THE FUNDAMENTALS OF PHYSICS

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Alhazen’s Book of Optics, a key

medieval science text

MEASURING THE

SPEED OF LIGHT

It’s the universal speed limit and the key to making sense

of the cosmos. But just how were scientists able to deduce

how fast light can travel? Frank Close investigates

A

ncient Greek mathematician
Euclid believed t hat sight
occurs because the eye emits
light. Hero of Alexa nd ria pronounced
that light must travel at infinite speed
as distant stars appear the instant that
one’s eyes open. And, in the 11th
century, the Basran mathematician
Alhazen wrote his Book of Optics in
which he argued that light moves from
object to eye with a finite speed that
varies depending on the medium
through which it passes. So for
example light moves more slowly
through water and glass than it does
through air.
Ideas continued to flow. In the 13th
century, Roger Bacon used Alhazen’s

ideas to support the theory that light

travels at a very high speed, faster

than sound but finite. The idea that

light travels infinitely fast in empty

space, but slows down in a medium,

was also believed at that time.

As late as the 17th century,

luminaries, such as Johannes Kepler

a nd René Desca r tes, insisted t hat light

travels infinitely fast. Kepler argued

t hat t his must be so, as empty space

would offer no resistance to its

passage. Descartes based his

a rguments on obser vation: du ring a

lunar eclipse the Sun, Earth and Moon

would be noticeably out of alignment

if light travelled at a finite speed – and

the absence of such misalignment

convinced him that light travels

instantaneously.

It was around this time that the

first attempts to make a direct

measurement were made. In 1629,

the Dutch philosopher Isaac

Beeckman proposed an

experiment wherein the flash

of a cannon was reflected by a

mirror about a mile away and the

time lapse measured. Galileo

independently proposed a similar

experiment, involving the

uncovering of a lit lantern, which

was carried out by his students in

1667. No time delay was detected,
      confirming the prejudice that light
      travels infinitely fast.
      With our modern knowledge of
      light’s speed, we k now it would have
      taken about one hundred-thousandth
      of a second for it to make the round
      trip. That’s less than the reaction time
      of the observers, hence their inability
      to measure any delay – the distances
      involved were simply too small. By
      contrast, the distances between the
      planets are so large that light takes
      several minutes to travel between
      them. All you need is some reference
      against which events can be measured.

International partners

In Paris, Giovanni Cassini had been

observing Jupiter’s moons, which

disappear behind the planet as they

orbit it. His measurements varied and

he attributed this variation to light

having a finite speed. Danish

astronomer Ole Rømer joined Cassini

and, in 1676, noticed that the time that

Jupiter’s innermost moon Io, takes to

reappea r is less when t he Ea r t h is

approaching Jupiter than when it’s

receding from it.

This confirmed Cassini’s conjecture

t hat when Ea r t h is approaching

Jupiter, it has moved nearer while

the light is en route and so the total 5