The Double-Slit Experiment

In 1801 the English scientist Thomas Young conducted an experiment identifying the dual nature of light. A finding that shocked the dominant understanding of light up until the time of the experiment. The paradigm in physics back then was still heavily dominated by the Newtonian formulations of the world. The prevailing definition of light – as postulated by the corpuscular theory – was that it is a composite of distinct particles.

In that sense, the double-slit experiment was a foregrounding step in blurring the lines between matter and energy. It proved how light can behave as a wave under certain conditions. About 80 years after the double slit experiment, Heinrich Hertz identified the photoelectric effect and demonstrated how light behaves as particle – but only under certain conditions as well. Those two seemingly contradicting – yet mutually inclusive – discoveries marked the shift from the deterministic classical physics to the probabilistic quantum understandings of the world.

The Experiment

The double-slit experiment itself is quite simple: a laser shines coherent beam of light (coherent light is light of unchangeable wavelength and phase) through a film disk containing two parallel slits. The light striking the screen behind the slit produces a classic interference pattern.

The results from the experiment would have been completely different if light could indeed be defined solely as a composite of distinct particles. Because if such was the case (according to the Newtonian corpuscular theory), there would have been only a pair of light bands projecting on the screen behind the double-slit disk. And those light bands would have had the shape and size of the slits through which the light has passed. The rest of the screen would have remained unlit.

But there instead were alternating bright and dark bands projected across the entire screen– and the width of those bands was greater than the width of the slits. The light reaching the slits was starting to act as secondary sources of spherical waves that go on to project themselves as bright and dark bands upon the screen. This could only be explained through the appearance of an interference pattern – a process strictly reserved for the behavior of waves. The results concluded that light does indeed behave as a wave is inevitable.

Different particles, same result

The double-slit experiment was recreated with electrons in 1989. The results confirmed the hypothesis of Louis de Brogile. According to him, all particles – and not just photons – could display a dual corpuscular-wave state of being.  The electron double-slit: cathode ray tube placed inside a glass container where the air has been sucked out – i.e. tube has been placed inside an environment of complete vacuum because the latter would provide a friction-free path to the motion of the electrons. A screen plate covered in luminophore (a substance that is glowing when “activated” by an electron flow – luminescence) placed at the other side of the double-slit plate. Any electron particle to hit the luminophore would leave a trace as a single dot on the screen plate. But instead, there was an interference pattern formed yet again.

The double-slit experiment has since been re-created with even bigger particles such as fullerenes (Anton Zeilinger, 1999) – having a diameter of 0,7 Nm (almost half millions times bigger than electrons) – but the result remained identical. All of the particles in the various versions of the experiment displayed wave behavior with their own specific frequency values. The outcomes from all the versions of the experiment are counter-intuitive to our everyday experience with discrete objects. And physicists were still not satisfied. They believed that the interference pattern might be due to the interaction between electrons that were simultaneously fired at the slit.

So, they decided to fire a single electron at a time. The electrons were initially appearing to leaving traces indicating they are particles. But it did not take too long for them to start forming interference bands once again. The conclusion was that a single electron is interfering with itself upon its passage from the slits. The luminescence detector responds by projecting fixated image indicating the dispersion of the electrons – i.e. the probability for the presence of a particle within a given area. The illustration below shows how the electron is transforming into a wave function upon its passage through the double slit. It is worthy noticing that the electron’s actual size is way smaller than the size of the probability cloud it forms. One can clearly see how the electron appears to be interfering with itself when passing through the slit. This results in the interference bands forming both in front of and behind the slit.

The quantum hide-and-seek

What would happen if there were detectors placed in close proximity to both slits? Would those be able to catch how the mischievous particles passing through only one of the slits before transforming into waves? But their dual nature turned out to be more wayward than expected. Whenever the quants were captured passing through either of the slid, they carried on with their particle behavior and they did not produce an interference pattern on the screen behind the slits. What about if a detector is being placed solely next to one of the slits? The result: even if the quantum has not been captured by the detector (it has passed through the other slit), it still goes on to project itself as a particle on the screen. It’s like the quantum knew it was being watched and decided to pretend to be a particle. The only way for identifying the slit through which the electron passes is by installing a measuring device working with identical electromagnetic waves as the electron. But such device of identical waves would also affect the electron by interacting with it. And the very interaction with it predetermines its behavior as a particle. If the interaction is reduced to zero, an interference pattern would appear but it would be impossible to identify the slit that the electron has passed through. It turns out that the processes within the quantum world are characterized by one extremely bizarre probabilistic behavior – the electron is a wave for as long as it has not been located, and it turns into a particle the very moment its location becomes identified.


What has become known as the wave-particle duality baffled the physicists for many years. But only those of them aiming a return to one deterministic and invariable explanation of the world. One that would draw clear lines between matter and energy in its endeavor to provide a throughout understanding. But the orientation in such direction in itself was fundamentally missing the point about the probabilistic nature of the quantum world. Perhaps the most telling display of such confusion is the EPR paradox. An alternative to this deterministic view was put forward by Niels Bohr and his complementarity principle. Instead of distinguishing between particles and waves, the complementarity is a tenet according to which the complete knowledge of phenomena on atomic dimensions requires a description of both wave and particle properties. A bit overwhelmed on physics?

The story below beautifully captures the idea for the unification of the opposites underlying the quantum understanding of the world through complimentary. Like and unlike are the same. All paradoxes might be reconciled:

Long time ago there lived a sage of great repute. He was devoted to his belief and knowledge in Everything that Is – he was famous for finding answers to the most difficult questions. People had great respect for him. They had complete trust in him and they often consulted him for advice. There was one student of the old sage who was blinded by envy because of his teacher’s devotion and knowledge. This student decided to embarrass his teacher in front of everybody. The young man went to the nearby lawn and where he caught a butterfly. He was holding it in his palms when he eagerly walked back to the sage. The young man asked his teacher the question in front of the gathered crowd:

Master, you have the answers to all the riddles. And at this very moment I am holding a butterfly in my hands.  Do you know if it’s dead or alive?

 While speaking these words out, the young man was prepared for both answers. If the teacher claimed the butterfly to be dead, the student would let it fly. If the teacher claimed the butterfly to be alive, he would crush it in his palms, thus proving it is dead. At this moment – with all his love and humility, and without even picking his head up – the old sage replied:

“Son, whether the butterfly is dead or alive, is entirely in your hands!”

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