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Q & A: Why is the speed of light what it is?

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Most recent answer: 03/26/2014
why is the speed of light at 299 792 458 m / s why cant it go faster is there a force holding it back?
- Sean (age 43)

Dear Sean,

Well, we just don't know why.  In principle it could be any number but c  is what we are stuck with.  You can't speed it up or slow it down in free space.  (Light does effectively slow down when passing through glassy substances with an index of refraction).  Einstein showed that in the special theory of relativity if you shine a laser beam from a speeding rocket ship, everyone will say that it still goes at c.   It is related to another fundamental value called the fine structure constant.   Certain astronomical observations have shown this value hasn't changed for billions of years.




p.s. You might also look at   and . /mw

(published on 03/26/2014)

Follow-Up #1: Why is the speed of light slow?

Why is the speed of light so slow? It does take million of years to travel between galaxies. And how our universe would have been different if the speed of light were much more faster?
- Anonymous

I may not be able to answer your question, but can help take a step or two toward making the issue clearer. 

You want to know why light is so slow. As Henny Youngman would ask, "compared to what?". 

When we say something is big or small or slow or fast, we always have some sort of comparison in mind. In this case, you're comparing the time it takes light to travel between visible galaxies, up to billions of years,  with some human scale, say a lifetime.  Now there are deep reasons why the visible universe is at least about as big as its age times the time since the Big Bang. It looks like inflation drove space to expand so rapidly that whatever was around ended up expanding so much that from any point you see things rushing away at up to the speed limit, the speed of light.

So now your question becomes "why is the universe so much older than a human lifetime?" The answer to that involves thinking about who we are. To be discussing such questions, we must be very complex beings. Organized complexity takes many, many steps to evolve. Each step is a generation. So nobody could be discussing these things except in a universe whose age was many, many lifetimes. 

Mike W.

(published on 03/26/2014)

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