Gravity seems very familiar.
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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