# Why is the speed of light what it is?

1. Oct 17, 2005

I'm a newbie here - this is my first post - and have interest in physics. Now we understand if force ‘a’ is applied to mass ‘b’ for ‘c’ seconds we can determine it velocity in the absence of any friction or other force.

So my question is do we have a similar fundamental understanding of the speed of light of why it is what it is?

2. Oct 17, 2005

### pervect

Staff Emeritus
We do know that the speed of light is $$\frac{1}{\sqrt{\mu_0 \epsilon_0}}$$

$\epsilon_0$ is the permittivity of free space, $\mu_0$ is the permeability. The speed of light is a cosequence of these two numbers via maxwell's equation.

Of course, you can always ask why these parameters have the values they have. (But in general one can always keep asking "why this? why that? why the other thing?).

3. Oct 17, 2005

### Mortimer

Speed is (measurement unit)/(measurement unit). The fact that the speed of light happens to be the seemingly random number 299792458 m/s is a result of mankind's choice of units for time and space, which is obviously not Nature's preference. These units are often redefined in relativistic calculations so that the speed of light becomes exactly 1, which is probably more in line with the system of units that Nature prefers.

4. Oct 17, 2005

### Staff: Mentor

The numerical value of the speed of light of course depends on our choice of units for distance and time. Nevertheless the speed of light has a sort of magnitude that is independent of those units, because we can relate the traveling of light to other physical phenomena.

For example, the speed of light (in whatever units) is such that it takes about 4.3 times as long for light to travel from Earth to Alpha Centauri, as it does for the earth to travel once around the sun, as measured in the Earth's reference frame. Why not 43 times as long, or 4300?

As far as I know there is no generally accepted explanation for why light travels at the first speed and not the other ones. I can't even think of any non-generally-accepted explanations, off the top of my head.