Intro
This video is sponsored by Magellan TV.
Welcome back to Launch Pad, I'm Christian Ready, your friendly neighborhood astronomer.
And, I'm sure you've heard by now that the 2020 Nobel Prize in physics was awarded to Sir Roger
Penrose for his theoretical work on black holes, and to astronomers Andrea Ghez and Reinhard Genzel
for discovering the supermassive black hole at the center of our galaxy.
This award is pretty consistent with the Nobel academy's trend of
splitting the prize between the theoretical and experimental sides of physics. It's also
very cool that for the second year in a row, astrophysicists were included in the award.
I'll be honest, my whole vibe toward the Nobel Prize is kind of mixed because on
the one hand it's very prestigious and garners a lot of public interest,
and certainly recognizes individuals who made significant contributions to physics.
But on the other, it's governed by some pretty outmoded rules. Notably, that the prize can only
be awarded to no more than three people at a time, even though modern science is done as
collaborations involving tens, or even hundreds of scientists all working on the same project.
And that may be why why the prize didn't go to the Event Horizon Telescope team
for producing the first direct image of a black hole. There's no
question it's perhaps the most compelling observation ever made in astrophysics,
but there's no way the Nobel Academy would award a prize to a team that large.
However, that should in no way detract from the award going to Penrose, Ghez, and Genzel because
there's no question they made huge contributions to our understanding of black holes.
So I thought it would be fun to talk about them today
and about the supermassive black hole at the center of our Galaxy in particular. But first,
I'd like to thank Magellan TV, who are very kindly sponsoring today's video.
Magellan is a new documentary streaming service that dives deep into topics such as History,
Art, Nature, and of course, Space and Science.
In fact, while researching this video, I watched "Monster Black Hole"
which talks about how supermassive black holes are thought to form,
how they drive the evolution of galaxies over time, and how researchers are recreating the
conditions that form a black hole in computer simulations and even in particle accelerators.
New programs are added weekly and can be watched on Roku, AmazonFireTV,
AppleTV, Google Play, and iOS. You can even cast directly from your phone to your TV.
Best of all, Magellan has a special offer just for my viewers. By signing up at the link below,
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of this video. History
When Albert Einstein published his theory of general relativity in 1915,
it was - to put it mildly - a revolution in how we perceived the Universe. Time and space were
no longer separate and absolute, but were woven together into a fabric of spacetime.
Matter and energy were no longer separate, but were equivalent and
could be transformed into each other. And if that weren't weird enough,
matter and energy can curve the fabric of spacetime, creating the effect we call gravity.
When Karl Schwarzschild solved Einstein's field equations,
he showed that if spacetime reached infinite curvature, it produced a singularity surrounded by
an event horizon at a critical radius. Events inside this horizon were forever
hidden from the rest of the Universe because light cannot escape from it. In other words,
Schwarzschild came up with the first mathematical description of a "black hole."
When Einstein learned of Schwarzschild's work, he was surprised that his field equations could
have such an exact solution. But, he and others didn't think it was actually possible matter could
ever compress itself to such incredible densities. So, black holes were regarded
as more of a mathematical curiosity but not a real thing in nature.
But in the 1960's, Roger Penrose and the late Stephen Hawking came up with a series
of theorems that showed that once matter collapsed beyond its own event horizon,
there was no force in nature that can prevent it from collapsing all the way down to a singularity.
But Penrose went a step further. He showed how something that wasn't a black hole
can transform itself into one, creating the event horizon and singularity in the process.
Along the way, he developed the mathematical tools to describe the behavior of light
at all points in space and time inside the event horizon. He even came up with an elegant way to
visualize the relationship between space, time, and light with his famous Penrose diagrams.
Thanks to Penrose, I now know that if I were to extend my hand beyond the event horizon of
the black hole, it would be pulled inexorably toward the singularity in the infinite future,
even as the rest of me remains here in the present. There's other complications,
such as whether or not I'm too busy being spaghettified and radiated to even notice.
But Penrose showed that black holes can exist in nature, and is credited with
making the most significant contributions to general relativity since Einstein himself.
But as far as who actually discovered the first black hole, well that's a different story.
Cygnus X-1 is widely accepted as the first black hole ever discovered. As its name implies,
it's a strong X-ray source in the constellation Cygnus the Swan. It was discovered in 1964
in an X-ray survey of the sky. It was later shown to be a close
binary system consisting of a blue supergiant star orbiting a black hole of about 15 solar masses.
The X-rays are coming from an accretion disk of material that's being drawn from the star.
As it falls into the black hole it spirals inward and gets
heated up to millions of degrees, producing X-rays in the process.
Both objects in the Cyg X-1 system are too close together to be resolved in a telescope,
so the mass of the black hole was determined by measuring the doppler shift of star's spectrum
as it orbited around the source.
Now, I don't know why the prize wasn't awarded for the discovery of Cyg X-1, but what I can say is
that Sagittarius A-star is widely considered to be the first empirical observations of a black hole.
It was initially detected by Karl Jansky in 1931 as a strong radio source coming from the center of
the Milky Way in the constellation Sagittarius. Later, it was later dubbed Sagittarius A.
Radio waves are very long, which means they're able to penetrate through the
26,000 light-years of gas and dust between Earth and the center of the Galaxy. However, their long
wavelength also makes them hard to resolve into a single object with just one radio telescope.
In 1974, Bruce Balick and Robert Brown studied Sagittarius A using the Green Bank interferometer
in West Virginia. This instrument consisted of four widely-separated radio telescopes
linked together through interferometry to produce a virtual telescope.. They showed that Sagittarius
A was in fact several radio sources, with the brightest - or "loudest" - source at the very
center of the Galaxy. In 1982, Brown dubbed this object Sagittarius A-star.
Many astronomers suspected that Sag A* was probably a supermassive black hole. However,
that's purely a speculative statement. And, there were some competing ideas that Sag A* could
instead be an ultra-dense cluster of neutron stars and white dwarfs
instead of a black hole. And this was because nobody really knew its mass.
But starting in the 1990's, large-aperture optical telescopes
started to become available. Now, viewing Sag A* at visible light wavelengths was
still out of the question because the intervening dust generates 25 magnitudes of extinction.
In other words, it reduces the otherwise visible light by a factor of around 9 billion!
However, these telescopes were built in locations where infrared observations are possible.
Since infrared waves are just long enough to avoid the dust,
these telescopes were able to actually see the stars orbiting around Sagittarius A*.
Now, let me just take a second here to say that we can actually see the individual stars orbiting
a compact, unseen object 26,000 light-years away
at the very center of the Galaxy. I mean, t hat still blows my mind!
Anyway, with these direct images, it became possible to
measure the orbits of these stars as they made their way around the black hole.
By measuring their orbital periods, their distances from the black hole, and their orbital
velocities, astronomers could use these stars to weigh the mass of the object they were orbiting.
The first astronomers to do this were Andreas Eckart and Reinhard Genzel using the New
Technology Telescope at the La Silla observatory in Chile. They calculated the mass of Sag A* at
2.45 million, plus or minus 400 thousand solar masses.
Now, this was the first direct measurement of Sag A*'s mass, and it's really hard
to describe anything that massive and that compact as anything other than a black hole.
But, this was just one measurement, and really needed confirmation before it could be considered
valid. So Andrea Ghez and her team at UCLA used the 10-meter Keck telescopes at Mauna Kea
to make their own observations from 1995 to 2007. Not only were the Keck telescopes much
larger than the telescopes used previously, they were also equipped with adaptive optics systems.
Adaptive optics uses lasers to create artificial stars that the system can
then use to cancel out the turbulence of the atmosphere and produce ultra-sharp images.
So for 12 years, Ghez and her team were able to make much more precise measurements of as
many as 580 stars orbiting the black hole. These meticulous observations
resulted in a mass of 4.5 million - plus or minus 400 thousand - solar masses.
This result sealed the deal.
Sag A* was far too massive and too compact to be anything other than a black hole.
Meanwhile, astronomers continue to make high-precision observations
of Sag A* to test General Relativity.
In 2018, Genzel and his team used the Very Large Telescope to catch the star S2 as it made its
closest approach to the black hole. To achieve the highest resolution possible, the team used
all four of the 8-meter unit telescopes, plus all four of the 1-meter auxiliary telescopes to create
a ginormous 130-meter virtual telescope. These were incredibly difficult observations to make,
because the light from all 8 telescopes had to converge at exactly the same place at precisely
the same time. But those observations showed that the light from S2 was being
gravitationally redshifted as it worked its way out of the black hole's gravity well.
Thus confirming yet another prediction of general relativity.
Then in April 2020, the GRAVITY team announced yet another confirmation of general relativity with
Sag A*. By then, the star S2 had been observed for its full 16-year orbit. S2's orbit precesses,
meaning that the location of its closest approach to the black hole changes with each orbit, so the
next orbit gets rotated ahead of the previous one, and the star traces out a rosette shape.
Orbital precession can be caused by several factors,
such as changing tidal forces on the orbiting object, quadrupole moments,
and gravitational perturbations from - in this case - other stars orbiting the black hole.
But general relativity predicts that spacetime itself can cause an orbit to precess as well.
As the black hole rotates underneath the star, it drags spacetime around it in an effect
called "frame dragging". This adds yet another kick to the precession of S2's orbit.
After 27 years of precise measurements, the GRAVITY team confirmed that S2's
orbit precesses exactly as relativity predicts. Einstein was right yet again!
Of course, nothing would be cooler than actually seeing the black hole. Last year,
the Event Horizon Telescope team blew our collective heads off when
they revealed the first-ever image of the black hole in the heart of the galaxy M87.
EHT did observe Sag A* as well.But,
M87 is 6.5 billion solar masses, so it's 1400 times larger than Sag A*. So,
even though M87 is 2000 times further away, both black holes are roughly the same size on the sky.
So, why then haven't the EHT team released an image of Sag A*? The reason has to do with its
comparatively small size. M87* has a radius of about 127 astronomical units. That's a little
less than the distance Voyager 1 has traveled from the Sun. At that size, the signal from any
flare-up on one side of the black hole takes about a week to propagate to the other side.
EHT's observations were taken over the course of a few days so
that was enough time to get an effectively still shot of the black hole.
But Sag A* has a radius somewhere around 30 million kilometers, or 0.2 astronomical units.
That's smaller than the orbit of Mercury. That means fluctuations happen on the timescales of
minutes, and that's too rapid for EHT to keep up with.
That's not to say they aren't trying to tease out an image from that noisy data. But,
perhaps the next generation of EHT will have a better chance of getting it.
But taking a step back, it is so cool that the Nobel prize went to Penrose,
Ghez, and Genzel. And, yes I'm particularly keen about the two astronomers getting the nod because
I remember a time when supermassive black holes weren't the widely accepted convention.
Now, we take it for granted that they dominate the centers of galaxies and influence their evolution.
Now, I've made a bunch of videos about black holes, including the gravitational wave
signal of the most massive black hole merger
ever detected, so please feel free to check that out when we're done here.
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