Columbia University physicist Brian Greene has become the public face of string theory. He has provided insight into the topology of those additional dimensions, and in 1999 he introduced the theory to nonscientists in a best-selling book, The Elegant Universe. In 2008 he cofounded the World Science Festival, an annual event that brings together scientists, artists, and ordinary people who are simply interested in the great questions of the universe. Greene talked to DISCOVER about how string theory has evolved, the attempts to find supporting evidence through new experiments, and the challenges of making science exciting to the general public.
What is the major problem string theory attempts to solve?
Our current theory of gravity—Einstein’s general theory of relativity—and our current theory of the behavior of atoms and subatomic particles—quantum mechanics—both work fantastically well in their respective domains: general relativity for big things, quantum mechanics for small things. But when you try to meld the two, there is an incompatibility, a hostility. It’s uncomfortable to have two laws of physics, each claiming that the other somehow doesn’t work. In reality, both sets of laws are meant to work everywhere.
How does string theory create a single worldview that applies everywhere—and what exactly is a string, anyway?
The fundamental idea is that the elementary constituents of matter—electrons, quarks, and so forth—might not be dots of no size, which is the traditional image, but rather little filaments. They could exist in either little loops of filament—tiny loops of energy—or little snippets of energy, open strings as we call them. When people stared at the mathematics governing the motion of these little filaments, they found, remarkably, that the math didn’t work in a universe that has only three dimensions of space. It required nine dimensions, and when you add in time it gives you 10 dimensions, which is an astoundingly bizarre idea. Nevertheless, it’s an idea that string theorists take seriously, because that’s where the math leads, and math has proved itself to be a very sure-footed guide to how the universe works.
How can we envision these extra dimensions, and how would they manifest themselves in our seemingly three-dimensional world?
The shape and size of the extra dimensions would affect the properties of particles. So if you asked me, “Why does the electron have its charge or its particular mass?” the answer in string theory would be because the extra dimensions have the shape that they do. An electron weighs what it does because it has a certain internal energy, and that energy, according to Einstein, equals mc². The energy depends on how its little string can vibrate, and the string vibrates in a manner that depends on its environment, so it depends on the shape of the extra dimensions. The dream in the 1990s was to find the shape of the extra dimensions and then calculate the values of all those properties that experimenters have found.
What is the current status of string theory research?
We have a range of possibilities for the shape of the extra dimensions. We have, in fact, catalogs of shapes. Literally, I could write out a book and turn page by page and show you different shapes for the extra dimensions that people have mathematically determined as being possible. The problem is we don’t know which page is the right one, and the number of pages has grown fantastically in the last few years. There are on the order of at least 10500 different pages now [a number that dwarfs the number of particles in the universe], and when you’re faced with a book of that many pages, some people throw up their hands in disgust. Others say that maybe all those shapes are out there in different universes. That’s the most recent and controversial approach that people have been following.
So there could be a multitude of other universes, each corresponding to a different solution or “page” of string theory?
As scientists we track down all promising leads, and there’s reason to suspect that our universe may be one of many—a single bubble in a huge bubble bath of other universes. And you can then imagine that maybe these different bubbles all have different shapes for their extra dimensions. This suggests a landscape of different universes with different forms of extra dimensions and therefore different properties within those universes. If that is true, our universe would be one of many, and then the question becomes why are we in this one and not in some other one.
One of your findings is helping scientists make sense of those extra dimensions and other universes, right?
We found that classical geometry, the kind you learn in elementary school, breaks down at extremely small scales. Instead, quantum geometry takes over, in which, for example, there can be two very different shapes in the extra dimensions that nevertheless yield exactly the same physics. In other words, there can be two different shapes from the perspective of a classical mathematician, but when dressed up with their quantum properties they become identical. What got us really excited was that horrendously complicated calculations framed in the language relevant to one shape became simple when reframed using the other shape. People like to talk about the Eskimos’ having 20 terms for snow and ice. It might take us a paragraph or a book to try to describe those distinctions, because our language is not set up to describe them. Similarly, with these shapes we’re basically rephrasing things from one language to another, and suddenly some very clunky and cumbersome descriptions become sleek, elegant, and completely solvable.
From the Southworth Planetarium
“A pinch of whimsy”
THE DAILY ASTRONOMER
March 10, 2010
Mother of All Neutrons
To JB, who has an irrational fondness for protons and neutrons,
In both outer and inner space, empty space is the rule, not the exception. Though one would expect the vacuum that separates worlds, stars and galaxies to be all void and little matter, it is surprising to discover that all our terrestrial materials are likewise stuffed to the gills with nothing. This stunning fact holds for anything you have ever encountered, even lead blocks, concrete slabs, and the half-ton dumbbells that those sissies use when they want to bulk up their biceps. So, if you grabbed the block and got the lead out, you would find that the actual matter occupies a minuscule amount of space. The powerful protons, neutral neutrons and ghostly electrons would be speck-small, but curiously heavy in your palm.
One wonders how a heavy lead block could actually be just emptiness with a smattering of subatomic particles? Well, it feels heavy because you are mostly empty space, yourself. (Don’t fret. We dig ‘uncomplicated’) Also, if one were to separate all the lead atoms, one would see a nucleus crammed with 82 protons and 125 neutrons. Buzzing around it like a swarm of tenacious gnats are 82 electrons. Here, we should mention that these electrons are hardly discrete little particles in space. The ‘electrons’ would seem more like a fog. However, if we could somehow command the electrons to stop being so quantum and stop moving, we’d count eighty-two of the negative little twerps.
This model suggests that the nucleus and electrons are in close proximity relative to their sizes. This is hardly the case. Even a lead nucleus is a tightly-packed little super nugget of protons and neutrons, while the electrons defining the atom’s outer boundaries fly far afield of the nucleus which constrains them through the electro static attraction between the positive protons and negative electrons. (In the electromagnetic realm, unlike charges attract, like charges repel.) In fact, as an analogy, imagine the nucleus as being marble-sized. The atom enclosing it would be as large as a big city football stadium. Even on this scale, the electrons would be barely visible.
The atom is mostly empty space. A public radio scientist type would say that there is a 15 order of magnitude difference between the atomic volume and the volume of the nucleon, the latter being the confines of the nucleus. This means that more than 99.99999% of the atom is nothing. Void. Nada. Zilch.
There is very little one could do to condense matter down to smaller volumes. The atomic structure is quite set and has worked nicely for lo these many aeons without the intervention of space efficiency experts, thank you very much. However, out in this hyper-energetic universe one could find a mechanism by which this surplus space could be squeezed out of the atoms like so much untapped vintage. These mechanisms are called stars. Within stars one finds plasma, the fourth state of matter in which protons, neutrons and electrons are all swarming about in a roiling inferno of fusion fire. These stars also have a great deal of empty space, especially near the outer layers.
However, things change when stars die.
A very massive star, much larger than the Sun, will end its life cycle in a cataclysmic event called a “supernova,” in which the stars’ layers collapse onto the core before rebounding with a tremendous explosion. The compressed core will either become a black hole or, if the star is toward the lower end of the super massive scale, a neutron star. When the layers fall onto themselves, the protons and electrons are crushed together. The combination of a positive and negative will result in a neutral particle. The neutron star spins rapidly to form a pulsar. These supernova remnants are the densest objects in the known universe. (Black holes, being pesky things enclosing singularities, don’t count.) A neutron star has no empty space. It is like a big neutron: a super-dense subway station of that single type of subatomic particle. These are so incredibly dense that a single teaspoon of neutron star material would weigh more than a million tons!
So, when that troublesome spouse or child complains bitterly about having to take the trash to the curb, just remind him/her that the fragrant flotsam is more than 99.999% emptiness. Therefore, their incessant carping about weighty waste is truly much ado about nothing. Tell me if that works.
Baseball
