We all know the number one traffic rule of the
universe – nothing can travel faster than the speed of light. And that
happens to be 299,792.458 kilometres per second. But why is it so?
Before the 1600s most people assumed light moved instantaneously.
Galileo was among the first to think that light travelled at a finite
speed.
In 1638 he tried to measure it. He and an assistant perched
themselves on distant mountaintops with covered lanterns. The idea was
that as soon as Galileo’s assistant saw the flash, he uncovered his
lantern. Galileo would then time how long it took to see the return
flash. The experiment failed dismally! To succeed, Galileo would have
had to register a time difference of microseconds. He had no such time
keeping device and his reaction time would be way slower than that.
Undaunted, Galileo concluded that light’s movement, “if not instantaneous, is extraordinarily rapid”.
But not long after, in 1676, we got a fair estimate of light’s speed
from a young Danish astronomer by the name of Ole Römer. One of the ways
sailors at sea checked their clocks was to observe the eclipse of
Jupiter by its moon Io. The time for Io to make one complete circuit
around Jupiter had been measured at 1.769 days. However there was a
minor problem.
Römer observed that the time between eclipses varied slightly
depending on the time of year. At times when the Earth was moving away
from Jupiter, the time between Io’s eclipses gradually increased; as it
moved closer the time decreased. The cumulative effect meant the
predicted times could be in error by more than 10 minutes.
Römer realised his observations could be explained by the varying
distance between Jupiter and Io, and Earth. The different times for Io’s
orbit reflected the different distances light had to travel. It also
allowed Römer to estimate the speed of light as 214,000 km/s. Not bad!
The first experimental measurement of the speed of light came 150
years later with Hippolye Fizeau. He came up with an ingenious advance
on Galileo’s method. In his experiment, a beam of light was projected
onto a rapidly rotating cog-wheel. The teeth of the rotating cog chop
the light up into very short pulses. These pulses travelled about 8
kilometres to where Fizeau had positioned a carefully aligned mirror. On
the return trip, the reflected light pulse could only reach Fizeau by
passing back through one of the gaps in the cog-wheel.
What happened? At slow speeds, the light pulse always got back to
Fizeau through the same gap in the cog’s teeth. But as Fizeau turned the
wheel faster, at a certain speed the pulse was blocked by the following
tooth. Knowing the rotational speed, Fizaeau thus could calculate how
long it took for light to travel 16 kilometres – and so how fast the
light must be travelling. His remarkable result of 315,000 km/s was
within about 5% of our most recent measurements using lasers.
The faster something travels, the more massive it gets,
and the more time slows – until you finally reach the speed of light, at
which point time stops altogether.
OK. We know that light travels at a finite speed. But why is it finite?
This question gave Albert Einstein pause for thought. If light has a
finite speed, what if you strapped a torch to the front of a moving
rocket? Wouldn’t the light coming from this torch be travelling faster
than the speed of light? Einstein puzzled over this issue with several
“Gedankens” (thought experiments) and came up with a crazy solution: the
motion of an object must somehow make time slow down. Time was no
longer constant and so relativity was born.
Many experiments have carefully tested Einstein’s predictions.
In 1964, Bill Bertozzi at MIT accelerated electrons to a range of
speeds. He then measured their kinetic energy and found that as their
speeds approached the speed of light, the electrons became heavier and
heavier – until the point they became so heavy it was impossible to make
them go any faster. The maximum speed he could get the electrons to
travel before they became too heavy to accelerate further? The speed of
light.
In another crucial test, physicists Joseph Hafele and Richard E.
Keating flew synchronised, super-accurate caesium atomic clocks on
various trips around the world on commercial airliners. After the
journeys, all the moving clocks disagreed with each other and the
reference clock back in the lab. Time ran slower for the moving clocks
just as Einstein predicted. So the faster something travels, the more
massive it gets, and the more time slows – until you finally reach the
speed of light, at which point time stops altogether. And if time stops,
well then, so does speed. And so nothing can travel faster than the
speed of light.
By the way, the next time you use your smart phone be aware that the
GPS satellites orbiting Earth have to take the slowing of time (time
dilation) into account. Disable these relativistic corrections and the
modern world would be lost forever.
