After all, it's what makes stuff fall,

it's what keeps the planets in their orbits,

but thinking about the fundamental physics of gravity

has led scientists to question the very existence of space

and time.

This is because gravity is very different

from the other forces.

So why is gravity so unique?

I'm Jared Kaplan and this is "Why is Gravity Different?"

You may know that there are four fundamental forces -

electromagnetism, the weak force, the strong force,

and also, of course, gravity.

The electromagnetic force is responsible

for the interactions of charged particles,

for magnetism, and holding electrons in their atoms.

The weak and strong nuclear forces

are responsible for subatomic processes,

and for holding protons and neutrons together.

To understand these three fundamental forces,

we take a reductionist approach.

We take matter apart into its most fundamental constituents

and then watch those constituents interact.

The enormous particle accelerators

that physicists use are really like giant microscopes.

So why do we build big particle accelerators

that use high energies?

It's because of Heisenberg's uncertainty principle.

To see small stuff we need big energies.

It might seem like gravity is very

similar to the other forces--

after all the orbits of the planets around the sun

were an inspiration for the idea that

electrons orbit atomic nuclei--

but in fact gravity is very different

from the other forces.

Gravity gets stronger and stronger and stronger

as you increase the energy of your accelerator,

so that if you were to try to probe gravity

at a fundamental level, all you'd do is make a black hole.

And that black hole would destroy your microscope.

So we have to study gravity a bit differently.

Our best theory of gravity is Einstein's theory

of general relativity.

Which famously says that the gravitational force

is due to the curvature of spacetime.

Fortunately, the very existence of black holes

points to a new way of thinking about quantum gravity.

We had the first hint of a major revolution when in 1972, Jacob

Bekenstein argued that the total information inside a black hole

is actually proportional to the surface area of the black hole

and not to its volume.

But why is this such a surprising and revolutionary

insight?

So if information is stored on areas rather than in volumes,

perhaps the laws of physics should be formulated

in fewer spacetime dimensions.

Why would we think something so surprising and crazy?

Well, let's take a step back and think about information.

Fundamentally, information is a description of the state

or the configuration of the universe.

On a practical level, we can think of it

in terms of a hard drive.

Hard drives store information with tiny little magnets,

and the more magnets you have, the more information you have.

But that means that information should

scale with the volume of our hard drive.

But What happens if our hard drive falls into a black hole?

The information on the hard drive won't be lost,

instead it will be encoded in the state of the black hole.

Yet, if the total amount of information

a black hole can hold is proportional to its surface

area, it means that, actually, that intuitive volume scaling

law of our hard drive was wrong.

This says that volume, which is essentially space itself,

isn't fundamental.

In other words, the fundamental theory of gravity

should have fewer dimensions.

The area law challenges many very basic principles.

It calls into question whether stuff can interact

with nearby stuff via forces.

This leads to a new way of thinking about physics, where

the Universe is a hologram.

Just as a hologram provides a three-dimensional image

from a two-dimensional plate, the fundamental description

of our Universe could be lower dimensional,

while we experience an illusory three-dimensional world.

Applying this idea to black holes,

we can look at the information in the lower dimensional

theory of physics to unpack the information in the black hole.

So because black holes would break our microscope,

we can't study quantum gravity in the same way

that we studied the other fundamental forces.

But fortunately black holes have provided us

with a way to think about gravity on a quantum level.

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