Читать книгу Quantum Physics is NOT Weird - Paul J. van Leeuwen - Страница 22

By the time of the rise of electric power, towards the end of the 19th century, the question of what was more efficient, gas light or electric light, became economically important. To investigate their efficiency scientifically, it was necessary to develop a standard light source with which various types of light sources could be compared. In 1887, inspired by Werner von Siemens, a special state laboratory was established in Berlin – the Physikalische-Technische Reichsanstalt – for physical research on applications of electricity and electromagnetic radiation.

The standard instrument for light intensity and color measurement, is basically a closed hollow cavity where the inner walls are heated to a precisely known temperature. The hot inner walls will start to emit and absorb heat radiation – infrared. On further increasing of the temperature, visible light and even UV – ultraviolet light will be emitted.

The intensity distribution of the emitted light, which is an EM-wave spectrum, depends solely on the inside temperature of the walls. Since the cavity is closed the radiation cannot leave it, resulting in an equilibrium condition within the cavity.

In 1859 Gustav Kirchhoff ^[14] (1824-1887) established, based on theoretical grounds, that the intensity distribution over the EM spectrum of the radiation in that cavity does not depend on the nature of the material of the inner walls, but solely on their temperature. This independency of the construction material is highly suitable for the manufacture of an industry standard for light measurement.

To observe the radiation intensity in the cavity, a small opening– small enough to interfere not with the equilibrium within – is made in the cavity wall. Such a hot EM emitting device in equilibrium is called a Black Body Emitter. It emits Black Body Radiation ^[15]. A Black Body is a theoretical device, which absorbs all EM radiation perfectly, so it would be perceived as an utterly black object. It had already become possible, at the end of the 19^th century, to compare the light output of different light sources very accurately with these Black Body Emitter devices. Edison's incandescent light bulbs came out as slightly more efficient than those of his competitor Siemens.

Physicists prefer – understandably – mathematical theories with which the behavior of physical systems can be predicted as precisely as possible from fundamental properties of nature. Firstly, this is a confirmation of the theory, secondly, it reduces the number of constants that physics requires for its models, and thirdly, the cost of a pen and paper exercise is considerably less than a real physical experiment.

Max Planck ^[16] (1858-1947), who had become in 1889 professor at the Friedrichs-Wilhems Universität in Berlin, had been commissioned by Siemens to do theoretical research on maximizing the light intensity from lightbulbs while minimizing energy. At the Physikalische-Technische Reichsanstalt he worked hard to find a theoretical solution for a problem that was known as the UV catastrophe. Statistical calculations in theoretical physics – based on the Maxwell equations for EM radiation – on the emission spectrum of a black emitter resulted in a ‘catastrophic’ prediction. The prediction of the theory was, that a black emitter, heated to 5000 Kelvin (^oK) – this is 5000 degrees above absolute zero – would emit almost all its energy as UV radiation. Just lighting a candle would cause you sunburn but would produce almost no visible light. Fortunately, reality was different. Which was, of course, a challenge for physics. Another theoretical approach of Black Body radiation predicted the emission in the UV range somewhat better but got out of hand in the infrared range. Both classical theoretical approaches misfired spectacularly.

Figure 3.12 illustrates the intensity distribution of EM radiation at different temperatures of a "Black Emitter", an emission spectrum. The far-right curve, – going through the roof at a wavelength around 1.3 micrometer (μm) – is the theoretically calculated behavior for a temperature of 5000 ^oK. It predicts a continuously sharply increasing radiation intensity at decreasing wavelengths, which is the UV catastrophe.

Figure 3.12: Black Body emission spectrum compared with UV-catastrophe.

Source: Wikimedia Commons.

The other three curves represent the reality, the measured intensity distribution for absolute temperatures of 3000, 4000, and 5000 ^oK. At a temperature of 5000 ^oK the maximum of the radiation emitted by a Black Emitter is located exactly in the middle of the visible light spectrum, between 0.4 and 0.7 μm. That is also – and certainly not by coincidence – the temperature of the photosphere, the radiant outer shell of our sun. Our eyes are optimized for our own local star. The real 5000 ^oK curve shows you that the intensity of Black Body emission at a temperature of 5000 ^oK drops quickly for the shorter wavelengths of UV radiation, and beyond 0.2 μm it vanishes entirely.