Now that we have a good grasp on what we

would see in the environment around a black hole,

we should now ask what an observer would feel in the environment around the black hole.

Normally when we think of a spacecraft in orbit around Earth,

we imagine an astronaut experiencing the sensation of weightlessness.

With gravity only serving to keep them from being flung out into deep space.

But there's another effect that becomes significant around black holes that will be

noticeable to an astronaut nearby, tidal forces.

Tidal forces are named after the tides here on Earth which we

experience as the rise and fall of sea levels that occurs periodically.

Humans have speculated about the cause of tides for millennia

and today we know that they're caused by a combination

of the gravitational forces of the moon and the

Sun gently pulling on the water in the ocean or

rather tidal forces act on everything on earth but

it's really only the oceans that we notice.

For a simple spherical object,

gravity acts to pull the objects towards the center of mass.

However, when a second gravitational body is introduced,

the forces are now the sum of the gravitational forces due to both of the bodies.

This presents an interesting dilemma.

Since the gravity from the second body changes strength with distance and thus

the tidal forces will have a different value and direction over the surface of the first.

These tiny differences in gravitational forces

are all that are needed for an object to experience tidal forces.

On Earth, we observe this tiny change in

force as a major change in the height of the seawater.

In some cases, like the Bay of Fundy in Canada,

the sea level can vary by as much as 16.3 metres,

tall enough to swamp an entire five story building.

If small forces like these can create big changes here on Earth,

what do you imagine the tidal forces near a black hole might be like.

In our daily lives,

we experience one Earth gravity worth of acceleration.

It's the force that keeps us stuck to the ground.

However, there's a very slight difference in the forces that pull on

our feet compared to the forces that pull on

our head unless of course you're laying perfectly level.

Let's calculate the difference in acceleration by rearranging

a version of Newton's formula for universal gravitation.

Here acceleration A is equal to two times G times M times H divided by R cubed.

Here A is going to be the difference between the acceleration of

two points separated by a height H above a body of mass M and

radius R. Let's see what the differences for

a person on the surface of the Earth by inputting Earth's mass

5.97 times 10 to the 24 kilograms and radius 6.3 times 10 to the six meters.

For someone my height about 1.8 meters,

they experience a difference in acceleration of a miniscule

5.5 times 10 to the minus six meters per second squared.

Compared to one Earth gravity,

that's less than two parts per million.

Definitely not something that we can sense.

But let's do the same thing again,

this time taking the mass and radius of the nearby black hole Cygnus X-1.

It has a mass of approximately 15 solar masses,

or three times 10 to the 31 kilograms.

For simplicity, let's just say it's

a Schwarzschild black hole with a radius of 44 kilometers.

Plugging in these numbers,

give us a difference in acceleration between my head and my feet

of 8,000,000 times the force of gravity.

Of course, we wouldn't survive such incredible differences and

forces and scientists have named this effect Spaghettification,

which isn't so much delicious as it is horrifying.

Essentially, as you approach a stellar mass black hole,

you'll eventually be pulled into a thin strand that once called itself a human.

Not a pretty way to go but that was for a small stellar mass black hole.

Let's see how would it affect someone around a super-massive black hole.

Sagittarius A star is the name of the black hole at the center of our Milky Way galaxy.

Weighing in at 4,000,000 solar masses Sagittarius A has

a corresponding Schwarzschild radius of 12,000,000

kilometers which is more than eight times the diameter of our own Sun.

Putting these values into our equation,

yields a difference in acceleration of a measly one 10,000s the gravity on earth.

So, what's going on here?