DI: Okay, so let's start with a real basic question: What the pluck is string theory?
GM: It's one of the ways that physicists have proposed to unite physics. Although nature has a unity to it, the laws we use to explain nature don't. Phenomena such as electricity, magnetism, and nuclear reactions are explained using one theory (quantum theory) and phenomena such as gravity and orbits are explained using another (Einstein's general theory of relativity). We get away with that because those phenomena cleanly separate, but they don't always. Black holes and the big bang require the use of both theories at once, and then you run into trouble, because the theories are incompatible. String theory aspires to reconcile them, to be a single theory that handles everything. I'd be tempted to call it the "uniter not a divider" if someone else hadn't already taken that phrase.
String theory may be the deepest level of physical reality -- the wellspring from which all else flows. It takes all the zillions of different types of matter and forces and suggests that they are aspects of *one* type of thing, a string, like a tiny guitar string or tiny rubber band. By vibrating in different ways, such a string can play the role of an electron or a quark or a photon or whatever other type of particle you like. You don't even need to pluck the string. Because of quantum effects, it plucks itself. Whether that is a mental image appropriate for a family-oriented website, I leave up to you.
[Read on for George's thoughts on string theory and time travel, the 10th dimension, D-branes, and a whole lot more.]
DI: If the first LHC tests are a success, will they help helped prove or disprove string theory?
GM: Well, the only way the LHC could really "fail" is to find nothing at all. Whatever it finds will guide physicists into a deeper level of nature. String theory could be that level. The LHC can't strictly prove or disprove string theory; "proof" is a very hard to achieve in any science. Usually there's more of a mounting weight of evidence one way or the other. But the LHC will either encourage or discourage string theorists. For instance, string theorists predict that for every type of particle we know, there is a partner we haven't met yet -- a giant physics blind date. If the LHC finds some of these partners, it will be a checkmark in the "string theory" column and an 'X' in the column of other theories.
DI: Why do we need such large instruments to measure something as small as particles?
GM: That's one of the great ironies of nature. To probe small sizes, you need high energy -- the two are inversely related. For instance, as you decrease the wavelength of light, you go from red to green to blue to violet to ultraviolet to x-rays. In so doing, you increase the energy of each individual packet of light -- that's why you get sunburned by ultraviolet light, but not by red light. That's also why x-rays are even more hazardous than ultraviolet. The same basic principle applies to the particles that physicists study. To look for new laws that kick in at short distances, you need high energy. That, in turn, requires a big machine.
DI: You've visited the LHC in person. Any first-hand accounts worth sharing? What impressed you about it?
GM: For starters, CERN -- the lab in Geneva where the LHC is based -- is such an exciting intellectual environment. There are thousands of people there from all over the world, and in the cafeteria you get Nobel laureates sitting down with students and talking about the nature of reality together. It takes such a huge variety of skills to make the accelerator work. Like the other great feats of humanity, from building the pyramids to organizing the Civil Rights movement, it's a collective effort of people pooling their abilities for a higher purpose.
The accelerator itself consists of a tunnel where the particles circulate among a series of giant caverns containing instruments. These instruments are massive and have an industrial feel to them, with giant cranes and gangways and hard hats. But the instruments are filled with fine filigree work of wires and detectors. So it's a case of steel mill meets Swiss watch.
DI: Would the proof of string theory shed any light on the evolution creation debate?
GM: That debate is settled: the world evolves. It changes and adapts in an ceaseless process of self-organization. We can see that with our own eyes.
What string theory and other proposed theories of its kind do is fill in the back story -- in particular, the evolution that occurred long before life existed on Earth, way back in the early days of the universe when matter, forces, space, and time were still coming into being. Moreover, string theory deepens the foundations of the theories of physics that underpin biological evolution. One of the great mysteries of physics is why our universe is so attuned to the needs of life. The natural world sometimes seems very hostile to life, but it could've been a heck of a lot worse. String theory sheds light on this very question.
I think a lot of religious believers have the gnawing sense that science seeks to take the mystery out of the world and deny a role for the divine. Sure, there are a lot of arrogant scientists, but most are deeply humbled by the beauty and complexity of the natural world. They seek to explain the "how", not the "why". By reflecting on their discoveries, I think believers deepen their own faith and appreciation of the subtlety of God's work.
DI: In your book you write that the first string theory was proposed in 1926 but then forgotten. You say that few string theorists even know that little bit of history. Who proposed it and why was it overlooked?
GM: The Nobel laureate physicist Steve Weinberg brings this up at http://arxiv.org/abs/hep-th/9702027. The physicists who proposed the first string theory were Max Born, Werner Heisenberg, and Pascual Jordan, three of the founding fathers of quantum theory. It wasn't really "overlooked"; their ideas played a role in the development of quantum mechanics. But the questions related to the full unification of physics hadn't yet been formulated, so it took a later generation to rediscover them in that context. It's often the case in science that theories are anticipated but have to be rediscovered. It's like when I buy another copy of a CD I already own -- you sometimes don't realize what you've got.
DI: You mention the Superconducting Super Collider that was being built in Texas in the 1980s. This was going to be the U.S.'s version of the LHC, no? Why did congress pull the plug on the accelerator? Is this another instance of a missed opportunity for the U.S. to make an impact on the scientific world or were we just ahead of our time?
GM: It was definitely a lost opportunity. The SSC would have been preceded the LHC by a decade and achieved even higher energies.
Physicists, frankly, bear some of the blame. The cost estimate for the collider kept climbing at the same time the U.S. was also facing cost overruns in the space program, and it was all getting a bit much for Congress. But there is a deeper issue with how science projects are proposed, funded, and managed in the U.S. which leads to budgetary low-balling and instability. For instance, budgets are approved by Congress on a year-by-year basis, making long-term planning hard. Also, sites and contractors are chosen to appease such and such a Senator or lobbyist. This really needs to be resolved for scientists' and taxpayers' sake alike. After all, the U.S. spent $2 billion on the collider and all it has to show for it is a big hole in the ground. Man cannot live on half-baked bread alone.
Europe often (not always) does better because, ironically, it's harder to get all those nations to agree to anything, but once they do, they're in it for the long haul.
DI: I really found your book fascinating. For instance, I didn't know anything about branes before I read it. Sounds like good marketing, right? Takes an Idiot's Guide to light up the brane. But seriously: tell us about branes, specifically D-branes.
GM: I think physicists came up with branes to act as a source for puns. Hey, you need to do something to entertain yourself during physics lectures, right? The basic idea is that in addition to the little loops that create particles, string theory predicts thingies called branes. They come in many varieties: dots, filaments, sheets, blocks, and even higher-dimensional structures floating through space. The interactions of strings give you particles, and the interactions of branes give you other phenomena, perhaps including the big bang itself. D-branes are a special type of brane that acts like flypaper, tying down the ends of strings. Our entire universe might be one.
DI: String theory space has 10 dimensions (11 if you count time, right?). We have trouble visualizing four, let alone 5 plus another 5. Can you explain how we might begin to think in 10?
GM: The trick is to start with an analogy you can easily visualize and work up from there. For instance, consider a parking lot. It looks two-dimensional: that is to say, it looks flat. But actually there is a third dimension, that of depth. You only really notice the third dimension if you're small -- like an ant walking across and forced to navigate the cracks. You might get hints of the third dimension if you have a shopping cart that rumbles when you push it across those cracks. So this is a good analogy to a situation where space appears to be three-dimensional but is actually four-dimensional, because the fourth dimension is tiny, like those cracks you don't see at first. You might indirectly see them if a particle "rumbles" as it passes through space.
For me, the best way to visualize extra dimensions is to read Edwin Abbott's novel "Flatland" or watch the animated film version from last year (http://www.flatlandthefilm.com/). By understanding what 3-D looks like to a 2-D creature, you can begin to grasp what 4-D would look like to us 3-D creatures.
DI: Could the LHC help prove there are other dimensions?
GM: One way is to look for particles that "rumble" for no visible reason. "Rumbling" would manifest itself as the appearance of new particle types. Another is to look for tiny black holes created by the accelerator. The machine has the power to make such holes only if gravity is unexpectedly weak, and such weakness could arise if space has extra dimensions into which gravity would spread and become diluted.
DI: Can you explain why string theory doesn't rule out the possibility of time travel but quantum theory does?
GM: Neither standard quantum theory nor string theory has anything definitive to say on time travel. In fact, both offer some hope and some disillusionment for would-be time-machine builders. Both suggest how you might obtain the ingredients for time machines, such as exotic energy sources, but both suggest that attempting to put those ingredients together would be doomed to failure. Physicists tend to think time travel isn't possible, because then you'd get all those contradictions made famous by science-fiction. For instance, in the recent TV adaptation of "The Andromeda Strain", (spoiler alert) the germ has no origin. It is discovered and then sent back in time to itself, so where did it come from?
DI: In the book you pose the following question when discussing the multiverse: Which would be creepier? An identical copy of you, on an identical copy of Earth, somewhere out in deep space? A nearly identical copy of you, differing only in eye color, but otherwise the same? Or a creature so unlike you, not even having eyes, made up of particles so alien that you could never meet without instant death to you both? I'd like to put that question to you, and, of course, to get you to explain a little bit about the concept of parallel universes.
GM: The basic idea is simple: the laws of physics can play themselves out differently in different regions of space. An analogy is the laws of planet formation. They're the same for Earth, Venus, Mars, etc., yet slight differences in the starting conditions (distance from the sun, etc.) produced such vastly different outcomes. The same thing goes for all the laws of physics. The distribution of matter, the masses of the particles, and the strength of the forces could be different in different regions, leading to vastly different outcomes. When the "region of space" in question is beyond our range of vision, we call it a parallel universe. Being "beyond our range of vision" can occur for various reasons, either because it's just too far away, or maybe because it's a hair's breadth away from us but light can't cross even that tiny gap.
The easiest type of parallel universe to understand is the type that's too far away. Light hasn't had time to reach us yet. Maybe light never will reach us, because of the expansion of space in between us and that region. Each region starts off with a slightly different arrangement of matter, leading to differently shaped galaxies, different-looking planets, etc. But it stands to reason that, if space is big enough, the conditions that we experience will appear elsewhere too. In that case, the laws of physics will play out the *same*, and you'll get an identical copy of Earth somewhere out there. Can you imagine more than one George Musser in the universe? Now *that's* scary.
DI: There's that great scene in Spinal Tap where the reporter asks David toward the end of the film if the band has seen it's last days. David says: " Well, I don't really think that the end can be assessed as of itself as being the end because what does the end feel like? It's like saying when you try to extrapolate the end of the universe, you say, if the universe is indeed infinite, then how - what does that mean? How far is all the way, and then if it stops, what's stopping it, and what's behind what's stopping it? So, what's the end, you know, is my question to you." My question to you, George, is, what is out there, at the end of space? What is space expanding into and how could string theory help us answer the question?
GM: Infinite space is enough to make your brain spontaneously combust, because as I said above, in an infinite space, there are copies of you out there, living out all the possible permutations of your life. There's only one thing weirder than infinite space, and that's finite space. If space comes to an end, what's beyond it? As it happens, astronomers have seen no signs of an edge or a looping-around to space, so space appears to be infinite or at least a good deal larger than Stonehenge.
What's space expanding into? It doesn't need to expand into anything. In fact, if you think about it, how could it? If it were expanding into something, that something would be space, and what would account for *that* space? At some point you have to cut things off and say, this amp only goes to 10.
Ultimately, it all comes back to the question of what space is, and answering that is a major goal of string theory. It and other similar theories suggest that space is not fundamental -- it arises from some ingredients that are spaceless. The concept of distance and therefore of infinity may be equally derivative. That's almost equally hard to envision as infinity. But what good would a theory of physics be if it didn't bend your brane, I mean brain?
DI: You talk a good deal about other theories and string theory critics in the book. What theory presents the biggest challenge to string? Do those theorists have a good argument?
GM: I think you want to get me into trouble, because when you start stacking up the theories against each other, physicists get very defensive about their babies, and will fill my inbox with irate comments. Like a good kindergarten teacher, I think each theory is special in its own way.
Since writing the book, though, I've grown more sympathetic to the idea I call in the book "tipping point" -- a loose term for the loose idea that the laws of physics we observe are not the fundamental ones. String theory, as radical as it can be, is conservative in many ways: it assumes that basic categories such as "particle", "field", and "gravitation" continue to be meaningful right to the deepest levels of nature. Those categories may be modified and extended, and they may serve merely as approximations to something deeper, but they're still basically right.
The "tipping point" is inspired by the behavior of fluids and solids, which can change _radically_, not merely incrementally. For instance, the concept of temperature is a collective property of a large group of particles; you can't really talk of the temperature of a single particle. Similarly, gravity might be a collective property of more fundamental ingredient, in which case even to talk of "quantum gravity" is to go about the unification of physics in the wrong way.
The trouble with the "tipping point" is that still it's still just the germ of an idea. And as the history of this field has shown time and again, a seemingly good idea can go poof as soon as you start to probe it. String theory is remarkable because it has survived despite all the efforts to blow it down.
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