Physics is Fun banner

NASA's Astronomy picture of the day

Every day a fun new picture!

Featured Topics

Quantum Mechanics:

Quantum mechanics (QM) is the branch of physics focused on very small things. Atoms are made of electrons, neutrons and protons. Neutrons and protons are made of quarks. Electrons and quarks are believed to be indivisible and so are called elementary particles. Thus, the behavior of elementary particles is described by quantum mechanics.

QM has been extensively proven via experiments. For example, it explains the periodic table of the elements in chemistry and how atoms bond together to form molecules. Fun fact: the origin of the term 'quantum leap' comes from an electron in an atom jumping from one discrete energy level to another. Such a transition involves the electron emitting or absorbing a particle of energy called a photon.

In QM the quantum state of a particle is described by a wave function. Wave functions are considered the set of all probability amplitudes. These probability amplitudes provide a relationship between the wave function of a system and the results of observations of that system. More specifically, the probability of obtaining any possible measurement outcome is equal to the square of the corresponding amplitude.

Significant ideas of quantum mechanics include wave-particle duality and the uncertainty principle. These ideas seem to conflict with our every day experiences but have been proven in experiments. Even Albert Einstein said about QM, "God doesn't play dice with the world."

Every elementary particle exhibits wave-particle duality, which means it has the properties of a wave as well as a particle. This idea really goes against our intuition. Think about how the waves on the surface of a lake can interfere with each other to get bigger or smaller. If a baseball was like a particle, wave-particle duality means a baseball could get bigger or smaller as its wave interferes with the wave of another baseball. Clearly, baseballs don't behave like particles and vice versa.

The uncertainty principle states we cannot know exactly two different qualities of a particle, such as position and momentum. This doesn't mean we don't have the technology yet to measure this. It means it is physically impossible due to the nature of the universe itself. The uncertainty principle actually follows from the wave nature of particles.

See more info below.


Interpretations of Quantum Mechanics:

Interpretations of quantum mechanics (QM) fall within the realm of the philosophy of physics. They are a conceptual way to relate QM mathematics and concepts with QM observations and the physical meanings of these observations. There are three main types of interpretations: collapse theories, Many Worlds theories, and hidden variables.

The Copenhagen Interpretation, a collapse theory, is the most popular interpretation of QM. In this interpretation, QM predicts the probabilities for different outcomes of pre-specified observations. During the act of observation the wavefunction describing the system collapses to one option. If there's no observation, there's no collapse and none of the options ever become more or less likely. All the collapse theories work similarly, with only the cause of the collapse being different. For example, in objective collapse theories, the collapse occurs randomly, with observers playing no special role.

The Many Worlds Interpretation is the second-most popular interpretation. It posits that rather than a wavefunction collapse and a resulting single outcome, all possible options come to pass, creating an infinity of worlds. An individual human experiences only one outcome because he/she exists in only one of these infinite worlds.

Hidden variable interpretations are less popular than collapse and Many Worlds interpretations. But an elegant hidden variable interpretation is John G. Cramer's transactional interpretation of quantum mechanics. This interpretation describes quantum interactions in terms of a standing wave formed by both forward-in-time and backward-in-time waves. Recall a standing wave, which can also be called a stationary wave, is a wave in which each point of the wave has an associated constant amplitude. Thus, in the transactional interpretation, the source and the receiver both emit physically real retarded and advanced waves that cancel each other out. Thus, the wavefunction collapse doesn't happen at any specific time, rather it occurs atemporally along the whole transaction. Notice here the emission and absorption processes are time-symmetric.

See more info below.


Quantum Mechanics is the branch of physics that describes really small things like elementary particles (e.g. quarks and electrons). It has been proven by extensive experiments in the fields of chemistry and physics, and is considered by many to be the foundation of all modern physics.

In quantum mechanics physical objects such as particles are also waves. Scientists call this wave-particle duality. The mathematical representation of this is a wavefunction. Quantum mechanics says we can only characterize these wavefunctions with probabilities. Furthermore, there are an infinite number of probabilities for every wavefunction until we make a measurement and observe the object, at which point we have one probability and its value is 100%.

What happens between having an infinite number of probabilities for an observation and having one probability? The Copenhagen Interpretation maintains that there must be a collapse of the wavefunction to instantiate a reality.

Read more about the Copenhagen Interpretation of Quantum Mechanics:

There are several other interpretations of quantum mechanics including:

  • the Many-Worlds Interpretation avoids collapsing the wavefunction by positing that the universe splits and every quantum possibility is implemented.
  • the Transactional Interpretation claims that quantum interactions require both a forward-in-time wave and a backward-in-time wave to describe them. The interference of these two waves avoids the problems of the Copenhagen Interpretation.
  • Read all about a variety of interpretations of quantum mechanics in the Wikipedia.