Thank you to Blinkist for supporting PBS. How do you see the unseeable - how do you
explore the inescapable? Our cleverest astronomers have figured out
ways to catch light that skims the very edge of black holes. Let’s find out what they learned. A few weeks ago a story made the rounds of pop-sci media proclaiming that for the first time light had been detected from behind a black hole. The reports were about a paper that claimed to have seen X-rays that came not from inside but from the back end black hole. This is obviously cool stuff - I mean, really
anything new with black holes captures the public attention. But this result is cool in ways most people
aren’t aware. And it brought us closer to a goal that I’ve
been personally striving towards for years - trying to understand what happens in the
vicinity of the largest black holes in the universe. I thought it would be worth doing a space
time journal club on the Nature paper by Wilkins, Gallo, Costantini, Brandt & Blandford so I
get to talk about my favorite subject in the universe. Now you might remember the picture of the
black hole captured by the event horizon telescope. We talked about this soon after it came out
- but to remind you, here we have radio light from charged particles whirling around the
black hole. And the more recent version of this in polarized
light shows the grain of the magnetic field right near the black hole’s edge. This image was produced through a herculean effort - a team of hundreds of scientists and engineers synthesizing data from radio
telescopes across the globe. They stitched the image together using the
very latest machine learning techniques to achieve an image resolution equivalent to
a telescope the size of the planet earth. All very impressive, very state-of-the-art. But the thing that I love about the result
we’re discussing today is its simple elegance. We’ve been using the same technique for
decades to understand quasars. This method doesn’t need an international
team or a planet-sized telescope - it can be done with a single, ordinary scope, and
one astronomer with lots of patience. That technique is reverberation mapping, in
which we watch as a flare of light from a violent event near the black hole radiates
and reflects - reverberates - its way through the complex structure of a quasar. When that light reaches us - many, many millions of light years away - it’s still imprinted with a wealth of information about the monster
that it escaped. Reverberation mapping is all about untangling
that information. We do this by watching how the spectrum of the quasar changes over time. A spectrum, by the way, is what you get when you split light into its component colors or wavelengths. So basically you watch the quasar, which is
just a faint point of light on the sky, and stretch it into an even fainter flickering
rainbow, and then try to pull from that full 3-D map of the quasar. To understand exactly how astrophysicists
do this, I’m going to take you on a journey - we’re going to follow a flare from its
origin deep in the belly of the beast - uncomfortably close to the supermassive black hole. Supermassive, by the way, is the actual technical term for the largest black holes in the universe - anything more than a million or so times
the mass of the Sun. These things are pretty much only found in
the very centers of galaxies, and pretty much every decent sized galaxy has one of these
at their center. They are mostly dark, except when a bunch of gas shows up in the galactic core - then you get this gigantic whirlpool of searing
plasma screaming around the black hole. AKA an accretion disk, and switching on the
quasar Quasars are not what you’d call stable. They sputter and flare with violent events,
for example as dense clumps of gas hitting the center, or magnetic instabilities shaking
things up. However it happens, when a flare is generated near the black hole it expands outwards, and it causes a lot of trouble on the way. The expanding shell first rips across the
surface of the accretion disk, causing different parts of the disk to brighten - first the
shorter wavelength which corresponds to the hot, inner disk, then to longer wavelengths
of the cooler, outer regions. The flare then continues outwards and upwards. Although it’s energetic, the accretion disk
is relatively well behaved - at least compared to the wild all stuff around it. A maelstrom of lower density gas rages above and below and beyond the accretion disk. That gas is accelerated by a combination of
the incredible gravitational field of the black hole and the continuous blaze of high
energy radiation from the inner accretion disk. It can be driven to up to 10% or more of the
speed of light. Pockets of gas within this flow are somehow shielded and so that gas can cool down a bit. This gas starts to glow in a different way
- not from heat, but from the motion of electrons between their atomic energy levels. This allows the maelstrom to reprocess some of the light of the accretion disk so that it shines bright in very specific colors depending on what atoms are present. In a normal spectrum we see the light from these electron transitions as sharp spikes at specific wavelengths - what we call emission lines. But in a quasar, the gas is moving fast, and
that motion shifts the wavelengths of the light as we see it. Now we’ve all experienced this Doppler shift
when the sound waves of an ambulance siren shift between higher and lower pitch as it
passes us on the street. In the case of the quasar’s emission lines,
gas is moving towards us and away from us at many different speeds. Sharp emission lines get shifted to many different degrees add together to produce the classic broad emission lines - perhaps the most striking feature of the quasar spectrum, and one that confused the hell out of the astronomers who first saw them. And, honestly, it’s been confusing us ever
since. From the Doppler shift we can tell that the
gas is moving fast. But that doesn’t tell us what drives the
gas. Is this stuff pouring towards the black hole? Orbiting it? Or being blasted out by the crazy radiation
from the accretion disk? Now this is where reverberating mapping can be the most powerful. The light from this gas is ultimately powered by the light from the accretion disk. So when our expanding shell of light rips
through this gas the broad lines get supercharged - but in a complicated way. First the gas near the center brightens, then
the gas further out. At the same time, the response of the gas
on the far side of the quasar is slower than the gas of near side because the far-side’s light has to travel further to get to us. Let’s look at a simplistic example. Two totally different scenarios for how gas
might be moving. One - it’s pouring in, rivers of gas dragged
down by the black hole’s gravity. And two: it’s being blasted out, riding
the flow of radiation from the inner accretion disk. Either of these scenarios could produce the
same Doppler-broadened emission lines. But reverberation mapping can tell between
them. Consider case one: if gas is pouring in then
it’s accelerating - moving fastest towards the center. This means this extreme velocity gas - the
far ends of the broad line - should respond first as our flare reverberates out. Also, in this scenario the gas on the far
side of the black hole is moving towards the black hole and so towards us. Its light is blue-shifted to shorter wavelengths. That gas responds late to the flare because
the light has to take this roundabout journey. Meanwhile the gas closer to us is actually
moving away from us as it falls towards the black hole - it’s redshifted to longer wavelengths. And it responds before the blue-shifted light. So in the in-flow model, the quickest response is for the red side of the emission line, the slowest for the blue side It’s exactly the opposite for case two - if
gas is being blasted out. Then the gas accelerates outwards, so we expect the faster moving gas to respond later than the slow gas. And now the gas on the far side of the black
hole is moving away while the gas on our side is moving towards us - so the redshifted gas responds after the blueshifted gas. Oh, and I forgot the third option: maybe the
gas is swirling around in a giant tornado above the accretion disk. In that case inner gas is moving faster like
in the infall case, but now the red and blue sides should respond at roughly the same time. Reverberation mapping has helped us tell between these three - well sort of, anyway. It turns out that all three types of motion
are seen in different quasars. The emerging picture is that gas falls in
along some trajectories, but then gets lifted off the accretion disk by the blazing radiation of the inner disk into a vortex and then is blasted outwards. That’s about as far as we’ve got with
the broad-line tornado. It’s really, really hard to do this stuff
because it takes years for big, powerful quasars to finish responding to a flare in the core. Over the next decade things will get better
as new telescopes like the Vera Rubin Observatory come online. We’ll be able to watch millions of quasars
all do their thing at the same time. OK, it’s time we got back to the discovery
that started our little journey - the light that was detected from behind a black hole. The events described in the paper happened extremely close to a relatively nearby supermassive black hole. The object is I Zwicky 1 - a so-called Seyfert
galaxy, which is like a mini-quasar - this one around 100 million light years away. Near the black hole there are sources of light that I haven’t mentioned yet. The intense radiation in this region strips
almost all atoms of their electrons. That leads to a haze of high-energy electrons surrounding the black hole. As light from the accretion disk passes through this haze it gains energy from the electrons, boosting it all the way up to X-ray energies. This is the X-ray corona. We also have winds of matter flowing at incredible speeds - but this close to the black hole only the heaviest elements like iron can hold on to any of their electrons. We see that iron because it shines at a specific X-ray wavelength - this is the iron K-alpha line. So here’s what happened. The X-ray corona flared bright, probably in
a small region right above the black hole. That light then spread in all directions - some traveled straight to us, some reflected off the disk in our direction, but some also reflected off the disk on the opposite side of the black hole. A portion of that light was then grabbed by
the black hole’s gravitational field and slung right back around towards us, and magnified in the process. It was as we say, gravitationally lensed. These three paths were apparent in the three distinct phases of the flare. They were also able to see there was a difference in the response of the different parts of the iron line. The blue side varied before the red side - and we talked about how this suggests an outflow. We have other reasons to think that this iron-laced wind also has a powerful rotational component. So it looks like we’re seeing an expanding
vortex of stuff that just narrowly escaped being sucked into the black hogle. The paper goes into a lot more detail than
this - lots of good stuff like measuring the mass of the black hole - 30 million Suns - to
verifying Einstein’s general theory of relativity. So you can check out the links in the description for more details. After decades of practice, and inventing better and better telescopes, we’re starting to get good at this game. I just love it when we can map the space around black holes by watching flickering points in the sky, and in that flickering reconstruct
how light reverberates around the most extreme regions of space time. We’d like to thank Blinkist for supporting
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trial, please go to the link in the description. Today we’re covering comments from the last two episodes: first was the Everett-Wheeler telephone, where we explored how to send messages across the quantum multiverse. And then the episode on how vacuum decay could destroy the universe .. or universes, if the ideas in the first episode are right. Mark V-A wonders whether faster than light
communication methods might be detectable from great distances - as in, could we see
the effects of FTL communication from distant alien civilizations? Well, warp fields are supposed to blast an
intense beam of radiation ahead of them when they come out of warp, which could potentially be seen if they were pointed at us. Some people have proposed that this is what gamma ray bursts might be. Those people are dead wrong because it would mean the number of FTL-capable civilizations in the galaxy would have to be enormous for the numbers to work out. That’s for actual FTL travel. For communication the warp bubble can be much much smaller, so probably wouldn’t be able to observe those. Same with entanglement-based communication. The way we’d most likely detect FTL activity
is because FTL-capable civilizations shouldn’t have any problem coming to visit us.- which
they haven’t, so there’s some evidence right there. Karina makes a similar style of argument - saying that this crazy idea just makes it even more convincing that Schrodinger's equation is
linear. In other words, if the non-linearity of the
schrodinger equation allows these paradox-generating phenomena, then we can take that as evidence against that non-linearity. And honestly, I think that’s the most useful
takeaway from Polchinski and Weinberg’s work. Moritz von Schweinitz and TBE_pryce both commented that the vacuum decay scenario sounds a lot like the big bang - what with it producing
an expanding bubble full of energetic particles. Excellent observation. In fact cosmic inflation - the event that
many physicists put the bang in the big bang - IS a type of vacuum decay. Some scalar field that may or may not have
been the Higgs field, is thought to have dropped from a much higher value into a stable minimum. The difference is that in the pre-inflation
universe, space was expanding exponentially quickly due to the high value of that field. That means a new vacuum decay event now wouldn’t
yield universe whose interior is expanding quickly - in the sense of particles racing
apart from each other. You can look at our past episodes on inflation for all the gory details Dr Patrick Bryant from Carnegie Mellon University popped into the comments section and told us a little about his work using the large hadron collider to try to map the shape of the Higgs potential by looking at Higgs boson pair production. Dr Bryant’s answered one question that I
also had. How big does the vacuum decay bubble need to be in order to propagate. If it’s too small and it’ll collapse on
itself. Actually, Dr Bryant didn’t know the full answer - but he tells us that it depends on the energy difference between the false and true vacuum
- presumably the bigger the difference the smaller the bubble needs to be. But I would also imagine that the bigger the
energy difference, the less probability of the field tunneling to the true minimum, so
maybe those cancel out? I’m just making stuff up now. Dr Bryant, if you wouldn't mind hurrying up
with mapping the Higgs potential so we know the answer to this, that’d be great. You mention you probably need a bigger collider to do that properly. Cool. Let’s do it. AAALE quips "If we figure out how to send
messages backwards in time, I'll make sure I'm the first to know.” If I figure out how to send messages backwards in time I’ll warn myself not to do video intros in which I say I’m going to send
messages between universes … in a Scottish accent. Mostly because everyone thought it was a Russian accent - except the Scots, who just thought it was really bad. But to that I answer - you people have no idea what Scottish accents sound like in parallel universes. That accent was as unfalsifiable as the Many Worlds interpretation, so there.
