# Why is the (maximum) speed of light the number it is?

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1. Nov 3, 2015

### gracie

What limits the speed of light to 3x10^8ms^-1? I was referred to Maxwells equations and have studied them but am not sure if this is the full answer and am unsure where Maxwell's relate anyway, the lectures I watched were unclear apart from relating to Ampere's Law.

As much maths as possible to answer please :)

2. Nov 3, 2015

### Staff: Mentor

Physics cannot answer "why"-questions at a fundamental level. Light in vacuum has a fixed velocity. Its numerical value comes from our arbitrary definitions of the units meters and seconds.
It is possible to express the speed of light via the electric vacuum permittivity and magnetic vacuum permeability: c2 = 1/(ε0μ0)

It is possible to invent laws of physics where light has infinite speed. We just don't live in such a universe.

3. Nov 3, 2015

### gracie

Okay but in our universe, through whatever units we use, the speed of light is limited, right? But what is it limited by, and why is it that particular number, why not bigger or smaller?

4. Nov 3, 2015

### gracie

Also why can it be expressed with that equation? Where has that come from?

5. Nov 3, 2015

### davenn

again you are asking why type questions

That is just the speed it is

6. Nov 3, 2015

### Khashishi

Historically, we measured the speed of light through various ways. But in modern times we just define the speed of light as a certain value. Or more accurately, we define the meter as the distance traveled by light in vacuum in 1/299792458 second. If we chose a different definition for the meter, then the speed of light would have some different numerical value. So the number itself is arbitrary, invented by humans.

If you want to remove the arbitrariness from the question, you have to take some ratio (or more complicated expression) so the human units all cancel out. Then you get a dimensionless number. We don't have explanations for some dimensionless numbers we find in the universe, like the fine structure constant.

7. Nov 3, 2015

### davenn

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