The double-slit experiment and wave-particle duality.

19th century:

It was the beginning of the 19th century. A man named Thomas Young, a British polymath, dared the unthinkable. He was going to stand up to the great Isaac Newton. His plan was simple. Newton believed that electromagnetic radiation behaved as particles only; Young’s experiment was going to attempt to prove him wrong. 

Young devised an experiment containing two slits, to pass light through, to then be observed on a screen.

If light did exist as particles, as it was believed at the time, the intensity of both slits will equal the intensity of the individual slits. 

If light did exist as a wave, we would instead see interference. 

Young knew this as he set up his experiment. He used a small slit to propagate sunlight rays onto another screen containing two parallel slits. The beam of light passed through and fell on a screen. This produced, what Young called, interference fringes which is a wave-like property that is formed when waves interfere with each other, either canceling out (destructive) or resulting in a combined wave of greater amplitude (constructive). He concluded this, due to his results showing bands of light of both constructive and destructive interference. Young immediately knew what he was looking at. The problem was that many recognized people at his time did not want to believe the possibility of Newton being wrong. Over time, people began embracing his theory of light and many more experiments followed to come to a better understanding of the subject. One example of such experiments is the Michelson–Morley experiment in 1887. Albert Michelson developed what we now call the Michelson interferometer, which was an apparatus to detect shifts in the interference pattern of light waves. Take a light wave, split it into components perpendicular to each other, travel identical distances, and then reflect them back. Now repeat this with movement to see if there is any change in speed, caused by relativity.  This would only work if the luminiferous aether existed, which was the suggested medium for light to travel in, yet the entire experiment failed. This result was so astounding, that it is the only Nobel prize to date, for the precise non-discovery of anything. It was later understood using Einstein’s special theory of relativity, proving that light is constant for all observers. 

20th century:

Finally, in 1905, Einstein published his Photoelectric effect paper which soon led to the term ‘wave-particle duality in the 1920’s by Louis de Broglie, a French physicist.

Following this, more experiments took place in order to get a better understanding of the subject. In 1961, Claus Jönsson of the University of Tübingen performed the first double-slit interference experiment with electrons, proving that they too, acted as both particles and a wave. Shortly after, it was shown that atoms and other molecules did so as well. 

Expanding the original experiment, suppose we fire single photons through the two slits, onto a screen. The results are the same! We again see the wave interference pattern just like before. This forced us to conclude that the photons must be interacting with themselves, going through both slits at the exact same time, quite counterintuitively. This effect is also known as quantum superposition, it takes place when a quantum system exists in multiple states simultaneously, which is a bizarre occurrence in quantum mechanics.

We also see another strange effect here. The simple act of measuring which slit the single photons go through causes the whole experiment to collapse, and in turn light will start behaving as a particle only. It is as if the particles know when they are being watched. According to the Copenhagen Interpretation, which is a specific collection of opinions on quantum mechanics, observing the superposition of paths of a particle corresponds to a measurement that collapses the superposition completely so no interference can be detected.

In 1978, an interesting proposition which was to trick the single particles and figure out which slit they are going through was suggested by John Archibald Wheeler. The idea was to quantumly entangle the particles, then set one-off to hit the screen and the other to a detector. The real trick here is that the detector is placed further back along the screen, meaning it would take longer for the particles to reach it. Somehow, these particles lose their wave-like property, as if they are being influenced by the future. This is known as the delayed choice quantum eraser and the theoretical possibility of retrocausality, which is when the effect of a system precedes its cause.

21st century:

A practical application of wave-particle duality was found recently. The idea to quantum mechanically link up optical telescopes, to allow the signals to be combined. Now imagine instead of two slits like in Young’s experiment, we use two telescopes. We can plug in a quantum hard drive at each telescope, to record the wave-like state of the photons once they reach earth. When we interfere with the signals from both telescopes, we can receive an extremely high-resolution image. 

To me, wave-particle duality is just another insane concept that arises in the world of quantum mechanics, and seeing real classical applications is fascinating.


1- Joseph,Jake.“Did the Delayed-Choice Quantum Eraser Experiment Break Causality?” Medium,Medium,11Aug.2020, 

2- “Do Atoms Going through a Double Slit ‘Know’ If They Are Being Observed?” Physics World,25Aug.2017, 

3- Lewton, Thomas, and substantive Quanta Magazine moderates comments to facilitate an informed. “Quantum Double-Slit Experiment Offers Hope for Earth-Size Telescope.” Quanta Magazine,5May2021, 

Writer: Golda

Editors: Omar Alturki/Uzay Kara