# Is the inverse of the Klein-Gordon equation ever used in physics?

The Klein-Gordon equation (scaling constants) is

$$\square u = -m^2 u.$$

I am wondering if the equation

$$\square u = m^2 u.$$

for real $m$ ever shows up in the physical literature?

In the normal equation the mass term is (of course) $m^2$, but in your modified form the mass term would be $-m^2$. The mass of a tachyon would be $im$ in your case. One example of this is in Revisiting Barry Cox and James Hill’s theory of superluminal motion: a possible solution to the problem of spinless tachyon localization in section 4.