String theory - Brian Greene

in the year 1919 a virtually unknown German mathematician named Theodor Kaluza suggested a very bold and in some ways very bizarre idea he proposed that our universe might actually have more than the three dimensions that we are all aware of that is in addition to left-right back-forth and up-down gluts to propose that there might be additional dimensions of space that for some reason we don't yet see now when someone makes a bold and bizarre idea sometimes that all it is both bizarre but has nothing to do with the world around us this particular idea however although we don't yet know whether it's right or wrong and at the end I'll discuss experiments which in the next few years may tell us whether it's right or wrong this idea has had a major impact on physics in the last century and continues to inform a lot of cutting-edge research so I'd like to tell you something about the story of these extra dimensions so where do we go to begin need a bit of backstory go to 1907 this is a year when Einstein is masking the glow of having discovered the special theory of relativity and decides to take on a new project to try to understand fully the grand pervasive force of gravity and in that moment there were many people around who thought that that project had already been resolved Newton had given the world the theory of gravity late 1600s that works well describes the motion of planets the motion of the moon and so forth the motion of apocryphal apples falling from trees hitting people in the head all of that could be described using Newton's work but Einstein realized that Newton had left something out of the story because even Newton had written that although he understood how to calculate the effect of gravity he had been unable to figure out how it really works how is it that the Sun 93 million miles away somehow defect the motion of the earth how does the Sun reach out across empty inert space and exert influence and that as a task to a Jain Stein set himself to figure out how gravity works and let me show you what it is that he found so Einstein found that the medium that transmits gravity is space itself the idea goes like this imagine space is a substrate of all there is Einstein said space is nice and flat if there's no matter present but if there is matter in the environment such as the Sun it causes the fabric of space to warp to curve and that communicates the force of gravity even the earth warps space around it now look at the moon the moon is kept in orbit according to these ideas because it rolls along a valley in the curved environment that the Sun and the moon and the earth can all create by virtue of their presence we go to a full frame view of this the earth itself is kept in orbit because it rolls along a valley in the environment that's curved because of the sun's presence that is this new idea about how gravity actually works now this idea was tested in 1919 through astronomical observations it really works it describes the data and this gained Einstein prominence around the world and that is what got Colusa thinking he like Einstein was in search of what we call a unified theory that's one theory that might be able to describe all of nature's forces from one set of ideas one set of principles one master equation if you will so Coolidge's said to himself Einstein has been able to describe gravity in terms of warps and curves in space in fact space and time to be more precise maybe I can play the same game with the other known force which was at that time known as the electromagnetic force we know of others today but at that time that was the only other one people were thinking about you know the force responsible for electricity and magnetic attraction and so forth so Colusa says maybe I can play the same game and describe electromagnetic force in terms of warps and curves that raised a question warps and curves in what Einstein had already used up space and time warps and curves to describe gravity didn't seem to be anything else to warp a curve so Colusa said well maybe there are more dimensions of space he said if I want to describe one more force maybe I need one more dimension so he imagined that the world had four dimensions of space not three and imagine that electromagnetism was warps and curves in that fourth dimension now here's the thing when he wrote down the equations describing warps and curves in the universe with four space dimensions not three he found the old equations that einstein had already derived in three dimensions those were for gravity but he found one more equation because of the one more dimension and when he looked at that equation it was none other than the equation that scientists had long known to describe the electromagnetic force amazing it just popped out he was so excited by this realization that he ran around his house screaming victory that he had found the unified theory now clearly Colusa was a man who took theory very seriously he in fact there's a story that when he wanted to learn how to swim he read a book a treatise on swimming then toven to the ocean this is a man who would risk his life on theory now but for those of us who are a little bit more practically minded to questions immediately arise from his observation number one if there are more dimensions of space where are they we don't seem to see them and number two does this theory really work in detail when you try to apply it to the world around us now the first question was answered in 1926 by a fellow named Oskar Klein he suggested that dimensions might come in two varieties there might be big easy to see dimensions but there might also be tiny curled up dimensions curled up so small even though they're all around us that we don't see them let me show you that one visually so imagine you're looking at something like a cable supporting a traffic lights in Manhattan in Central Park that's it's kind of irrelevant but the cable looks one-dimensional from a distant viewpoint but you and I all know that it does have some thickness it's very hard to see it though from far away but if we zoom in and take their perspective to say a little ant walking around little ants are so small that they can access all the dimensions the long dimension but also this clockwise counterclockwise direction and I hope you appreciate this it took so long to get these ants to do this but this illustrates the fact that dimensions can be of two sorts big and small and the idea is that maybe the big dimensions around us are the ones that we can easily see but there might be additional dimensions curled up sort of like the circular part of that cable so small that they have so far remained invisible let me show you what that would look like so if we take a look say at space itself I can only show of course two dimensions on a screen some of you guys will fix that one day but anything that's not flat in the screen is a new dimension goes smaller smaller smaller and way down in the microscopic depths of space itself this is the idea you could have additional curled up dimensions here is a little shape of a circle so small that we don't see them but if you were a little ultra-microscopic ant walking around you can walk in the big dimensions that we all know about that's like the grid part but you could also access the tiny curled up dimension that's so small that we can't see with the naked eye or even with any of our most refined equipment but deeply tucked into the fabric of space itself the idea is there could be more dimensions as we see there now that's an explanation about how the universe could have more dimensions than the ones that we see but what about the second question that I asked does the theory actually work when you try to apply it to the real world well it turns out that Einstein includes and many others worked on trying to refine this framework and apply it to the physics of the universe as was understood at the time and in detail it didn't work in detail for instance they couldn't get the mass of the electron to work out correctly in this theory so many people worked on it but by the 40s certainly by the 50s this strange but very compelling idea of how to unify the laws of physics had gone away until something wonderful happened in our age in our era a new approach to unify the laws of physics is being pursued by physicists such as myself many others around the world it's called super string theory as you were indicating and the wonderful thing is that super string theory has nothing to do at first sight with this idea of extra dimensions but when we study super string theory we find that it resurrects the idea in a sparkling new form so let me just tell you how that goes super string theory what is it well it's a theory that tries to answer the question what are the basic fundamental indivisible uncuttable constituents making up everything in the world around us the idea is like this so imagine we look at a familiar object just a candle and a holder and imagine that we want to figure out what it is made of so we go on a journey deep inside the object and examine the constituents so deep inside we all know you go sufficiently far down you have atoms we also all know that atoms are not the end of the story they have little electrons that swarm around a central nucleus with neutrons and protons even the neutrons and protons have smaller particles inside of them known as quarks that is where conventional ideas stop here is the new idea of string theory deep inside any of these particles there is something else there's something else is this dancing filament of energy it looks like a vibrating string that's where the idea of string theory comes from and just like the vibrating strings that you just saw on a cello can vibrate in different patterns these can also vibrate in different patterns they don't produce different musical notes rather they produce the different particles making up the world around us so these ideas are correct this is what the ultra-microscopic landscape of the universe looks like it's built up of a huge number of these little tiny filaments of vibrating energy vibrating in different frequencies the different frequencies produce the different particles the different particles are responsible for all the richness in the world around us and there you see unification because matter particles electrons and quarks radiation particles photons gravitons are all built up from one entity so matter and the forces of nature all are put together under the rubric of vibrating strings and that's what we mean by a unified theory and here is the catch when you study the mathematics of string there you find that it doesn't work in a universe that just has three dimensions of space it doesn't work in a universe with four dimensions of space nor 5 nor 6 finally you can study the equations and show that it works only in a universe that has ten dimensions of space and one dimension of time leads us right back to this idea of Kaluza and Klein that our world when appropriately described has more dimensions than the ones that we see now you might think about that and say well ok you know if you have extra dimensions and they're really tightly curled up yeah perhaps perhaps we won't see them if they're small enough but you know if there's a little tiny civilization of green people walking around down there you make them small enough and we won't see them either that is true one of the other predictions of string theory no that's not one of the other predictions of string theory but it raises the question are we just trying to hide away these extra dimensions or do they tell us something about the world and the remaining time I'd like to tell you two features of them first is many of us believe that these extra dimensions hold the answer to what perhaps is the deepest question in theoretical physics theoretical science and that question is this when we look around the world as scientists have done for the last hundred years there appear to be about 20 numbers that really describe our universe these are numbers like the mass of the particles like electrons and quarks the strength of gravity the strength of the electromagnetic force a list of about 20 numbers that have been measured with incredible precision but nobody has an explanation for why the numbers have the particular values that they do now the string theory often answer not yet but we believe the answer for why those numbers have devised they do may rely on the form of the extra dimensions and the wonderful thing is if those numbers had any other values than the known ones the universe as we know it wouldn't exist this is a deep question why those numbers so finely tuned to allow stars to shine and planets to form when we recognize that if you fiddle with those numbers if I had 20 dials up here and I'll let you come up and fiddle with those numbers almost any fiddling makes the universe disappear so can we explain those twenty numbers and string theory suggests that those twenty numbers have to do with the extra dimensions let me show you how so when we talk about the extra dimensions in string theory it's not one extra dimension as in the older ideas of Kaluza and Klein this is what string theory says about the extra dimensions they have a very rich intertwined geometry this is an example of something known as a claw be a shape name isn't all that important but as you can see the extra dimensions fold in on themselves and intertwine in a very interesting shape interesting structure and the idea is that if this is what the extra dimensions look like then the microscopic landscape of our universe all around us would look like this on the tiniest of scales when you swing your hand you'd be moving around these extra dimensions over and over again but they're so small that we wouldn't know it so what is the physical implication though relevant to those twenty numbers consider this if you look at the instrument of French horn notice that the vibrations of the airstreams are affected by the shape of the instrument now in string theory all the numbers are reflections of the way strings can vibrate so just as those air streams are affected by the twists and turns in the instrument strings themselves will be affected by the vibrational patterns and the geometry within which they are moving so let me bring some strings into the story and if you watch these little fellas vibrating around they'll be here in a second right there notice that the way they vibrate is affected by the geometry of the extra dimensions so if we knew exactly what the extra dimensions look like we don't yet but if we we should be able to calculate the allowed notes the allowed vibrational patterns and if we could calculate the allowed vibrational patterns we should be able to calculate those 20 numbers and if the answer that we get from our calculations agrees with the values of those numbers that have been determined through detailed and precise experimentation this in many ways would be the first fundamental explanation for why the structure of the universe is the way it is now the second issue that I want to finish up with is how might we test for these extra dimensions more directly is this just an interesting mathematical structure that might be able to explain some previously unexplained features of the world or can we actually test for these extra dimensions and we think this is I think very exciting that in the next five years or so we may be able to test for the existence of these extra dimensions here's how it goes in CERN Geneva Switzerland a machine is being built called a Large Hadron Collider it's a machine that will send particles around a tunnel opposite directions near the speed of light every so often those particles will be aimed at each other so there's a head-on collision the hope is that if the collision has enough energy it may act some of the debris from the collision from our dimensions forcing it to enter into the other dimensions how would we know it well we'll measure the amount of energy after the collision compare it to the amount of energy before and if there's less energy after the collision than before this will be evidence that the energy has drifted away and if it drifts away in the right pattern that we can calculate this will be evidence that the extra dimensions are there let me show you that idea visually so imagine we have a certain kind of particles called a graviton that's the kind of debris we expect to be ejected out if the extra dimensions are real but here's how the experiment will go you take these particles you slam them together you slam them together and if we are right some of the energy of that collision will go into debris that flies off into these extra dimensions so this is the kind of experiment that we will be looking at in the next five seven to ten years or so and if this experiment bears fruit if we see that kind of particle ejected by noticing that there's less energy in our dimensions then when we began this will show that the extra dimensions are real and to me this is a really remarkable story and a remarkable opportunity going back to Newton with absolute space didn't provide anything but an arena a stage in which the events of the universe take place Einstein comes along and says well space and time can warp and curve that's what gravity is and now string theory comes along and says yes gravity quantum mechanics electromagnetism all together in one package but only if the universe has more dimensions than the ones that we see and this is an experiment that may test for them in our lifetime amazing possibility thank you very much

Channel: TED-Ed

Views: 2,451,553

Rating: 4.9263787 out of 5

Keywords: \Brian Greene\, \string, theory\, \superstring, TED, TED-Ed, \TED, Ed\, TEDEducation

Id: kF4ju6j6aLE

Length: 19min 10sec (1150 seconds)

Published: Fri Aug 09 2013

Reddit Comments

[u/haplo_and_dogs]

The experiment he describes were run, and at the distance scales available to the large hadron collider extra dimensions were ruled out. Extra dimensions could exist at much smaller scales, but we are unable to test.

Low Energy Super Symmetry ( near the Higgs Scale < 1TeV ) has been ruled out. Without Super Symmetry string theory doesn't work, however it could be present at a much much higher energy that we can never test this.

Dark Energy has become more experimentally grounded. A anti de sitter universe is basically ruled out. There is a lot of active conversation if a de sitter string theory is possible, or is stuck in the swamp of incompatible solutions.

Given the above it appears that we will be unable to test any compatible string theory in our life times, as all testable versions are ruled out.

[u/Pisgahstyle]

I’m just a lowly high school physics teacher but thanks for these conversations that make it easier for me to understand it. Powerful stuff. Wish I would’ve been more interested in higher Physics instead of bio while in college.

[u/jaekx]

Also watch Sean Carroll Podcast Episode 18: Clifford Johnson on What’s So Great About Superstring Theory. Full episode list over at r/seancarroll

[u/20mcgug]

I've met Brian Greene before. He's a very kind person and his books really sparked my interest in physics.

[u/Zophike1]

I'm just a lowly Math Undergrad but how was our mathematical understanding of ST(String Theory) evolved from when the theory was first discovered till know ?

I understand that String Theory is being pursed in a different light then when it was first contrived.

[u/ShinyLeptonWhale]

Brian Greene is very cool and an excellent physics writer. If you're looking for a physics book to read, The Elegant Universe by Brian Greene is a fantastic book for almost any level.

[u/moschles]

As the video shows -- this talk was actually given in 2005 , not 2013.

In Elegant Universe, Greene definitely claims that gravity is weak because it 'leaks out' into large extra dimensions. (He showed a pool table and the sound leaking off the table dimension into the dimensions of the room.)

Recent observations of gravity waves from neutron star collisions have ruled this out pretty squarely. The energy in the gravity waves did not diminish as if it were leaking into a fourth dimension.

[u/wiserone29]

This video was the one that finally helped me understand gravitational fields. The graphics also helped grasp the concept of fields in general. Another great one is the ted talk by Brian Cox.

Also, after it was mentioned in the movie Limitless, I read The Elegant Universe and didn’t get it into. After watching this video, I really got into it and read it again and LOVED it.

Plus he is in my favorite Big Bang Theory episode.

Scientist like him and the great NGT have piqued my interest in the sciences and I ended up taking more courses in the physical sciences after I’d already become establish in my current field.

[u/ExclusiveGrabs]

Tangentially related. Any advice on how to go from Brian Greene ted talks to his talks at a slightly more technical level? E.g. https://youtu.be/PBOwargPdJ4

Not really sure what to search for to start chasing down the concepts he's building on (don't know the names to use).