I have been recently reading Neil Turok & Paul Steinhardt's new book "The Endless Universe - Beyond the Big Bang" which I bought at the Quantum to Cosmos festival held at Waterloo a few months ago. This book provides a fascinating glimpse into the process of cosmological and theoretical physics research. Both authors recount their personal history, their introduction to cosmology, and how they became involved in research related to the Big Bang theory. Overall, a great read for those anxious of physics related advancements since the 2000s. Here is a snippet of one of my favourite chapters.
Let’s talk physics history. An ever-so exciting tale of young physicists that changed the mindset of physics for decades to come since the late 1960s.
“Make everything as simple as possible but not simpler.” – Albert Einstein.
Nowhere was the optimism of particle physicists in the early 1980s more evident than at the annual Workshop on Grand Unification, known by the acronym WOGU (pronounced “whoa-goo”). Each spring the leading physicists, their postdoctoral fellows, and their students would gather at a different site to discuss the latest experimental breakthroughs and theoretical advances. Every year, the exciting presentations at WOGU seemed to engender new confidence that quantum field theory and grand unification were on track … until the fourth WOGU, when a soft spoken young theorist politely suggested that a sharp turn in the current thinking might be needed.
The meeting took place in April 1983 at the University of Pennsylvania, in Philadelphia, about fifty miles from Princeton, New Jersey, the home of Edward Witten. Only thirty-two years old at the time, he was already recognized as a theoretical physicist of great vision. For years, he had been a much admired pioneer in exploring the theoretical underpinnings of grand unification.
When Witten was invited by one of WOGU’s organizers to give a presentation, surprisingly, Witten was reluctant to accept. He explained that he was working on something new and was not sure the topic would be appropriate for a meeting on grand unified theories. That only made the prospect more intriguing, and so with persistence, Witten finally agreed to speak.
When the time came for Witten to talk, the last of the meeting, the auditorium was packed to standing room only. In his characteristic calm and gentle voice, Witten began by noting ways in which the current attempts at grand unification were failing. The most dramatic prediction, the instability of protons, had been tested, but no decays had been seen. The predictions of the masses of matter particles had also turned out wrong. Physicists could adjust the models to evade these problems, but only at the cost of adding ugly complications that made the whole framework implausible.
Witten then suggested that it might be time to consider a totally new approach. He proposed three guiding principles. First, the new approach should include gravity from the outset. Particle physicists were used to ignoring gravity because the gravitational attraction between elementary particles is normally negligible. However, when particles were smashed together at high energies, their collective mass rises in accordance with Einstein’s famous equations E = mc2, and the effects of gravity become stronger and stronger. At the very high energies where the strong and electroweak forces seem to merge into a single unified force, gravity is nearly as strong. For this reason, Witten argues, gravity has to be included in any theory of unification.
Dealing with gravity would be no easy task. Einstein had developed his theory of gravity in the early part of the twentieth century, at the same time that quantum theory was emerging. Despite all attempts, the two strands of physics had never been successfully joined. Einstein’s theory works tremendously well on large scales for describing gravity on the Earth, the solar system, and in the universe. But just like electromagnetism and light, gravity must be formulated in a way that is consistent with the laws of quantum physics in order to make sense on microscopic scales. For the other three forces, the quantum field approach had been spectacularly successful. But for gravity, every attempt to quantize Einstein’s theory had failed, leading to infinities, negative probabilities, or, at best, an infinite number of indeterminate parameters. A totally new approach was needed, one that would give a sensible answer.
Everyone in the audience knew about these difficulties in building a quantum theory of gravity. So all in attendance were naturally anxious to learn what Witten had in mind. Witten emphasized that he did not deserve credit for the idea he was going to suggest. Hard work had been done by a small, intrepid group of theorists working largely unnoticed and unappreciated. But Witten was now advocating, as his second principle, considering their daring proposal: a conceptual framework known as string theory.
Many in the auditorium had heard of string theory before, but most knew little about its history because it had had little impact on mainstream particle physics or cosmology up to that point. String theory had been developed in a rather roundabout way.
In 1968, Gabriele Veneziano at the European Organization for Nuclear Research (CERN) had proposed a formula for describing the scattering of nuclear particles interacting via the strong nuclear force. In 1970, Yoichiro Nambu at the University of Chicago, Holger Nielsen at the Niels Bohr Institute in Copenhagen, and Leonard Susskind, then at Belfer Graduate College in Israel and now at Stanford University, showed that Veneziano’s formula could be interpreted as a model of vibrating one-dimensional strings. Unfortunately, it was soon discovered that the model had various pathologies, such as a tachyon, a physically impossible particle that moves faster than light. But this problem was cured as people realized that string theory was much more than a theory of nuclear particles. First, Joel Scherk at the Ecole Normale Superieure in Paris and John Schwarz at the California Institute of Technology showed that string theory included a particle behaving like a graviton, the troublesome quantum of Einstein’s theory of gravity. Then, by incorporating matter particles using a powerful new quantum symmetry called super-symmetry, Scherk with David Olive and other physicists managed to construct a completely consistent model with no tachyon.
In this way, the theory originally designed to describe the strong nuclear force was suddenly transformed into a unified theory with the potential to describe all the forces and particles in nature, including quantized gravity. But these developments went largely unnoticed. The 1970s were the heyday of quantum field theory, and string theory was seen as a speculative backwater. A few lonely theorists continued to struggle to develop the theory and iron out its remaining mathematical difficulties. This was a daunting and slow process, since few people were willing to risk working on the subject.
Witten’s talk went on to describe the advantages of reinterpreting elementary particles as tiny spinning bits of string. Just as Einstein pictures three-dimensional space as an elastic substance that can be stretched and distorted, you can think of string as a geometrical curve with no width that can bend and turn in all possible ways, like an infinitely thin strand of rubber. The string is perfectly elastic, so it can shrink to a point or be stretched out to an arbitrary length. If you stretch a piece of string out in a straight line, the free ends pull together with a fixed force called the string tension.
Some of the properties of string are actually very similar to those of cosmic strings. But whereas cosmic strings are really twisted-up configurations of fields with a minuscule but finite width, fundamental strings are ideal one-dimensional mathematical curves.
The string picture is beautiful in that one basic entity – string – can potentially account for the myriad of elementary particles observed in nature. Bits of string vibrate and spin, in certain specific quantized motions. Each new quantized state has a set of physical attributes: mass, charge, and spin. The little pieces of string describing photons, electrons, or gravitons are far too tiny to be seen, much less than a trillionth the diameter of a proton. To us, they appear like pointlike particles. But if string theory is correct, the masses, charges, ad spins of these little bits of string should precisely match the physical properties of all of the particles ever discovered.
Witten was especially attracted to this picture because it included gravitons as a hidden bonus, as Scherk and Schwarz had first shown. Bits of string with two free ends could account for all known types of matter particles. But the mathematics of string also allows for closed loops, like tiny elastic bands. When vibrating and spinning in just the right way, these loops have the same properties as gravitons, the quanta of the gravitational field. Even better, while calculations assuming pointlike particles and gravitons give nonsensical, infinite answers, calculations for stringy particles and loopy gravitons produce sensible, finite results. Although not designed for the purpose, string theory appears to automatically incorporate a theory of quantum gravity without infinities.
The reason string theory works where the particle description of quantum field theory fails can be explained by simple geometry. If two pointlike particles collide, their energy is concentrated at a point. Such pileups of energy cause a large gravitational field, curving space and drawing even more energy into the region. A runaway process ensues in which space curls up irretrievably into a tinier and tinier knot: a singularity. This catastrophe leads to mathematical infinities signaling a breakdown of the theory. On the other hand, if particles are tiny vibrating strings, their energy is spread out. If a collision causes a momentary pileup of energy, the string rapidly wriggles away and spreads out the energy, preventing the gravitational distortion from concentrating in one spot. Calculations of what happens when two bits of string collide, join, and break apart again give sensible, finite results. There are no singularities, and no infinities.
Witten’s third guiding principle dealt with the major hitch theorists had previously discovered about string theory. The equations describing the quantized vibrations of strings give sensible answers only if the number of spatial dimensions is nine. Nine!? To most physicists, this seemed absurd. Why study a theory that predicts six extra dimensions of space that have never been seen?
Witten addressed the problem of extra dimensions head-on: Learn to live with them, he said. Just accept the six extra dimensions of string theory; they are an essential aspect of the geometry of the universe. He reminded the audience that back in the 1920s the Swedish physicist Oskar Klein, building on the work of the German physicist Theodor Kaluza, had dreamed up a way of linking Maxwell’s electromagnetic theory with Einstein’s theory of gravity, in a model of the universe where one extra dimension of space was hidden from view.
To see how this works, consider the surface of a long soda straw. From a long distance away, it appears to be one-dimensional because you cannot detect its thickness. But up close, you can see the surface of the straw. To prove to yourself that the surface is two dimensional, slit the straw along its length and flatten it out. You will get a rectangle, a shape that is obviously two dimensional because it has both length and width.
Klein supposed that in addition to the three familiar dimensions of height, width, and length, there is a fourth dimension of space that is curled up in a circle so tiny that it cannot normally be seen. Kaluza and Klein’s remarkable discovery was that Einstein’s theory of gravity in four space dimensions, with one of the dimensions curled up as described, contained both Einstein’s theory of gravity in the remaining three extended dimensions and Maxwell’s theory of electromagnetism. Electric and magnetic fields arise, in this picture, from a “twisting” of the small extra dimension as you move along one of the large everyday dimensions.
According to Witten, theorists simply had to adapt Klein’s idea to the six extra spatial dimensions in string theory. There is no problem having strings wiggle in nine spatial dimensions, so long as six of the spatial dimensions are too small to be seen.
The extra dimensions would exist at every point in three-dimensional space. As an analogy, I’m going to use a rather famous one - consider a pile carpet made of woolen loops. To us, looking from above, it appears as a two-dimensional surface. But to an ant it seems like a huge forest of loops. At any point, the ant can choose to run along the direction of the floor, that is, along one of the two extended dimensions, or around one of the woolen loops that describe the curled-up dimension. In the same way, the extra dimensions in Kaluza and Klein’s approach are invisible, because their tiny size is too small to be seen. But in principle, with a very powerful microscope using very short wave-length radiation, one would be able, like the ants on the pile carpet, to see the convoluted structure of the extra dimensions on tiny length scales.
Witten framed his lecture carefully and peppered it with qualifications, but his message was clear. In a mere forty minutes, he made a compelling case that theories of grand unification were incomplete and that gravity, strings, and extra dimensions ought to be considered. Research on the fundamental laws of physics could be headed toward a revolution, he quietly suggested. You could have heard a pin drop in the auditorium as many physicists described. The audience was stunned, unsure how seriously to take Witten’s remarks.
Through the remainder of 1983, there were few signs that anything was going to change. During the Aspen summer workshop that year, for example, the talk was almost all about grand unification and field theory. But, sure enough, Witten’s lecture was the harbinger of a revolution that would soon sweep the world. The “first string revolution,” as it has been called, was ignited a year later at the 1984 Aspen workshop when Michael Green, then at Queen Mary College, London (now at Cambridge), and John Schwarz overcame a key mathematical roadblock in the construction of realistic string theories.
Until that point, there were many versions of string theory with different ways of folding the extra dimensions, but they all seemed to be fatally flawed. Witten had recently shown that many versions of string theory are unacceptable because they violate the conservation of energy through a quantum effect known as an anomaly. Green and Schwarz’s breakthrough was the identification of a special version of string theory that had realistic matter particles and no anomalies. Now, for the first time, one could point to a quantum theory that incorporated gravity and other forces and gave finite, sensible answers.
Working at Princeton, David Gross, one of the leading pioneers of unified quantum field theories (now director of the Kavli Institute for Theoretical Physics in Santa Barbara), along with Jeffrey Harvey and Emil Martinec (both now at the University of Chicago) and Ryan Rohm (now at Boston University) produced a compelling example known as heterotic string theory. The word heterotic, meaning hybrid, was added because it combined different versions of string theory to obtain one that has more of the ingredients needed to make a realistic theory of elementary particle physics. (A later, further improved form, heterotic M theory, was the stimulus for the Cyclic model of the universe.) These successes, and others that followed in rapid succession, captivated the international community of theoretical physicists. Almost overnight, it seemed, the focus of research shifted from particles to strings. And the merger of fundamental physics and cosmology that had seemed imminent in 1983 was put on hold.
On an unrelated note, i'm going to be posting less often as exam season is approaching. Oh the suspense ... Hooray for new banner & layout!
