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The biggest black holes in the universe are, on average, lighter than air. Strange, right? Black holes hide several more surprises like this. Let's take them one at a time. Number one: near a black hole, time slows down.
An astronaut falls toward a black hole, holding a lamp that blinks once a second. We watch from far away. He seems to move slower and slower, and so does the lamp. Its light turns redder and dimmer, until it fades away.
We never see him fall in. He seems frozen at the black hole's doorstep. Number two: the bigger the black hole, the gentler it is. Close to a black hole, your feet are pulled harder than your head. You get stretched out, like a noodle.
A small one weighs about ten Suns. You'd be stretched into a noodle before you even reached the door. A giant one weighs billions of Suns. You might cross its doorstep and feel nothing at all. Of course, once you're in, there's no coming back out.
Number three is the one we started with: the biggest black holes are lighter than air. When a black hole gets twice as heavy, it also gets twice as wide. But the space inside becomes eight times bigger. The weight can't keep up with the space, so spread it out, and the bigger ones come out thinner.
Take a galaxy in the constellation Virgo. The black hole at its center weighs six and a half billion Suns. Spread that weight through a ball as big as the black hole, and each cubic meter holds less than half a kilogram. A cubic meter of air weighs about one point two kilograms.
Number four: black holes sing. Two hundred fifty million light-years away, a black hole blasts out matter. It pushes ring after ring of ripples through the gas around it. Those ripples are sound. Its note is fifty-seven octaves below middle C on a piano.
A whole piano spans only about seven octaves. Our ears could never hear it. Number five: black holes slowly evaporate. In 1974, Stephen Hawking worked out that black holes leak a tiny bit of light, and shrink little by little.
For a black hole as heavy as the Sun to leak away completely would take ten to the power of sixty-seven years. The universe is only thirteen point eight billion years old. And no one has actually seen this happen yet. The nearest known black hole is fifteen hundred sixty light-years away. And at the very center of our galaxy sits another. It's more than four million times as heavy as the Sun.
Look up tonight, and you won't see them. But they're always there. The darkest places in the universe still hold puzzles we haven't solved. Got a strange question about the universe? Leave it in the comments, and it might be our next episode.
Near one, time slows. Bigger ones are gentler. The biggest are lighter than air. They sing. And they slowly evaporate.
Let’s take them one at a time.
An astronaut falls toward a black hole, holding a lamp that blinks once a second.
From far away, we see him move slower and slower. The lamp blinks slower, turns redder and dimmer, and finally fades away.
We never see him fall in. He seems frozen at the black hole’s doorstep.
Close to a black hole, your feet are pulled harder than your head. You get stretched out, like a noodle.
Of course, once you’re in, there’s no way back out.
When a black hole gets twice as heavy, it also gets twice as wide — but the space inside becomes eight times bigger. The weight can’t keep up, so the bigger ones come out thinner.
The black hole at the center of a galaxy in the constellation Virgo weighs 6.5 billion Suns. Spread that weight through a ball as big as the black hole, and each cubic meter holds less than half a kilogram.
A cubic meter of air weighs about 1.2 kg.
That’s just an average. The inside of a black hole is nothing like thin air.
250 million light-years away, a black hole blasts out matter, pushing ring after ring of ripples through the gas around it. Those ripples are sound.
Its note is 57 octaves below middle C on a piano.
A whole piano spans only about seven octaves. Our ears could never hear it.
In 1974, Stephen Hawking worked out that black holes leak a tiny bit of light and shrink, little by little.
A black hole as heavy as the Sun would take 1067 years to leak away — a 1 followed by 67 zeros. The universe is only 13.8 billion years old.
So far this comes from the math. No one has seen it happen.
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