There are a lot of interesting ideas in physics, especially in modern physics. Matter exists as a state of energy, while waves of probability spread throughout the universe. Existence itself may exist as only the vibrations on microscopic, trans-dimensional strings. Here are some of the most interesting of these ideas, in modern physics. Some are full-blown theories, such as relativity, but others are principles (assumptions upon which theories are built) and some are conclusions made by existing theoretical frameworks.

All, however, are really weird.

Wave Particle Duality PASIEKA/Science Photo Library/Getty Images 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.

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.

Quantum Probability & The Measurement Problem Quantum physics is defined mathematically by the Schroedinger 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 led to some intriguing theories.

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.

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 Schroedinger 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 this quantum entanglement "spooky action at a distance," illuminated this concept with his EPR Paradox.

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. There are several specific theories that fall under the heading of unified field theory including Quantum Gravity, String Theory / Superstring Theory / M-Theory, and Loop Quantum Gravity

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.

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.

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.