Dwarkesh Patel: uncertainty

Transcript context

…ccelerate towards the black hole, getting faster and faster as you approach it. You just sail across the event horizon totally as normal. The event horizon is not a particularly violent place for you. You can calculate the tidal forces as you approach and then cross the event horizon. They’re not particularly big, or rather, for large black holes, they’re not particularly big. For a solar mass black hole, they would be pretty big and would be pretty painful. You’d find that your feet are being attracted to the black hole much more vigorously than your head is, because they’re closer, and you end up getting stretched. But if I take a big enough black hole, you wouldn’t notice anything funny happening whatsoever. The bigger the black hole, the smaller the tidal effects. If I took a black hole the mass of the galaxy, you’d be basically fine as you cross the event horizon. If I took an even bigger black hole than that, you could live out your entire life having crossed the event horizon, before you hit the singularity, which is fatal. When you cross the event horizon, you are doomed. You are doomed because once you cross the event horizon, you must proceed to the singularity. There’s no way you can fire a rocket to stop yourself hitting the singularity. You are doomed, but you are not dead. You are only for sure dead once you hit the singularity and get spaghettified, mangled by the tidal forces. But for a large enough black hole, you can be doomed and not even know it. The event horizon is really a not locally measurable quantity. It is a teleological fact. It says that once you have crossed the event horizon, you must proceed to the singularity. But it can take a long time to get there for a large enough black hole. In principle, for a black hole that was many light centuries across, you could live out your entire life. You could have descendants, all of whom live inside the black hole. Only once you really approach the singularity do the tidal forces get strong and kill you. As you were saying, GR explains or predicts a lot of phenomena. Some we think are correct, some we don’t know are correct. Why do we think black holes are correct but not wormholes? That’s a great question. People did not believe it to begin with. Schwarzschild wrote down his solution almost immediately after Einstein wrote his field equations. People thought that that equation was sick in some way, that it was a measure zero thing that would never happen. It was some mathematical monstrosity, but it was impossible to make black holes naturally in the real universe. They were wrong, because black holes do exist. We’re extremely confident now. There were theoretical developments, and there was experimental evidence that black holes exist. The biggest theoretical development was Penrose, and then later Hawking and Penrose—for which he won the Nobel Prize—who showed theoretically that the formation of black holes is a generic feature of general relativity. It’s not just some sick thing that happens if you fine-tune the initial conditions. If you start off with generic initial conditions, the development of black holes is a generic feature. That was a huge development. Then there was the experimental side. The pieces of experimental evidence we have for black holes are now huge. They did not exist in Einstein’s day, and for 50 years after Einstein, people were extremely confused about black holes and thought they didn’t exist. But there are numerous pieces of evidence. I think the most visually appealing piece of evidence is just observing the center of our galaxy. If you look at the center of the galaxy—spoiler alert—there is a black hole there. We call it Sagittarius A*. It’s a huge black hole, weighing many millions of times the mass of the sun. You can’t see the black hole directly, because it’s black. What you can see is the stars around it. If you watch these stars over the course of decades—and we now have a number of decades of observations of them—you will see the stars not moving along what we would call straight lines, but instead moving in nice little ellipses, or precessing ellipses. Those ellipses look like they are orbiting something. You cannot see the something, but you can see the stars that are orbiting it. You can calculate how big it is, how massive it is. What you find is that it’s very massive indeed, and it’s also very small indeed. You know it’s small because the stars get super close to it but don’t seem to collide with it. So by tracing these orbits, you can tell that there is something super heavy, super dark, and super compact at the center of the galaxy. That is Sagittarius A*, the black hole at the center of our galaxy. That’s one compelling piece of evidence. Another piece of compelling evidence: about a decade ago, we not only saw black holes, we felt them. LIGO is this huge laser interferometer that we built at a number of different sites, that is super attuned to vibrations in spacetime itself. There’s a famous event pretty much immediately after we turned it on in late 2015, where we felt spacetime shaking.…

Stored transcript either side of the excerpt. The highlighted words are the published quote; the surrounding text is unedited source, never generated.