So what exactly is the infamous double-slit experiment of quantum mechanics and how does it defy reality? To understand this, let us first recall the behavior of particles and waves when they pass through a slit. Consider that you are bombarding a wall with marbles through a barrier containing a single slit. Some of the marbles would bounce off the barrier and some would pass through the barrier and hit the wall. If you mark the spots where the marbles hit the wall, it will roughly take the shape of the rectangular slit. Now suppose that instead of the marbles you project a monochromatic light at the slit; then you will get an illuminated rectangle on the wall. Moving on, let us consider a similar case with two slits instead of one. The particles passing through the slits would produce two similar rectangular impressions on the wall. But a light wave projected at this barrier would split into two waves – one at each of the slits. These waves, as they propagate beyond the barrier would interfere with each other. Where a crest meets the trough of the wave, they will cancel out each other and there would be no light, that is, it will form a dark band. And where two crests of the waves meet, they will reinforce each other and there will be light of greater intensity, that is, it will form a bright band. Thus, a pattern of alternate light and dark bands forms on the wall, which is called an interference pattern.
This is what the double slit experiment also does, but at a quantum level with electrons instead of marbles. An electron is a subatomic particle with negative charge. So when they are fired by an electron gun onto a screen through a single slit, as is expected, a single band is obtained. Similarly, when electrons are bombarded on a screen through a barrier with two slits, a pattern of two rectangular bands is expected. Surprisingly, the scientists got an interference pattern instead. To remove any possibility of the electrons having bounced off, interfered or co-ordinated with each other while passing through the slits, they were now fired one by one. Each electron hit the screen at only one point, like a particle, but the end result was still an interference pattern. This can suggest only one thing- that the electron split into two, passed through the two slits and interfered with each other like waves before hitting the screen like a particle. Baffling, isn’t it? To further uncover the mystery of the motion of the electron that was fired, they placed a detector at one of the slits that would give a signal each time an electron passed through the slit. And this time, as unpredictable and bewildering as it sounds, a particle pattern of two bands was produced. The mere act of observing collapsed the wave function. And with it, collapsed the whole idea of reality as we see it.
Naturally, many scientists and theories stepped in to explain this ‘weird’ phenomenon of electrons, among which most of you would be familiar with Niels Bohr and Heisenberg’s Copenhagen Interpretation. It is based on Born’s statistical interpretation which follows from the fact that we can only determine the probability of finding a particle at a given location; that we cannot pinpoint the location of the particle for certain. The Copenhagen Interpretation proposes that physical objects actually possess indefinite properties until the time that they are measured. So the electron that was fired from the electron gun can be considered as a wave of possibilities and when it is measured, either by the photosensitive screen or the electron detector, all the possibilities collapse and reduce to only one of the possible locations, where it hits the screen or passes through the slit respectively. So now we can rightfully ask whether all of reality is just an enormous set of probabilities that collapses into something definite yielding to our perception. And if you have read part 1 of my post, you can also ask, is the scrap of paper real?
Particles are what form the matter or the substance around us. The solidness of what we touch and the feeling that something exists is because they are made up of matter. What if there is no matter? What if everything is just a wave held together as matter only because we are observing it? In that sense, reality would be, simply put, a figment of our imagination. And where does that put us in the whole scheme of the universe? Are we waves or particles or nothing at all? Even as these questions remain unanswered, we have to understand that this is just one point of view among many. There have been a number of arguments made against the Copenhagen interpretation, the most popular one being Schrodinger’s cat experiment. Can we really decide the fate of the cat in the box by looking at it? It definitely sounds absurd. Or as Einstein put it very deftly, “I’d like to believe that the moon exists even when I am not looking at it.”
Irrespective of what quantum mechanics says, our immediate sense-interactions with the world do not change. So even though I began the essay by asserting that reality is not real, I do not intend to force the idea on you. My intention was only to make you question what you call reality, to make you realize that the whole lot of material things you assign so much value to MAY not even be real. That the reality you take on face value is just skin-deep but if you dig deeper, you do possess some power, no matter how big or small, over your reality through controlling your perception.