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fluidistic
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Homework Statement
I must show that the one dimensional wave equation ##\frac{1}{c^2} \frac{\partial u}{\partial t^2}-\frac{\partial ^2 u}{\partial x^2}=0## is invariant under the Lorentz transformation ##t'=\gamma \left ( t-\frac{xv}{c^2} \right )## , ##x'=\gamma (x-vt)##
Homework Equations
Already given. Chain rule for a function of several variables.
The Attempt at a Solution
Rather than posting pages of latex litterally, I'm going to explain what I did.
Hypothesis: u(t,x) is a solution to the wave equation ##\frac{1}{c^2} \frac{\partial u}{\partial t^2}-\frac{\partial ^2 u}{\partial x^2}=0##. With this assumption, I want to see whether ##u _L(t,x)## is a solution to the wave equation ##\frac{1}{c^2} \frac{\partial u _L (t,x)}{\partial t^2}-\frac{\partial ^2 u _L (t,x)}{\partial x^2}=0## where ##u_L(t,x)=u(t',x')##.
My goal is to express, first, ##\frac{\partial ^2 u_L(t,x)}{\partial t^2}## in terms of ##\frac{\partial ^2 u (t',x')}{\partial t'^2}## (and ##\frac{\partial ^2 u_L(t,x)}{\partial x^2}## in terms of ##\frac{\partial ^2 u (t',x')}{\partial x'^2}##) in order to later use the hypothesis and simplify the math to reach the goal.
I've done it and I'm left to show that ##\frac{\gamma v }{c} \{ \frac{\partial ^2 u(t',x')}{\partial x \partial t'} - \frac{\partial ^2 u(t',x')}{\partial t \partial x'} +\gamma \left [ \frac{\partial ^2 u(t',x')}{\partial t' \partial x'} - \frac{\partial ^2 u(t',x')}{\partial x' \partial t'} \right ] \}=0##. If the left hand side is worth 0 then ##u _L(t,x)## is a solution to the wave equation ##\frac{1}{c^2} \frac{\partial u _L (t,x)}{\partial t^2}-\frac{\partial ^2 u _L (t,x)}{\partial x^2}=0## and the wave equation is invariant under the Lorentz transform.
However I haven't been able to show that this last equation holds. Any ideas are welcome. At first glance it doesn't look obvious to me.