Hi. Welcome back to
the Cosmic Classroom.
We'll now see what
it would be like to
visit a Black Hole.
So strange things
happen as you get
close to a Black Hole.
Lets see what, what happens.
First of all let me
ask you question.
Would you rather visit a
Low-Mass Black Hole or
a High-Mass Black Hole.
You know you would die if
you go up too close, right?
So you think you would
survive a few more
seconds, visiting a
High-Mass Black Hole or
Low-Mass Black Hole.
Well let's see.
Let's imagine two Black
Holes,one that is, has
the mass of the Sun and
the other that has three
times the mass of the Sun.
Well, a Black Hole with
the mass of the Sun will
have a radius of 3 km.
A Schwarzschild Radius,
which is the event horizon.
And the three solar mass
Black Hole would have
a radius of 90 km.
And now lets
imagine a penguin.
And the penguin is deciding
which one to visit.
His first idea is, well I
don't really want to die
so soon so I'm going
to visit the low-mass one.
It's probably safer.
So there he goes he is
visiting the low-mass one.
And as he goes he
gets stretched and he
is not very happy there.
Right?
He is completely stretched,
he is thin and tall.
He is feeling more,
greater pull in
his feet than in his head.
That's why it's being stretched.
This is the Tidal Effect and if
you're not sure about Tidal Effects
go look back at when I talked
about tides because I referred
about Black Holes too in tides.
So the penguin, the poor
penguin gets stretched.
It's, this is called
spaghettification also.
So then the penguin
decides to try again,as
if he could, lets just
imagine for a minute
he could try again.
Right?
He couldn't, but lets see.
So the two penguins
the, the two experiences
are as follows, the penguin
went into, gets close to the -----
event horizon of the three
solar of mass sun, the two
solar mass Black Hole is
less stretched, its suffers
less of a Tidal Effect than,
than it was,than when it
was at the event horizon
of the one solar mass one.
Notice that the feet, in this
case, are been less attracted
than they were before they
half, the head is also being
less attractive than it was
before, and if you don't
believe me just remember
that the force of gravity
depends on the mass and
it's inversely proportional
to the radius square.
So the radius is more
important than the mass.
Ok?
Because it's farther from the
the sun, farther from the Black
Hole that the,the amount,the
pull of gravity also decreases.
But what's really important
here is not the, the forces
themselves but the difference
in forces, because that's what's
stretching that penguin.
If the force in the head
and the force were in the
feet were the same, it
would just be falling fast
and it wouldn't be stretched.
But it's the fact that they
are different in the head
and in the feet that coises,
causes the Tidal Effect.
So, if you're still wondering
better to visit a high-mass Black
Hole than a low-mass one.
But there's more missing
from this picture.
Alright?
Now I added another
penguin here this is
the observer penguin.
It came in a spaceship with
the other penguin,but is smart
enough to know to stay away
from the Black Hole and
observes while the fool one --------
goes into the Black Hole.
So we're going to analyze
this picture here and try to
see a more complete picture,
but before I do that, I need
to explain some things that
happen near a massive
object, near a Black Hole.
One of the things that
happen is that when light
is leaving a strong gravitational
field, it gets red shifted.
So imagine that the penguin
was holding a flashlight and
pointing that flashlight to its
friend sitting in the spacecraft.
The light, as it try,as it
tries to leave the Black
Hole you can think about
it as it gets tired, it loses
energy,it becomes red shifted.
So as it's trying to get
away from this object
that's attracting it it gets
redder and redder and redder.
So if the flashlight is for
example, emitting visible
light at 100 km from
the Black Hole it will be
emitting infrared at 10,000
that light will be radio,it
would be red, red shifted.
If you know, if the flashlight
is emitting in the x-ray
then maybe the penguin's
sitting far away will see it
in the visible, but the fact is that
the light gets red shifted.
Another way of seeing that
red shift is by looking at
the space time diagram again
so here we have the Black
Hole this is similar to the
space time diagram that
I showed for the sun before.
And the, and the light, kinda
the light is trying to climb
out of his hole, gets tired,
loses energy, gets red
shifted, this is called the
Gravitational Red Shift.
A consequence of the
Gravitational Red Shift is
that time, there's time dilation
and the space contracts.
Now let's see if you can follow that.
The penguin is near a massive
star and sends a signal,
light of a certain frequency.
And the penguin decides to
that one second is the
time between two, the
two quests here, it's the
wavelength of the, the
wavelength of, of light,
and it decides that this
is defined as one second.
Now the penguin sitting far
away from the Black Hole
will receive this light sometime
later and the light would've
been stretched, it's red shifted.
So,the penguin that's sitting
here counts one second in
its clock, but a full wavelength
has not passed yet.
So the penguin here would
have to count a bigger
amount of time to, to see
the same, the same thing
that was referred to as
one second by the penguin
near the massive star.
So a consequence of
Gravitational Red Shift
is that the time as seen
by the observer far away
is dilated, it looks like
this penguin here is
taking forever to send
the two peaks of light.
Ok?
Takes more than a second.
So if they had agreed
that the penguin would
arrive there and send
light ever, a peak of
light every one second
this guy here is thinking
that,the guy falling
is thinking he is not
really his job he's lazy,
he's just standing it once
every two seconds
Ok?
So it'll see the guy as slow.
So, the consequence is that
the one that's in falling the
amount of time that passes
is smaller than the amount
of time that passes for someone
that's observe, that's observant.
So let's see what really
happens when, let's
complete the picture.
What happens is that
first of all the penguin
that's falling it gets
redder because of the
Gravitational Red Shift.
Right?
So the penguin would be
observed this is how this
one here will see it.
The penguin that's staying
still will see it as one
getting redder and redder
and redder because
it will be red shifted.
And also it will notice
that this penguin is
taking forever to do
anything is really slow,
is lazy, it's taking forever
to fall into the Black
Hole, and it's taking
forever to send any
signal to, to him waiting
in the spacecraft.
So time will be passing differently.
This penguin here will
continue to be young
while this one is aging and
waiting for light to arrive.
So what the odd, the old
penguin that stays has to
wait forever to get any signal
from the young penguin.
And it never sees the penguin
really fall into the Black Hole.
Ok?
So very funny things
happen when you get
too close to a Black Hole.
And if you haven't figure
that out yet, you can't
really visit the Black Hole,
not this close, you're much
more likely to move into it.
And for the penguin that's
falling in it everything
happens fast, he doesn't
notice anything, it'll take
a fraction of a second.
But for the penguin that's
observing he never sees
his friend really falling
into the Black Hole.
It just becomes redder and
redder and redder and it
freezes in space, like this.
And that's what I wanted to
tell you about Black Holes.
I hope it helped.
See you next time.
