Could the speed of light never have changed, if not, why not?
Briefly: it could have changed a teeny bit, it probably hasn't, and it's because we don't see what we would if it had changed.
More detail:
The speed of light is not a really fundamental quantity. We can tell this because it has units (distance divided by time); since units are arbitrarily sized inventions of humans, nothing with units is truly fundamental. The speed of light is related mathematically to several other quantities that do fundamentally describe our universe and do not have units.
From this it follows immediately that the speed of light cannot change on its own; if it does change, there will be other effects. What these effects are and why is a subject beyond the scope of this kind of forum, unless Eta or someone else wants to try.
The conventional thinking, based on lots of theoretical and experimental studies of these possible effects, is that the speed of light has either never changed and will never change, or maybe it's changed in the distant past by a tiny bit (more on that below).
There are, however, mainstream scientists investigating the possibility of significant changes in the speed of light. Their work has not gained widespread acceptance, but it hasn't been censored or suppressed; when and if they come up with some truly convincing results people will listen to them.
The main creationist proponents of a changing speed of light are Barry Setterfield and Russel Humphreys. They have different and independent models.
Setterfield's model is based on a mathematically invalid analysis of measurements of the speed of light over many years, using data that he fudged and from which he discarded data that he didn't like. He's been tweaking his model ever since in an attempt to make it fit the observed universe, without success. The last I heard his model predicted that stars don't shine. See
Acts and Facts Magazine | The Institute for Creation Research,
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The Decay of c-decay, and
Re: Flood dating discrepancies
Humphreys' model is more sophisticated, but ultimately fails the test of agreeing with what we observe. See
The Unravelling of Starlight and Time and both criticisms and responses at
http://www.trueorigin.org/ca_rh_03.asp.
About tiny changes:
There is a quantity called the "fine structure constant".
Here's a definition and some discussion. The fine structure constant is a combination of the speed of light and some other important quantities and does not have units; it's truly fundamental.
There were recently some studies that indicated that the fine structure constant may have changed slightly in the very early universe. I don't know the current status of these studies (e.g. whether they've been replicated, repudiated, accepted widely, rejected, or what). Mabe Eta knows and will stop by.
If indeed the fine structure constant changed, one possiblity for how it changed is a small change in the speed of light.
What is the criteria for calling an ancient water body a shallow sea, rather than a regular, or deep sea?
Haven't got a clue.
[This message has been edited by JonF, 02-04-2004]