When we detected the very first gravitational
wave, a new window was opened to the mysteries of the universe. We knew we’d see things previously thought
impossible. And we just did - an object on the boundary
between neutron stars and black holes, which promises to reveal the secrets of both. By now we’re becoming used to announcements
that a new gravitational wave event has been detected. As though it’s no big deal that we regularly
read the infinitesimal ripples in the fabric of spacetime due to a cataclysmic collision
of black holes billions of light years away. As the LIGO and VIRGO gravitational wave observatories
spot event after event, the excitement is shifting from the holy-crap-we-did-it phase
to giddy excitement about what we’re actually learning. And the latest event is one of the most informative
- and perhaps most surprising so far. It seemed innocuous enough at first glance
- two compact bodies spiraling together. From the shape of the gravitational waveform,
and based on calculations using Einstein’s general theory of relativity, the masses of
those bodies were calculated. One was a hefty 23 times the mass of our Sun
- making it definitely a black hole, and pretty similar to other LIGO mergers. Its companion was puny by comparison - a mere
2.6 solar masses. And there lies the surprise. At that mass, it’s pushing the limit for
what was thought possible for a neutron star, and it’s lighter than what was thought possible for
a black hole. So what exactly is this thing? Today on Space Time Journal Club we’re going
to try to figure it out - and we’ll do that by studying the paper that reported this detection,
published by the LIGO science collaboration just a few weeks ago. We’ve done gravitational wave astronomy
before, but this event is so mysterious we had to cover it. But here’s a quick refresher nonetheless: The LIGO observatories in Washington State and Louisiana and the VIRGO observatory in Italy, consist of kilometers-long vacuum tubes set at right angles. A laser beam is split, sent down these tubes,
then recombined. The passage of a gravitational wave causes
extremely tiny changes in these arm lengths, which in turn causes the peaks and valleys
of the laser’s electromagnetic wave to line up differently, and so those changes can be
measured. On August 14 2019, a gravitational wave hit
the LIGO and VIRGO observatories one after the other in close succession, consistent
with a wave traveling through the entire earth at the speed of light. From the shape of the detected waveform, the
masses of the merging objects were figured figured out as 23.2 and 2.59 solar
masses - and we’ll get back to why those are weird. From the arrival times at the three observatories,
the location of the event could be narrowed down to some small arcs on the sky. Unfortunately, there are countless galaxies
in a region that size, so to start with we have no idea in which galaxy the merger happened. Nonetheless, MANY telescopes quickly swiveled
to scan that region, hoping to spot a faint flash of light - any indication that the merger
of these objects may have been accompanied by an explosive event. Now that’s only been seen once before - with
the merger of two neutron stars in 2017 - which we obviously covered back then. In that case it corresponded to an explosion
observed across the electromagnetic spectrum - energy released as the neutron stars tore
themselves apart in their collision before they collapsed into a black hole. That light carried with it an enormous amount
of information about what happens when neutron stars collide - and we’re going to be using
that information in just a bit. But there was no such luck with the more recent
event. No electromagnetic counterpart was found. That’s not entirely surprising - that event was
6 times further away than the 2017 neutron star merger, so would probably have been too
faint to see anyway. And besides, there may have been no accompanying
explosion at all. That would be the case if both objects were
black holes, but even if the smaller object was a neutron star it could well have been
swallowed whole by the larger black hole without so much as an electomagnetic peep. So, we get to the mystery. What was that smaller body? And why are we all so excited to spot something
with this mass at all? To understand that, we have to understand
a bit more about black holes and neutron stars. A neutron star is what’s left after some
massive stars explode as supernovae. Once it was the burning heart of the star
- a fusion engine that allowed the star to resist the inward crush of gravity. But when it ran out of fuel, gravity took
over and the entire star collapsed. The innermost part of the core turned into
an ultradense nugget of matter while the rest of the infalling core rebounded causing the
supernova explosion. So you end up with at least one and a half
suns worth of matter locked in a ball that would fit inside a small city. That insane density gives the neutron star
a surface gravity around 100 billion times stronger than the surface of the Earth. Scientists believe that it would be very difficult
to get out of bed on the surface of a neutron star. And much more difficult to escape the neutron
star - the escape velocity at the surface is up to half the speed of light. In fact neutron stars are on the verge of
being black holes, which by definition have an escape velocity at the event horizon equal
to the speed of light. If only you could cram a little more matter
into the neutron star, the escape velocity would increase and it would become a black
hole. Now in the case of normal matter, you can’t
just add mass to make a black hole because as you do so the radius of the object increases. That means the surface gets further away from
the center, which means you don’t get the full impact of that extra mass. But neutron stars are NOT made of normal matter. That ball of neutrons is a fundamentally quantum
mechanical object. One of its weird properties is that as you
add mass the size does not necessarily increase, and at the highest masses the size actually
gets smaller. If you want to know why, check out the episode
we did a while ago. So more mass in a neutron star means higher
surface gravity means higher escape velocity. For any given mass, there’s a certain size
that if you could crunch an object down below that size it would be a black hole. It’s like a phantom event horizon. In the case of the earth the phantom event
horizon is about a centimeter in diameter. In the case of a neutron star it’s several
kilometers. As you increase a neutron star’s mass, its
phantom event horizon grows while its actual surface shrinks. When they overlap you have a black hole. This basic picture is pretty well accepted,
but we still aren’t sure just how massive a neutron star can be before becoming a black
hole. It’s not because our theories are wrong
- it’s because the calculations required to understand the bizarre states of matter
in a neutron star are horrendous, and there’s still some stuff that we don’t know. That’s especially true towards the center
of the neutron star, where the neutrons themselves probably break down into different types of
quark matter. Up and down quarks that comprised the
neutrons may even transform into strange quarks - something we’ve talked about before. The details of the state of matter in the
neutron star determines how a neuron star’s size changes with mass - and that’s what
determines the maximum possible mass. Those models have predicted maximum masses
in the range 2 to 3 times the mass of the sun. Now, we can do better at making this theoretical
prediction if we can catch a glimpse of the innards of a neutron star. And we can - when they merge. In the 2017 neutron star merger we learned
a lot about the structure of these objects by the way they warped as they spiraled together,
and by the stuff they spewed out after colliding. We also see the results of these mergers in
gamma ray bursts - frequent flashes of energetic light from the distant universe. We’ve estimated a maximum neutron star mass
of between 2.2 to 2.4 solar masses. More direct measurements of neutron star masses
come from pulsars - cosmic lighthouses that result from a neutron star’s precessing
jets sweeping past the earth. Most pulsars are closer to the minimum neutron
star mass of around 1.4 solar masses. But, the most massive so far is around 2.1 Suns. All of these numbers are quite a bit lower
the 2.6 solar masses of this new guy. And that’s what makes it so cool. If it IS a neutron star then it puts us at
the theoretical limit, and can tell us a lot about the crazy states of matter inside. That’s if it’s a neutron star at all. So why can’t it just be another black hole? Well that would perhaps be even more surprising. So far we’ve never observed a black hole
with masses lower than around 5 times that of the Sun. We see those in X-ray binaries - when a black
hole is orbiting and cannibalizing another star. It may seem weird that there seems to be a
gap in masses between the biggest neuron stars and the smallest black holes, but actually
we very much expect this. New black holes are formed when the most massive
stars die and the core is too big to become a neutron star. But you don’t get this smooth transition
from neutron stars to black holes. Like I said earlier, a neutron star forms
when a star’s core collapses, but most of the material rebounds as a supernova explosion. But if that neutron star then becomes a black
hole, some of the infalling material just gets sucked into the black hole. That increases the black hole’s mass quite
a bit. Based on our calculations and simulations
of how stars die, that minimum black hole mass of 5 Suns seems about right. A black hole with 2.6 solar masses is difficult
to explain. If we figure out that this object CAN’T
be a neutron star then we’re going to have to rework our models of how stars die - or
find some other way to make extra-teensie black holes. But if it IS a neutron star then we’ve learned
a ton about the most extreme states of matter in the universe. This is just the beginning. With new gravitational wave events coming
every week or two, we’re sure to see more of these sorts of mergers. Each will be rich in information on the nature
of stars, and gravity, and strange quantum states of matter. Billion-year-old secrets carried to us on
ripples in spacetime As always guys, I want to give our deep thanks
to all of your support. Subscribing and tuning in each week goes a long way to keeping the show going. And special thanks to our Patreon contributors - we know you’d get this stuff for free anyway, so it's huge that you'd throw in a bucks each month. For those who don't know we have a very active discord channel available to all patreon tiers. And finally I want to give an extra extra special thanks to Ahmad Jodeh, who’s supporting us at the
big bang level. Ahmad, we got you a small gift to show our
appreciation. It’s a brand new 2.6 solar mass black hole. I know, it’s small as far as black holes
go - but we hope it adequately reflects our thanks. We’re shipping it to you from a little star
cluster in Sagittarius - made locally from humanely sourced o-giant star of course. It should arrive at your doorstep in the next few million
years, sadly destroying the solar system in the process. OK, In our last two episodes we talked about
how to dissolve a black hole event horizon by adding rotation or charge to the black
hole, and also about experiments at CERN to study the nature of antimatter. Let’s do comments for both. Regarding trying to disolve an event horizon
by throwoing more and more electric charge into it - Ultimantis points out that it would
be increasingly difficult to do so as the black hole gain charge. That’s right - and that means we shouldn’t
expect highly charaged balck holes in nature. However it never actually becomes impossible
to keep charging up a black hole if you can put enough energy behind your electron beam. So, you know, still worth trying. Fabrizio Cimo has a good one: What if I cross
the event horizon just for a plank lenght and the instant later the black hole shrinks
due to evaporation? Would I find myself outside again
and have seen the inside of the black hole? From your point of view - no, not really. Hawking evaporation is excrutiatingly slow
- much slower than your rapid plummet to doom from your pespective. For someone watching from far away, you appear
to freeze smeared out at the event horizon - and then yes, they would see your energy
exit again as the black hole shrank - but it would itslef be as hopelessly scrambled
hawking radiation. So, in a sense both happen, but you don’t
get to report what you saw to anyone. A couple of you asked why we think there had
to be an actual imbalance in the number of antimatter versus matter particles in the early universe. Couldn’t the two just have become separated
in the early universe and now occupy different parts of the universe? So, maybe then, there are antimatter galaxies
out there. Cool thought - but that doesn’t work because
when they are created, each particle - antiparticle pair is close together. Matter and antimatter would have been smoothly
mixed, and there’s no mechanism to separate them. Now there’s no way to directly test if distant
galaxies are actually antimatter galaxies - they should emit exactly the same light
as regular galaxies. But if there WAS an antimatter section of
the universe, you’d have a great wall thorugh the universe where hydrogen and anti-hydrogen
came into contact to explosive effect.
Any person who understand this stuff who would bother explaining the implications here? If neutron stars can get heavier than predicted, then what?
Neutron Star: dense
Black hole: denser
I wanna watch this just because of interstellar lol