There are a lot of interesting theories in physics. Matter exists as a state of energy, while waves of probability spread throughout the universe. Here are some of the most interesting theories, to my mind, in modern physics (in no particular order, despite the enumeration).Many are in our courses.

1) Wave Particle Duality

Matter and light have properties of both waves and particles simultaneously. The results of quantum mechanics make it clear that waves exhibit particle-like properties and particles exhibit wave-like properties, depending on the specific experiment. Quantum physics is therefore able to make descriptions of matter and energy based on wave equations that relate to the probability of a particle existing in a certain spot at a certain time.

2) Einstein's Theory of Relativity

Einstein's theory of relativity is based upon the principle that the laws of physics are the same for all observers, regardless of where they are located or how fast they are moving or accelerating. This seemingly common sense principle predicts localized effects in the form of special relativity and defines gravitation as a geometric phenomenon in the form of general relativity.

3) Quantum Probability & the Measurement Problem

Quantum physics is defined mathematically by the Schrodinger equation, which depicts the probability of a particle being found at a certain point. This probability is fundamental to the system, not merely a result of ignorance. Once a measurement is made, however, you have a definite result.

The measurement problem is that the theory doesn't completely explain how the act of measurement actually causes this change. Attempts to solve the problem have lead to some intriguing theories.

4) Heisenberg Uncertainty Principle

The physicist Werner Heisenberg developed the Heisenberg Uncertainty Principle, which says that when measuring the physical state of a quantum system there's a fundamental limit to the amount of precision that can be achieved.

For example, the more precisely you measure the momentum of a particle the less precise your measurement of its position. Again, in Heisenberg's interpretation this wasn't just a measurement error or technological limitation, but an actual physical limit.

5) Quantum Entanglement & Nonlocality

In quantum theory, certain physical systems can become "entangled," meaning that their states are directly related to the state of another object somewhere else. When one object is measured, and the Schrodinger wavefunction collapses into a single state, the other object collapses into its corresponding state ... no matter how far away the objects are (i.e. nonlocality).

Einstein, who called these influences "spooky action at a distance," illuminated this concept with his EPR Paradox.

6) Unified Field Theory

Unified field theory is a type of theory that goes about trying to reconcile quantum physics with Einstein's theory of general relativity. The following are examples of specific theories that fall under the heading of unified field theory:
Quantum Gravity
String Theory / Superstring Theory / M-Theory
Grand Unified Theory
Loop Quantum Gravity
Theory of Everything
Supersymmetry

7) The Big Bang

When Albert Einstein developed the Theory of General Relativity, it predicted a possible expansion of the universe. Georges Lemaitre thought that this indicated the universe began in a single point. The name "Big Bang" was given by Fred Hoyle while mocking the theory during a radio broadcast.

In 1929, Edwin Hubble discovered a redshift in distant galaxies, indicating that they were receding from Earth. Cosmic background microwave radiation, discovered in 1965, supported Lemaitre's theory.

8) Dark Matter & Dark Energy

Across astronomical distances, the only significant fundamental force of physics is gravity. Astronomers find that their calculations & observations don't quite match up, though.
An undetected form of matter, called dark matter, was theorized to fix this. Recent evidence supports dark matter.
Other work indicates that there might exist a dark energy, as well.

Current estimates are that the universe is 70% dark energy, 25% dark matter, and only 5% of the universe is visible matter or energy!

9) Quantum Consciousness

In attempts to solve the measurement problem in quantum physics (see above), physicists frequently run into the problem of consciousness. Though most physicists try to sidestep the issue, it seems that there is a link between the conscious choice of experiment and the outcome of the experiment.
Some physicists, most notably Roger Penrose, believe that current physics cannot explain consciousness, and that consciousness itself has a link to the strange quantum realm.

10) Anthropic Principle

Recent evidence shows that were the universe just slightly different, it wouldn't exist long enough for any life to develop. The odds of a universe that we can exist in are very small, based on chance.
The controversial Anthropic Principle states that the universe can only exist such that carbon-based life can arise.
The Anthropic Principle, while intriguing, is more a philosophical theory than a physical one. Still, the Anthropic Principle poses an intriguing intellectual puzzle.



Thanks for patience.