1-The origins
While relativity delivered the first important shake to established concepts in physics and created a new view of space and time, the second revolution was the discovery of quantum phenomena and the theory developed to describe them. The birth of quantum theory is related to the following 4 questions that made people wonder at the beginnings of the 20th century.
Mystery 1
To gain a better understanding of the microscopic structure of matter in 1910 Rutherford conducted experiments, where he scattered alpha particles
1 on thin metallic sheets (he used gold sheets). Most of the alpha particles went through the metal sheet without being scattered. This indicated that there was a lot of empty space in the structure. The experiment also indicated that atoms were made of heavy positive charges at the center (nucleus) and light negative charges (electrons) around. Thus the electrons were supposed to move around the nucleus like planets around the sun.This model of the atom however presented a problem.
The Maxwell equations that are the fundamental equations describing the dynamics of electric and magnetic fields say that an electric charge that is in a nonuniform2 motion radiates. Therefore an electron in an orbit around the nucleus should radiate . Loosing energy through this radiation, it should fall in a spiral-shaped path inwards towards the nucleus. Therefore atoms shouldn’t exist at all. It was known that atoms radiate when a material is heated. This indicates that excited electrons fall down towards the nucleus as expected. But according to the Maxwell equations there this radiation should continue until the electron meets the nucleus. It seemed that on atomic scales Maxwell equations ceased to be valid, or some unknown additional "repulsive" force seemed to be in effect at small scales balancing the attractive electrostatic force and preventing the electron to come closer to the nucleus.
The mystery was Why do electrons not fall on the nucleus.
Mystery 2
As suggested in the 19 century, the temperature of an object reflects the kinetic energy of the atoms or molecules flying/moving around (gases/liquids) or vibrating (solids). Some part of the heat energy is related to he motion/vibration of the atoms as a whole , some part of it is related to the motion of electrons in the atom . At a given temperature all these phenomena exist together . The thermal motion of electrons and the thermal motion of ions3 relative to each other lead, because of Maxwell equations, to electromagnetic radiation as mentioned above. This is called thermal radiation.
The electromagnetic radiation can occur at several frequencies depending on the frequency of the vibration of charges . Physicist tried to understand how the intensity of thermal radiation depends on frequency. Experimentally obtained results seemed to refuse to fit the calculations. The theory predicted higher contribution from higher frequencies but the reality was that the intensity dropped drastically at higher frequencies.
The Mystery was :
Why is the spectral distribution of purely thermal radiation (so called black body radiation) not in accordance with classical radiation theory?
Mystery 3
It was discovered that when some materials were exposed to light, electrons could free themselves from the atoms and build a cloud around the material. One can verify this by applying a voltage between the specimen of this material and a plate at some distance where the specimen is charged negatively and the plate is charged positively . The free electrons around the specimen are attracted by the positively charged plate and fly towards it through empty space so that an electrical current suddenly emerges when light falls on the specimen .
This is known as the photoelectric effect.
However the strange fact was that below a certain frequency there was no electric current at all, which meant that electrons could not be pulled away from the atoms at these radiation frequencies. Above this frequency an electric current emerged and the energy of the electrons seemed to be proportional to the frequency of the incident light.
This was surprising since the energy of the electromagnetic radiation was believed to depend only on the magnitude of the electric and magnetic fields but not on the frequency of radiation. (This is how it stays in text books in order not to confuse students "unnecessarily". In fact, as known from the resonance phenomena, it is not so surprising that the energy transfer between two coupled oscillating systems (electromagnetic radiation/vibrating electrons) may be related to frequency. However the linear form of energy/frequency dependence didn’t resemble a resonance phenomenon, so it was not considered as a possible explanation. We will discuss this in chapter 14
Figure 3 the photoelectric effect
The Mystery was:
Why does the energy of electrons in photoelectric effect depend on frequency of light, and does not depend on the intensity ?
Mystery 4.
Light or electromagnetic radiation emerge in a large class of phenomena, for example when a material is heated, or in some chemical reactions, or when electric voltage is applied to some gases. All these processes feed atoms with energy . What happens is that electrons leave their mysteriously stable orbits (see Mystery 1 ) and move to larger orbits and fall inward again by emitting light. In such a process a radiation of a continuous frequency range was expected
The radiation frequency spectrum of electrically excited gases revealed some strange findings. There were frequency regions without any radiation with very sharp frequencies of radiation at some mysterious frequencies. These were called line spectra in contrast to continuous spectra. The line spectra of hydrogen was discovered as early as 1853 before any models were available for atoms.
The Mystery was:Why is there radiation at some very sharp frequencies with no radiation at all between them?
As you can see, all 4 questions are somehow part of the more general question about “how the microscopic electric charges interact with the electromagnetic fields”. The problems however appeared and were tackled at different times by different physicists in a time span of several decades. The line spectra and the black body radiation were the earlier discovered problems while the discovery of photoelectric effect and the discovery of atoms structure by Rutherford’s experiments and happened later. The discovery of the Schrodinger equation in 1926 marks the culmination of the efforts dedicated to an understanding of these mysteries.
1. Positively charged particles that are product of radioactive decay of some elements
2. Any motion where either the magnitude or the direction of the velocity (or both) changes in time
3. Atoms that are electrically charged because of missing or excessive electrons