Introduction
The puzzling properties of light and the ether remained through the turn of
the century and up to 1904: the speed of light (as described by the equations
of electromagnetism) did not depend on the motion of the observer and,
stranger still, the medium in which light propagates could not be described
consistently.
A final effort was made in order to understand in a “fundamental” way
the negative result of the Michelson-Morley experiment. It was postulated
(independently) by Fitz-Gerald and by Lorentz that matter moving through
the ether is compressed, the degree of compression being just so that there
is a negative result in the M&M experiment. The claim was that the ether
wind does slow down and speed up light, but it also contracts all objects
and these two effects conspire to give no effect in all experiments.
A calculation shows that an object of length ` moving with velocity v
with respect to the ether should be contracted to length " l' " is given by
l'=l(1-v²/c²)½
(where c is the speed of light) in order to get the null result required.
So in order to understand the gamut of experimental results the ether
had to be a very tenuous medium that could not be felt or tasted, nonethe-
less the strongest materials would be squashed by it by an amount which
makes it impossible to see the ether’s effects. The amount a material would
be squashed, though admittedly very small, would always be there and is
independent of the composition of the object going through the ether (see
Fig. 1). This is a situation like the one I used in the “ little green men on
the moon” example (see Sect. ??): the ether has was awarded the property
that no experiment could determine its presence; the ether hypothesis is not
falsifiable.
Figure .1: The idea behind the Lorentz–Fitz-Gerald contraction.
Enter Einstein
In 1905 Einstein published three papers. The first (dealing with the so-called
“photoelectric effect”) gave a very strong impulse to quantum theory, and
got him the Nobel prize in 1921. The second dealt with the movement of
small particles in a fluid (Brownian motion).
The third paper (Fig. 3) of 1905 was called On the electrodynamics of
moving bodies, it changed the face of physics and the way we understand
nature.
This paper starts with a very simple (and well known) example: if a
magnet is moved inside a coil a current is generated, if the magnet is kept
fixed and the coil is moved again the same current is produced (Fig. 6.4).
This, together with the difficulties in detecting the motion with respect to
the ether, led Einstein to postulate that
Figure 6.2: Albert Einstein (in his later years)
the same laws of electrodynamics and optics will be valid for all
frames of reference for which the laws of mechanics hold good
which is known as the Principle of Relativity.
In order to understand the implications of the Principle of Relativity we
need (again) the concept of an inertial observer (see Sec. ??). This is a
person which, when observing an object on which no forces act, finds that
it moves with constant speed in a straight line, or else is at rest. In terms
of inertial observers we can restate the Principle of Relativity:
all the laws of physics are the same for all inertial observers. All the laws of physics are
the same for all inertial
observers
Galileo made a very similar statement but he referred only to the laws of
mechanics, Einstein’s achievement was not only to provide a generalization,
but to derive a host of strange, surprising, unexpected and wonderful con-
sequences from it.
The puzzling properties of light and the ether remained through the turn of
the century and up to 1904: the speed of light (as described by the equations
of electromagnetism) did not depend on the motion of the observer and,
stranger still, the medium in which light propagates could not be described
consistently.
A final effort was made in order to understand in a “fundamental” way
the negative result of the Michelson-Morley experiment. It was postulated
(independently) by Fitz-Gerald and by Lorentz that matter moving through
the ether is compressed, the degree of compression being just so that there
is a negative result in the M&M experiment. The claim was that the ether
wind does slow down and speed up light, but it also contracts all objects
and these two effects conspire to give no effect in all experiments.
A calculation shows that an object of length ` moving with velocity v
with respect to the ether should be contracted to length " l' " is given by
l'=l(1-v²/c²)½
(where c is the speed of light) in order to get the null result required.
So in order to understand the gamut of experimental results the ether
had to be a very tenuous medium that could not be felt or tasted, nonethe-
less the strongest materials would be squashed by it by an amount which
makes it impossible to see the ether’s effects. The amount a material would
be squashed, though admittedly very small, would always be there and is
independent of the composition of the object going through the ether (see
Fig. 1). This is a situation like the one I used in the “ little green men on
the moon” example (see Sect. ??): the ether has was awarded the property
that no experiment could determine its presence; the ether hypothesis is not
falsifiable.
Figure .1: The idea behind the Lorentz–Fitz-Gerald contraction.
Enter Einstein
In 1905 Einstein published three papers. The first (dealing with the so-called
“photoelectric effect”) gave a very strong impulse to quantum theory, and
got him the Nobel prize in 1921. The second dealt with the movement of
small particles in a fluid (Brownian motion).
The third paper (Fig. 3) of 1905 was called On the electrodynamics of
moving bodies, it changed the face of physics and the way we understand
nature.
This paper starts with a very simple (and well known) example: if a
magnet is moved inside a coil a current is generated, if the magnet is kept
fixed and the coil is moved again the same current is produced (Fig. 6.4).
This, together with the difficulties in detecting the motion with respect to
the ether, led Einstein to postulate that
Figure 6.2: Albert Einstein (in his later years)
the same laws of electrodynamics and optics will be valid for all
frames of reference for which the laws of mechanics hold good
which is known as the Principle of Relativity.
In order to understand the implications of the Principle of Relativity we
need (again) the concept of an inertial observer (see Sec. ??). This is a
person which, when observing an object on which no forces act, finds that
it moves with constant speed in a straight line, or else is at rest. In terms
of inertial observers we can restate the Principle of Relativity:
all the laws of physics are the same for all inertial observers. All the laws of physics are
the same for all inertial
observers
Galileo made a very similar statement but he referred only to the laws of
mechanics, Einstein’s achievement was not only to provide a generalization,
but to derive a host of strange, surprising, unexpected and wonderful con-
sequences from it.
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