118 The Poetry of Physics and The Physics of Poetry
mass of the particle at rest and v is its velocity relative to the aether. This
increase in mass was actually detected in 1900 by W. Kauffman. This
result would also be incorporated in Einstein’s Theory of Relativity five
years later with a more satisfactory explanation.
Although the Lorentz–Fitzgerald interpretation of the Michelson–
Morley experiment contained elements of Einstein’s future theory of
relativity, they represented a rear-guard action to preserve the notion of
the aether. Einstein, unburdened by the traditions of the past, regarded
the negative result of the Michelson–Morley experiment as evidence that
the luminiferous aether of Maxwell simply did not exist. It is likely that
he was aided in arriving at this conclusion in 1905 by the results of his
work on photoelectric effect in the same year. In 1900, Planck had
discovered that, apparently, the energy of light was quantized in discreet
bundles of energy called photons. Einstein’s work on the photoelectric
effect in 1905 corroborated this view and showed that light in the form of
photons sometimes behaves like a particle. We shall discuss this result in
greater detail in Chapter 18.
The possible significance of Einstein’s view that light can also behave
like a particle is that the concept of an aether is no longer necessary for
understanding the propagation of light through a vacuum. Light can
propagate in the form of particles, and hence, the whole notion of an
aether can be jettisoned. This is exactly what Einstein did. He went
further. He also concluded that the light that traveled along the two
different paths of the Michelson–Morley interferometer arrived at the
same time because the velocity of light remained constant over the
two paths and did not change as a result of the Earth’s velocity. This
deceptively simple interpretation of the Michelson–Morley result forms
the foundation of Einstein’s special theory of relativity: the velocity of
light in free space is the same in all directions of space and is
independent of the motion of the light source or the observer. This
extremely revolutionary idea violates our intuitive notions of space, time
and motion.
Let us first consider the addition of velocities. If I were to throw a ball
from a moving car in the same direction as the motion of the car, then we
all know from our own experience that the velocity of the ball is greater
than the same ball thrown with the same force from a stationary car. It is
common knowledge that the velocity of the ball and the car add. Is the
same thing true of the velocity of light emitted from the headlights of our
car? If we were to carry over our experience from the ball thrown from