From an early age David Gross was attracted to physics and mathematics. “I was fascinated by theoretical physics and determined to become a theoretical physicist. I had no real idea what that meant, but it seemed incredibly exciting to spend one's life attempting to find the secrets of the universe by using one's mind ”, he says at his Nobel Prize biographical note.
The “2nd symposium of the seven wise men of the world in cosmology” took place in Athens and in Ancient Messinia, 2-4 October 2015. The aim of this international meeting was to present cutting edge knowledge and concerns faced by the contemporary science of cosmology such as the birth, the energy content and the evolution of the universe. David Gross presented his view inspired by the program for the unification of all forces in nature, a program to which he contributed a lot. His ability to present complex scientific ideas simply to general audience is striking.
Raised in United Stated and Israel, he studied at the Hebrew University and received his PhD at Berkeley, California. He was Fellow at Harvard University and professor at Princeton. Gross finds that his best ideas “emerge in heated exchanges with others.” His approach to dealing with students was to involve them in ambitious projects that he was working on. The fact that two of the students that he advised, Frank Wilczek and Ed Witten were awarded the Nobel Prize and the Field Medal respectively manifests this.
Democrat and antimilitarist, when referring to religion he declares atheist. Optimist about the progress of the physical sciences, Gross draws this faith “from the capacity of the human civilization to push the frontiers of knowledge to regions obscured by ignorance. And if our civilization has the will and the means to proceed, then one day, we may find a final theory given that our “sea of ignorance” has a finite amount.” Of course if this happens, he remarks, “this will not mean the end of the physics: structures and forms to explore and describe coming from such a fundamental theory will be considerably unlimited.”
During his stay in Athens, Gross visited the School of Applied Mathematics and Physical Sciences of the National Technical University. He gave a remarkable two-hour talk that fascinated the students, who occupied the main amphitheatre of the School, triggering profound questions and discussion to the audience.
The dynamics of the nuclear microcosm
In 1973 David Gross, a thirty-two years old researcher, tenure in the University of Princeton, together with his PhD student F. Wilczek managed to unravel the dynamics of quarks: the concealed constituents of the proton and neutrons that were never observed directly or found in isolation. They introduced the theory of quantum chromodynamics characterized by eight force carriers, called the gluons, which have a unique and highly unexpected property: they interact not only with quarks but also with each other!
But how is it impossible to find quarks in isolation? Why the dynamics of quarks differ so much from that of electrons and we cannot use quarks to construct “chromo-circuits” or generate “chromo-waves” as we daily do so in the circuits and antennas of the countless electronic devices around us?
The complicated structure of the chromo-vacuum on a scale of a proton
“Remarkably, all has to do with the properties of the vacuum”, David Gross mentions. In a quantum field theory we cannot just say that the ground state of the empty space is the state with no quanta, with no energy content. Actually, the ground state of the fields does contain quanta and the vacuum is never empty. This effect is called the vacuum polarization and in chromodynamics this effect is extremely strong. The empty space contains a soup of spontaneously appearing, interacting, and disappearing gluons as well as a sea of virtual quark-antiquark pairs that also emit and absorb more virtual gluons.
“Putting quarks in such a vacuum is like putting quarks into a medium. And it is this active vacuum itself that affects the force law between quarks and makes it to differ so dramatically from the Coulomb and the Gravitational force laws that fall off with the squared of the distance” Gross explains.
In the subnuclear realm of chromodynamics when the distance between quarks is increased the binding force also increases. This phenomenon is known as colour confinement. That is why a quark can never be removed from an atomic nucleus and appear in isolation. On the contrary, the closer quarks come to each other the force weakens. The theoretical description of this property, called asymptotic freedom, gave the Nobel Prize to Davis Gross, David Politzer and Frank Wilczek.
The vacuum of quantum chromodynamic fluctuations has indeed an extremely complicated structure. This sea of quark-antiquark pairs and gluons dominates the energy density of our body and any visible matter. It is striking that most of our weight is due to the movements and processes in the interior of the protons and neutrons! The inevitable conclusion is that, quantum chromodynamics – the interactions between quarks- rule our energy content even though we have never felt this strong microscopic force!
The way the forces of Nature unify influences the evolution of the early Universe
From quarks and gluons to string theory
One of the most intriguing features of QCD asymptotic freedom is that forces of nature depend on what scale you are looking at! This opens up the possibility of a unified description of Nature's forces.
Unification is the old dream of physicists, who would like to describe the laws of Nature in the simplest language possible. Unified theories, are schemes that go beyond the Standard Model of particle physics. The Standard Model, although it accounts for an extremely successful theory, needs modifications to incorporate the discovered properties of neutrinos that they have a non-zero mass (Nobel Prize 2015!) and cosmological enigmas such as the dark matter. According to Gross string theory provides the best framework for realizing this concept.
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A visualization of the 10 dimensional string theory
Geometry: The guide to the ultimate unification
In the early 1980's, Gross began to work on string theory again and in 1984, together with a group of younger collaborators. They discovered the heterotic string, which at the time seemed to offer the possibility of explaining the Standard Model from string theory.
Heterotic was named after the Greek name “heterosis”, which means “hybrid vigor”. “We looked up a dictionary to find the proper word”, Gross says. The reason is that string consists of a closed string that has two types of vibrations, clockwise and counterclockwise, which are treated differently. The clockwise vibrations live in 10-dimensional space. The counterclockwise live in 26-dimensional space, of which 16 dimensions have been compactified!
David Gross emphasises in front of the charmed audience of students of NTU Athens, “string theory builds matter itself from geometry. Matter and the forces of the nature emerge in the same way gravity emerges form geometry.”
And he adds, “The Greek philosophers and mathematicians would love that the beauty and simplicity of superstring framework come from an underlying geometric principle.”
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David Gross with his wife and Professor Zoupanos visiting Poseidon Temple after the talk at Natl. Tech. U. Athens, October 2015
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David GrossDavid Gross is a renowned theoretical physicist. He was awarded the 2004 Nobel Prize in Physics for the discovery of asymptotic freedom. He is one of the minds behind the formulation of the heterotic string theory in 80s. David Gross visited Greece to participate in the “2nd symposium of the seven wise men of the world in cosmology” 2-4 October 2015.