[MUSIC PLAYING] As if black holes and neutron
stars aren't weird enough, physicists have very good reason
to believe that there are even stranger stellar
remnants out there, stars made entirely of quarks. [MUSIC PLAYING] The mathematics
of modern physics that emerged through
the 20th century explained so much
about our universe. But the same mathematics
also hid some surprises. There are monsters lurking
in that math, predictions of phenomena so
extreme and bizarre that, as with any
monster, it was hard to know whether to
credit them with reality. The most wonderfully
monstrous of these are the remnant corpses
of the most massive stars, stellar zombies like neutron
stars and black holes. Einstein's general
theory of relativity tells us that the
core of a dead star must collapse under its
own incredible weight. What happens to the resulting
ultra-dense material depends on quantum theory. We've already talked about
how quantum processes save a neutron
star from collapse, but ultimately also
doom the most massive to collapse into a black hole. But just shy of that
final transition, and on the fringe of our
understanding of the quantum universe, a star may
become very strange indeed. Literally, I'm talking
about strange stars. Before we can understand
strange stars, we need to start with
a stellar remnant that we know for sure
exists, the neutron star. These are weird enough
all on their own. They are created in the final
collapse of a very massive stellar core after it has
exhausted all possible fusion fuel supplies. In that collapse, most of
the electrons and protons are crunched together
to form neutrons. At a radius of
around 10 kilometers, the collapsing core
is suddenly halted when those neutrons hit an
absolute limit of density, which I'll come back to. The rest of the in-falling star
collides with the new neutron star and ricochets outwards
in the most powerful explosion in the universe, a supernova. The remaining neutron
star is millions of Kelvin in temperature,
and may be spinning thousands of times per second. Its immense magnetic field
drives jets of material at extreme speeds. These jets may sweep
across the Earth due to the spinning-top-like
procession of the neutron star. We see these as pulsars. Our understanding
of neutron stars seems to fit the
behavior of pulsars very well, at least
for most of them. But for some, we see hints of
weird things happening deep beneath the star's surface,
which we'll get to. For ordinary neutron
stars, that surface is a thin crust of iron,
which quickly gives way to a fluid of almost pure
neutrons, neutronium, the densest known
substance in the universe. A cubic centimeter
weighs a billion tons. Like I said, this is the
absolute limit of density. Well, nearly, as we'll see soon. Neutronium is
degenerate matter, and I don't mean that in the same
way that your parents probably used the word. Degenerate matter
is so compressed that particles can't
get any closer together without occupying the
same quantum states. The Pauli exclusion
principle states that this is forbidden
for fermions, the family of particles
that neutrons belong to. We don't know nearly
as much as we'd like about the nature of neutronium. We can't make this
stuff in labs. And we certainly can't
test what happens to it when subjected to the insane
pressures at a neutron star's core. In those conditions,
individual neutrons are packed so tight that
they begin to overlap. This may cause neutrons to
dissolve into their component quarks. This so-called quark matter is
its very own type of bizarre. We think that a type
of gas-like quark matter, a so-called
quark-gluon plasma, filled the entire universe until
around a millionth of a second after the Big Bang,
the Quark Epoch. And we have good
reason to think that, because we can actually make
this stuff in our largest particle accelerators. Minuscule flecks of
quark-gluon plasma exist for tiny
fractions of a second after very high-speed
particle collisions. We can study its nature
based on the particles that decay from it. However, the quark matter
in a neutron star is forged by insane pressures, not by the
greater-than-a-trillion-Kelvin temperatures of the Quark Epoch
or the Large Hadron Collider. In that state, it forms
a superfluid rather than a plasma, a superfluid even
denser than neutronium. We sometimes call a neutron
star with such a quark matter core a quark star. Now, neutrons are comprised
of one up and two down quarks. Quark matter made
of these quark types would need to be confined
by incredible pressures to maintain stability
outside the atomic nucleus. So that probably rules
out having an entire star made of this stuff, unless the
quark matter is also strange. It may be that when
neutrons disintegrate under high pressure,
half of the down quarks are converted to strange quarks. The result is strange matter. It's a special type
of quark matter. It has three quark
types instead of two, and that means more particles
can occupy the lowest quantum energy states. It's as though the quarks trick
their way around the Pauli exclusion principle
by having some of them put on silly disguises. This lower energy state
means that strange matter may be the most stable form
of matter in the universe, more stable even
than iron, which is the most stable atomic nucleus. A star made entirely
of this stuff should be completely stable. And if they exist, they
could exist forever. We call these strange stars. Not content even with this
level of weirdness, physicists have proposed even more mad
ideas for neutron star cores. If the density is high enough,
the conditions at the core may be so extreme that
they resemble the time even before the Quack Epoch. At less than a billionth of
a second after the Big Bang, the fundamental forces of nature
were not as we know them today. The electromagnetic force
and the weak nuclear force were unified as the
electroweak force. It could be that neutron
stars have an electroweak core, an apple-sized ball with
the mass of two Earths in which quarks themselves burn. The outflowing energy is
almost all in neutrinos. And those may provide
the final pressure that halts the collapse of
some stars into a black hole, at least for another
million years or so. OK. This is fun stuff. And it keeps theoretical
physicists off the streets. But can we test any
of this madness? Well, yes. Have we? Maybe. Take the case of 3C58. In the year 1181, Chinese
and Japanese astronomers recorded a new star in the
constellation of Cassiopeia. It faded over six months. But nearly 1,000 years later,
after some small technological advancements, we pointed
our radio telescopes and then the orbiting Chandra
X-ray Observatory to that spot and found a young pulsar,
a rapidly rotating neutron star 10,000 light years distant. Now, this wasn't so unexpected. After all, the pulsar
in the Crab Nebula was also observed as a supernova
by Chinese astronomers in 1054, except something was
weird in the case of 3C58. The x-ray data revealed a
surface temperature of a more million Kelvin, much
cooler than expected for a neutron star of its age. A possible explanation is
that a quark matter core formed at the heart
of this neutron star and is slowly transforming
into strange matter. As down quarks flip into the
more massive strange quarks, they have to absorb
energy from somewhere to provide for that extra mass. That energy would be the heat
energy of the neutron star. There are other
candidates that could be quark and/or strange stars. Some appear a little bit
too small for their mass, suggesting quark
matter densities. Then there are supernovae
that appear way too bright and last too long. And it's been
hypothesized that these may be due to a second explosion
as the neutron star collapses further into a quark star. Even the famous
supernova that exploded in the Large Magellanic
Cloud in 1987 has been hypothesized to have
left behind a quark star. The dying star shouldn't
have been massive enough to leave a black hole,
yet astronomers still haven't found the
expected neutron star at the location
of the supernova. Nothing is confirmed yet, but
there are tantalizing hints that these exotic stars,
these monsters in the math, may be very real. Who knows what other
strange denizens lurk in yet to be discovered
laws of physics? And who knows which
are actually out there, waiting to be discovered in
the expanse of spacetime? Hey, guys. Two exciting announcements. Number one, we thought
about it long and hard and we decided that we want to
keep making Space Time forever and ever and ever and ever,
and to keep making it better along the way. That leads to number two. We could really use your
help with number one. And now there's a way. We're finally on Patreon. If you care to, head over
there and maybe throw a buck or a few our way each month. It'll really help us keep
pushing Space Time forward. Also, there are some
pretty sweet rewards. But mostly, thanks for watching. That's incredible
support all by itself. So last week we talked
about a new study that suggested that
dark energy may not be all that we thought it was. Turns out dark energy
probably is still real. But it did provoke some
really interesting discussions in the comment section. Now, pixel girl asks whether
in a curved 3D space, shouldn't the triangles appear flat to us? OK. So this is a great
opportunity for me to correct some misconceptions
that other people had. And I'll get back
to this question. So when I say that the
universe is flat, of course I don't mean that it's
flat like a pancake. It's definitely spatially
three-dimensional. A flat 3D space
means that the rules of geometry in that
space work just like on an actually flat 2D surface. I referenced a
previous video when I made that statement in last
week's dark energy episode. So if I say something
that sounds dumb, I encourage you to take a
second to make sure you at least glance at the resources I
provide to justify or explain that statement. If I still sound
dumb, by all means hit the comments in
all caps, because I'm sure I say a good amount
of genuinely dumb stuff. Anyway, back to you, pixel girl. So a giant triangle in
geometrically curved space will definitely have weird
angles, less than 180 degrees for a hyperbolic
geometry, more than 180 for a spherical geometry. But you are correct in thinking
that besides the angles, the triangles will
actually look flat. Your comparison was
for people living in a 2D flatland universe
that has the geometry of the surface of a 3D sphere. A flatlander analyzing a
triangle in that universe would measure its angles to
be greater than 180 degrees, but would also not
see the triangle curve over in the sense that
there would be no horizon. That's because light
would also have to follow the curve of that space. Same with triangles in
a curved 3D universe. We wouldn't see the
triangle arc over. That arc occurs in an
imaginary fourth dimension that is analogous to the radial
dimension of the 3D sphere. But it isn't actually
part of this universe. A lot of you asked for a video
on De Brogile-Bohm pilot wave theory. OK, but only because
you asked nicely. Now, a lot of you,
and I mean a lot, also want me to talk about Erik
Verlinde's entropic gravity, and the possibility that it
might explain both dark matter and dark energy. To do that, I'm going to have
to go through the black hole information paradox, Hawking
radiation, some string theory, the holographic
principle, other stuff. Fine. But don't say you
didn't ask for it. Pipe2DevNull suggests that the
finger slit light diffraction test can be used as a secret
salute when the science deniers finally take over. Stay strong, comrades. [MUSIC PLAYING]
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