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.