Originally Posted On: https://scalarlight.com/the-double-slit-experiment/


Physics struggles to provide explanations for some of the strangest phenomena ever discovered.  Some would argue that an understanding of physics means that one essentially understands the fundamentals of life, itself.  Since the subjects that physics explores underscore everything we know, see, sense, and experience, in a sense, those arguments are correct.  After all, physics has shown us that we are a collection of atoms which form everything from the skin on our bones, to the bones themselves.  Atoms comprise the trees we see outside our door and the cars we drive.  Atoms are generally thought to behave in certain ways, and many experiments have shown that they do follow a universal set of rules or laws.  Still, in the quantum world, things are not as they seem.  In this regard, one of the experiments that first showed just how strange things get in the quantum realm was called “The Double Slit Experiment.”  The Double Slit Experiment and Quantum Physics provide snapshots into how strange it gets once you take things down to levels so small that atoms themselves seem giant.

The Double Slit Experiment and Quantum Physics

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. This type of experiment was first performed, using light, by Thomas Young in 1801, as a demonstration of the wave behavior of light. At that time it was thought that light consisted of either waves or particles. With the beginning of modern physics, about a hundred years later, it was realized that light could in fact show behavior characteristic of both waves and particles. 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.

Variations of the Double Slit Experimentmeasuring the wavelength of laser light using a double slit

The experiment itself has been divided into a broad heading of such experiments called “double path” experiments.  In each of these, photons are streamed in two separate waves that travel through a space and then through a barrier with two slits open through it.  The photons are then catalogued as to where they land on a flat surface behind the double-slit barrier.  The original Double Slit Experiment and Quantum Physics were not thought of as necessarily related at first.  But the experiment demonstrated that when photons or the building blocks of light are streamed through the two slits, they do not hit the flat surface behind the barrier in lines that mirror the stream.  What is more, it demonstrated that light acted as both particles and a wave.  This was paradigm-shifting in that beforehand, science thought of photons as traveling as particles.  Yet, when the Double Slit Experiment met Quantum Physics, the theories of the quantum realm helped shed light on the idea that light not only behaved as a stream of particles, but also as a wave.  Further, and even more oddly, if the substance being streamed through the double slits were measured, they proved impossible to nail down.  Stranger still, when directly observed by scientists, the quantum entities behaved differently than when they were not directly observed, leading some to speculate that somehow, the entities that made up the quantum-level substance, somehow were able to detect that they were being observed or measured.



Quantum Healing Free trial

Probably, Not Certainly

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 appears 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 and Quantum Physics have given scientists much in the way of thought experiments.  The experiment demonstrated that an observer would find it impossible to predict the results with precision, but could, however, predict them with probability.  In other words, when measuring the fundamental position of quantum entities in the Double Slit Experiment, one could never say, or even wasn’t allowed to say that a certain position would be where those entities would be found.  One was only allowed to say that they would probably be there.

What Are The Philosophical Implications?

What could the double slit experiment and quantum physics have to say about anything having to do with the philosophical or the metaphysical, some might ask.  Aside from demonstrating that reality forms itself to us only when we observe things, the implications also reach into the realm that some call a fallacy — the Spirit Realm.  After all, if reality is reality only when observed, would that not lead to saying that there are other realities that we do not see?  If so, then what are they?  Are there realities of which we see only glimpses?  If the fundamental building blocks of all that we are do not follow the laws of physics and thereby science, does this not open questions as to how much of what we call “reality” really is real?  It seems rather humanocentric to say that just because human beings don’t, can’t, or do not readily observe something, that it cannot possibly exist.  After all, science and philosophy teach us that of all the knowledge there is out there, we understand and are aware of a mere fraction of a fraction.

So if one considers the Double Slit Experiment, Quantum Physics, and Philosophy, entire new worlds of inquiry open. At first, it is almost an anxious sort of thought to say that our reality is only so because we observe it.  But after the initial shock subsides, we begin to see that we have only just scratched the surface of what is real.  This points us to a future of discovery in the purview of the Double Slit Experiment and Quantum Physics.  It also opens up brilliant new ideas about how those two entities may actually support the teachings of philosophers and metaphysicists, ideas that are age-old.  We don’t know where the line of query leads, but what an exciting journey it will be, without a doubt.

Informational Sources


The Physics arXiv Blog.  Particularly https://medium.com/the-physics-arxiv-blog/physicists-smash-record-for-wave-particle-duality-462c39db8e7b


Eibenberger, Sandra; et al. (2013). “Matter-wave interference with particles selected from a molecular library with masses exceeding 10000 amu”. Physical Chemistry Chemical Physics15 (35): 14696–14700.


Robinson, Andrew (2006). The Last Man Who Knew Everything. New York, NY: Pi Press. pp. 123–124.