Birth of the Quantum Physics

For centuries and millenniums, our ancestors looked towards the sky and were amazed by the presence of the celestial bodies. From Aristotle, Copernicus, Galileo and Kepler, all these men who devoted their entire lives to study the cosmic beings much bigger than human lives, till Newton and Einstein who were to formally explain how our entire cosmos worked using gravity, physics was slowly breaking out of its cocoon and spreading its wings into the world.?

After being mesmerized by the cosmic events of the stars, galaxies black holes and all things unfathomably larger than us for all these years, finally in the early 1800s the focus of physics was shifting from something immeasurably large to the immeasurably small - the world of particles. The physicists were now interested in knowing how the constituents that made up our universe behaved. Or more fundamentally in some sense, they were trying to figure out what we were exactly made up of? The fact that everything in the world could be reduced to atoms was starting to become common knowledge by the start of the century. They were now digging for more treasures to unravel in the microscopic world. And at this moment in history, the physicists of that time were looking at what we know today as the Quantum Realm.??

We have all seen a football getting kicked and flying in air doing a projectile motion. It forms an arc shape in the air and gracefully falls to the ground, thanks to gravity. Nothing weird here. Now what if I tell you instead of the ball smoothly gliding in air and performing its projectile motion, the energy gained by the ball is weirdly increasing in steps and decreasing in steps as well? Well it should look something similar to the image below.?

Now this is weird right? Why would anybody see this happen in normal everyday life? When we kick a ball, the ball receives the energy given by us and it should just normally increase and decrease continuously as it flies up and down.?

Well yes you're right. That's exactly what happens. But here’s a catch. The energy seems to be flowing continuously only because we are looking at this entire scenario at the macroscopic level.?

Alright then lets try zooming in on the football that we already kicked and try watching what's happening to the atoms present in the ball that absorbed the energy given by us. When you look at the microscopic world, the exchange of energy between the atoms present at the tip of our toes and the atoms present on the surface of the point of contact of the ball seem to exchange energy discontinuously in certain fixed bits. Even when you look at the atoms that were earlier in ground state and later went to an excited state on receiving energy, did that in certain fixed jumps but never continuously. As weird as it seems, the energy in the microscopic world is discontinuous or rather quantized. Energy seems to be increasing only by certain fixed amounts. Like for example, if you had 2 Joules of energy with you, the law dictates the increment can happen only by adding 1 Joule of energy, so that you have 3 Joules with you now. The increment is always going to be an integral increase, it can never happen by 0.5 Joules or 1.5 joules per say. It's forbidden by nature and these are the rules with which she plays in the Quantum Realm.? This is the meaning of quantum physics in its truest essence, all the physical parameters like energy, momentum etc are quantized.?

Now you can argue that if this is the case then why don't we come across it in everyday lives? Why is our ball still smoothly and continuously increasing and decreasing its energy then? Well the simple answer is because even though at the microscopic level the energy is discontinuous, macroscopically it appears continuous because it's all about the scale with which you look at things. Closely spaced lines can look like a block of colour, but on magnifying it you get to see two separate lines, isn't it? That's exactly what is happening here as well.?

Now that we know what it means for the Quantum Realm to be quantum. Let's go back two centuries in the past and see how it all began.?

In the 1890s when the blackbody spectrum was first found out, all the physicists were extremely excited about it. Well before we head into it let me explain a little about blackbodies and their radiation.?

Blackbodies are ideal objects which can absorb or emit thermal radiation of all frequencies. So when these blackbodies absorb and emit radiation of different frequencies, what we get is called its blackbody spectrum. And when the experimental graph of energy density vs wavelength for different temperatures was obtained for the blackbody radiation it looked something like this.?

The theoretical physicists of that time were now given a task to explain this graph and come up with a formula that could exactly fit the experimentally obtained graph. The first attempt was made by a German physicist Wilhelm Wien but he was only able to explain shorter wavelengths with his formula. The next flicker of hope to explain this came from two scientists Rayleigh and Jeans, who together popularly gave the Rayleigh-Jeans formula. But again even this formula had problems. For shorter wavelengths this formula was tending to infinity which doesn't happen in reality. Experimentally the graph needs to come to zero for shorter wavelengths. And because shorter wavelength range is where UV radiations are found, this problem is even today infamously referred to as the Ultra-Violet Catastrophe. ?

Now in the pursuit of solving this problem, another German physicist Max Planck took a close look at the Rayleigh-Jeans Law. He observed that the only way this problem could be solved was if our initial assumption didn't assume the energy to be continuous. Planck in his blackbody radiation theory made a ground-breaking assumption that the energy exchanged between the walls of the cavity containing the radiation and the radiation itself happened in certain packets of energy. And this produced astonishing results. Now the formula was exactly matching with the experimental graph.?

Planck’s assumptions on blackbody radiation were revolutionary. Who had even in their wildest dreams ever imagined that energy does not flow continuously but rather came in bits of fixed amounts. It was very difficult for the physicists of that time to absorb this new piece of information. But nevertheless nature played by her own rules and this was the kind of universe we lived in where energy came in small packets. January 1901 when Planck published his paper, a new sun had risen on the coast of physics. The century-long monarchy of the classical physics was shattered and overthrown by one single equation. The world of physics underwent a revolution no one saw coming - the Quantum Revolution.

?Four years later, in 1905 Einstein applied the same theory and gave us the Photoelectric Effect, which established once and for all that light/radiations are made of small energy packets called quanta. Even though the term quanta was coined much later in 1926 by Gilbert Lewis in his letter to the reputed publication ‘Nature’. Further, not long enough in 1913, Bohr used this theory in the atomic realm and gave us what we know today as the Modern Theory of Hydrogen Atom. One could see that the revolution was fast spreading. Neils Bohr and Max Planck became the founding fathers of Quantum Physics, later followed by Einstein.?

Einstein’s photoelectric effect had spurred the debate of wave-particle duality of light. Further to only discover that even matter behaves like waves by De-Broglie who gave us his infamous De-Broglie equation. Only much later in 1926 the bewildering weirdness of the quantum world started to surface when Erwin Schrodinger gave us his Schrodinger’s equation. Spooky phenomena like superposition and entanglement came into picture and since then the perplexing mysteries of the quantum world haunt us till date.????

No matter how far ahead we have come today with our theories in understanding the microscopic universe better, it was the genius of Max Planck which broke out of the shackles of the classical universe and gave us an entirely new way of looking at things. It was the paper of 1901 on the Theory of Blackbody Radiation by Planck where Quantum Physics was first born.?