what’s the reason why it’s impossible to enter a white hole event horizon?
To enter a white hole horizon, you’d have to be travelling faster than light — just as you’d have to be travelling faster than light to cross outwards through a black hole’s horizon. A horizon is a null surface, which means it’s generated by paths that light rays would follow. At any event in spacetime, you (or any massive test particle) can only be following a timelike worldline, which lies inside the light cone at that event. Every light cone comes in two pieces: one facing into the past, one facing into the future (with the definition being a matter either of convention or thermodynamics; there’s nothing in local spacetime geometry to tell you which is which). At a point on the event horizon of a black hole, the future-pointing half of the light cone also points entirely inwards, into the hole (except for a sliver that remains exactly on the horizon, i.e. the cone is tangent to the horizon). Equally, the past-pointing half of the light cone points entirely outwards (except for that
