Physics of the Universe: Exploring the Unknown

Transition from Classical Physics to Quantum Mechanics:

At the end of the nineteeth century, classical physics had progressed to such a level that many scientists thought all problems in physical science had been solved or were about to be solved. After all, classical Newtonian mechanics was able to predict the motions of celestial bodies, electromagnetism was described by Maxwell's equations, the formulation of the principles of thermodynamics had led to the understanding of the interconversion of heat and work and the limitations of this interconversion, and classical optics allowed the design and construction of scientific instruments such as the telescope and the microscope, both of which had advanced the understanding of the physical world around us. In chemistry, an experimentally derived classification of elements had been achieved (the rudimentary periodic table), although the nature of atoms and molecules and the concept of the electron's involvement in chemical reactions had not been realized. The experiments by Rutherford demonstrated that the atom consisted of very small, positively charged, and heavy nuclei that identify each element and electrons orbiting the nuclei that provided the negative charge to produce electrically neutral atoms. At this point, the question naturally rose: Why don't the electrons fall into the nucleus, given the fact that opposite electric charges do attaract? A planetary-like situation where the electrons are held in orbits by centrifugal forces was not plausible because of the (radiative) energy loss an orbiting electron would exeperience. This dilemma was one of the causes for the development of quantum mechanics. In addition, there were other experimental results that could not be explained by classical physics and needed the development of new theoretial concepts, e.g. the inability of classical models to reproduce the blackbody emission curve, the photoelectric effect.

The general properties of Quantum Chromodynamics:

QCD possesses two remarkable properties. The first is the property of confinement: quarks and gluons cannot leave the region of their strong interaction and cannot be observed as real physical objects. Physical objects, observed experimentally at large distances, are hadrons- mesons and baryons. The second important property of QCD is the spontaneous violation of chiral symmetry. The masses of light up (u), down (d), strange (s) quarks that enter the QCD Lagrangian, especially the masses of "u" and "d" quarks from which the usual (non-strange) hadrons are built, are very small as compared to the characteristic QCD mass scale M~1GeV. In QCD, the quark interaction is due to the exchange of vector gluonic field. Thus, if light quark masses are neglected, the QCD Lagrangian (its light quark part) becomes chirally symmetric, i.e. not only vector, but also axial currents are conserved. In this approximation the left-hand and right-hand chirality quark fields do not transform into each other. However, this chiral symmetry is not realized in the spectrum of hadrons and their low energy interactions. Indeed, in chirally symmetric theory fermion states must be either massless or degenerate in parity. It is evident that baryons (particularly, the nucleon) do not possess such properties. This means that the chiral symmetry of the QCD Lagrangian is spontaneously broken. According to the Goldstone theorem, the spontaneous breaking of symmetry leads to the appearance of massless particles in the spectrum of physical states- the Goldstone Bosons. When QCD was developed, it was proved that the appearance of Goldstone bosons is a consequence of spontaneous breaking of chiral symmetry of the QCD vacuum which leads to the vacuum condensates violating the chiral symmetry. It was also established that baryon masses are expressed through the same vacuum condensates.

THE THREE BODY PROBLEM:

The three body problem arises in many different context in nature. It is an old problem and logically follows from the two body problem which was solved by Newton in his Principia in 1687. Newton also considered the three body problem in connection with the motion of the Moon under the influences of the Sun and the Earth. There are good reasons to study the three-body gravitational problem. The motion of the Earth and other planets around the Sun is not strictly a two-body problem. The gravitational force by another planet constitutes an extra force which tries to steer the planet off its elliptical path. The solution of the three-body problem remained elusive even after two hundred years following the publication of Principia. In the general three-body problem all three masses are non-zero and their initial positions and velocities are not arranged in any particular way. The difficulty of the general three-body problem derives from the fact that there are no coordinate transformations which would simplify the problem greatly. Thus, unlike the two-body problem, the three-body problem has no general closed-form solution, meaning there is no equation that always solves it. When three bodies orbit each other, the resulting dynamical system is chaotic for most initial conditions. Because there are no solvable equations for most three-body systems, the only way to predict the motion of the bodies is to estimate them using numerical methods. In an extended modern sense, a three-body problem is any problem in classical mechanics or quantum mechanics that models the motion of three particles. Atomic systems e.g. atoms, ions, and molecules, can be treated in terms of the quantum n-body problem. The three-body problem is special case of the n-body problem, which describes how n objects move under one of the physical forces, such as gravity.

Albert Wallace Hull (19 April 1880 - 22 January 1966)

An American physicist who invented the magnetron and the thyratron. A magnetron is a high-vacuum tube containing a cathode and an anode, the latter usually divided into two or more segments. A constant magnetic field modifies the space-charge distribution and the current-voltage relations. In modern usage, the term "magnetron" refers to the magnetron oscillator in which the interaction of the electronic space charge with the resonant system converts direct-current power into alternating-current power, usually at microwave frequencies. A thyratron is a type of gas-filled tube used as high-power electric switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Hull also independently discovered the powder method of X-ray analysis of crystals. This method allows the study of crystalline material in a finitely microcrystalline state. He also studied noise measurements in diode and triode and he took interest in metallurgy and glass science. He wrote a paper on the effect of uniform magnetic field on the motion of electrons between coaxial cylinders.

"The World As I See It."

18 April 1955 Einstein died at the age of 76. This is how he saw himself throughout his life: "The physicists say that I am a mathematician, and the mathematicians say that I am a physicist. I am a completely isolated man and though everybody knows me, there are very few people who really knows me." The influence of Einstein's work in physics has been enormous. It spans from the smallest (standard model of elementary particle physics, with the theory of relativistic quantum fields as a framework) to the largest (the structure of the Universe, with general relativity ruling the global geometry and dynamic evolution in cosmology). From the simplest (the gyromagnetic ratio of the electron) to the most complex (the collapse of a supernova). From the lowest energies (Bose-Einstein condensates at temperatures as low as a few nanoKelvin) to the higher(quark-gluon plasma). From the commonest application (global positioning system) to the most sophisticated techniques (non-linear atomic optics). Einstein's impact is not restricted to physics and associated disciplines. His ideas have also made a mark on modern culture from art to poetry. They have shaped the theory of knowledge and philosophy as well. Einstein rejected an empirical explanation for the origin of the physical concepts, which he considered a free creation of the human mind. But mere logical thinking does not provide us with knowledge of the external world, which only experience gives us. Nevertheless, concepts help organize sensory experiences and, so far, Nature has always appeared on the side of simplicity and mathematical beauty. In the realm of philosophy, relativity theory forced philosophers to revise their conception of space, time and matter, and later, to take part on the local realism which impregnated the critical position of Einstein on quantum physics. Experimental results appear to stubbonly reject this position.

THE GEOMETRIZATION OF PHYSICS:

Geometry is a methamatical model for describing both invariant geometric properties and their representation by local coordinates. In ancient times, one only considered invariant geometric properties. The description of geometric properties by coordinates dates back to René Descartes (1596-1650). In 1667 Descartes published his "Discours de la méthode" which contains, among a detailed philosophical investigation and its application to the sciences, the foundation of analytic geometry (e.g., the use of Cartesian coordinates). Einstein geometrized gravitation in his 1915 general theory of relativity. Quantum mechanics was geometrized by Dirac, as a unitary geometry of Hilbert spaces. In the introduction to his book "The Principles of Quantum Mechanics," Clarendon Press, Oxford, 1930, the young Dirac (1902-1984) wrote:
"The important things in the world appear as invariants...The things we are immediately aware of are the relations of these invariants to a certain frame of reference...The growth of the use of transformation theory, as applied first to relativity and later to the quantum theory, is the essence of the new method in theoretical physics."

STRING THEORY: HOPES AND DISSAPOINTMENTS

By 1984, physicists had spent the previous decade looking without much success for a new theory that would transcend the limitations of the Standard Model. A very small number worked not on quantum field theory, but on string theory. In string theory, the role of point-like particles is replaced by one-dimensional elementary objects, the "strings." During the summer of 1984, Michael Green and John Schwarz were able to show that one possible source of inconsistences canceled out in two string theories. This encouraged Witten to start working in this area and many others soon joined. Witten and others quickly priduced examples of string theories they hoped would both contain gravity and reproduce the Standard Model at low energies, finally providing a truly unified theory, a so-called 'Theory of Everything.' String theories at low energies have a massless spin two particle that can play the role of a graviton and provide a gravitational force. Consistency of string theory requires that it have a property called "supersymmetry", and that the strings move in a space-time of ten dimensions. To connect string theory to the observed world, one has to find some way to make 6 of these dimensions invisible, and one has to account for the fact that observed physics does not have the property of supersymmetry. From 1984 to the present day, efforts to find a way around these problems have dominated efforts to find a way to use string theory to unify physics. "String Theory" does not actually refer to a well-defined theory, rather to a set of approximate rules for calculating what happens with quantum strings interacting weakly with each other.The rules are only fully consistent in the limit of zero interaction between strings, but in this limit one cannot get non-trivial physics. String theory can be described as not really a theory but a hope that a theory exists which in weakly interacting approximation is described by rules understood

The Nobel Prize in Physics has been awarded 117 times to 224 Nobel Prize Laureates between 1901 and 2023. John Bardeen is the only laureate who has been awarded the Nobel Prize in Physics twice, in 1956 for the invention of the transistor and in 1972 for a fundamental theory of conventional superconductivity known as the BCS theory. The first Nobel Prize in Physics was awarded to Wilhelm Röntgen in 1901 for the discovery of X-rays. We last witnessed the 2023 nobel prize awarded to Pierre Agostini, Ferenc Krausz and Anne L'Huillier for their discovery of "experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter." The attosecond laser pulses reveal the hidden world of electron dynamics within atoms and molecules. The techniques helps us to peer inside atoms to the scale of electrons, which were previously moving too fast for us to see- we didn't have a strobe light fast enough to resolve the motion. That new window into the natural world allows us to probe electron dynamics and molecular systems, which are at the heart of the chemical and physical interactions of materials that underpin all our electronic, chemical, and medical innovations and technology. The 2024 Nobel Prize in Physics will be announced on Tuesday 8 October 2024. Who do you think will be the 2024 laureates and which discoveries do you think are worth a Nobel Prize to be announced this week?

On this day, 4 July, in 2012, the discovery of the Higgs Boson was announced:

Origins in condensed matter physics:

A Higgs Boson is a particle whose existence is predicted in a class of quantum field theories in which a symmetry under a Lie Group of transformations of the fields is spontaneously broken by an asymmetric vacuum state. It is a quantum of certain excitations of the order parameter. Such spontaneous symmetry breaking was first proposed as a feature of theories of elementary particles in 1960, but it has a much longer history in the context of condensed matter theory. The earliest relevant example is ferromagnetism, as explained by Heisenberg in 1928; an array of electron spins with nearest-neighbour interactions which favour energetically parallel over antiparallel configurations has a ground state in which all the spins are aligned in some direction, thus breaking the rotational symmetry of the dynamics. Another example, which comes closer to the kind of symmetry breaking which is of interest in particle physics is superfluidity. In 1947, Bogoliubov studies Bose condensate of an infinite system of neutral spinless bosons with short-range repulsive two-body interactions. Such a condensate is characterised by a "macroscopic wave function" (the order parameter) which is complex; its modulus squared is a measure of the observable condensate density, but its argument (which is unobservable) is arbitrary, thus breaking the symmetry of the dynamics under rotations of the boson wave functions in the Argand diagram. The short range interactions are represented in the second-quantized Hamiltonian by a term proportional to the square of the particle density, that is, to a quartic in the components of the scalar quantum field.

Searches for free quarks and limits of the model:

Quarks were introduced in order to explain the regularity and the symmetry properties of hadron spectroscopy. Since the first formulation of Gell-Mann and Zweig in 1964 the question regarding the possible existence of free quarks fuels continuing interest. Many searches for free quarks have been performed, all without success (although some have indicated possible, but not confirmed, signals.) Quantum Chromodynamics is consistent with the hypothesis of quark confinement within hadrons. However, the search for free quarks continues at increasing levels of precision. The research is based on the fact that any free quark or quark tied in nuclei corresponds to a fractional charge. Two research lines are followed: (1) the search for free quarks in terrestrial and extraterrestrial stable matter and (2) the search for fractional charged particles from high energy collision or in the penetrating cosmic radiation. Examples of the type (1) methods are experiments like that used by Millikan for the measurement of the electron q/m ratio or experiments dealing with magnetic levitation. These experiments used microscopic samples of matter. The best limits obtained are less than one free quark over 10²² nucleons of stable matter. Particles with fractional electric charge were searched for amongst the products of hadron-hadron, lepton-nucleon and e⁺e⁻ inelastic collisions. The searches are based on the fact that particles with +/- 1/3 and +/- 2/3 electric charge ionize 1/9 and 4/9 compared to a muon with the same momentum. The best limits obtained are at the level of less than one quark for many millions of ordinary particles. Some studies were devoted to the search for free quarks after a possible deconfinement phase in nucleus-nucleus collisons at high energies.

RIPPLES IN THE FABRIC OF SPACETIME:

Soon after the proposal of general relativity (GR), Einstein predicted the existence of gravitational waves (GWs) and estimated its strength from the wave equation he obtained in his 1916 paper on "Approximative Intergration of the Field Equations of Gravitation". Toward the end of his paper, he obtained the expression of the radiation 'A' of the system per unit time in GR as A= (κ/24π)∑αβ(∂³Jαβ/∂t³)² with Jαβ defined as the time-variable components of moment of inertia of the radiating system (κ=8πGₙ interms of the Newtonian gravitational constant Gₙ). He then continued that "The expression (for the radiation 'A') would get an additional factor 1/c⁴ if we would measure time in seconds and energy in Ergs (erg). Considering κ= 1.87x10⁻²⁷ (in units of cm and gm), it is obvious that 'A' has, in all imaginable cases, a practically vanishing value." Indeed at that time, possible expected source strengths and the detection capability had a huge gap. However, with the strides in the advances of astronomy and astrophysics and in the development of technology, this gap is largely bridged. The existence of GWs is the direct consequence of GR and unavoidable consequence of all relativistic gravity theories with finite velocity of propagation. The role of GW in gravity physics is like the role of electromagnetic waves in electromagnetic physics. The importance of GW detection is two-fold: (i) as probes to explore fundamental physics and cosmology, especially black hole physics and early cosmology and (ii) as a tool in astronomy and astrophysics to study compact objects and to count them, complement to electromagnetic astronomy and cosmic ray (including neutrino) astronomy. The existence of gravitational radiation is demonstrated by binary pulsar orbit evolution. In GR, a binary star system would emit radiation in the form of GWs.

Deciphering Gravitational Waves:

Gravitation is not presently included in the Standard Model of particle physics, nor in advanced extensions, such as those foreseen by Grand Unification Theories. However, there is an aspect of gravitation that is strongly connected to particle physics, astrophysics and cosmology: gravitational waves. Gravitational waves are traveling ripples in space-time, generated when heavy cosmic objects accelerate. These distortions, described as waves, move outward from the source at the speed of light. They were predicted based on Einstein's general theory of relativity. Gravitational waves transport energy in a form of radiant energy similar to electromagnetic radiation. In contrast to the incoherent superposition of emissions from the acceleration of individual electric charges, gravitational waves result from coherent, bulk motions of matter. Because they transfer very small amounts of energy to matter, gravitational waves are able to pe*****te the very densely concentrated matter that produce them. The years 2016 and 2017 saw the dawn of astrophysics and cosmology with gravitational waves, awarded with the 2017 Nobel Prize in Physics. This first direct observation of a gravitational wave is a milestone, and not only for proving a means to investigate general relativity in a previously inaccessible regime. In fact, gravitational waves allow for exploring the distant non-thermal Universe in a way completely independent of electromagnetic radiation. The events of September 14th, 2015 (denoted as GW150914, the first black hole-black hole merger) and August 17th, 2017 (GW170817, the coalescence of two neutron stars producing a short gamma-ray burst and follow-up observed by 70 observatories on all continents and in space) represent true milestone in science. Gravitational waves are the last probes for multimessenger studies, after charged cosmic rays, γ-ray and neutrinos.