Explanation of Modern Physics

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  • Topic: Photon, Quantum mechanics, Special relativity
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  • Published : October 22, 2012
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Explanation of Modern Physics

While the term “modern physics” often suggests that all that came before it was incorrect, 20th and 21st century additions to physics simply modified and expanded the phenomena which Newton and his fellow scientists had already contrived. From the mid-1800’s onward, new advances were made in the way of physics, specifically the revolutions of Einstein’s relativity, removing mankind further from the absolute, and quantum mechanics, which replaced certainty with probability. All of this led to an advance in nuclear weaponry, the advancement of laser technology, and the information age of computers.

Although it directly contradicted the classical equipartition theorem of energy, black body radiation was one of the first discoveries in modern quantum mechanics. This theorem states that within thermal equilibrium, where each part of the system is the same temperature, each degree of freedom has 12kBT, kB representing the Boltzmann constant, of thermal energy associated with it, meaning that the average kinetic energy in the translational movement of an object should be equal to the kinetic energy of its rotational motion. By this point, it was known how heat caused the atoms in solids to vibrate and that atoms were patterns of electrical charges, but it was unknown how these solids radiated the energy that they in turn created. Hertz and other scientists experimented with electromagnetic waves, and found that Maxwell’s previous conjectures that electromagnetic disturbances should propagate through space at the speed of light had been correct. This led to the explanation of light itself as an electromagnetic wave. From this observation, it was assumed that as a body was heated, the atoms would vibrate and create charge oscillations, which would then radiate the light and the additional heat that could be observed. From this, the idea of a “black body” formed, an object that would absorb all radiation that came in contact with it, but which also was the perfect emitter. The ideal black body was a heated oven with a small hole, which would release the radiation from inside. Based on the equipartition theorem, such an oven at thermal equilibrium would have an infinite amount of energy, and the radiation through the hole would show that of all frequencies at once. However, when the experiment was actually performed, this is not the result that occurred. As the oven heated, different frequencies of radiation were detected from the hole, one at a time, starting with infrared radiation, followed by red, then yellow light, and so on. This proved that high oscillators are not excited at low temperatures, and that equipartition was not accurate. This discovery led to Stefan’s Law, which said that the total energy per square unit of black body per unit time, the power, is proportional to the absolute temperature to the fourth power. It also led to Wien’s Displacement Law, stating that the wavelength distributions of thermal radiation of a black body at all temperatures have essentially the same shape, except that the graphs are displaced from each other. Later on, Planck characterized the light coming from a black body and derived an equation to predict the radiation at certain temperatures, as shown by the diagram below.

For each given temperature, the peaks changed position, solidifying the idea that different temperatures excite different levels of the light spectrum. This was all under the assumption that radiation was released in quanta, now known as photons. All of these laws help modern physicists interpret radiation and make accurate estimations at the temperature of planets based on the radiation that comes from them.

Einstein used the same quantization of electromagnetic radiation to show the photoelectric effect, which disproved the idea that more intense light would increase the kinetic energy of the electrons radiated from an object. Photoelectric...
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