Janna Levin on Black Holes, Gravitational Waves, and the Human Story of Physics

This episode features theoretical physicist and cosmologist Janna Levin discussing the profound nature of black holes, their formation, and their role in the universe. She clarifies that black holes are not merely super-dense dead stars but rather regions of spacetime defined by an event horizon, beyond which nothing, not even light, can escape. Levin explains the process of stellar gravitational collapse, where massive stars exhaust their thermonuclear fuel, leading to a supernova explosion, and if the remaining core is sufficiently massive, it collapses further to form a black hole. She highlights the concept of gravitational waves, describing them as ripples in spacetime generated by accelerating massive objects like merging black holes, which can even resonate in the human auditory range.

The conversation delves into the historical context of black hole theory, starting with Carl Schwarzschild's exact solution to Einstein's equations in 1915, which initially surprised Einstein himself. Levin emphasizes that these early ideas were thought experiments, not initially believed to manifest in nature. She then discusses J. Robert Oppenheimer's visionary 1939 paper, co-authored with his student, which predicted black holes as the end state of gravitational collapse, a conclusion initially met with skepticism by figures like John Wheeler, who later coined the term "black hole" and eventually conceded Oppenheimer's correctness. A key distinction made is that while the event horizon is a boundary, the interior of a black hole remains largely a mystery, with current theories suggesting a singularity where spacetime breaks down and quantum mechanics becomes essential.

The podcast also explores the broader implications of scientific discovery, particularly through the lens of nuclear physics and the development of the atomic bomb. Levin reflects on the ethical quandaries faced by physicists like Oppenheimer and Heisenberg, highlighting the agnostic nature of science itself, which can be used for both creation and destruction. She discusses the geopolitical context of World War II and the Cold War, emphasizing how the pursuit of scientific knowledge became intertwined with national security and global power dynamics. The discussion touches upon the importance of intellectual freedom and a vibrant scientific community, citing the disproportionate number of Nobel laureates from the United States, often immigrants who fled oppressive regimes.

Ultimately, the episode underscores the human element in science – the rivalries, collaborations, and personal struggles that shape discovery. Levin's insights reveal that while physics explores the most extreme and abstract phenomena, its progress is deeply rooted in human curiosity, ingenuity, and the complex interplay of scientific theory, experimental observation, and societal forces. The episode leaves listeners with a sense of wonder about the universe's mysteries and a sober reflection on the profound responsibility that accompanies scientific advancement.

Black holes curve space and time around them in the way that we've been describing. Things follow along the curves in space.

It's really important to emphasize that it's not light. None of this has to do literally with light that we can detect with normal things that detect light.

If you're near enough to two colliding black holes, they actually ring spacetime in the human auditory range.

Black holes are more general and more fundamental than just the death state of a star.

The black hole is the event horizon. The event horizon is really just a point in spacetime or or a region in spacetime. It's actually in this case a surface in spacetime.

I think everybody would say the closer you get to the singularity for sure you have to include quantum mechanics.

I do do like to say that black holes are no thing. They're nothing.

The star like the Cheshire cat fades from view. One leaves behind only its grin, the other only its gravitational attraction.

It's the same technology and that's what I mean by science is agnostic, right?

One of the most beautiful things that we suspect happens on the inside of a black hole is that space and time in some sense swap places.