Black holes are some of the most mysterious and powerful objects in the universe. They are formed when massive stars collapse at the end of their life cycles. Their gravitational pull is so strong that nothing can escape them, not even light. Black holes cannot be directly observed, but astronomers can detect their presence and study their effects on surrounding matter. When two black holes get close enough, they exert tremendous gravitational forces on each other and eventually collide in spectacular fashion. But what exactly would happen when two of these dark cosmic giants crash together?
Formation of a New Black Hole
The most direct outcome of a black hole collision is the formation of a new, more massive black hole. According to Einstein’s theory of general relativity, when two black holes collide, they will merge together into a single black hole. This merged black hole will have a mass that is greater than the sum of the two original black holes’ masses.
This is because some of the mass is converted into energy during the collision, through a process called gravitational radiation. This energy is released in the form of gravitational waves – ripples in space-time that travel outward at the speed of light. The gravitational waves carry away energy, and as a result, the mass of the final black hole is less than the initial total mass. However, it still ends up being larger than either of the original colliding black holes.
For example, if two black holes with masses 30 times and 50 times the mass of the Sun collided, they could merge together into a single black hole with perhaps 75 times the mass of the Sun (the exact amount would depend on how much energy was radiated away as gravitational waves). The event horizon of the new black hole, which defines the boundary beyond which no light can escape, would expand dramatically as a result of the increased mass.
Production of Gravitational Waves
One of the most important consequences of a black hole collision is the generation of powerful gravitational waves. Gravitational waves are vibrations in the fabric of spacetime, predicted by Einstein’s theory of general relativity. They transport energy across space just like electromagnetic waves transport light. Black hole mergers are some of the most energetic events in the universe and can produce gravitational waves that distort spacetime over enormous distances.
As two black holes spiral inward towards each other, they create gravitational perturbations that travel outward in all directions at the speed of light. These perturbations intensify as the black holes accelerate their orbital motion and finally merge. At the moment of peak collision, the black holes violently shake the surrounding fabric of spacetime, emitting a burst of gravitational waves.
These gravitational waves propagate across the universe, stretching and squeezing any matter they pass through. If the gravitational waves pass over Earth, they could theoretically stretch and squeeze our planet by tiny amounts. Gravitational wave observatories like LIGO have detected many black hole mergers by looking for these miniscule distortions.
Electromagnetic Signals
Surprisingly, black hole collisions can also produce visible light and other electromagnetic radiation. This seems counterintuitive since isolated black holes cannot emit light. However, in some cases the environment around two merging black holes can become intensely heated and emit high-energy photons.
One way this can happen is if one or both of the black holes are surrounded by accretion disks – disks of gas and dust particles orbiting around the black holes. As the black holes spiral closer, these accretion disks can collide at nearly the speed of light, generating immense friction and heat. This heat causes the disks to glow brightly across the electromagnetic spectrum, particularly at X-ray and gamma ray wavelengths.
Jets of fast-moving particles may also be produced, ejected outward perpendicular to the accretion disks at nearly the speed of light. These jets can extend for thousands or even millions of light years. The energetic particles in the jets interact with magnetic fields and surrounding material, producing synchrotron radiation visible as light, radio waves, and other electromagnetic signals.
Astronomers have directly observed a number of cases where black hole mergers produced observable light, confirming Einstein’s theory that black holes can collide. Electromagnetic signals from merging black holes provide insights into their environments and precise measurements of their masses and spin rates.
Acceleration of Matter
As two black holes spiral towards each other, they stir up any surrounding matter, like accretion disks or clouds of gas. This matter can be accelerated and heated to tremendous speeds and energies. At the moment of peak collision, a burst of gravitational waves radiates outward, driving matter away from the merging black holes at nearly the speed of light.
Matter that gets ejected can travel vast distances, carrying energy far beyond the two black holes. Some of this high-speed matter may collide with slower moving debris, producing shock waves and particle acceleration. Radio telescopes see evidence of this in the form of jets and radio lobes extending far from the centers of merging galaxies.
The hot accelerated matter produces visible light and other electromagnetic radiation as it interacts with magnetic fields or slams into interstellar gas. This extended glow of energized matter helps astronomers identify galaxies and other objects where black hole collisions may have occurred in the past.
Disruption of Accretion Disks
Both of the initial black holes likely possess accretion disks – rotating structures containing hot gas and dust particles orbiting rapidly around each black hole. A merger between two black holes can dramatically impact these surrounding accretion disks. As the black holes spiral closer together, their extreme gravitational tugs can essentially shred each other’s accretion disks.
Tidal forces stretch and bend the disks, while hydrodynamical instabilities further tear them apart. When the black holes finally collide, the remaining debris is tossed outward at high speeds by gravitational recoil. This can eject a significant fraction of the disks’ matter far from the merging black hole.
The ejected disk material may collide with other slower debris, creating shock waves and high energy radiation. While the accretion disks are disrupted by the merger, over time new accretion disks can form around the final combined black hole from surrounding matter.
Production of Ripples in Space-Time
One of the most intriguing effects of a black hole collision is its distortion of space-time itself. According to Einstein’s theory of general relativity, space and time are interwoven into a single fabric called space-time. This fabric can be stretched, bent and twisted by the gravitational influence of massive objects like black holes.
As two black holes begin orbiting around each other closely, they create major ripples in their surrounding space-time. The empty space becomes distorted, like the surface of a trampoline with heavy weights rolling around on it. These distortions intensify as the black holes spiral inward and collide.
At the moment of collision, the black holes can violently shake the space-time around them, sending out ripples propagating at light speed. These gravitational waves, as discussed earlier, stretch and squeeze space along their direction of travel. The merging black holes generate the strongest space-time distortions of any event in the universe.
While space-time quickly settles down after the collision, the large-scale ripples can travel across the entire cosmos. Advanced gravitational wave detectors use lasers to measure tiny spatial distortions caused when the ripples from distant black hole mergers pass by Earth.
Expansion of Event Horizons
Every black hole has a boundary around it called the event horizon, which represents the point of no return – any matter or radiation that crosses this threshold is sucked in and cannot escape the black hole. When two black holes collide, their individual event horizons also collide and merge together.
The event horizon of the final black hole rapidly expands outward as it consumes the horizons of the progenitor black holes. The larger the final black hole, the bigger its event horizon. Exactly how the event horizons merge and settle down after a collision is quite complex and represents an active area of current research.
Astrophysicists are studying how the oscillating and distorted event horizons interact with surrounding matter and magnetic fields. This can cause electromagnetic flares and eject high-energy particles. The event horizon’s evolution during a merger also determines how much matter and radiation ultimately get pulled into the final black hole.
Creation of Relativistic Jets
Many black holes, including the ones produced by mergers, can generate powerful jets of matter and radiation along their rotation axes. As surrounding material falls toward the black hole, magnetic fields channel some of this infalling plasma into narrow beams shooting out perpendicular to the accretion disk.
The black hole’s rapid rotation and magnetic field lines accelerate particles within these beams to tremendously high speeds. The particles interact and produce emission across the electromagnetic spectrum, concentrated in bright jets emerging from the poles of the black hole.
When two black holes merge, the collision violence, magnetic realignment, and accretion disk disruption can trigger the formation of a transient jet lasting months to years. In some cases, the merging black holes may already have pre-existing jets that interact and reshape during the collision.
The newly formed black hole remnant can also generate longer lived jets as fresh accretion disks form and feed it. Detailed observations of black hole merger jets provide information about the particle acceleration mechanisms at work near these extreme objects.
Effects on Galaxies and Stars
The collision of two black holes releases an enormous amount of energy, sending powerful ripples through the host galaxies. The gravitational waves generated can traverse vast distances, subtly distorting any celestial objects they pass through. Stars and planets far removed from the collision may wobble around slightly in response.
Closer in, the merging black holes can induce much more pronounced effects. The rapidly changing gravitational fields around the collision excite vibrations in the stars, gas clouds and dark matter of the two galaxies. Tidal forces tear apart stars and disrupt the shape of the galaxies during the final orbits and collision.
The ejection of fast matter by the newly formed black hole can also blast winds through both galaxies. This drives interstellar shocks, triggers new star formation, and feeds growing supermassive black holes later on. Ultimately, the galaxies merge together into one larger galaxy surrounding the merged black hole.
Contribution to Gravitational Wave Background
The gravitational waves produced by a single black hole merger spread out across the universe at the speed of light. As these ripples pass through the cosmos, many black hole mergers happening throughout space and time contribute to a background of gravitational wave noise permeating the universe.
This gravitational wave background is analogous to the cosmic microwave background radiation left over from the Big Bang. While microwave radiation gives astronomers insights into the early universe, the gravitational wave background contains clues about black holes and other dark objects that reshape galaxies over cosmic history.
Advanced LIGO, Virgo and future gravitational wave observatories study the gravitational wave background to learn about the population and history of black hole collisions across cosmic time. Discerning this growing chorus of merging black holes provides a new window into the dynamic, evolving universe.
Boosts to Black Hole Growth
Over cosmic timescales, black hole mergers may be a primary mechanism for black hole growth. Individual black holes formed from the collapse of huge stars may start out with masses up to 20-30 times the Sun. But over multiple generations of collisions and mergers, black holes can steadily increase in size to become the supermassive black holes found at the centers of galaxies.
These giant black holes, with masses millions or billions of times the Sun, likely gained much of their bulk through repeated mergers. When galaxies themselves collide, their central black holes sink toward the core of the newly formed galaxy and eventually spiral together in a black hole collision. Such mergers can rapidly build up black hole mass in the hearts of galaxies.
Conclusion
The collision and merger of two black holes is one of the most extreme and energetic events in the cosmos. When their tremendous gravitational fields interact, all manner of fireworks can ensue: gravitational waves crisscrossing the universe, electromagnetic flares, relativistic jets, distortions of space-time itself and accelerations of matter to tremendous speeds. These mergers can even shake galaxies, influence star formation, and leave imprints on the gravitational wave background.
Most importantly, black hole mergers contribute to the growth of supermassive black holes over cosmic time. By combining together over and over through galaxy collisions, smaller black holes build up into the giants we see today anchoring the centers of galaxies. The collision of two black holes represents a tiny glimpse into the remarkable life cycle of these dark, mysterious objects.