16
R. N. Mohapatra
2.6 Matter and Anti-Matter from Einstein’s
Theory of Relativity: The Power of an
Algebraic Sign
Protons, neutron, and electrons were fundamental to understanding the
atomic nucleus. As noted, the existence of the neutron was already suspected by
Rutherford and was discovered by Chadwick in 1932. Are there any other kind
of particles in nature and, if so, what are they? To understand this further, we
need to understand the other key developments in physics in the early 1900s.
In 1905 one of the five papers that Albert Einstein wrote was entitled “On
the Electrodynamics of Moving Bodies,” where the cardinal concepts of the
theory of relativity were introduced. Einstein was a patent clerk when he wrote
these papers. The theory of relativity provided a unified approach to space and
time which until then were thought to be two separate components of physics
thinking. The long held concept was based on the experience about time
behaving so differently from space. For instance, time flows in one directions—
past goes to future through the present but never the other way, whereas in
space, we can go backward and forward, as well as up and down. Einstein’s
theory was therefore a radical departure from this common experience.
Einstein came to this conclusion in a strange way. He had been thinking
about light from his very early years. A lot was known about light in Einstein’s
time. Light is a special kind of wave. It was known to travel at finite speed
ever since Danish astronomer Ole Christenson Roemer measured its speed in
1676. The speed of light is now known to be about three-hundred thousand
kilometers per sec (actual measured value is 2.99792458 × 10
8 meters per
second). This is a huge speed but not infinite as people had thought before
Roemer.
Einstein’s thinking about light can be summarized as follows. A moving car
or moving train moves at finite speed like light does. Is there then a difference
between a moving train or car and a traveling beam of light? Imagine two
trains traveling side by side. If one of them is traveling next to the other at
the same speed, for a passenger in one train, the other train will look like it
is standing still or moving at zero speed relative to this train. In other words,
the relative speed between the two trains is zero. Einstein always wondered if a
train traveled at the speed of light next to a beam of light, will a light beam also
stop moving like the train traveling in the previous example? In other words,
does a traveling light beam next to a moving train behave the same way as a
train traveling next to a moving train? An attempt to answer this question came
from two American physicists, Albert Michelson and Edward Morley, working
R. N. Mohapatra
2.6 Matter and Anti-Matter from Einstein’s
Theory of Relativity: The Power of an
Algebraic Sign
Protons, neutron, and electrons were fundamental to understanding the
atomic nucleus. As noted, the existence of the neutron was already suspected by
Rutherford and was discovered by Chadwick in 1932. Are there any other kind
of particles in nature and, if so, what are they? To understand this further, we
need to understand the other key developments in physics in the early 1900s.
In 1905 one of the five papers that Albert Einstein wrote was entitled “On
the Electrodynamics of Moving Bodies,” where the cardinal concepts of the
theory of relativity were introduced. Einstein was a patent clerk when he wrote
these papers. The theory of relativity provided a unified approach to space and
time which until then were thought to be two separate components of physics
thinking. The long held concept was based on the experience about time
behaving so differently from space. For instance, time flows in one directions—
past goes to future through the present but never the other way, whereas in
space, we can go backward and forward, as well as up and down. Einstein’s
theory was therefore a radical departure from this common experience.
Einstein came to this conclusion in a strange way. He had been thinking
about light from his very early years. A lot was known about light in Einstein’s
time. Light is a special kind of wave. It was known to travel at finite speed
ever since Danish astronomer Ole Christenson Roemer measured its speed in
1676. The speed of light is now known to be about three-hundred thousand
kilometers per sec (actual measured value is 2.99792458 × 10
8 meters per
second). This is a huge speed but not infinite as people had thought before
Roemer.
Einstein’s thinking about light can be summarized as follows. A moving car
or moving train moves at finite speed like light does. Is there then a difference
between a moving train or car and a traveling beam of light? Imagine two
trains traveling side by side. If one of them is traveling next to the other at
the same speed, for a passenger in one train, the other train will look like it
is standing still or moving at zero speed relative to this train. In other words,
the relative speed between the two trains is zero. Einstein always wondered if a
train traveled at the speed of light next to a beam of light, will a light beam also
stop moving like the train traveling in the previous example? In other words,
does a traveling light beam next to a moving train behave the same way as a
train traveling next to a moving train? An attempt to answer this question came
from two American physicists, Albert Michelson and Edward Morley, working