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The Double Slit Experiment: Does Consciousness Change Reality?

Carolina is a journalism student based in Canada who enjoys learning and sharing information about how to lead a meaningful life. She is passionate about traditional culture, handmade crafts, and the connection between humans and nature.
Published: August 17, 2022
Quantum mechanics continues to be an enigmatic field. According to some physicists, the quantum world is defined and modified by human consciousness. For others, all possible quantum events are physically realized in independently coexisting worlds or universes. Why is the world at minute scales so difficult to study? (Image: geralt via Pixabay)

Have you ever wondered what the world would be like without human beings? Some scientists claim that it would simply continue its natural course with countless species inhabiting it and becoming extinct. Others, challenging existing conventions, have dared to assert that it would not exist whatsoever, since reality may be a creation of human consciousness. But what is the basis for these claims? Let’s look at one of the enigmas that physics has yet to unravel.

Particle vs. wave behavior

According to quantum mechanics, all existing particles behave as either a particle or a wave. While particles are distinct and occupy a specific space in the universe, waves — as propagating entities — are described in terms of probabilities, that is, the likelihood of finding that particle at a given point.

To illustrate their behaviors, imagine two parallel walls, facing each other, with one of them having two slits in the center. If we stand on the side of the wall with the slits and splash ink on it, the traces of ink — those that passed through the slits in the wall — will most likely form shapes on the opposite wall, similar to those of the slits. This is how particles behave.

Waves behave differently. When a wave — such as light — is made to pass through the double slit wall, it will split into two new waves that will interfere with each other after crossing the first wall. This interference pattern will appear on the other wall as a series of luminous streaks. 

Photo of the double-slit interference of sunlight. (Image: Aleksandr Berdnikov via Wikimedia Creative Commons)

Where things get strange

By observing the bright stripes that light forms after passing through the slits, it can be concluded that light is a wave. But isn’t light composed of photons, and aren’t photons particles? If light is a stream of particles, shouldn’t they behave as such? 

In fact, if light is made to flow through a single slit, it indeed behaves as a particle, drawing a slit-shaped strip. It is only when a second slit is open that the wave behavior is seen. 

But what if we fire each photon one by one through the double-slit so that they don’t interfere with each other? Intriguingly, photons released individually still draw the interference pattern on the rear wall, with each photon contributing a “dot” of the overall pattern. 

Puzzled by the dual behavior, scientists have tried to observe how photon particles create the interference pattern. Is it possible that each particle somehow splits itself to pass through the wall openings, interferes with itself and then comes back together to meet the rear wall as a single distinct particle?

After several years, this phenomenon remains unexplained — not because of scientists’ poor understanding of the data — but because of their inability to collect it.

It turns out that as soon as a detector is placed on the model to witness firsthand when and how these entities begin to behave as waves, they mysteriously change their behavior to that of particles, drawing the double-slit shape on the back wall. But once the detector is turned off, the wave pattern reappears. 

Results of the double-slit experiment when the detection screen is located close to the slits and when: a) Only the top slit is open (left figure) b) Only the bottom slit is open (center figure) c) Both slits are open (right figure) The blue dots correspond to the impacts of electrons or photons. (Image: Gondran via Wikimedia Creative Commons)

Is it that the particles know they are being observed and consciously decide to behave differently? Or is it that the detector itself somehow interferes with the wave causing it to collapse and show a different pattern on the wall? Interestingly, after numerous tests, physicists have discovered that the phenomenon persists even if the detector changes locations. 

The Measurement Problem

The inability to determine why and how a wave collapses when measured has been referred to as the “Measurement Problem” in quantum mechanics. Although scientists have tried to study the moment right before the particles enter the slits — where the decision to collapse is presumably made — and the moment right after traversing the first wall — where the interference apparently takes place — they have found that the experiment cannot be cheated: each time a measurement attempt is made, the wave pattern disappears only to reappear when the particles are not observed. 

But this raises many questions: What is measurement, anyway? Does it take place when the detector is turned on? Is it when the instrument records the data? Or is it when a consciousness interprets this data? Would the system collapse without an observer? 

While there is no universal agreement among researchers, most explanations fall within the framework of two opposing concepts. 

Two main physical interpretations

The Copenhagen interpretation is the most widely taught today. Formulated by the Danish physicist Niels Bohr and the German theoretical physicist Werner Heisenberg, it establishes a clear distinction between the observer and the observed system and asserts that quantum descriptions are objective and independent of the mental arbitrariness of the observer. 

Thus, its position on the measurement problem is that wave collapse in the double-slit arrangement is not subject to the presence of an observer — that is, to the interpretation of a conscious mind — but is an event that will always take place according to physical principles and calculated probabilities. 

Conversely, the von Neumann–Wigner interpretation claims that wave collapse is caused by the consciousness of the observer. By considering the human mind to be the only genuine measurement instrument, this interpretation places “subjective perception” at the core of its postulate. 

John von Neumann (right) was a polymath considered the greatest mathematician of his time, with an unprecedented mastery of both pure and applied sciences. (Image: Dencey via Wikimedia Commons)

The scientists concluded that “factors associated with consciousness, such as meditation experience, electrocortical markers of focused attention, and psychological factors including openness and absorption, significantly correlated in predicted ways with perturbations in the double-slit interference pattern.”

Although these results are consistent with the consciousness-based interpretation of quantum mechanics, further research is required to confirm and systematically replicate this interpretation, as it would open the door to challenging philosophical implications.   

An alternative explanation

Aiming to give sound resolution to the wave-particle duality, the American physicist Hugh Everett proposed the Many-worlds interpretation in 1957. According to this theory, all possible outcomes of the quantum measurement are physically realized in a different world or universe.

This means that in the double slit experiment, when we pass a beam of light through the first wall, there is one reality in which the wave collapses and another reality in which it does not. There is also another world in which we make observations and another universe in which no measurements of any kind are made. According to Everett, all these probabilities take place simultaneously and do not depend on each other, nor do they interact with each other.

Although the many-worlds interpretation was heavily criticized at first, its popularity has grown in recent years, with prominent physicists such as Sean Michael Carroll espousing this theory. The question of why these alternative realities are inaccessible to humans remains. 

The Many-worlds interpretation also offers an explanation for the Schrödinger’s cat paradox, in which the cat is alive and dead in different branches of reality. (Image: Physikinger via Wikimedia Creative Commons)

Recent findings

According to a research paper published in 2010 by Shan Yu and Danko Nikolic, wave collapse occurs when it’s measured even if nobody is watching. To obtain this information while avoiding interference from any observer, the measurements were recorded in the state of an atom and preserved for later interpretation. 

This breakthrough made scientists reevaluate the concept of “observer.”  Could it be possible that particles were being observed by entities that are not perceptible to humans? What if the Universe were the observer? Would that mean that the Universe is a consciousness as a whole? 

In addition, a 2019 research paper by Massimiliano Proietti, a PhD student at Heriot-Watt University, provided evidence of the coexistence of different realities for the first time. Through a carefully designed experiment, Prioetti and his team discovered that quantum mechanics can allow two observers to experience different and irreconcilable realities. The existence of an objective reality, on which science bases its measurements and fundamental facts, is now being reevaluated.