8
R. N. Mohapatra
2.1 Protons and Electrons
All matter we see around us is built out of three “fundamental” building blocks:
the proton, electron, and neutron. It took years to realize that the protons
and neutrons in turn are made out of more basic constituents called quarks.
There is no evidence today for anything more fundamental than quarks and
electrons. How did this particle picture of matter emerge? Below we give a
brief historical perspective on the development of these concepts.
Democritus of ancient Greece suggested the existence of the atom, but it
took almost two millennia before the atom was placed on a solid ground as a
fundamental object in physics. It goes back to John Dalton (1766–1844), who
used his own analysis of chemical reactions to conclude that matter must be
composed of tiny particles and suggested that they must be the atoms suggested
by Democritus. Microscopist Robert Brown, who in 1827 was looking at
pollen of plant Clarkia Pulchella under a microscope, found them emitting
such tiny particles. This random motion of particles in the liquid became
known as Brownian motion (after Robert Brown). The theory of how these
particles jiggle around incessantly because they were colliding with the atoms
and the molecules in the medium was given much later by Albert Einstein
in one of his five famous papers in the miracle year 1905, and the idea was
subsequently verified by Jean Perrin 3 years later.
The atomic picture kept getting more refined and detailed as J. J. Thompson observed the cathode rays and discovered the electron, and Rutherford
scattered alpha particles, against atoms and established the existence of atomic
nucleus. In 1886, Eugene Goldstein observed that while cathode rays traveled
from the cathode to the anode, there were also rays traveling from anode to
cathode. Those were the hydrogen nuclei (protons) although Goldstein did not
know what they were or if they had a wider role in understanding the atom.
Also, these experiments made it clear that electrons are negatively charged and
the other rays (to be known later as protons) are positively charged. Already
since the work of Charles Augustin de Coulomb in 1785, it was known that
opposite charges attract each other and like charges repel, a fact that was used
to determine that the cathode rays which were moving towards the anode
had negative charge. They were the negatively charged electrons, and anode
rays traveling in the opposite direction were positively charged and were the
protons (the hydrogen nucleus). Rutherford gave the name proton to the
hydrogen nucleus and argued that they are part of all atomic nuclei. Protons
had positive electric charge but Rutherford also suggested that there might
be other heavy particles in the nucleus, which had no electric charge or were
R. N. Mohapatra
2.1 Protons and Electrons
All matter we see around us is built out of three “fundamental” building blocks:
the proton, electron, and neutron. It took years to realize that the protons
and neutrons in turn are made out of more basic constituents called quarks.
There is no evidence today for anything more fundamental than quarks and
electrons. How did this particle picture of matter emerge? Below we give a
brief historical perspective on the development of these concepts.
Democritus of ancient Greece suggested the existence of the atom, but it
took almost two millennia before the atom was placed on a solid ground as a
fundamental object in physics. It goes back to John Dalton (1766–1844), who
used his own analysis of chemical reactions to conclude that matter must be
composed of tiny particles and suggested that they must be the atoms suggested
by Democritus. Microscopist Robert Brown, who in 1827 was looking at
pollen of plant Clarkia Pulchella under a microscope, found them emitting
such tiny particles. This random motion of particles in the liquid became
known as Brownian motion (after Robert Brown). The theory of how these
particles jiggle around incessantly because they were colliding with the atoms
and the molecules in the medium was given much later by Albert Einstein
in one of his five famous papers in the miracle year 1905, and the idea was
subsequently verified by Jean Perrin 3 years later.
The atomic picture kept getting more refined and detailed as J. J. Thompson observed the cathode rays and discovered the electron, and Rutherford
scattered alpha particles, against atoms and established the existence of atomic
nucleus. In 1886, Eugene Goldstein observed that while cathode rays traveled
from the cathode to the anode, there were also rays traveling from anode to
cathode. Those were the hydrogen nuclei (protons) although Goldstein did not
know what they were or if they had a wider role in understanding the atom.
Also, these experiments made it clear that electrons are negatively charged and
the other rays (to be known later as protons) are positively charged. Already
since the work of Charles Augustin de Coulomb in 1785, it was known that
opposite charges attract each other and like charges repel, a fact that was used
to determine that the cathode rays which were moving towards the anode
had negative charge. They were the negatively charged electrons, and anode
rays traveling in the opposite direction were positively charged and were the
protons (the hydrogen nucleus). Rutherford gave the name proton to the
hydrogen nucleus and argued that they are part of all atomic nuclei. Protons
had positive electric charge but Rutherford also suggested that there might
be other heavy particles in the nucleus, which had no electric charge or were