Einstein's general theory of relativity dominates the science of the very large, including gravity and cosmology. Quantum theory reigns supreme on the scale of atoms and smaller. What is more natural than to try to unify these two theories into a single quantum theory of gravity? Modern theories of gravity, including superstring theory, give quantum properties to space and time. These theories have been inspired by Dirac's principles of beauty and elegance. But Dirac had the advantage of experimental evidence, and modern theories of quantum gravity do not. Moreover, beauty and elegance are subjective – they are a guide to the correct theory, not a guarantee. For any scientific theory, experiment is the final test.
Einstein's general theory of relativity dominates the science of the very large, including gravity and cosmology. Quantum theory reigns supreme on the scale of atoms and smaller. What is more natural than to try to unify these two theories into a single quantum theory of gravity? Modern theories of gravity, including superstring theory, give quantum properties to space and time. These theories have been inspired by Dirac's principles of beauty and elegance. But Dirac had the advantage of experimental evidence, and modern theories of quantum gravity do not. Moreover, beauty and elegance are subjective – they are a guide to the correct theory, not a guarantee. For any scientific theory, experiment is the final test.
Modern physics relies on two distinct fun- damental theories, General Relativity and Quantum Mechanics. Both describe on one hand macroscopic and cosmological phenomena such as gravitational waves and black holes and on the other hand micro- scopic phenomena as superfluidity or the spin of par- ticles. The unification of these two theories remains, so far, an unsolved problem. Interestingly, candidate Quantum Gravity theories predict a violation of the principles of General Relativity at different levels. It is, therefore, of a timely interest to detect violations of these principles and determine at which level they occur.
“Despite almost a century’s worth of study, it is still unclear how general relativity (GR) and quantum theory (QT) should be unified into a consistent theory. The conventional approach is to retain the foundational principles of QT, such as the superposition principle, and modify GR. This is referred to as ‘quantizing gravity’, resulting in a theory of ‘quantum gravity’. The opposite approach is ‘gravitizing QT’ where we attempt to keep the principles of GR, such as the equivalence principle, and consider how this leads to modifications of QT. What we are most lacking in understanding which route to take, if either, is experimental guidance. Here we consider using the Dirac–Pauli representation of free Dirac equation to search for the clues. In particular, we study how the double-slit diffraction experiment, when only one Dirac particle (‘say’ electron) at a time is in the apparatus, could test a gravitizing QT proposal in which the wavefunction collapse emerges on the detector screen from the Dirac–Pauli representation, as an objective process, resolving the measurement problem of QT.” [1] We found that the Dirac–Pauli representation is inherently a unified theory embodying a gravitizing QT proposal that Dirac–Pauli representation as wave nature is manifestation of inertial mass and the Foldy-Wouthuysen representation as particle nature is manifestation of gravitational mass such that the fact, that same physics is involved in these two representations, is simply the validity and exhibition of the principle of equivalence, of GR, in QT. [1] Richard Howl et al 2019 New J. Phys. 21 043047 DOI 10.1088/1367-2630/ab104a
Copernicus broke with 1300 years of tradition, and revolutionized science by moving from earth-centric to sun-centric view. But, moving the viewpoint to the center of our expanding universe explains the origin and true nature of time itself. From this God’s view, Relativity and Quantum Mechanics are two sides of the same coin. Relativity is inside the light cone phenomena (since nothing can travel faster than light), while Quantum Mechanics is outside the light cone phenomena (allowing instant communications in quantum entanglement experiments). This recent paper [Quantum principle of relativity; Andrzej Dragan, Artur Ekert, New. J. Phys. 22 (2020) 033098] has shown that every exotic Quantum effect like superposition, entanglement, probabilistic behavior, multiple paths etc. can be explained just by allowing superluminal possibility. The inside the light cone phenomena and the outside the light cone phenomena together span the entire region within the space and time axes. Only in unison they complete the entire picture. Both are dictated just by the scale (i.e. whether we use classical/human scale or sub-atomic scale). We failed to realize that the same spacetime is getting split into space like and time like regions based on size scale. And the reason behind this is not the magical (?) speed of light. That would have turned both relativity and quantum mechanics into mere branches of electromagnetism. The true reason is much deeper and at a very fundamental level of our universe. It turns out that the c is the radial expansion velocity of our universe. But why does the transition from Relativity to Quantum Mechanics happens with decreasing size scale? That’s because, this is what is happens to spatial and temporal dimensions at different scales: 3+1 (Classical regime) <=> 2+2 (Compton regime) <=> 1+3 (Planck regime). From the center of universe viewpoint, simultaneity is absolute (as demanded by Sagnac effect), and there is indeed absolute universal time (as demanded by quantum mechanics). From our viewpoint (located at an awkward position in the universe), locality is absolute, and velocity c is the upper limit, and remains constant for every observer. This turns (our) space and time into inseparable twins, and makes (our) time a relative concept. This solves the time problem which had so stubbornly resisted the reconciliation of Quantum Mechanics and General Relativity. General Relativity makes things a bit tricky since it involves curvature (warping) of spacetime, but it does not become incompatible with Quantum Mechanics. The only difference between the warped spacetime of General Relativity, and the flat spacetime of Special Relativity is very similar to the difference between a stretched rubber membrane, with and without a metal ball placed on it. General Relativity and Quantum Mechanics are the two pillars of modern Science. Reconciliation of the two greatest theories of physics leads us to Theory of Everything.
For over nine decades, quantum mechanics has profoundly shaped our understanding of nature, offering explanations for the stability of atoms, the formation of mole cules, the properties of materials, and phenomena such as light or radioactivity. It is essential to modern technologies—from microchips to lasers—and underpins the physics of elementary particles. Yet, despite its successes, it remains a deeply mysterious theory, confronting a central enigma: why does the quantum world behave so differently from everyday reality? Elements such as superposition and quantum entanglement—which allow particles to exist simultaneously in multiple states or remain interconnected regardless of distance—contradict classical world intuitions. This gap between microscopic and macroscopic behavior has given rise to what is known as the “measurement problem,” raising fundamental questions about the nature of reality. Diverging interpretations have led to two major schools of thought: anti-realism, promoted by Niels Bohr, which holds that reality depends on the observer and their interaction with measurement instruments; and realism, supported by Albert Einstein, which argues that quantum mechanics is incomplete and that eality exists independently of observation. Bohr’s complementary philosophy and Heisenberg’s uncertainty principle were central to the formulation of the “Copenhagen interpretation,” which became dominant in the 20th century. However, the persistent opposition of scientists like Einstein and Schrödinger inspired generations of physicists to seek a unification between quantum mechanics and relativity, culminating in new theories such as loop quantum gravity and string theory. Thus, the debate between realism and anti-realism remains alive, and philosophy continues to play a crucial role in exploring the new foundations of physics.
The wave properties of matter in quantum mechanics first postulated by de Broglie in 1923 as well as Einstein’s theory of general relativity have radically changed our perception of the world at the beginning of the twentieth century. While each theory is extremely successful and well tested within its range of validity, a unification of both theories has so far resisted any attempt. However, the advances in precision of modern matter-wave interferometers have paved the way to designing experiments at the interface of gravity and quantum mechanics. Indeed, quantum mechanical devices are on the brink of becoming sensitive enough to challenge predictions of general relativity such as the weak equivalence principle or set bounds on alternative gravitational theories. Reaching sensitivities required for these experiments necessitates a careful assessment of deleterious effects some of which might be atom-atom interactions or the influence of the gravitational potential of the laboratory setup itself. Estimation of the size of such effects calls for refined theoretical tools for the description of light-pulse atom interferometry which is the subject of the present thesis.