The term quantum mechanics (QM) generally refers to a mathematical formulation of quantum theory. Quantum mechanics often describes and predicts the movement and behavior of particles at the atomic and subatomic levels. These include particles such as atoms, electrons, protons, and photons. According to quantum mechanics, the behavior and movement of particles at these microscopic levels are counter-intuitive, radically differing from anything observed in everyday life. This requires a theory like quantum mechanics in order to better explain this different world.

In the macroscopic world, there usually are two types of phenomenon that are classified by scientists: particles and waves. Particles can be thought of as being localized, transporting mass and energy in their movements. Waves are a type of phenomenon that never is localized and has no mass, but nevertheless carries energy. The micro-world particles that are examined in quantum mechanics are different from those of the macro-world, because in certain situations, particles may act like waves, while conversely, waves may act like particles. According to quantum mechanics, this would mean that on the quantum level an apple could behave like a wave, while a wave could behave like an apple.

Quantum mechanics uses the quantum unit to roughly assign and measure quantities of the physical properties of these particles. Particles generally are measured by considering the wave function recorded at the time of the measurement. Yet because particles may behave like waves, the simultaneous location or velocity of a particle can never completely be determined.

This apparent conundrum is known as Heisenburg’s uncertainty principle, which in other words states that position and momentum cannot be exactly measured simultaneously. This is not entirely because there are any limitations in measuring equipment or ability, but because this uncertainty is believed to be inherent to the particles themselves. Quantum mechanics therefore is a mathematical theory that tries to calculate probabilities according to the wave function.

Many scientists do not agree with the postulates of quantum mechanics. Albert Einstein was a scientist who sometimes was critical of quantum theory, as it conflicted with his own theory. Thus, one of the biggest problems in contemporary physics is to create a theory that would unify the ideas of quantum mechanics with those of Einstein's general theory of relativity. Physicists such as Julian Barbour have proposed some innovative solutions to this problem, basically stating that if humans consider that time itself does not exist, there is no obstacle to unifying quantum mechanics and the theory of relativity.