The little secret behind the first picture of a Black Hole

In a series of six papers (I,II,III,IV,V,VI) published on 10 April 2019, more than 200 authors present and discuss the first event-horizon-scale images of the super massive black hole candidate M87* from an Event Horizon Telescope very long baseline interferometry campaign conducted in 2017 April at a wavelength of 1.3 mm. In other words, they report the first image of a black hole:

What you’re looking at is a ring of fire created by the deformation of space-time. Light goes around, and looks like a circle.

Nature, Black hole pictured for first time — in spectacular detail

The images show an irregular but clear bright ring, whose size and shape agree closely with the expected lensed photon orbit of a 6.5 billion solar mass black hole.

Black holes are a fundamental prediction of the theory of general relativity (Einstein 1915). A defining feature of black holes is their event horizon, a one-way causal boundary in spacetime from which not even light can escape (Schwarzschild 1916). The production of black holes is generic in General Relativity (Penrose 1965), and more than a century after Schwarzschild, they remain at the heart of fundamental questions in unifying General Relativity with quantum physics (Hawking 1976; Giddings 2017).

Black holes are common in astrophysics and are found over a wide range of masses. Evidence for stellar-mass black holes comes from X-ray and gravitational-wave measurements. Huge black holes, with masses from millions to tens of billions of solar masses, are thought to exist in the centres of nearly all galaxies, including in the Galactic centre of the Milky Way and in the nucleus, M87*, of the nearby elliptical galaxy Messier 87.

Active galactic nuclei are central bright regions that can outshine the entire stellar population of their host galaxy. Some of these objects, quasars, are the most luminous steady sources in the universe and are thought to be powered by super massive black holes accreting matter at very high rates through a geometrically thin, optically thick accretion disk. In contrast, most Active Galactic Nuclei in the local universe, including the Galactic centre and M87*, are associated with super massive black holes fed by hot, tenuous accretion flows with much lower accretion rates.

Soon after Einstein introduced general relativity, theorists derived the full analytic form of the photon orbit, and first simulated its lensed appearance in the 1970s. By the 2000s, it was possible to sketch the “shadow” formed in the image when synchrotron emission from an optically thin accretion flow is lensed in the black hole’s gravity. A steady progression in radio astronomy enabled very long baseline interferometry observations at ever-shorter wavelengths.

Very long baseline interferometry (VLBI) at an observing wavelength of 1.3 mm (230 GHz) with Earth-diameter-scale baselines is required to resolve the shadows of M87* and of the Galactic Centre, the two super-massive black holes with the largest apparent angular sizes. At 1.3 mm and shorter wavelengths, Earth-diameter VLBI baselines achieve an angular resolution sufficient to resolve the shadow of both sources, while the spectra of both sources become optically thin, thus revealing the structure of the innermost emission region. The Event Horizon Telescope collaboration was established to assemble a global VLBI array operating at a wavelength of 1.3 mm with the required angular resolution, sensitivity, and baseline coverage to image the shadows in M87* and the Galactic Centre.

The image of the shadow confines the mass of M87* to within its photon orbit, providing the strongest case for the existence of super-massive black holes. These observations are consistent with Doppler brightening of relativistically moving plasma close to the black hole lensed around the photon orbit. They strengthen the fundamental connection between active galactic nuclei and central engines powered by accreting black holes through an entirely new approach. In the coming years, the Event Horizon Telescope Collaboration will extend efforts to include full polarimetry, mapping of magnetic fields on horizon scales, investigations of time variability, and increased resolution through shorter wavelength observations.

The papers provides the full scope of the project and the conclusions drawn to that date.

This work signals the development of a new field of research in astronomy and physics as we zero in on precision images of black holes on horizon scales. The prospects for sharpening our focus even further are excellent.

As it happens, the highly anticipated and publicised photo has provided the perfect excuse for all sort of memes:

That’s great, and it also grants me the opportunity to share a dirty little secret.

Using interferometry, eight telescope stations over six geographical locations can be combined to emulate a larger telescope with a size equal to the maximum separation between the telescopes. Using science, the effort of hundreds or thousands of scientists are combined across geographies and over the years to reveal the more precise image of reality we have. The image of the black hole epitomises “collective” intelligence at its best. Science is the “interferometry” we use to combine human minds.

Unfortunately, not everybody can see this in the image of the black hole. As an anecdote, I will tell you that I had never been blocked by anyone in Twitter before, until I shared this naive tweet with which I tried to highlight this magic.

An offended twitter user blocked me. I suspect in her opinion, collective effort does not live up to her “incorrect” expectations on how science is made.

Sometimes, it takes a lot of energy not to fall down the black hole of despair.

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