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StarTalk Radio

Cosmic Queries - Death of a Black Hole

Published October 14, 2025
View Show Notes

About This Episode

Neil deGrasse Tyson hosts a Cosmic Queries Grab Bag edition, answering listener questions on topics ranging from why eclipses do not happen every month to the evaporation and final moments of black holes. He discusses dark energy and why external gravitational tugs are unlikely to explain it, defends the term "black hole," explores time travel paradoxes and Hawking's chronology protection idea, and explains Jupiter's shielding role in the solar system. The episode also covers entropy and why life on Earth does not violate the second law of thermodynamics, relativistic addition of velocities, the distinction between space and time dimensions, the value of scientific literacy, and what "vacuum" and "nothing" really mean in physics.

Topics Covered

Black holes Dark energy Time travel General relativity Second law of thermodynamics Vacuum of space Scientific literacy Observational astronomy Jupiter Asteroid impacts

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Quick Takeaways

  • Lunar and solar eclipses do not occur every month because the Moon's orbital plane is tilted relative to the ecliptic, so alignments only occasionally place the Sun, Earth, and Moon in a straight line.
  • According to Hawking's calculation, a black hole evaporates via Hawking radiation and ends in a brief, extremely energetic burst of gamma rays, leaving nothing behind.
  • Dark energy behaves like a property of the vacuum that becomes more significant over larger volumes of space, making a simple external gravitational pull an unlikely explanation.
  • The term "black hole" is accurate in that it describes a three‑dimensional gravitational "hole" from which nothing, not even light, can escape, regardless of the direction of approach.
  • Time travel to the past raises causality paradoxes, inspiring ideas like Hawking's conjecture that laws of physics may ultimately forbid backward time travel.
  • Jupiter's immense gravity affects any comet or asteroid entering the solar system, effectively helping shield all the inner planets, not just Earth, from many impacts.
  • Life and complex order on Earth do not violate the second law of thermodynamics because Earth is an open system continuously receiving low‑entropy energy from the Sun.
  • Near light speed, velocities no longer add linearly; relativity's velocity‑addition formula ensures that no observer ever measures anything moving faster than the speed of light.
  • In relativity, time is a coordinate but behaves very differently from the three space dimensions, so our spacetime is not simply a four‑dimensional "cube" or tesseract.
  • Even the best vacuums contain particles, radiation, quantum fluctuations, and spacetime itself; a true "nothing" would require removing not just matter but the fabric of spacetime and its laws.

Podcast Notes

Introduction and episode setup

Banter about questions and pronunciation

Neil compliments Chuck on a "brilliant" set of questions for this Cosmic Queries episode[0:56]
• They joke that some questions bordered on the philosophical or even spiritual
Chuck jokes about taking credit for all the questions and getting a C+ on name pronunciation[1:07]

Framing of Cosmic Queries Grab Bag format

Neil announces this is a Cosmic Queries Grab Bag edition of StarTalk[1:19]
They emphasize the random, mixed-topic nature of a "grab bag" of listener questions[1:43]

Why we don't get a lunar eclipse every month

Question from Roger McVeigh about monthly eclipses

Roger from Wisconsin (currently in Thailand) asks why there is not a lunar eclipse every month and whether it's due to distance, wobble, or something else[2:06]

The ecliptic and the Sun's apparent motion

Neil explains that the Sun appears to move against the background stars over the year because Earth orbits the Sun, changing our line of sight[2:39]
• Each month, the Sun appears in front of a different set of background stars along a path called the ecliptic

Moon's tilted orbit and eclipse conditions

The Moon's orbital plane is tilted with respect to the ecliptic, so it is often above or below the Sun's apparent path[3:08]
• You can only get an eclipse when the Sun and Moon are in the same plane and location so that Earth, Sun, and Moon line up
Neil notes his description is "pre-Copernican" in the sense that he talks as if the Sun has an orbit, but really it's Earth's orbit changing the view[2:57]
If a full Moon occurs without crossing the ecliptic, it will not enter Earth's shadow and no lunar eclipse occurs[4:18]
Similarly, a new Moon not crossing the ecliptic will not pass directly between Earth and the Sun, so there is no solar eclipse[4:18]
• You need a full or new Moon that coincides with the points where the Moon's orbit crosses the ecliptic to get an eclipse

Difference between solar and lunar eclipses

For a solar eclipse, you must be located on the specific path of the Moon's shadow sweeping across Earth to see it[5:14]
In a lunar eclipse, the Moon enters Earth's shadow in space, so anyone on the night side of Earth who can see the full Moon can see the eclipse[5:25]
• Neil notes that lunar eclipses are slightly less common than solar eclipses, but they are visible over an entire hemisphere instead of a narrow track

Death of a black hole and dark energy speculation

Question from Elysiel about black hole evaporation and the Big Bang

A listener from Newcastle, Australia asks whether a black hole's final explosion would contain all the energy it ever accreted or if that energy escapes earlier via Hawking radiation[6:28]
They further speculate whether the Big Bang could have been an exploding supermassive black hole whose event horizon was larger than our observable universe, with a larger, older galaxy beyond pulling on us and mimicking dark energy[6:41]

Hawking radiation and final gamma-ray burst

Neil confirms that as a black hole evaporates via Hawking radiation it loses mass, and the smaller it gets, the faster it evaporates[7:43]
According to Stephen Hawking's original paper, the final stage of evaporation produces a very high-energy burst of gamma rays[8:01]
• The characteristic wavelength of light emitted is comparable to the size of the black hole, so as the black hole shrinks, the emitted light's wavelength shrinks and its energy increases, ending in gamma rays
Neil emphasizes that after this final gamma-ray burst, "there's nothing left" of the black hole[8:01]
• Because nothing remains, it is not like an explosion that then fills a void with expanding matter in the way the listener imagined

Why a prior black hole is unlikely to explain the Big Bang or dark energy

Neil restates the idea that perhaps something beyond our cosmic horizon could be pulling on our universe, giving a false impression of an outward-pushing "dark energy"[8:42]
He notes that even if such an external pull existed, it would still be ordinary gravity, and the object would more likely be a whole other universe rather than just another galaxy[9:14]
Dark energy appears strongest when you consider larger regions of space, suggesting it is a property of the vacuum: the more vacuum, the more dark energy effect you have[9:22]
• If an external massive object were just pulling gravitationally, its influence would weaken with distance as the universe expands, opposite to how dark energy behaves
Neil concludes that while all ideas are considered because dark energy remains mysterious, this specific explanation is probably not correct[10:18]

Debate over the name "black hole"

Question from Dennis (Paradox) about renaming black holes

Dennis from Indiana suggests removing the word "hole" from "black hole" because he finds it misleading, proposing instead the term "Black Omega Star"[11:22]
• Neil and Chuck riff on "Black Omega Star" as sounding like a blaxploitation superhero character from the 1970s

Neil's defense of the term "black hole"

Neil states he is generally not inclined to argue over word definitions if a term is working well[12:35]
He argues that "black hole" is appropriate because, regardless of direction, you "fall in" due to gravity, making it effectively a three-dimensional hole in spacetime[13:10]
• Unlike the everyday image of a hole in a 2D surface you fall through, a black hole is a 3D region where approaching from any direction leads inward
Light cannot escape from inside this region, so "black" is accurate, and he calls it "the best named thing there ever was" in astrophysics[13:49]

Etymology of "galaxy" and cultural naming differences

Neil explains that "galaxy" comes from the Greek "galactos" meaning milk, giving us "Milky Way"[14:03]
In Chinese, where milk is less culturally central, the Milky Way is called the "silver river," which Neil finds more poetic[14:27]
The sugar "galactose" and "lactose" derive from the same root, showing how astronomical naming influenced chemistry[14:51]

Comedy aside: superhero domestic life and Key & Peele

Imagining Black Omega Star's home life

Chuck humorously imagines the domestic struggles of a superhero named Black Omega Star whose partner is frustrated with his household neglect[15:28]

Key & Peele skit about Neil and his wife

Neil recounts a Key & Peele sketch where Jordan Peele plays him and Keegan-Michael Key plays his wife, who demands he stop looking through the telescope and do chores[16:17]
In the sketch, "Neil" deflects by invoking alternate universes and quantum ideas to avoid the chores[16:41]
Neil says that when he met them at the Emmys he pointed out that his wife has a PhD in mathematical physics, so the real-life conversation would not have gone that way[17:09]

Spacetime, causality, and time travel paradoxes

Question from Sam Green about spacetime splitting to preserve causality

Sam from Tulsa asks whether spacetime could behave in a "meiotic" or "mitotic" way-splitting into branches-to preserve itself when causality is violated[21:50]
• He imagines spacetime splitting when encountering acausal events so that the universe maintains internal consistency

Conditions needed to violate causality

Neil notes that the only way to truly mess up causality is to travel back in time[23:30]
He cites J. Richard Gott, a colleague and co-author, who showed that certain paths around black holes can be solutions where you return to your starting point before you left[24:31]
• These are solutions to Einstein's equations that form closed timelike curves, though the mathematical details are not worked through in the discussion

Time travel paradoxes and universe splitting idea

Neil frames the classic paradox: you go back in time and tell your past self not to go back, raising the question of what happens to the "you" who originally went[25:00]
He acknowledges this motivates ideas like the universe splitting into separate timelines so the original trip still exists in one branch while another branch avoids it[25:09]
Popular depictions often resolve paradoxes by forcing events to conspire such that you end up going back anyway, or by having people fade from photographs, but these are narrative choices, not physics[24:55]

Hawking's chronology protection conjecture and time travel party

Neil describes Stephen Hawking's idea (chronology protection conjecture) that a law of nature may ultimately be found that forbids travel backwards in time[25:48]
He recounts that Hawking held a "time travel party" at Caltech, inviting time travelers (by announcement after the fact) to come back in time to the party[26:07]
No one showed up at the party, which Hawking jokingly took as informal evidence against time travelers visiting us[26:18]

Pop-culture time travel: Time Tunnel and the Titanic

Neil references the TV show "Time Tunnel," where characters travel through time but cannot easily return, with the Titanic as the first destination[26:48]
He mentions a hypothesis that, given popular fascination, so many future time travelers would visit the Titanic that their presence might explain the disaster in a tongue-in-cheek way[27:54]
Neil suggests two broad possibilities: either backward time travel is impossible, or if it occurs, the universe might split into separate timelines preventing alteration of a single history[28:18]

Jupiter as protector of the inner solar system

Question from Parker Mann about Jupiter shielding planets

A retired geophysicist asks if Jupiter's orbit could similarly protect other planets (like Mars or Venus) if it were located further out or in[29:15]

Jupiter's gravitational role

Neil explains that Jupiter protects not just Earth but all planets interior to its orbit-Mercury, Venus, Earth, and Mars-by its strong gravitational influence on incoming comets and asteroids[30:09]
Any comet entering or leaving the solar system will feel Jupiter's gravity; it cannot traverse the system without being affected by Jupiter in some way[29:59]

Exponential spacing of planetary orbits

Neil notes that the distances of the planets from the Sun, in units of Earth's distance, grow roughly exponentially: Mercury ~0.4, Venus ~0.7, Earth 1, Mars ~2.5, Jupiter ~5, Saturn ~10, Uranus ~20, Neptune ~30[30:09]
• He highlights that compared to these large outer distances, the inner planets are relatively huddled together, all deep within Jupiter's orbit

Planet formation and hot Jupiters

Neil says our understanding of solar system formation is still evolving; early on, astronomers assumed other systems would resemble ours[31:28]
Exoplanet discoveries have revealed systems with "hot Jupiters"-Jupiter-mass planets orbiting as close to their star as Mercury is to the Sun[31:40]

Entropy, life on Earth, and open vs closed systems

Question from Freddie Abden about Earth's "pause" in entropy

Freddie asks how Earth could maintain extraordinary order and complexity for a long time, seemingly in contrast to the second law of thermodynamics that entropy must increase[32:55]
He wonders whether this rare pocket of low chaos might indicate unique initial conditions or even influences beyond natural forces[33:20]

Proper statement of the second law of thermodynamics

Neil stresses that the second law strictly applies to closed systems: in any closed system, entropy will inexorably increase[34:31]
Earth is not a closed system; it is "open to the universe" and continuously bathed in energy from the Sun[34:39]
Local decreases in entropy (like the emergence of life and complex structures) are allowed as long as entropy increases elsewhere, such as in the Sun[35:04]

Sun's role and a religious parallel

Neil says the Sun is effectively "dying" so that we might have life, framing it as a different kind of sun giving up its life for ours[35:21]
When the Sun eventually dies, the energy input that supports Earth's low-entropy structures will cease, and the system will then tend towards entropy[35:37]

Enclosed marine ecosystem example

Neil describes a sealed glass sphere at the Rose Center containing water and three life forms: tiny krill, snails, and an underwater plant similar to kelp[35:52]
• Krill produce waste, snails eat the waste, and their activity fertilizes the plant, forming a closed-loop ecosystem in terms of matter
Despite being sealed, the system is not fully closed because sunlight passes through the glass and drives photosynthesis in the plants[36:44]
During construction of the Rose Center, workers covered the sphere with a tarp to protect it from dust, unintentionally cutting off sunlight[37:07]
• After a few days, some krill died, but once the tarp was removed the system rebalanced and the ecosystem has persisted for decades

Life, metabolism, and entropy in the human body

Neil points out that being alive requires continuous energy consumption, which we obtain from food (calories)[38:28]
• He notes that in French, the word for energy in this context is "calorie," emphasizing that calories are fundamentally energy units
After death, metabolism stops and no more energy is imported, making the body effectively a closed system that proceeds to decay and increased disorder[39:46]

Relativistic velocity addition and the speed of light

Question from Nicholas Hayes about appearing faster than light

Nicholas asks whether, if he traveled near the speed of light in a starship and passed a planet moving in the opposite direction, observers on the planet would see him moving faster than light[40:04]

Classical vs relativistic velocity addition

Neil reviews low-speed intuition: two cars each going 60 mph in opposite directions pass each other at a relative speed of 120 mph[40:55]
He gives an airplane example: planes each going about 500 mph in opposite directions appear to zip past each other at ~1000 mph when viewed from another plane[40:55]
At such low speeds, we simply add velocities linearly; this works well when speeds are small compared to the speed of light[40:29]
At high speeds near light speed, this simple addition breaks down and must be replaced by the relativistic velocity-addition formula[42:36]
• The relativistic formula depends on each speed expressed as a fraction of the speed of light, and as those fractions get large, the result never exceeds the speed of light
Neil notes that, for example, two objects each going at half the speed of light in opposite directions will not see each other at light speed but at a value computed by the relativistic formula, still below c[42:36]
He emphasizes that the formula is descriptive: it reflects what we actually observe in experiments, not a rule the universe is trying to obey[43:10]
He mentions that the formula for adding relativistic velocities involves squares and square roots and is accessible with basic algebra, and can be found on reference pages[43:45]

Space dimensions, tesseracts, and cosmic expansion

Question from John Stam about 3D space and expansion

John from Tampa asks whether the fabric of space is better described as four-dimensional (a tesseract) because it is expanding, and what would happen if the universe stopped expanding[47:07]

Unknown links between expansion and physical laws

Neil says we do not know which aspects of physics are fundamentally tied to the expanding universe, or how laws like the second law of thermodynamics might behave in a collapsing universe[47:49]
Some have suggested that entropy's behavior could be connected to expansion, but it is unknown whether entropy would reverse behavior in a contracting cosmos[47:22]

Difference between space and time dimensions

Neil clarifies that our time dimension is not the same as a space dimension, so spacetime is not literally a four-spatial-dimensional tesseract[48:01]
He describes humans as prisoners of the present, always transitioning between an inaccessible past and an unknowable future, unable to move freely back and forth along the time axis as we do in space[48:27]
We can move forward/backward, left/right, and up/down in space (x, y, z), but cannot similarly traverse time at will, underscoring its different character[48:35]
He notes that a universe with four space dimensions plus time would be a five-dimensional spacetime, conceptually different from our 3+1[48:46]

Pop reference to The 5th Dimension musical group

Neil briefly mentions that an arranger for the band The 5th Dimension was a family cousin and recalls attending their concerts as a child[48:58]

Value of scientific understanding and data before theory

Question from Marcus Gustafsson about what is gained from scientific literacy

Marcus from Sweden reflects that scientific literacy has given him perspective on truth and humility, and asks what else is gained beyond knowledge itself[50:49]

Science literacy as empowerment against misinformation

Neil says the number one feature of science literacy is that it empowers you to know when someone else is "full of" nonsense[51:19]
He gives the example of someone selling crystals with supposed healing powers: understanding physics and medicine helps you avoid being taken in by charlatans or misled believers[51:29]
He also praises the ability to quantify one's ignorance as a major self-awareness benefit that many people never achieve[52:01]

Measuring phenomena before understanding them

Neil notes that there are many things we can measure but do not yet understand, such as dark matter, dark energy, or the origin of life from organic molecules[52:10]
He offers the historical example of early stellar spectroscopy: once photographic plates became sensitive enough, astronomers collected spectra of stars without yet understanding their detailed meaning[52:37]
• These spectra showed bright and dark lines, with some matches to laboratory spectra (e.g., hydrogen, carbon) but the atomic-level explanation was missing
With the advent of quantum physics in the 1920s and a deeper understanding of atomic structure and electron transitions, those previously mysterious spectral lines became the foundation for models of stellar evolution[54:08]
Neil calls this one of the great triumphs of 20th-century astrophysics, illustrating how data collected before theory can later become crucial[53:42]

Vacuums, nothingness, and the fabric of spacetime

Question from Alejandro Guardado about vacuums and fans in space

Alejandro from Washington State asks how to conceptualize partial or complete vacuums, including why fans would not work in space[55:44]

What is a vacuum and what is "nothing"?

Neil says the deeper underlying question is: what is nothing?[55:46]
He describes creating high-quality vacuums in laboratories, such as in parts of the Large Hadron Collider, where pumps remove gas from a cavity[56:14]
• After initial pumping, heating the cavity walls releases gas molecules trapped in surface irregularities, causing the pressure to rise again before further pumping reduces it
Even after these steps, some particles remain; our best man-made vacuums are still not "perfect"[57:45]

Natural vacuums from interplanetary to intergalactic space

Neil notes that the vacuum between planets in our solar system is better (emptier) than any vacuum humans have created[58:15]
The vacuum between stars in the galaxy is better still, and the space between galaxies offers an even more rarefied vacuum[58:15]
He cites a number: the intergalactic vacuum might have about one particle per cubic meter[58:34]
• Given such low particle density, he jokes that you should "leave your fan at home" because there is virtually nothing to push on

Beyond matter: fields, radiation, and spacetime itself

Neil argues that even if you removed all particles from a region, it would still not be "nothing" because light can pass through it and there is a pervasive cosmic microwave background radiation[59:24]
Quantum physics predicts the presence of virtual particles popping in and out of existence even in a vacuum with no real particles[59:31]
Spacetime itself-the fabric of space and time-still exists in an empty region and carries with it the laws of physics[1:00:00]
Neil suggests that a truly empty "nothing" would require removing not only all particles but also virtual particles, radiation, spacetime, and the laws of physics themselves[59:55]
• He speculates that if spacetime were removed, the laws of physics might be inseparable from it, so eliminating spacetime could eliminate those laws

Merlin column and best-vacuum question

Neil mentions writing under the pen name "Merlin" for a long-running Q&A column where one reader asked about the best possible vacuum[1:01:09]
In that column, he laid out a spectrum of vacuums from everyday to intergalactic, including estimates of particles per cubic meter, similar to his explanation here[1:01:15]

Closing of the Cosmic Queries grab bag

End of questions and episode wrap

Neil notes they are out of time and that all questions in this episode came from listeners participating in a Q&A format[1:01:24]
They identify the episode as another installment of "Cosmic Queries Grab Bag Edition" and Neil signs off with his customary encouragement to "keep looking up"[1:02:43]

Lessons Learned

Actionable insights and wisdom you can apply to your business, career, and personal life.

1

Local increases in order, like life and complex ecosystems, are possible because real-world systems are rarely closed; when energy flows in from elsewhere, you can build structure while exporting entropy to the wider environment.

Reflection Questions:

  • • Where in your work or life are you trying to create order without accounting for the energy or resources that must flow in to sustain it?
  • • How could you redesign a project or system you manage so that it has a clear, sustainable "energy source" instead of relying on hidden or one-time inputs?
  • • What is one area this week where you could intentionally bring in new resources (time, knowledge, people, capital) to make positive change more thermodynamically realistic?
2

Scientific literacy is a form of protection and empowerment: understanding basic principles lets you recognize when claims conflict with reality and avoid being misled by confident but incorrect people.

Reflection Questions:

  • • In the past year, when have you later realized you believed something that didn't fit basic scientific or logical principles?
  • • How might improving your understanding of one scientific topic (like statistics, physics, or biology) change the way you evaluate news, products, or health advice?
  • • What is one concrete step you can take this month to strengthen your ability to spot unsupported or pseudoscientific claims before acting on them?
3

Our models and formulas are tools that describe observed behavior, not rules the universe "obeys"; when observations change, the right response is to refine the model, not to force reality to fit our expectations.

Reflection Questions:

  • • Where in your professional or personal life are you clinging to a mental model that no longer matches what you actually observe?
  • • How could you set up a simple feedback loop to compare your assumptions against real-world data more regularly?
  • • What is one belief or framework you could purposefully stress-test this week by seeking out evidence that might contradict it?
4

Collecting good data before you fully understand it can be extremely valuable, because future insights or theories may turn that raw information into breakthroughs.

Reflection Questions:

  • • What kinds of data or records could you start capturing now, even if you are not yet sure how you will analyze or use them?
  • • How might systematically documenting your experiments, projects, or decisions give your future self a richer foundation for insight?
  • • What is one ongoing activity where you could begin a low-effort habit of measurement or journaling to build a useful dataset over time?
5

Concepts like "nothing" or "empty" are often more complicated than they first appear, reminding us to question intuitive labels and look for hidden structure or assumptions in any situation.

Reflection Questions:

  • • Where are you currently treating something as "simple" or "empty" without examining the underlying mechanisms or context?
  • • How could you dig one layer deeper into a familiar concept at work or in life to uncover complexity you have been glossing over?
  • • What is one idea you commonly use (like value, risk, or success) that you could redefine more precisely to avoid misleading simplifications?

Episode Summary - Notes by Micah

Cosmic Queries - Death of a Black Hole
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