# A quick question of light

1. Jun 10, 2007

### prasannapakkiam

I was explaining SR to someone today. I explained why c is the threshold. I think my explanaiton is invalid - is it?
*I said that with x amount of energy, a lower mass would go faster. thus something with no mass would have no inertia, thus would go the fastest.
*Also something that requires no medium would also go faster than something that does.

Well the guy buyed it (for now) But I better come up with a better explanation. So can anyone quickly explain or give me a link to tell me why c is the limit?

2. Jun 10, 2007

### neutrino

I think a kinematical explanation may be simpler. The axioms of the principle of relativity and finite invariant velocity leads to the (correct) velocity addition rule. From there you can explain why no inertial observer can measure an object to be travelling a velocity greater than that invariant velocity.

It turns out light also travels at that invariant velocity; although these days, the speed of light is a definition, if I'm not mistaken.

3. Jun 10, 2007

### prasannapakkiam

No I understand Lorenz to relativistic mass. All I wish to know is that reason for why the speed of light is chosen

4. Jun 10, 2007

### Chris Hillman

If the question is, why c has the value that it has, it might help slightly to note that:

1. in natural units we should and do set c=1,

2. its not easy to cook up kinematical theories with more than one characteristic speed associated with the symmetry group of the fundamental equations. In particular, the Lorentz group only admits one characteristic speed, and it makes sense to adopt units in which c=1.

(Kinematics is the study of motion. The two types of kinematics most of us at PF are familiar with are Galilean kinematics, which is used in Newtonian physics, and relativistic kinematics or "special relativity", which is used in relativistic physics.)

5. Jun 10, 2007

### neutrino

The linking of the "speed of light" with relativity is mainly due to historical reasons. Maxwell's equations, aether, and Einstein resolving all the mess, eventually.

But I find the more modern treatment simpler: SR starts with the postulate there exists an finite, invariant speed, i.e. one which is same for all inertial frames(call it c). In comparison, in pre-Einsteinian mechanics, if there is an invariant speed, it should be infinitely large. This c turns out to be equal to the measured speed of light.

6. Jun 10, 2007

### prasannapakkiam

interestingy, that's what I thought... Well there is definitely a c - and it is not infinity.