Probably the most important and famous experiment on quantum mechanics is the double slit experiment, where one basic quantum principle is manifested, and give us a glimpse of the weird world around this branch of Physics.
The grandfather of the experiments that demonstrate the properties of the quantum effects is the experiment of the double slit, where the duality of wave and particle is clearly evidenced and is the classic example of how strange the world of quantum mechanics is.
And to elaborate on this experiment we can start by reviewing the behavior of light and the different phenomena it presents. Knowing that light a type of electromagnetic wave, which is part of the fundamental forces of nature, and are the photons, the particle associated with this force, the ones who generates the light effects who have been studied for centuries, starting with Francesco Grimaldi, an Italian priest who in the middle of the seventeenth century studied the behavior of light and who created the term “diffraction”; his studies on light continued with several scientists, including cases such as Isaac Newton; but it was Thomas Young at the beginning of the nineteenth century who made the first experiment of the double slit with light.
In this experiment that Young did, noticed how a light source that passes through a double slit, has a particular behavior where light interferes with itself due to a phase shift of its waves, having a reinforcement when two wave “crests” combine, and cancelling when a crest and a through of these waves meet; resulting in a series of maxima and minima, and having a so-called diffraction pattern.
So far there are no surprises with this experiment, with the well-known diffraction and interference of light. This experiment was later repeated by Claus Jönsson using electrons. And here we must consider that these are particles of matter that, unlike photons, have mass, and therefore it is to be expected that they behave like small “marbles” or particles. If these particles are fired in a slit, they form a pattern with the shape of the slit. And the same was to be expected if these marbles were shot in a double slit, a pattern where two columns of “impacts” would be detected.
But the result with the double-slit experiment was not what was expected, this experiment generated a diffraction pattern, just as light does, but … with particles!
More refined versions of this experiment were created, considering that might be likely that the electrons were “colliding” with each other, generating this diffraction pattern. To discard this possibility, the new version of this experiment triggered one electron at a time, yet the result was the same, the electrons, fired one by one, also formed a diffraction pattern; this showed the first quantum effect:
“The electron interferes with itself!”
Even when this effect could not be explained using classical physics, it is a phenomenon perfectly congruent with quantum physics theory. And this has been considered “The most beautiful experiment in physics”; Which has been reproduced also with atoms and even with some molecules, made up of thousands of particles, in all these cases demonstrating the “wave – particle” duality of matter.
This quantum effect of wave and particle duality is clearly understood, but not without additional surprises; in another version of the same experiment a sensor was included, which measures how each one of these electrons act when crossing through the double slit, this being the equivalent of “observing” how electrons behave. When this new version of the experiment was executed another new surprise was detected:
“Being observed, the electrons start to behave again like particles!”
By setting this sensor, to measure and “observe”, what happens with these electrons, their wave function collapses, and the electrons manifest a deterministic behavior where they act only as particles, presenting a result where there is no interference, and generate a pattern where only two columns of impacts are distinguished, and the interference effect disappears.
The double-slit experiments continued, creating increasingly sophisticated versions of it, trying to understand in more detail this dual behavior of particles of matter; one of these was realized in 1978 by the physicist John Wheeler; this experiment called “Wheeler’s Delayed Choice Experiment”, the concept of the observer is applied; but in such a way that it presented a paradox that is even more intriguing.
In this version of the experiment, the sensor or “observer” is activated only after the electron has crossed the barrier with the double slit; this implied that, when not observed, the electron must have a wave behavior and present an interference pattern; since the decision to measure it or not is made until after the electron has crossed the barrier. But the big surprise was that again the result was a pattern with no interference, where the electrons behave like particles. But … if these were not observed when crossing the double-slit barrier, how can they behave like particles before being observed? The answer is even more intriguing and somewhat spooky:
“The causality of electrons behaving like particles goes back in time”
This is, the electrons know before crossing the double-slit barrier, that they are going to be measured, and behave like particles before knowing they will be measured, this happens in the past. The choice of what we are going to do as observers determines the previous state of the electron.
This will sound like fantasy, and could be considered to be an effect that occurs only in controlled conditions and short distance, but this is not the case; in astronomical observations it is feasible that these effects may occur in photons emitted by galaxies very long distances from us, and therefore the decision to behave as a particle was determined billions of years before they were “observed” by our monitoring devices.
Similar and even more complex double-slit experiments have been made; in 1999, the “Delayed choice quantum eraser” experiment was carried out by the physicists Yoon-Ho Kim and Marlan Scully, manifesting simultaneously and from the same electron source, a wave – particle behavior; making it clear that this duality is a fundamental property in nature.
But, what does all this means?
The quantum physics theory that explains this dual behavior of matter, was a work done by several physicists including Planck, Einstein, Compton and Bohr, but mainly by Louis de Broglie and Erwin Schrödinger around 1925. And it has become one of the great postulates of quantum physics. In essence, we all are subject to these quantum effects, yet the probability that these will manifest is much higher on a quantum (very small), scale.
But then, can these effects occur on non-quantum scales? In essence, yes. But the probability that an object at our scale has a behavior similar to the electron’s duality is so low that it is very likely that it won’t occur during the existence of the universe.
And this is just the beginning; as Niels Borh commented, “If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.”