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It’s been over 50 years since black holes were first detected, and we now know
there may be 40 billion billion of them in the observable universe.
They’re all over the place, including at the center of almost every large galaxy.
And yet they’re still among the most mysterious objects in the universe.
Given that they’re completely invisible, scientists have struggled to answer even
relatively simple questions about them, like how fast they spin,
and whether or not they obey fundamental laws of the universe.
But now, a pioneering space telescope is giving us a brand-new view
of black holes… and a chance to answer these questions at last.
Back in 1915, Einstein published his theory of general relativity,
which predicted the existence of black holes.
It indicated that there were objects in the universe so dense that they created
a deep gravitational well in the fabric of spacetime,
trapping everything that got too close, including light.
And they squeezed all this matter into an infinitely dense point.
But the whole swallowing-light thing made them hard to actually detect.
Black holes don’t emit or reflect any radiation.
So, it wasn’t until 1964 that astronomers first detected one indirectly,
based on its gravitational influence on another star.
Since then, scientists have indirectly observed many black holes.
Some are just a few times the mass of our Sun,
while others are many billions of times more massive.
But beyond calculating their masses, there’s a lot astronomers still
don’t know about these bizarre, extreme objects.
One of the biggest mysteries is about their spin.
Pretty much everything in the universe spins like a top,
including planets, stars, and galaxies.
Black holes are no exception.
And as an object contracts and gets denser, it generally spins faster.
Just like how a figure skater who tucks in their arms and legs
will start spinning so fast they become a blur.
It’s an example of a law of physics called the conservation of angular momentum.
And nothing in the universe is exempt.
So you’d think that black holes, with their infinite density,
would rotate so fast that their spin approaches infinity too.
But mathematical solutions to Einstein’s theory
of general relativity suggest that’s not true.
They indicate that there’s actually a maximum speed that black holes can spin.
And any extra angular momentum will be radiated away as gravitational waves.
The problem is, the blackness of black holes has made it tough to figure out
whether or not this math holds up in practice.
For the longest time, no one could see how fast black holes really spin.
But in recent years, that’s changed.
In 2012, NASA launched an X-ray telescope called the
Nuclear Spectroscopic Telescope Array, or NuSTAR, into space.
They actually launched it from underneath a plane, which is pretty cool in itself.
But what followed was even cooler.
Once the telescope was in low Earth orbit, around 600 kilometers
above Earth’s surface, it unfolded a 10 meter-long mast and
began to scan the sky for the highest-energy X-rays.
These X-rays are especially useful for probing the universe because they can
pass straight through the dust and gas that obscure many objects in space.
So NuSTAR unveiled many phenomena that had never been explored before.
And one thing that it looked at was gigantic black holes.
Now, it’s not like NuSTAR could see the black holes themselves,
because as long as an object is invisible,
it doesn’t matter how powerful your telescope is.
But, while black holes themselves are invisible,
they’re often surrounded by a huge, turbulent disk of material.
As material in that disk swirls in toward the black hole,
it gets superheated and radiates energy away as X-rays.
So it glows in wavelengths that NuSTAR can detect.
In 2012, scientists focused the telescope on the supermassive black hole at
the center of a galaxy some 56 million light-years away.
They were hoping to use the information in the X-rays
radiating from its disk to measure the black hole’s spin.
General relativity suggests that the faster a black hole spins,
the tighter its disk will hug the horizon of the black hole.
And the closer it is, the more its radiation will be affected by twisted spacetime.
Previous observations by other telescopes had looked at X-rays emitted by iron,
which has a characteristic signature that the telescopes could pick out.
And they found that these X-rays were warped and distorted.
But, unlike NuSTAR, these older telescopes observed
lower-energy X-rays that could potentially be obscured by dust.
So, astronomers weren’t sure whether the distortion they saw was
from the dust clouds or warped spacetime.
Which meant they couldn’t draw any conclusions
for sure about the black hole’s spin.
That’s where NuSTAR came in.
NuSTAR detected this same iron signal, but in higher-energy wavelengths
that would cut through any dust.
And it found that the signal was still distorted,
proving that it wasn’t clouds causing the distortion, but the black hole’s spin itself.
Now, scientists could precisely measure the distortions
in the iron signal to work out exactly how fast the black hole was spinning.
Through this, they found that the black hole was rotating at
84 percent of the speed of light.
That’s very close to the maximum possible speed predicted by Einstein’s theory.
It’s the best evidence yet that black holes do have a maximum spin speed,
and that Einstein got this right decades before we’d even detected a black hole.
Beyond providing real-life evidence supporting theoretical physics,
measuring the spin of supermassive black holes is important for
understanding how they formed and evolved throughout the early universe.
NuSTAR has given us a rare look at objects we have limited ways to explore.
And today, this telescope is still going strong,
pulling back the curtain on these giants and the universe as a whole.
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