Skip to main content

Reality of Double-Slit Experiment

More than two hundred years passed after Thomas Young performed the famous double-slit experiment as a demonstration of the wave behavior of visible light and still it's revelation puzzles our sanity ever since. In short, if we shoot a beam of light at a panel with two small slits (less than a millimeter apart), the photons - elementary particles that light is made of, have to figure out how to get through the slits to radiate out the other side. If they are truly particles, like in macro world, they would project a solid image of two piles on the background wall behind the slits. If they travel similar to the waves, like water does in macro world, the image would resemble a wave-like interference pattern: alternating locations, equidistantly spaced, where particles leave a mark on the wall.

Thanks to the outcome of the experiment, we know that light is capable to do both. It always travel in wave like fashion, even if we shoot photons in a row towards slits, one after another. Quantum mechanics explanation is that photons are in superposition, meaning they can exist in different states and even multiple places at the same time. The weirdness comes 'only' if we try to tag particles with a pass-through detectors in order to detect which slit they are choosing to go through. At that instant they brake superposition and continue to travel as a macro-objects, just like bullets.

This is very similar to the coin flipping or well-known Schrödinger's cat from the macro world analogy. If we use quantum terminology, these two are in a simple binary superposition, they both have only two outcomes, the coin ending either head or tail or in cat's version being either dead or alive. Superposition in double-slit experiment is way more complex as for the photons being in multiple places at the same time between slits and the detection wall.

However, the weirdness is only present in the quantum level of the micro world and in the case of light in double-slit phenomenon, the puzzle is not the nature of how light travel but rather why it behave the way it does in the moment of being observed? Certainly, it creates profound questions for which we still have no definite answers. The most interesting one is did we find the puzzle here which shows us how nature really works? What is reality behind the engine in quantum world? More important is even the question is this reality objective throughout the universe or it is subjective and created for the observer only?

Let's think about reality problem for the moment first. This behavior of breaking superposition for the sake of observer is very reminiscent of graphically demanding video games in which the reality is never objective - the scenes are always rendered for the gamer sensory inputs - if you play those kind of games and decide to enter a closed room through the door, the room and everything in it doesn't exist at all until you open the door. Only then the CPU starts creating it for you and there's certain superposition of the room and your actions in it which breaks to the only one outcome, depending of what you do.

Now, double-slit is way to complex to test it this way, but binary superposition could be simple enough to create in lab in quantum equivalent and monitor what happens. This is what experiment made by Massimiliano Proietti and his team at Heriot-Watt University in Edinburgh, tried to perform on a small-scale quantum computer made up of three pairs of entangled photons. The idea is to experimentally test Wigner's thought experiment of an observer of the quantum outcome being also observed by second observer. The resulting statements of the two observers is that their interpretations of the outcome contradict each other. The same happens in the lab, with entangled photons in the role of non-conscious observers - the inequality in the data is definitely violated which points in the direction that quantum mechanics might indeed be incompatible with the assumption of objective facts. To put it simply, multiple observers of the same event can have different outcomes as the process in superposition breaks in different patterns. Just like in video games, the reality of the nature could also be subjective and rendered for the observer eyes only.

Of course this raises more questions and the main one is what observation and observer really mean? In quantum mechanics an observation is defined as the interaction of two quantum states that can collapse each other’s probability wave function. In one way or another this also means that by observing something we disturb it to the point of ruin the process we are trying to understand. If we add philosophical point of view we can also ask ourselves does consciousness play a role in the observation process? There's an interesting philosophical thought experiment starting with question: "if a tree falls in a forest and no one is around to hear it, does it make a sound?" followed by "can something exist without being perceived by consciousness?".

Well, if you ask me, consciousness or not, if we go this path with reflection to the original double-slit experiment it is all going toward the direction that unobserved world only exists in sort of superposition state, all possibilities of the all possible outcomes only waiting for an observer to disrupt it to the point of the ultimate collapse as the result of the reaction the two - process and observer. The light is no different, it's wave-like behavior is it's own superposition only waiting for somebody to play with. Preferably with lasers and Lego cubes, just like in above YouTube video. Science is fun, perhaps because it is so mysterious from occasion to occasion. I know I had tons of fun creating this video with my son couple of years back. Please find more stories within physics thread of the blog in below link.

Strange world of physics at MPJ:

Science refs:

© 2023 Milan's Public Journal