If we look at the forces of nature, it's surprising that the diversity, and complex phenomena of the natural world can be explained by only four fundamental forces. These forces have been known for a number of years, but are not completely understood. These four forces of nature have very different strengths. Gravity is by far the weakest of these forces. The next strongest is the weak force that operates within an atomic nucleus responsible for radioactivity, and next is the electromagnetic force responsible for electromagnetic radiation including light. The strongest is the force that binds the atomic nucleus. In addition to the very different strengths of these four forces, they operate in different ways. Gravity and the electromagnetic force are so-called inverse-square laws, their strength or intensity diminishes with the square of the distance from the source of radiation. So they have infinite range, because one over the square of a very large number never goes to zero. However, the nuclear forces operate over a short range, and are only responsible for binding together the atomic nucleus on a scale of 10 to the minus 15 meters. They do not operate beyond the atom. Putting these four forces in view together begs the question of how are they related? How are they related in an underlying sense in terms of physics? The job of physics is to explain the natural world as simply as possible, and to physicists, the four forces just like the large number of subatomic particles, is unnecessary complexity implying a deeper theory that might be able to explain them more simply. A hint of what might be going on emerges from accelerator experiments over the last few decades. If we take these four forces and see how they behave as you raise the energy or temperature in an experiment, an interesting thing happens. In the 1970s at CERN, it was shown that the electromagnetic force and the weak nuclear force, the one responsible for radioactivity, actually start to merge or melt together at very high energy. In the CERN experiments in the 1970s, the particles that carry these forces were understood, and the two forces were unified in a so-called electroweak force. This was a profound discovery, because it showed that the particular forces were part of a situation that is more general and more broadly described at high-energy. Speculating along the same lines, it was hoped or anticipated that if sufficient energy or temperature could be created, perhaps in an accelerator, we might see the merger of the electro weak force with the next strongest force, the strong nuclear force. These theories we called grand unified theories, and they're being tested right now. The final step in this unification conceptually is to bring in the weakest force of nature into the mix. The weakest force is so much weaker than the other forces that the temperature required or implied to bring them all into the same landscape or merge or melt them together is a phenomenal temperature, unobtainable in any accelerator. This force, of course, would be a force of everything, a theory of everything, requiring us to combine the theory of gravity and the theory of quantum physics. In this video clip, physicist Michio Kaku will talk about theories of everything based on strings, tiny sub-atomic entities that underlie all particles and allow us to create a theory that combines gravity with other forces. String theory is currently being tested we hope, but it's possible that those tests may be so difficult, this can't happen for another few decades. Einstein spent the last 30 years of his life trying to create a theory of everything, a theory of black holes of galaxies, and a theory of atoms of light of force. So we have two great theories of physics: the theory of the very big, Einstein's Theory of Relativity, and theory that very small the, Quantum Theory. These two theories don't like each other. They are incompatible. One is smooth, beautiful like marble, and the other one is coarse and grainy like wood. To get them to meet together has been the object of the last 50 years of intense investigation. Today we think we have it. We think we have the Superstring theory, which is perhaps the most fantastic, the most marvelous theory ever proposed in the history of science. String theory borders on mysticism. It contemplates a universe strewn with minute strands of space-time. Strings are extremely tiny like a hundred billion times smaller than a proton. So let me explain. Take a atom and expand it to the size of the solar system. The atom where the size of our solar system, and a string is much smaller than that, a string is the size of an atom. That is how incredibly tiny this is but we also think that once upon a time, the universe was the size of a string. String theory or superstring theory is based on an extraordinary premise. The particles themselves are illusions, that what's really going on is that nature is made of fundamental one dimensional entities called strings that are only manifest on scales dozens of orders of magnitude smaller than atoms, and are only accessible by energies dozens or of orders of magnitude higher than are available at room temperature, or even in accelerators. It's an extraordinary concept. It also predicts that all the phenomena of the universe, including all the things we consider subatomic particles, are produced by oscillations, interactions, mergers, and splits between these fundamental entities called superstrings. Physicists did not dream of string theory out of their heads. They were led to it by the mathematics of how to describe high-energy phenomena and particles themselves. They were led in the direction of a particular form of theory, where multidimensional space time is involved. Typically 7, 8, 10, or maybe even 12 dimensions of space-time, of which the three dimensions of space and one of time that we live in are just the subset manifest at low energies. At the extraordinarily high energies of string theory, these strings would be manifest along with these extra dimensions of space time, that do not exist in our everyday world. The physicists were drawn to the theory in part because of its mathematical beauty. Even though it's complex and extremely difficult to do calculations, it considered a beautiful theory. One of the strong threads in the history of physics is that beautiful mathematics underlies our most successful theories, plus the best example of this is general relativity, which Chandrasekhar, one of the greatest astrophysicists of the 20th century said is the most beautiful theory known. There's nothing extraordinary in principle about multi-dimensional space-time. In fact, the theory of multidimensional space time actually goes back a 150 years to Russian mathematicians such as Bolyai and Lobachevsky, who were extending beyond Euclidean space-time to countenance other dimensions. The mathematics of multidimensional space time is easy to countenance. You're simply adding in mathematics higher dimensions, whether or not they're observable in physical space. So we can simply extrapolate from three-dimensions: up, down, and sideways, to other spatial dimensions in realms that we can't visualize because we live in three dimensions. So String Theory embeds hidden dimensions. They're hidden because the energy does not exist in the everyday world to realize or manifest these extra dimensions. Tests of string theory are trying to figure out ways in which hints of these extra dimensions might be accessible through accelerators, or even through lab experiments. Probably, the only true manifestation of string theory would be in the very first fractions of a second of the universe in the Big Bang itself, which did embed these incredibly high temperatures. Another implication of string theory isn't the smoothness of space time, which is a premise of Newtonian gravity, and also of general relativity, is an illusion, that space-time is chaotic at the quantum level. We can imagine an evanescent and seething foam of space-time events based on string interactions, of which gravity is just one manifestation. String Theory represents a profoundly different concept of the physical universe. We've seen that the bottom-down theory starts with the biggest things we can observe and drills down to see what they're made of, working down from galaxies, to stars, to individual atoms, to subatomic structure. String theory is called a bottom-up theory. It starts with the fundamental entity of strings, and then says that everyday objects and the universe itself is built up from strings, and so atoms and molecules are just manifestations of this fundamental entity. It's a conceptually different way of thinking about nature, matter, and the universe itself. String theory is tentative, we have no theory of everything, it does not exist. Einstein spent the last 20 years of his career fruitlessly trying to unify gravity and the other forces of nature, which is what String Theory seeks to do too. If someone as smart as Einstein didn't succeed in this task, you know it's difficult. Some of the smartest physicists in the world are working on this problem, and it even led to a backlash where so many young physicists for a generation went into string theory that other physicists started to complain that their fields were being neglected, or that the progress in string theory was so slow that perhaps that was a waste of human talent. However, it is addressing one of the most profound questions of nature, what is matter made of? In the meantime with no theory of nature and no theory of everything to go with, everyone's free to have their own theory of everything. Another issue arises from imagining a theory of everything. What does that actually mean in terms of understanding the universe? Conceptually, imagine the universe consists of things that are and things that aren't, things that do happen and things that don't happen. The goal of physical theory is to explain everything that happens. That would be a theory of everything. Perhaps at the moment, our theories explain some small fraction of all the things that actually happen. But how do we know we have a theory of everything as opposed to a theory of too much? A theory that encompasses and perhaps even predicts phenomena that don't occur in the universe. Until we've tested some of these frontier ideas of String theory, we don't know whether we're on the right track. There are four forces of nature, and on the face of it they appear quite distinct. They spent dozens of orders of magnitude of range of strength to have infinite range, and to have short range within the subatomic nucleus. To try and make sense of these four forces, physicists have imagined that if the energy could be high enough, these forces would melt together into a super force. The first stage of this was demonstrated at CERN in the 1970s. The ultimate merger of all four forces involves a theory that unites gravity and quantum mechanics. No such theory exists. In the meantime, theoretical physicists are speculating on String theory. String theory holds that normal particles are in fact made of tiny one dimensional entities called strings, that are visible on scales, many orders of magnitude smaller than normal atoms, and only manifest on energy scales many orders of magnitude higher than energies available to us in the lab. String theory is based on beautiful mathematics of 10 dimensional space-time, as yet it's unverified. Some people think it may be a false direction in physics, but it's an extraordinary and ambitious attempt to understand the fundamental nature of matter.
