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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,

  • All of our physics essentially has been based on...

  • Essentially we were talking about billiard balls.

  • The very idea of the string is such a paradigm shift,

  • Because instead of billiard balls,

  • You have to use little strands of spaghetti.

  • But not everyone is enamored of this new theory.

  • So far no experiment has been devised that

  • Can prove these tiny strings exist.

  • And let me put it bluntly.

  • There are physicists and there are string theorists.

  • It is a new discipline,

  • A newyou may call it a tumor

  • ——you can call it what you will,

  • But they have focused on questions

  • Which experiment cannot address.

  • They will deny that, These string theorists,

  • But it's a kind of physics which is not yet testable,

  • It does not make predictions that have anything

  • To do with experiments that can be done in the laboratory or

  • With observations that could be made in space or from telescopes.

  • And I was brought up to believe, and I still believe,

  • That physics is an experimental science.

  • It deals with the results to experiments,

  • Or in the case of astronomy, observations.

  • From the start, many scientists thought

  • String theory was simply too far out.

  • And frankly,

  • The strange way the theory evolvedin a series of twists,

  • Turns and accidentsonly made it seem more unlikely.

  • In fact, even it's birth has turned something . which goes like this

  • In the late 1960s

  • a young Italian physicist,

  • Named Gabriele venetian,

  • Was searching for a set of equations

  • That would explain the strong nuclear force,

  • The extremely powerful glue that

  • Holds the nucleus of every atom together

  • Binding protons to neutrons.

  • As the story goes,

  • He happened on a dusty book on the history of mathematics,

  • And in it he found a 200-year old equation,

  • First written down by a Swiss mathematician, Leonhard Euler.

  • Veneziano was amazed to discover

  • that Euler』s equations, Long thought to be nothing More than a mathematical curiosity,

  • Seemed to describe the strong force.

  • He quickly published a paper and was famous

  • Ever after for this "accidental" discovery.

  • I see occasionally, written in books,

  • That, uh, that this model was invented by chance or was, ,

  • Found in the math book

  • ,,this makes me feel pretty bad.

  • What is true

  • Is that the function was the outcome of a long year of work,

  • And we accidentally discovered string theory.

  • However it was discovered, Euler』s equation,

  • Which miraculously explained the strong force,

  • Took on a life of its own.

  • This was the birth of string theory.

  • Passed from colleague to colleague,

  • Euler's equation ended up on the chalkboard

  • In front of a young American physicist, Leonard Susskind.

  • To this day I remember the formula.

  • The formula was...

  • And I looked at it, and I said,

  • "this is so simple even I can figure out what this is."

  • Susskind retreated to his attic to investigate.

  • He understood that this ancient formula described

  • The strong force mathematically,

  • But beneath the abstract symbols

  • He had caught a glimpse of something new.

  • And I fiddled with it, I moneyed with it.

  • I sat in my attic, I think for two months on and off.

  • But the first thing I could see in it,

  • It was describing some kind of particles

  • Which had internal structure which could vibrate,

  • Which could do things, which wasn't just a point particle.

  • And I began to realize that what was being described here was a string,

  • An elastic string, like a rubber band,

  • Or like a rubber band cut in half.

  • And this rubber band could not only stretch and contract,

  • But wiggle.

  • And marvel of marvels, it exactly agreed with this formula.

  • I was pretty sure at that time That I was the only one in the world who knew this.

  • Susskind wrote up his discovery

  • Introducing the revolutionary idea of strings.

  • But before his paper could be published

  • It had to be reviewed by a panel of experts.

  • I was completely convinced that when it came back it was going to say,

  • "Susskind is the next Einstein," or maybe even,

  • "the next Newton." and it came back saying,

  • "this paper』s not very good, Probably shouldn't be published."

  • I was truly knocked off my chair.

  • I was depressed, I was unhappy.

  • I was saddened by it.

  • It made me a nervous wreck,

  • And the result was I went home and got drunk.

  • As Susskind drowned his sorrows

  • Over the rejection of his far out idea,

  • It appeared string theory was dead.

  • Meanwhile, mainstream science

  • Was embracing particles as points, not strings.

  • For decades,

  • Physicists had been exploring the behavior of microscopic particles

  • By smashing them together at high speeds

  • And studying those collisions.

  • In the showers of particles produced,

  • They were discovering that nature is far richer than they thought.

  • Once a month there'd be a discovery of a new particle:

  • The rho meson, the omega particle, the b particle,

  • The b1 particle, the b2 particle, phi, omega...

  • More letters were used than exist in most alphabets.

  • It was a population explosion of particles.

  • It was a time when graduate students

  • Would run through the halls of a physics building

  • Saying they discovered another particle,

  • And it fit the theories. and it was all so exciting.

  • And in this zoo of new particles,

  • Scientists weren't just discovering building blocks of matter.

  • Leaving string theory in the dust,

  • physicists made a startling and strange prediction:

  • That the forces of nature

  • Can also be explained by particles.

  • Now, this is a really weird idea,

  • But it's kind of like a game of catch

  • In which the players like me

  • And me are particles of matter.

  • And the ball we're throwing back and forth

  • Is a particle of force.

  • It's called a messenger particle.

  • For example, in the case of magnetism,

  • The electromagnetic forcethis ballwould be a photon.

  • The more of these messenger particles or photons That are exchanged between us,

  • The stronger the magnetic attraction.

  • And scientists predicted

  • That it's this exchange of messenger particles

  • That creates what we feel as force.

  • Experiments confirmed these predictions

  • With the discovery of the messenger particles for electromagnetism,

  • The strong force and the weak force.

  • And using these newly discovered particles

  • Scientists were closing in

  • On Einstein's dream of unifying the forces.

  • Particle physicists reasoned that

  • If we rewind the cosmic film

  • To the moments just after the big bang,

  • Some 14 billion years ago

  • When the universe was trillions of degrees hotter,

  • The messenger particles for electromagnetism

  • And the weak force would have been indistinguishable.

  • Just as cubes of ice melt into water in the hot sun,

  • Experiments show that as we rewind

  • To the extremely hot conditions of the big bang,

  • The weak and electromagnetic forces meld together

  • And unite into a single force called "the electroweak."

  • And physicists believe

  • That if you roll the cosmic film back even further,

  • The electroweak would unite with the strong force

  • In one grand "super-force."

  • Although that has yet to be proven,

  • quantum mechanics was able to explain how three of the forces

  • operate On the subatomic level.

  • And all of a sudden

  • We had a consistent theory of elementary particle physics,

  • which allows us to describe all of the interactions

  • weak, strong and electromagneticin the same language.

  • It all made sense, and it's all in the textbooks.

  • Everything was converging

  • Toward a simple picture of the known particles and forces,

  • A picture which eventually became known

  • As the "standard model."

  • I think I gave it that name.

  • The inventors of the standard model, both the name and the theory,

  • Were the toasts of the scientific community,

  • Receiving Nobel prize after Nobel prize.

  • But behind the fanfare was a glaring omission.

  • Although the standard model explained three of the forces

  • That rule the world of the very small,

  • It did not include the most familiar force,

  • gravity.

  • Overshadowed by the standard model,

  • String theory became a backwater of physics.

  • Most people in our community lost, completely, Interest in string theory.

  • "okay, that was a very nice elegant thing

  • But had nothing to do with nature."

  • It's not taken seriously by much of the community,

  • But the early pioneers of string theory are convinced

  • That they can smell reality and continue to pursue the idea.

  • But the more these diehards delved into string theory

  • The more problems they found.

  • Early string theory had a number of problems.

  • One was that it predicted a particle which we know is unphysical.

  • It's what's called a "tachyon,"

  • A particle that travels faster than light.

  • There was this discovery that the theory requires ten dimensions,

  • Which is very disturbing, of course

  • Since it's obvious that that's more than there are.

  • It had this massless particle

  • Which was not seen in experiments.

  • So these theories didn't seem to make sense.

  • This seemed crazy to people.

  • Basically, string theory was not getting off the ground.

  • People threw up their hands and said, "this can't be right."

  • By 1973,

  • Only a few young physicists were still wrestling

  • With the obscure equations of string theory.

  • One was john Schwarz,

  • Who was busy tackling string theory's numerous problems,

  • Among them a mysterious massless particle

  • Predicted by the theory but never seen in nature,

  • And an assortment of anomalies

  • Or mathematical inconsistencies.

  • We spent a long time trying to fiddle with the theory.

  • We tried all sorts of ways of making the dimension be four,

  • Getting rid of these massless particles

  • And the tachyons and so on,

  • But it was always ugly and unconvincing.

  • For four years,

  • Schwarz tried to tame the unruly equations of string theory,

  • Changing, adjusting,

  • Combining and recombining them in different ways.

  • But nothing worked.

  • On the verge of abandoning string theory,

  • Schwarz had a brain storm.

  • Perhaps his equations were describing gravity.

  • But that meant reconsidering the size

  • Of these tiny strands of energy.

  • We weren't thinking about gravity up 'til that point.

  • But as soon as we suggested that

  • Maybe we should be dealing with a theory of gravity,

  • We had to radically change our view of how big these strings were.

  • By supposing that strings were

  • A hundred billion billion times smaller than an atom,

  • One of the theory's vices became a virtue.

  • The mysterious particle john Schwarz

  • Had been trying to get rid of

  • Now appeared to be a graviton,

  • The long sought after particle

  • Believed to transmit gravity at the quantum level.

  • String theory had produced the piece of the puzzle

  • Missing from the standard model.

  • Schwarz submitted for publication his groundbreaking new theory

  • Describing how gravity works in the subatomic world.

  • It seemed very obvious to us that it was right.

  • But there was really no reaction in the community whatsoever.

  • Once again string theory fell on deaf ears.

  • But Schwarz would not be deterred.

  • He had glimpsed the holy grail.

  • If strings described gravity at the quantum level,

  • They must be the key to unifying the four forces.

  • He was joined in this quest by one of the only other scientists

  • Willing to risk his career on strings,

  • Michael green.

  • In a sense, I think, We had a quiet confidence

  • That the string theory was obviously correct,

  • And it didn't matter much if people didn't see it at that point.

  • They would see it down the line.

  • But for green's confidence to pay off,

  • He and Schwarz would have to confront the fact

  • That in the early 1980s,

  • String theory still had fatal flaws in the math

  • known as "anomalies."

  • An anomaly is just what it sounds like.

  • It's something that's strange or out of place,

  • Something that doesn't belong.

  • Now this kind of anomaly is just weird.

  • But mathematical anomalies can spell doom for a theory of physics.

  • They're a little complicated, So here's a simple example.

  • Let's say we have a theory

  • In which these two equations

  • Describe one physical property of our universe.

  • Now if I solve this equation over here,

  • And I find x=1,

  • And if I solve this equation over here and find x=2,

  • I know my theory has anomalies

  • Because there should only be one value for x.

  • Unless I can revise my equations

  • To get the same value for x on both sides,

  • The theory is dead.

  • In the early 1980s,

  • String theory was riddled With mathematical anomalies kind of like these,

  • Although the equations were much more complex.

  • The future of the theory depended

  • On ridding the equations of these fatal inconsistencies.

  • After Schwarz and green battled the anomalies

  • In string theory for five years,

  • Their work culminated

  • late one night in the summer of 1984.

  • It was widely believed that

  • These theories must be inconsistent Because of anomalies.

  • Well, for no really good reason, I just felt that had to be wrong because

  • I, I felt, 「string theory has got to be right,

  • Therefore there can't be anomalies."

  • So we decided, 「weve got to calculate these things."

  • Amazingly it all boiled down to a single calculation.

  • On one side of the blackboard they got 496.

  • And if they got the matching number on the other side

  • It would prove string theory was free of anomalies.

  • I do remember a particular moment,

  • When john Schwarz and I were talking at the blackboard

  • And working out these numbers which had to fit,

  • And they just had to match exactly.

  • I remember joking with john Schwarz at that moment,

  • Because there was thunder and lightningthere was a big mountain storm in aspen at that moment

  • And I remember saying something like, you know,

  • "we must be getting pretty close, because the gods are trying

  • To prevent us completing this calculation."

  • And, indeed, they did match.

  • The matching numbers meant the theory was free of anomalies.

  • And it had the mathematical depth

  • To encompass all four forces.

  • So we recognized not only that the strings could describe gravity

  • But they could also describe the other forces.

  • So we spoke in terms of unification.

  • And we saw this as a possibility of realizing the dream

  • That Einstein had expressed in his later years,

  • Of unifying the different forces in some deeper framework.

  • We felt great.

  • That was an extraordinary moment,

  • Because we realized that

  • No other theory had ever succeeded in doing that.

  • But by now, it』s like crying wolf.

  • Each time we had done something,

  • I figured everyone's going to be excited, and they weren't.

  • So I, I figured...by now I didn't expect much of a reaction.

  • But this time the reaction was explosive.

  • In less than a year, the number of string theorists

  • Leapt from just a handful to hundreds.

  • Up to that moment, the longest talk I'd ever given on the subject

  • Was five minutes at some minor conference.

  • And then, suddenly, I was invited all over the world

  • to give talks and lectures and so forth.

  • String theory was christened

  • "the theory of everything."

  • In early fall of 1984,i came here, to oxford university,

  • To begin my graduate studies in physics.

  • Some weeks after,

  • I saw a poster for a lecture by Michael green.

  • I didn't know who he was,

  • But, then again, I really didn't know who anybody was.

  • But the title of the lecture was something like "the theory of everything."

  • so how could I resist?

  • This elegant new version of string theory

  • Seemed capable of describing all the building blocks of nature.

  • here is how

  • Inside every grain of sand

  • Are billions of tiny atoms.

  • Every atom is made of smaller bits of matter,

  • Electrons orbiting a nucleus made of protons and neutrons,

  • Which are made of even smaller bits of matter called quarks.

  • But string theory says this is not the end of the line.

  • It makes the astounding claim

  • That the particles making up everything in the universe

  • Are made of even smaller ingredients,

  • Tiny wiggling strands of energy that look like strings.

  • Each of these strings is unimaginably small.

  • In fact

  • If an atom were enlarged to the size of the solar system,

  • A string would only be as large as a tree!

  • And here's the key idea.

  • Just as different vibrational patterns

  • Or frequencies of a single cello string

  • Create what we hear as different musical notes,

  • The different ways that strings vibrate

  • Give particles their unique properties,

  • Such as mass and charge.

  • For example, the only difference between the particles Making up you and me

  • and the particles that transmit gravity and the other forces

  • Is the way these tiny strings vibrate.

  • Composed of an enormous number of these oscillating strings,

  • The universe can be thought of as a grand cosmic symphony.

  • And this elegant idea resolves the conflict

  • Between our jittery unpredictable picture of space on the subatomic scale

  • And our smooth picture of space on the large scale

  • It's the jitteriness of quantum theory versus

  • The gentleness of Einstein's general theory of relativity

  • That makes it so hard to bridge the two, to stitch them together.

  • Now, what string theory does,

  • It comes along and basically calms the jitters of quantum mechanics.

  • It spreads them out by virtue of

  • taking the old idea of a point particle

  • And spreading it out into a string.

  • So the jittery behavior is there, but it's just sufficiently less violent

  • That quantum theory and general relativity stitch together

  • Perfectly within this framework.

  • It's a triumph of mathematics.

  • With nothing but these tiny vibrating strands of energy,

  • String theorists claim to be fulfilling Einstein's dream

  • Of uniting all forces and all matter.

  • But this radical new theory contains a chink in its armor.

  • No experiment can ever check up what's going on

  • At the distances that are being studied.

  • No observation can relate to

  • these tiny distances or high energies.

  • That is to say, there isn』t no experiment that could be done,

  • Nor is there any observation that could be made,

  • That would say, 「you guys are wrong."

  • The theory is safe, permanently safe.

  • Is that a theory of physics or a philosophy?

  • I ask you.

  • People often criticize string theory for saying

  • That it's very far removed from any direct experimental test,

  • And it's...surely it's not really, um, um, a branch of physics, for that reason.

  • And I am response to that is simply that

  • They're going to be proved wrong.

  • Making string theory even harder to prove,

  • Is that, in order to work, the complex equations require something

  • that sounds like straight out of science fiction

  • Extra dimensions of space

  • We've always thought, For centuries,

  • that there was only what we can see.

  • You know, this dimension, that one, and another one.

  • There was only three dimensions of space and one of time.

  • And people who've said that there were extra dimensions Of space

  • have been labeled as,

  • You know, crackpots, or people who were bananas.

  • Well, string theory really predicts it.

  • To be taken seriously,

  • String theorists had to explain

  • How this bizarre prediction could be true.

  • And they claim that the far out idea of extra dimensions

  • May be more down to earth than you'd think.

  • Let me show you what I mean.

  • I'm off to see a guy who was one of the first people

  • To think about this strange idea.

  • I'm supposed to meet him at four o'clock at his apartment

  • At fifth avenue and 93rd street, on the second floor.

  • Now, in order to get to this meeting,

  • I need four piece of this information

  • One for each of the three dimensions of space

  • A street, an avenue and a floor number

  • And one more for time, the fourth dimension.

  • You can think about these four information in your common experiences

  • Left-right, back-forth, up-down and time.

  • As it turns out, the strange idea that

  • There are additional dimensions

  • Stretches back almost a century.

  • Our sense that we live in a universe

  • Of three spatial dimensions

  • Really seems beyond question.

  • But in 1919,theodor Kaluza,

  • A virtually unknown German mathematician,

  • Had the courage to challenge the obvious.

  • He suggested that maybe, just maybe,

  • Our universe has one more dimension

  • That for some reason we just can't see.

  • Look. he says here, 「I like your idea."

  • So why does he delay?

  • You see, Kaluza had sent his idea

  • About an additional spatial dimension to Albert Einstein.

  • And although Einstein was initially enthusiastic,

  • He then seemed to waver,

  • And for two years held up publication of Kaluza's paper.

  • Eventually, Kaluza's paper was published

  • After Einstein decided extra dimensions were his cup of tea.

  • Here's the idea.

  • In 1916,einstein showed that gravity is nothing

  • But warps and ripples In the four familiar dimensions of space and time.

  • Just three years later,

  • Kaluza proposed that electromagnetism might also be ripples.

  • But for that to be true,

  • Kaluza needed a place for those ripples to occur.

  • So Kaluza proposed an additional hidden dimension of space.

  • But if Kaluza was right, where is this extra dimension?

  • And what would extra dimensions look like?

  • Can we even begin to imagine them?

  • well, Building upon Kaluza's work,

  • The Swedish physicist Oskar Klein

  • Suggested an unusual answer.

  • Take a look at the cables supporting that traffic light.

  • From this far away I can't see that they have any thickness.

  • Each one looks like a line

  • ——something with only a single dimension.

  • But suppose we could explore one of these cables

  • Way up close,

  • Like from the point of view of an ant.

  • Now a second dimension

  • Which wraps around the cable becomes visible.

  • From its point of view,

  • The ant can move forwards and backwards,

  • And it can also move clockwise and counterclockwise.

  • So dimensions can come in two varieties.

  • They can be long and unfurled like the length of the cable,

  • But they can also be tiny and curled up

  • Like the circular direction that wraps around it.

  • Kaluza and Klein made the wild suggestion

  • That the fabric of our universe

  • Might be kind of like the surface of the cable,

  • Having both big extended dimensions, The three that we know about,

  • But also tiny, curled up dimensions,

  • curled up so tinyBillions of times smaller than even a single atom

  • That we just can't see them.

  • And so our perception that we live in a universe With three spatial dimensions

  • may not be correct after all.

  • We really may live in a universe

  • With more dimensions than meet the eye.

  • So what would these extra dimensions look like?

  • Kaluza and Klein proposed that

  • If we could shrink down billions of times,

  • We'd find one extra tiny,

  • Curled up dimension located at every point in space.

  • And just the way an ant can explore the circular dimension

  • That wraps around a traffic light cable,

  • In theory an ant that is billions of times smaller

  • Could also explore this tiny, curled up, circular dimension.

  • This idea that extra dimensions exist all around us

  • Lies at the heart of string theory.

  • In fact the mathematics of string theory

  • demand not one,

  • But six extra dimensions,

  • Twisted and curled into complex little shapes

  • That might look something like this.

  • If string theory is right we would have to admit

  • That there are really more dimensions out there,

  • And I find that completely mind-blowing.

  • If I take the theory as we have it now, literally,

  • I would conclude that the extra dimensions really exist.

  • They're part of nature.

  • When we talk about extra dimensions

  • We literally mean extra dimensions of space

  • That are the same as the dimensions of space that we see around us.

  • And the only difference between them

  • Has to do with their shape.

  • But how could these tiny extra dimensions,

  • Curled up into such peculiar shapes,

  • Have any effect on our everyday world?

  • Well, according to string theory, shape is everything.

  • Because of its shape,

  • A French horn can produce dozens of different notes.

  • When you press one of the keys

  • you change the note,

  • Because you change the shape of the space

  • Inside the horn where the air resonates.

  • And we think the curled up spatial dimensions In string theory

  • work in a similar way.

  • If we could shrink down small enough To fly into

  • one of these tiny six-dimensional shapes predicted by string theory

  • We would see how the extra dimensions are

  • Twisted and curled back on each other,

  • Influencing how strings,

  • the fundamental ingredients of our universe,

  • Move and vibrate.

  • And this could be the key

  • To solving one of nature's most profound mysteries.

  • You see, our universe is kind of like a finely tuned machine.

  • Scientists have found that there are about 20 numbers,

  • 20 fundamental constants of nature

  • That give the universe the characteristics we see today.

  • These are numbers like how much an electron weighs,

  • The strength of gravity,

  • The electromagnetic force and the strong and weak forces.

  • Now, as long as we set the dials on our universe machine

  • To precisely the right values for each of these 20 numbers,

  • The machine produces the universe we know and love.

  • But if we change the numbers by adjusting the settings

  • On this machine even a little bit...

  • The consequences are dramatic.

  • For example,

  • If I increase the strength of the electromagnetic force,

  • Atoms repel one other more strongly,

  • So the nuclear furnaces that make stars shine break down.

  • The stars, including our sun, fizzle out,

  • And the universe as we know it disappears.

  • So what exactly, in nature,

  • Sets the values of these 20 constants so precisely?

  • Well the answer could be the extra dimensions in string theory.

  • That is, the tiny, curled up,

  • Six-dimensional shapes predicted By the theory

  • cause one string to vibrate in precisely the right way

  • to produce What we see as a photon

  • And another string to vibrate in a different way

  • Producing an electron.

  • So according to string theory,

  • These miniscule extra-dimensional shapes really

  • May determine all the constants of nature,

  • Keeping the cosmic symphony of strings in tune.

  • By the mid 1980s,string theory looked unstoppable,

  • But behind the scenes the theory was in tangles.

  • Over the years, string theorists had been so successful

  • That they had constructed not one,

  • But five different versions of the theory.

  • Each was built on strings and extra dimensions

  • But in detail,

  • The five theories were not in harmony.

  • In some versions, strings were open-ended strands.

  • In others they were closed loops.

  • At first glance,

  • A couple of versions even required 26 dimensions.

  • All five versions appeared equally valid,

  • But which one was describing our universe?

  • This was kind of an embarrassment For string theorists

  • because on the one hand, we wanted to say that this might be it,

  • The final description of the universe.

  • But then, in the next breath we had to say,

  • "and it comes in five flavors, five variations."

  • now There's one universe

  • you expect there to be one theory and not five.

  • So this is an example where more is definitely less.

  • One attitude that people who didn't like string theory could take was,

  • "well, you have five theories, so it's not unique."

  • This was a peculiar state of affairs,

  • Because we were looking just

  • To describe one theory of nature and not five.

  • If there's five of them,

  • Well maybe there's smart enough people would find twenty of them.

  • Or maybe there's an infinite number of them,

  • And you're back to just searching around at random for theories of the world.

  • Maybe one of these five string theories

  • Is describing our universe

  • on the other hand, which one? and why?

  • What are the other ones good for?

  • Having five string theories,

  • even knows big progress, revises the obvious question

  • If one of those theories describes our universe

  • Then who lives in the other four worlds?

  • String theory seemed to be losing steam once again.

  • And frustrated by a lack of progress,

  • Many physicists abandoned the field.

  • Will string theory prove to be a "theory of everything"

  • Or will it unravel into a "theory of nothing?"

It's a little known secret

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エレガントな宇宙 02 (The Elegant Universe 02)

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    姚姚 に公開 2021 年 01 月 14 日
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