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
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