Quantum mechanics is believed to be the underlying theory of the constituents of all matter and energy. Every particle in the universe has a quantum wavefunction from which all of its behaviour and properties can be predicted. From drug design to microelectronics, the wavefunction’s importance cannot be overstated.
The wavefunction embodies the idea that every particle is also a wave. This wave is much like the set of ripples travelling out from a pebble dropped in a pool. And the shape of this set of ripples is what is analogous to the wavefunction. A feature of quantum mechanics is that, unlike a water wave, the very act of observing the wavefunction changes it, making it a slippery object to measure.
For example Heisenberg showed that if one observes exactly where a particle is then you disturb it, forcing its velocity and direction to become completely random. In our analogy, this would be like placing a coconut at a particular position in the pool to see if the ripples would cause it to bob. As well as bobbing, the coconut would have the unwanted side effect of reflecting the incoming ripples and sending them in every which direction. Because of this inevitable disturbance, right now scientists only measure the wavefunction indirectly, much like looking only at the shadow a wave casts on the bottom of a pool.
The obvious solution to this problem, to use a lighter float such as a wine cork, is analogous to the solution for measuring the wavefunction. If one simply measures the position of the particle gently in a ‘weak measurement’ then the velocity does not become random. The catch is that one does not get much information about the particle’s position from this weak measurement. The trick is to repeat the gentle position measurement followed by the normal velocity measurement over and over again on many identical wavefunctions until one has enough information to say what the average result of the position measurement is. This average is equal to the wavefunction itself.Having a way to see the wavefunction directly gives scientists a new tool to investigate the elementary units of nature. It also liberates the wavefunction from abstraction by giving it a simple and pragmatic definition in terms of how we measure it.