Ask Ethan: Can matter escape from the event horizon during the merger of black holes?

Black holes should have accretion disks from which matter falls into them, and after crossing the event horizon, matter should no longer have a way to get out. Can something affect this order of things?

Once having fallen beyond the event horizon of a black hole, it will not work to get out. There is no speed that could get out of there, even the speed of light is not enough for this. But, according to the general theory of relativity, space in the presence of mass and energy is curved, and the merging of black holes is one of the most extreme options in nature. Is it possible to fall into the black hole, cross the event horizon, and then run away from there while this horizon is distorted as a result of a massive merger? This question arose from our reader:

If the two BHs merge, is it possible that the matter inside the event horizon of one of the BHs could escape from there? Can she run away and move to another, more massive BH? And run away immediately beyond both horizons?

The idea, of course, is crazy. But is she crazy enough to work? Let's find out.


When the lifetime of a massive star comes to an end, or when sufficiently massive remains of stars merge, a black hole may appear as a result. The event horizon will be proportional to its mass, and around it there will be an accretion disk of matter falling into it.

Typically, a BH is formed during the collapse of the core of a massive star, which occurs either after a supernova explosion, or when neutron stars combine, or during direct collapse. As far as we know, each BH consists of matter that was previously part of the star, so BH in many senses is the final form of stellar remains. Some BHs appear in isolation, others are part of a binary system or even a system of several stars. Over time, BHs can not only spiral and merge, but also absorb other matter falling inside the event horizon.


In the case of the Schwarzschild BH, falling into it leads to singularity and darkness. No matter which direction you move, how much you accelerate, and so on, crossing the horizon will inevitably lead to a meeting with a singularity.

When something crosses the BH event horizon from the outside, this matter is doomed. In just a few seconds, it will inevitably meet with a singularity in the center of the BH: in the case of a non-rotating BH this will be a point, and in the case of a rotating BH it will be a ring. The BH itself has no memory of what particles fell into it, and what was their quantum state. From the point of view of information, only the total mass, charge and angular momentum of the BH remain.


In the last moments before the merger, the space-time around the BH pair will be distorted, and matter will continue to fall into both BHs from the space surrounding them. Not a single moment is visible at which it would be possible to escape from the inside of the event horizon to the outside.

Then we can imagine a situation when matter falls into the black hole at the last stages of the merger, when the black hole is ready to merge with the other. Since the BH should always have accretion disks, and there is always matter flying somewhere in the interstellar space, particles must constantly cross the event horizon. Everything is clear here, and we can consider a particle that has just fallen beyond the horizon of events, in the last moments before the merger.

Can she run away? Can she “jump” from one BH to another? Let's examine the situation in terms of space-time.


Computer simulation of the merger of two BHs and space-time distorted by them. Gravitational waves are emitted in abundance, but matter should not break out.

When two BHs merge, the merger itself occurs after a long period of spiral approach, during which energy is radiated outward in the form of gravitational waves. It is radiated up to the very last moment before the merger. But because of this, the event horizons of both BHs do not shrink; this energy appears due to the ever-increasing deformation of space-time in the region of the center of mass. One can imagine a similar process in which the energy of the planet Mercury would be lost - as a result, the planet would approach the Sun, but from this the properties of the Sun and Mercury would not change.

However, at the very last moments before the merger of the BH, the event horizons begin to distort due to their gravitational influence on each other. Fortunately, specialists in numerical methods of the theory of relativity have already accurately calculated exactly how this merging affects the event horizons, and this is an amazingly informative calculation.



Despite the fact that up to 5% of the total BH mass before the confluence can leak out in the form of gravitational waves, it can be noted that the event horizons never shrink; a connection appears between them, they are slightly distorted, and then increase in volume. The last point is important: if you take two BHs of the same mass, their event horizons will occupy a certain volume. If you merge them and create one BH of double mass, then the volume occupied by the event horizon will be four times the total volume occupied by the event horizons of the two BHs. The mass of the BH is directly proportional to its radius, and the volume is proportional to the cube of the radius.


We found many BHs, and for all of them the radius of the event horizon is directly proportional to the mass. Double the mass - the radius will double, the surface area of ​​the horizon will increase four times, and the volume - eight!

It turns out that even if you keep the particle stationary inside the BH and make it fall as slowly as possible to the singularity in the center, it still cannot get out of the event horizon. The total volume of the general event horizon increases, but not decreases, and regardless of the path of the particle crossing the event horizon, it is destined to be forever swallowed by the combined singularity of both BHs.

In many collision scenarios in astrophysics there is an “ejecta” when matter from inside an object breaks out during a cataclysm. But in the event of a merger of the BH, everything that was inside remains inside; most of what was outside gets inside; only a small part of what was outside can, in principle, escape. If something has fallen in, it is doomed, and nothing will change it, no matter what you throw in the black hole - even another black hole!

Ethan Siegel - astrophysicist, science popularizer, author of the Starts With A Bang! He wrote the books “Beyond the Galaxy” [ Beyond The Galaxy ], and “Tracknology: the science of Star Trek” [ Treknology ].