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From: Brian Green
Professor of Physics and Mathematics, Columbia University
Author of The Elegant Universe
Newton's Law of Gravity
Einstein�s Theory of Relativity
Black Holes
String Theory
Einstein�s Unified Theory of Physics
Hadron Collider

Physicists often use the term elegance or beauty in describing the universe. And often what we mean by that is that there is a real coherence, a logical connnectedness between the laws of the universe that really fills us with an incredible sense of awe and wonder that it all fits together, so perfectly, so swiftly with such economy. And it�s that economy of thought that can give rise to the wealth and richness of the world around us. Which is what we mean by the universe having an inner elegance.

The laws of the universe, to a physicist, are those ideas, those principles which seem to govern the fundamental constituents of the universe, the basic ingredients of matter, and govern the ways in which those ingredients influence each other. So we often ask, what is the most fundamental ingredient that makes up the stuff from the world around us? And what are the fundamental forces that allow those ingredients to interact and ultimately drive the evolution of the cosmos?

Newton's Law of Gravity
A simple example of a law of physics perhaps is Newton�s law of gravity. That�s a law that Newton wrote down in the late 1600s and it tells us how strongly one massive body in the universe will attract another massive body in the universe. So the sun and the earth are good examples of two massive bodies. Newton�s law of gravity tells us how strongly the sun and the earth interact through the gravitational force. And that force is what is responsible for keeping the earth in orbit around the sun and the strength of it allows us to predict the motion of the earth around the sun with incredible precision. And that�s why, for instance, we can predict a thousand years into the future when a solar eclipse is going to occur, because those laws allow us to understand the motions of those bodies with great accuracy.

Einstein�s Theory of Relativity
Relativity comes into the laws of physics in about 1905. And it comes in in two different flavors. The first in 1905 was Einstein�s special theory of relativity. And that�s a theory that really talks about the fundamental nature of space and time and how they appear to different people in the universe. Previously Newton said, space and time, they are out there, they are an inert backdrop, a stage on which the events of the universe take place. But Einstein said no, space and time are more fluid ideas, more malleable ideas and your perception of space and time depends on your state of motion. Strange idea but two individuals that are moving relative to one another will not have the same perception of the amount to time that elapses between two given events. Very strange. Unfamiliar in day to day life because these effects only become relevant when things are moving really quickly. At every day speeds, speeds of cars or planes or space shuttle, those speeds are too small for these effects to really be visible without incredibly precise equipment. But Einstein fundamentally showed that space and time in a sense are very much in the eye of the beholder. And that�s the first version of relativity. In 1915, a second revolution in our understanding of physics came with Einstein�s general theory of relativity. And that�s a theory of gravity. That�s a theory which replaces in a sense Newton�s earlier version of the gravitational force with one that�s based upon warps and curves, ripples in the fabric of space and time. And this again is a complete over haul of our understanding of space and time giving rise to a theory of gravity that�s more accurate than Newton�s theory and also solves some very outstanding puzzles at that time.

The fabric of space and time is one of the most illusive ideas that we freely make use of in our laws of physics, but if you push us to the wall and say what are space and time, what do you mean by that fabric? Where is it? Why can�t I grab it? We can�t really answer that question. It is a very powerful organizing idea. It is very hard to imagine discussing the universe without saying where things are in space and when things happen in time. And through the theory of relativity those basic ideas of space and time are woven into what we call the fabric of space and time. It�s an arena if you will in which the events of the universe play themselves out but it�s an arena that responds according to Einstein to those very events. But if you say, where is it�it�s very hard to make it more concrete. The best I think I can do is this. Einstein told us that gravity is a manifestation of curves in the fabric of space. So right now we all feel gravity, we feel it pulling us to the earth. And that feeling, according to Einstein, is really us sliding down an indentation in the fabric of space caused by the presence of the earth. So, if you say: �Why can�t I feel the fabric of space?� One answer is you are feeling it because you are feeling gravity right now and that is the way the fabric of space talks to us.

The strength of gravity depends on two things. One thing it depends upon is how massive a body is. The bigger something is the greater the gravitational pull it can exert while the less massive it is, the less the gravitational pull it can exert. So right now, the earth is pulling us down. But in fact, every object in the world around us is also pulling us as well with gravity but those objects are much smaller than the earth typically. And therefore, we don�t feel them. The second thing that gravity depends upon is how far away you are from the massive body. So, the sun is far more powerful than the earth fundamentally in terms of the gravity it can exert cause it is so much bigger. But we are far away from the sun, so we feel the earth more easily than we feel the sun right now. If we go into empty space, that means we are far away from all massive bodies, so the gravitational pull that they exert on us gets less and less and less. It�s still there but its strength is diminished by virtue of the fact that we are far away from it. So in empty space, in a sense, the fabric of space gets flatter and flatter and therefore there is less and less of a slope for us to slide down. And from that perspective we don�t feel gravity out in empty space.

Black Holes
Black holes are one of the most striking features of the general theory of relativity. A black hole can be thought of as a region of space in which an incredible amount of matter has been crushed, compressed to an incredibly small size. So there is matter, but the matter is so powerful that the gravity field it exerts is so strong that nothing can escape its grip if it gets too close to the black hole itself. Even light, as it tries to escape from the black hole so to speak, is pulled back by the force of gravity in a sense that it is forced to go into a u-turn and can�t escape into outer space. And that�s why the region looks black. No light can escape it. So gravity, in it�s most extreme form, yields these warps in the fabric of space that are so severe, so steep that nothing can escape if it gets too close and falls into the gravitational well produced by this incredibly crushed gravitational mass.

The force the black hole exerts on its environment is due to gravity. It�s the same gravity that we are feeling here on earth. It�s the same gravity the earth is feeling as it goes around the sun. It�s just more extreme because the mass that�s responsible for exerting that gravitational pull is huge and crushed to a very small size. And the combined effect of those two features yields this extreme gravity field that is more powerful than anything that one would experience in more ordinary terrestrial settings. So powerful, in fact, that if we were to fall into the gravity-well formed by the black hole, as we approach the center of the black hole, if we are falling feet first, our feet will be pulled down a little more strongly than our head, because our feet are closer, so our body will be stretched. So as we get closer and closer to the center of the black hole, we�ll become spagettified, we�ll be stretched, ultimately ripped apart because the gravitational field is so strong that it can do that, it can tear apart a human body, in fact it would tear apart anything at all that was to fall into the center.

So, the question as to what is really at the center of a black hole (forgetting about the fact that you would be destroyed before you got there), imagine you could go on a journey in your mind to the center of a black hole: What would you find? We don�t know. That�s one of the big puzzles. Is it a little nugget in some sense? We don�t know. Is it a gateway perhaps to another universe? You go to the center of the black hole and then space opens up into another universe that�s only tenuously connected to ours at the center of the black hole? Speculative idea. We don�t know the answer. So, that is one of the hot topics that people are still trying to figure out.

String Theory
String theory is a subject, which, was in a sense, discovered by accident. In the last 60s, people were trying to understand the so-called strong nuclear force. That�s the force that keeps quartz inside of protons and neutrons and protons and neutrons crammed inside the center of atoms. And people came upon some formula. They understand what it meant, but it seemed to describe the features of that force fairly well. So people started to pursue this idea. It was then realized that equation that people had stumbled upon was an equation describing strings and in that context, at first, it looked like the idea would work well, but then a few years later, the theory didn�t seem to describe the strong force. Well, people by and large gave up on it. Except for two guys: John Shwartz and Michael Green kept at it. Two lone physicists pursuing this idea and by the mid-1980�s they came to the striking conclusion that if you broadened your scope and didn�t just use this idea of strings to describe the nuclei of atoms, but used it to describe all of nature�s forces and all matter, then the theory seemed to work incredibly well. And that launched what we call the first super string revolution. Thousands of physicists around the world, dropped what they were working on at that time and started to think about string theory. The theory progressed extremely well for many years, but as with any field of endeavor, it had its ups and downs. And many people got a little bit disillusioned by it not reaching the ultimate aim of the unified theory of physics that people thought it would achieve. And dropped out of the field. But by the mid-1990s, a second revolution, it�s called �The Second Super String Revolution� took the field by storm allowing us to again make incredible strides in understanding this theory. And we feel that we are probably right now still within the midst of that revolution, taking us many, many steps closer to realizing the potential of this powerful theory.

String theory attempts to answer what perhaps is the deepest problem that theoretical physics has faced over the last fifty to seventy-five years. And that problem is this: in the early part of the 20th century, through Einstein�s general theory of relativity, we got a theory of gravity that works incredibly well when you apply it to big things. Stars, galaxies, clusters of galaxies, the things where gravity seems to really matter. In the 1920s and 1930s, another road of development gave us quantum mechanics. That�s a theory of the smallness things in the universe, molecules, atoms, sub-atomic particles, and in that domain quantum mechanics works incredibly well, too. So, we have a great theory for the small stuff: quantum theory. We have a great theory of the big stuff: Einstein�s general theory of relativity. But the problem is that each of these theories claims the other is wrong. They are mutually incompatible. Any attempt to merge together the laws of the small and the laws of the big has failed. Until the advent of string theory. String theory tries to mediate between the laws of the small and the big giving us a consistent theory of the small stuff and the big stuff and perhaps everything in between as well.

Einstein�s Unified Theory of Physics
For about 30 years, the last 30 years of his life, Albert Einstein sought what he called �The Unified Theory of Physics.� And by this, he meant a theory that would from one basic equation, one basic over-arching principle able to describe, in a sense, everything in the physical universe. And he never achieved this aim. Never found the unified theory. And, in retrospect, it�s actually not too hard to understand why. There are many fundamental features of the universe that were either completely unknown during his lifetime or at best poorly understood. But now, with the incredible advances in the last twenty or thirty years, we�ve been able to pick up where Einstein left off and we think through this new approach called String Theory, may have realized this Unified Theory. Again a theory which can describe all of nature�s forces, all of the matter that makes up the world around us, using essentially one basic idea.

Well, the basic idea of string theory is pretty straightforward. About 2,500 years ago, the ancient Greeks asked a question which has been with us every since: What is the stuff in the world made of? What is the fundamental ingredient making up matter? So, you take anything: a block of wood, a chunk of iron, slice it in half, slice that piece in half again, keep on cutting into ever-smaller pieces, what finally do you get to? Now we have learned in our age that sooner or later you find atoms. But, we�ve also learned that atoms are not the end of the line because they can be split. They are made up of smaller ingredients, little electrons that swarm around a central nucleus, it is called, which has in it smaller particles called neutrons and protons. And that�s not the end of the line either. It�s sort of like a sequence of Russian dolls inside the neutrons and protons are smaller particles called quarks. Now conventional theory and experiment stops there. Electrons and quartz are the basic ingredients of matter. String Theory comes along and says that there is one more layer of structure inside an electron, inside a quark. This theory suggests strongly there is a little filament of vibrating energy. The filament looks like a string. That�s why we call it string theory. And the great idea is that just like the string on a violin or a cello can vibrate in different patterns which our ears sense as different musical notes. The little strings in string theory also can vibrate in different patters. Now we don�t hear them. Rather we see them as different particles. So, the electron is a string vibrating in one pattern. A quark is a string vibrating in a different pattern. In that way, all of matter arises in a sense from the notes that the little strings in string theory can play. Now an important question is can you actually see these little strings? No, not yet, and perhaps we never will, because according to this theory, strings are really, really tiny. Just to give you a sense of how tiny, if you were to take a single atom and magnify it to be as large as the entire known universe. A little string in string theory would magnify under the same scale factor to be the size of a tree. So a tree is to the entire universe as a little string is to an atom. So we do not have the equipment necessary to actually see something that small so nobody has seen a string, nobody has in any way confirmed the ideas of string theory yet. But we hope that will happen sometime soon through indirect probes of the theory.

It�s unlikely that we�re going to have the equipment necessary to see strings in our lifetime or perhaps ever. But it�s a very common state of affairs in science that you don�t only test a theory by the brute force approach. The brute force approach would be you would peer down and you see the strings and that would settle it pretty definitely. We�re not going to do that. But there are indirect tests. For instance, this theory says that our world should have more particle species than we currently are aware of. We know of electrons and quarks and some other particles as well. But for each of the known particles that have been seen, this theory claims there should be a partner particle. It�s called a super partner particle. For the electron it�s the super symmetrical electron it�s called, or selectron for short. Quark, squarks, nutrinos, snutrinos, strange names, but that�s what they are called. Nobody has every seen the sparticles. We�ve seen the particles, but nobody has seen the sparticle partners yet. And we think the reason is this they are probably heavier than the known particles. A heavier particle means you need an atom smasher with greater energy to conjure the particle into existence.

Hadron Collider
So right now, people are building a large atom smasher in Geneva, Switzerland. It�s called the large hadron collider http://lhc.web.cern.ch/lhc/. It�s going to slam matter against matter at incredibly high energy and look for these new species of particles in the debris from those collisions. And if those species are found, it won�t prove string theory is correct, but it will be a very strong piece of circumstantial evidence, indirect evidence, that this theory is in fact on the right track. This should happen in the next five to ten years.

The idea that there have to be more particle species than the ones that we have seen is based upon a symmetry principle that is deeply ingrained within the structure of string theory itself. The symmetry principle is called super �symmetry.

It�s hard to know what the framework of the ultimate theory is going to be, if we ever find it. What we tend to do in science is change our previous theories incrementally so we have ideas that go back to Newton which were changed over the years and ultimately went through a significant upheaval with Einstein where the framework of space and time was changed. Quantum theory came along which then completely changed the notion of what it means for an object to have certain characteristics and what it means to be able to predict the future based upon what you know about the present. Things became probabilistic. Completely new idea from the 1930�s. And now with string theory we seem to going through another upheaval where the number dimensions that our universe has seems to be challenged and changed by string theory. The basic kind of ingredients that make up the theory, little strands of energy, little blobs of vibrating energy, compared with the previous idea of point objects, complete change. We can only guess that in the next 20, 40, 50 , 100 years there are going to be other revolutions which are likely to completely change the framework once again. I wish I could predict what that framework will be because I would be working on it if I could. But what we do now is we go step by step and we now are in the midst of the string theory upheaval. And we�re pursuing those ideas of extra dimensions and a complete change in our understanding of what the fundamental ingredients of matter are.

The reason that I got into mathematics and physics I can actually trace it fairly well because I remember the moment when I was walking to junior high school and I just kind of got gripped for a moment by the questions that everybody struggles with at that age and ever since. You know, questions of why am I here? What is this all about? What�s the point? And the thing that struck me at that moment though was that it was very unlikely that I was going to contribute anything to answering questions like that because people much smarter than me had struggled with them for thousands of years and apparently had not come up with anything particularly convincing. So I thought, well maybe the best thing for me to do, is not to try to answer those questions but just to become deeply familiar with the questions themselves. And deeply familiar with the arena in which the questions are asked, namely the universe itself. So it really seemed to me and it has seemed to me ever since that understanding how the universe came to be, how it evolved to take the form that we currently witness, the fundamental forces that are responsible for the way it evolves over time, understanding those questions and perhaps contributing a little bit to answering those much more limited questions would be the most satisfying and gratifying thing that I could do.
 







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