International, Technical

The letter that triggered Quantum revolution

In the late 19th century, experimental techniques had improved sufficiently such that it was possible to make fairly precise measurements of the energy distribution of thermal radiation. Measurements showed that the intensity of radiation from a hot body at a particular temperature peaks in the middle of the visible part of the spectrum. It decreases beyond the low frequency infrared and high frequency ultraviolet radiations.

When Lord Rayleigh and James Jeans analyzed the aforementioned characteristics of thermal radiation using the physics of the time –radiation is wavelike in nature and radiation energy could have any continuous value, their result differed drastically from experimental observation. In particular, it diverged as the frequency of radiation became greater than the frequency of ultraviolet radiation. This divergence was dubbed by physicists the “ultraviolet catastrophe”.

The solution to this grave anomaly was found by Max Planck (1858-1947), a German physicist whose tutor Johann von Jolly advised him in 1878, when Max Planck approached him as a prospective undergraduate student, not to study physics because all the important problems of physics had already been solved and there was nothing left to discover except to “fill a few unimportant holes.” Nevertheless, Planck studied physics and on 19 October, 1900, he proposed a simple formula that explained thermal radiation across the entire spectrum perfectly. He had no explanation for the formula – it was an empirical formula that just happened to work.

Prof. Max Planck

According to Planck, the amount of energy contained in radiation of any specific frequency could only be a multiple of the fundamental frequency, ν multiplied by an infinitesimally small quantity denoted by the letter h. Stated simply, the energy of radiation could be hν, 2hν, 3hν, and so on, but nothing in between. And the energy can jump from one value to another in discrete steps only.

When he presented his theory of radiation to the Prussian Academy in Berlin in December 1900 and introduced the now famous constant h that bears his name, Planck thought he was offering a refinement to classical theory, fixing a little flaw in a solid edifice. Instead, he destroyed it with his remarkable prescient suggestion that energy comes in discreet packets known as quanta. Five years later, in 1905, Einstein gave the seal of approval to Planck’s idea when he established that energy quanta were real entities, not merely mathematical abstractions. In 1918, Planck was awarded the Nobel Prize for giving physics a new lease of life.

Max Planck did not see himself as a revolutionary. Yet, his formula, particularly the letter h of the Latin alphabet, triggered the quantum revolution that eventually unraveled the entire fabric of what had passed for reality. The revolution overthrew the common sense laws of science and replaced them by a bizarre set of rules in which causes were not guaranteed to be linked to effects, rules that allow a subatomic particle like an electron to be in two places simultaneously, and rules that characterise light as a particle.

Planck’s constant is the fundamental constant of quantum theory that sets the scale of the subatomic world. It determines the smallest amount by which energy can be changed. If it were smaller, atoms would be smaller and the oddities of quantum theory would be even farther removed from everyday experience. If it could magically be made larger, nature would be “lumpier” and quantum phenomena would be more evident.

Since this constant governs the scale of the quantum effects, it had profound ramifications in technology. It enabled the construction of microcircuits, quantum computers, transistors and semiconductors, lasers, iPods, cell phones, high-resolution imaging and nuclear reactors, to name a few, that have changed the trajectory of our lives beyond measure, from ordinary to extraordinary.

Just imagine living in a world before quantum mechanics. No microcircuits, no personal computer, no laser, no CDs or Blu-ray players, no iPods, no portable digital device for storing audio and video files, no cell phones, no text messages, no e-mails, no web surfing, no nuclear reactor, no Neutron Capture Therapy – a cutting-edge treatment method that uses neutrons captured during operation of a  reactor to treat certain cancers. The list is endless.

By combining this constant with Newton’s gravitational constant and the speed of light, Planck formed the “natural units” of mass, length and time. One of the more enduring applications of his units is in the very early stages of the Universe when everything was extraordinarily, unfathomably small. How small? The size of the Universe just after the big bang was one Planck length, which is one billionth of a trillionth of a trillionth centimetre and Planck time, the smallest unit of time we can think of, is the time it takes for light to travel a Planck length.

The marriage of the gravitational constant, which governs the strength of the gravitational force, with Planck constant is believed to be the cornerstone of Quantum Gravity, broadly construed as a theory incorporating both the principles of Einstein’s general relativity and quantum theory. Such a theory is expected to be able to provide a satisfactory description of the microstructure of the early Universe when space and time were so small (the Planck values) that quantum effects of gravity is assumed to have dominated over all other effects.

Finally, Planck’s constant that came not from studies of matter but from a problem in thermal radiation launched the greatest revolution in physics since Newton’s time. It distinguishes the subatomic world from the “classical” world of everyday experience. It forced us to rethink and change our view of the Universe.


–  The author, Quamrul Haider is a Professor of Physics at Fordham University, New York, USA

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