so this is the moment you've all been
waiting for no not the end of the course
when we talk about black holes so
remember we said that for a neutron star
the star has to die with no more than
about a couple times the mass of the Sun
about two two and a half times the mass
of the Sun well what happens if it dies
with more than that a neutron star
doesn't collapse because the neutrons
are snobbish and like nope nope not
getting in my space now the neutrons
will get a lot closer to each other than
those electrons will because the
neutrons don't have any electric charge
but the neutrons you know that they have
a little bit of self respect and don't
want to get completely in each other's
face but if the dead star is more than
about two and a half times the mass of
the Sun there is no force in the
universe that can overcome gravity and
that will collapse literally down to a
point imagine something heavier than a
couple times the mass of the Sun
literally being crushed down to a point
that is a black hole another way to
think of this is in terms of general
relativity remember we had our warped
space-time remember we had a flat rubber
sheet and remember we said that if you
had for example the Sun it would warp
remember the elephant sitting in the
trampoline well what if we had something
with a stronger gravitational field it
would make even a longer divot deeper
hole deeper hole and the deeper this
gravitational hole the harder it's
going to be to get out you do not play
chicken with a black hole got it because
remember this is the curvature of
space-time once you go in you need to
expend energy to get out and at some
point you would need an infinite amount
of energy to get out you can't have an
infinite amount of energy you fall in
you keep falling until you get to the center
that's a black hole ok so a black hole is
imagine taking the warp of space-time and
sort of like pinching it and
pulling it down infinitely far you fall
in you can't get out again okay nothing
can get out of a black hole you would
have to travel faster than the speed of
light to escape a black hole because you
can't see it because light itself cannot
escape that's why it's called a black
hole okay now I can draw a diagram of a
black not like this I think if you can
imagine a sphere I'll draw it as a circle
this is the Thou shalt not cross here you
shall not pass to quote Gandalf because
if you do you can't get out again okay
again don't play chicken with a black
hole oh I can get away oh it's harder
but I can get away oh man it's really
hard to get away ahhhh okay there's a point
where you get too close to the black
hole you can't escape that is something
called the event horizon because we
cannot see events inside of there that
information cannot get out okay it's also
called the Schwarzschild radius after
the physicist who first came up with
this concept now this is not a circle in
space it's actually like a
three-dimensional spherical boundary you
can't like sneak in from the back okay
at the center that's where all the
matter got crushed all the matter
several times the mass of our Sun gets
crushed down to a mathematical point
that is called the singularity it's
called a singularity because not only
does the mathematics break down there
but our knowledge of physics breaks down
this is where Einstein's general theory
of relativity breaks down okay now what
happens when something falls into a
black hole I get asked this question by
children all the time and the answer is
don't try this at home boys and girls
all right let's imagine that for some
crazy reason you wanted to volunteer for
this one way suicide mission
and let's imagine that you feel that
well you know I'm just gonna like enjoy
my trip in I'm gonna jump in feet-first
because they tell you not to dive so if
we jump in feet-first maybe it won't
hurt as much no because your feet are
closer to the black hole they will
experience a stronger gravitational pull
they will experience a greater curvature
of space-time your head will not
encounter quite as much what that means
is that your feet will be pulled
stronger than your head let's think
about this for a moment remember the
medieval torture device called the rack
where we stretch people out that's what
happens to you what happens is that your
feet get pulled in faster than your head
if you fall in feet first you actually
get stretched out and ripped to shreds
the end you die screaming it's horrible
don't try this at home boys and girls
there is actually a name for this really
cool awesome horrible thing it's called
spaghettification yeah like spaghetti
like being stretched thin like a strand
of spaghetti really cool now you can't
really do this experiment because there
aren't any nearby black holes that you
can jump in but we can actually watch
material falling into a black hole and
see it being ripped to shreds see it
being spaghettified and in fact that's
how we actually find a black hole
because black holes don't emit any light
how are we going to find them
we're going to find them by their eating
habits remember when I had that white
dwarf which had the red giant near it that that
material was spiraling in well if you
had instead of a star orbiting a white
dwarf imagine you had a star orbiting a
black hole the material would also be
pulled in you'd also have one of those
accretion disks okay as the material
starts spiraling in towards the black
hole and remember I said the material
in the accretion disk gets very very hot
because of
the particles rubbing against each other
due to friction and it gets so hot that
it actually emits x-rays so in order to
find a black hole we need several things
to happen we need to have a binary star
system we have an object that we can see
orbiting around something that we can't
see we know that object is there because
stars don't like just walk around in
circles okay it's not like you're drunk
star go home it's not the way stars work
if a star is orbiting it's orbiting
something okay so we need to have a star
orbiting around something that we cannot
see we need to have the accretion disk
with the x-rays to help us find this
thing in the first place and then we
have to rule out all other possibilities
for this invisible object it has to be
something that's too heavy to be a white
dwarf or neutron star or anything else
it has to be more than about three times
the mass of the Sun and the great thing
about binary star systems is that
through Kepler's laws of motion we can
actually estimate the mass of this
invisible object and that is how we have
found many black holes throughout our
galaxy we also have ginormous humongous
like millions of times the mass of our
Sun supermassive black holes at the
Centers of galaxies that are eating
entire stars for breakfast lunch and
dinner
