Chapter 7 – Reality

Gravity

The force conspicuously missing from the Standard Model is gravity.

One approach to adding gravity is to include a graviton particle which would elegantly complete the explanation of forces:

• Electromagnetic force → photon
• Strong nuclear force → gluon
• Weak nuclear force → W and Z bosons
• Gravity force → graviton

This doesn’t quite work because

• The math for a graviton doesn’t work at high energies
• A graviton has not yet been detected
• It doesn’t explain curved spacetime of General Relativity

There may be a way to address these issues but it has not yet been discovered.

General Relativity

Quantum mechanics describes the world and very small scale. General relativity describes large scales and high densities. It is the difference between these two theories that makes addressing gravity complex. The inconsistencies become important for very small scales with high densities – especially black holes or the Big Bang. Key differences include:

Quantum Mechanics	General Relativity
Energy comes in chunks	Energy is continuous
Space is fixed	Matter and energy cause space to curve
Gravity is not explained by QM	Gravity is fundamental to GR

Having two incompatible theories of how the universe operates is a problem and it has been a goal of physicists to find a unified theory that explains everything. These are two approaches being developed but are not yet complete:

• String theory: the elementary particles are made of small vibrating strings in a 10 or 11-dimensional space.
• Loop Quantum Gravity: space and time come in chunks so concepts like volume and area will always be multiples of a minimum size
• Holographic universe: The world we experience is a projection of a lower dimensional quantum world – the two worlds are equivalent! Entanglement in the quantum world drives space curvature and gravity