Two magnets collide in a dark room, stick together into a clump, and a dull thud echoes. Black holes behave much the same way, except instead of magnets they carry electric charge, and instead of a thud, there's a burst of gravitational waves. But there's a third player in this story — an invisible field that physicists call scalar. It's like the temperature in a room: at first it's not there, but flip on the radiator and the air warms up.
In the new calculations, two charged black holes fly toward each other. Initially, there's nothing around them — no scalar field. But as they approach, something changes. The hole's charge acts like that radiator: it 'switches on' the field, and on the event horizon, the invisible boundary from which not even light escapes, a scalar 'fur coat' begins to grow. That's hair growth: a previously smooth hole sprouts an invisible layer. After the collision, a single hole remains. If its charge is small or the field is weakly coupled to electromagnetism, the 'coat' slips off and vanishes — the hole goes bald again. But if the coupling is strong and charge is preserved, the hair stays forever.
The most curious part is the moment of merger. The burst of scalar radiation goes hand in hand with the gravitational wave: when one peaks, the other shudders too. Like two musicians playing the same tune on different instruments. This means future gravitational-wave detectors could hear not only the trembling of space but also the 'rustle' of scalar fields — if they truly exist. For now, the calculations show: the fate of the hair depends on how much charge the final hole carries away. Little charge — bald. Lots — hairy.
So the collision of two dark bodies reveals a mechanism previously seen only in isolated holes. A binary system is like a litmus test: dip it in, and the color shows. And if telescopes ever pick up such a 'colored' signal, we'll know that the emptiness around black holes isn't so empty after all.
The scalar field around a charged black hole behaves like the fur of a statically charged cat: the stronger the charge, the thicker the 'fur' and the farther it sticks out.
In Stanisław Lem's novel 'Solaris', the planet's ocean reacts to human thoughts — much like the scalar field around a black hole responds to electromagnetism.