Black holes and extreme phenomena

A blurry orange ring of light surrounds a black disc in the centre, on a black background. — The first image of a black hole: the shadow of the giant black hole at the centre of the galaxy M87, obtained by the Event Horizon Telescope and published on 10 April 2019. EHT Collaboration, CC BY 4.0, source

This image went around the world: a blurry orange ring surrounding a black disc. The image of the black hole in the galaxy M87, published on 10 April 2019.¹

It is not a photograph.

The way it was made is more interesting than the image itself.

First, what we are talking about

A black hole is a region of space where gravity is so strong that nothing can get back out — not even light. Its boundary is called the event horizon: whatever crosses it does not come back. It is not a hole in the sky, but an enormous mass compressed into a tiny space.²

The one in the image lies at the centre of a giant elliptical galaxy called M87 (Messier 87). Its mass is about 6.5 billion times that of the Sun.³

The ring visible around it is formed by matter rotating around the black hole at a speed close to that of light. The black disc in the centre is its shadow: the zone from which no light reaches us. The ring is ten times brighter than this central depression.⁴

The dark zone, however, is not the black hole itself. It is wider than the horizon, because gravity bends light rays around the black hole: what we see is an enlarged mirage, and not the edge itself.⁵

What it really is

In April 2017, a network of radio telescopes spread across the planet — eight stations at six sites — observed the same target at the same moment. Atomic clocks, hydrogen masers, ensured that they all stayed synchronised.⁶

Put together, these telescopes form a virtual instrument the size of the Earth. This is what is called very long baseline interferometry.⁷

An image then had to be reconstructed. No telescope directly “saw” the ring: the instruments recorded radio waves at a wavelength of 1.3 millimetres, invisible to the eye, and algorithms calculated what structure could produce these measurements.⁸

The colour, meanwhile, is a matter of choice. The radiation captured has no visible colour. The orange represents the intensity of the signal: the lighter the shade, the stronger the emission.

None of this is fakery. But an image like this one is, for that very reason, a claim: here is what these instruments captured, reconstructed in this way, translated into these colours.

How they guarded against themselves

The researchers had a problem, and they knew it: they expected to see a ring. The theory of general relativity had predicted it for decades. The risk was therefore of unintentionally producing the image they hoped for (confirmation bias).

So they set up safeguards against themselves.

They formed four teams, each blind to the work of the other three. The teams produced their images of M87 with two different methods: an established method, called CLEAN, and a newer one.⁹

The four results were then compared.

All of them contained the same ring, about forty microarcseconds across — and this size matches what theory predicts for the path of light bent around the shadow of a supermassive black hole.¹⁰

That is what made the result convincing. Not the beauty of the image, but the fact that isolated groups, working with different methods, arrived at the same result. The ring also appears on all four nights of observation, with the same diameter and the same width.¹¹

The aim of this set-up is stated in black and white in their publication: to avoid shared human bias.¹²

And now, what happened next

In the days following publication, the world’s press presented a single person, the computer scientist Katie Bouman, as the sole creator of the image. A photograph showing her discovering the reconstruction on her laptop went viral.

She spent the following weeks correcting this narrative.

Her clarification never varied: no algorithm and no person made this image alone. The list of the article’s signatories bears this out: it runs to several pages, and it ends at the letter Z.¹³

Why this story is told here

Elsewhere, this site recounts several stories of people who were erased — the page on light and distances brings them together (notably the story of Henrietta Leavitt, whose law on Cepheids was used without her name at first being given its rightful place).

Here is the reverse case: a person who is credited with too much, and who refuses that monopoly.

This is not a sentimental anecdote. It is proof that the problem is not limited to personalising or forgetting people. Attributing a complex discovery to a single person distorts the very nature of science in both directions — and it also harms the person one claims to honour, by erasing the ecosystem of engineers and researchers without whom nothing would have been possible.

A modern discovery is almost always a large-scale collective effort. That is harder to tell briefly — and that is why the media often prefer to reduce the story to a single face.

Scientific debates and controversies

A scientific image is not a passive snapshot. It is a result produced by methods — and those methods are legitimately open to criticism.

The 2022 objection: an instrumental artefact?

Japanese researchers took the public data on M87* and published a reanalysis: according to them, the ring could be a processing artefact, a structure produced by the way the array samples the sky, and not by the black hole. They later raised a similar objection to the image of the black hole in our own galaxy.¹⁴

What to take away

Neither “the image is false” nor “science is a matter of faith”.

Here are the data, here is the method, here is the published objection, and here is the discussion that follows. This is how knowledge advances — and it is worth noting that this objection was only possible because the collaboration had made its data public.

A millisecond, ten thousand years

Since this page deals with the extreme phenomena of the Universe, here is another, which a simple comparison of scale helps to grasp.

In a few milliseconds, a fast radio burst (FRB) releases a colossal amount of energy.

They fall into two families: those that repeat, and those that have been observed only once. It is not yet known whether they share the same origin.¹⁵

The day a nearby signal was picked up

In April 2020, a radio burst was detected coming from a magnetar located in our own galaxy — a neutron star with an extremely intense magnetic field. Two observatories recorded it at the same moment.¹⁶

This was proof that a magnetar can generate this type of pulse.

The essential logical distinction

That burst, however, was much less powerful than those detected in distant galaxies.

It has been shown that a magnetar can produce a radio burst. It has not been shown that all radio bursts come from magnetars.

Confusing “A can cause B” with “all B is caused by A” is a common error of reasoning. Keeping the two apart is a fundamental tool of critical thinking.

Note 1

This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87.

Event Horizon Telescope Collaboration, “First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole”, The Astrophysical Journal Letters, vol. 875, L1, 2019, preprint en accès libre