What is Quantum Mechanics

The topic though very fundamental, yet doesn't seem to be a common question, as it has never been asked to me, and I believe, to any of us in any of the exams. And specially in these days, when many of us are studying physics, so as to ensure a first division and get a seat in a professional course, it is a neglected question – for its not found in the question bank.

But dear friends, I tell you the answer is pretty interesting. Well if you can spend five minutes reading this article till end; I tell you, it won't be that bad a deal.

Now coming to the central theme; what is quantum mechanics? It is the stream of science that deals with objects small in absolute sense. In my day to day life I can't imagine that I put a meter scale straight to measure the length of an object and the object changes position. Exactly this is the case quantum mechanics deals with. The sense of large and small in physics is not what we say in our day to day life. Small object means any such object that gets disturbed the moment we try to observe it. In fact all the larger objects we see around are composed of these tiny objects called atoms and molecules. And these objects are so small, that it becomes difficult to locate them or make any precise statement about them. So, if we want to know more about the larger things around us, we need to look at these tiny particles and that vision is enabled when we resort to quantum mechanics.

I give you one example. I go for a walk along the city streets in the evening. I see that the leaves of trees are moving with gust of air. If I ask you a question, what do you think, is the air gust causing the leaves move up and down or is it the other way, i. e. since the leaves are moving we are feeling this gust of air? You would laugh at me and say how can it be the other way. Very correct you are. Its the gust of air that causes the leaves move and not the other way round. But what if I ask you that, there is a single photon that I let fall on a beam splitter, which way will it go. Will it be transmitted or will it be reflected? Puzzled you are, yes, so would any undergraduate student be. As none of our classical observations can answer this question – for we always talk about ray of light or a light wave that is half reflected and half transmitted by the beam splitter. A beam or ray of light is nothing but congregation of many photons. So what we see is the combined effect and no one of us thinks about the fate of a single photon. What I meant to say is we are habitual with the world we see and that is a world of larger objects that are constituted by smaller objects. Since we see the larger objects only, we never in general, think of the behavior of the smaller objects constituting them – just like we don't think of a single photon and think of a beam of light. And its very true that our classical observations and knowledge fails to answer the questions related to these smaller constituents. Quantum mechanics is a tool that enables us to find answers to such questions. For it can talk about a single photon or a single electron, if not with absolute certainty, within an admissible limit of error. This error can be tolerated; for nothing is perfect. And I believe, its always better to know something partially true than not at all knowing it, so far as science is concerned.

(This article is the extract of a discussion with Dr. J. T. Andrews, Reader, Department of Applied Physics, SGSITS, Indore.)