# Why String Theory is Right

MATT O’DOWD: Thanks

to the Great Courses Plus for supporting

PBS Digital Studios. Some see string theory

as the one great hope for a theory of

everything that will unify quantum mechanics and

gravity and so unify all of physics into one great,

glorious theory of everything. Others see string theory

as a catastrophic dead end, one that has consumed a

generation of geniuses with nothing to show for it. So why are some of the

most brilliant physicists of the past 30-plus

years so sure that string theory is right? [MUSIC PLAYING] Why has string theory been

the obsession of a generation of theoretical physicists? What exactly is so compelling

about tiny vibrating strings? In our last

string-theory episode, I talked about what

these things really are and covered some history. In short, the strings

of string theory are literal strands

and loops that vibrate with

standing waves simply by changing the vibrational mode

and you get different particles analogous to how different

vibrational modes on guitar strings give different notes. And, by the way,

these strings exist in six compact

spatial dimensions on top of the familiar three. In this episode,

I’m going tell you why string theory is

right, at least why so many of those

geniuses think it is. Maybe I can summarize. It’s pretty, or at least

it started out that way. Its mathematics seem to

come together so neatly towards a unified description

of all forces and particles, and most importantly that

unification includes gravity. I want to try to

give you a glimpse into this mathematical elegance. I also want to give you a

teaser on why string theory is actually wrong. Don’t worry, that topic will

get its own whole episode. The greatest criticism

of string theory is that it’s never made

a testable prediction. The space of possible

versions of string theory is so vast that nothing can

be calculated with certainty, so string theory can neither

be verified nor ruled out. It’s unfalsifiable. But string theorists

might disagree. They might say,

maybe half jokingly, that string theory does

make one great prediction. It predicts the existence

of gravity, which is stupid, of course. Everyone knows that

Isaac Newton discovered gravity when he fell

out of an apple tree, or something like that. There was definitely

an apple tree involved. But the fact is when

you start to work out the math of string theory,

gravity appears like magic. You don’t need to try to fight

gravity into string theory. In fact, it will be

difficult to remove it, and the quantum gravity

of string theory is immune to the main difficulty

in uniting general relativity with quantum mechanics. It doesn’t give you

tiny black holes when you try to describe

gravity on the smaller scales. We did talk about this

and other problems with developing a quantum theory

of gravity in a recent episode, but before we get to the

nuts and bolts of how string theory

predicts gravity, it’s worth taking a moment to see

how stringy gravity avoids the problem of black holes. Let’s actually start with the

regular old point particles of the standard model. When a point particle is

moving through space and time it traces a line. On a spacetime diagram, time

versus one dimension of space, this is called its world line. In quantum theories of gravity,

the gravitational force is communicated by

the graviton particle. When the graviton acts

on another particle, it exerts its effect at an

intersection in their world lines over some distance. But in very strong gravitational

interactions, that intersection itself becomes more

and more point like. The energy density at that

point becomes infinite. More technically, you start to

get runaway self-interactions, infinite feedback effects

between the graviton and its own field. If you even try to

describe very strong gravitational interactions,

you get nonsense black holes in the math. OK, let’s switch

to string theory where particles are not points. They’re loops or

open-ended strands. The graviton in

particular is a loop. When strings move on

a spacetime diagram, they trace out

sheets or columns. In fact, you can think of a

string not as a 1D surface but as a 2D sheet

called a world sheet. Now let’s look at the

interaction of two strings. The vertex is no

longer point like. It can’t be point like. Even the most

energetic interactions are smeared out over

the string, so you avoid the danger of black

hole creating infinities. OK, put a pin in

these world sheets. We’re going to need them later. They illustrate why quantum

gravity isn’t hopelessly broken in string theory, and that’s a

huge point in favor of string theory, but these

world sheets will also help us see why

string theory predicts gravity in the first place. And this is the second point

in string theory’s favor. You see, it turns out that

tiny vibrating quantum strings automatically reproduce the

theory of general relativity and, in the same mechanism,

seem to promise to reproduce all of quantum theory too. This is part of the

elegance I spoke of earlier. This stuff appears a little too

naturally in the math of string theory to be a coincidence,

or so a string theorist might tell you. For some reason,

vibrating strings are bizarrely well

suited to quantization. By quantization I mean taking

a classical large-scale description of something

like a ball flying through the air or a

vibrating rubber band and turning it into a

quantum description. To do this, you basically

take the classical equations of motion and follow a

standard recipe to turn them into wave equations with

various quantum weirdness added in like the

uncertainty relation between certain variables. I say basically. This is a tricky

process, and it only works if your equations of

motion are especially friendly. Schrodinger’s equation is the

first and easiest example. It quantizes the

equations of motion of slow-moving,

point-like particles. A while ago, we talked about

Paul Dirac developed a wave equation for the electron that

took into account Einstein’s special theory of relativity. It was a mathematical mess until

Dirac added some nonsense terms to the electron-wave function

that caused a lot of the mess to cancel out. Those nonsense terms turned out

to correspond to antimatter. The resulting Dirac equation

is incredibly elegant, and in the pursuit

of that elegance Dirac predicted the

existence of antimatter. This is a powerful

example of how following mathematical

prettiness could bring us closer to the truth. Quantizing the motion of

strings also starts out ugly, but there are also some

math tricks to make it work. A big part of it is

making use of symmetries. If the physics of

a system doesn’t care about how you define

particular coordinates or quantities, we say

that that parameter is a symmetry of the

system or that the system is invariant to transformations

in that parameter. Finding symmetries

can massively reduce the complexity of the math. A really important type of

symmetry in quantum mechanics is gauge symmetry. It’s when you can redefine

some variable in different ways everywhere in space and

still get the same physics. I want to remind you of one

particularly crazy result of gauge symmetries. It’s a reminder

because we covered it, but it’s so relevant that

it’s worth the review. So, we expect the phase

of the quantum wave function to be a gauge

symmetry of any quantum theory. That means you should be able to

shift the location of the peaks and valleys in different ways

at different points in space without screwing up the physics. And guess what? In the raw Schrodinger

equation, you can’t. It breaks various

laws of physics. But it turns out that

you can add a very special corrective term to

the Schrodinger equation that fixes these phase differences

preserving local phase invariance. That term looks

like what you would get if you added the

electromagnetic field to the Schrodinger equation. So in a way,

electromagnetism was discovered in its quantum form

by studying the symmetries of quantum mechanics. It turns out that exploring

a very different symmetry of string theory both makes it

possible to quantize the theory and gives us a very different

field, the gravitational field. So, like I was

saying, when we try to quantize string theory,

of course it’s a huge mess. Applying the usual

old symmetries got physicists some of the

way, but to succeed, they needed an extra

weird type of symmetry. That symmetry is Weyl

symmetry or Weyl invariance. This is a weird one. It says that changing

the scale of space itself shouldn’t affect

the physics of strings. Hermann Weyl actually came

up with this symmetry right after Einstein proposed his

general theory of relativity. He tried to use it to

unify general relativity with electromagnetism. Fun story– Weyl invented

the name gauge symmetry to describe this scale

invariance inspired by the gauge of railroad

tracks which measures the separation of the tracks. Anyway, Weyl symmetry

doesn’t work. Turns out that in 4D

spacetime it does matter whether you change

the scale of space and the separation

of its tracks. But it turns out that

there’s a very particular geometric situation that

does have Weyl invariance. That’s on the 2D

dimensional world sheet of a quantum string. Remember that? Mysteriously, the 2D sheet

traced out in spacetime by a vibrating 1D

string has this symmetry that lets us redefine the scale

on its surface however we like. That means we can smooth out

that surface mathematically and write a nice,

simple quantum wave equation from the

equations of motion, but only for 1D strings

making a 2D world sheet, not for any other

dimensional object. This is part of what makes

strings so compelling. They are quantizable in a way

that other structures aren’t. But there’s a cost to

using this symmetry. Just as local phase

invariance required us to add the electromagnetic

field to the Schrodinger equation, adding

Weyl invariance means we need to add a new field. That field looks like a 2D

gravity on the world sheet. It’s a projection of the

3D gravitational field. So, with our quantized

equations of motion in hand, you can predict the quantum

oscillations of our string. These are particles,

and the first mode looks like the

graviton, a quantum particle in the aforementioned

gravitational field. If you use string

theory to write down the gravitational field in what

we call the low-energy limit, which just means not in

places like the center of a black hole,

then it looks just like the gravitational

field in Einstein’s theory. OK, a caveat– you

can only get the right particles, including the

graviton and the photon, out of string theory for

a very specific number of spatial dimensions,

nine to be precise. In fact, if string theory

makes any predictions, it’s the existence of

exactly this number of extra dimensions. And this is where string theory

starts to look less attractive. Our universe has three

spatial dimensions. String theorists hypothesize

that the extra dimensions are coiled on themselves

so they can’t be seen, but that seems like a

hell of an extra thing to add in order to

make your theory work. There’s also no

experimental evidence of the existence of

these dimensions. And that’s just the first of

many problems of string theory. But like I said, we’re going to

need a whole episode for that. Physicists were lead

to string theory by the elegance of

the math and the fact that it appeared, at

least in the beginning, to converge on

the right answers. That convergence is also seen

in the union of different string theories by M theory

and in the discovery of AdS/CFT correspondence– again, for future episodes. But can such an elegant and rich

mathematical structure really have nothing to do with reality? There’s plenty of

historical precedent for mathematical beauty

leading to truth, but there’s no fundamental

principle that says it has to. Perhaps we’re now

overly distracted by the elegance

of string theory. Philosophical points to

consider as we continue to follow the mathematical

beauty hopefully towards an increasingly true

representation of spacetime. Thanks to the Great Courses

Plus for supporting PBS Digital Studios. The Great Courses Plus is

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visit thegreatcoursesp lus.com/spacetime. Last week we talked about one of

the most misunderstood concepts in quantum mechanics, the

idea of virtual particles and their tenuous

connection to reality. You guys asked pretty much

every question that I avoided. Uri Nation asks

about the photons that mediate the magnetic

field or the contact force between two bodies. Aren’t they virtual? Well, they are, but

they don’t exist. These fundamental

forces are mediated by fluctuations in the quantum

fields of the relevant forces. Those fluctuations

can be approximated as the sum of many

virtual particles, but the particles

themselves are just convenient mathematical

building blocks to describe a messy

disturbance in the field. Eddie Mitch asked whether the

virtual particles are required to explain the Casimir force. So the Casimir

effect is sometimes explained as resulting from the

exclusion of virtual particles between two very closely

separated conducting plates which results in the plates

being drawn together. So, if the Casimir effect

really is due to a change in the zero-point energy– and there are those

who say it isn’t– but if it is, then it’s still

misleading to attribute it to virtual particles. More accurately, the

conducting plates create a horizon in what would

otherwise be a perfect infinite vacuum. In fact, you create two

horizons between the plates and one horizon on the outside. Those horizons

perturb the vacuum which can lead to the creation

of very real particles, as in Hawking radiation. But in the Casimir

effect, the double horizon between the plates restricts

what real particles can be produced

there whereas there’s less restriction on the

outside of the plates with their single horizon. That leads to a net pressure

pushing the plates together. David Ratliff asks if a

quantum tree falls in a vacuum and nobody is around to measure

it, does it still have energy? Well, believe it or not,

it’s a serious question as to whether the universe has

counter-factual definiteness, whether or not we can make

a meaningful statement about the state of the

universe without conducting an experiment. To address this

seriously, I want you to imagine this

gedankenexperiment. You have a box containing

a vial of poison connected to a radioactive isotope

that could either decay or not, releasing the poison. You put a mime in the box. Quantum mechanics can’t tell

us whether anyone cares.

Am I starting to think about this on the right track or am I way off?

Is compactification of dimensions to be thought of as a similar type of warping to the way gravity warps the 4 dimensions we know and love? Just as, if the universe had positive curvature it could be unbounded but finite; should I consider these dimensions as extremely positively curved, so that the whole dimension loops back on it self within something around the plank scale?

That seems strange and would leave me with a picture of space as made up of a huge number of individual looped dimensions stacked together. Maybe this doesn't mean that each individual unit is uniquely perpendicular to every other individual unit, but if they are finite because of extreme curvature it does suggest that there are individual units stacking up to make space time no?

It also seems weird to me to think of space having any kind of structure like this because it sounds a bit like a stationary "ether"; something about space it self relative to which an object could move. Of course this flies in the face of relativity, so something about my thinking hear is almost certainly wrong.

Why is almost every video from pbs space time slightly out of focus?!

Nobody knows (after 30 yrs) how many dimensions may be involved.

It you could relate string theory to the fundamental understandings of Pi, it can be validated. I believe there's a way to do this that I have realized. Proving the existence of a string would be paramount before you could jump on the strung out symphony or imagine how well angled lines magically emit energy through radiation with in an understanding of particle physics.

why does he even try to joke

I beg to differ, millennials and generation-Xers have dumped string theory and replaced it with rap, voice dynamics, polyrhythms and live streaming. One problem was that a 7 or 8-string guitar may have been better choice than a 6. Either way, rock played on either a 6 or 7-string guitar was doomed. The great hippies party is long over. Death destruction nuclear holocaust genocide natural disaster famine disease will suddenly FOLLOW WORLD PEACE like a lightning storm on a clear day.

The entire physics science is completely wrecked by our almost complete lack of understanding of gravity. It is supposed to point to vast quantities of the universe being invisible while completely wrecking quantum mechanics. As long as gravity remains a mystery, nothing can and will be reconciled.

You just have to Believe

Everting came out of Nothing

And with No First Cause

BS. Bad Science…

It's right because Tony Stark figured it out. He needs to be the narrator for this channel and not Tyrion Lannister.

quadrature of the circle is theory of everything.nothing else

Muy interesante la teoría me gustó mucho el vídeo síganme

Nothing string theory predicts – gravity, axions – can be used to confirm or reject it. String theory is unfalsifiable and therefore unscientific.

So that's cleared that up…

is there a symmetry between dimensions and particles?

String theory is wrong! Where is the evidence?

"String theory" is simply scientific fraud. You might read the great book from physicist Dr. Sabine Hossenfelder, book title: "Lost in Math" …

So the string theory is right cuz it's wrong? I get that pbs ain t a fan of ST but you should at least try to leave the personal biases outside the videos..Talking about reality and truth in science, lmao!

16 minutes ? this is just a tip of the ice berg, you need years to gain en-light-en-ment in this STRING THEORY!

The only reason string theory is right is because it was written by people who spent 20 years reading books, they need to keep their jobs, yup, even physicists need a paycheck

It is designed for the majority of the population that live in a 2 dimensional reality, COMMON SENSE, look at a piece of string, its 2 dimensional,

The advent of the clock is string theory a lie, before the clock was invented, everything was done in 10. Mathematics, they. Added the 4 and 8

And 12

The correct no. Sequence is , 123579 11

Get your calculus book out

Gdnite

3 has always been just a theory, the r y, tree

Trying to get you to do your homework since school was invented,

Read

my diarrhea > string theory

the object you show at 1:20 is only 3 dimensions… you physicists keep cheating, not one of you dumb motherfuckers can even explain what a 4th spatial dimension would be.

Listen, I don't really understand why people think String Theory is right, I only know what the basics are. But it seems to me that it is better to believe in it until we discover a more certifiable theory. Denying it and giving it no credit while also not yielding anything but criticism is the worst possible thing to do. At least recognize it, state its problems, then move on to find a better alternative.

Well all the smartest people in the world think string theory is right but this youtube comments section begs to differ. So i guess it’s wrong

The natural first (Occam’s) assumption to explain how or why a particle like a photon (or electron, etc) might behave as an uncertain location particle while also like a polarizable axial or helical wave ''packet'', given that everything in the universe from electrons to solar systems are in orbit with something pulling them into polarizable axial or helical apparent waves depending on the orientation of their orbits as they travel thru space in our expanding universe, is that they (all particles) might be orbiting something, no?

And since we know we’re in a sea of undetectable dark matter but don’t know where it’s disbursed, it seems this would be a natural and common first assumption by people beginning to study physics, no? If it’s the case, it would explain the double slit, uncertainty, and wave particle duality in general and the amplitude of waves would be the orbit diameter of the particle and the wavelength would be its orbit rotation speed relative to travel speed. Surely this idea has been considered and proven not possible, right?

Could someone please point me to the specific experiments, calculations, theories, etc that have disproved this possibility (other than the fact that dark matter does not otherwise interact with baryonic matter)? Thx

String theory is basically physicists being drawn into pure mathematics like a moth to a flame

What does string theory say about singularities? Isn't the whole point of searching for a theory of everything, being able to apply our physics to singularities (inside a black hole and the instant of the big bang) because this is where our current physics break down.

6:28 his cat was a lot easier to understand. Or so I thought, apparently others think that his cat is the most baffling thing in the world.

String theories can prove anything using Mathematics. For example – 5 of them prove that the universe is 10 dimensional and one of them proves it is 11 dimensional. So either 10 is equal to 11 or universe is N dimensional where N can be more than just 1 number. It is all about the magic of mathematics and nothing related to 1 universal reality.

True that! No one cares about mimes

ads-cft correspondence is correct, string theory itself is WRONG

For those who deny string theory, answer me a question: how do YOU reconcile quantum mechanics and relativity? Can you provide a better solution than string theory? I think not. Even the competition (like loop quantum gravity) is miles behind string theory.

Smoke some DMT and you will see those extra dimensions in no time.

Thanks! Very good about the Schroedinger equation and local phase invariance!

Thank You, Lord Tyrion.

"welll atleast why scientists "think" it's right"…… Change the title of the video.

You're wrong. My thoughts are controlled by quantum mechanics and they prove nobody cares.

Me: What the bleep do we know?

String Theory: YES

Who is this phoney?

From Wave vibrations we get WAIT FOR IT- -PARTICLES? The logic ends before….or is it befive? It starts……You bring in unresolved “units or factors” from No Where? And then continue with this umbrella over your and our heads! You cannot for hundreds of years continue to evade or misrepresent the quintessentials and bring in Fundamentals expecting realistic progress……Even if you think you can…..Super Artificial Intelligence Dictates You cannot!….So start out with original Thinking….from your Thought bank! To acquire progression! RDR

String theory, and quantum mechanics fit rather well with one particle or stream of energy theory.

I’m just a big dum’ dum’ because I don’t understand the math in string theory but from what I’ve read, if string theory was actually the unifying theory of everything then it’d make math obsolete for other dimensions. Or something along those lines. So I’d like to believe that there has to be a different theory of everything.

Ah yes tree discovered gravity when Issac Newton fell out of an apple

Apart from the math involved, which is totally beyond me, my biggest problem with string theory is imagining a string that is essentially a perpetual motion machine. Isn't that in itself impossible in basic physics?

You have got to be joking? String theory is not even wrong. It is a bad quantisation of linearised General Relativity, it has an unphysical number of dimensions, it needs Supersymmetry, which was all but ruled out at the LHC. It has no predictive power whatsoever. There is no obvious geometrical interpretation of spacetime. String Theory does NOT quantise General Relativity, it quantises a non-renomalizable Free field theory of graviton., that continually has to be fudged, because starting from linearised Gravity produces a false theory, with no resemblance to the real world.

In Loop Quantum Gravity the curvature of space-time, can be written in terms of two Yang-Mills fields, allowing the gravitational field to be rewritten in terms of Yang-Mills theory, and transformed into the Wilson Loop basis, and transformed into the the spin network basis, which are irreducible representations of SU(2)xSU(2) which are eigenvalues of volume and area.

Three dimensions, finite, geometrical which can describe the micro-states of area, giving the correct entropy for a stationary black hole horizons.

String theory is right if you're trying to hamstring the scientific community from discovering anti-gravity technology or curvature beam technology. String theory also doesn't describe things that we can do, like quantum entanglement. It doesn't explain the mechanism that causes the speed of light to be invariant, doesn't explain time, doesn't explain how space-time geometry is laid out. If there was a conspiracy to prevent the world from advancing technologically, it would be called string theory. But other than that, it is a great magic theory that can never be disproven.

Proton or neutrons or neurons

Start out as strings attached to each other by energy the more strings the more connects you get the more positive or negative space you get.

Only peaceful families and species get neutral energy.

The more and denser the strings the more connections the older space is. Less is younger space.

If a ship moves thru space it can reprogram positive young growing space to negative energy space

Hide and go seek

WHERE IS THE ORIGINAL STRING

If your reality does not recognize energy then no it is not reality

What does strong theory predict or explain that no rival theory can explain? What aspect of macroscopic (scale of humans, stars, galaxies, universe) physics does string theory explain?

My dear Matt, this is how dimensions work, a tiny bit more vibration and you already got another dimension, in which you have another universe, and so on and so on.

The problem I find with most modern theoretical physics is it's all maths and trying to make things as hard and complex as possible, we should focus on practical experiments like they used to?

He said Gayge Symmetey lol

You can basically make math do whatever you want it too, which is why it's such a very important part of Science to be able to test a hypothesis. The point being to attempt to prove it wrong.

String theorists seem to be trying to prove it's validity, that's not science, that's religion… This is not to say that I believe string theory is empirically a dead end, only that as it stands it cannot be tested and so the validity of it as an answer should naturally be critically challenged. That is Science after all.

No LHC tests indicate anything about string or supersymetrie to be the case,

regarding the dimensions wrapping into each other explaining why we can’t observe them, what is the reason why we are assuming we should be able to observe them at all? I mean, just because we can’t see something doesn’t mean it doesn’t exist, so why do we need to explain their existence like that just because we can’t observe them? Is it simply that we find it uncomfortable to claim something exists if we can’t observe it, even if all the math checks out?

I was totally excited about string theory, but it met with a catastrophic end for me when my cat, Schrodinger, stole the whole ball.

It’s not a scientific theory. It can’t make testable predictions. It may be mathematically complete as a theory but it’s not scientific.

We have 3 dimensions because we perceive 3 dimensions. How could we ever fathom 5+ dimensions. I'm a big fan of the mind being a 4th dimension.

string theory is total bs, want proof?. any formula that requires you to invent things like parallel universes to make them work has gone way to far down the rabbit hole to ever find its way back out.

Will it b proven someday.

I guess never.

Unfortunately this theory does not provide results after many years of digging. It looks like this episode is based on 10yrs old info at least and very very basics of it. It was a great idea that as it seems is leading to dead end of nothing. again Unfortunately :/

String theory is wrong. It doesn't look like nature, doesn't look like quantum mechanics or relativity.

Hawking: "… I shall take the simpleminded view that a theory is just a model of the universe, or a restricted part of it, and a set of rules that relate quantities in the model to observations that we make. It exists only in our minds and does not have any other reality (whatever that might

mean). A theory is a good theory if it satisfies two requirements. It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations…"

E8

id like it if you could slip in one quick "fuck" in every episode. dont signal it, and put it somewhere it doesnt belong, for maximum impact. make it count. you can do it, i believe in you.

how many strings are there per cubic meter?

how long does a string live before it disappears? or is it eternal?

can string split or merge?

do strings bounce when they collide, or do they pass through each other like photons?

are all strings the same or are there various types? if yes how many types?

how long are the strings?

do the strings change length?

what is the speed of the waves along the string?

do strings interact with electrical field?

do string posess electrical charge?

how fast do strings move?

why dont they just add a corrective term to the equations or divide by some infinities to fix the mess between gravity and quantum mechanics? they do this all the time

how many MeV mass does the graviton in string theory have?

what is the size of the coiled dimension? cm? mm? um? nm? fm? am?

Mime in a box? No. Nobody cares.

Why String Theory is not even Wrong?

……What?

came straight from seeker is string theory wrong?? haha the irony

In which order should I watch these videos to understand progressively?

Why string theory is right? … I'ts right? says fucking who? Show me proof? While you are not doing that.. Could you go and give a refund on life to all the poor fuckers who spent yeeears.. And whole careers not being ale to answer that question… But yet a putts on a propaganda promoting yt channel did. I think not. And your vid on why string theory is wrong is also wrong. Unified field theory holds more water, not only mathematically and logically but quite literally!.

Schroedinger equation is the EASYEST example? I'm stupid, hopelessly dumb…at least feel like one.

What if you had to calculate the black holes from GR into string theory in order to prove where the mass comes from?

It is all very much like coding, where every variable can contain its own string. It makes so much sense when listening to what Degrassi has said about the codes written into our natural forms of existence. #God is so beautiful!

Tyrion Lannister, I found you!

If you have an apple and expand it to the size of the observable universe , and take another apple that apple will be the size of planck length and the apple expanded to the size of the universe will be the observable universe. What if we are all in microscopic world. What if we zoom out away from the outside universe. Maybe we are some cells from a animal… So on and on