Michelson-Morley Experiment & Black-Body Radiation: Modern Physics
Michelson-Morley Experiment
Phenomenon: The Michelson-Morley experiment aimed to determine the velocity of the Earth in the frame of the ether, based on the characteristics attributed to light. The experiment revealed that the speed of light is constant and independent of the observer’s motion and the motion of the source, i.e., not dependent on the reference system. This conflicted with the classical conception of movement.
Vision of Classical Physics
Nineteenth-century physicists made assumptions about the nature of light based on a comparison of light to mechanical waves known at that time, such as sound. From these assumptions came the need to assume the existence of a substance called ether: a material that filled space and penetrated all bodies. Ether could not have mass, as light travels through space, and must have elastic properties, like solids, since it transmits transverse vibrations inherent in the wave motion of light.
Explanation Provided by Modern Physics
Einstein’s special relativity theory provides an explanation with two postulates:
- The laws of physics are the same in all inertial reference systems. From this premise, it follows that there is no specific reference system (system of ether) for electromagnetic phenomena and that all inertial frames are equivalent descriptions of any physical phenomenon.
- The speed of light is the same in all inertial reference systems, whatever the source velocity.
New Branches of Physics
Relativity.
Thermal Black-Body Radiation
Phenomenon: Thermal black-body radiation. A black body is a body that absorbs all radiation that arrives. As a result, its temperature increases to the point where it begins to emit radiation outwards. Furthermore, this black body radiation depends only on temperature and not its composition. Because the thermal radiation emitted is not visible at ordinary temperatures, it appears black. However, as the thermal radiation emitted by a black body depends on the temperature, the color is not always black. First, it emits infrared radiation, which we do not see. Then, as the temperature increases, it acquires a reddish color, eventually turning white or even bluish-white.
Vision of Classical Physics
It is noted that the maximum wavelength (which corresponds to the highest energy emitted) is inversely proportional to temperature. This led to the “Ultraviolet Catastrophe.” Classical physics, in attempting to explain black body radiation, predicted that the radiation energy should increase indefinitely with decreasing wavelength. However, it had been verified experimentally that the energy tends to zero for very short wavelengths, such as the ultraviolet. (1eV = 1.6×10-19 J).
Explanation Provided by Modern Physics
Planck’s Hypothesis: “The atoms that emit radiation behave as harmonic oscillators.” Each oscillator absorbs or emits radiation energy by an amount proportional to its frequency of oscillation, f: E0 = hf (h = 6.62607015 × 10-34 J⋅s). Thus, the total energy emitted or absorbed by each atomic oscillator can only have an integer n lots of energy E0: E = nE0 = nhf (for n = 1, 2, 3 …). The hf energy packets are called quanta, so that energy is quantized for oscillators, and n is a quantum number. In developing this quantum hypothesis, Planck obtained an expression which allowed him to explain the experimentally observed energy distribution.
New Branches of Physics
Quantum mechanics.