These microscopic models do not contain gravity and they are defined either in a lower dimensional fixed space, or in some cases, with no space at all. At the microscopic level, physical processes are described in terms of very different concepts which can be treated in a quantum mechanical fashion. In this current view of gravity, smooth dynamical space itself is such an emergent concept, pretty much like a smooth density of water. Notions like density and viscosity are examples of emergent concepts. There is a well-known equation called the Navier Stokes equation which describes this motion. These concepts are nonexistent at the molecular level. Rather, it is much more useful to describe the water in terms of completely different concepts, such as density, viscosity, etc. In fact, even if it is possible to do so, such a description will not be useful. However, since there are a huge number of molecules, the problem of describing the motion of water in terms of molecules becomes enormously complicated. Microscopically this a collection of molecules which are streaming along and colliding with each other and with the walls of the pipe. Emergent concepts are ubiquitous in physics.Ĭonsider, for example, water flowing through a pipe. In this approach, which originated in String Theory, the very notion of dynamical space is not fundamental. Recent theoretical developments have led to a radically different approach to the problem. Attempts to apply the laws of quantum mechanics naively to dynamical space-time have failed. In this sense, points and distances between points are dynamical. A gravitational wave stretches or contracts space along its line of propagation. However, in the well tested theory of gravity - Einstein’s General Relativity - space- time itself is dynamical. For example, light is an electromagnetic wave which propagates in this fixed space-time. The other forces can be understood as processes happening in a fixed space-time background, where spatial distances and time intervals are fixed once and for all. One reason for this difficulty is the following. However ever since Einstein discovered the laws of gravity in 1915, reconciling gravity with quantum mechanics has been famously problematic. It is natural to think that gravity should also be governed by quantum mechanics. The first three are governed by the laws of quantum mechanics. By Professor Sumit Das, College of Arts & Sciences Distinguished Professor (2019)Īs far as we know, almost all natural phenomena stem from four fundamental interactions: electromagnetism, weak interaction, strong interaction, and gravity.
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