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If you’ve heard only one thing about black holes, it’s probably that, once inside a
black hole’s event horizon, nothing, not even light, can escape.
At which point it’s natural to wonder, if nothing can escape a black hole, how could
we ever observe them?
How do we even know they exist?
Well, only things inside the event horizon are stuck – black holes also gravitationally
pull on stuff outside their event horizons, and by looking at that stuff we can get a
really good sense that there’s a black hole nearby.
For example, lots and lots of stars orbit in pairs , but we also see stars orbiting
things that aren’t normal stars, but instead emit crazy amounts of x-rays – and x-rays
in space often come from dust and gas that gets superheated while spiraling into a very
dense, very heavy object.
Anyway, by figuring out the mass and orbital characteristics of the stars whose partners
emit x-rays, we can determine how heavy the partners are.
Some parters are lightweight enough to be neutron stars , but neutron stars can only
get so big before they collapse in on themselves – theoretical calculations put their upper
size limit at around 2-3 times the mass of the sun, and the biggest ones we’ve observed
all fall inside that limit . And yet, there are plenty of stars whose orbits clearly show
that their x-ray-emitting partners are 5-10 times the mass of the sun, and we simply don’t
know anything else these could be other than black holes.
Sometimes you don’t even need an orbiting star at all, and just the x-rays and radio
waves from the hot infalling material can be used to determine the mass of a solitary
non-star object.
In some cases they turn out to be neutron stars, but in others they turn out to be way
too heavy, and can only be black holes.
There are also objects at the centers of lots of galaxies (including our own), that emit
lots of x-rays, radio waves and infrared radiation, but not much visible light, and we know these
objects are stupendously heavy because of the way that nearby stars and hot glowing
dust orbit them.
These orbits tell us the objects are both so heavy and so small they can’t possibly
be a star or cluster of stars or distributed clumps of other invisible matter; the only
thing they could be is supermassive black holes.
For example, in the middle of the Milky Way there’s an x-ray, radio wave and infrared-emitting
object called “Sagittarius A*” with nearby stars orbiting it in such such small, fast
orbits that we know it weighs 4 million times as much as the sun!
And finally, we’ve also directly observed, on multiple occasions, gravitational waves
that were emitted from the inspiralling collisions of two very heavy dense objects.
Some of those waves have the signature of a collision between objects lightweight enough
to be neutron stars.
But other waves could only have come from collisions between objects far too heavy to
be anything but pairs of black holes merging to become single, bigger, black holes.
And in these cases, the details of the wave signatures looked exactly like what theoretical
black hole collision calculations predict.
So, in many different places throughout the universe, we’ve detected very dense high-mass
objects by their gravity – either indirectly via their affect on nearby bright stuff like
stars or accretion disks of gas and dust, or directly via their gravitational waves.
Many of these dense high-mass things are too dark to be regular stars, too compact AND
too dark to be clusters of stars, and too heavy to be neutron stars.
They exist, they behave pretty much exactly the way physics predicts black holes would
act, and there’s literally nothing else they could be.
To quote an astronomer: we have “strong confidence that black holes, or at least objects
that have many of the features of black holes, exist”
In other words, if it looks like a black hole and acts like a black hole… we call it a
black hole.
Thanks to NASA's James Webb Space Telescope Project at the Space Telescope Science Institute
for supporting this video.
The James Webb Space Telescope will be able to observe the most distant emissions from
some of the earliest supermassive black holes in primordial galaxies and hopefully help
us understand how black holes drive galaxy evolution and development.
Webb will also spot black holes via the stars, gas and dust they attract, and help us understand
black hole energy dynamics, including the powerful relativistic jets they can produce.