NOVA

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Narrator

One of the biggest puzzles in physics today is one that sounds straight out of science fiction. Just what would happen if you fell into a black hole? Make no mistake, the answer to this bizarre question is that you would die, that's not up for debate, but it's how you would die that's keeping physicists up at night. There are currently two major camps fighting over this gruesome scenario and the outcome of this fight could actually revolutionize the fundamental laws of nature. To begin to understand this controversy you first need to know what a black hole is. Black holes form when stars collapse under their own weight. For most of the past century, scientists have thought that due to the intense pull of gravity all the matter that made up the collapsed star gets squished into a teeny tiny dot. This dot is called a singularity. And it's incredibly massive and incredibly dense. It's basically a pit of gravity in the fabric of space time. And the closer you are to this dense dot of mass, the stronger the pull of gravity is. Some physicists described this pull as a river that leads to a waterfall. If you're significantly upstream of the precipice you can easily steer your raft to safety, but once you get far enough down stream, no matter how fast you paddle in the opposite direction, you can't escape the river's pull, and you'll topple over the edge. For black holes this point of no return is called the event horizon. And it's the place where nothing, not even the fastest thing in the universe, light, can escape the singularity's gravitational pull. So what would happen if you passed through an event horizon? For years, scientists thought they knew how you would meet your demise. Say you were traveling feet first toward the singularity, initially, you wouldn't notice that you passed any significant boundary, but your feet would experience a greater gravitational pull than your head, and would begin moving faster than the rest of your body. Thus, you would get stretched into a long person-noodle until your body eventually snapped. Each noodley piece of you would stretch and break until you were nothing but a collection of subatomic particles smushed against the rest of the mass at the singularity. This stretching hypothesis actually has a name, physicists call it spaghettification. The spaghettification theory satisfied most physicists for years, but then in 1974, the physicist Stephen Hawking proposed what is now a widely accepted idea about the nature of black holes. He said that black holes will eventually evaporate. Basically, they'll disintegrate. This is really complicated stuff, but essentially they'll start radiating particles from their event horizon. And even though these particles don't come from the singularity, the mass within the black hole will start to shrink, but there's just one problem with this idea,

it violates a key principle of quantum physics

Information cannot be destroyed. And just to clarify, when physicists say information, they're referring to quantum mechanical properties of the particles that make up everything in the universe. So here's the issue. If by definition matter and information that gets sucked into a black hole can't escape, then when the black hole evaporates, all that information stored inside of it would just disappear. Until recently, many scientists satisfied their frustration with this information paradox by saying that, maybe we just need to think about the inside and outside of black holes as different realms that can't communicate. We could save any information that fell inside a black hole by allowing it to exist in two places, both stuck inside and leaking out with the radiation. Usually, physicists would say that you can't have two different copies of identical information at once, but since the realms can't communicate, the particles would get a free pass. But then in 2012, the physicist Joseph Polchinski and his colleagues realized that the paradox might be even more complicated than anyone previously thought. They imagined what would happen if you had two entangled particles, particles that are quantum mechanically linked, and you tossed one of them into the black hole and kept an eye on the other one, this would create a problem. By their very nature the entangled particles would need to be able to respond to one another. But if you bought into this whole two realms concept the particles couldn't do that. So the physicist came up with a new revolutionary idea about how black holes might work. They got rid of the problem of these two realms. They entirely threw out the idea of a black hole that could spaghettify you, saying that matter doesn't get sucked into a black hole at all. Instead matter might hit the event horizon and incinerate. The information about where all that mattered came from and where it goes would get conserved. It might go bouncing off into the universe, but nothing would get sucked past the horizon. And that's because there is nothing beyond the horizon. According to Polchinski and his colleagues, black holes don't have centers at all. This is a totally wild and problematic idea. It's essentially saying, that beyond this event horizon firewall, you hit the end of the universe. And that throws into question Einstein's theory of general relativity, because it's literally a rip in the fabric of space-time itself. This firewall idea really rankled a bunch of physicists, including Stephen Hawking, who released a two page statement that's left the rest of the physics community perplexed. He's now said that both concepts of what black holes are, are wrong. He wrote that there are no singularities, but that there are also no event horizons. Instead, there are apparent horizons, which store and scramble all matter and energy that they suck in. This may solve the entangled particle problem. The two particles are still quantum mechanically linked, and it may even solve the information loss issue, well, kinda, the information is scrambled, but theoretically not irretrievable, but without a singularity to create a strong gravitational pull, it still messes with general relativity. If this leaves you scratching your head, you're in good company. Other physicists are confused why Hawking proposed this idea. So far it just seems like a third equally problematic solution. So for now you decide. If you fell into a black hole, would you rather be spaghettified, toasted, or scrambled? Frankly, they all sound pretty unappetizing to me. (mysterious music fades)