The dawn of Quantum Physics.

Biggest discoveries often came from the observation of simple effects, so ordinary that most of us just dismiss them, such is the case of the origin of quantum mechanics, the most successful physic theory so far.

It all started close to the end of the eighteen Century, at that time it was considered that physics was at its peak with the three main domains in physics, mechanics, thermodynamics and electromagnetism; considering that there were no longer great theories to discover, and the future of Physics implied only better precision on measurement and calculation. However, it was at that time when the greatest revolution in physics was about to arrive, with the introduction of Relativity and quantum theory.

The origin of quantum theory started from a simple observation, it was well known then, as it is now, how metals glow when they are subject to high temperatures. In physics the termblackbody radiation” was used to describe this phenomenon and to explain how an object emits radiation. A black body is considered an object that can absorb radiation of any kind; and the radiation coming out of it as the object gets hotter is called blackbody radiation, this radiation has a characteristic wavelength depending on the object’s temperature. All objects in essence act as a black body, and all radiate energy; take for example the human body, as our temperature is on average 98 degrees Fahrenheit (37 degrees Celsius or 310 Kelvin); our bodies emit radiation in the infrared wavelength, which is not visible to the human eye, yet we can feel that our body is warm, and we can see our radiation using an infrared camera.

The challenges

The case of the described glow coming from a hot metal is also blackbody radiation; and it changes in color depending on how the temperature changes; as it can go from red glow to white glow to blueish glow as the object’s temperature increased. This effect was the key to the engineering and development of the incandescent lamp; for which tests and experiments began since the early 1800’s, looking for a viable alternative to provide lighting using electric lamps. As history goes the first commercially viable solution was Edison’s lamp patented in November 1878. But with the recently popularized incandescent light, the underlying question about why the filament in these lamps glowed and emitted light was still an unanswered question as this phenomenon didn’t had a physics theory to describe this phenomenon.

While studying this effect in more detail, an approximation based in classical physics was used to derive an equation that describes the relationship between the object’s temperature and the wavelength of the emitted light; this was the Rayleigh-Jeans law. But this law had a problem, it only worked for low frequency light like microwaves or infrared light, but as the frequency increased, the calculated energy spiked to very high values, approaching an infinite value; these calculations of course did not coincide with the experimental results, making obvious that this law was flawed; this situation was called the Ultraviolet catastrophe. As this problem was evident around the wavelength of ultraviolet light where the calculated energy spiked.

Ultraviolet catastrophe

Ultraviolet catastrophe –

Another experiment that also had unanswered questions was the photoelectric effect, which is the emission of electrons from a metallic surface when light shines on a metal, the best experiment was the use of a gold leaf electroscope, using gold foil on an insulated metal plate, inside a grounded camera (so as not to affect the charge). When the metal plate was subject to a negative charge, by having the same electric charge the sheet of gold separate from the plate as it was repelled (knowing that equal charges repel and opposite attract each other). While doing an experiment by shining light in this plate It was noticed that red light had no effect on the electroscope’s charge, but by using ultraviolet light, the charge decreased immediately, provoking that the gold leaf touched again the metal plate. Therefore, it was concluded that low energy radiation (such as red light) had no effect on the charge, while high energy radiation caused the metal to emit electrons due to this light.

Here comes Planck.

Radical problems require radical thinking, and just before the start of the twentieth new century Planck started working on this problem, entrusted by electric companies to produce lamps that give the maximum illumination using the minimum of energy. After the first attempts to explain the effect on blackbody radiation, Planck finally presented his postulate, which in later years would become one of the fundamental principles of quantum mechanics. Planck postulates that the energy emitted by a black body is quantized, this means that the energy can come only in multiples of an elementary unit, or quanta. Back then it was considered that physics phenomena were always a continuum, imagine pouring water in a glass, where you have an infinite range of possible water levels; now Planck proposal considered that in this phenomenon energy was delivered in small packets, the equivalent of pour jellybeans in a glass, with the consideration that you cannot have fractions of jelly beans in it.

He explained, the energy emitted was in proportion of the frequency and for this purpose he calculated the so called Planck constant “h”, with a value of 6.62606896 × 10−34 Joules per second; the new formula was an adjustment to the Raleigh-Jeans equation, where by adding this new Planck constant the blackbody radiation could be calculated for all the frequency spectrum, including high frequency light, matching experimental results, and therefore resolving the dilemma of the ultraviolet catastrophe. Finally, in 1900 Planck proposed its Planck law which describes “The spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature T”. Neat right? And this description of how a black body radiates energy as it gets hotter. Oh, and yes, the result of this research made him credited with a Nobel prize in 1919.

Although initially his assumption about quantized energy was incompatible with classical physics and was severely attacked by fellow physicists, yet, with additional experiments, the Planck constant was verified repeatedly and was a key value on the study of quantum phenomena and for the definition of a new set of universal physical units, including the Planck length, Planck time, Planck mass or Planck temperature; each one of them interpreting effects in this realm of the quantum physics.

We will not address in detail the case of the photoelectric effect for now, as this is a very interesting story in itself, but is worth mention that this was also solved by a well-known physicist, which by taking this new proposal from Planck for quantized energy, he combined this quantum principle to state that light itself was composed of small energy packets (or quanta), instead of being formed by waves, and called these energy packets photons; determining that the higher the frequency of associated light was, the more energetic these photons were. Returning then to the gold leaf electroscope experiment, and how different kinds of light have very different effects in it; with this new theory it was considered that, when the red light illuminated a negative charged metal the low energy photons were incapable of “knock out” the electrons from the metal; but with the ultraviolet light, as its photons were more energetic, they had sufficient energy to knock out the electrons from the metal therefore removing the negative charge in it.

Photoelectric Effect

Photoelectric Effect –

This theory also made him a Novel prize winner, this well-known physics was no other than Albert Einstein. And yes, his novel prize wasn’t awarded due its far more famous proposal on relativity, but by the quantum principle of light. The combined work of Planck and Einstein gave rise to the so-called “Planck-Einstein relationship” which states that the energy of a photon is proportional to its frequency, and is reflected in the formula E = h ν (The energy is equal to the product of the Planck constant and the light frequency)

Max Planck always followed closely Einstein’s work, and was a key promoter for Einstein to join the Prussian Academy of Sciences. It is fair to say that, after Quantum Physics, perhaps Planck’s main discovery was Albert Einstein itself. Both were pioneers in the development of this new branch of physics, which is at the center of today’s modern physics and is, undoubtedly, the most mind-bending and most successful theory of physics so far.

Regards – Alex



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