How We Know Black Holes Exist

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.

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