Index Key:  PHY074
Author:     Mike S.
Subject:    Light speed
Text:       Has the speed of light slowed down over the centuries?

Response #:  1 of 1
Author:      R.C. Winther
Text:        Earthbound measurements of the speed of light show no tendency of 
this quantity to change over time.  But reliable measurements have been made 
for less than two centuries; perhaps this is too short a time span over which 
to detect a change.  However, looking out at various objects in the universe, 
we can observe light from many past times, even billions of years ago.  This 
has in fact been done.  According to the theory, light from remote objects 
such as quasars, some of which recede from Earth at a speed nearly equal to 
the speed of light, will travel at the same speed as light from other sources.  
The "nearly constant" is a bit of hedging.  It is risky to make absolute 
statements based on measurement; there is always some uncertainty in a 
measurement.