The event Horizon telescope (EHT)
Using black holes to test Einstein’s theory of gravity[1]
Einstein’s general theory of relativity has never been tested in places where gravity is extremely strong such as at the edge of a black hole. Black holes are difficult to observe from earth by virtue of their distance and they are obscured from our view in two ways. Firstly, they occur at the very centre of galaxies, deep within dense clouds of gas and dust that block most of the electromagnetic spectrum and. Secondly, even material that emits the light we want to detect – that glowing whirlpool of crushed material spiralling in towards the horizon – is itself opaque to most wavelengths of light. Consequently, there are only a few wavelengths of light that can escape from the black hole’s edge to be observed by us on Earth.
All that is about to change with the Event Horizon Telescope (EHT), an international effort consisting of least nine radio-telescopes and arrays around the globe assembled with the aim of resolving [achieving an image of] the event horizon at the heart of Sagittarius A*, the black hole at the heart of the Milky Way, situated some 26,000 light-years from Earth and 4 million times the mass of our Sun in size, which sounds big but by supermassive black hole standards it is rather small. To achieve the highest possible angular resolutions possible from the Earth, the EHT exploits a technique known as very long baseline interferometry (VLBI), in which astronomers at radio dishes around the globe observe the data they collect on hard drives and then combine all those data using a supercomputer to form a single image. In this way, many independent radio antennae separated by hundreds or thousands of miles can be used in concert to create a 'virtual' telescope with a diameter of the entire planet, making an image of the radio sky with detail far surpassing the magnifying power of any optical telescope.
Each telescope is located at high altitude to minimise the absorption of the signals in Earth’s atmosphere. By spanning the globe and operating at millimetre wavelengths, the array will achieve an effective angular resolution at the shortest possible radio wavelengths close to one mm in size. At these wavelengths, the Milky Way is largely transparent, enabling the EHT to observe Sagittarius A* with a minimum of blurring from the intervening gas. These same wavelengths are also able to pierce the matter falling towards the black hole, allowing access to the innermost regions surrounding Sagittarius A*’s event horizon. The magnifying power of a globe-spanning VLBI array at millimetre wavelengths is well suited to the event horizons of the nearest supermassive black holes.
Each year since its first data capture in 2006, the EHT array has moved to add new sites around the world in the anticipation that the first image of Sagittarius A* could be produced as soon as Spring 2016. Data collected on the hard drives must be transported via jet airliner from the various telescopes to the MIT Haystack Observatory in Massachusetts, USA, where the data are cross-compared and analysed on a grid computer made of about 800 CPUs all connected through a 40Gbps network.[2]
The EHT's finest achievement
Over several nights in April 2017, the EHT turned its dishes towards a galaxy known as M87, a neighbouring galaxy to our own Milky Way, ultimately revealing a bright fringe of gas being squeezed, heated and accelerated as it falls towards the event horizon of a supermassive black hole at the galaxy’s centre. The bright ring is caused by the incredible pull the black hole exerts on nearby matter. It's surrounded by a swirling disc of gas, which gets superheated and emits bright radio waves as it accelerates towards the event horizon — getting very, very close to the speed of light.
This galactic monster sits 55 million light-years from Earth and is 6.5 billion times heavier than the Sun. Its event horizon is spherical in shape and about three times bigger than the path Pluto traces around the Sun. A video of what the scientists discovered may be found at https://www.youtube.com/watch?v=DNIAYYOZbIU The finding is described in a series of six research papers published in a special issue of Astrophysical Journal Letters
The EHT's finest achievement
Over several nights in April 2017, the EHT turned its dishes towards a galaxy known as M87, a neighbouring galaxy to our own Milky Way, ultimately revealing a bright fringe of gas being squeezed, heated and accelerated as it falls towards the event horizon of a supermassive black hole at the galaxy’s centre. The bright ring is caused by the incredible pull the black hole exerts on nearby matter. It's surrounded by a swirling disc of gas, which gets superheated and emits bright radio waves as it accelerates towards the event horizon — getting very, very close to the speed of light.
This galactic monster sits 55 million light-years from Earth and is 6.5 billion times heavier than the Sun. Its event horizon is spherical in shape and about three times bigger than the path Pluto traces around the Sun. A video of what the scientists discovered may be found at https://www.youtube.com/watch?v=DNIAYYOZbIU The finding is described in a series of six research papers published in a special issue of Astrophysical Journal Letters
What else is it anticipated the EHT will achieve?
- The EHT will assist in resolving such problems as is Sagittarius A* a black hole or a naked singularity (a black hole where the event horizon does not form or is exposed to view)?
- General relativity predicts that the universe began in a singularity an initial moment when all the contents of the cosmos were concentrated into a single point of infinite density. It also tells us that a singularity, where gravity is infinite and matter is compressed to infinite density, lies at the centre of every black hole. General relativity does not require all black holes to be “clothed” with a horizon.
- Unlike black holes, naked singularities are highly theoretical. The controversial cosmic censorship hypothesis formulated by Roger Penrose in 1969 postulates the idea that physics somehow censors the nakedness of singularities by always enshrouding them with an event horizon.
- If the EHT reveals that the black hole at the centre of the Milky Way is in fact a naked singularity and not clothed by an event horizon, we will be able to directly observe phenomena at conditions where modern physics breaks down.
The no hair hypothesis
- General relativity dictates that any black hole that is surrounded by an event horizon can be completely described using just three properties: mass, spin and electrical charge. In other words, they must have ‘no hair’, that is, no geometric irregularities or distinguishing characteristics.
- A black hole casts a shadow on the emission from the hot matter surrounding it. The shape and size of the shadow depend, in principle, on how fast the black hole is spinning, on the amount that light rays are bent in its vicinity and the orientation of the observer. Computer simulations are in line with the no hair scenario, but if Sagittarius A* has an event horizon, and if the size and shape of its shadow deviates from these predictions, that would contradict a violation of the no-hair theorem, and consequently general relativity.
Mapping the event horizon’s magnetic field
The EHT will record the full polarisation of the radiation emitted by the black hole, enabling the creation of maps of the magnetic fields near the event horizon. This will help scientists understand the physics behind the powerful “jets” emanating from the centres of some galaxies. Are these powered by magnetic fields near the event horizon of supermassive black holes?
Monitoring hotspots
It will also assist in shedding some light on localised, short-lived flare ups or ‘hotspots’ - regions of increased temperature in the accretion flows that orbit the black hole before dissipating, causing brightness variations often seen appearing in the vicinity of Sagittarius A*. Measuring these signals will make it possible to monitor the accretion flow as it orbits the black hole and thus map the spacetime of the black hole and test the prediction of Einstein’s theory.
Measuring rates of orbital precession
Finally, measuring the rates of orbital precession for small compact objects orbiting around supermassive black holes in nearby galaxies at different distances from a black hole (per medium of the evolved Interferometer Space Antenna (eLISA), will lead to a complete three-dimensional reconstruction of spacetime around a black hole, providing many tests of general relativity in the presence of extreme gravity[3].
Black holes of the supermassive variety: how they eat and grow
Instead of watching for the steady glow of quasars or by clocking the speeds of stars whizzing around them, astronomers can also look for brief, bright flares of light from a black hole’s vicinity called tidal disruption events (TDEs), which occur when a supermassive black hole consumes an unlucky star in order to measure them and see how they grow.
Unfolding over months, rather than millennia, they allow researchers to track the feeding process from beginning to end and are bright enough to be observed in galaxies both near and far. Stars wandering too close to these “cosmic monsters” can be shredded by intense gravitational fields, sending gas streaming into the black hole. The gas then compresses, heats and glows as it falls in. The process begins when a supermassive black hole exerts a greater gravitational pull on the near side than on the far side of a passing star, similar to the way our moon exerts a tidal force on the Earth’s tides. The strength of these “tidal forces” depends on the black hole’s mass and the star’s density. [4]
As Cenko and Gehrels go on to explain, the tidal forces increase as the star gets closer to the black hole, until they overcome the star’s self-gravity and rip it apart into arcing filaments of gas. Half the filaments are flung away from the black hole, never to return. The rest swirls into decaying orbits, forming a white-hot accretion disk that funnels the stellar debris into the black hole. Astronomers detect TDEs via the glowing accretion disks left in their wake. [5]
A TDE is the only known way for astrophysicists to witness a supermassive black hole awaken, feats on a meal and return to quiescence. By tracking how long the accretion disk takes to form, reach peak brightness and fade, a scientist can gauge the size of the consumed star as well as the black hole’s mass and spin. [6]
[1] Unless otherwise indicated, this is an edited summary of material in “The black hole test” by Dimitrios Psaltis and Sheperd S Doeleman, Scientific American, Special Issue – 100 years of General Relativity, September 2015, 64-69.
[2] https://en.wikipedia.org/wiki/Event_Horizon_Telescope
[3] The EHT will work in conjunction with the optical interferometer GRAVITY which is being built for use on the European Southern Observatory’s Very large Telescope in Chile and which will track the orbits of stars in our galaxy that lie fairly close to SA*s event horizon; also the Square Kilometre Array (SKA) which is being built in Australia and New Zealand and which will monitor the orbits of rapidly spinning neutron stars called pulsars around the same black hole; as well as eLISA.
[4] Cenko and Gehrels, "How to swallow a sun", Scientific American, 28-35 at 31-32.
[5] Ibid, 33.
[6] Ibid.