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R. Barrett and P. P. Delsanto
In other words, an experiment on electromagnetism would be able to reveal
our absolute velocity, c ontrary to what was affirmed in the previous Section.
So, who is right: Galileo or Maxwell? Or are they both wrong?
In the last Chapter, we saw that electromagnetic radiation can propagate
in a vacuum. This is in contradistinction to sound, which requires a material such as air to maintain the wave motion, and sea waves, which of course
require water. In the 19th Century it was thought that an as yet undiscovered, but all pervading, medium must exist to enable the propagation of light
waves. This was called the quintessence 3 or ether (aka aether ). According to
the physics of Newton and Galileo, the velocity of light on earth with respect
to the ether should be given by the same Galilean equations discussed in the
previous Section for the motion of a person on a train (i.e. the velocity of
light as measured on earth should be equal to the velocity of light through
the ether plus or minus the speed of the earth along its direction of motion).
As we mentioned earlier, we are always in motion with respect to the ether,
so the time taken for a ray of light to travel a certain distance in our direction of motion, and return, is expected to be different from the time taken
to travel the same distance at right angles to our direction of motion, and
return. It is analogous to a person swimming in a moving stream of water.
The time taken to swim 100 m upstream and then return downstream to
the starting point is different from the time taken to swim 100 m across the
stream, and return. The huge magnitude of the velocity of light (300,000 kms
per second) compared with our planetary speeds means that this time difference is very small, but we should still be able to measure it, if Newtonian
physics is correct.
In a series of brilliantly conceived experiments, beginning in 1881, Albert
A. Michelson and Edward W. Morley split a beam of light into two rays,
sent these off on perpendicular paths, bounced them back from mirrors and
then let the returning two rays produce an interference pattern on a screen.
By this approach they were able to show that the velocity of light in the two
perpendicular directions was actually the same. This did not seem possible. It
was analogous to Mary, when measuring the velocity of the ball thrown by
Peter in the last Section, obtaining a velocity of 20 kph, even though she was
located on a moving train. It made no sense.
In 1892, the Dutch physicist, Hendrick Lorentz, remarked that the negative results of the Michelson-Morley experiments could be accounted for by
replacing the Galilean Transformations by another, more complicated set of
3 The fifth classical element after earth, fire, water, and air. The name has made a reappearance as a
form of Dark Energy (see Chap. 11).
R. Barrett and P. P. Delsanto
In other words, an experiment on electromagnetism would be able to reveal
our absolute velocity, c ontrary to what was affirmed in the previous Section.
So, who is right: Galileo or Maxwell? Or are they both wrong?
In the last Chapter, we saw that electromagnetic radiation can propagate
in a vacuum. This is in contradistinction to sound, which requires a material such as air to maintain the wave motion, and sea waves, which of course
require water. In the 19th Century it was thought that an as yet undiscovered, but all pervading, medium must exist to enable the propagation of light
waves. This was called the quintessence 3 or ether (aka aether ). According to
the physics of Newton and Galileo, the velocity of light on earth with respect
to the ether should be given by the same Galilean equations discussed in the
previous Section for the motion of a person on a train (i.e. the velocity of
light as measured on earth should be equal to the velocity of light through
the ether plus or minus the speed of the earth along its direction of motion).
As we mentioned earlier, we are always in motion with respect to the ether,
so the time taken for a ray of light to travel a certain distance in our direction of motion, and return, is expected to be different from the time taken
to travel the same distance at right angles to our direction of motion, and
return. It is analogous to a person swimming in a moving stream of water.
The time taken to swim 100 m upstream and then return downstream to
the starting point is different from the time taken to swim 100 m across the
stream, and return. The huge magnitude of the velocity of light (300,000 kms
per second) compared with our planetary speeds means that this time difference is very small, but we should still be able to measure it, if Newtonian
physics is correct.
In a series of brilliantly conceived experiments, beginning in 1881, Albert
A. Michelson and Edward W. Morley split a beam of light into two rays,
sent these off on perpendicular paths, bounced them back from mirrors and
then let the returning two rays produce an interference pattern on a screen.
By this approach they were able to show that the velocity of light in the two
perpendicular directions was actually the same. This did not seem possible. It
was analogous to Mary, when measuring the velocity of the ball thrown by
Peter in the last Section, obtaining a velocity of 20 kph, even though she was
located on a moving train. It made no sense.
In 1892, the Dutch physicist, Hendrick Lorentz, remarked that the negative results of the Michelson-Morley experiments could be accounted for by
replacing the Galilean Transformations by another, more complicated set of
3 The fifth classical element after earth, fire, water, and air. The name has made a reappearance as a
form of Dark Energy (see Chap. 11).