# Open Problems in Physics

Here we present a list of open problems in Physics.

## Quantum Gravity

The present rules of gravity formulated by Einstein, Hilbert and others is known to break down at the quantum level. A quantum theory of gravity is thus still elusive. Several proposals for a quantum theory of gravity have been proposed over the last 40 years, one of the most prominent being String Theory, but empirical evidence for these theories is still lacking.

## Supersymmetry

Supersymmetry is a proposed symmetry between fermions and bosons, and says that for every fermion, there exists a "sister" boson that differs only in its spin. Many quantum theories of gravity require supersymmetry, and it may also solve convergence issues in calculating certain quantum amplitudes. It is also thought to be desirable by some theoretical physicists purely for aesthetic reasons, dating back to Dirac and his remarks on magnetic monopoles in Maxwell's laws of Classical Electrodynamics. However, no evidence for supersymmetric particles has ever been found, and runs of the LHC at CERN have actively ruled out some of the proposed Supersymmetric theories.

## Hierarchy Problem

The hierarchy problem is the problem in explaining why the gap in relative strengths between the two weakest forces in nature, gravity and the weak nuclear force, is so large. The weak force is $10^{25}$ times stronger than gravity. The relative strengths of the four fundamental forces in nature are:

fundamental force relative strength to strong force relative strength to next stronger force
strong interaction 1 -
electromagnetism $10^{-3}$ $10^{-3}$ weak interaction $10^{-16}$ $10^{-13}$ gravity $10^{-41}$ $10^{-25}$ Such a discrepancy between the relative strengths between the forces is thought to be unlikely to occur by chance, and therefore it is supposed that there is some mechanism or reason as to why gravity is so much weaker than the other forces. This mechanism is currently unknown.

## Horizon Problem

The horizon problem is the problem in trying to explain why regions of the sky that are so far apart that no interaction could possibly have travelled between them could have the same temperature to within $10^{-3}\,K$ . In other words, the visible Universe has almost exactly the same temperature everywhere even though most regions of the visible Universe could not have had time to thermally equalize their temperatures. The commonly accepted solution to this problem is the hypothesis of Cosmic Inflation, However a physical source for the inflation field is still to be found.