A QUEST FOR NEW PHYSICS?

The challenges facing cosmology (and astrophysics) to describe the universe, its origins and evolution are behind the quest for new physics theories. This is because there are still too many enigmas and issues which have not been resolved (for instance dark matter, dark energy, etc.).
It is often possible to resolve an enigma by imagining a mysterious new effect: hidden mass (dark matter) to account for the dynamics of galaxies or clusters of galaxies; exotic energy (dark) to account for the acceleration identified in cosmic expansion; inflation to understand some aspects of the formation of galaxies. However these "solutions", introduced somewhat empirically and without true justifications, may only be fully accepted and understood if they fit into a true physical theory founded on solid principles.
Moreover, it is physics itself which has to be redefined, due to its lack of unity and the weakness of its foundations.
It is today divided into two distinct and partially antagonistic bodies of knowledge: quantum physics and general relativity, each with its inadequacies, which are exacerbated by attempts to unify or bring them closer together. This dichotomy is blocking any vision of a unified and coherent world.

SOME PROBLEMS ARISING IN QUANTUM PHYSICS

Interactions
While quantum physics has been well confirmed by observations at microscopic scales, its direct manifestations are only rarely detected during experiments or observations conducted at our scales. However it is responsible for the cohesion of matter and for interactions between matter and radiation. It describes the three types of interactions (excluding gravitation) to which elementary particles are subject and which intervene in atomic and nuclear physics, i.e. theories which are grouped together in what is known as the Standard Model, describing the interaction between elementary particles (the interactions are described by so-called field quantum theories). However and in spite of irrefutable success in prediction power, the standard model is somewhat arbitrary.
In the 1980s, physicists placed a lot of hope in unified field theories since these theories aimed to unify the three interactions (electromagnetic, strong and weak) without however including gravitation. However, these unified field theories were abandoned due to a lack of experimental proof.
Furthermore, it has since been observed that the convergence of interactions was even better in the framework of a new approach called supersymmetry. Hopes have now shifted to this theory, which has the further advantage of at least partly incorporating gravity.

Quantum divergences
Moreover, the field quantum theory has run into a problem (some calculations of energy yield infinite values, which are called "divergences" and physicists have taken this into account of in the renormalisation procedure). But this lack of coherence means that the current field quantum theory cannot be considered as a physical theory in its own right. It is considered to be an "effective" theory, in other words as the approximation of a more fundamental theory which still has to be discovered.

Interpreting quantum physics?
Other unresolved aspects of quantum physics or at least problems in interpreting it, have been much written about. However, quantum physics raises an important issue. If everything is quantum as it stipulates, how can we explain the non-quantum appearance of our universe? This is one of the measurement problem aspects. Quantum effects are extremely discrete in our every day physics, in ordinary cosmology. There are, however, tentative answers; some physicists such as Roger Penrose, consider that we will never truly understand quantum physics without incorporating gravitation. Its effect on "ultramicroscopic" scales much lower than those which characterise normal quantum phenomena, may have been under-estimated. Whence the interest in a quantum gravity theory or at least in a quantum theory which takes gravitation into account.

SOME PROBLEMS ARISING FROM GENERAL RELATIVITY
While the general relativity theory does not involve interpretation problems, which are as crucial as for quantum physics, it sometimes raises questions which have no answers.

The quantum content of the Uuniverse
General relativity theory explains most of the problems of dynamics, gravitation and cosmology quite satisfactorily. But it runs into problems when describing the first moments of the Big Bang or the physics of black holes, "singularities" which have been bothering cosmologists and physicists for many decades. These are areas of Space-time (or its margins) where some parameters, such as curvature, become infinite. It is a fairly old idea that the existence of these singularities might only be an illusion due to our imperfect physics, to our inability to take into account some quantum processes, in particular processes related to gravitation.
In other words, the use of general relativity theory requires "forgetting" the fundamental quantum character of the content of the universe. No doubt this is not a problem when we are dealing with ordinary matter.
But it is already more complicated when one has to involve "exotic" components which cosmologists are taking more and more seriously, such as vacuum energy, dark energy, etc.
One might thus wonder whether some quantum effects have not been "forgotten", simply because physicists do not know how to describe them.
This is another argument as well in favour of a quantum approach to the universe and to gravity.

Gravity and other interactions
Three interactions may be described with quantum theory, but not gravitation. Paul Dirac declared that it was "difficult to imagine how some aspects of Nature could be described by quantum physics and others not". The idea of a world which was half "quantum" and half "classical" appears to be even more difficult in that the most fundamental concepts, time and space, matter and vacuum are dealt with very differently depending on which school of thought is describing them!!!
Furthermore, gravitation is described geometrically but not the other interactions (in any case, not in the same way).
Gravitation thus leads to wave phenomena: the gravitational waves predicted by general relativity have only been indirectly observed until now. Since all the other types of waves have been quantified it would be surprising if the former were an exception to the rule: the corresponding quantum still to be discovered would quite naturally be called a graviton.
Finally, the intensity of gravitation appears to be much less than that of other interactions. Some physicists believe that there is something unnatural in this and believe that a future theory would be able to explain this state of affairs. Supersymmetry or string theory might effectively explain these phenomena satisfactorily.

Should the concepts converge?
It would appear that we might be satisfied with these different theories (quantum and relativistic) to the extent that we do not have to deal with any situation which requires a synthesis of the two approaches: the relativistic and quantum spheres of application are more or less separate. We might thus put up with this diversity.
However conceptual questions have always led to breakthroughs in physics. Such questions and not the practical problems to be resolved led to the theories of relativity and - at least partly – to quantum physics. This historical lesson alone would be a good reason for trying to synthesize them. The desire for a unified theory, the need for a harmonious description of the world, requires that we transcend the quantum /non-quantum difference. This means dealing with gravity and other interactions in the same way while retaining the geometrical aspect of the one and the quantum aspect of the others.
These attempts to unify theories are called: supersymmetry, string and superstring theories, which should lead to a still mysterious M-theory; quantum gravity and cosmology and non commutative geometry.