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  • It's a little known secret

  • But for more than half a century

  • A dark cloud has been looming over modern science.

  • Here is the problem

  • Our understanding of the universe

  • Is based on two separate theories.

  • One is Einstein's general theory of relativity

  • That's a way of understanding The biggest things in the universe,

  • Things like stars and galaxies.

  • But the littlest things in the universe,

  • Atoms and subatomic particles,

  • Play by an entirely different set of rules called,

  • "quantum mechanics."

  • These two sets of rules

  • Are each incredibly accurate in their own domain

  • But whenever we try to combine them,

  • To solve some of the deepest mysteries in the universe,

  • Disaster strikes.

  • Take the beginning of the universe, the "big bang."

  • At that instant a tiny nugget erupted violently.

  • Over the next 14 billion years the universe expanded And cooled

  • into the stars, Galaxies and planets we see today.

  • But if we run the cosmic film in reverse,

  • Everything that's now rushing apart comes back together

  • So the universe gets smaller, hotter and denser

  • as we head back to the beginning of time.

  • As we reach the big bang,

  • When the universe was both enormously heavy And incredibly tiny

  • our projector jams.

  • Our two laws of physics, when combined, break down.

  • But what if we could unite quantum mechanics

  • And general relativity

  • And see the cosmic film in its entirety?

  • A new set of ideas called "string theory"

  • May be able to do that.

  • And if it's right,

  • It would be one of the biggest blockbusters In the history of science.

  • Someday, string theory may be able to explain all of nature

  • From the tiniest bits of matter

  • To the farthest reaches of the cosmos,

  • Using just one ingredient, tiny vibrating strands of energy

  • called strings.

  • But why do we have to rewrite the laws of physics to accomplish this?

  • Why does it matter if the two laws That we have are incompatible?

  • Well, you can think of it like this.

  • Imagine you lived in a city ruled not

  • By one set of traffic laws,

  • But by two separate sets of laws that conflicted with each other.

  • As you can see it would be pretty confusing.

  • To understand this place, You'd need to find a way

  • To put those two conflicting sets of laws together

  • Into one all-encompassing set that makes sense.

  • We work on the assumption that there is a theory out there,

  • And it's our job,

  • If we're sufficiently smart and sufficiently industrious,

  • To figure out what it is.

  • We don't have a guarantee

  • It isn't written in the stars that we're going to succeed

  • But in the end

  • We hope we will have a single theory

  • That governs everything.

  • But before we can find that theory,

  • We need to take a fantastic journey

  • To see why the two sets of laws We have

  • conflict with each other.

  • And the first stop on this strange trip

  • Is the realm of very large objects.

  • To describe the universe on large scales

  • We use one set of lawsEinstein's general theory of relativity,

  • And that's a theory of how gravity works.

  • General relativity pictures space as sort of like a trampoline

  • A smooth fabric

  • that heavy objects like stars and planets can warp and stretch.

  • Now, according to the theory,

  • These warps and curves create what we feel as gravity.

  • That isThe gravitational pull that keeps the earth In orbit around the sun

  • Is really nothing more than our planet Following the curves and contours

  • that the sun creates in the spatial fabric.

  • But the smooth, Gently curving image of space

  • predicted By the laws of general relativity

  • is not the whole story.

  • To understand the universe on extremely small scales,

  • We have to use our other set of laws, quantum mechanics.

  • And as we'll see,

  • Quantum mechanics paints a picture of space

  • So drastically different from general relativity

  • That you'd think they were describing

  • two completely separate universes.

  • To see the conflict between general relativity

  • And quantum mechanics

  • We need to shrink way, way, way down in size.

  • And as we leave the world of large objects behind

  • And approach the microscopic realm,

  • The familiar picture of space In which everything behaves predictably

  • Begins to be replaced by a world

  • With a structure that is far less certain.

  • And if we keep shrinking,

  • Getting billions and billion of times

  • Smaller than even the tiniest bits of matter

  • Atoms and the tiny particles inside of them

  • The laws of the very small, quantum mechanics,

  • Say that the fabric of space becomes bumpy and chaotic.

  • Eventually we reach a world

  • So turbulent that it defies common sense.

  • Down here,

  • Space and time are so twisted and distorted

  • That the conventional ideas of left and right, up and down,

  • Even before and after, break down.

  • There's no way to tell for certain that I'm here,

  • Or here or both places at once.

  • Or maybe I arrived here before I arrived here.

  • In the quantum world you just can't pin everything down.

  • It's an inherently wild and frenetic place.

  • The laws in the quantum world

  • Are very different from the laws that we are used to.

  • And is that surprising?

  • Why should the world of the very small, at an atomic level,

  • Why should that world obey the same kind of rules and laws

  • that we are used to in our world, With apples and oranges

  • And walking around on the street?

  • Why would that world behave the same way?

  • The fluctuating jittery picture of space and time predicted

  • By quantum mechanics

  • is in direct conflict With the smooth, orderly,

  • Geometric model of space and time

  • described by general relativity.

  • But we think that everything,

  • From the frantic dance of subatomic particles

  • To the majestic swirl of galaxies,

  • Should be explained by just one grand physical principle,

  • One master equation.

  • If we can find that equation,

  • How the universe really works at every time and place

  • Will at last be revealed.

  • You see,

  • What we need is a theory

  • That can cope with the very tiny and the very massive,

  • One that embraces both quantum mechanics and general relativity,

  • And never breaks down, ever.

  • For physicists,

  • Finding a theory that unites general relativity

  • And quantum mechanics is the holy grail,

  • Because that framework would give us a single mathematical theory

  • That describes all the forces that rule our universe.

  • General relativity describes

  • The most familiar of those gravity.

  • The quantum mechanics describes three other forces.

  • The strong nuclear force

  • That's responsible for gluing protons and neutrons together inside of atoms;

  • Electromagnetism,

  • which produces light, Electricity and magnetic attraction;

  • And the nuclear forces.

  • That's the force responsible for radioactive decay.

  • Every event in the universe

  • From the splitting of a atom, to the birth of a star

  • Is nothing more than these four forces into acting with manner

  • Albert Einstein spent the last 30 years Of his life

  • searching For a way to describe the forces Of nature

  • in a single theory,

  • And now string theory may fulfill

  • His dream of unification.

  • For centuries,

  • Scientists have pictured the fundamental ingredients of nature

  • atoms and the smaller particles inside of them

  • As tiny balls or points.

  • But string theory proclaims

  • That at the heart of every bit of matter

  • is a tiny, Vibrating strand of energy called a string.

  • And a new breed of scientist believes

  • These miniscule strings

  • Are the key to uniting the world of the large

  • And the world of the small in a single theory.

  • The idea that a scientific theory That we already have in our hands

  • could answer the most basic questions

  • Is extremely seductive.

  • For about 2,000 years,