Planetary motion (almost!) completely predicted on the basis of Newton’s laws. Unification of terrestrial and planetary phenomena. Confirmed Copernican theory. Newton’s Law of Universal Gravitation
General relativity predicts that rotating objects will emit gravitational waves. Measurements of binary pulsars show a spin down rate precisely consistent with general relativity. General Relativity
General relativity is a completely successful theory. There are no experimental anomalies. Predictions from General Relativity are verified to extremely high accuracy. Status of General Relativity So why are we still working on gravity?
Current Issues 1. Quantum Mechanics and Gravity 2. Black Holes and the Information Paradox 3. Cosmological Constant problem
Quantum Mechanics and Unitarity
A theory which violates unitarity is presumably inconsistent. For example, if your theory predicts that the electron will form a hydrogen atom with probability 200%, something is wrong, and most likely, it’s the theory. But could anything as crazy as this ever happen? Sure! Quantum Mechanics and Unitarity
The probability of an electron hitting a proton and forming a neutron can be calculated in Fermi’s theory of the weak interactions. Quantum Mechanics and Unitarity
Result: The probability is P = GFE2where G is the Fermi constant, E is the energy. Quantum Mechanics and Unitarity
Probability grows with energy, eventually becoming greater than 1. Theory loses unitarity at high energies! Quantum Mechanics and Unitarity
The Fermi theory must break down at high energies. One possibility: there may be new massive particles at high energies which modify the behavior of the theory. These cannot be any random set of particles; their behavior must reproduce the Fermi theory at low energies, but make the probability less than 1 at high energies. Can we find such a modified theory?