At the heart of every galaxy lies one of the most mysterious objects in the universe:
a supermassive black hole.
Millions to billions of times the mass of our sun,
these giants power astrophysical jets,
one of the most energetic processes known to physics.
In 2019, the Event Horizon Telescope released this picture of the heart of M87 —
the first-ever image of a black hole.
A second image, released in March 2021,
is helping astronomers understand the inner workings of astrophysical jets
and the celestial giants that fuel them.
A quasar is a massive black hole that resides
in the nucleus of most galaxies like our own.
It's smaller than our solar system, and yet
through a variety of mechanisms, it is able to outshine
the whole galaxy around it
so it is a very mighty engine
that's very small,
but is very, very powerful.
If you look at the essential features
of what we currently think is going on
in the tiny nuclei of these galaxies,
is that firstly, there is a black hole there, which has this
mass, but it has one other property,
which I believe to be very important,
which is that it is spinning.
As a black hole spins, it draws charged particles,
known as plasma, into its gravitational pull.
These particles create a magnetic field that ejects energy
thousands of light-years into space.
Jets themselves comprise very hot gas,
and a magnetic field, which together move at a speed which is very close to the speed of light.
Following the precepts of special relativity,
that means that any radiation that they emit will be beamed
along their direction of motion.
In 1918, Heber Curtis discovered the first jet,
which he described in his notes as a “curious, straight ray.”
As it turned out, that jet belonged to the supermassive black hole in M87.
So it's a rather special object in the life of an astronomer.
I was a research student in Cambridge University in the early 1970s.
This was, in some sense, a golden time for black holes.
My friend and colleague Roman Znajek and I came up with a process
which was able to extract energy from the spin of a black hole
with an efficiency of roughly 1/2,
and then we said, well maybe this is the power source for the jets,
and much else besides.
The Blandford-Znajek process describes
how a spinning black hole twists its magnetic fields into coils.
This motion creates a current of electrons and positrons
that flow out from the black hole in both directions,
accelerated by the twisting magnetic fields.
But until the recent Event Horizon Telescope image,
this process was just a theory.
So what we see in our image is a spiral structure
of polarization, which tells us that the magnetic field is also spiral.
To have a Blandford-Znajek process, you need ordered magnetic fields
which is what we see.
Our image is ordered.
And the spiral structure tells us that there is a component
in the magnetic field around the disk,
but there's also a vertical component.
This means there's magnetic pressure that pushes out gas very close to the black hole.
This is also what the Blandford-Znajek process predicts.
This vertical component, we think, is a direct link
to how the black hole is able to launch this jet.
It is the most energetic, powerful process in our universe.
These tiny nuclei,
smaller than the solar system,
but containing massive black holes,
can produce as much energy over their lifetimes
as the stars that surround them.
And so when we ask, "How is the universe powered?"
Yes, there's gravity.
There are stars.
But these tiny but mighty galactic nuclei
are big players, too.
And so they have a large role
in the growth of what we see in the universe around us.