Black holes are notorious for gobbling up, well, everything. They're icons of destruction, ruthless voids, ambivalent abysses from which nothing can return — at least, according to pop culture. But black holes have another side: Astrophysicists have seen powerful jets, sometimes millions of light-years long, shooting out of supermassive black holes – including the one at the center of our own galaxy. So today, we're getting to know the other side of black holes, and the powerful role they may play in creating and shaping the cosmos. Read more about the Blandford-Znajek process.Got other cosmic curiosities? Email us at [email protected] more about sponsor message choices: podcastchoices.com/adchoicesNPR Privacy Policy
Chapter 1: What are black holes commonly misunderstood?
You're listening to Shortwave from NPR. In pop culture, black holes have developed this reputation for gobbling things up. Being these points in the universe where all matter, even light, is inescapably sucked up into this extraordinarily dense black void.
They're often seen as sort of cosmic vacuum cleaners. Just sucking in all the material, gas and stars that stray close.
That's Priyambada Natarajan, an astrophysicist at Yale University who focuses on black holes, specifically how extremely large ones came to be. And Priya says these supermassive black holes, like the one in the center of our own Milky Way galaxy, are more than just cosmic vacuum cleaners.
Because when black holes eat material immediately around them, they create this bright disk, like a glowing donut.
But... What is counterintuitive is that we do see very powerful jets of material that are actually expelled from them as well.
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Chapter 2: How do black holes create powerful jets?
Basically, black holes are really messy eaters, so not all the dust and gas they eat make it down the hatch. And for a supermassive black hole, this can look like beams of white-hot plasma and radiation shooting out of that glowing donut just outside the event horizon that makes up the edge of the black hole. Sometimes these beams are millions of light-years long.
Roger Blanford says you can think of these black hole jets carrying this massive amount of energy, kind of like nuclear power.
Of course, they can be famously destructive, but also it can be a source of power in a nuclear reactor.
Rogers is an astrophysicist and a professor at Stanford University. In the 1970s, when he and his friend Roman Znayek were at the University of Cambridge over in the UK, they started to look at how these black hole jets were created. And they came up with a hypothesis for how these jets were powered.
But it would take some time for all these pieces to come together to see if this explanation held true. So today on the show, a look at the most energetic objects in the universe, supermassive black hole jets. What are they, how they might be created, and what new images can tell us about these mysterious objects? I'm Regina Barber, and you're listening to Shortwave, a science podcast from NPR.
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Chapter 3: What insights do we have about black hole jets?
Okay, so funny thing about black hole jets, they were first imaged before astronomers could even agree on what a galaxy was. Heber Curtis is one of the astronomers at the center of that debate, and he's the one who first identified a jet in 1918. At first, it seemed like a strange bright streak in the cosmos.
And he saw what he called a curious straight ray, and that was an optical photograph of a jet flying.
The ray was coming out of a fuzzy thing named M87, we now know to be an enormous galaxy 50 million light years from Earth.
And M87... Harbors a supermassive black hole that's like 6 billion times the mass of the sun. And it has a huge jet that's coming out that extends all the way and whose origin we can actually link to the black hole itself.
Fast forward to the early 70s when Roger was starting graduate school and astronomy was buzzing with new discoveries. Humans had landed on the moon in 1969. The first black hole was confirmed to exist in 1971 after decades of mathematical theory. And astronomers were looking at the really bright centers of galaxies in our universe. The centers of these galaxies have extremely massive black holes.
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Chapter 4: How were black hole jets first discovered?
And some also seem to have these bright streaks or jets of energy coming from the middle. Just like the one Curtis saw coming out of M87 decades earlier.
So I was watching my colleagues and friends learn about these jets using their wonderful radio telescopes, which have got more and more wonderful as the years have rolled by.
And these jets they found that were coming from these black holes were huge, four times as long as they were wide. These blazing streaks were stretching far past the width of the galaxies they lived in.
This was a truly extraordinary thing because an object that's no bigger than the solar system, and in many cases significantly smaller, is making enough power to outshine the surrounding galaxies by more than a thousand in some cases. That's even more impressive than a tiny little atomic nucleus in an atom producing all the power that it can produce.
But how do these jets exist in the first place? Well, to solve any good mystery, we need clues. And we're better to look for clues than the black holes themselves. Black holes have two important clues. First, black holes spin, or as we say in physics, they have angular momentum. But why are black holes spinning?
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Chapter 5: What role do black holes play in the universe?
We believe that black holes are spinning because of the way in which material accretes onto them.
The dust and gas getting sucked up into the black holes, they're also spinning and have angular momentum. But along the way to entering a supermassive black hole, that dust and gas swirling in, it loses angular momentum.
They swirl less. That's why it's able to swirl all the way in. But there's angular momentum that you get and that spins a black hole. Plus, we also know that's one way black holes grow. So the spin is a consequence, we believe, of how black holes actually assemble and grow in mass.
So stuff accumulates around these black holes and causes them to spin. But how is that material getting shot out into space in these huge energetic jets? Clue two, magnetic fields. Black holes have magnetic fields like stars and some planets do. And as black holes spin, these magnetic fields get tangled up.
And charged particles in the gas get carried away along field lines and eventually into the jets.
there is a source of power which is the spin of the black hole, and the magnetic field is the agency for removing it, if you like. But in addition, there's all this gas that is falling inwards, swirling around the black hole. And just like a satellite in the atmosphere, it gets hot, loses energy, falls down,
And then it goes out. And with all these charged particles, you get radiation.
So by the time I was in graduate school, we already knew that the primary mechanism for jet production in black holes arises from sort of the interaction of strong magnetic fields with the spinning black hole.
where the rotation of the black hole kind of twists and amplifies these magnetic field lines and pushing and propelling charged particles along the poles of the black hole into these powerful jets. And this process is often referred to as the Blanford-Znaik mechanism. And that's Roger, one of those names. Absolutely.
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Chapter 6: Why do black holes spin and how do they grow?
The popular view is that you can't get anything out of a black hole. But if the black hole is spinning and there are magnetic field lines... like those you would associate with a picture of a magnet or the Earth, if they go through the surface of the black hole, which is called the event horizon, then it is possible to take power out of the black hole.
And that comes at the expense of the rotational energy of the black hole.
This hypothesis and its equations bring together big concepts in physics, like Albert Einstein's equations of general relativity, how gravity is just a warping of space-time, and James Clerk Maxwell's older electromagnetism equations that describe how electricity and magnetism interact.
It's just really beautiful because it combines these two very deep and profound ideas and shows you how energy can actually be extracted. But all of this is really hard to observe.
And it wasn't until the last decade or so that scientists were able to really see the jets up close, with a bunch of radio telescopes around the world showing high-resolution images of the jet coming from that supermassive black hole at the center of M87, the one Heber Curtis stumbled upon in 1918.
Images also showed a magnetic field around the black hole, one step in supporting the Blanford-Sniak process.
The fields are very strong and somewhat organized, as might have been expected. And also, they look like they're being created by gas swirling around the black hole, and they have the sort of distortion you might expect with it.
These images are the closest we can get to studying black holes directly.
They're as up close as we can possibly get to a black hole's event horizon. You're able to see where the jet starts. And so that is pretty unprecedented.
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