Question

Explain why mass is invariant under Lorentz transformations (boosts)
Thank you

Answer 1

Any scalar is invariant under Lorentz transformations,

In particularly: $p_{\mu} p^{\mu}=E^2-p^2c^2= m^2c^4$

where $p_{\mu}=(E,pc)$

So, the speed of light is constant, the mass of the object is Lorentz invariant.