What is Double-slit experiment ?

hi

In modern physics, the double-slit experiment is a demonstration that light and matter can display characteristics of both classically defined waves and particles; moreover, it displays the fundamentally probabilistic nature of quantum mechanical phenomena.

The experiment was first performed with light by Thomas Young in 1801. In 1927, Davisson and Germer demonstrated that electrons show the same behavior, which was later extended to atoms and molecules.

Thomas Young's experiment with light was part of classical physics well before quantum mechanics, and the concept of wave-particle duality. He believed it demonstrated that the wave theory of light was correct, and his experiment is sometimes referred to as Young's experiment or Young's slits.

The experiment belongs to a general class of "double path" experiments, in which a wave is split into two separate waves that later combine into a single wave. Changes in the path lengths of both waves result in a phase shift, creating an interference pattern. Another version is the Mach–Zehnder interferometer, which splits the beam with a mirror.

In the basic version of this experiment, a coherent light source, such as a laser beam, illuminates a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen behind the plate. The wave nature of light causes the light waves passing through the two slits to interfere, producing bright and dark bands on the screen — a result that would not be expected if light consisted of classical particles

However, the light is always found to be absorbed at the screen at discrete points, as individual particles (not waves), the interference pattern appearing via the varying density of these particle hits on the screen.Furthermore, versions of the experiment that include detectors at the slits find that each detected photon passes through one slit (as would a classical particle), and not through both slits (as would a wave).

However, such experiments demonstrate that particles do not form the interference pattern if one detects which slit they pass through. These results demonstrate the principle of wave–particle duality.

The Double Slit Experiment was first conducting by Thomas Young back in 1803, although Sir Isaac Newton is said to have performed a similar experiment in his own time.

During the original experiments, Newton shone light on a small hair, whereas Young used a slip of card with a slit cut into it

Other atomic-scale entities, such as electrons, are found to exhibit the same behavior when fired towards a double slit. Additionally, the detection of individual discrete impacts is observed to be inherently probabilistic, which is inexplicable using classical mechanics.

More recently, scientists have used a point light source to illuminate a thin plate with two parallel slits, and the light passing through the slits strikes a screen behind them.

The experiment can be done with entities much larger than electrons and photons, although it becomes more difficult as size increases. The largest entities for which the double-slit experiment has been performed were molecules that each comprised 810 atoms (whose total mass was over 10,000 atomic mass units).

Classical particles do not interfere with each other; they merely collide. If classical particles are fired in a straight line through a slit they will all strike the screen in a pattern the same size and shape as the slit.

Where there are two open slits, the resulting pattern will simply be the sum of the two single-slit patterns (two vertical lines). But again and again, the experiment demonstrated that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen.

The double-slit experiment (and its variations) has become a classic thought experiment, for its clarity in expressing the central puzzles of quantum mechanics. Because it demonstrates the fundamental limitation of the ability of the observer to predict experimental results, Richard Feynman called it "a phenomenon which is impossible […] to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery [of quantum mechanics].

the bands on the screen were always found to be absorbed as though it were composed of discrete particles (aka. photons). To make matters even more confusing, measuring devices were put in place to observe the photons as they passed through the slits.

By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more!

This could only be resolved by claiming that light behaves as both a particle and a wave, and that observing them causes the range of behavioral possibilities to narrow to the point where their behavior become predictable once more.

The Double Slit experiment not only gave rise to the particle-wave theory of photons, it also made scientists aware of the incredible, confounding world of quantum mechanics, where nothing is predictable, everything is relative, and the observer is no longer a passive subject, but an active participant with the power to change the outcome

The double-slit experiment is an experiment in quantum mechanics and optics demonstrating the wave-particle duality of electrons, photons, and other fundamental objects in physics.

When streams of particles such as electrons or photons pass through two narrow adjacent slits to hit a detector screen on the other side, they don't form clusters based on whether they passed through one slit or the other. Instead, they interfere: simultaneously passing through both slits, and producing a pattern of interference bands on the screen.

The double-slit experiment is a nineteenth-century investigation into the properties of light that has since been found to demonstrate both the duality of photons and the concepts of superposition and quantum interference.

If light consisted strictly of ordinary or classical particles, and these particles were fired in a straight line through a slit and allowed to strike a screen on the other side, we would expect to see a pattern corresponding to the size and shape of the slit.

Debate over whether light is made up of particles or waves dates back over three hundred years. In the seventeenth century, Isaac Newton proclaimed that light consisted of a stream of particles.