Quantum Physics Part 1 : The world inside an atom


If we throw a ball up in the sky, we can use the well-established laws of physics to predict its motion and various other properties such as speed , velocity, maximum height and various other factors at a given moment in time, if we are provided with the right set of tools. The result we obtain are concrete and are more or less accurate with what is observed in practical life. These laws of physics that govern motion and dictate the behavior of macroscopic objects (collectively known as “mechanics”), disappear while looking at the world many times smaller than what can be seen through the naked eyes . Sub-atomic particles at the quantum level, such as electrons exhibit very different properties and, the laws that are used to describe them are known as quantum mechanics.

Advent of Quantum Theory

An experiment conducted in 1913  by Neils Bohr, showed that when the light emitted by gases is passed through a prism, it displays light of specific colors, not as a continuous spectrum of a rainbow, but as distinct sharp lines with gaps in-between. Bohr, in an attempted to explain the phenomenon proposed that electrons in a orbit exists in distinct orbits, and jumps to outer or inner orbit, depending on whether it gains or loses electrons. This theory was counter-intuitive and unlike any other physical phenomenon experienced in daily life. This is because any fundamental unit– whether its mass or length or even the compound ones such as velocity or volumes were known to have existed in theoretically infinite amount of states(the number of real numbers in between two intervals. An example of this would temperature. Although the minimum value of temperature is the absolute zero or 0k, we known that it can be of endless values – 1K, 2K and so on , not to mention the decimal values in between). This was the earliest known experiments that started unraveling the strange behavior of particles at a quantum level. Due to this behavior, the energy level of electrons around an atom was said to be quantised.i.e. able to exist in distinct states and nowhere in between.

Another ground-breaking experiment conducted was known as “Young’s Double Slit Experiment” (abbreviated as YDSE) by Thomas Young in 1801 that shows the wave behavior of electron particles. Before this experiment, electrons were considered only as solid objects, that exhibit properties similar to that of macroscopic objects such as a tennis ball.

Understanding the Wave Nature of Electrons

Instead of delving right into the experiment , let us understand this wave property through a simplified analogy . Consider a pack of 100 tennis balls and two screens A and B such that A is placed behind B. Screen A has two distinct holes/slits, with sizes just enough for the tennis balls to pass. In order to record the positions, the balls are dipped in blue paint, and are shot in the direction of screen A perpendicular to its surface (in a rectilinear manner). Every time we throw a ball, that there might be two possible cases.

Tennis ball.jpg
Figure A

Case I:

Either, the ball passes through the slit and makes a round blue mark on B, in the area directly behind the two slits of screen A as shown in figure A.

Case II:

The ball hits the screen A and bounces back.

The whole point of this experiment was to demonstrate that the balls can hit B only on two distinct areas behind the slits and nowhere else. This observation is constant with any number of painted balls and is applicable for any matter particles.

But if the sizes of these balls could shrink substantially to a size comparable to that of an electron, then the outcome would have been different as shown in Fig B. The balls(or electrons) would not only hit the area behind the mark, but a larger area(linearly blocked by A) behind the slits as distinct lines. This tells us something about these smaller particles that couldn’t be explained by classical mechanics (properties of macroscopic objects).

Interference Pattern.jpg
Figure B

The pattern made by electrons is more similar to light(photons), than tennis balls, i.e. it moves in wave-like manner, with crests and troughs and not unlike a wave, interferes with each other after passing through slits, either increasing the amplitude, decreasing the amplitude or cancelling each other out, in the process.

The electron-wave in question, doesn’t entirely comprise electrons and is different from a wave of water , where the given volume comprises matter particles. It is instead, a probabilistic wave, that help us determining the probability of finding an electron given by wave function below, at a specific point in space. The volume of the wave is probability density cloud, and this density varies at different point in space. The higher the density, the higher is the probability of finding the electron in that place.


Thus, electrons exist throughout its entire orbit as a probability distribution. Albert Einstein, one of the greatest scientists to ever live, did not believe that the fabric of reality at the sub-atomic level relied purely on chance and said “God does not play dice with the universe”. To conclude our discussion, quantum theory , despite a large number of discoveries,  has still a long way to go to be qualified as a complete theory and has a large set of debatable theories to be discussed in the later articles.



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