The year 2019 is
here. With it, we've been promised a splendid moment in astronomy. For
years, the
Event Horizon Telescope has been working to bring us the first ever
telescopic photograph of the event horizon of a black hole.
A simulation of a black hole from a paper by Thorne and colleagues on the CG techniques used to develop Gargantua. (James et al./Classical and Quantum Gravity)
Indeed, for all
their popularity in public imagination, we have never actually seen a
black hole. And the reason for that is laughably simple. Black holes, you
see, are literally invisible. The pull of their gravity is so immense that,
past a certain point, nothing escapes. This includes the electromagnetic
radiation - such as X-rays, infrared, light and radio waves - that
would allow us to detect the object directly.
That point of no
return is called the event horizon, and apart from being a terrifying location
you never want to find yourself in, it's also our key to actually visualising a
black hole. While we may not be able to see the black hole itself, there's a
chance that its event horizon can be photographed; and we are tantalisingly
close to seeing the results thanks to the Event Horizon Telescope (EHT),
due for a public announcement any day now.
But long before
the EHT, there was an astrophysicist named Jean-Pierre Luminet. All
the way back in 1978, he already gave us what could be thought of as the very
first image of a black hole's event horizon. It's not, of course, an actual
photo. Luminet, whose background was in mathematics, used his skillset to
perform the first computer simulation of what a black hole might look like to
an observer, using a 1960s punch card IBM 7040
computer.
“At the time it was a very exotic subject, and most astronomers did not believe in their existence,” Luminet told ScienceAlert. “I wanted to explore the strange physics of black holes and propose specific mechanisms that could help to get indirect signatures of their very existence. Also, to pursue the pun, with my name 'Luminet' I liked much the idea of how a perfectly non-luminous star can give rise to observable phenomena.”
What data the
computer returned, Luminet then painstakingly plotted by hand with pen and
India ink on negative paper, as if he were a human printer. That fuzzy image -
seen above - shows what a flat disc of material falling into a black hole might
look like if we were close enough to see it. It doesn't look flat, because the
intense gravity of the black hole is bending light around it.
“Indeed the gravitational field curves the light rays near the black hole so much that the rear part of the disk is 'revealed',” Luminet explained in a paper published on arXiv last year. “The curving of the light rays also generates a secondary image which allows us to see the other side of the accretion disc, on the opposing side of the black hole from the observer.”
Luminet was the
first, but he wasn't the only one captivated by the mystery of what a black
hole might look like. Others have attempted to visualise these objects since
then, and even put their efforts on the silver screen.
The 2014
Christopher Nolan film Interstellar was lauded for its
supposedly "scientifically accurate" depiction of a black hole, based
in large part on the work conducted by Luminet decades earlier, and created in
consultation with theoretical physicist Kip Thorne of
Caltech. Ultimately, the film opted for a simplified version, to be less
confusing and look pretty on screen. It was certainly impressive; but,
according to both Luminet and Thorne, it's not really what a black hole would
look like.
The primary and
secondary images created by the gravitational field are present and correct.
But, unlike Luminet's image, the disc's brightness is uniform.
“It is precisely this strong asymmetry of apparent luminosity,” Luminet wrote, “that is the main signature of a black hole, the only celestial object able to give the internal regions of an accretion disk a speed of rotation close to the speed of light and to induce a very strong Doppler effect.”
He penned a 15-page paper on the film's
science, and Thorne himself wrote a
book on the topic. You may notice that all of these versions of a
black hole look very different from another type of black hole image you may
have seen, most famously for the LIGO
discovery in 2016. These are based on the work of
astrophysicist Alain Riazuelo, of the French National Centre for Scientific
Research and International Astronomical Union, who first simulated such a black
hole in 2016.
The reason these
black holes look different is because the artwork shows a quiescent black hole
- one without an accretion disc. Denuded of that shroud of dust and gas, the
black hole's gravity warps the space behind it; if we were close enough to be
seeing the black hole like this, we would be in motion, captured by its gravity
in orbit. This is why it appears to move across the field of stars.
In the case of two
black holes together, as seen in the LIGO video, each black hole has a small
banana-shaped secondary image of the other hole appearing behind it. (Gravity
is neat.)
The EHT has been
focusing on Sagittarius A*, the supermassive black hole at the centre of our
own galaxy, the Milky Way. We don't know what we're going to see; it's possible
that the data will only return a few blurry pixels. (If that's the case, more
telescopes will join the collaboration, and the scientists will try again.)
Given the black
hole had an accretion disc during observations, we're anticipating something
that looks a lot like the work of Luminet. In addition, the collaboration will
hopefully help us understand more about the polarisation of radiation, the
structure of the magnetic field, and the black hole's relativistic jets. It's
already yielded up clues about the structure
of space around the black hole.
But what's the
most exciting part about the work of the EHT? We're totally with Luminet on
this one. “The photo of the accretion disc!” he said. And we can hardly wait.
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