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The mind-blowing science of black holes

What we know about black holes is both fascinating and scary.

MICHIO KAKU: First we think that out of the Big Bang came dark matter, invisible matter. If I held dark matter in my hand it would literally ooze its way right though my fingers, go right to the center of the Earth, go to China and then go back and forth between China and my hand. That's dark matter. We think that dark matter began to clump first because of gravity. Then matter was attracted to the clumpiness creating the super massive black hole and then later the galaxy itself began to form. We have computer simulations about this, but still the relationship is not yet clear. Now remember, stars. We know almost everything about stellar evolution. That's because the pentagon has given us physicists billions of dollars to model hydrogen bombs, and a star is nothing but a hydrogen bomb. However, a galaxy consists of over a hundred billion stars so it's much more difficult to tell which came first, the black hole or the galaxy itself.

BILL NYE: The way I like to describe a black hole. It's a star. A black hole is a star. Now when you and I think of stars we think about the sun which is giving off all this light. But the other thing about the sun to keep in mind is it has a lot of gravity because it's huge. One of Einstein's discoveries, Albert Einstein's discoveries was that gravity changes the path of light. It can bend light. It's just not in our everyday experience. To measure it we usually find objects way out in space and we have known brightness and we see where we think they're going to be and then where they really appear to be and then we infer or figure out that they're not where we thought they were going to be because gravity bent the beam of light. It's amazing. Anyway, so a black hole is a star so massive that not even light can escape from it.

MICHELLE THALLER: What you're looking at is something called the shadow of a black hole. Now, black holes tend to have material orbiting around them. Black holes have a lot of gravity and gas begins to fall in towards the black hole and it begins to spin up into a disk around the black hole. And as that gas gets closer and closer to the black hole its accelerated faster and faster. And so in this disc of gas some of it is traveling very close to the speed of light. You have a lot of friction. You have lots of things rubbing up against each other at very high speeds and incredible amounts of heat and light are generated in this disk. So black holes usually are surrounded by disks of very, very bright, very hot material and that's how we find them. Black holes themselves give off no radiation at all. Any light gets absorbed into the black hole and when I say light I mean every possible form of light from gamma rays, x-rays, infrared light that we think of as heat, radio waves. Nothing comes out of a black hole at all. So what you're looking at in this image is the black hole is sort of framed by this bright ring. And that bright ring is this hot material that's orbiting around the black hole.

One of the first things you'd say well okay, it's really kind of a wonderful stroke of luck that the particular black hole we're looking at the ring was right face onto us. You see this bright ring exactly around the black hole. And, in fact, that's probably not the case. The disc of material could be at many different orientations around the black hole. Light itself has no mass. Light should not be attracted by gravity, right. I mean gravity is the force between two things that have mass. Light has no mass. It just flies straight through space. So why should light be affected by a black hole? And amazingly this is what happens. A black hole's gravity is so strong it actually bends the space itself. So light thinks it's traveling through straight space, just traveling in a straight line. The disk can be pretty much any orientation you like. What will happen is light from any part of that disk will get bent around the black hole. So you'll be able in a very real way to see the underside of the disk at the front of the black hole. The backside all the way over the front because light itself is being bent around. So if there's any hot, glowing material at all you're going to see it sort of surrounding the entire black hole because the light's been bent around it.

KAKU: We're not swallowed up by a black hole because we orbit around them. However, are there wandering black holes? And the answer is yes. In fact, we've been able to track wandering black holes as they wander through the galaxy. One day one of them may catch up with us and eat us for breakfast and it wouldn't even burp in the process. Now, a black hole is black. It's invisible so how the hell do we know that there's a wandering black hole in our vicinity? The answer is quite easy. It was found by accident. By taking a picture of the night sky and taking the same picture at a different time you see a distortion. A distortion of light and then like time lapse if you put these photographs together you see that the distortion goes in a straight line. And then you say aha, that's the black hole. Even though it's invisible it distorts light. For example, many people wear glasses. There's glass inside your glasses, but how do you know that? How do you know that there's glass inside your glasses when glass is invisible. Well, it's obvious. Glass distorts light. That's how you know that something that is invisible is actually there.
And the same thing with black holes. They are invisible but they distort starlight as they move. So one day if a wandering black hole snuck up behind is how would we know? First of all Pluto and Neptune would begin to perturb. Some of them would be, in fact, flung into outer space. As the black hole got closer and closer to planet Earth we would see more and more disruptions in the solar system as more planets got flung into outer space. And, in fact, as it whizzed by the Earth it could even gobble up the Earth, in fact, eat up the sun and hardly even notice. And so the appetite of a black hole would be enormous and it's something that at some point in the future we may encounter.

THALLER: If you were around a black hole which is a dead star and say the mass of the black hole was about 20 times the mass of the sun. A black hole like that is actually not very physically large. You have all that mass, but the black hole itself may only be say on the order of about 30 miles across. That means you have all that mass packed into a tiny little area. If you were nearby a black hole that means there really would be a detectable gravity stretching across something as small as your body. And not just the water in your body would feel that. As you got closer and closer to a black hole you would actually feel your head stretched away from your feet. There would be tidal forces just like the Earth goes through with the sun and the moon, but next to a black hole the gravity is so extreme there would be tides over something as small as a human body. Get closer and closer to a black hole and your head keeps getting stretched more and your feet keep getting stretched that way and you would actually turn you into a stream of particles. Scientists have a really cool name for this. It's called spaghettification from the word spaghetti. If you got close to a black hole there would be tides over your body that small that would rip you apart into basically a strand of spaghetti that would fall down the black hole.

CHRISTOPHE GALFARD: Picture yourself in outer space. There is a black hole right in front of you. You don't see anything. No light can come out of it so it's like a dark patch that distorts the stars that are around. The matter that surrounds you, the light that surrounds you obeys a different kind of law. They obey quantum physics. They obey the law of the quantum world. And everything we had known about black holes until the mid-1970s was only related to gravity. Now what Hawking did at the mid-1970s is to add some quantum aspects to all this. He took a quantum particle and threw it in his mind towards the black hole to see what would happen to it and he found out that some part of that particle was getting out of the black hole. That the black hole was evaporating. It was not the exact same particle he had sent inside and that's what's tricky about this problem and that's what's interesting about this problem. In a way that means that the information gets bleached by a black hole. Why is that? You could imagine that it's the same with an encyclopedia that you would throw in the fire. You take an encyclopedia and you threw it in the fire it's gone. Well, not quite. If you could get back the ashes, if you could collect all the lights that was shown during the fire, if you could get the heat and everything you could build back the encyclopedia. It's not gone. It's just difficult to retrieve.
For a black hole it's worse. If you throw an encyclopedia inside what the black hole will evaporate has nothing to do with the encyclopedia whatsoever. You could have thrown in something else. So why is that a problem? It is a problem because it means that our universe has memory losses. It means that whatever a black hole has swallowed would get inside, never again back out and we would never be able to understand the past of our universe. There are some things that are not retrievable at all, not just in practice but also in theory. And as it evaporates the black hole eventually disappears completely maybe. And if it's gone where did the information go about the encyclopedia. Where did it go? We don't know.

THALLER: Information can be almost anything. All of the different atoms in my body have angular momentum. They have charge. They have mass. There's all sorts of little bits of information that make me me. At the quantum mechanic level, at the tiniest of levels there are different amounts of energy. There are different probabilities that are contained in the structure of my matter. And information in some ways is a form of energy. Energy and mass are the same thing. They're equivalent. You can actually make mass into energy and you can make energy into mass. Around a black hole where there's very hot gas, very high temperatures, very strong magnetic fields perhaps. There's a lot of energy. And that energy can actually manifest itself as particles, mass. And the energy always creates particle anti-particle pairs. They're called virtual particles. And matter and antimatter, the thing you know about it is that it annihilates immediately. So these tiny little particles come into existence then annihilate and you're back to energy. And this happens all around us all the time.

So if this happens near a black hole it's possible one of these little particles can go into the black hole and the other one escapes. And all of a sudden there's a particle that shouldn't be there. The universe basically has a new particle, energy from nowhere and how can that work? And the information theory people say that what happens is that energy has to come out of the black hole. The black hole's mass begins to decrease if there is this poor little orphan particle that shouldn't have been there in the first place. So over time tiny particle by tiny particle these black holes can evaporate away. And maybe there's something about those virtual particles that contain some information about the black hole and what fell into it. Black holes may be the key to where the next physics has to go. We all know that we need a next Einstein, a next quantum theory, something that actually describes how gravity works in very intense situations like a black hole. Now we're actually observing black holes well enough that we really have to get on this. We really have to figure out how the universe works around one of these things. And we may end up learning what the universe itself really is.

  • When it comes to black holes, science simultaneously knows so much and so little, which is why they are so fascinating. Focusing on what we do know, this group of astronomers, educators, and physicists share some of the most incredible facts about the powerful and mysterious objects.
  • A black hole is so massive that light (and anything else it swallows) can't escape, says Bill Nye. You can't see a black hole, theoretical physicists Michio Kaku and Christophe Galfard explain, because it is too dark. What you can see, however, is the distortion of light around it caused by its extreme gravity.
  • Explaining one unsettling concept from astrophysics called spaghettification, astronomer Michelle Thaller says that "If you got close to a black hole there would be tides over your body that small that would rip you apart into basically a strand of spaghetti that would fall down the black hole."

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