Knowable Magazine · How black holes morphed from theory to reality

Black holes, from theory to reality: Physicists were mostly skeptical when the idea of black holes emerged a century ago from Einstein’s work. This podcast episode tells the story of how the evidence evolved from circumstantial to conclusive.


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Introducing the Knowable podcast. What are the limits to what’s knowable — and how does our thinking about big questions in science and technology evolve over time? Take an audio journey with Knowable Magazine from Annual Reviews as we explore puzzles as diverse as the existence of black holes and how to build an artificial heart — with plenty of surprises along the way. More episodes


When does a concept become concrete? How does an abstract idea evolve into something we believe — maybe even know — is a part of our universe? How does the lens of time transform an artifact of an equation from something unknown into something real, something tangible? And what evidence do we require when our idea is, by its very nature, invisible?

This is Knowable. And I’m Adam Levy.

It’s hard to imagine something less visible than a black hole. By their very definition, virtually nothing can escape these overwhelmingly dense bodies — not even light. An isolated black hole would look like exactly that: a black hole, a complete absence of light. Anything that falls past the boundary called the “event horizon” is doomed — destined never to be seen again by the outside world. The huge “tidal” forces would rip apart any object falling towards the very center.

Today, astronomers and physicists understand that black holes can form from the remains of massive dead stars. Observations suggest that these once fantastical objects are in fact regular parts of our universe. Supermassive black holes have been identified in the center of many galaxies, including our own, where they have been shown to play a key role in galaxy evolution. But the evolution of our black hole knowledge has been just as tumultuous as the formation of these shadowy stellar corpses.

Black holes can trace their beginnings back to Albert Einstein. His 1915 theory of general relativity called for a radical rethinking of one of the most fundamental forces in the universe: gravity. Physicists had long pictured space and time as the passive stage on which objects — such as planets and people -— exist and events take place. But general relativity breathes life into space and time, turning them into active players in their own right. Einstein’s theory proposed that gravity — the force that holds us to the Earth, and moves the Earth around the sun and the sun around our galaxy — that gravity operates by distorting this space-time fabric.

And the theory wasn’t just radical; it also appeared to be right. It explained why Mercury’s orbit disagreed with predictions from Newton’s laws, and it predicted how light rays bend as they pass the sun. The latter was quickly confirmed during a solar eclipse, as one 1919 New York Times article announced:

Lights all askew in the heavens. Men of science more or less agog over results of eclipse observations. Einstein theory triumphs.

Einstein himself had been agog over the consequences of general relativity. Only a few months after he published his theory, German physicist Karl Schwarzschild found an “exact solution” to Einstein’s equations, showing how they described the structure of space-time around a stationary, spherical object, such as a star.

G equals minus, open parentheses, 1 minus RS over R, close parentheses, C squared, DT squared plus, open parentheses, 1 minus RS over R...

He solved this mathematical problem while serving as a soldier in the German Army in World War I. Einstein responded:

I had not expected that the exact solution to the problem could be formulated so simply. Your analytic treatment of the problem appears to me splendid.

But there was something strange lurking in Schwarzschild’s solution.

Clifford Will: “At the time, of course, Einstein, Schwarzschild, didn’t really understand what he had found.”

This is Clifford Will, a physicist based at the University of Florida, who has been peering at the mathematical theory and experimental observations of general relativity for decades. Looked at from a particular perspective, the Schwarzschild solution seemed to suggest that a mass gathered in a small enough space would lead the equation to blow up to infinity in a way that surely couldn’t correspond to anything in the real world.

Karl Schwarzschild would die just a few months later, long before the full consequences of his equation were understood.  His name (Schwarzschild) translates to “black shield” — foreshadowing the darkest shadow of all.

But although the mathematics of the solution provided the groundwork for the idea of a black hole, the concept was still overlooked by physicists. Two decades later, in 1939, a paper titled “On Continued Gravitational Contraction” by J.R. Oppenheimer and H. Snyder made the concept a little more real. They suggested that a massive star at the end of its life could collapse under the force of its own gravity until it becomes invisible, not even letting out any light:

The star thus tends to close itself off from any communication with a distant observer; only its gravitational field persists.

Here’s Clifford again.

Clifford Will: “And from today’s point of view, this is like, this is a perfect description of what happens when a star collapses and forms a black hole. The trouble was, in 1939, general relativity just wasn’t on anyone’s mind, so this paper was almost completely ignored and forgotten.”

After the second world war, physicists were focused on ideas like nuclear physics, lasers and transistors. It wasn’t until the 1950s and 1960s that researchers began really tinkering with the Schwarzschild solution, appreciating it for all its mathematical oddities, though they were still some way from thinking of black holes as existing out in the universe. They started to understand that the mathematics was pointing to a point of no return: the event horizon.

Clifford Will: “People started to understand that what was going on was this horizon. But it still, I would say in the late ‘50s, people were not seriously thinking of these as real objects.”

And why should researchers have thought of black holes — then referred to as “completely collapsed gravitational objects” — as real? After all, nothing had been seen in the universe that evoked the need for the immense gravitational fields emanating from black holes. That is, until quasars.

Quasars, or quasi-stellar objects, were unfathomably bright sources of radio waves first identified by telescope in the late 1950s. The strength of their signal suggested cosmic sources with fantastical forces.

Clifford Will: “The fact that they were so bright, people could not imagine what could possibly be the source of such energy output. So this was a big mystery. But a few people started to wonder — well, maybe the strong gravitational field associated with things like this Schwarzschild object might be relevant.”

Quasars, people theorized, may mark massive releases of energy from immense black holes sitting at the center of galaxies, devouring matter and spewing out energy in the process.  But this was an extraordinary idea. And extraordinary ideas require extraordinary evidence. More quasars were spotted in the 1960s, the decade in which black holes also became known as black holes.


And astronomers collected further circumstantial evidence with the advent of X-ray telescopes that identified a new set of mysteries. The first of these signals was spotted in 1971. They originated from stars quickly revolving around a massive, but invisible, dance partner. Anne Cowley, an astronomer at Arizona State University, remembers studying photographic plates that captured the movements of one particular star. The star was undergoing huge accelerations, suggesting it was rapidly rotating around another object.

Anne Cowley: “I remember being in the dark room and looking at several consecutive plates and seeing that there were huge changes in the velocity. And so that night we knew we had something extremely unusual and interesting.”

And no one doubted that astronomers were finding unusual and interesting things.  But there was still doubt that any observations were black holes themselves, even with evidence ranging from low-frequency radio waves to rapid X-rays.  This reluctance persisted for years — decades even. In 1992, Anne published an article in the Annual Review of Astronomy and Astrophysics titled “Evidence for Black Holes in Stellar Binary Systems,” stating in the introduction:

Although numerous black-hole candidates have been put forward, conclusive proof of their presence has been much harder to find. The fact that even the most likely examples are still referred to in the literature as ‘candidates’ shows our hesitancy to accept their existence.

Anne Cowley: “I know particularly about astronomers there are always people who think, ‘Oh, no, there must be some other explanation,’ of almost anything that you claim that’s new. Yeah, I mean, I don’t remember particularly who were the grouches. But they tend to be the older people who are stuck in their ways. I mean, I’m old now, but I hope I’m not like that!”

But this skepticism wasn’t just from a few voices, as astronomer and physicist Heino Falcke of Radboud University in the Netherlands, recalls.

Heino Falcke: “In fact I remember conferences even in the late ’90s when we would have a vote: ‘Do you believe there are black holes?’ And you would see at least a third of the colleagues raising their arms and say, ‘We can’t be sure, we don’t know.’”

For some, the evidence still wasn’t conclusive. Even so, it seems by this point black hole doubters were something of a minority. And evidence continued to mount close to home as observations pointed to an incredibly massive object at the center of our own galaxy, the Milky Way. Heino coauthored a review in 2001 titled “The Supermassive Black Hole at the Galactic Center,” stating:

The Galactic Center is now known to contain arguably the most compelling supermassive black hole candidate, weighing in at a little over 2.6 million suns.

With all these lines of evidence, black holes gradually transitioned over the 20th century from strange unreal artifacts of the Schwarzschild solution, to objects that physicists and astronomers strongly believed to exist out there in the universe. But all this evidence was still circumstantial.

Heino Falcke: “You could just see their smoke.  But you couldn’t see the fire.”

But this wasn’t the end of the journey for black holes. Because the 21st century has seen new ways of peering behind the smoke screen.

The first is gravitational waves. General relativity predicted not only that objects should distort the fabric of space-time, but that motion of massive objects should radiate space-time ripples out through the cosmos, causing distances to contract and expand ever so slightly as they pass. These ripples were expected to be so small — dwarfed by a proton even when originating from the dance of incredibly massive objects. Incredibly massive objects like two black holes spiraling and merging together.

Astrophysicist Samaya Nissanke of the University of Amsterdam has spent her career thinking about gravitational waves. The possibility of observing gravitational waves, those tiny ripples in spacetime predicted by general relativity, had long been discussed. And as Samaya’s career developed, the incredibly sensitive equipment required was finally coming of age.

Samaya Nissanke: “You know, I did grow up I think believing one day it will happen, but whether it was in the 2020s, 2030s, I think that was an open question.”

And then in late 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO to its friends) began taking measurements before resuming full operations and happened to catch a signal.


Samaya Nissanke: “And I remember when the email basically came through a few days before, actually, the advanced LIGO detectors were supposed to switch on. And I remember thinking, ‘Oh no, this can’t be real!’”

The observation came almost exactly a hundred years after the publication of general relativity.  The signal seemed to be emanating from two black holes smashing together, and would earn LIGO the 2017 Nobel Prize. And since that first gravitational wave measurement, a cacophony of black hole mergers have been observed.

Samaya Nissanke: “And now it’s sort of like I wake up in the middle of the night — ‘Oh yes, another binary black hole merger has just been detected’ — so it’s a bit surreal actually.”

The detected vibrations, reminiscent of a sound wave, were exactly as expected for two colliding black holes.


Here’s Heino again.

Heino Falcke: “You were listening to the fire indeed. You were no longer seeing just the smoke, you heard the fire.”

But astronomers haven’t been content just hearing these snippets of black hole collisions. They have been pushing to catch a glimpse as close to a black hole as possible. For some 20 years, Heino and others formulated a plan to take an actual photograph of supermassive black holes. To do this would require a telescope about the size of the Earth.

Of course, Earth’s not going to be replaced with a telescope any time soon, so instead Heino and his collaborators enlisted telescopes around the world. Carefully combining their observations would allow these instruments to act as one single giant telescope: the Event Horizon Telescope. This needed high-tech equipment, a global collaboration...

Heino Falcke: “And you had to have perfect weather around the entire world at the same time.  The last thing is almost the most challenging aspect.”

All this effort to make the evidence for black holes that much less circumstantial.

Heino Falcke: “The great thing about the Event Horizon Telescope was that you would actually see the fire, and that gives a very different perspective.”

The first image from the Event Horizon Telescope — of a black hole at the center of galaxy Messier 87 — provided just that. Published in 2019, the false-color image of the radio signal displays flaming orange and yellow streaks, encircling an eerily dark central disk. Just over a century since Schwarzschild’s solution hinted at the possibility of these objects, physicists could now marvel at — and study — a portrait.

So is this what it takes to make something real? An image? For an astronomer, this isn’t necessarily any more convincing than all that hard data gathered from X-ray and radio telescopes before.

Anne Cowley: “The individual observations that show that there’s a massive object there are more convincing for me than any picture that you can make.”

But these direct observations, from LIGO and from the Event Horizon Telescope, do offer new tests for these objects. Black holes appear to look as the mathematics predicts for the relatively small black holes detected by gravitational waves, right up to the supermassive black hole at the center of a galaxy:

Heino Falcke: It turns out the theory is right for small objects of 10 to 20 times, or 30 times, the mass of the sun, to 6 billion times the mass of the sun. So over a factor of 100 million, the theory works. And so with these two perspectives — hearing and seeing black holes — I think we really nailed it now.

As our tools to observe the universe have built up — from radio waves to X rays; gravitational waves to the Event Horizon Telescope — so has our understanding and belief in black holes.  What started as a strange artifact of the mathematics of general relativity is now seen, by the vast majority of researchers, as a fact of our universe.

Samaya Nissanke: “For me it’s just incredible. You know, we do think of them as being sort of just astronomical beasts in their own right, as sort of the stars that we can actually see with the naked eye.”

Testament to how far black holes have come, in October 2020 three researchers were awarded the Nobel Prize in physics for their work on black holes — both theoretical and observational.

So when does an idea or a theory become known? Did it happen with Einstein and Schwarzschild? Or with quasars? Perhaps black holes were still unknown until we heard them with gravitational waves. Or maybe, just maybe, we still don’t know whether all these observations are really just black holes. Perhaps they’re just the shadows of something we still don’t understand. And now we have even more tools to find out.

Heino Falcke: “Are these really exactly the object as described by Einstein, or do we have to modify the theory?”

Samaya Nissanke: “Yeah, I’d say most of the community believe that we are seeing black holes, but I don’t think we should ever sort of sit back and say, ‘Oh it’s done and dusted.’”

The journey from an idea to knowledge is not one that has a clearly defined beginning or end. And this continuing quest is what defines the march of science towards the known.

Clifford Will: “This work came originally out of some just purely mathematical curiosity about these new field equations of Einstein. I don’t think anyone at the time, or even in the ’30s or the ’50s, could have imagined that black holes would be such an important part of astronomy and physics today.”

Heino Falcke: “And this is so beautiful — this is the beauty of physics. Something you really, as a physicist, you live for. When suddenly, you know, an idea, an abstract concept, becomes reality in front of your eyes.

Make sure to subscribe to Knowable wherever you get your podcasts. That way you won’t miss our next episode where we take a look at the quest to build an artificial heart. How do you engineer something, when nature has already come up with a solution?

It was really a disaster. And it was a disaster because we were trying to use technology to replace the heart in the image of the human heart.

That episode will be available from tomorrow. And if you enjoyed exploring the cosmic unknowns with us, be sure to drop us a review on your favorite podcast provider — it helps us reach even more ears. If you have any feedback, ideas, queries, whatever, send us an email at [email protected].

This podcast was made by Knowable Magazine from Annual Reviews, a journalistic endeavor dedicated to making scientific knowledge accessible to all. Through smart storytelling and sound science, Knowable aims to build understanding and fascination with the world around us. Knowable Magazine is free, and always will be. Read more at

In this episode you heard from Heino Falcke, Clifford Will, Samaya Nissanke and Anne Cowley.  There were also quotes from three papers: J. Robert Oppenheimer and Hartland Snyder, 1939; Anne Cowley, 1992; and Fulvio Melia and Heino Falcke, 2001. I’m Adam Levy, and this has been Knowable.