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  • 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.

  • [MUSIC PLAYING]

Gravity seems very familiar.

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Why is gravity different?

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    joey joey posted on 2021/04/28
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