A quantum wave (running through the red mesh) interacts with a detector screen (green film) and creates a particle (yellow/orange spot). If this image were accurate, the wave would disappear simultaneously with the appearance of the particle. [Image source: stills from Fermilab video by Dr. Don Lincoln, “Quantum Field Theory” (in the public domain) Jan. 14, 2016; .]
When it’s not interacting, the matter is in a superposition of many possible states. A superposition is more than one wave on top of another in the same spacetime. That’s really the definition for a superposition in ordinary (classical) physics. For example, we can have a superposition of sound waves in air either reinforcing each other or flattening out each other.
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In the case of a quantum superposition, the waves are not in a known medium. Their physical nature is not understood or at least there’s no consensus among physicists as to their physical nature. The superposition represents the mathematical idea that there are many possibilities for properties of the particle. Let’s say, for example, we’re interested in the position of an electron. Until observed, the electron is in a superposition of many possible positions.
The mathematical equation which describes this superposition looks like an equation for a sound wave or a water wave in classical physics. So, the superposition state is called the “wave state” of the quantum particle. The equation (the famous Shrodinger Wave Equation) tells us the probabilities of where we will find the electron as a particle if measured. In the meantime, until measured, it’s a set of possible positions.
Quantum superposition (on left) and particles forming objects in spacetime (on right). The superposition is described by an equation (the “wavefunction”) derived from the Shrodinger Wave Equation. Upon interaction with parts of the physical universe (observation/measurement), the superposition instantaneously becomes the particles forming the objects that we perceive in spacetime. This is called the “collapse of the wavefunction.” [Image source: David Chalmers and Kelvin McQueen, “Consciousness and the Collapse of the Wave Function”]
The superposition is described by an equation that looks like a wave equation, but also the superposition state acts like a wave. For example, physicists and biologists now believe that the superposition state is important in photosynthesis. To create sugar in photosynthesis, a photon excites an electron in chlorophyll. Then, the electron needs to find the right spot in the plant leaf (the “reaction center”) to interact with. The electron finds the right spot much faster than a particle is capable of traveling; it seems to be able to check out many parts of the leaf at the same time, as a spread-out wave could.
This is also described as the electron being in more than one place at the same time or exploring all possible paths to the reaction center simultaneously.Then, the electron gets itself to the right spot, gives the reaction center a particle of energy, and helps to make a molecule of sugar.
So, the electron, in its superposition wave-state, surveys and travels through the leaf. Upon interacting in the reaction center, it creates a particle of energy. This wonderful trick is described in a video.