One of the most famous experiments in physics is the “double-slit experiment.” This experiment teaches us with an unparalleled strangeness that light or matter sometimes behaves as a wave and at other times as particles. Also, the experiment shows that observation of a particle influences its behavior. This seemingly simplistic experiment is invaluable in quantum mechanics.
What is the Double Slit Experiment?
In the experiment, the researchers sent a beam of light through a barrier with two slits. Two slits are cut on a metal plate. They sent the light through both slits. First as a constant wave, then into separate particles. However, what is happening is anything but easy to explain and has sparked one of quantum mechanics’ biggest debates.
Particles in waves
In the early 1800s, most scientists believed that light was made up of particles and not waves. English scientist Thomas Young had a premonition that particle theory was not the end of the story and wanted to prove that light was a wave. He knew that waves interact in predictable ways (that is, in fixed patterns), and if he could demonstrate those interactions with light, he could prove that light was indeed a wave. So he started an experiment: he cut two slits in a metal plate and shone light through them on a screen.
If light were indeed made of particles, the particles hitting the metal plate would bounce off, and the particles that would pass through the slits would show the two slits on the screen. However, if light were a wave, the experiment would be very different: once the light had passed through the slits, the light waves would fan out and interfere with each other.
The highlights of the waves converge. They would amplify each other and leave a brighter spot on the screen. They would converge with a peak wave on one side and a trough on the other. They would cancel each other out and leave a dark spot on the screen. That would create a so-called “interference pattern,” showing a very bright slit shape surrounded by “echoes” that appear fainter on both sides.
This was the outcome of the experiment, and light travels in waves.
Refinement Double Slit Experiment
Around the turn of the 20th century, some scientists began to refine this idea. Max Planck suggested that light and other radiation types occur in quantified amounts (i.e., not a constant amount). Albert Einstein proposed the idea of the photon, a “quantum” of light that behaves like a particle. He said that light was both a particle and a wave.
Let’s go back to the double-slit experiment. If the light had been a particle, there would have been an irregular pattern on the screen instead of an interference pattern. The equipment available to the scientists enabled them to send the light particles through the slits one by one. When the scientists did this, something strange happened. The interference pattern was still visible, and no irregular pattern appeared on the screen.
The interference pattern suggests that something very strange is going on: the photons seem to “know” where they would go if the researchers sent them in a wave.
You can imagine it as a school of fish that knows exactly where to swim, with each fish individually flawlessly knowing which spot to occupy without disturbing the pattern.
This means that “all possible paths of these particles can disrupt each other, even if only one of the possible paths occurs.” The analogy here is that all realities exist simultaneously until the end result occurs.
When the scientists placed detectors at each slit to determine which slit each photon passed through, the interference pattern disappeared. This suggests that observing the photons alone kills many realities.
What do we learn from the double-slit experiment?
What does the experiment tell us? It suggests that particles, like electrons, somehow take on the properties of particles that are sent through the slits as a wave. That’s the famous wave-particle duality within quantum mechanics.
It also suggests that observation, the measurement of a quantum system, has a profound effect on the system. The question of exactly how that happens is the measurement problem of quantum mechanics. For that reason, it will take a while before a good explanation can be found for the results of this experiment.
What the Double Slit Experiment teaches us is that our consciousness – that is, the measurement of the photons in this experiment – probably determines our reality.