From Einstein's doubts to photographs: how black holes became real objects

The Short Version

Black holes began as a solution to Einstein's equations that Einstein himself did not accept. Karl Schwarzschild found it in early 1916, and about 25 years later Einstein wrote a paper arguing that such objects do not exist [1]. Today, three physicists speaking in separate conversations treat them as plain facts of the sky: Andrew Strominger says "We've seen those regions," Michelle Thaller describes how eight observatories were turned into one Earth-sized telescope to photograph one, and Brian Cox says the picture "looks like the prediction" made from a theory published in 1915 [1][2][3]. The change came from several independent kinds of observation, images, stars whipping around an unseen mass, glowing gas disks and gravitational waves, all pointing the same way [2][3]. What observation has not done is settle what a black hole is on the inside. There the three agree that the theory that predicted black holes also breaks down, and that this breakdown is now pushing physics toward the idea that space and time are not fundamental [1][2][3].

Table of contents

What They Said

Andrew Strominger: what a black hole is, and why Einstein got it wrong

Strominger, a theoretical physicist at Harvard, gives the definition first. A black hole is a region of space-time from which light can never escape [1]. He explains it with escape velocity. A rocket needs about 11 kilometers per second to leave Earth, and less to leave a smaller body like the moon. Gravity's pull grows with mass, and light carries energy, so it feels gravity too. Pack enough mass into a small enough region and the escape velocity reaches the speed of light, which is the universe's absolute limit, and at that point even light is trapped [1]. He is clear that this is settled as a fact: "It definitely happens. We've seen those regions." [1] He says there are thousands or millions of black holes in the sky and that we now have spectacular pictures of them, while adding that the proper explanation of why light cannot escape is still debated [1].

The history he tells is short and awkward for its hero. Einstein wrote his field equations in 1915. Schwarzschild found the black hole solution in early 1916. About 25 years later Einstein wrote a paper arguing that black holes do not exist, a paper Strominger jokes would earn a student in his general relativity course a C minus [1].

Asked why the idea was so hard for Einstein, Strominger says "I think his reasoning was ultimately wrong" and begins his account at the center of the black hole, behind the horizon, in a region nobody can see and live to tell about [1]. That is where the singularity sits, the place where Einstein's equations become infinite and, as Strominger puts it elsewhere, computers crash [1]. He sets this beside a second error of the same period. In the 1930s Einstein also wrote a paper arguing that gravitational waves do not exist, because physicists were confused about coordinate transformations [1].

Strominger explains that confusion with weather stations. Two people mapping the weather across a country with different sets of stations should end up with the same map and give a driver the same route to stay dry. The labels are an artifact, not physics [1]. That is almost true, he says, but subtleties arise where space-time has edges, and physicists are only now, a century later, close to resolving them [1]. Einstein's own path shows how slippery this was. His 1914 version of general relativity was only partly coordinate invariant, and it predicted the bending of starlight by the Sun wrong by a factor of two. An expedition sent out during World War I to measure the bending was captured before it could, "And that gave Einstein four more years" to get the theory right [1].

Strominger draws a lesson about how to judge a scientist from this record: "It's not how many things you got wrong. It's not the ratio of how many you got wrong, how many you got right. It's the number that you got right." [1] Great discoveries look obvious afterward, he says; after he and his collaborators solve a problem, they cannot reconstruct how confused they were two weeks earlier [1].

He also reverses the usual reading of the singularity. An infinity at the center of a black hole is not, for him, an embarrassment. It shows where the theory needs improving. "My view is that there's no exact equation. Everything is an approximation," he says [1]. Newton's gravity needed small corrections, Maxwell's electromagnetism needed quantum mechanics, and general relativity is special because it predicts its own demise: a perfectly ordinary system can evolve into a singularity where the equations no longer apply [1]. "Edges are wonderful because that keeps us in business," he says [1].

Strominger again: what the photograph can and cannot tell us

Strominger belongs to the Event Horizon Telescope collaboration, which produced the famous image of the black hole in the galaxy M87, though he stresses he had nothing to do with the experiment [1]. He is precise even about its date, correcting the suggestion that the picture was taken in 2019: "Not taken in 2019. Present in 2019." [1]

His contribution starts from a limitation. The image mostly shows the hot matter swirling around the black hole, and the makeup, temperature and magnetic fields of that matter are unknown [1]. So how do you learn about the hole and not just its surroundings? Working with Michael Johnson, Alex Lupsasca and others, he found an answer in the way light behaves near it. "A black hole is a mirror," he explains [1]. A photon heading straight in is lost. One that just misses can swing around the back and return, so that in principle you would see yourself. It can also loop twice, or three times, giving infinitely many copies with small delays between them [1]. Those orbiting photons are the photon ring [1].

His comparison is a department store's three-way mirror, where the relationship between each reflection and the next depends only on the mirrors. "It doesn't matter what clothes you're wearing." [1] You could go a thousand times in different clothes and find the same relation between successive images. In the same way, comparing successive images in the photon ring reveals the black hole's geometry and spin whatever the swirling matter is doing. "And that is how we're going to learn about the black holes," he says [1].

He notes that despite volumes of research on black holes, this "was a physical question that had never been asked in exactly this way" and that it is now shaping plans for future images [1]. The photograph, in other words, changed the questions theorists ask. "the black holes just have so many magic tricks and they do so many weird things. And the photon ring is among the weirdest of them," he says [1].

Michelle Thaller: how the picture was taken

Thaller, an astrophysicist and retired NASA executive, describes the photograph as something she would have bet could not be done [2]. The Event Horizon Telescope linked eight observatories around the planet, one at the South Pole, others in the United States and Chile, into a single virtual instrument [2]. The requirement was severe. Every site had to catch the very same wavefront of light. If one had caught a wavefront a millionth of a meter later, she says, there would have been no image: "the same wave of light, had to be caught in all of those telescopes at once." [2] The data were so large that the teams shipped hard drives instead of sending them [2].

The result, in her words: "They managed to make a telescope that's actually as big as the Earth" and with it they took a picture of the dark part of a black hole [2]. She names what that dark part is, the "shadow of the event horizon," which appears slightly larger than the event horizon itself because space and time bend around the black hole [2].

Thaller places the image among other lines of evidence. Astronomers have watched stars such as S2 whip around the black hole at the center of our own galaxy at nearly 20 million miles an hour, and another at more than 50 million, which lets them test time dilation [2]. Discussing the film Interstellar, she says that "time really does slow down close to a black hole. And again, we observe this." [2] She also describes an older puzzle that black holes ended up solving. Astronomers once wondered whether certain hugely bright objects were white holes, the far end of a tunnel through space. That idea had more of a following in the 1960s and 70s and has fallen out of favor: "But now we know that these super bright objects are actually hot gas disks around black holes." [2] They are among the brightest things known in the sky, visible billions of light years away [2].

Then there is the second kind of signal altogether. Thaller recalls the two LIGO detectors, with four-kilometer laser arms sensitive to distortions thousands of times smaller than an atomic nucleus. When both recorded the same signal from two merging black holes, offset by exactly the light-travel time between the sites, she says her heart dropped with joy [2].

From all this comes a census. "every major galaxy has a big black hole in the center," she says [2]. Ours is about four million times the mass of the sun, and the largest reach tens of billions of solar masses, with event horizons about the size of Pluto's orbit [2].

She is just as careful to mark where seeing stops. Of the region the photograph outlines she says, "We don't even know if there really is an interior to a black hole. All the equations blow up." [2] And she passes on a joke from NASA colleagues who study neutron stars and think their objects are the more exciting ones, on the grounds that "with a black hole, you can't see anything." [2] Everything collapses behind an event horizon and nothing comes out, while a neutron star is right there in front of you [2].

Brian Cox: what they do to physics

Cox, a particle physicist at the University of Manchester, sums up the present state of the subject in one sentence. Asked how anyone studies a black hole, he says, "It's mainly theoretical, although we have now got photographs of them." [3]

He counts two photographs, both from radio telescopes: Sagittarius A* at the center of our galaxy, and the black hole in M87, which is 55 million light years away, about six billion times the mass of the sun, and has an event horizon larger than our solar system [3]. He explains what the image shows. The material around the hole forms a flat disk. "So you think, think Saturn's rings." [3] What appears in the photograph is light from that flat disk bent around the black hole, which is what Einstein's theory, published in 1915, says such an object should look like. For the first time in history there was an image, "And it looks like the prediction." [3]

Cox adds gravitational waves as the other direct evidence. Colliding black holes ripple space-time, and the LIGO detectors pick up distortions far smaller than an atomic nucleus [3]. Kip Thorne calls these ripples "a storm in time," and Cox notes that they are passing through the room as he speaks, very slightly changing the rate at which time passes [3].

Having become real, black holes pose a question Cox calls the simplest one could ask: "what happens to stuff that falls in." [3] Stephen Hawking calculated in the 1970s that black holes glow, which means they shrink and eventually disappear, and his mathematics said the radiation carried no information about what had fallen in [3]. That would make black holes the only thing in physics that truly erases information, since in principle even a burned notebook's information survives in the ash and the light [3].

Cox also describes the inside as Einstein's theory gives it. You would not notice crossing the horizon of a supermassive black hole, but once inside you are headed to the singularity. Asked what that is, he says one answer is that we do not know. "But in Einstein's theory, it's the end of time." [3] Space and time are so distorted that they in a sense flip over, and the singularity is better pictured as a moment you cannot avoid than as a place, no more escapable than tomorrow [3].

He reports where the paradox stands. "So we now think that black holes do not destroy information. We're pretty sure." [3] It has been shown mathematically, to most people's satisfaction, that the information ends up out again, possibly through some kind of wormhole connection, though the physical picture is not yet understood [3]. And he draws the consequence that gives black holes their importance beyond astronomy. The study of how information gets out is leading physicists to suspect that space and time are not fundamental but emerge from something else, which he describes as something like a network of entangled qubits [3]. He notes the order in which this happened: nobody started with a theory of emergent space and time and went to black holes to check it. The black holes forced the idea [3].

Where They Agree, and Where They Don't

They agree that black holes are observed, not merely inferred. Strominger says we have seen those regions [1]. Thaller separates what follows from what came before with the words "this is not observation," used only when she turns to speculation about universes inside black holes; everything before that point, she says, is something we see [2]. Cox says the image matches the prediction [3].

They agree that Einstein's theory both predicted black holes and fails inside them. Strominger says the equations become infinite at the singularity [1]. Thaller says all the equations blow up [2]. Cox says that in Einstein's theory the singularity is the end of time, and that one honest answer to what it is remains "we don't know" [3].

They agree that black holes point beyond space and time as we know them. Cox says space and time appear to emerge from something else [3]. Strominger, who says time like space must ultimately be emergent, answers a question about how to picture this with "They're both illusions." [1]

They give different masses for our galaxy's black hole. Thaller says the one at the center of the Milky Way is about four million times the mass of the sun [2]. Cox's account puts Sagittarius A* at about six million [3]. Neither discusses the other's figure, and the sources do not explain the difference.

They describe the photograph differently. Thaller calls it a picture of the shadow of the event horizon, the dark part [2]. Cox describes the bright part, light from the disk bent around the hole [3]. Strominger stresses that the image mostly shows hot swirling matter with unknown properties, which is exactly why it is hard to learn about the black hole from it [1]. These are three descriptions of one picture, but the emphasis matters: Thaller and Cox present the image as confirmation, and Strominger presents it as a starting point that needed a new idea, the photon ring, before it could say much about the hole itself.

They differ in confidence about the information paradox. Cox says physicists are pretty sure information gets out and that this has been shown mathematically to most people's satisfaction [3]. Strominger is more guarded about his own part in it. Working with Cumrun Vafa, he used string theory to count the information stored in certain black holes and found it matched the amount that Bekenstein and Hawking had shown a black hole must hold, an amount proportional to its area and not its volume [1]. He compares that to counting the memory chips in a phone: it tells you how 64 gigabytes are stored, not how the data gets in and out, and how information escapes is the harder problem [1]. On his later work with Hawking and Malcolm Perry, which showed that things thrown into a black hole leave subtle imprints on the horizon called soft hair, he says it exposed an error in the argument that all black holes are the same, and that whether it is the key error or a technical point remains to be seen, though Hawking was sure it was critical [1]. Both men agree the full physical picture is missing.

They differ on whether there is an inside. Cox narrates the fall through the horizon to the singularity [3]. Thaller doubts that there is an interior at all if space and time do not exist there [2]. Strominger says that in the simplest description a black hole is a hole in space with nowhere to store anything, and yet it must store information, which is what led to the holographic idea that the information in a region can be held on its boundary [1].

Why It Works

The sources explain why an object defined by letting nothing out could nonetheless be observed.

Gravity bends light, so a black hole has an outline. Because space and time curve around it, the dark shadow appears slightly larger than the horizon itself, and light from the surrounding disk is bent into a shape that can be predicted in advance [2][3]. A prediction of that kind can be checked against a picture.

Matter falling in is bright even though the hole is dark. The hot gas disks around black holes are among the brightest things in the sky [2]. The surroundings give the hole away.

Things nearby move in ways only a huge unseen mass explains. Stars close to the center of our galaxy orbit at tens of millions of miles an hour, and observations near black holes show time itself running slow [2].

A telescope can be as large as the planet if the timing is exact. Catching the same wavefront at eight sites turned separate dishes into one instrument the size of the Earth [2].

Collisions shake space-time, and two detectors can confirm each other. The LIGO sites saw the same signal offset by the light-travel time between them, which is what a real wave crossing the planet would do [2][3].

Successive images depend only on the black hole. Strominger's photon ring works because the relationship between one looped image and the next is fixed by the hole's geometry and spin and is indifferent to the messy matter that supplies the light [1].

Theory had independent checks of its own. Wheeler, Bekenstein and Hawking showed that quantum mechanics and gravity cannot be consistent unless a black hole stores information proportional to its area [1]. Putting the central equations of Hawking and Boltzmann together, Strominger says, "you get a formula for the number of gigabytes in a black hole." [1] He calls the string theory calculation a moment of truth, since a mismatch would have shown the theory inconsistent [1].

Where It Applies, and Where It Doesn't

Observation reaches the horizon and stops. The photographs show the shadow and the light around it. They do not show an interior, and Thaller is unsure there is one to show [2].

The image does not test the deepest ideas. Strominger's recent work with Dan Kapec and Alex Lupsasca explores whether the holographic plate of a real astrophysical black hole lies in a region that includes the photon ring. He says they have evidence from simpler lower-dimensional examples, not proof, and that he does not expect imaging to test it directly [1]. He also does not expect a yes-or-no experimental test of string theory in the foreseeable future, and says he does not believe it himself without an experiment [1].

How the giant ones formed is unknown. Thaller calls it one of the most fascinating unanswered questions in astronomy: how do you gather ten billion times the mass of a star so early [2]? The James Webb Space Telescope has found "little red dots" from the early universe. She says the leading explanation is that huge gas clouds collapsed directly into black holes wrapped in hot gas, which later merged, and adds, "But at the moment, it's one of the best explanations we have." [2] Cox, shown a report on the same dots, declines to guess: "I don't know. It says there that we don't know. So I'm going to go with that." [3] He says of supermassive black holes generally, "We don't quite know how they form actually" [3].

Some ideas remain conjecture, and one has been dropped. Whether a new universe forms inside a black hole, and whether the Big Bang was such an event, are questions Thaller labels as conjecture: "We don't yet have the physics to answer them." [2] White holes as an explanation for the brightest objects have fallen out of favor [2].

Why light cannot escape is still argued over. Strominger offers escape velocity as a way in, and says the proper explanation is still debated [1].

What To Do With It

1. Read the photograph correctly. The bright ring is hot matter in a flat disk, with its light bent around the hole. The dark center is the shadow of the event horizon, a little larger than the horizon itself. It is not a picture of the inside [1][2][3].

2. Keep three categories apart. Thaller's habit is to say when she moves from observation to conjecture [2]. Black holes existing is observation. Information escaping is mathematics most physicists accept [3]. Universes inside black holes is conjecture [2].

3. Watch for the photon ring. Strominger says the idea is shaping plans for future black hole images. Successive ring images are what would reveal a hole's geometry and spin [1].

4. Follow gravitational-wave astronomy. Cox mentions proposals to put a detector in space, one of them called LISA, using lasers between satellites to build something far larger than LIGO [3].

5. Follow Webb's little red dots. They may be the seeds of the giant black holes, and Thaller says astronomers will keep observing them and finding new ones [2].

6. Treat an infinity as a signpost. When a theory produces a singularity, Strominger's reading is that it has told you where it stops working, which is where the next discovery is likely to be [1].

7. Judge a scientist by what they got right. Einstein rejected black holes and gravitational waves and is still Einstein. Strominger's rule is to count the number right [1]. Cox's version is that in science "you have to be delighted when you're wrong," [3].

8. Watch Interstellar with this in mind. Strominger praises its scientific accuracy and Kip Thorne's role, and Thaller says it has good teaching moments, including time slowing near a black hole [1][2].

Sources

1. Andrew Strominger on Black Holes, String Theory, the Holographic Principle, and Soft Hair · Lex Fridman Podcast
2. Michelle Thaller on Black Holes, Time, Quantum Weirdness and the Limits of Science · The Joe Rogan Experience
3. Brian Cox on Black Holes, the Great Silence and the Meaning of Life in the Universe · The Joe Rogan Experience