“The double-slit experiment demonstrates that the act of observation alters the outcome of quantum events.”

Peer-reviewed article examining the double-slit experiment from a 'reality-without-realism' perspective, directly addressing interpretations of observation in quantum mechanics and the measurement problem.

The double slit experiment provides a classic example of both interference and the effect of observation in quantum physics. When particles are sent individually through a pair of slits, a wave-like interference pattern develops, but no such interference is found when one observes which “path” the particles take. Using this model, we demonstrate how the loss of interference in the system is correlated with the information gain by the measuring apparatus/observer.

MIT physicists performed an idealized version of the double-slit experiment and confirmed the predictions of quantum theory: 'The more information was obtained about the path (i.e. the particle nature) of light, the lower the visibility of the interference pattern was.' The researchers tested Einstein's proposal to detect the photon's path and found that when they removed the 'spring-like' detection mechanism, the same phenomenon occurred, demonstrating that the act of measurement—not a physical apparatus—determines whether wave or particle behavior is observed.

The researchers thus confirmed the predictions of quantum theory: The more information was obtained about the path (i.e. the particle nature) of light, the lower the visibility of the interference pattern was. In previous realizations of the double slit experiment, physicists have incorporated such a spring-like ingredient, and the spring played a major role in describing the photon’s dual nature. But Ketterle and his colleagues were able to perform the experiment without the proverbial springs.

The measurement process — by the very definition of “measurement” as a procedure that segregates left-slit cases from right-slit cases — has resulted in the two parts of the superposition being different even when they both have the particle reaching the same point P. Therefore, in contrast to Fig. 3, quantum interference between the two parts of the superposition cannot occur. And yet, it’s crucial that the path of each object on its way to the screen remain unknown. If one measures which of the slits each object passes through, the interference pattern never appears.

One of the most famous experiments in physics is the double slit experiment. It demonstrates, with unparalleled strangeness, that little particles of matter have something of a wave about them, and suggests that the very act of observing a particle has a dramatic effect on its behaviour.

The experiment 'questions the role of the observer in the outcome of events and demonstrates the fundamental limitation of an observer to predict experimental results.' It establishes 'wave-particle duality' and shows that 'matter and energy can display the characteristics of both waves and particles.'

The double slit experiment performed with particles and particle detectors is used to clearly demonstrate the nonclassical behavior of microscopic particles including the delayed choice experiment and causality issues. The screen on the right side of Figure 3 is covered with many closely spaced particle detectors whose positions are indicated by the variable x.

In quantum mechanics, the collapse of the interference pattern in the double-slit experiment occurs due to the physical interaction of the measuring device with the particle, such as a photon scattering off an electron to detect its path. This is known as the observer effect, but it refers to measurement apparatus, not conscious human observation.

However, it turns out that any attempt to measure which slit the particle “actually” passes through destroys the interference pattern and produces the classically-expected result of all the particles striking the screen directly behind one of the slits. This measurement by an observer and his/her equipment effectively removes the wave aspect from the particle!

Oddly enough, experimental evidence shows that when the electrons are observed, they go back to behaving like the marbles above and pass through the slits as particles, almost as though they were never waves!

The double-slit experiment defies our understanding. It involves a laser shooting beams of light particles, called photons, at a screen with two parallel slits. Instead of two strips, we see an alternating pattern of light and dark bands characteristic of waves interfering.

How Quantum Particles Behave when no one is looking; This video illustrates the central mystery of quantum mechanics where particles change behavior upon observation.