### Re: Why is the speed of light considered the highest speed ?

Area: Physics
Date: Fri Oct 17 15:41:40 1997
Area of science: Physics
ID: 876881858.Ph
Message:
```Your first statement is "Because it was a supposition and unproven...".
Actually, there are MANY experimental confirmations that the theory of
relativity is a good and correct theory, and, therefore, we can take the
speed light, c, as the fastest possible speed.

Another of your statements is "Because of e=mc^2, light has mass."
Unfortunately you have misapplied the equation.  It is intended to give the
equivalence of energy to mass, but not to assign mass.  It has been shown
very well that photons do NOT have mass.

Another of your statements is "Light will slow down when entering an
optically dense medium."  Actually this is not quite true.  While it is
true that we measure a lower AVERAGE speed of the light through a medium,
the propagation of light through the medium, between atoms, is actually at
the normal vacuum speed of light, c.  What happens is that the light moves
at speed c between atoms, but photons are "absorbed" by the medium's atoms.
By "absorption" I mean that the energy of the photon causes an electron of
the atom to be kicked to a higher energy level, and the photon ceases to
exist.  Then, after a very small time delay, the electron goes back to its
original (usually ground state) energy and "emits" a photon of the same
energy (and thus same frequency and thus same wavelength) as the original
"absorbed" photon.  (In fluorescent materials the energy of the photon is
downshifted, but I am talking here of "elastic", or non-energy shifting,
absorptions.)  It is this very small time delay which makes us measure the
average "speed of light" through the medium as slower than the vacuum speed
of light.  But, again, between atoms the light does travel at the speed c.

```

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