Black holes are intriguing objects, allowing scientists to probe physics under extreme conditions – and with GRAVITY we can now probe them both near and far.

Next: the rotation of space-time

Even more bizarre than time dilatation, gravitational redshift, or the existence of an event horizon is another aspect of the nature of black holes: even space and time rotate as the rotating black hole pulls its surroundings along.

Over the coming years, GRAVITY (and further instruments under development at MPE) will measure this rotation of space-time as it reveals itself in the motion of the stars and in-falling matter.

In a first step, very soon the continuing observations of the star S2 orbiting the black hole are expected to reveal a relativistic effect called ‘Schwarzschild precession’ – a small rotation of the star’s orbit – as S2 moves away from the black hole. This would mark another milestone in our understanding of the Theory of General Relativity.

It is a remarkable prediction of General Relativity that all observable characteristics of a black hole are determined by just two parameters, its mass and rotation, irrespective of its inner structure and its complex evolution.

With advanced technology such as GRAVITY, we will soon be able to measure both with unprecedented accuracy for the black hole at the centre of our Milky Way – with other galaxies to follow.

Dr Eckhard Sturm
GRAVITY Project Manager
Max-Planck-Institute for Extraterrestrial Physics
+49 89 300003806
sturm@mpe.mpg.de
www.mpe.mpg.de/personnel/18102

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