“Quantum”, what?
Quantum, what?
Max Planck (often called the father of quantum physics) is said to have narrated a story during one of his lectures. Around 1874, while beginning his studies in theoretical physics, Planck was apparently advised by his teacher Philipp von Jolly, that he might be getting into a boring field, as there appeared to be not much left to do in physics research. The teacher felt that everything that needed to have been discovered by physicists to understand how the universe works, had already been done!
Of course the erudite teacher would not have known, if this tale is correct, that he was addressing a young man who, along with many other geniuses like Einstein, Dirac, Pauli, Schrodinger and Heisenberg, would change the way physics would be viewed for the next 100+ years. He would in fact be hailed as the Father of Quantum Physics! However the story serves to mark an interesting point in time, when all that was possible in “Classical Physics” was thought to have already been achieved.
While the “Classical Physics” of Newton and those following him for the next hundred and fifty years had been brilliant at describing the world that we experience, it’s now known to not be enough, especially when :
(a) Things get really tiny (at the size of atoms and below), or for
(b) Things really far from each other (at astronomical distances), or for
(c ) Things are moving very quickly (at near the speed of light).
So does this set of deficiencies mark the end of Classical Physics?
Well, just in case you’re wondering, Classical Physics is by no means over and done with. It still works very well to describe our every-day world around us. Secondly, there are many things yet to be discovered, many open and interesting problems still around for Classical Physics to solve, hence there are many areas of active research in this field, including a lot of Engineering problems. Some of the current areas of research in Classical Physics include :
- Turbulent flows in fluids,
- Properties of materials,
- Acoustics, Optics, Electromagnetism,
- Nonlinearity, Chaos theory, and so on.
We can talk more about these in another piece.
However let’s pick up the story from where we left off. As Planck was getting into studying physics in the late 19th century, the English physicist James Clerk Maxwell had pretty much summarised the known laws of classical physics including different laws of conservation (eg mass, energy, momentum), electromagnetism, and thermodynamics into his laws. However, just a few years later, around 1905 when Albert Einstein published his papers on the theories of Relativity, it marked a key new area, that became part ofwhat began to be called Modern Physics.
Modern and Classical Physics co-exist in the 21st century
Modern Physics today includes fields like Relativity, Nuclear Physics, and Quantum Theory. SO Modern Physics has developed in the early 20th century and is going strong into the 21st, even as the older Classical Physics continues its merry march into the 21st century as well!
Rabbit holes
Lets open one of the doors of Modern Physics, choosing to descend into the rabbit hole that’s marked “Quantum Theory”. (We can get into “Relativity” and some of the others another time). There’s more than enough happening inside the Quantum Theory rabbit-hole for now!
Thomas Young and his double slit experiment
Young’s double slit experiment is perhaps the most interesting, and a somewhat spooky way to get an introduction into Quantum phenomena.
You can watch Nobel Laureate Richard Feynman’s lecture of over fifty years ago, at https://www.youtube.com/watch?v=2mIk3wBJDgE. It is a long but interesting video that shows the master deliver his brilliant lecture in a huge, packed classroom of students hanging on to every word.
A different, shorter, and very lucid explanation is available at https://www.youtube.com/watch?v=A9tKncAdlHQ.
What one realises through understanding the experiment is this :
- When waves, say light waves, are passed through two slits on an opaque barrier (i.e. two holes in a wall) onto a screen, alternate dark and bright bands known as “interference patterns” are seen to form on the screen.
- These interference patterns are formed due to the overlapping of the light waves emerging from the two slits. Bright bands are seen where the waves emerging from the two slits constructively interfere (add up), and dark bands are seen in areas where the waves destructively interfere (i.e. cancel out). This is an expected and well understood pattern since the days of Classical physics i.e. since over 200 years.

- Now when we switch from waves to tiny subatomic particles, say electrons, and fire them from an electron source through the same two slits made in the wall, we should expect to see two areas of accumulation of electrons on the screen behind (formed more or less in line with the two slits) — and not much else happening at other parts of the screen.
- In reality however, the electrons also form the same alternate dark and bright “interference-type” bands on the screen. So in some ways the subatomic “quantum” particles are showing us that they have wave nature as well i.e. they seem to have a duality about them— being both like a particle that we’ve fired, and like a wave that creates interferences with other waves, as it reaches the screen.

Duality sounds strange? Wait — it gets stranger!
- To better understand what’s behind this strange behaviour of the electrons, if we fire our electrons one by one thru the slits, and watch the screen behind, we see a steady build up of flashes as they hit the screen, abut in the end the flashes mysteriously form the interference pattern again! View this magic happen in this telling video of just over a minute: https://www.youtube.com/watch?v=ToRdROokUhs. It is quite difficult to explain this. You see the two clusters of electron hits being built up on the screen, approximately corresponding to the two slits..then suddenly they form alternate and multiple light and dark bands…
- Now, what if we set up sensors just beyond the slits, to investigate which slit each electron individually passed through, and what might have been happening between the slit and the screen that suddenly created these interference bands?
- On doing this we find that, exactly as expected, ~50% of all the electrons go through one slit, and 50% through the other but this time there is no interference pattern formed on the screen.
Huh? Why did they change behaviour? Did they know that we’re watching?
- Now remove, or switch OFF the sensors, and then fire the individual electrons. The interference patterns form once again! If now we switch the sensors back ON, no patterns! Weirder and weirder! It is as if the very act of observation changes the results!
And that’s the explanation that eventually emerged (attributed to Ernest Schrodinger and others) — that at quantum levels, there are neither particles nor waves, quantum entities are probabilistic events which get “forced” to take a value whenever we observe them.
If we choose to view the electrons at the final screen, they have to take up a value there (the bright bands are locations of their higher probability and dark bands are locations of lower probability).
If however we begin to track them with our sensors just as they pass the slits, each one is forced to choose a single path — either through slit 1 or slit 2 — as if it is a particle. So that determines the final image we see on the screen — of particle impacts, with no wave-like interference pattern!
These are the two great principles of Quantum entities — their ability to exist as a superposition of two states (wave and particle) at the same time, and their probabilistic nature!
There’s a third property in the Quantum world, which is that of Entanglement : irrespective of distance, whatever happens to one Quantum object also simultaneously happens to another one as if the two are connected or entangled.

This third principle has led to many interesting effects including Quantum Computing. But enough of complexity now, lets first mull over this much so far!