So, what would it feel like to fall into a black hole? “Well, at the moment you crossed the horizon, you wouldn’t feel anything – there would be nothing dramatic,” Peter Galison, co-founder of the Black Hole Initiative at Harvard University, says over the phone. Huh. Doesn’t sound too bad. “But inevitably, you would be pulled towards the centre,” he continues. “There’s no going back; everything that falls into a black hole just keeps falling; there’s no resisting that pull and things don’t end well.” Ah. Go on. “Physicists have an expression called ‘spaghettification’ because if you were falling in feet first, your feet would be more attracted towards the centre than your head, and your sides would be pushed towards your middle and this process would extend and compress you.” Right. So, terrifying, then. Especially when Galison adds with cosmic understatement: “In the long term that’s not a good survival event.” We are talking about his documentary film, Black Holes: The Edge of All We Know, four years in the making and available on Netflix from 1 June, which follows two scientific collaborations to understand the most mysterious objects in the universe. Among the highlights is being a fly on the wall as the late Stephen Hawking tries to figure them out. It is Hawking’s voice, that instantly recognisable computer speech synthesiser, that opens the film: “A black hole is stranger than anything dreamed up by science fiction writers. It’s a region of space where gravity is so strong that nothing can escape. Once you are over the edge, there’s no way back.” City-sized black holes form when certain stars run out of fuel to burn and collapse under the force of their own gravity. Supermassive black holes – millions or billions of times bigger than our sun – are found at the centre of almost every galaxy including our own, the Milky Way. Given that there are 100bn galaxies in the visible universe, there are probably 100bn supermassive black holes. They are a source of fascination for astrophysicists, mathematicians and philosophers and a ready-made metaphor for artists. But television is an insatiable visual medium so Galison – Pellegrino University professor of the history of science and physics at Harvard – had an unenviable task in making a film about “something that struggles with all of its might to be unseen”, as one of his interviewees puts it. “I think that I’m perversely drawn to topics of invisible things,” he says cheerfully, noting that his previous films include national security secrecy and the need to bury nuclear waste. “Black holes are the hardest to see objects in the universe because they reflect no light, they emit no light. The one we’re looking at is 55m light years away so its imprint on the sky, so to speak, is like trying to read the date on a coin in London from New York. “But the first good news in the effort is that these black holes accrete gas and matter, stuff that flows around it and gets heated up in that swirl to a temperature of roughly 10bn degrees, and that glows. So what you’re seeing is a kind of shadow of the black hole and by looking at this glowing ring around the black hole, we can deduce a lot about its size and its properties. We use the visible things that are near a black hole to deduce things about the black hole itself.” The black hole at the heart of a galaxy known as Messier 87 is so far away that it would ideally require a telescope the size of Earth. That being impractical, the next best thing is the Event Horizon Telescope collaboration, an array of eight radio observatories on six mountains spanning four continents, acting together like shards of a single mirror. The effort produced spectacular results two years ago by capturing the first ever image of a black hole, displayed on newspaper front pages around the world and now printed in large format at the Museum of Modern Art in New York. Galison, himself a member of the Event Horizon Telescope team, recalls: “It resonated for people because it did seem to recalibrate our relationship with the world. Even if you know there are black holes – from the equations or from more indirect observations – to actually look at it and say, ‘There is a black hole,’ I think was thrilling for the public and thrilling for the scientists involved.” He adds: “The scientists were not divorced from broader cultural philosophical meanings. We’d seen simulations of this thing for years so it wasn’t a matter of not having any idea what it would look like, but seeing an actual black hole was something different. “One colleague said she looked at it and she felt terrified of it. Another said the image on the cellphone was captivating for hours – just walking around staring at this image before it was released. So I think there is something arresting about the image beyond its technical achievement.” Galison’s film also follows a separate group of theoretical physicists – Sasha Haco, Malcolm Perry, Andrew Strominger and Hawking – working on a problem known as “the information paradox”. Strominger describes how he thinks about the paradox 24 hours a day, seven days a week, even when he’s brushing his teeth and when he’s dreaming. “It is the most interesting, well-posed question in modern physics,” he says, looking like a decade-older version of Mark Ruffalo’s Bruce Banner in the Avengers movies. “So interesting that I was ready to devote my life to trying to understand it.” So what is it? That goes back to Hawking in 1974 when it was thought that a black hole had only two properties: how big it was and how much it was spinning. No one could say how it was formed. Galison continues: “What Hawking realised was that this was in fundamental contradiction with something essential to what physicists believed, which was that if you knew the state of the world at a given moment, you could figure out what it was like in the past. If you knew what was in the present, you could predict the future. “But black holes defy that. You can’t tell whether they were made from stars or made from pianos or made from giraffes. You can’t say anything about how they formed. So they seem to defy our extracting, even in principle, what the past was, and that really bothers physicists. They like to look at a table of billiard balls move in motion: you tell us how fast they’re moving and where they are, we’ll tell you where they were and where they’re going to be. “Black holes didn’t seem to be like that. So the information loss paradox is that we’ve lost the information about how they were formed and this seems to violate the several-hundred-year aim of physics, since the time of Newton at least. If you knew things now you could say where things were and where they’ll be.” The film shows Hawking and collaborators puzzling over the paradox and suggesting that the fields around a black hole have enough capacity to store information that mean it is not lost, creating a kind of record of what went in. Galison, in turn, has captured a precious record of Hawking, who died in 2018 at the age of 76, in intellectual flight. Galison says: “I wanted to show Hawking working, not just the iconic Hawking making delphic pronouncements about the world, but the Hawking doing what he really does, which is doing science. I thought that would be really interesting for people to see.” His respect and affection for Hawking kept growing. “He remained in such good humour and curiosity about the world. There’d be a party in Cambridge and he would come to the party. He visited here in Boston and went to a dinner party and enjoyed that. He wanted to take a boat trip. He liked life and I thought that was great and I admired him all the more. “I knew his physics before then but I came to appreciate his engagement with the world. Once I asked him about what he thought of the history of black holes and he said, ‘J Robert Oppenheimer really discovered black holes. Too bad about the bomb.’ It’s funny and lapidary. Each word, each sentence had to be laboriously formed so there wasn’t a lot of extraneous talk. That doesn’t mean it was all business either but it was compressed, like a good joke.” But this is a film that also subtly pushes against the heroic myth of individual genius. Science, in practice, is a team sport. Galison reflects: “All the biggest problems that we face now, which are not particularly astronomical, like global climate change or the pandemic, require collaborative efforts, not just the multiplication of the same sort of person a million times over. “I wanted to show how central, how vital, how productive collaborations could be, whether it was the small collaboration on the theory side or the much bigger one on the observational side, and to see the human complementarity that made it possible to find out new things when people work together.” Black Holes: The Edge of All We Know is available on Netflix on 1 June