History of Science - 13
History of Science - 13.
Heinrich Geissler - (1814-1879).
Julius Plucker - (1801-1868).
Eugen Goldstein - (1850-1930).
J J Thomson - (1856-1940).
Rontgen - (1845-1923).
Henri Becquerel - (1852-1908).
Marie Curie - (1867-1936).
Pierre Curie - (1859-1906).
Ernest Rutherford - (1871-1937).
Francis Aston - (1877-1945).
Max Planck - (1858-1947).
Robert Millikan - (1868-1953).
Niels Bohr - (1885-1962).
Louis de Broglie - (1892-1987).
Arthur Compton - (1892-1962).
Erwin Schrodinger - (1887-1961).
Werner Heisenberg - (1901-1976).
Paul Dirac - (1902-1984).
Carl Anderson - (1905-1991).
Wolfgang Pauli - (1900-1958).
Enrico Fermi - (1901-1954).
Sinitiro Tomonaga - (1906-1979).
Julian Schwinger - (1918-1994).
Richard Feynman - (1918-1988).
One of the most important inventions, in the history of science,
is the invention of the vacuum pump.
Michael Faraday wanted to investigate the behaviour of electricity,
in the absence of air, in the 1830’s.
He used an apparatus which was far from airtight.
The pressure inside the vessel was kept low, by constant pumping,
but it was no where near a vacuum.
It was the German, Heinrich Geissler, who made the break through in 1850.
His improved vacuum pump used mercury to make airtight contacts.
His process was able to create a vacuum in a glass jar.
Geissler was trained as a glassblower.
He sealed two electrodes into the evacuated glass vessel,
creating a tube in which there was a permanent vacuum.
He had invented the vacuum tube.
It was this technology that led to the discovery of cathode rays, or electrons,
and x-rays.
This encouraged the work, which led to the discovery of radio activity.
Julius Plucker, a professor of physics, at the university of Bonn,
had access to Geissler’s new vacuum tube technology.
He carried out experiments, investigating the nature of the glow seen in the tube,
when an electric current flows between the electrodes.
His student Johann Hittorf, noticed that the glowing rays from the cathode,
seem to follow straight lines.
Eugen Goldstein gave these glowing lines, the name, ‘Cathode rays’.
He showed that these rays cast shadows, and that they are deflected by magnetic fields.
He thought they were electromagnetic waves, similar to light.
In 1886, Goldstein discovered another form of ray,
being emitted in the holes in the anodes.
He called them ‘Canal rays’ from the German term for these holes.
We now know that these rays are streams of positively charged ions.
William Crookes was born in 1832 in London, the eldest of 16 children.
He inherited enough money, to be financially independent.
He setup a private chemistry laboratory, and founded and edited,
the weekly, ‘Chemical news’.
He had wide interests, including spiritualism.
He improved the vacuum tube, which was called, the Crookes tube.
He carried out experiments, which seemed to prove,
the corpuscular nature of cathode rays.
He showed that the rays caused the shadow of the cross, he placed in its path.
He also placed a tiny paddle wheel in the beam of the rays,
and showed the impact of the rays, causes the wheel to turn.
By 1879, he was championing the corpuscular nature of cathode rays,
which was accepted by most British physicists.
In Europe, however the scientists thought it was a electromagnetic wave.
This was partly because of the discovery of X-rays.
It wasn’t until the end of 1890’s , that the situation was finally resolved.
Evidence that cathode rays, are not a form of electromagnetic radiation, came in 1894,
when JJ Thomson, in England showed that they move much slower than light.
Maxwell’s equations tell us that all electromagnetic radiation, moves at the speed of light.
In 1895, Jean Perrin showed that the rays are deflected by magnetic fields.
He also showed that, when the cathode rays hit a metal plate,
the plate becomes negatively charged.
Walter Kaufmann in Germany, studied the deflection of cathode rays, in vacuum tubes,
containing different residual gases.
He was able to calculate the ratio of the charge of the particles to their mass,
called ‘e by m’.
He expected to find different values for this ratio, for different gases.
He was surprised to find that he always got the same value for e by m.
JJ Thomson did experiments to calculate the e by m.
He was not surprised that he always got the same value.
He always thought that the cathode rays had identical particles.
Expressing his results as m by e, he pointed out the smallness of the number.
He compared it with hydrogen ions, and concluded that this smallness meant,
that the particle involved was very small, or the charge was very high.
He speculated that the particle was smaller than an atom.
Some scientists thought he was ‘pulling their legs’.
In 1899, Thomson succeeded in measuring the electric charge, ‘e’.
This enabled him to provide a value for ‘m’.
He showed that the cathode ray particles, have one two thousandths of the mass,
of a hydrogen atom.
He concluded that the part of the atom, was getting detached from the original atom.
This was a startling discovery, that the atom was not indivisible.
Thomson was born in England, in 1856.
At the age of 14, he started to study engineering, in what is now Manchester University.
His father died 2 years later, resulting in constraints, in the family finances.
He had to switch to a course in physics, chemistry and mathematics,
for which he was awarded a scholarship.
He graduated in mathematics, from Trinity college, Cambridge, in 1880.
He stayed there for the rest of his working life.
Thomson worked at the Cavendish laboratory.
He became the head of the laboratory in 1884, and held the post till 1990.
He became the first scientist to be appointed as, Master of Trinity,
a post that he held till his death, in 1940.
He received the Noble prize, for his work on electrons, in 1906.
He was knighted in 1908.
He had the uncanny ability to device experiments which would reveal fundamental truths,
about the physical world.
We could say he was the ultimate theoretical experimentalist.
He lured many of the best physicists, to work at Cambridge.
Seven of the physicists who worked as his assistants, received Noble prizes.
Thomson played a significant part, as a teacher, guide,
and much liked, head of department.
Rontgen was born in Germany in 1845.
He became professor of physics, at the university of Wurzburg in 1888.
In 1895, Rontgen was studying the behaviour of cathode rays.
In 1894, Philipp Lenard had shown that cathode rays could pass through thin metal foils.
This was interpreted as evidence, that the rays, were waves.
Rontgen, when experimenting with cathode rays, discovered, what he called as, X-rays.
This was published in 1896.
It caused a sensation, because of the ability of the rays to penetrate human flesh,
and provide a photographic image of the underlying skeleton.
One of the first X-rays that he did was of his wife’s hand.
He demonstrated this phenomena to emperor Wilhelm 2 in Berlin.
He received the first Noble prize for physics, in 1901.
He died in 1923.
Physicist knew a great deal about the behaviour of X-rays,
even though they did not know, what these rays were.
The rays were produced, where the cathode rays struck the glass wall of the vacuum tube.
It spread out from the source in all directions.
Like light it traveled in straight lines, and affected photographic material.
It was not deflected by electric or magnetic fields.
Unlike light they did not seem to be reflected or refracted.
For many years it was not clear, whether they were waves or particles.
But it was widely used in medical applications.
It was also used in science, for example to ionise gases.
This is only after 1910, that it became clear,
that X-rays are a form of electromagnetic waves.
Its wave length was much shorter than ultraviolet light.
They do reflect and refract, given the appropriate target.
The most important thing about the discovery of X-rays, is that it led to the discovery,
of a far more puzzling kind of radiation.
Henri Becquerel was a scientist in the right place, at the right time.
His grandfather was a pioneer in the study of electric and luminescent phenomena.
He was so successful, that a chair of physics was introduced at the French museum,
of Natural History.
His father worked with his grandfather, and was also interested in the behaviour,
of phosphorescent solids.
Henri Becquerel followed in the family tradition of physics.
He received his PhD from the faculty of sciences, in Paris in 1888.
He became a professor of physics, at the French museum of Natural History.
Later his son also became a professor, in the museum.
The physics chair at the museum was occupied by the Becquerel family for 110 years.
Henri Becquerel heard the hot news about the discovery of X-rays.
It suggested to him that phosphorescent objects, which glow in the dark ,
might produce X-rays.
He set out to test his hypotheses using a variety of phosphorescent materials,
accumulated in the laboratory, since his grandfathers time.
The key feature of these phosphorescent materials, was that they had to be exposed,
to sunlight to make them glow.
This exposure in some unknown way, charged them up with energy,
and they would glow in the dark for a while.
Becquerel conducted experiments with the phosphorescent substances.
He found that the rays from the phosphorescent substances,
could penetrate the black wrapper of photographic plates,
and create a image on them.
It seemed as though, that X-rays, could be produced by the action of sunlight,
on phosphorescent salts.
Becquerel conducted an experiment, with a phosphorescent uranium salt.
He waited for the sun to come out in Paris.
After several overcast days, he decided to anyway develop the photographic plate.
To his astonishment they created an image in the plate.
It seemed that uranium compounds produce X-rays, even without sunlight.
The most dramatic aspect of the discovery was that the salts were producing energy,
out of nothing.
This contradicted one of the most cherished tenets of physics,
the law of conservation of energy.
The discovery did not have a popular impact, like the discovery of X-rays.
Most scientists thought that it was another type of X-ray.
In 1899, Becquerel showed that the radiation from the uranium salts,
could be deflected by a magnetic field.
This showed that it could not be X-rays, but must comprise of some charged particles.
Detail investigation of this was taken up by Marie and Pierre Curie,
and later by Ernest Rutherford.
Becquerel shared the Noble prize with the Curies, for their discovery.
Marie Curie is strongly associated with the investigation of radioactivity.
She introduced the term radioactivity.
She played an important role, and worked under difficult conditions.
Her popularity was also partly due to the fact, that she was a woman.
This seems to have influenced even the Noble committee,
which gave her the Noble prize twice, essentially for the same work.
She got the Noble prize for physics in 1903, and chemistry in 1911.
Marie Curie was born in Poland in 1867.
With great difficulty she scraped up funds to study at the Sorbonne in Paris, in 1891.
She almost was starving as an under graduate.
It was at Sorbonne that she met and married Pierre Curie.
Pierre Curie was born in 1859, and established himself as a highly regarded expert,
on the properties of magnetic materials.
Marie Curie got pregnant, and only in 1897, settled down to do her PhD work,
in uranium rays.
At that time no woman had completed PhD in any European university.
She was given a leaky shed for her work, and was banned from the main laboratories,
fearing that she might distract the other researchers.
Marie made her first great discovery in 1898.
She discovered the ore from which uranium is extracted, called pitchblende,
is more radioactive then uranium, so must contain another radioactive element.
The discovery was so dramatic, that Pierre abandoned his research project,
and joined Marie to isolate this unknown element.
After intense research, they found two new elements.
One they called as polonium, and the other as radium.
It took until 1902, to isolate 1/10th of a gram of radium from tonnes of pitchblende.
This was enough to be analysed chemically, and located in its place,
in the periodic table.
She got her PhD, and her first noble prize in 1903.
Pierre went on to measure the astonishing output of energy from radium.
Each gram of radium could heat a gram and third of water,
from freezing to boiling point in an hour.
A single gram of radium was capable of repeatedly heating gram after gram of water,
to boiling point, seemingly endlessly.
It was an important discovery, and brought the team much acclaim.
In 1906, Pierre was killed when he slipped crossing a road,
and was run over by a horse drawn wagon.
It is believed that the slip was the result of a dizzy spell, caused by radiation sickness.
Marie also died in 1934, of leukemia, caused by exposure to radiation.
Her laboratory notebooks are still so radioactive, that they are kept in a lead lined safe.
The discovery of X-rays and radioactivity, was a first step,
in the understanding of the subatomic world.
It was Ernest Rutherford, who gave the first understanding of the structure of the atom.
Rutherford was born in NewZealand in 1871.
His parents were among the first wave of settlers, from Britain,
which claimed NewZealand in 1840.
He was one of the 12 siblings.
He obtained his B.A. from Canterbury college, Christchurch, in 1892.
He received his M.A. partly based on original research,
in electricity and magnetism, in 1893.
In 1895, he got a scholarship of 150 pounds a month,
and joined the Cavendish laboratory, in Cambridge University, as a research student.
This was just a couple of months before Rontgen, discovered X-rays,
and a couple of years before Thomson measured e by m, for the electron.
In Cambridge, he carried out experiments in long range radio transmissions.
This was about the same time that Marconi,
was carrying out similar experiments in Italy.
Marconi was interested in the commercial possibilities of wireless telegraphy.
Rutherford was interested in the scientific aspects of this research.
He soon moved on to research of subatomic particles.
In the spring of 1896, Rutherford was working on X-rays,
under the supervision of JJ Thomson.
Their joint work provided evidence that X-rays are a more energetic form of light.
That is they were electromagnetic waves with a shorter wave length.
He moved on to investigate, the radiation discovered by Becquerel,
and found that it had two components, which he called as alpha radiation,
and beta radiation.
The alpha radiation, had a shorter range, and less penetration.
The beta radiation, had a much longer range, and more penetration power.
Rutherford also identified another type of radiation, which he called gamma radiation.
We now know that alpha rays comprise of particles, same as the helium nucleus.
Beta rays are high energy electrons.
Gamma rays are a form of electromagnetic radiation with wave length,
shorter than X-rays.
Rutherford’s scholarship in Cambridge expired.
He moved to McGill university in Montreal, Canada in 1898.
He worked with the scientist Fredrick Soddy, and found that the radiation,
discovered by Becquerel, now called radio active decay,
resulted in an atom being converted, to an atom of a different element.
They also solved the puzzle of seemingly inexhaustible supply of energy,
from radioactive materials.
They discovered that a certain proportion of the atoms originally present in a sample,
will decay in a certain time.
This is expressed as half-life.
For example, radium decays in a way, that in 1602 years, half the atoms would decay,
into atoms of the gas radon.
In the next 1602 years, half the remaining atoms will decay, and so on.
Radium itself is produced from the decay of the much longer lived Uranium.
They also found that radioactive materials represents a finite storehouse of energy.
Rutherford also pointed out, that this storehouse of energy gives the Earth,
a possible lifetime of at least 100’s of millions of years.
This paved the way, for the work of Bertram Boltwood and Arthur Holmes.
Rutherford returned to England in 1907, as professor of physics,
at the university of Manchester, which had outstanding research facilities.
Within a year, Rutherford’s team established that alpha particles,
are the nucleus of helium atoms.
In 1909, alpha particles produced by radioactivity,
was used to probe the structure of the atom.
Hans Geiger was involved in this research.
Geiger later developed the radiation detecter, named after him.
At that time, JJ Thomson’s model of the atom was the most popular.
In this model the atom, was a sphere of positively charged material,
in which negatively charged electrons were embedded.
Rutherford’s team conducted experiments, where positively charged alpha particles,
were fired at, a thin gold foil.
Some went right through, some were deflected slightly, and a few bounced straight back.
Rutherford interpreted these results, implying that most of the mass and the charge,
of the atom, is concentrated in a tiny central nucleus, surrounded by a cloud of electrons.
The term nucleus was coined by Rutherford.
Most of the alpha particles go straight through the electron cloud.
If an alpha particle comes near the nucleus of an gold atom, it is nudged sideways,
by the positive charge of the nucleus of the gold atom.
In rare cases, where the alpha particles goes directly towards the nucleus,
it is repelled back.
Later experiments showed that the nucleus occupies,
100 thousandth of a diameter of an atom.
Typically a nucleus 10 to the power of -13 cm across,
is embedded in an electron cloud, 10 to the power of -8 cm across.
Atoms are mostly empty space, with the web of electromagnetic forces,
linking the positive and negative charges.
Rutherford’s model of the atom would be his most important achievement.
In 1919, he succeeded Thomson as the head of the Cavendish laboratory.
He found that nitrogen atoms bombarded with alpha particles,
were converted into a form of oxygen, after the ejection of a proton.
Rutherford coined a term proton in 1920.
It was the first transmutation of an element.
It was clear that this process involved a change in the nucleus of an atom.
It marked the beginning of nuclear physics.
During 1920-24, Rutherford’s team showed that most of the lighter elements,
ejected protons, when bombarded with alpha particles.
He was knighted in 1914.
In 1932, Chadwick’s discovery of the neutron, completed the nuclear model of the atom.
Rutherford died in 1937.
After the discovery of the nuclear model of the atom, the most important discovery,
was that the same element comes in different varieties.
This was discovered by Francis Aston, working with Thomson in the Cavendish laboratory,
in the 1920’s.
Fredrick Soddy, in 1911, suggested that elements which had identical chemical properties,
could have different atomic mass.
He gave the name ‘isotopes’ to these varieties.
The proof of the existence of isotopes came from Aston’s work.
This came from studying, how the positively charged rays of ions,
are deflected by electrical and magnetic fields.
This was a development of the technique used by Thomson, for measuring the e by m,
for the electron.
Aston measured the e by m for ions, and since e was known, was measuring the mass m.
Heavier ions will be deflected less than lighter ions.
This is the basis of the mass spectrograph.
Oxygen has an atomic mass of 16, but the oxygen atoms,
made when Rutherford bombarded nitrogen, had an atomic mass of 17.
The explanation became clear only after Chadwick discovered the neutron in 1932.
Soddy received the Nobel Prize for chemistry in 1921,
and Aston received the same in 1922.
Walter Bothe, Joliot Curies, Frederic and Irene, laid the foundation,
for the discovery of the proton.
Chadwick proved the existence of a neutral particle, which he called as the neutron.
Its mass was slightly greater than the proton.
Chadwick received the Nobel prize for physics in 1935.
Rutherford had envisaged a neutron, as a proton bound with an electron, in 1920.
Chadwick built on this and discovered the neutron, in 1932.
Though the nuclear model of the atom, was now established, it was not clear,
why the negatively charged electron cloud does not collapse,
into the positively charged nucleus.
This involves yet another puzzle about the nature of light.
The puzzle concerned the nature of electromagnetic radiation from a perfect radiator,
called a black body.
A perfect black body absorbs all the radiation that falls on it.
When it is hot, it emits radiation in a manner,
that is entirely independent of what the object is made of.
A sealed container with a small hole acts like a black body.
When the container is heated, the radiation bounces around inside the container,
before escaping from the hole as black body radiation.
Robert Kirchhoff studied black body radiation in 1850’s.
But researches found it difficult to come up with a mathematical model,
that would accurately describe the spectrum of radiation from a black body.
The key feature of the black body spectrum,
is that it has a peak in a certain band of wavelengths.
Less energy is radiated at longer and shorter wavelengths.
The position of the peak, shifts to shorter wavelengths,
as the temperature of the black body increases.
For example, a red hot iron, is cooler than iron which glows yellow.
This relationship is vital in astronomy,
where it is used to measure the temperature, of the stars.
Max Planck was a professor of theoretical physics, at the university of Berlin, in 1892.
He tried to find a way to derive the black body radiation law,
in terms of the entropy of an array of electromagnetic oscillators .
At that time the electron had not been discovered, and Planck was in the dark,
of what the oscillators might be.
In his model, the total energy of all these oscillators in a blackbody,
is divided into finite and large number of equal but tiny parts.
It was determined by a constant of nature,
’h’, which became known as the Planck’s constant.
Planck announced his model in the Berlin Academy of Sciences, in 1900.
This date is often given as the beginning of the quantum revolution in physics.
There are no suggestions by Planck,
that there was a physical reality to the notion of energy quanta.
The real quantum revolution began five years later,
when Albert Einstein made his first dramatic contribution to the subject.
In 1905, Einstein published a revolutionary paper on light quanta.
He eventually received the Nobel prize for this.
He found that electro magnetic radiation behaves,
as if it consisted of mutually independent energy quanta.
He calculated that when an oscillator, an atom,
emitted or absorbed electromagnetic radiation, it would do so in discreet units,
which were multiples of ‘hv’.
’v’ is the frequency of the radiation, being emitted or absorbed.
In the same paper Einstein discussed how electromagnetic radiation could knock electrons, out of the surface of a piece of metal.
This is the photo electric effect.
In 1902, Philipp Lenard found that when light of a particular wave length is shone,
on a metal surface, the emitted electrons all have the same energy.
But the energy is different for different wavelengths of light.
This could be explained, in Einstein’s model, if light of a particular wavelength,
consisted of a stream of individual light quanta, each with the same energy, ‘hv’.
This would give the same amount of the energy to an electron in the metal,
which is why the ejected electrons, all have the same energy.
Although Einstein emphasised the provisional nature of his ideas,
he seems to have been convinced, that light quanta was real.
Light quanta was given the name photon, only in 1926.
Einstein’s work marked the true beginning of the quantum revolution.
Depending on the experiment, light behaves as a wave, or as a stream of particles.
When it acts as a stream of particles, it is called as the photoelectric effect.
Robert Millikan was an experimental physicist, who did not accept the idea of light quanta.
He set out to prove that Einstein’s interpretation of the photoelectric effect was wrong.
He spent 10 years testing the 1905 equation of Einstein.
In 1915, he admitted that Einstein’s equation was unambiguously correct.
In the process, he derived a very accurate measurement of Plank’s constant,
as 6.57 into 10 to the power of minus 27.
Millikan’s work established the concept of light quanta.
He also made a superbly accurate measurement of the charge of the electron.
He received the Noble Prize for physics in 1930.
Einstein was awarded the Nobel Prize in 1922.
By this time quanta proved its worth, by explaining the behaviour of electrons in atoms.
The problem with Rutherford’s model of the atom, with a tiny central nucleus,
surrounded by a cloud of electrons, was that there was nothing to stop the electrons,
falling into the nucleus.
Planets orbit the Sun, and would be attracted by the Sun, to fall into it,
but the centrifugal force balances the pull of gravity.
Electrons cannot orbit the nucleus in the same way,
because when they have to change direction to move in an orbit, around the nucleus,
they are accelerated.
An electron which accelerates will radiate energy away,
in the form of electromagnetic waves.
In this way the electron would loose energy, and would spiral into the nucleus,
and the atom would collapse.
This would happen in one ten-billionth of a second.
The reason why atoms are stable is entirely due to quantum physics.
The first person to appreciate this was, Niels Bohr.
Niels Bohr was born in Copenhagen in 1885.
He came from an academic family.
He received a good scientific education, and a PhD in physics, in 1911.
He went to work under JJ Thomson, at the Cavendish laboratory.
He found it difficult to fit, partly because JJ Thomson,
in his 50’s was no longer as receptive to new ideas, as he had once been.
Rutherford gave a talk in Cambridge, about his latest work,
and made a strong impression on young Bohr.
He also met him at a family dinner, and the two hit it off very well.
Bohr moved to Manchester in 1912.
It was there that he worked out the first quantum model of the atom,
directly based on Rutherford’s model.
Bohr returned to Denmark in 1912, and married his fiancée Margrethe.
He took up a junior teaching post, at the university of Copenhagen.
It was there he completed a trilogy of papers on the structure of the atom.
It was published in 1913.
He received the Nobel prize for this in 1922.
The Rutherford-Bohr model of the atom, contains classical theory,
like the idea of orbiting electrons, and pieces of quantum theory,
like that energy is emitted or absorbed in discreet quanta, hv.
The physical insight that the model provides is so good, that it is still taught in schools.
In Bohr’s model an electron can emit quanta of energy, one at a time.
This would correspond to jumping from one orbit to another.
Stable orbits correspond to a certain fixed amount of energy.
The model could not explain entirely why the electrons cannot jump to the nucleus.
The explanation had to wait for more than 10 years,
for Werner Heisenberg to discover the uncertainty principle.
In Bohr’s model the jump of an electron from one orbit to another,
corresponds to the release of a precise quantum of energy.
This corresponds to a precise wavelength of light.
If a large number of hydrogen atoms are radiating energy, the quanta will add together,
to make a bright line in the spectrum, at that wave length.
Bohr calculated the energy that would be emitted or absorbed,
when electrons jumped from one orbit to another.
He found that the spectral lines predicted by the model,
precisely matched the position of lines in the absorbed spectra.
Quantum physics had explained why, and how each element produces,
its own unique spectral fingerprint.
The Rutherford-Bohr model raised as many questions as it answered.
Einstein’s work and Millikan’s experiments helped to formulate,
a complete quantum theory model by 1920.
Rutherford offered Bohr, an appointment in Manchester as a reader.
Bohr seized the offer to work alongside Rutherford, for a while.
He returned to Denmark in 1916.
He established a research institute in Copenhagen, now known as the Neils Bohr institute.
In the 1930’s Bohr became interested in nuclear fission.
When Denmark was occupied by German forces, in the second world war,
he became concerned about the possibilities of the Nazis, obtaining atomic weapons.
He escaped via Sweden to Britain.
With his son, Aage Bohr, he acted as a advisor, on the Manhattan project.
Aage Bohr also received the Nobel prize in 1975.
After the war Neils Bohr promoted peaceful uses of atomic energy.
He was a leading figure in the foundation of CERN,
the European particle research institute in Switzerland.
He died in 1962.
Bohr’s model provided the basis for the understanding of chemistry.
It explained how and why some elements react to form compounds.
The new quantum physics led to an understanding of the nucleus,
and opened up a new world of particle physics.
The next big step in quantum physics came in 1924, when French physicist Louis de Broglie,
published his doctoral thesis in 1925.
He proposed that electro magnetic waves could be described in terms of particles,
and all particles such as electrons could be described in terms of waves.
De Broglie’s aristocratic family wanted him to train as a diplomat.
He started studying history in Sorbonne in 1909, before switching to physics,
against the wishes of his father.
During the first world war, he served as a radio specialist at the Eiffel Tower.
He made up for lost time and produced a key insight into the subatomic world.
This earned him the Nobel prize, in 1929.
His ideas is simple, but flies completely in the face of common sense.
De Broglie started out from two equations that apply to light quanta or photons.
One is E=hv.
The other was derived by Einstein, from relativity theory,
and related to the momentum of photon.
Equation is E=pc.
E is the energy carried by the photon, p is the momentum of the photon,
and c is the speed of light.
Putting these two together as hv=pc, he derived that p=hv divided by c.
The wavelength of electromagnetic radiation is denoted by Lamda, is related to frequency.
Lamda=v by c, which means that p lamda is = h.
That is the momentum of a particle, multiplied by its wavelength,
is equal to Planck’s constant.
In 1924, this was not a startling idea, as far as light was concerned.
De Broglie suggested this applies to other particles, in particular to electrons.
On this basis he devised a model of the atom , in which the electrons,
were represented by waves, running around the orbits.
He said that different energy levels of the electrons, correspond to different harmonics,
of these waves.
Only orbits in which these harmonics fitted exactly, with the peaks and troughs,
reinforcing one and another, instead of cancelling out, were allowed.
His thesis supervisor, was nonplussed, and showed the thesis to Einstein.
Einstein certified that it was sound work.
De Broglie was asked in his PhD interview, how his idea could be tested.
De Broglie pointed out that electrons ought to have the right wavelength,
to be diffracted from crystal lattices.
Two scientists, Davison and Thomson, in independent experiments confirmed this.
The two scientists shared a Nobel prize in 1937.
JJ Thomson received the Nobel prize, for proving that electrons are particles.
Interestingly, his son George Thomson received the Nobel prize for proving,
that electrons are waves.
Both of them were right.
The definitive proof that photons exists had been provided by Arthur Compton.
He conducted experiments involving X-rays scattered from electrons.
Compton established that this scattering, could only be explained,
in terms of an exchange of momentum between particles.
He received the Nobel prize for this in 1927.
In another example, of the bizarre logic of the quantum world,
it was his work which treated electrons as particles,
established that electromagnetic radiation is both wave and particle.
This helped to inspire De Broglie, to show that electrons can also behave like waves.
De Broglie’s equation tells us that everything has dual wave-particle character.
Momentum is related to mass.
Light is a special case where photons have no mass, in the every day sense of the term.
Because Planck’s constant is so small, ‘the waviness’ of an object, like a football,
is so utterly tiny, that it can never be detected.
The waviness becomes important,
when an object is about the same size as Planck’s constant.
This is an important factor in describing the behaviour of protons and neutrons,
and is absolutely crucial in describing the behaviour of electrons.
We have no hope of understanding what an electron really is,
in terms of our everyday common sense experience.
It is like nothing we have ever seen.
All we can hope is to find mathematical models,
which tell us how electrons behave in different circumstances.
Sometimes it behaves more like a wave, and sometimes more like a particle.
This is exactly what happened in quantum mechanics.
A complete mathematical model describing the behaviour of electrons in atoms,
was developed twice, since De Broglie’s ideas.
The Austrian physicists Erwin Schrodinger developed a model entirely based on waves.
The German scientist Werner Heisenberg, developed a mathematical model,
which emphasised the particle approach, and the quantum jumping,
from one energy level to next.
British physicist Paul Dirac, developed a more abstract mathematical model to describe,
the behaviour of electrons in atoms.
He showed that both of the other two approaches were contained within his model.
All these scientists received a Nobel prize for their contribution to quantum theory.
By 1927, physicists had a choice of mathematical models, which they could use,
in calculating the behaviour of quantum entities, such as electrons.
Most like Schrodinger preferred the wave model.
The particle model was equally relevant.
They are simply different facets of a whole.
Sometimes the electromagnetic radiation behaves like a wave, and sometimes as a particle.
Heisenberg made another contribution to quantum physics, his famous uncertainty principle.
It is related to the wave-particle duality.
It says that, certain quantum properties such as position and momentum,
can never both be precisely defined at the same time.
There is always a residue of uncertainty.
This is related to the size of the Planck’s constant.
These effects only show up on very small scales, in the value of one of these parameters.
The more accurately one of the pair is constrained,
the less accurately the other one is constrained.
It is a fundamental feature of the quantum world.
An electron does not know both, precisely where it is, and precisely where it is going,
at the same time.
Heisenberg published this in 1927, where he states, we cannot know,
as a matter of principle, the present in all its details.
It turned out that this is a fundamental aspect of the way the world works.
It is possible to construct the entire edifice of quantum mechanics,
starting out from the uncertainty principles.
This explains why electrons in an atom don’t fall into the nucleus.
When an electron is an orbit around the nucleus,
its momentum is determined by the properties of the orbit.
Any uncertainty in the momentum-position pair, it is forced to be in its position.
The very size of atoms, and the fact that atoms exists at all,
are determined by the uncertainty principle of quantum mechanics.
In 1927, Dirac published a paper on the wave equation for the electron.
It fully incorporated the requirements of the special theory of relativity.
It was the definitive last word on the subject, the equation of the electron.
Curiously his equation had two solutions.
It is similar to the equation x squared equal to 4, which has the solution,
x is equal to 2, or -2.
The negative solution to Dirac’s equation seem to describe a particle,
which had the opposite properties to an electron.
Notably it had a positive charge, instead of a negative charge.
By 1931, he realised that the equation was predicting the existing of a unknown particle,
with the same mass as the electron, but with a positive charge.
Further research suggested that if enough energy was available, it could be converted,
in line with Einstein’s equation, E=mc squared, into a pair of particles.
One an ordinary electron, and another a negative electron.
Carl Anderson, in his experiments in 1932 and 1933,
found the trace of such a positive charged particle, in his studies of cosmic rays.
He called it the positron.
He did not realise that it was manufactured by the pair production process,
predicted by Dirac.
The connection was soon made by other people.
It came to be known as antimatter.
It was a real feature of the physical world.
Every type of particle is now known to have an antimatter equivalent,
with the opposite quantum properties.
In the 1920’s the neutron was not discovered.
It was thought at that time, that an alpha particle is a combination of four protons,
and two electrons.
In 1921 Chadwick wrote that if this model of the alpha particle was correct,
it must be held together, by forces of very great intensity.
This must apply to all nuclei, which are essentially balls of protons and neutrons,
with an overall positive charge.
Some strong force, stronger than the electric force, over very short distances,
represented by the diameter of an atomic nucleus, must overhelm the electric repulsion,
and hold everything together.
This became known as the strong nuclear force.
Later experiments showed that this force is about a 100 times stronger,
than the electric force.
This is why there are about a 100 protons, in the largest stable nuclei.
Any more would cause the electric repulsion to overcome the strong nuclear force,
and blow the nucleus apart.
Unlike electric, magnetic, and gravitational forces, the strong nuclear force does not obey,
the inverse square law.
It is very strong over a limited range of about 10 power minus 13cm.
It cannot be felt beyond this range.
This force determines the size of the nuclei.
If the force had a longer range, the nuclei would have a larger size.
In the 1920’s there was a puzzle in the process of beta decay.
In this process an atom ejects an electron, and transformed to an element,
which is next in the periodic table.
After the neutron was discovered, it became clear, that this process involves,
a neutron being transformed into a proton.
What happens when a neutron decays, is that the mass-energy of the neutron,
is converted into the mass-energy of a proton and a electron,
with some leftover to provide kinetic energy, for the electron to speed away.
The puzzle was the electron speeding away, was able to carry any amount of energy,
upto a well defined maximum value.
This was quite different from the behaviour of alpha particles, ejected during alpha decay.
In alpha decay, all the particles ejected from a particular kind of nucleus,
may emerge with the same kinetic energy.
In some cases they may emerge with the smaller energy,
but accompanied by a energetic gamma ray.
The energy carried by the alpha particle, and the gamma ray,
always adds to the same maximum energy, for that particular kind of nucleus.
The energy liberated is equal to the difference in mass-energy,
between the original nucleus, and the one left after the decay.
In this way energy is conserved.
The alpha particles can only have certain discreet energies,
because the gamma ray photons are quantised.
Momentum and angular momentum, are also conserved in alpha decay.
In beta decay limited electrons from a particular nucleus seem to emerge,
in any lesser amount of energy.
There was no accompanying photon to carry off the excess.
By the end 1920’s it was clear that there is a continuous spectrum of electron energies,
in beta decay.
Wolfgang Pauli came up with a speculative suggestion of what was going on.
He suggested that there could be a electrically neutral particle,
which he called as the neutron.
He said in beta decay a neutron is emitted along with a electron,
in such a way that the sum of energies of the neutron and the electron is constant.
The neutron could carry any amount of energy, upto the maximum, and was not quantised,
in the way gamma ray photons are quantised.
The name neutron was given to the particle, identified by Chadwick.
This was definitely not the particle Pauli had in mind.
The problem of the continuous beta spectrum, refused to go away.
In 1933, Enrico Fermi took up Pauli’s idea, and developed it into a complete model.
In his model the decay process is triggered by the action of a new field of force.
This force came to be known as the weak nuclear force.
The weak nuclear force had a short range, which could cause a neutron,
to decay into a proton and a electron, plus another uncharged particle.
He called this new particle, a neutrino, which is Italian for little neutron.
Fermi came up with a mathematical model, to indicate how the energy of the electrons,
emitted during beta decay, was distributed.
The prediction agreed with experiments.
The journal ‘nature’ however refused to publish his paper, as it was too speculative.
The neutrino proved elusive to detect.
It was finally detected only in the mid 1950’s.
This was not surprising because, if a beam of neutrinos, travelled through a wall of lead,
3000 light years thick, only half of them would be captured by the nuclei.
The discovery of the neutrino, completes the set of particles and forces,
that are responsible for the way things behave in the everyday world.
We now had four particles, proton, neutron, electron, and neutrino,
and their associated antiparticles.
We had four forces, electromagnetism, strong and weak nuclear forces, and gravity.
This is sufficient to explain what is detectable to our senses.
The interactions of the known particles and forces, are governed by quantum mechanics.
This was pieced together as a complete theory,
of electromagnetic radiation and matter, in the 1940’s.
That theory is known as quantum electrodynamics, or QED.
It is probably the most successful scientific theory yet developed.
QED was independently developed by 3 different scientists.
Sinitiro Tomonaga was the first to develop it in Tokyo.
Julian Schwinger and Richard Feynman developed it in the US.
All 3 shared the Nobel prize in 1965.
Tomonaga and Schwinger both worked,
within the traditional mathematical framework of quantum mechanics.
Feynman used a different approach.
He essentially reinvented quantum mechanics from scratch.
All these approaches were mathematically equivalent.
There is a neat physical picture, which gives a feel of what is going on.
When two charged particles, such as two electrons, or an electron and a proton,
interact, they do so by the exchange of photons.
Two electrons, say, move towards one another,
exchange photons and be deflected on to new paths.
It is this exchange of photons which produces the repulsion,
which shows up as a inverse square law.
This law emerges naturally from QED.
The strong and weak nuclear forces can be described in terms of the exchange,
of photon like particles, in a similar way.
The weak force has been incorporated into electromagnetism to form a single model,
called the electroweak interaction.
It is thought that gravity should also be described by the exchange of particles,
called gravitons.
No complete model of quantum gravity has yet been developed.
The accuracy of QED can be gleaned from looking at just one property of the electron,
called its magnetic movement.
In the early version of QED developed by Dirac, this value was predicted to be 1.
In the same units, experiments measure the value of the electron magnetic moment,
as 1.00115965221, with the uncertainty of 4% in the last digit.
The final version of QED predicts a value of 1.00115965246 with an uncertainty of plus,
minus 20, in the last digits.
The agreement between theory and experiment is 0.00000001%.
This is by far the most precise agreement between theory and experiment,
for any experiment carried out on Earth.
This illustrates how far we have come since scientists like Galileo and Newton,
began to compare theory with observation and experiment in a scientific way.
In the second half of the twentieth century, scientists proved within the nucleus,
an investigated high energy events, using giant particle accelerators.
They discovered a world of subatomic particles.
They found that protons and neutrons, comprised of entities called quarks,
held together by the exchange of entities, analogous to photons.
They found that the strong nuclear force is just a outward manifestation,
of this deeper force at work.
In the beginning of the 21st century many scientists thought,
that it is better understood as the manifestations,
of even deeper levels of activity involving tiny loops of vibrating ‘strings’.
This research is still ongoing.