What nobody knows yet

So far you've seen two kinds of claims. Some are observed, such as stars orbiting nothing visible, X-ray disks, gravitational waves, and the images. Others are strong theory, such as what the horizon is like and what falling in would feel like. This lesson is the third kind, where the best physicists alive disagree.

Black holes might not be black

In 1974 Stephen Hawking combined Einstein's gravity with quantum physics, the rules for very small things, and got a shock. Black holes should glow very faintly and slowly lose mass. This is now called Hawking radiation. The usual picture involves pairs of particles appearing near the horizon, with one escaping. Hawking himself said that's a rough cartoon of the real math.

"Faintly" is an understatement. A black hole with the Sun's mass would have a temperature of about 60 billionths of a degree above absolute zero. Empty space is warmer than that. It still glows at about 2.7 degrees above absolute zero, left over from the Big Bang. So real black holes today take in more energy than they give off. Even once the universe cools down, a black hole like that would take around 10 to the power of 67 years to evaporate. The universe is about 14 billion years old.

Hawking radiation has never been detected from a real black hole. Lab experiments using sound in ultracold gases have produced a lookalike effect. Most physicists expect it's real, because it comes from two theories that have each been tested very well. Expecting isn't the same as seeing.

Where does the information go?

Physics has a deep rule. Information is never truly destroyed. Burn a book and, in principle, the smoke, ash, and heat still carry every word, scrambled beyond any practical recovery but still there.

Now throw the book into a black hole and wait for it to evaporate. Hawking's calculation says what comes out is a random glow that depends only on the black hole's mass, spin, and charge. Throw in a dictionary instead and you'd get the same glow. The information seems to vanish. That's the information paradox.

For decades Hawking argued the information really is lost. In 2004 he changed his mind and conceded a bet to the physicist John Preskill, paying him with a baseball encyclopedia. Since 2019, new calculations have suggested how information could leak back out with the radiation, and many physicists now lean that way. Nobody has shown exactly how it happens.

Where things stand

Pick a claim to inspect its evidence. A successful prediction and a direct observation answer different questions.

Black holes are where Einstein's gravity and quantum physics are both needed at once, and the two don't fit together yet. That makes them the best natural laboratory we have for the biggest unsolved problem in physics.