Quantum Real Umm…

The theoretical minimum

"Anyone who claims to understand quantum theory is either lying or crazy." Richard Feynman

Physicists are outnumbered, and abominably so. Not even those of us who somehow graduate from this sizzling hot mess of a subject wish to pursue it any further than the sophomore year, let alone the first degree. Let us leave alone graduate, the absolute near majority of us physics graduates understand zero, zip, nada of it and I say it with no shame inside my chest.

Most modern physics finds its abode in Quantum Mechanics one way or the other, so it would be safe to extrapolate Feynman's quote to the entirety of the subject, as the subject is so pervasive it even found its way inside Sir Newton's (staunch classicist Pun intended. ) great works.

When the greatest of our breed lament over the condition of the subject, who the f*ck am I? I am a f*cking graduate is what I am!

So, to all the rookies and the fledglings: you will listen, and I will talk, and when you finish reading this masterpiece, you will bear the illusion that you understand what Quantum Mechanics is, but you can't.

Here goes absolutely nothing.

The What?

So, what is QM Quantum Mechanics will be referred to as 'QM' henceforth. you ask? Everything. And nothing.

Everything is made up of fundamental particles and is clear to all. Newton gave the laws of motion and gravity along with countless other contributions in optics, thermodynamics, mathematics (calculus being the stellar achievement here) and my personal favourites: alchemy and theology.

Everything was fine up until the moment a German with a peculiar moustache went berserk because a certain population was not following up onto the calculated way of life. That man found the solution to the problem: segregated it into the lowly and discarded its way of life as impure and puny.

And so, he gave birth to the ultimate society, to which those who believe in its superiority concede any criticism to be treason; where no lowly member was allowed to survive, and if found lingering: was somehow forced to follow in the German's footsteps or fall, and so they did.

Then emerged a superstar champion: Albert Einstein. Being the free minded and those from the oppressed, vehemently opposed the dictator's propaganda.

Wait, what? Who are you thinking I am referring to? I am not talking about Adolf Hitler and the holocaust; it is rather Max Planck and his QM. If you fell for it, that was precisely the point. The parallel was intentional — and yes, I am aware of how unhinged this reads.

A world where the champions of those fallen right before him were persecuted and so were their prophecies. Classical Mechanics: the physics from before the 1900s, the type we study in high school was persecuted, its laws were objectively, demonstrably inferior.

They were non-uniform and untidy. So came the final solution which made everything tidier once and for all, right? Right. Or so do the madmen say.

What actually happened was the tiny atoms and subatomic particles were following certain aspects observed by experimentation and predicted by advanced mathematics and theory. Planck fixed the erroneous classical physics to unfurl an even bigger mess, you guessed it right: The QM.

What QM tells you is that nothing can be said with absolute certainty, and what scientists and research tells you is that everything QM tells you is correct in the sense that it is amongst the most rigorously tested scientific theories, so what it tells you about uncertainties and probabilities is certain to be accurate to the highest degree. Clubbing the two results in informing us that we can be certain that everything is uncertain.

Take the example of a subatomic particle on a 1D axis for the time being. On the x-axis is the plausible degree of freedom — of all where the particle can be — and on the y-axis is how plausible is it for the particle to be on all those plausible points on the x. Reference: Introduction to Quantum Mechanics, David J. Griffiths.

If you deep it, it does not really make sense. If a car moving at 50 km/h has to travel a distance of 150 kms, assuming an ideal path, you can state with absolute confidence where it would be at the end of any given moment of time within the three hours long journey, but QM tells you cannot do it for anything smaller than a certain limit. Weird that the very stuff that is unpredictable makes up everything that is the exact opposite.

And you would be a fool to think this exotic sh*t happens only to matter, light is no exception: a moment it walks and talks like matter (anything that has mass and occupies space) and another it acts like a wave. Which, yes, of course, is quantum mechanics as well.

The particle (matter) nature of light was demonstrated by Albert Einstein through his evergreen classic of an experiment: The Photoelectric Effect — for which he won the Nobel Prize in Physics, 1921 — tells you that light is a particle.

In the world of the photoelectric effect, light's dance moves (photons) show that it doesn't behave like a wave here. Waves are like continuous, flowing motions, where energy spreads out as the wave moves along. But when we look at the photoelectric effect, we see something different. Light doesn't smoothly spread its energy to the material's electrons, like a wave would do. Instead, it comes in packets, like tiny individual dancers (photons) with specific energies.

These dancers approach the material's electrons like a knock on the door, one dancer at a time. And remember, to convince the electrons to come out and dance, the dancers (photons) must have enough energy ($E$) based on their frequency ($\nu$), according to:

where $h$ is Planck's constant — the physical constant that started this entire mess.

This "one-at-a-time" behavior of the dancers (photons) and the fact that they have specific energies is a clear sign that light doesn't act like a wave in the photoelectric effect. Instead, it behaves like a group of individual dancers, each with their unique energy, knocking on the material's door and interacting with the electrons in a discrete, particle-like manner.

Back in time by a few hundred years before the ultimate confirmation of the particle nature of light, Sir Isaac Newton was quietly brewing a revolution of his own in the field of optics; hell-bent on proving the particle nature of light and crushing any and all attempts to prove otherwise, he attempted to stick a needle in his eye, Yes, Newton literally poked his own eye with a bodkin to test if pressure affects color perception. The man was a menace. as they say, to check if pressure plays a role in our perception of light and its component colours. But one legendary Christiaan Huygens was hell bent on proving him wrong by pursuing his proposed 'wave-theory of light'.

More than a hundred years passed, and it was relatively quiet in the world of light, before a certain Thomas Young shook the physics community down to its last nerve — considering it came more than 50 years before the father of Quantum Physics, Planck, was even born — it was a groundbreaking deal.

Can light and light produce darkness?

A romantic would tell you otherwise, heck, even a hardened physicist would have told you a resounding no. But welcome to the world of physics, the answer came out to be a blinding yes!

What Young did was devise an experiment where light was made to pass through two tiny holes on an otherwise uncompromised thin sheet opposite a screen to host the coming light from the slits. And the result was rather baffling: if light was all particles, the result would have been in the exact pattern of the slits — spots proportional to the size of the slits in straight line paths — rather than a repetitive pattern of brightness and darkness. The double-slit experiment. If you only learn about one experiment from all of physics, let it be this one.

Waves have a characteristic property of interference: by construction and destruction, depending on the sync of their relative motion to each other — in physics we call it the path difference — they can either add up en-route or nullify each other. Reducing the magnitude is also an option on the table.