In fact, nobody can. Our picture of a Newtonian universe, where objects move around intuitively according to logical conclusions that physicists can draw, simply does not seem to apply at a sub-atomic level. Mathematics might explain the phenomena that we observe in reality; but does not tell us why reality happens. At some point, it must be accepted that there is no intuitive explanation that we can reside our understanding on.
Young’s “Magic Trick”
I will begin by admitting that there is no magic involved in the “trick”.
The experiment, first performed by Thomas Young in 1801, involves a beam of light fired at a sheet of material containing two slits, both of which the light can go through. Beyond the sheet is a screen, where interactions with the light can be observed. Before I explain the result of the experiment, I ask that you predict what you might see on the screen. If light behaved as a particle, you would expect each particle of light to travel through one of the two holes and carry on its trajectory towards the screen – at which we would see two distinct “piles” of light particles. On the other hand, if light behaved as a wave, you would expect to see an interference pattern on the screen: where the light waves travel through both slits, creating two new waves that interact with each other. This would result in an alternating pattern of light density when observed, known as an interference pattern.
However, what we observe is both. The light on the screen clumps up in a rough interference pattern reminiscent of a wave, however discrete points of greater and lesser density are shown, only possible if light were made up of particles. Since either one cannot exhibit both properties, then light must somehow be a particle and a wave. This is known by physicists as wave-particle duality.
Fascinatingly, this experiment has been repeated with electrons – with the same results gained! What this suggests is that electrons – which we previously knew to be particles – demonstrate wave-like properties. In fact, this experiment has been done with particles up to 2000 atoms in size, although results diminish in clarity by this point. In fact, everything can be described as a wave-particle. Anything bigger than a few atoms begins to demonstrate more particle-like properties, and anything smaller demonstrates wave-like properties.
So what then? The particle acts as a wave until it interacts with a surface, for example, in which case it collapses into a particle? A strange theory, yes, but not completely unreasonable. This, unfortunately, is not the case. If a detector is placed at each slit, which can tell if a photon (in the case of light) has passed through its respective slit, we observe that each detected photon travels through one slit only (just as a classical particle) rather than both (just as a wave). Stranger still, the particles do not form the interference pattern if one detects which slit they go through, as if the particle knew it was being watched and changed its behaviour. There is no definite conclusion as to why this is.
The double-slit experiment is well known for its clear representation of the fundamental mysteries that lie in quantum mechanics, all of which cannot be explained in any classical way.
The Wave Function and Superposition
When the double slit experiment is done with electrons, it can be helpful to think of the wave as one of probability – simply a mathematical “tool” as opposed to a real, physical wave. It is where the amplitude of the wave is greatest that provides the most likelihood of the particle being at that location.
Imagine a particle travelling towards the two slits. The particle will have its wave function, which is a function of position and time, mathematically represented by the Greek letter psi and written as Ѱ(x, t). The wave function is said to travel through both slits, as a spread-out wave would do. The new waves (which are actually extensions of the original wave function) created by the two slits interact with each other, causing the interference pattern to appear on the screen. However, remember that the wave function is simply a mathematical entity which we use to describe the position of the particle; the position of the particle on the screen is a result of the probability distribution of the wave function. If the experiment were repeated with many identical particles, we should see (due to the nature of probability) the interference pattern build up on the screen, using the density of particles at a given location to represent the greatest probability amplitudes.
This view of reality as probabilistic, that is, based on chance and not deterministic, made scientists at the time of its discovery uncomfortable, notably Einstein, who failed to accept it throughout his life. He famously stated, “God does not play dice with the universe”. It is true that this interpretation is difficult to comprehend for us as “macro beings”, where the probabilities of all individual particles amount to uncertainties in position that are too minute to ever notice at a large scale. For us, if an object moves forward, the object moves forward. It is a simple as that.
Another question occurs when you think about how the wave function evolves through the two slits, most importantly the logical fact that it does travel through both slits. What does this mean? Does the particle travel through both slits? We certainly could not imagine that. After all the particle is one object that can only go through one slit; indeed if a measurement is to be made, we can see that the particle does adhere to these classical rules. However, quantum mechanics states that the particle does in fact travel through both slits; it is in a superposition of both. When a measurement is made, the wave function collapses and the probability of the particle travelling through one of the slits is 0 – thus no interaction between waves on the other side and no interference pattern. Described here is one popular interpretation, loosely referred to as the Copenhagen interpretation, attributed mainly to Niels Bohr and Werner Heisenberg.
“Shut up and calculate!”
N. David Mermin coined the above phrase (often wrongly accredited to Richard Feynman), agreeing that whilst the Copenhagen interpretation can provide astonishingly accurate predictions, it does not attempt to explain the phenomena that we observe in physical terms. After all, the wave function is only a mathematical entity; it is not real (although some researchers are now debating this), so how can it govern the behaviour of a particle? How can the chance that a particle goes through one slit affect the particle's motion through the other? How can our interference affect the outcome of all this? It is like a video-game being rendered in the background whilst the player is not looking; the world does not have to exist to the player if it is not seen: only when the player observes something does it have to be created and displayed.
[Quantum superposition] is like a video-game being rendered in the background whilst the player is not looking; the world does not have to exist to the player if it is not seen: only when the player observes something does it have to be created and displayed.
The Copenhagen interpretation does not concern itself with this, opting to simply use the knowledge to generate predictions and meaningful conclusions. It does not matter why it happens, only that it happens. It is an interesting debate: should we seek knowledge that we cannot seem to understand? Some wonder how a theory so fundamental and meaningful to our society can simply be brushed aside. Quantum physics is used in computers, MRI scanners, GPS networks, toasters! How can we ignore its inner workings? Is it not human nature to understand?
One might argue that there is arrogance in this approach. What right do we have to demand anything from the universe? Why should the quantum realm be tailored to our inferior minds? Niels Bohr disputed Einstein’s famous quote (above): “Einstein, stop telling God what to do”, and indeed, just because we have absent knowledge, does not mean we can draw conclusions about a “greater scheme" at play.
Should we seek knowledge that we cannot seem to understand?
I have, of course, only scratched the surface of quantum theory throughout this article. Every path you find yourself travelling down in understanding only seems to open the door to more mystery. Throughout history, civilisations turned to higher beings when their knowledge of the world around them did not match their observations. If we just wait, through extended research and new experimental conclusions, will we eventually discover the truth behind the “magic”, or is it too off-putting to the macro mind? Will we never understand how the universe at the micro level really works? As exciting as the future of quantum understanding might be, for now, all we can do is shut up and calculate.