Wave-particle duality

In physics, wave-particle duality holds that light and matter can exhibit properties of both waves and of particles. This concept is a key part of quantum mechanics.

The 'double-slit' experiment

An intriguingly simple and well reported experiment, the double-slit experiment, summarizes the duality. An electron gun is pointed at a screen with two slits and the positions of detection of the electrons recorded by a detector behind the screen. An interference pattern just like the one produced by diffraction of a light or water will be observed on the screen.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

This pattern will even appear if the electron source is slowed down so that only one electron's worth of charge per second comes through. In the classicall sense, every electron is a point particle and must either travel through the first or through the second slit. The same interference pattern should appear if the experiment lasts twice as long, but closing one slit for the first half, then closing the other slit for the second half of the experiment. However, it is found that the same pattern does not emerge. Furthermore, if detectors are placed around the slits in order to determine which path a particular electron takes, this very measurement destroys the interference pattern. THis result shows that something much more profound is taking place.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The interference pattern can be explained as a result of the charge wave being diffracted by both slits and interfering with itself. In quantum mechanics, the state function is a complex-valued function of space and time. The square of the magnitude of this function describes the probability of finding the electron at a given location at a given time. Interference is due to the fact that the square of the magnitude of the sum of two complex numbers may be different from the sum of the squares of their magnitudes.

Related Topics:
Function - Magnitude - Complex numbers

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The experiment also illustrates an interesting feature of quantum mechanics. Until an observation is made the position of a particle is described in terms of probability waves, but after the particle is observed, it is described as a fixed value. How to conceptualize the process of measurement is one of the great unresolved questions of quantum mechanics. The standard interpretation is the Copenhagen interpretation which leads to interesting thought experiments such as Schrödinger's cat. Due to this confusion, some theorists (including Stephen Hawking and Murray Gell-Mann) believe the many-worlds interpretation is true. However, there is currently some doubt over the validity of both the Copenhagen interpretation and the many-worlds interpretation, due to the controversial Shahriar Afshar's experiment http://www.irims.org/quant-ph/030503/, a variation of the two-pin-hole "which way" experiment.

Related Topics:
Copenhagen interpretation - Thought experiment - Schrödinger's cat - Stephen Hawking - Murray Gell-Mann - Many-worlds interpretation - Shahriar Afshar's experiment - Two-pin-hole "which way" experiment

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Notes

~ ~ ~ ~ ~ ~ ~ ~ ~ ~