For anyone who loves physics, this is a must read. The magnitude of breakthroughs in theoretical physics between the late 1800’s and early 1900’s is staggering. Kumar takes you through those breakthroughs one at a time, carefully including personal anecdotes of the great scientists as their leaps of understanding changed the world.
Here is a collection of my favourite moments:
The birth of quantum (1900):
“Applying Boltzmann’s techniques, Planck discovered that he could derive his formula for the distribution of blackbody radiation only if the oscillators absorbed and emitted packets of energy that were proportional to their frequency of oscillation.” “The problem for Planck was that Boltzmann’s procedure for slicing energy required that at the end the slices be made ever thinner until mathematically their thickness was zero and they vanished, with the whole being restored. To reunite a slice-up quantity in such a fashion was a mathematical technique at the very heart of calculus. Unfortunately forPlanck, if he did the same his formula vanished too. He was stuck with quanta, but was unconcerned. He had his formula; the rest could be sorted out later.”
Ref:
- https://hsm.stackexchange.com/questions/11725/how-to-get-to-planck-s-radiation-law-as-planck-did-it-warts-and-all
- http://strangepaths.com/files/planck1901.pdf
- http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Planck%20(1900),%20Distribution%20Law.pdf
The birth of photons (1905):
The photoelectric effect was a measure of the energy of electrons emitted when light was shown at them. “It was expected that increasing the intensity of a light beam, by making it brighter, would yield the same number of electrons, but with each having more energy. However, the opposite was found: a greater number of electrons were emitted with no change in their individual energy.”
“Whereas Planck had only quantised the emission and absorption of electromagnetic radiation so that his imaginary oscillators would produce the correct spectral distribution of blackbody radiation, Einstein had quantised light itself. The energy of a quantum of yellow light was just Planck’s constant multiplied by the frequency of yellow light.”
Evidence for atoms (1905):
“Einstein realised that Brownian motion was due to water molecules regularly deviating from their ’normal’ behaviour as some of them got bunched up and struck the pollen together, sending it in a particular direction.
Using this insight, Einstein succeeded in calculating the average horizontal distance a particle would travel as it zigzagged along in a given time. He predicted that in water at 17 degrees C, suspended particles with a diameter of one-thousandth of a millimetre would move on average just six-thousandths of a millimetre in one minute. Einstein had come up with a formula that offered the possibility of working out the size of atoms.”
3 years later, in 1908, his predictions were confirmed, for which he received the Nobel Prize in 1926.
More timelines:
1911: Rutherford announces the discovery of the atomic nucleus at a meeting in Manchester, England.
1913: Bohr hears about Balmer’s formula for the spectral lines of hydrogen for the first time (Feb). He presents his new theory of the quantum atom at the British Association for the Advancement of Science (Sept).
1914: The Franck-Hertz experiment confirms Bohr’s concept of quantum jumps and atomic energy levels.
1916: Sommerfeld proposes a theory for the fine structure of hydrogen, and introduces a second quantum number to replace Bohr’s circular orbits with ellipticals. Einstein discovers the phenomena of spontaneous and induced emission of a photon from an atom. Sommefeld adds the magnetic quantum number to Bohr’s original atomic model.
1919: Experimental evidence for general relativity is found (light deflected by gravitational fields). Einstein becomes a global celebrity overnight.
1923: Scattering of X-ray photons by atomic electrons is found (the ‘Compton effect’), bringing irrefutable evidence in support of photons. De Broglie links waves with electrons extending wave-particle duality to incorporate matter.
1925: Pauli discoveres the exclusion principle. Heisenberg develops the beginning of matrix mechanics, the first theory of quantum. Goudsmit and Uhlenbeck propose the concept of quantum spin. Pauli applies matrix mechanics to the hydrogen atom.
1926: Heisenberg, Born, and Jordan publish their paper on matrix mechanics. Schrodinger’s first paper on wave mechanics is published. Shrodinger and others prove that wave mechanics and matrix mechanics are mathematically equivalent. They are two forms of the same theory - quantum mechanics.
1927: Evidence for wave-particle duality applied to matter is found via diffracting electrons. Heisenberg discovered the uncertainty principle. Bohr presents his principle of complementarity and the central elements of what would become known as the Copenhagen interpretation of QM.
Quantum Mechanics:
“The Bohr-Sommerfeld quantum atom could account for the frequency of hydrogen’s spectral lines, but not how bright or dim they were. Heisenberg’s idea was to separate what was observable and what was not. The orbit of an electron around the nucleus of a hydrogen atom was not observable. So Heisenberg decided to abandon the idea of electrons orbiting the nucleus of an atom.
“Neils Bohr would soon argue that until an observation or measurement is made, a microphysical object like an electron does not exist anywhere. Between one measurement and the next it has no existence outside the abstract possibilities of the wave function. It is only when an observation or measurement is made that the ‘wave function collapses’ as one of the ‘possible’ states of the electron becomes the ‘actual’ state of the probability of all the other possibilities become zero.”
“Bell was able to calculate the limits on the degree of spin correlation between pairs of entangled electrons in a Bohm-modified EPR experiment. He found that in the ethereal realm of quantum there is a greater level of correlation if QM reigns supreme than in any world that depends on hidden variables and locality. Bell’s theorem said that no local hidden variables theory could reproduce the same set of correlations as QM. Any local hidden variable theory would lead to spin correlations that generated numbers called the correlation coefficient between -2 and +2. However, for certain orientations of the spin detectors, QM predicted correlation coefficients that lay outside the range known as ‘Bell’s inequality’ that ran from -2 to +2. (1964).
“The testing of Bell’s inequality directly or indirectly helped spawn new areas of research including quantum cryptography, quantum information theory, and quantum computing. Among the most remarkable of these new fields is quantum teleportation.”
The electron microscope (1931):
“No particle smaller than approximately half the wavelength of white light can absorb or reflect light waves so as to make the particle visible through an ordinary microscope. However, with wavelengths more than 100,000 times smaller than that of light, electron waves could. The construction of the first commercial electron microscope began in England in 1935.”
4/5.