What Survives Inside A Black Hole?

SPEAKER_1 (0:01)

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SPEAKER_2 (0:30)

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Matt O'Dowd (1:02)

Thanks to The Great Courses Plus for supporting PBS Digital Studios.

I got a physics joke for you. A proton, an electron, and an antineutrino walk into a black hole.

That's it.

We've established by now that black holes are weird. The result of absolute gravitational collapse of a massive body, a point of hypothetical infinite density surrounded by an event horizon. At that horizon, time is frozen, and the fabric of space itself cascades inwards at the speed of light. Nothing can travel faster than light, and so nothing can escape from below the event horizon. Not matter, not light, not even information. These ideas are pretty mind-blowing, but as crazy as black holes are, they're also kind of simple. Don't get me wrong, the math is complicated, but the objects themselves are simple. In fact, every black hole in the universe, no matter how it formed or what happened to it afterwards, can be perfectly described with only three properties. Those properties are mass, electric charge, and angular momentum. Or at least, this is the proposition behind the famous no-hair conjecture or no-hair theorem. Today, we're going to see why black holes are bold. And in an upcoming episode, we'll combine the no-hair conjecture with everything we learned recently about the conservation of quantum information. The result will be the black hole information paradox, one of the greatest unanswered questions in modern physics, and also the gateway to black hole thermodynamics and the holographic principle. Jacob Bekenstein was a graduate student at Princeton when he proposed that no other properties besides mass, electric charge and angular momentum should emerge from beneath the event horizon. Or, as Bekenstein put it, black holes have no hairs. This is why you don't let graduate students name things. Bekenstein's advisor, the legendary John Archibald Wheeler, took up the phrase, and so now we're stuck with it. The basic reason for the no-hair conjecture is straightforward enough. The interior of a black hole is cut off from the external universe. It's causally disconnected. Nothing beneath the event horizon can influence the exterior universe because no signal can escape the event horizon to carry that influence. The black hole could have formed from a collapsed star or entirely out of antimatter or photons or monkeys. The only thing we can know about the material that went into the black hole are its mass energy content, electric charge and angular momentum.

Given the one-way nature of the event horizon, the loss of this information makes some intuitive sense. Perhaps more mysterious are the three exceptions, the three remaining hairs around our otherwise bald black hole. After all, the forces involved, namely the gravitational and electromagnetic forces, are only communicated at the speed of light. So how does mass electric charge and angular momentum communicate their influence across the uncrossable horizon?

Let's think about this in terms of fields. And we'll start with the gravitational field. Gravity may be caused by mass, but a gravitational field is a very real thing all on its own. In Einstein's general theory of relativity, we think of the gravitational field as curvature in the fabric of space-time. In the good old rubber sheet analogy, a heavy ball stretches the sheet downwards, affecting the motion of objects moving on that sheet. We can think of the gravitational field at any point as being caused by the gravitational field at surrounding points. Each point on the rubber sheet doesn't actually see the source of the gravitational field. Instead, it stretches and curves in response to the behavior of the surrounding patches.

The Earth orbits the sun, but more directly, it orbits the sun's gravitational field.

SPEAKER_1 (5:01)

Strength is one of the most important investments in your long-term health. That's why I love orange theory fitness.