Brief review into the history for the search for the speed of light. Its significance and problems.

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LIGHT SPEED IN RENAISSANCE
Infinite Speed

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Infinite Speed of Light
Since the days of Greek cultural
revival, the idea of an infinite
speed of light was persistent and
influential mainly because of the
influence of Aristotle.
Even in the days of renaissance
in the16th century, the great
French philosopher and
mathematician René Descartes
(1596-1650 A.D.) was in favour
of such an infinite speed idea.

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Instant Transmission
Descartes believed that space is
filled up with a stable fluid made of
microscopic spherical particles (the
Plenum), acting as a medium for
the propagation of light. The light
particles transmitting motions in a
straight line across the medium
instantly, “like a stick transmits a
push on one end to the other end”
In other words, light is a kind of
pressure transmitted through a
medium at infinite velocity. . . . .
Descartes' Principia Philosophiae
(Principles of Philosophy) published in
1644.

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Johannes Kepler
Johannes Kepler (1571-1630), the
renowned astronomer who
established the first two laws of
planetary motion, also put his faith
on an infinite velocity of light. He
believed that the speed of light
was infinite since empty space
presents no obstacle to it.
In fact, most of philosophers of the
time were quite happy with the
ancient idea. So the idea of light
with an infinite speed remained
popular for a further 200 years
after Descartes.

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Wave theory demands
finite speed.
Christiaan Huygens and
Robert Hooke (1635-
1703) were in favour of
the wave theory of
light. They held that
these waves were
disturbances
transmitted through the
plenum proposed by
Descartes. Like all
disturbances through a
medium a finite velocity
is therefore the
necessary consequence.
However, not many of their contemporary philosophers
were doubtful about the idea of infinite velocity. If there
was any, Galileo Galilee (1564-1642), was one – the most
prominent one.

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Galileo Galilei
Galileo Galilei (1564-1642) was an Italian
physicist, mathematician, engineer,
astronomer, and philosopher who played a
major role in the scientific revolution during
the Renaissance.
His achievements include improvements to the
telescope so much so that data could be
collected to support Copernicanism. Galileo
has been variously called the “father of
modern observational astronomy”, the "father
of modern physics”, the “father of science”,
and “the father of modern science”. When
Newton said that he could see further because
he was on the shoulder of giants.” Galileo is no
doubt the giant he referred to.

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The Speed of Light by
Galileo Galilee
Galileo was quite convinced that
light did not suddenly appear out
of nowhere when a light source
was unshielded. It sounded
physically absurd that a ray of light
could be anywhere in the universe
at the same time.
His conviction was fully reflected in
his book entitled “Dialogues
Concerning Two New Sciences”
published in 1638. In the book,
there were three interlocutors
named Salvanti, Sagredo and
Simplicio discussing certain
scientific problems. The following
quotation from the book embraced
all the problems Galileo knew
about the speed of light.

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Dialogues Concerning
Two New Sciences [01]
Salvanti (who represents Galileo’s
opinion): “We see that fires and
dissolutions are accompanied by
motion, and very swift motion;
behold the action of lightning and
of gunpowder used in mines and
bombs..... So I cannot believe that
the action of light, however pure,
cannot be a kind of motion which
is of the swiftest kind.”
Sagredo: (The inquirer) “But what
kind and how great should we take
the speed of light to be? Is it
instantaneous or momentary? Or
does it, like other movements,
require time? Could we assure
ourselves by experiment?”

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Dialogues Concerning
Two New Sciences [02]
Simplicio (An Aristotalian): “Everyday experience shows
that the propagation of light is instantaneous. When we
see an artillery fired far away, the brightness of the flames
reaches our eyes in no time, but the sound comes to our
ears only after a noticeable interval.”
Sagredo:“Well, Simplicio, from this well-known
experience, I can only deduce than that sound travels
slower than light. It does not assure me whether the light
is instantaneous or very fast. Your observation is no more
conclusive than it would be to say: “As soon as the sun
reaches the horizon, its splendour reaches our eyes.” For
who will assure me that the rays did not reach the horizon
before they reached our vision?”

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The Galileo Experiment
With this conviction, Galileo did try to set up an experiment to measure the speed of light in
1638. Galileo and his assistant each took a shuttered lantern, and positioned themselves on
hilltops one mile apart. As soon as the assistant saw Galileo flashing his lantern, he would
reply by opening the shutter to his own lantern. Galileo would then mark down how long it
took before he saw the light from the other lantern. Dividing the return trip of two miles with
the time, he would obtain the speed of light in just the same way of measuring the speed of
any moving daily object.

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Result of Experiment
However the velocity of light is too great for such a
crude set up. In the tick of the clock, a ray of light
would have travelled around the earth seven and a
half times. Light from Galileo’s lantern would have
only taken 11 microseconds to cover the return
trip [a microsecond is a unit of time equal to one
millionth (10-6) of a second]. With his naked eye,
flickering lantern, mechanical shutter, and crude
timing instrument, the total reaction time would
have well over 0.2 second. It would have been a
miracle that Galileo could have detected such an
interval of time for light. He could only conclude
that the speed of light must have been ten times
faster than sound – that is, about 3000 metres per
second, as compared to the modern figure of
299,792,458 metres per second.

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Father of Modern
Science
Although Galileo have failed in the
attempt to produce any precision figure,
he was undoubtedly the first man in
history making an effort to carry out
such a measurement. This, together with
his other achievements in astronomy,
mathematics and philosophy, he was
made the pioneer genius of all time and
was crowned with the honour of being
the “Father of Modern Science”.
What is more, Galileo discovered three
of Jupiter's four largest moons with a
telescope devised by himself, paving the
way for the discovery of the speed of
light by celestial measurement in a later
time by others.