Black Holes from Theory to Reality

Episode Overview

Once dismissed as mathematical absurdities, black holes are now recognized as real, powerful features of our universe—cosmic wells where space, time, and even light collapse. In this episode of Math! Science! History!, we explore the astonishing story of how black holes evolved from a rejected theory to an accepted reality. From Einstein’s reluctance and Oppenheimer’s overlooked models, to John Wheeler’s advocacy and Stephen Hawking’s revolutionary radiation theory, this episode traces the full arc of scientific discovery—and what black holes reveal about our own place in the cosmos.

 

Three Key Take-Aways

Why Karl Schwarzschild’s World War I-era math predicted black holes decades before anyone took them seriously

How John Wheeler changed the game by naming—and championing—the black hole

What modern observations like Cygnus X-1, Hawking radiation, and LIGO’s gravitational wave detection tell us about collapsed stars and spacetime

Resources & References (the books include affiliate links)

Oppenheimer & Snyder (1939): On Continued Gravitational Contraction

David Finkelstein (1958): Past-Future Asymmetry of the Gravitational Field

Kip Thorne’s book: Black Holes and Time Warps: Einstein’s Outrageous Legacy

Marcia Bartusiak’s book: Black Hole: How an Idea Abandoned by Newtonians, Hated by Einstein, and Gambled on by Hawking Became Loved

LIGO and gravitational wave discovery (2015)

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Episode Transcript

(00:01):
Welcome to Math Science History. I'm Gabrielle Birchak, your host, and today
we're going to do some deep, dark research into the history of
understanding black holes and how it even got its name. By the time you are
done listening to this podcast, you're going to know so much more about the
study of these dark vacuums of nothingness. In the year 415, the infamous

(00:31):
philosopher and mathematician Hypatia of Alexandria, Egypt, was savagely murdered
by church monks. This murder shocked the Roman community and its government
leaders. Hypatia was known far and wide as a respected philosopher, mathematician,
government advisor, and a professor. Hypatia, the sum of her life, is a book

(00:53):
that I wrote that looks not just at the circumstances surrounding her death, but
also at the sum of her entire life. I weave in the details of her education,
disciples, Neoplatonic philosophies, female contemporaries, and the many
mathematics that she wrote and taught about. There is truly more to Hypatia's

(01:14):
life than her death. Hypatia, the sum of her life, written by me, Gabrielle Birchak,
is now on sale on Amazon. Buy your copy today. Imagine a place in the universe
where time stops, where space folds into itself, and where not even light can

(01:37):
escape. A place that devours everything, matter, radiation, even information. Now,
imagine the scientists who tried to explain it and were laughed at, ignored,
or dismissed as absurd. This is the story of black holes, perhaps the most
mysterious and misunderstood objects in the cosmos. And believe it or not, the

(02:00):
journey to understanding black holes is as strange and fascinating as the holes
themselves. It is a story that spans centuries, from the Enlightenment era,
stargazers to physicists in trench coats, from the elegant math to the cosmic PR
disasters. And yes, even a moment when the term black hole was considered

(02:22):
scandalous. The idea that gravity might be powerful enough to trap even light
isn't new. In 1783, the English natural philosopher Reverend John Mitchell wrote
a letter to Henry Cavendish proposing certain properties of a, quote, dark star.
Using Newtonian physics, he reasoned that if an object were massive and compact

(02:46):
enough, its gravitational pull could exceed the speed of light. Light, he
argued, wouldn't be able to escape. This concept predated Albert Einstein's
theory of black holes by over a century. A few years later, the French
mathematician Pierre Simon Laplace made a similar suggestion, noting that the

(03:07):
escape velocity of such a body would exceed the speed of light. And for more
information about Pierre Simon Laplace, please be sure to check out my previous
episode where I actually go back in time and interview him. Anyhow, these
musings that he had were clever, but they lacked physical evidence and were
based on classical mechanics, which assumed that light was made of particles.

(03:31):
When the presentation of wave theory gained traction in the 19th century, the
idea of dark stars fell out of favor and the notion faded into obscurity. In
November 1915, Albert Einstein presented his general theory of relativity, a
radical new model of gravity in which mass and energy bend the very fabric of

(03:52):
space and time. Just a few months later, in 1916, Karl Schwarzschild, writing
from the front lines of World War One, discovered the first exact solution to
Einstein's field equations, revealing a peculiar critical radius, which is now
known as the Schwarzschild radius, beyond which a star's gravity would

(04:12):
become so intense that not even light could escape. Yet Einstein himself
remained skeptical that nature would allow such extreme objects to exist. He
reportedly remarked that nature abhors a singularity, doubting that the universe
would permit mass to collapse into an infinite point of density. So needless

(04:34):
to say, there was a reluctant revolution. And up until this point, most
scientists believed that the death of a star meant it collapsed into a white
dwarf. So enter the brilliant theoretical physicist Subrahmanyan
Chandrasekhar, an Indian born scientist whose highly awarded career spanned the

(04:55):
years at Cambridge, Harvard, University of Chicago and Yerkes Observatory. At
the age of 19, yes, just 19 years old, he began to determine the calculations
within a white dwarf star and found that remnants within a star that are 1.4
times larger than our solar system's sun would be too large to create a white

(05:17):
dwarf. This value that he determined is known as the Chandrasekhar limit. It
was brilliant. However, at a 1935 meeting of the Royal Astronomical
Society, physicist Sir Arthur Eddington publicly ridiculed Chandrasekhar
stating that quote, there should be a law of nature to prevent a star from

(05:38):
behaving in this absurd way, unquote. Eddington backed up his statement with
an argument that perfectly reflected what Chandrasekhar was theorizing. In
other words, he used Chandrasekhar's theories to argue against himself.
Regardless, Chandrasekhar was embarrassed and actually considered quitting the
field of physics. He was only 19 years old. And this one gets me because he was

(06:02):
just a kid. So heads up tenured professors, sometimes these young kids
know what they are talking about. So in 1939, physicist J. Robert Oppenheimer
and Hartland Snyder published a groundbreaking paper that mathematically
described how a massive star could undergo unstoppable gravitational

(06:23):
collapse forming what we would now recognize as a black hole. Their model
titled quote, on continued gravitational contraction, unquote, predicted that
beyond a certain point, no known force could halt a star's collapse into a
singularity hidden behind an event horizon. Yet, at the time, almost no one

(06:46):
paid attention. World War Two erupted just months after their paper appeared
shifting the world's and the scientific community's attention toward wartime
research like radar, nuclear fission and weapons development. When physicists
finally returned to pure theoretical work after the war, it would take decades
before Oppenheimer and Snyder's startling insight was fully appreciated

(07:10):
and folded into mainstream astrophysics. It was like Oppenheimer and Snyder
discovered a monster in the basement, but everybody on the main level was
fighting zombies and couldn't go into the basement to look. After Oppenheimer
and Snyder published their 1939 paper, there was a silent period on the study
of black holes. However, in the late 1940s and through the 1950s, a few

(07:35):
important but quiet theoretical developments about gravitational
collapse and extreme objects did happen before John Wheeler really made black
holes a mainstream discussion in the 1960s. Almost immediately after
Oppenheimer and Snyder published their paper, Einstein published a counter
paper titled On a stationary system with spherical symmetry consisting of

(07:58):
many gravitating masses. In this paper, he argued that true gravitational
collapse wouldn't happen in nature. He believed that internal pressures inside
matter would prevent a singularity from forming and due to his long existing
credibility, it added to mass skepticism over Oppenheimer and Snyder's work. In

(08:19):
the 1950s, Italian mathematician Tullio Levi Civita analyzed Einstein's field
equations that were related to strong gravitational fields and the collapse of
a star. Though he didn't fully address black holes, he laid a mathematical
groundwork about extreme spacetime behaviors. In the late 1950s, David

(08:40):
Finkelstein published a paper titled Past-Future Asymmetry of the
Gravitational Field, which described the event horizon as perfectly all part of
spacetime. He referred to it as a one-way membrane in which matter could fall
inward but never escape. This one-way membrane directly refers to the

(09:00):
Schwarzschild radius. Then, in the late 50s, physicist Werner Israel began to
formalize an idea that once a collapsing star is formed, it remains simply mass,
charge, and spin, stating that all the other details do not matter. Theoretical
physicist Jacob Bekenstein later credited Israel's theory stating that black

(09:25):
holes have no hair, meaning that black holes are simply as Israel states. They
have mass, charge, and spin, and any other matter that might fall into the black
hole's event horizon is not visible. It eventually came to be known as the
quote, no hair theorem. Eventually, John Wheeler ran with it. John Wheeler's

(09:45):
influence went far beyond clever naming. In the late 1950s and 1960s, he played a
crucial role in pushing the physics community to take the idea of a total
stellar collapse seriously. Wheeler's work helped transform black holes from a
mathematical curiosity into a cornerstone of modern astrophysics. We'll

(10:07):
be right back after a quick word from my advertisers. At the heart of it all is
the dramatic fate of massive stars. When a star much larger than our Sun exhausts
its nuclear fuel, it can no longer produce the internal pressure needed to
counteract the relentless force of gravity. For moderately sized stars,

(10:29):
collapse halts at the white dwarf stage, supported by electron degeneracy
pressure. And for more information about electron degeneracy pressure, please be
sure to listen to my next podcast on Flashcard Fridays this Friday on
Subrahmanyan Chandrasekhar. But back to black holes. For even heavier stars,

(10:50):
neutrons resist collapse, creating neutron stars. But for the most massive
stars, even these quantum pressures fail. There is no known force strong enough to
resist gravity's pull. Collapse becomes unstoppable. As the star implodes, its
core compresses further and further. Eventually, it reaches a critical

(11:11):
threshold known as the Schwarzschild radius. As I noted before, the
Schwarzschild radius marks the boundary beyond which escape is impossible, not
just for matter, but even for light. This boundary is what we now call the event
horizon. Not to be confused with the Sam Neill cringe movie Event Horizon. It's

(11:31):
really a cringe movie. It's best reserved for nights of drinks, weed, and cringe
movie watching. And definitely popcorn. Lots of popcorn. But again, back to black
holes. Sorry, I love to digress. Inside the event horizon, space-time itself is so
warped that all paths, even those that would normally move outward, are dragged

(11:53):
inward. The core of the collapsing star is theorized to continue shrinking,
collapsing down to a single point of infinite density known as a singularity.
At this singularity, the known laws of physics, particularly general relativity,
break down. It's a region where quantities like density and curvature
become infinite, and our current scientific models can no longer predict

(12:17):
what happens. Wheeler's genius was not only in recognizing this grim end
point, but also in framing it in a way that physicists could grapple with. In
particular, he emphasized that black holes were not merely exotic anomalies.
Rather, they were a natural consequence of Einstein's equations, demanding

(12:38):
exploration, not dismissal. Wheeler's analysis also helped introduce a new way
of thinking about black holes. Instead of being pathological, they could be simple
and elegant. Working with physicists like Kip Thorne and others, Wheeler added to
the no-hair theorem, stating that everything else about the collapsing

(12:58):
star, its complex internal structure, its chemical makeup, its quirks, are lost
forever beyond the event horizon. This idea that black holes are simple
objects, despite their violent birth, helped scientists not only accept their
existence, but also to model them mathematically and explore their

(13:18):
behavior through thought experiments and, eventually, observational evidence.
Wheeler's advocacy came at the perfect time. Through the 1960s, technological
advances like radio astronomy and x-ray telescopes were uncovering phenomena
that fit beautifully with black hole models. Mysterious, powerful sources of

(13:38):
with no visible counterpart, like Cygnus X-1. Meanwhile, theoretical work by Stephen
Hawking, Roger Penrose, and others were making it increasingly clear that
singularities were not rare freaks of nature, but inevitable outcomes under
many conditions in general relativity. In this way, Wheeler's efforts closed the

(14:01):
gap between theory and observation by championing black holes not just as
possibilities, but as predictable, observable objects. Wheeler changed the
scientific landscape forever. His contributions reframed black holes from
speculative oddities into crucial testing grounds for understanding
gravity, space-time, and quantum theory. Today, thanks in part to Wheeler's work,

(14:25):
black holes are no longer cosmic mysteries hiding in mathematical margins.
They are essential pieces of the universe's story. Labs where the
boundaries of physics are pushed to their breaking point. The now familiar
name black hole was not always part of the scientific lexicon. Wheeler is
widely credited with popularizing the term between 1967 to 1968, but it had

(14:49):
surfaced a few years earlier in more informal contexts. In fact, the phrase
appeared in print as early as January 1964. Science newsletter reporter Anne
Ewing wrote that if enough mass were added to a dense star, quote, such a star
then forms a black hole in the universe, unquote, describing findings at a 1963

(15:15):
American Association for the Advancement of Science meeting. This was the first
published use of black hole in the astronomical sense, beating a Life
Magazine story by Albert Rosenfeld one week later. Neither Ewing nor Rosenfeld
identified who actually coined the phrase at those meetings. It was likely

(15:35):
tossed around informally by scientists chatting over drinks, or as I like to
call it, drinking and deriving. Historian Marsha Bartusiak later traced the root
of the term to Princeton physicist Robert H. Dick, who around 1960 had
jokingly likened a completely gravitational collapse to the black

(15:58):
hole of Calcutta, which was a notorious prison from which no one escaped. This
morbid quip, suggesting an object one can enter but never leave, apparently
evolved into the pithier label black hole that began circulating at
conferences in the early 1960s. For a few years, black hole remained a casual, even
controversial term. Many physicists still used more technical descriptions like

(16:24):
completely collapsed object or frozen star, and the concept of such an object
was itself viewed with skepticism by some. Wheeler, as well as Roger Penrose,
initially were no exceptions. Wheeler had doubts about truly collapsed stars,
but by the late 1960s, he had come around to Oppenheimer's 1939 prediction

(16:46):
of collapse to a singularity. In December 1967, during a lecture in New
York, Wheeler found himself repeatedly saying the cumbersome phrase
gravitationally completely collapsed object. As he later recalled, someone in
the audience interrupted and suggested, why not just call it a black hole?

(17:06):
Wheeler immediately recognized the value of the name and said it was
perfectly appropriate for such an object as he put it. Just a few weeks later, on
December 29th, 1967, Wheeler boldly used black hole in an address to the
American Association for the Advancement of Science annual meeting in New York.
He even included the term in the published write up of that talk titled

(17:30):
Our Universe, The Known and Unknown. It was in the spring 1968 edition of
American Scientist, marking the term's formal entry into scientific literature.
By attaching his considerable prestige to the new catchy name, Wheeler
effectively gave his authority to the term, as one historical analysis notes.

(17:52):
Physicist Kip Thorne later quipped that Wheeler became the, quote, enthusiastic
baptizer of black holes after overcoming his own earlier doubts. From 1968
onward, use of the term exploded both in academia and in popular culture, firmly
replacing the clunkier alternatives.

(18:12):
We'll be right back after a quick word from my advertisers.
When Wheeler began using black hole publicly, the scientific community's
reaction was mixed intrigue and mild discomfort. Many younger astronomers and
physicists embraced the term for its brevity and vivid imagery. It, quote,

(18:32):
immediately captured the imagination of scientists, unquote, according to some
historians. However, some establishment figures and editors were initially wary
of its informal, almost irreverent tone. Wheeler himself noted the advertising
value of the name. It was attention grabbing, but that quality also made it

(18:53):
sound almost too colloquial for formal discourse. In fact, science writer
Marsha Bartusiak observed that what Wheeler really provided was permission.
He never claimed to have invented black hole, but his use of it legitimized the
term in scientific circles. Bartusiak writes that he had the authority to give

(19:13):
the scientific community permission to use the term black hole. She implied
that without Wheeler's blessing, others might have hesitated to adopt such a
punchy phrase. Indeed, before 1967, researchers often kept black hole in
quotes or opted for technical jargon. Even Soviet physicists preferred terms
like frozen star in the early 60s. And in France, the literal translation,

(19:38):
tronois, raised eyebrows. For a time, French scientists used the more
genteel astre, oculus, which meant occluded star instead, until tronois
eventually won out. These hesitations show that the term was initially seen
as slang, evocative and handy, but not yet wholly respectable. Yet any serious

(20:01):
pushback against black hole quickly faded as evidence for these objects
mounted at the end of the 1960s. Once Cambridge astronomers announced the
first pulsars and candidates for black holes in 1967, the community needed a
convenient name and Wheeler's choice fit the bill. By 1970, research papers

(20:21):
freely used black hole without apology, and the term was appearing in journal
titles and conference proceedings. In short, what began as an informal quip
became an ideal name for the phenomenon. Succinct, descriptive and memorable.
Any initial unease was outweighed by the term's explanatory power. As one

(20:42):
Physics Today article put it in 1971, the name black hole conveys in two
words, the chief properties of these objects, a hole in spacetime that is
black because not even light escapes. Although black hole is commonplace now,
it provoked some amused reactions and some off color jokes in its early years.

(21:05):
And really, who could refrain from making off color jokes? I couldn't. The
term's stark literalness and possible double entendres did not go unnoticed.
At the 1967 lecture where Wheeler first adopted it, the audience reportedly
chuckled at the suggestion. Then when Wheeler added that black holes have no

(21:27):
hair to describe how these objects lack distinguishing features, it, quote,
prompted some controversy and generated a series of problems with the editor of
the Physical Review, who found the phrasing too flippant for a series
journal. This incident hints that even black hole initially struck some as too
irreverent. In hindsight, black hole was the perfect name for one of the most

(21:51):
radical predictions of modern physics. Its journey into acceptance was not
instantaneous. Early on, it was mocked by some as jargon from the hotel bar
circuit of astronomers and even deemed unseemly in certain languages. What
started as a conversational nickname, even a bit of a joke, is now an
indispensable concept in astrophysics. The initial chuckles and critiques have

(22:15):
long been overshadowed by the terms utility and popularity. It was even
useful during Stephen Hawking's theories as he stated that black holes,
quote, ain't so black. In the 1970s, black holes transformed from theoretical
oddities into dynamic players in the universe, thanks in large part to

(22:37):
Stephen Hawking. In 1974, Hawking stunned the scientific community when he
did propose that black holes aren't so black. Instead, black holes emit tiny
amounts of radiation, now famously known as Hawking radiation. This
groundbreaking idea suggested that black holes could eventually evaporate and
disappear, challenging the notion that nothing could ever escape from them.

(23:01):
Around the same time, astronomers made another monumental leap when they
identified Cygnus X-1, a strong X-ray source in our galaxy. Observations
revealed that Cygnus X-1 was a binary system with one visible star and an
unseen companion so massive and compact that the only plausible explanation

(23:23):
was a black hole, the first real observational evidence of such an object.
Decades later, in 2015, black holes again dominated headlines when the
Laser Interferometer Gravitational Wave Observatory detected gravitational
waves for the first time. The ripples in space-time caused two black holes

(23:43):
merging more than a billion light years away. I will never forget that. It
was a stunning confirmation of Einstein's century-old predictions and a
thrilling new way to observe the cosmos. Once, black holes were nothing
more than rejected mathematics, strange predictions scribbled in the margins
of Einstein's equations, too wild, too impossible for nature to allow. They

(24:07):
were the monsters science dared not believe in, haunting the theoretical
shadows where few physicists wanted to look. But over time, these shadows
sharpened into reality. Though relentless questioning, brilliant insight,
and a willingness to follow the math wherever it led, black holes moved
from theory into confirmation, from skepticism to the front page of human

(24:32):
discovery. Today, we know they are real. We have seen the fingerprints of
their existence in the X-rays of Cygnus X-1, heard their cosmic mergers
through LIGO's detectors, and glimpsed their ghostly silhouettes through
the Event Horizon Telescope. Black holes, once banished ideas, now anchor
our understanding of space, time, and the limits of existence itself. And

(24:56):
in their journey from rejection to revelation, they tell a story that
mirrors our own. We, too, live in a universe that challenges what we
believe is possible. We, too, wrestle with invisible forces, with
mysteries we cannot yet name. Black holes remind us that truth does not
vanish simply because it seems too strange. It waits for us to be brave

(25:20):
enough to see it. In the end, the story of black holes is not just the
story of collapsing stars. It's the story of human curiosity, how we
confront the darkness, how we fall inward, how we rise again with new
understanding. Somewhere in every black hole's silent pull is the echo
of our own search for meaning in the cosmos. Thank you for listening to

(25:46):
Math Science History. And until next time, Carpe Diem. Thank you for
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(26:09):
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