The Speed of Light: Historical Perspective and Experimental Findings

TRENT SCARBOROUGH AND BEN WILLIAMSON
the speed of light must be finite. Alhazen affirmed that light is
movement; therefore it is at one instant in one place and at
another instant in another place. Since it is impossible for light
to be in both places at the same time, the transmission of light
cannot be instantaneous. Sir Francis Bacon stated in 1620
“Even in sight, whereof the action is most rapid, it appears that
there are required certain moments of time for its accomplishment…
[for there are] things which by reason of the velocity of
motion cannot be seen – as when a ball is discharged from a
musket…” There was still uncertainty seen by Bacon’s contemporary,
Johann Kepler. Kepler was famous for his work on planetary
motion. Kepler reasoned that since light could be propagated
into illimitable space, it required no time. Since light did
not need time, it was immaterial and could offer no resistance to
a moving force. Thus according to Aristotelian mechanics (a now
archaic form of mechanics that would be superseded by
Newtonian mechanics at least 40 years after Kepler’s death)
light must have an infinite velocity. 2
The First Experiments to Determine the Speed of Light
A contemporary of Kepler and Bacon, was Galileo Galilee.
Galileo was the first person to actually attempt to measure the
speed of light. Galileo devised that two people should go stand
with covered lanterns a known distance apart. When one uncovered
their lantern, the other would quickly uncover his lantern
and the time difference would be measured. After testing this
experiment at a distance less than one mile, Galileo concluded:
“I have not been able to ascertain with certainty whether the
appearance of the opposite light was instantaneous or not; but if
not instantaneous it is extraordinarily rapid – I should call it
momentary.” 2 It could be said that Galileo at least established a
lower limit for the speed of light. It is known today that about a
thirtieth of a second is the minimum time interval distinguishable
by the unaided, human eye. Then Galileo’s experiment put a lower
limit of about 9.65 x 10 4 meters per second (m/s) on the speed of
light; which is about 3000 times less than the currently accepted value.
Renee Descartes sought to get an actual number by greatly increasing
the distance. Descartes proposed that if light was finite with time
than eclipses of the moon would be visible on earth later than when
the sun, moon and earth were calculated to be in a straight line. When
no delay was detected, Descartes proceeded to declare that light
must be instantaneous. 3
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