Speed of Light

Experimental measurements of the speed of light have been refined in
progressively more accurate experiments since the seventeenth century. Recent
experiments give a speed of

but the uncertainties in this value are chiefly those of comparisons to
previous standards for the length of the meter. Therefore the above speed of
light has been adopted as a standard value and the length of the meter is
redefined to be consistent with this value.

The speed of light in a medium is
related to the electric and magnetic properties of the medium, and the speed of
light in vacuum can be expressed as

c as Speed Limit

The speed of light c is said to be the speed limit of the universe because
nothing can be accelerated to the speed of light with respect to you. A common
way of describing this situation is to say that as an object approaches the
speed of light, its mass increases and more force must be exerted to produce a
given acceleration. There are difficulties with the "changing mass"
perspective, and it is generally preferrable to say that the relativistic
momentum and relativistic energy approach infinity at the speed of light.
Since the net applied force is equal to
the rate of change of momentum and the work done is equal to the change in
energy, it would take an infinite time and an infinite amount of work to
accelerate an object to the speed of light. (Sorry, Captain Kirk. We can't give
you warp speed!)

A common resistance to the speed limit is to suggest that you just accelerate
two different objects to more than half of the speed of light and point them
toward each other, giving a relative speed greater than c. But that doesn't
work! Time and space are interwoven in such a way that no one observer ever sees
another object moving toward them at greater than c. The Einstein velocity
addition deals with the transformation of velocities, always yielding a relative
velocity less than c. It doesn't agree with your common sense, but it appears to
be the way the universe works.