Thank you to 23andme for supporting PBS Digital Studios
You may have heard the usual pop sci description of string theory.
There are these tiny vibrating strings and
That's where all the forces particles including gravity in the entire universe come from
This raises more questions than it answers
Like why strings? What are they made of? And what's all this nonsense of extra dimensions?
In physics we like to reduce our description of the mechanics of reality down to the simplest possible form
We expect the most fundamental machinery to have the fewest possible moving parts or free parameters
This is why the standard model of particle physics is considered incomplete.
Its equations predict many things with stunning accuracy
But they first require us to tune many mathematical knobs and dials
we need to use physical measurement to fix 19 free parameters like the masses of particles and
Then there's gravity which doesn't fit into the standard model at all
so surely there exists a deeper set of cogs and wheels
A theory that brings all observable phenomena into the same mechanical framework
That would be a theory of everything and this is the great hope of string theory.
In the following episodes
We're going to explore the gory details of string theory, but today it's string theory 101
Where did this crazy idea come from? I mean why tiny vibrating strings?
Versus literally any other tiny vibrating anything? What exactly are the strings of string theory first?
Let's do a quick primer on the origins of string theory.
The idea started in the 60s with efforts to understand the behavior of hadrons
collections of quarks bound by the gluons of the strong nuclear force
That includes protons and neutrons as well as mesons, which are a combination of a quark and an antiquark
Peculiarities in the interactions between pairs of mesons as well as an odd relationship between their angular momenta and masses
Suggested that the quarks in mesons are connected by, you guessed it, strings
In this case, the strings are stretched out tubes of strong nuclear force
Vibrating elastic bands made of gluons.
A lot of work went into figuring out a quantum theory for the
the strong interaction based on the physics of strings
The theory had some success but kind of got stuck and was ultimately replaced by quantum chromodynamics
One of the reasons this strong force version of string theory got stuck
is that it predicted the existence of unexpected and unwanted
vibrational modes in the gluon field of these strings.
What's a vibrational mode in a quantum field?
It's a particle.
And one of those modes appeared to be a massless spin-2 particle
But the only hypothetical massless spin-2 particle is the graviton, the conjectured quantum particle of the gravitational field
If the gravitational field is made of quantum particles
Which it might be we really don't know. But if it is, then the quanta of gravity should have an uncanny
resemblance to the type of particle produced by this little investigation into hadronic strings
except that there's no way anything like the graviton should appear in that sort of string.
This realization came in the early 70s
A bold new proposal emerged
forget mesons
What if the math of this theory could be used in a theory of quantum gravity?
in fact, what if all force carrying particles result from oscillations in tiny strings?
All we needed was to make the strings a bit smaller. Like 20 orders of magnitude smaller
Shrinking from the size of a proton to the Planck scale
Roughly the scale of the difference between the Milky Way galaxy and your living room
Oh, and we needed to add 22 dimensions to the familiar 4. No biggie
This was so-called bosonic string theory
If it worked it would have been a candidate for a grand unified theory combining all known forces
But why stop there? If wiggly strings can explain force carrying bosons,
Why not also the fermions that comprise matter?
Through the 70s and 80s, several proposals
Introduced the idea of supersymmetry to bring the fermions and bosons into the same theoretical framework
The resulting super string theory sought to become an all-encompassing
mechanism to explain the underlying workings of our entire reality.
A theory of everything.
As an added bonus this ambition also shaved off a bunch of dimensions
Only ten were needed once fermions were added.
Then in 1995 Ed Witten brought the many forms of super string theory together into the single framework of M theory
All for the low price of adding only one more spatial dimension for an eleven dimensional theory
Okay enough for the history lesson. Let's talk strings.
So wiggly strings could explain the whole universe
That's a hell of a claim.
To understand quantum strings, first we need to look at regular strings
they're cooler than you think. The key is that strings can carry waves and
If the string has ends or is tied in a loop then a wave will end up overlapping with itself
In that case you get a standing wave
Roughly speaking when these travelling waves overlap each other they can either stack up or cancel out
constructive or destructive interference
Constructive interference only happens if the wavelength of the wave fits a neat number of times along the length of the string
Then the phases of the overlapping wave match in the right way
and that wavelength / frequency of the wave is enhanced
All other frequencies tend to die out
The result is that for a given string only certain frequencies
Corresponding to certain energies are possible. These resonant frequencies depend on the length of the string
Also, it's tension which defines wave velocity and so relates frequency to wave length
For example, this leads to the specific frequencies of vibration on a guitar string
But this sort of behavior, where only specific discrete energy modes are allowed
sounds very quantum like
String theorists weren't the first to notice this
Niels Bohr came up with the first quantum model for electron orbits by thinking of them as ring like standing waves
around the hydrogen atom but
Quantum strings are much more ambitious than boring electron orbits
The hope is that tweaked just right, those discrete vibrational modes can be made to match the properties of known particles
Particle mass just comes from the length of the string and it's tension tension is after all just energy per unit length
string length defines mass
But also defines which complex vibrational modes are possible and those modes in turn define particle properties
like electric charge and spin
So this is the great promise of string theory. By defining a single parameter the string tension
Or equivalently string length scale all of the possible particles should be automatically defined
Compare that one parameter to the 19 free parameters of the standard model
It sure sounds closer to a fundamental theory.
Okay recap, we have these Planck scale
One-dimensional structures that can be in loops or extended they have vibrational modes that define particle properties
By the way, those vibrations the standing waves
You're not some abstract internal wave the strings are real physical strands and the waves are wiggles in actual space
But physical strands of what?
common answers include pure mass energy
fundamental irreducible existence
Topological irregularities in the fabric of reality or the most common answer. It's a meaningless question.
They are fundamental so not made of anything or in other words a material known as shut-up-and-calculate-onium
Most string theorists are more interested in what strings do not what they're made of
So, what do they do?
Well vibrate obviously they can hold energy
They can stretch they can also merge and split apart
These last properties are important because it gives a mechanism for the particles of string theory to interact and to decay into other particles
This picture of strings coming together joining and splitting apart is a huge strength of the theory
It solves one of the main problems with quantizing gravity
Maybe you remember from our episode on quantum gravity if you try to describe
gravitational interactions on the smaller scales, the energies required to interact on that scale produce black holes
There's no way to even think about the shape of the gravitational field on the Planck scale
That doesn't produce a hopeless conflict
string theory fixes this because the graviton is a loop not a point particle its interactions are
Smeared around that string handily avoiding the explosion of mathematical infinities
You get below the Planck length
All this stuff sounds great
and by the way
doesn't work for any other geometrical structure other than a 1d string
So vibrating guitar strings, yes. Drum skins, no.
Unfortunately, it's not going to be this easy
Yeah, the strings themselves are 1d but to even start to produce the properties of known particles
They need to vibrate in more than just the three dimensions of space
In fact, the theory only works in precisely nine spatial dimensions plus one for time
Plus one for M theory, which we'll come back to in short without exactly this number of dimensions
You don't get gravitons or any other massless particle. We'll look into why in future episodes
But this is awkward to say the least. It's a theory that works in a universe
That is clearly not our own with its measly three dimensions of space.
But this sort of thing doesn't deter string theorists
There's a way to add extra spatial dimensions that is still consistent with our perceived 3d universe
to get our heads around this
Imagine we lived in a 2d flatland universe. We only perceive the giant x and y directions
But what if flatland isn't truly flat? What if the z-direction?
Has a tiny tiny width
This is a pac-man dimension. Travel the tiny width of this dimension and you'll find yourself back where you started
Very tiny objects like quantum strings could explore that extra dimension and importantly oscillate in it
But we giant lumbering Flatlanders would have no clue it existed
Okay now scale this up three large dimensions of space and six tiny pac-man dimensions
That only strings experience. Voila string theory is saved
Modern m-theory proposes an additional large spatial dimension our universe of 3d space and
1d time is like flatland on this 5d object called a 5-brane
m-theory unites different string theories because it demonstrates some
philosophically fascinating dualities between different ways of thinking about the dimensions
Ultimately, it also leads to the ultimate duality. That is the holographic principle
Patience, grasshoppers. We will get there.
The exact behavior of strings depend on the shape of their compact dimensions
In fact the single free parameter in string theory becomes the configuration of the extra dimensions
Find the right location in this string landscape and you perfectly describe the universe
the only issue is that there are an estimated 10 to the power of
500 possible choices and almost no way to figure out which one is ours
Right now string theory appears to be at an impasse. It has produced no confirmed predictions
Some would say it's made no testable predictions
Tuning that string landscape to match our universe is daunting and perhaps impossible
Yet despite this impasse its promise and its elegance has convinced many that it must be right
or at least the right path
In coming episodes will look deeper into both the successes,
the failures, and the profound weirdnesses of string theory then you can decide for yourself whether you accept the
fundamental stringy nature of space-time
Thanks to 23andme for supporting PBS Digital Studios and space-time
23andme is a personal genetic company created to help people understand what their DNA says about them
the month of October is Family History Month, which is a great time to explore and learn more about your own family and ancestry a
discovery that can lead to new connections with others
You could learn more by going to 23andme.com/spacetime
Last week we talked about the fundamental computational limits of our universe and
Incidentally what it would take to compute a universe simulation on the event horizon of a black hole
Let's see what you had to say
RomanR asks
Whether computation at an event horizon would experience massive time dilation relative to an outside observer
So how do we see the results of the computation?
Yeah, that's an issue
Really? You can't read off the results of an event horizon computation until practically ever
I mentioned in the video that you'd read off the result in Hawking radiation
Which would take until long after the last star in the universe has died to even give you a small fraction of that read out
The slow read out by Hawking radiation is equivalent to the time dilation issue
You know what these black hole computers suck. Let's not build one
A few of you pointed out that a black hole computer couldn't store the information about other black holes and
You're right
this is one of the assumptions we made in the calculation
Our supermassive black hole computer is only large enough to contain all information in radiation and matter
But most of the information in the universe is in black holes or more accurately
Most entropy or hidden information is in black holes
So our black hole computer can't contain the information hidden in all black holes. It can't even contain the information from black holes
larger than itself
Yuval Nehemia went back to an old video that quotes me saying to Neil deGrasse Tyson
To simulate the universe you need a computer the size of the universe.
In direct contradiction with everything I've said recently about the bekenstein bound
As hard as it is to believe that I have ever said anything wrong before
I'm afraid Yuval has caught me out. You can build a universe simulator smaller than the universe
That's it. The universe simulator that you'd build inside this universe has limits
It couldn't simulate a universe so perfectly that the simulated universe could also contain an equally good universe simulator
There's no infinite set of nested simulators
Like I said our black hole computer is only simulating particles not black holes
It's like the simple logical gate arrays that people build inside Minecraft
Emulators are never as efficient as the original hardware
Sam Gil tells us this was the most boringest video I've ever seen. I have to say I'm kind of flattered
Have you been on YouTube?
Even PBS Digital Studios has spectacular contenders
What about that one where Johansson does nothing but
lick a lollipop for over 17 minutes?
Or Vanessa Hill's literal "The Most Boring Video Ever"?
That one's actually quite interesting