# B Meaning of the constant c

1. Jul 26, 2017

### davidge

For systems whose motion is discribed by the wave equation

$$\bigg(\frac{1}{c^2} \frac{\partial^2}{\partial t^2} - \vec{\nabla^2} \bigg)u \big(\vec{x},t \big) = 0$$ $c$ is the speed of light. It corresponds to different quantities depending on what the system under consideretion is. For instance, for a simple vibrating string, $c = \sqrt{T / \rho}$ where $T$ is the tension and $\rho$ is the mass density per unit length.

My question is, What is the meaning of the ratio $T / \rho = c^2$? Maybe, tension propagates at the speed of light throughout the string?

2. Jul 26, 2017

### NFuller

Not necessarily, c is the speed of whatever wave you are dealing with.

3. Jul 26, 2017

### davidge

Oh, I forgot about that. Thanks.

But, still, could the ratio $T / \rho = v^2$ be interpreted as the tension propagating across the string at the speed $v$?

4. Jul 26, 2017

### NFuller

$v$ is the speed at which a disturbance in the string would travel. If you plucked the string, the distortion in it's shape would travel down the string at this speed.

5. Jul 27, 2017

### Preston Leggett

c is speed of light that has a numerical value of 3*10^8 m/s.

6. Jul 27, 2017

### weirdoguy

Did you even read OP and other posts? c does have more general meaning in the context of OP.

7. Jul 27, 2017

### rumborak

If I understand the OP post correctly, he is trying to use the string derivation of the wave equation to glean insight into the "cause" of c, i.e. apply that insight into space-time.
I don't think you can. I think you just have to take the 1/c^2 as a factor that, while eventually causing the wave speed, can have very different and unrelated derivations.

8. Jul 27, 2017

### davidge

Thank you all