[MUSIC PLAYING] MATT O'DOWD (VOICEOVER):
This episode is brought to you
by Curiosity Stream. Last year, LIGO
announced the detection of gravitational waves from
the merger of two black holes. The science world
went a little crazy. But only a few weeks
ago, a new rumor emerged, that LIGO
had for the first time spotted gravitational
waves from the collision of a pair of neutron stars. If this is true, some
longstanding astrophysical mysteries are about
to be unlocked. When the Laser Interferometer
Gravitational Wave Observatory, LIGO, detected
gravitational waves from a pair of
merging black holes, an entirely new realm of the
universe opened up to science. We now have an
observatory that can explore the most extreme
gravitational phenomena in the universe. Check out our episodes on the
LIGO detections for more info. Since the first, LIGO has
announced the detection of two more black hole mergers. As the data comes
in, we're learning a ton about black
holes, how they grow, and the stars that produce them. But the merger of
binary black holes isn't the only game in town. LIGO was supposed to also
detect some other crazy stuff like certain types of
supernova explosion and the merger of
binary neutron stars. It may have just done so. Rumors abound that
LIGO has finally spotted the long expected
neutron star, neutron star merger, and that the event was
accompanied by a bright flash of gamma rays. The rumor has already been
hyped all over the press, so let's dissect it
with a skeptical eye. But before we figure out
whether the rumor is true, in fact, before we talk about
the supposed signal at all, let's refresh our
memory on neutron stars. When a massive star ends its
life in a supernova explosion, it leaves behind an
ultra dense core. For the most massive
stars, that core will collapse into a black hole. But there's an
intermediate range. A remnant core between
1.4 and around 3 times the mass of our sun instead
ends up as a neutron star. These insane objects carry the
mass of a star within a sphere the size of a city, around
18 kilometers in diameter. They are mostly
composed of neutrons at the density of
an atomic nucleus and are held up by a quantum
mechanical force called degeneracy pressure. We talk about the bizarre
physics of these quantum and gravitational
monsters in this video. Neutron stars can rotate up to
thousands of times per second and have enormous
magnetic fields that result in jets
of near light speed particles that sweep through
space like a lighthouse. When those jets
sweep past the Earth, we see the regular
flashes of a pulsar. In fact, the first real
evidence of the existence of gravitational waves
came from a pulsar. This was the Hulse-Taylor
binary, two neutron stars in orbit around each other,
one of which is visible to us as a pulsar. This binary pair stirs up
spacetime in its vicinity, creating ripples
that travel outwards as gravitational waves. And that gravitational
radiation sucks energy from the orbiting system,
causing the neutron stars to spiral inwards. By monitoring the pulses
of one of those stars, this inspiral was measured. The rate of loss
of orbital energy exactly matches
the expected rate of emission of
gravitational radiation. Any neutron stars or black holes
in close orbit with each other will eventually collide as they
leave gravitational radiation. We now know of plenty of neutron
star pairs in binary orbits. In fact, we expect them
to be much more common than black hole binaries. Why? Well, because the
universe makes far more neutron stars than black holes. See, black holes only
form in the deaths of the most massive stars, those
over approximately 20 times the Sun's mass. But these are also
the rarest of stars. Neutron stars form
from the not quite as rare stars of around
8 to 20 solar masses. That means neutron stars should
be more common than black holes and neutron star binary
systems should merge more often than black hole binaries. So why isn't LIGO
seeing lots of them? Well, again, it's
because of mass. The remnant core
of a dead star must be less than 3 solar masses
to make a neutron star. But that's a factor of 10
smaller than the 30 solar mass black holes that merged in
the first LIGO detection. Smaller mass means weaker
gravitational waves. In fact, a typical
neutron star merger needs to be about 10
times closer to us than a typical black
whole merger for LIGO to be able to see it. If we can see neutron star
mergers only out to 1/10 the distance, then that
translates to being sensitive to 1/1,000 of the volume. We can see black hole
merges across 1,000 times more universe compared
to neutron star mergers. So even though the
latter are common, we have to wait longer for one
to happen close enough to us to be detectable. Neutron star mergers
do have one advantage over black hole mergers. They last a lot longer, at
least from LIGO's point of view. LIGO is sensitive to a
specific frequency range. Inspiraling black holes only hit
that range in the final second before merger,
while neutron stars ring at audible gravitational
wave frequencies for at least several seconds. If we did spot a neutron
star merger as rumored, we'll have a lot more juicy
data to analyze compared to a black hole merger. Let's talk about this rumor. It was started by a tweet
from astronomer J Craig Wheeler about a LIGO detection
with an optical counterpart. "Optical counterpart"
means that there's a source of visible
light associated with the gravitational wave. And in this case, it's
from the suspected galaxy that the wave came from. But how do we locate the galaxy? After all, LIGO
can only constrain the origin of its signals to
a wide band across the sky. Well, there's also the rumor
that the Italian Gravitational Wave Observatory, VIRGO, also
spotted the signal, which helps triangulate the
location, but not enough to get an exact origin. Well, here's how we know. The day before the fateful
tweet, August 17th, the Fermi satellite had spotted
a flash of gamma radiation-- so the highest energy light-- from a galaxy 130
million light years away. Now, that in itself
isn't unusual. These gamma ray bursts
are relatively common. Most are believed to result
from supernova explosions. But around 30% of them,
the short-lived ones which last for less
than 2 seconds, are believed to come from
merging neutron stars. The observed gamma ray
burst, GRB, was of that type. Logs from the follow-up
observation by the Chandra X-ray satellite confirms this. This is still not
especially convincing. But check this out. A Hubble Space
Telescope observation was triggered a few days
later to look at the location of this gamma ray burst. And the particular observing
program that was triggered is one specifically
intended for following up on gravitational
wave detections. Now it's suddenly compelling. Astronomers don't trigger
Hubble observations lightly. Someone in the know
decided that this gamma ray burst was very likely associated
with a gravitational wave. That blob is the origin
of the gamma ray burst. It's NGC 4993, a known
lenticular galaxy. We rarely see supernovae
from this galaxy type because their
most massive stars have long since
exploded to leave neutron stars and black holes. But that makes them the perfect
environment for neutron star mergers. NGC 4993 is 130 million
light-years away, which is about at the limit
for LIGO sensitivity to neutron star mergers. Compare that to the
around 1 billion light-year distance of the
earlier black hole mergers. OK, so assuming the rumor
is true, why do we care? Well, beyond raw
curiosity, it may be that neutron star collisions
produce many of the heavier elements of the periodic table. Most heavy elements like
gold, lead, uranium, et cetera, are produced when the
nuclei of lighter elements capture fast-moving neutrons. This is the r-process. It definitely happens
in supernova explosions, which for a long
time were thought to be the primary source
of heavy elements. But it turns out that merging
neutron stars can do this too. As these stars coalesce,
most of their material goes into forming
a new black hole. But the neutron
stars' thin iron crust is likely bombarded
with neutrons and blasted outwards, spraying
a ton of r-process elements into the galaxy. Depending on how much of
the outer layer is ejected, neutron star mergers
could produce most of the heavy r-process
elements that exist. Seeing a gravitational
wave signal from merging neutron
stars would allow us to determine pretty
exactly how much mass is lost in the merger. This will be an
extremely important piece of evidence in either killing
or helping confirm the idea. Besides their importance
to nucleosynthesis, the simple fact that we can
see neutron star mergers in regular light is
extremely powerful. Black hole mergers
are dark, so we have to infer almost everything
from the gravitational waves alone. But colliding neutron
stars are bright across the electromagnetic spectrum. Comparing the EM and
gravitational wave signatures will teach us a lot. If this is real, why
hasn't LIGO announced it? Well, the LIGO team has
always shown admirable caution before making big
announcements in the past. They want to analyze
the data fully to ensure the signal meets
their very strict statistical standards for
claiming a detection. The last LIGO
observing run ended on August 25, at which point
they announced that there were "promising candidates." Probably that means
more black hole–black hole mergers in addition
to this rumored neutron star merger. But to go from
promising candidate to confirmed detection
requires meticulous statistical analysis. Public announcements
will happen when the team is sure of the
significance of the signal. Fingers crossed on this one. We may have just spotted
for the first time a long theorized
astrophysical catastrophe, one that may have birthed
a new black hole and created half the
Earth's mass in gold. And we will have learned it from
over 100 million light-years' distance by collecting only
a handful of gamma rays and by sensing the
faint ripples it made in the very
fabric of spacetime. This episode is brought to
you by Curiosity Stream, a subscription streaming service
that offers documentaries and nonfiction titles from some
of the world's best filmmakers, including exclusive originals. As it happens, Curiosity Stream
has a really excellent overview of LIGO and gravitational waves. "Gravitational Waves--
Rewinding Time" includes some fascinating
behind-the-scenes footage at the observatories. Get unlimited access today. And for our audience, the first
two months are free if you sign up at
cruiositystream.com/spacetime and use the promo
code "spacetime" during the signup process. I want to thank all of our
Patreon supporters once again. Your contributions really add
up and are an incredible help. And an extra huge thanks to
Mark Rosenthal for supporting us at the quasar level. Mark, we've made arrangements
for half a planet's worth of gold from the next
neutron star merger to be shipped directly to
your address, priority mail. Unfortunately, that will
still take at least 130 million years. Now, we missed comment responses
on the last two episodes because I was traveling. There was the small matter of
an eclipse to be witnessed. We're going to catch up today
by doing a comment from each of the last three episodes. For our episode on
extraterrestrial super storms, Feynstein 21 and
David Nelson asked whether it would be possible
to stop a hurricane by dropping a nuke in its eye. Well, the answer is
no, thank the stars. Nuclear fallout is dangerous
enough, extra dangerous with a bit of wind. Stir up the fallout from a
large bomb with a hurricane, and it becomes a
global catastrophe. At any rate, by simple
energy arguments, it just doesn't work for
any weapon we now possess. A full fledged hurricane
generates anywhere from 50 to 200 trillion
watts of power, which is the equivalent of a
megaton hydrogen bomb exploding every 2 minutes. Even assuming a large enough
bomb could be brought to bear, it wouldn't reduce the
pressure in the eye for longer than
it takes the shock front to leave the hurricane. Pressure depends on the
actual amount of air present relative to the surroundings. And exploding the
eye isn't going to add hundreds of
millions of tons of air. If anything, it'll
reduce air pressure and add heat to the ocean,
making things worse. Regarding our episode
on detecting life by observing
extraterrestrial atmospheres, Alex asks whether we can observe
the atmosphere of Proxima Centauri B. Well, the answer
is no, at least by the method described in the video. That method is to watch the
effect on the parent stars' light as it passes through
the planet's atmosphere. And for that to
happen, the planet needs to pass directly
in front of its star from our perspective. Only a small fraction
of planetary systems are conveniently aligned
so as to do that, and the Proxima Cen
system isn't one of them. On the other hand,
the TRAPPIST-1 planets are perfectly aligned for this. After all, that's how we found
them, using the Kepler Space Telescope. Proxima Cen B was instead found
by the Doppler method, which can measure the tiny wobble
in a star's motion caused by the planet's
gravitational influence. Regarding our episode on
white holes, a few of you wanted some clarification
of a point I made. I said that the Big
Bang is mathematically similar to a white
hole, except that it doesn't possess a singularity. What I mean by that is that the
Big Bang happened everywhere at once, not at an
infinitesimal point in space. The origin of all
space is the Big Bang, and so all space came
into being in that event. Now, we can imagine both the
white hole and the earliest instant of the Big Bang as
possessing infinite or at least extremely high density. But for the Big
Bang, that density is everywhere, not
concentrated at a single point, as in the case of
the white hole. BrendanBlake42 points out
that Karl Schwarzschild looks like Simon Pegg
in a comedy mustache. Oh, my god, that
makes so much sense. No wonder he's such
a Star Trek fan. And Hot Fuzz is obviously
a reference to black hole singularities in string theory.
Question: If there is a humongous black hole, and a smaller one merging with it, the bigger one doesn't move around as much, but the smaller one orbits it in a bigger circle (or ellipse). They basically orbit their barycenter point. Would the gravitational wave of such system be symmetric? I mean can we understand if one is big and one is small by just looking at the gravitational wave? If we can, how do we do it? And If not, do we see the waves as if it's coming from two black holes of the smaller size or two black holes with a size of the average of the two?