Over three hundred years ago, Sir Isaac
Newton revolutionised the study of the natural world by putting
forth laws of nature that were stated in mathematical form for the
first time. Newton's book, The Mathematical Principles of
Natural Philosophy [1] forever changed how
scholars would study the physical world. Newton's formulation
of physical laws was so powerful that his equations are still in
use today. [2]

By the start of the 20th century, physicists had worked with
Newton's laws so thoroughly that some of them thought that they
were coming to the end of physics. In their opinion, not much
was left to do to make physics a complete system. Little did
they know that the world they described was soon to be understood
in a completely different way. The quantum revolution
was about to happen.

This revolution was begun by a very unlikely person, a physicist
named Max Planck, who was very conservative in all his
views. It speaks well of Planck's intellectual honesty that
he was able to accept the reality of what he discovered, even
though he found the consequences of his discoveries distasteful and
unpleasant for the rest of his life.

Max Planck *

Born in 1853, Max Planck came from a conservative and
respectable family in Kiel, Germany. Young Max was very
bright, and had a variety of fields from which to choose to study
for his professional life. Planck chose physics because he
felt that it was the field in which he was most likely to do some
original work. At the young age of 21, he received his
doctorate in physics from the University of Munich.

Planck was investigating the properties of heat- and
light-emitting bodies. Classical physics had theories which
predicted that the brightness of a body increases
continuously as the frequency [3] of its
electromagnetic radiation [4] is increased.

Fig. 2-1: Intensity vs. Frequency Plot

Unfortunately, experiments revealed a totally different picture.
The brightness did increase initially, but only upto a
limit. Then, actually, it began to fall. We thus
get a bell-shaped curve if we plot frequency against brightness.

Besides, another observation was made: as bodies become
hotter, their maximum brightness shifts towards higher
frequencies. This is why an object, heated to 300-400 C,
emits mostly infra-red or heat waves. As the temperature is
increased, the object appears to be red, then orange, and finally
white or even blue.

Classical theories totally failed to explain this discrepancy
between the known facts and the observations. Then, in the
winter of 1900, Max Planck found a solution to this problem.
Planck ushered in the quantum era by making a bizarre assumption:

Emission and absorption of energy can occur only in
discrete amounts.

This might seem totally unsurprising to you, but believe me, it
shook the scientists of that period. Planck himself did not
know he would end up with this statement!

Imagine for a moment that you are a sculptor, and you have
obtained a piece of stone in the shape of a cube. To begin
your sculpture, you take a chisel and place its edge against the
stone, and then strike the chisel with a hammer. What do you
imagine would happen? I think you would imagine that a piece
of the stone would be split off, as well as some smaller splinters
and pieces of stone. Imagine instead that when you struck the
stone, it broke into hundreds of small cubes, each one of them
exactly the same size, 3 centimeters per side. Wouldn't you
be surprised, even shocked? Imagine furthermore that no
matter how hard you tried, these smaller cubes could not be broken
into smaller pieces at all!

We think that the reaction that a sculptor would have in such a
circumstance would be similar to what Planck and other physicists
felt upon discovering that energy only occurred in discrete
amounts. It was a completely unexpected discovery, and yet it
was only the beginning of what would come later.

Planck called these discrete lumps as quanta. This
was against the entire world-view that had been built from the time
of Newton onward. In the physics that had been built up since
the time of Newton, and indeed in the minds of most thinkers before
Newton, matter and energy were thought to be smooth and
continuous. Even by the time of Planck, the idea that matter
could be ultimately broken down into tiny indivisible 'atoms' was
only held by a few physicists.

And so the quantum revolution began.

Footnotes

1. Natural Philosophy was the term
used in Isaac Newton's time for what we now call 'the
sciences'. 'Science' would only become distinct from
philosophy much later. The term 'scientist' was only coined
in the middle of the 19th century. Back

2. Newton's physics gave a very good
understanding of the working of gravity, as well as other natural
phenomena. Although Newton's interpretation of his
discoveries has mostly been abandoned, the mathematical laws that
he discovered are close enough to what is actually found in the
real world that they are still useful for many things, from
engineering to astrophysics. Back

3. Frequency is a term associated with
waves, and refers to the number of waves that pass through a point
in a given time interval. For example, if you can count three
'heads' of ripples pass by you in a pond in ten seconds, then the
frequency is 3/10 Hz. Hertz is the unit of frequency.
Back

4. Electromagnetic radiation consists of
fluctuating electric and magnetic fields. In the increasing
order of frequency, radio waves, microwaves, infra-red radiation,
the visible light (red through violet), ultra-violet radiation, X
rays and gamma rays are all electromagnetic waves. They are
caused by changes in the electric field or the magnetic field, and
they travel at the speed of light, which is 300,000 km per
second. Back