Measuring the speed of light with chocolate.
Part 1: The Speed of Light
More of this Feature
• Part 2: Measuring the speed of light with chocolate
• Part 3: Joe's Measurements Related Resources
• The Speed of Gravity - Measured?
• More on Galileo
• Albert Einstein
• Light and Electromagnetism
Nothing travels faster than light - it only takes 8 minutes for it to reach
the Earth from the nearest star, the Sun, which is 150 million kilometers away.
This means that when you see the sun (remember not look directly at the sun),
you’re really seeing light that left the sun 8 minutes ago – you’re
seeing the sun as it was, and where it was, 8 minutes earlier.
One of the first to try to measure the speed of light was Galileo: In the early
17th century, the general belief amongst scientists (or natural philosophers
as they were often called then) was that the speed of light was infinite; that
is, light could travel any distance in no time at all. Just as with many other
important discoveries made by Galileo, he disagreed with most of his contemporaries.
One of Galileo’s great strengths as a scientist was his ability to conceive
experiments to test his theories. To measure the speed of light, he and his
assistant each took a shuttered lantern to hilltops one mile apart. Galileo
flashed his lantern, and the assistant was supposed to open the shutter to his
own lantern as soon as he saw Galileo's light. Galileo would then time how long
it took before he saw the light from the other hilltop. Then, he could divide
the distance by the time he measured to get a speed.
Unfortunately for Galileo, this time he had not conceived an experiment sufficiently
clever to measure the extraordinary speed with which light traveled. We now
know that light travels at approximately 3x10 8m/s (that is approximately 1100000000
km/Hr), so it would travel the one mile (1.6 km) between the hills in 0.000005
s (5 microseconds), whereas, even if Galileo could time such a short trip, the
assistant could not possibly unshutter his lantern fast enough that Galileo
could tell what part of his measurement was the travel time!
In fact, due to the extraordinarily high value of c (c is the standard symbol
physicists use for the speed of light), there was no where on earth any two
people could stand so that they could conduct this experiment. In order to make
such a direct measurement of the speed of light, one needed a laboratory much
larger than the earth! Remarkably, Galileo had effectively created such a Laboratory
with another of his discoveries – the moons of Jupiter.
In the 1676, Danish Astronomer Ole Roemer made the first reasonable observation
of the finite speed of light. Since Galileo’s discovery of the larger
Jovian moons, a great deal of telescopic observation lead to extremely precise
measurements of the orbital period of Io (1.76 Days). Testing these calculations,
Roemer observed the eclipses over the course of a Jovian year. Roemer found
that as Jupiter moved further from the Earth, his predictions of when Jupiter’s
moons would cross its face became less and less accurate. The times that he
saw Io cross the face of Jupiter became steadily later than the times predicted,
as much as one and a quarter hours late. However, as he continued to observe,
he was able to discount the idea that his predictions were simply wrong, as
on its approach to the earth, the events once again approached his predicted
times. In a stroke of genius, Roemer attributed this discrepancy to the finite
speed of light, and he even published an estimate of that speed, approximately
two thirds the currently accepted value (due to the inaccurate estimates of
the size of Earth and Jupiter’s orbits of his contemporaries). With the
correct measurements of the orbits, Roemer’s data gives a speed of light
equal to 3x10 8m/s.
In an unwitting homage to Roemer, the motion of Jupiter now seems to have allowed
measurement of the speed gravity propagates.
Since then, the speed of light has been measured in many ways – as technology
increased, the speed became easier and easier to tap into. In the 1920’s
Fizeau and Foucault (of Pendulum fame) in France competed to measure the speed
of light using high speed rotating objects. Foucault bounced light from a stationary
mirror to a mirror that was quickly rotating – the angle the light was
reflected through allowed a precise measurement of the speed of light. Eventually,
Foucault was able to determine that light traveled at 299,796 Km/s, an extraordinarily
accurate value.
Even more recently, we have been able to essentially conduct Galileo’s
Experiment in two ways – Using high speed electronics and long lengths
of optical fiber in a laboratory, or by timing Laser light reflected off mirrors
placed on the moon by the Apollo astronauts.
Early in the 20 th century, Einstein’s special theory of relativity established
the speed of light not just as finite value, but also a universal constant and
“speed limit”.
Now anyone can measure this speed - with chocolate and a microwave oven!