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Because black holes can swallow anything, could a black hole follow a quasar or another black hole?
Asked by: Amira Brown
School: Jennie F. Snapp Middle School
Teacher: Mrs. Coury
Hobbies/Interests: Reading, painting, learning
Career Interest: Lawyer, college professor, interior designer, florist, artist
Answer from Hiroki Sayama
Director, Collective Dynamics of Complex Systems Research Group
Research area: Complex systems, artificial life, mathematical biology, computer and information sciences Interests/hobbies: Traveling, walking, swimming
Great question, Amira! This is a very timely one because, this year, scientists have finally obtained the very first direct observations of two black holes chasing each other and eventually merging into one. So the answer is yes -- a black hole definitely can follow whatever has mass around it, even if it is another black hole or a quasar.
As you may know already, black holes are made of a large amount of mass concentrated in a very small area in space. Their huge mass causes strong gravitational force, pulling anything that has mass toward them, including other black holes nearby. And if a black hole keeps absorbing a large quantity of mass continuously, this process releases an excessive amount of energy in the form of light and other electromagnetic radiations, making it a very bright quasar.
But it is not just light or electromagnetic radiations that black holes can emit. In 1916 (precisely 100 years ago!) Albert Einstein predicted that accelerating objects with asymmetric mass distribution, such as two black holes chasing each other, could also emit "gravitational waves," in other words, ripples of spacetime. Gravitational waves were predicted to carry energy by themselves and propagate at the speed of light. This means that two black holes chasing each other should gradually lose energy due to emission of gravitational waves, eventually falling into each other to form a single black hole.
Gravitational waves are thought to be detectable by measuring tiny variations in time needed for light to travel across a certain distance. Astrophysicists have been working hard for more than forty years to achieve this. Their project, named LIGO (Laser Interferometer Gravitational-Wave Observatory), finally detected gravitational waves produced by two merging black holes in September 2015, which was publicly announced in February 2016. And the second set of gravitational waves from another merger of two black holes was detected only a few months later, in December 2015. This implies that mergers of black holes are happening much more frequently than originally thought. You can even hear the "sounds" of gravitational waves from LIGO’s website (http://bit.ly/1sLYjc8). They are kind of cute!
So, why is this important? Obviously, because scientists were able to show Einstein’s prediction was right. But perhaps what’s more important is that humanity has obtained a whole new way to observe the world. Gravitational waves are fundamentally different from light and other electromagnetic waves. Being able to "see" (or "hear") such gravitational waves means we now have a completely new sensory ability, which enables us to see, for example, places far distant in space and time from where light could not reach us. We are very fortunate to be alive in this very exciting era.