Development of Quantum Physics I – Introduction

Almost everyone has heared of this quantum concept, but for most people the laws of quantum physics are unfamiliar and incomprehensible. This is probably due to the discrepancies between the theory and our everyday experiences. Everybody knows catchphrases like “Schrodinger’s Cat“, “Heisenberg’s Uncertainty Principle“, “Quantum Teleportation“, and so on – but they often are misunderstood and misinterpreted. Moreover, I’ve been asked questions like “What is the ‘quantum’ in ‘quantum physics?'”, “Does quantum mechanics describe everything?”, and “Why is nobody able to understandably explain this quantum stuff?” many times.

These are just a few reasons for me to write several articles on the topic of quantum physics.
Where is the best way to start? – Exactly: Let’s start at the very beginning…

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What is the nature of light?

There was quite a debate on this subject in the 18th century. Newton and his proponents believed that light was made out of particles (“corpuscles”), whereas Huygens et al. were convinced of the light’s wave character. The corpuscular theory was able to describe the linear propagation of light and phenomena like refraction in the (classical) optical sense satisfactorily. But it failed in explaining things like interference and diffraction at that time. These effects could only be described in the context of the wave theory. (Step by step, the wave model of light gained acceptance – not least because of Heinrich Hertz’s discovery of electromagnetic waves which revealed that even the visible light was just ordinary electromagnetic waves.)

Isaac Newton (1689)

Christiaan Huygens (1671)

What is the true nature of light?

According to quantum physics, both the wave and the particle model are corrent – or more precisely: not wrong.
(The ideas of this paragraph are often linked to the term “wave-particle dualism”, but I’ll try to avoid this expression because it does more harm than good, I think.)
Funnily enough, both models are not contradictory (If they had been, they wouldn’t have lasted next to each other for this long time!), they are rather complementary: Depending on the intention of the experiment, sometimes the wave model works better (e.g. interference, diffraction), sometimes the particle model gives better explanations (e.g. absorption and emission).

It turns out that all of the known electrical and optical phenomena at that time could be quantitatively and correctly described through the wave model of light. Hence, the following question arises:

Why should we revise our previous theories?

Well…of course there also were things that couldn’t be explained in terms of the wave model. (Needless to say, there are lots of incomprehensible things today despite our evolved knowledge.)
Theoretical considerations and experimental investigations of the cavity radiation yielded early indications of the need of correcting the continuous energy of the electromagnetic field.

Let’s think of a way to realize an ideal absorber, i. e. some body that “swallows” one hundred per cent of the incoming radiation. You can accomplish that by taking some hollow body consisting of absorbing walls and drilling a tiny hole into it. If light falls into this hole, it is reflected in all directions many times before it can reach the opening again. Practically, light which falls into a cavity doesn’t get out anymore. (A little DIY: Pierce a hole through a closed shoe box and take a look into it – quite dark in there, right?!)
The absorbtivity of such a little hole is equal to 1, which means that it absorbes 100 per cent of the incoming radiation.
If you heat the walls of the cavity, the opening functions as a radiation source. This source has the maximum emittance possible: compared to other bodies of the same temperature it radiates the most. You can think of it this way: In thermal equilibrium, emission and absorption of the cavity walls have to be of the same amout. Otherwise the temperature would change and the state of the system wouldn’t be stationary anymore (stationary = not changing with time). Kirchhoff’s Law says that the (sprectral) emittance in proportion to the (spectral) absorbtivity at a given frequency is equal to the (spectral) radiance of the cavity radiation. $E/A=L$.
For a black body (like our tiny hole), the absorptivity is equal to 1, as we found. Therefore, the equation $E/A=L$ turns into $E=L$ (because $A=1$). In other words: The emittance of a black body is equal to the radiance of the cavity radiation.

By the way, the black-body radiation has to be homogenous (“location-independent”) and isotropic (“direction-independent”). Otherwise it would be possible to build a perpetuum mobile and break the second law of thermodynamics.

But this was just the beginning!

In the next part of this series we will use the knowledge we’ve yet obtained to increase the frequency of the black-body radiation. The so-called ultraviolet catastrophe will occur and physics will break down.

Thanks to an ambitious idea of physicist Max Planck there’s a way out of this mess. His flash of insight lays the foundation of the successful theory of quantum mechanics.

(You can always find the other articles of this series via the navigation bar above. I’ve also linked it here, for reasons of comfort.)

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 Next part: The Quantum Hypothesis