# What is it that sets the speed limit of light?

1. Oct 22, 2011

### Lino

There are quite a number of threads (and FAQ's) that discuss why the speed of light is the "number" that it is, but I'm having difficulty finding some information on what is causing, or setting, the limit. So, let me ask and answer a different question as an example of the what I am tryinh to find out here. Q: Why is the speed limit on our national roads the number it is? A: This number is caused by goverment / legislation that causes the speed limit to exist.

So, without a reference to a measurement or unit, what is it that causes the speed of light to exisit? If anyone can help / direction me to a relevant paper / article on this, it would be greatly appreciated.

Regards,

Noel.

2. Oct 22, 2011

### Pengwuino

If you're asking about why specifically the 299,792,458 m/s number is what it is, it's because we defined it that way. I'm always sketchy about the history/details, but we define SI units such that that is true.

If you're asking why, without reference to any units or numerical value, it is the way it is, it just is. We have no underlying theory that would predict the speed of light from some more fundamental concept.

3. Oct 22, 2011

### Naty1

As noted, we have no complete theory yet Just like we have no theory to predict the mass
nor the charges of any fundamental particles....All that stuff in the standard model is based
on observations rather than first principles.

4. Oct 23, 2011

### Lino

Thanks all.

Naty1, any pointers observational evidence?

Regards,

Noel.

5. Oct 23, 2011

### Staff: Mentor

About the speed of light? See the sticky post at the top of this forum, "Experimental Basis for Special Relativity."

6. Oct 23, 2011

### Lino

Thanks JTbell. Much appreciated. It'll keep me (good) busy for a while!