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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,
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As it happens, Curiosity Stream has a really excellent overview
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"Gravitational Waves-- Rewinding Time"
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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.