A projector and background display setup with two slits (Image: CU).
Basically, light fired at a thin plate with single slit cut in it will travel through and onto a background surface appearing as a one-band pattern. Interestingly, light in the double-slit version of this experiment reaches the two slits and diffracts like a wave, interfering with itself either constructively or destructively to create a background pattern of bright and dark fringes (see above).
Now, if you were to perform the experiment with individual particles of matter such as electrons, it is surprising that a similar interference pattern begins to take shape in the background as well. Classical particles fired one by one would be expected to go through either slit by chance and create a simple pattern of just two bands. According to quantum theory, a fringe interference pattern could only happen if a single electron behaves as a physical wave of potentials and then interferes with itself after going through both slits, maintaining its wave-like nature until it arrives at the background surface as a particle. Other proposed solutions include the electron passing through one slit, the other slit, or even neither slit.
Many thought this explanation deviated too much from the presumed behavior of only going through one slit at a time. However, when scientists place an observing device at the slits in order to see what an approaching electron actually does, its classical particle nature suddenly emerges and changes the background pattern into a mere two bands. It acts as if the information gathered about which way a particle goes through prevents any wave-like behavior from taking effect. This suggests that certain observation techniques will affect how objects interact at a quantum level. A successful attempt to determine the path of a particle, while leaving the fringe interference pattern produced by wave-like behavior intact, was completed in January of 2012 by using entangled photons and a light source with two intensity maxima. This method also allows particle and wave-like natures to be viewed simultaneously.
In the Copenhagen interpretation of quantum mechanics, the act of observation instantly measures and reduces a system's set of possible outcomes to randomly assume one probable value. This phenomenon is referred to as a collapse of the wave function, and it links an object or a system's unobserved state to recognizable properties such as momentum or position. In fact, a quantum entity is thought to exist in all of its theoretically possible states until one of them is observed or naturally evolves with time via physicist Erwin Schrödinger's famous equation. For this case, directly observing a particle as it goes through the slits is enough to break the delicate quantum superposition of states necessary to achieve an interference pattern from wave-like behavior. This experiment is significant for showing light and matter's wave-particle duality and how an observer can have a role in determining the reality of a quantum-mechanical situation.