All language subtitles for General Relativity Explained simply & visually

When Albert Einstein first published the Special
Theory of relativity in 1905, he was either

vehemently ridiculed or ignored.

People thought it was just too weird and radical
to be real. This guy is not even a working

scientist, he’s just a patent clerk, some
said. How dare he challenge the greatest scientist

that ever lived – Isaac Newton, whose theories
have been proven to be correct for hundreds

of years.

Some politicians even insulted his religious
heritage and called it "Jewish science" – a

way to subvert traditional culture and thinking.

How did Einstein feel about this? Well, he
wasn’t satisfied with his theory either.

He was unhappy because the theory only applied
to observers moving in a straight line at

a constant speed. The theory did not apply
if Gravity was present or if the observer

was accelerating.

Einstein was known, however, to have a very vivid imagination. And one day, as legend has it,

while observing a window washer on a ladder
near his patent office, he had one of his

famous thought experiments that would go on
to change the course of scientific history.

He imagined what would happen if the worker
were to fall. But he didn’t think of it

the way you and I would think of it. What
was his thought experiment? And how did it

lead to perhaps the greatest single scientific theory of the past 100 years…that’s coming

up right now!

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While watching the window washer on a ladder,
Einstein thought about what would happen if

the window washer fell. For most people, imagining
this would just conjure up disturbing images

of the poor guy landing on the ground below,
and the story would not have a happy ending.

Einstein thought about it differently. He
put himself in the window washer’s perspective,

and imagined not what would happen when he met the ground, but what he would experience as

he was falling.

What he realized was that if he was falling,
gravity would be the only force acting on

him. He would be accelerating towards the
ground, but since the ground would not be

pushing up on his body, he would feel no weight.
With no wind resistance, he would be in free

fall. And this would be no different than
being weightless in space.

In a way, gravity and acceleration were different
ways to describe the same thing. This is where

Einstein had a huge insight. The way to connect
gravity with the theory of relativity was through

the idea of acceleration, since the two are equivalent.

Einstein imagined being in a room with no
windows. And if the room had a bathroom scale

handy, what would happen if you stepped on
the scale. Well if anywhere stationary on

earth, you would weigh 80 Kgs, or whatever
your weight is.

Now he imagined being in the same room in
space. Now, what if the room was on a space

on a space ship that accelerating in an upward
direction at 9.8 meters/second/second, which

happens to be exactly the same as gravitational
acceleration on earth. What would happen

if he stepped on the scale then?

Well, the scale would read 80 kgs, just like
it did on earth. The acceleration on a spaceship

would appear to him, inside the room as being
indistinguishable from being stationary on

earth. If he didn’t know he was on a space
ship, he could just as well presume he was

on earth.

There would be no way to tell the difference.
Or would there?

Einstein thought about this, and asked himself
if there was a way to tell the difference.

He imagined what would happen if he took a
flashlight or a laser beam and pointed it

from one side of the room to the other, as
the space ship was accelerating upwards.

If he had a sensitive measuring device, he
could measure the height of the light on the

other side of the room. He realized that the
height he would find on the other side would

be slight lower than the source of the light.

Why? Because the floor of the room would be
rushing upwards at ever faster speeds, as the

light was propagating across the room. since
the room was accelerating upwards at 9.8 meters/second/second.

The light beam would appear to curve downward.

However, If you were on earth, and you measured
the two heights, you may think that there

should be no difference. That light should
go straight to the other side of the room.

Although this appears to be common sense.

Einstein thought it can’t be because it
violate the principle of equivalence. Acceleration

of the room on a space should be no different
than the room under the influence of gravity

on earth.

He realized that this meant light must bend
in the presence of a gravitational field.

But how could this be, because light always
takes the shortest path between two points?

It should be going straight.

Then he realized, wait a minute, maybe the
light IS taking the shortest path between

two points. Maybe the shortest path is not
a straight line.

He imagined the curved surface of the earth. The shortest path between any two distant points on earth,

if you're restricted to the surface of the earth, is never a straight line, because you have to traverse

the curvature of earth. So the shortest path
is always curved.

So maybe gravity somehow causes a curvature
of space itself. He hypothesized that in space,

perhaps a straight line is NOT the shortest
path between two points, and that perhaps

in the presence of mass and energy, space
somehow becomes curved, so that the shortest

path that light can take is a curved path.

This was the key insight that Einstein had
about gravity.

But in order to express this mathematically,
it required very complicated mathematics that

even a genius like Einstein could not easily
figure out.

He contacted an old buddy of his from college days, mathematician Marcel Grossman.

Grossman had just finished his PhD dissertation on the topic of, wouldn’t you know it, the geometry

of curved spaces, called Reimannian Geometry.
With his help, Einstein figured out the mathematics

of curved space time. And this curved geometry
is really the basis of General relativity.

Now, you have to realize how different this was than the status quo of the time which was

Newtonian space and time, which presumed that
time was fixed, space was fixed, and gravity

was a mysterious force that could act at a
distance from one massive object to another

without touching it. In this model, Gravity
did not affect the underlying space and time,

but acted within it.

Einstein’s theory was now that gravity was
not a force between massive objects, but something

that emerges from the interaction of space
and massive objects. John Wheeler would later

summarize this theory in 12 short words: “Space-time tells matter how to move; matter tells space-time

how to curve.” That’s it. That’s General
relativity in a nutshell.

And orbits of planets could now be explained
not by some mysterious force that acts at

a distance, but rather an interaction that
takes place locally with mass or energy, and

the space around it. And this can be visually
represented by the kind of graphic you see

here to show how massive objects like planets
form orbits around other massive objects.

It’s important to note that the trampoline
analogy you normally see on TV shows and youtube

videos like this is a 2D plane used for visualization
purposes only. The interaction occurs obviously

in three dimension, not just two. It looks
more like this graphic. This is much more difficult

to visualize and animate, so it is typically
not used. But it is more accurate.

However, in order for this theory to really
be taken seriously, it had to make a prediction

that could be tested. And that prediction
could not be explained in any other way.

This test came in the form of Mercury. Mercury’s
orbit had been a mystery for decades because

it was unusual. All planets orbited the sun
in an ellipse. The planet closest to the sun,

Mercury, also orbited in an ellipse. But it did something weird. It had something called

a precession.

What this means is that its elipse never closes.
The point of the orbit that was farthest from

the sun advances a little bit every time Mercury goes around the Sun. It’s as if the ellipse

itself is orbiting the sun.

No one could ever figure this out using Newton’s equations.

When Einstein applied his new curved space theory to this orbit, the new theory predicted

exactly the precession that Mercury actually
has. Finally, a theory perfectly matched the

observation which had been a mystery for decades.

You can only imagine how Einstein felt when
he figured this out. There was a time when

Einstein was the only person in the world
who realized that the universe actually works

this way.

But many skeptics still remained. Many scientists
still had doubts about Einstein’s theory.

But the most fool-proof confirmation of his
theory came 4 years after he published it.

That’s when a team led English Astronomer,
Arthur Eddington. in 1919, photographed stars

near the sun during a total solar eclipse.

If Einstein was right, then the position of
the stars near the sun would appear different

than predicted location based on where they
should be as seen at night. This would happen

because as light passed near the sun, it should
be bent by the curvature of space due to

gravity. And that’s exactly what he found,
confirming that the theory was correct. This

is the moment Einstein became a celebrity.

You might ask, ok I get it. I get space curvature.
But it’s called space-time. How does time

enter into the picture? Why is this not just
a distortion of space but also of time?

This is where Einstein’s first theory, special
relativity comes in. The essential presumption

in special relativity is that light always
moves at the same speed regardless of perspective

or reference frame. This means that light
will have the same speed in an accelerating

reference frame as it will in a resting reference
frame.

If this is the case then it means that the
speed of light in the presence of gravity

will be the same as its speed in empty space.

Speed equals distance over time. S = D/T

But since the distance traveled by the beam
of light in a gravitational field is longer

due the curving of space, in order for the
speed of light to remain constant, time itself

must pass slower in the gravitational field
relative to time in empty space.

In other words, time increases proportionately
with the curvature of space near a gravitational

field, compared to empty space, to keep the
speed of light constant in both reference

frames. This is why time is considered distorted
by gravity along with space. It is really

just part of the same fabric called space-time.
This has some massive implications. It implies

that the observer experiencing no gravity at all, sees the clock in a gravitational field running

slower. This means that the clocks on earth
run slightly slower than clocks on the international

space station. This effect has been confirmed
by many experiments, and is taken into account

in order keep the clocks of GPS satellites
in sync with the clocks on earth. Otherwise

your GPS apps like google maps would give
you inaccurate locations.

You should know that although General relativity
is an astounding achievement by one the greatest

scientists of all time, it does not answer
everything. Questions remain. Although it

tells us how gravity works, it doesn’t
tell us what exactly it is. Why do massive

objects distort space time? What is the underlying
connection between mass and space-time?

It also predicts regions of space where space
time can get so distorted that nothing escapes

including light. This is called a black hole.
But it shows that within these black holes

lies something that seems impossible, and
that is a mass concentrated to an infinitely

small point with infinite density. This is
called the singularity and is theorized to

exist within the black hole. General relativity
fails to work at this singularity.

But infinities like this in science usually
indicate some sort of incompleteness of theories rather

than things that actually exist.

To figure this out what happens at these really
small scales, we need our old friend Quantum

mechanics. But alas, the equations of quantum
mechanics make no sense in terms of singularities,

or in terms of general relativity.

So for now the two theories remain incompatible. If we can bring these two theories together, and

truly understand how gravity behaves at the tiniest scales, we may answer the question of what

gravity actually is. We will need a new theory
to figure this one out – and that theory

is called quantum gravity.

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