Quantum Mechanics- A conceptual introduction

“ Anyone who thinks they can talk about Quantum Mechanics without getting dizzy hasn’t yet understood the first word of it. “ - Niels Bohr

Quantum Mechanics is not something that can be predicted by using normal senses or intuition. From the dawn of civilization,we’ve been doing basic physics.Even in the stone ages,when someone threw a brick at us,we didn’t calculate the trajectory or the velocity vector of that brick to know where it’s path. We used our intuition,in a macroscopic world, we learned to predict as we became more and more evolved species.Everything follows our intuition when we do something in our day to day life,by using our classical mechanics and logical absolute way of thinking. But things are different in a microscopic level,those things are beyond our senses,and classical mechanics fails to describe it in an orderly manner.

I think our leap into microscopic world started when we really tried to push classical mechanics to objects at a very small level,in the nineteenth century,when we started using modern equipment to further experiment on small things. Even in the small level we tried to describe quirky behaviors of particles at a subatomic level using classical mechanics. But something different happened in the start of the nineteenth century, one radical physicist named Albert Einstein, published 4 scientific papers in his annus mirabilis year of 1905, one of which was asking the question,if time is absolute or not. According to classical mechanics,time was something absolute based on observation, we observed the change of an object into another state or the movement of an object to another place to give a definition to time, we used time in most of our day to day things,for example we used velocity which had change of time in its denominator,but the definition of velocity was completely mute,unless we can define time. Einstein did his thought experiments,like travelling on top of a light beam. Could we go past light? Or was it the physical limit of what we call speed. One another question he asked was how could we define time when we go to deep space,where there is no inertial reference frame to compare the object to,how can we define time over there based upon observation? As his special theory of relativity stated,time was not absolute,he successfully proved his radical proposition against 200 years of human thought and progression, standard Newtonian mechanics. One restriction that it had was Einstein at first could only do it to prove his theory in objects at a constant motion,not objects at a changing velocity or acceleration. But Einstein did fix that issue leading up to his General Theory of Relativity,which is far more sophisticated and radical,and described gravity as the curvature of spacetime.

Physicists started to think and question if Classical Mechanics was effective anymore to describe everything or not. Then one experiment changed the landscape of our observation of particles at a subatomic level for forever, which was the Quantum slit experiment, the Quantum slit experiment was completely counter intuitive as the classical way of thinking and completely denied what was observed. And it was actually an accident, not a controlled experiment. They repeated the same experiment at different speeds and even one particle at a time but the same unexpected results kept happening.

The result on the other side completely denies our intuition,as particles behave like when we throw two pebbles at a pond. It starts to behave like a wave,and it starts intersecting with each other creating a weird pattern on the other side as we see in the picture. An important version of this experiment involves single particles. Sending particles through a double-slit apparatus one at a time results in single particles appearing on the screen, as expected. Remarkably, however, an interference pattern emerges when these particles are allowed to build up one by one each and every time. This demonstrates the Wave-Particle Duality,which describes that the state of every single object has both particle and wave like behaviours. Where particles show us the exact pulse of something or a location of something in a certain spot and wave shows the probabilistic behaviour of a location of an object. And it also concludes that each particle can have a wave function,which completely opens up a new world of reality in front of us,the principles of quantum entanglement,where two of the particles can have same wave function,and can be entangled in a way that their spin would always be opposite no matter how far apart in the universe they are. And also one thing that is hugely weird is that the spins of these particle is completely at random,and it has the phenomenon named quantum superposition,which states that these particles can have two different spin states at the same time and it only breaks out this superposition state once we observe them. Schrodinger’s equation gave us the probabilistic outcome of the state or position of a particle,and he described superposition phenomenon using a cat and a box in his famous cat experiment.

All these different radical ideas has faced a lot of critics,even Einstein included,Einstein concluded that nothing in the universe is at random,as he said “God does not play dice”,he also argued that the probabilistic behavior in quantum mechanics came from the ignorance of pre-existing local variables,and was completely contradictory to his general theory of relativity,because for Quantum Mechanics to be true,information had to be transferred faster than light,he described this phenomenon with two of his fellows in Princeton in the famous “Einstein Podolsky Rosen” Paradox.

Einstein was proven wrong eventually as the ideas of Niels Bohr obtained more and more experimental proof,but the debate on quirky behaviors of subatomic particles,and whether they are completely random or not,still running wild.

Saikanam Siam

10th Grade

Brooklyn Technical High School,New York
Natore Govt. Boy's High School,Bangladesh