Monsters in the Dark: Supermassive Black Holes and Their Destructive Habits
by Louise Good
In this image taken by the Hubble Space Telescope, the luminous red galaxy known as LRG-3-757 (center) is the galaxy acting as a gravitational lens that distorts the light from a more distant blue galaxy, which we see as a blue Einstein ring. LRG-3-757 has a mass about ten times that of the Milky Way. This image gives us a view of the early Universe: the blue galaxy’s redshift (a measure of how the wavelength of its light has been stretched by the Universe’s expansion) is approximately 2.4. This means we see it as it was about 10.7 billion years ago, only 3 billion years after the Big Bang. Credit: NASA.
Nicholas McConnell, the IfA’s Beatrice Watson Parrent Postdoctoral Fellow, delivered an informative and entertaining pre-Halloween talk about the largest (“supermassive”) black holes in the Universe at a Frontiers of Astronomy Community Event held on October 29 on the Mānoa campus.
The first thing McConnell explained is that supermassive black holes live in galaxies. In keeping with the Halloween theme, he began by explaining that spiral galaxies are “living healthy galaxies” with lots of gas and stars (a normal family dwelling), while elliptical galaxies are “red and dead” with little gas for star formation and very old stars (more like haunted houses). Black holes may trigger a key process that transforms spirals into ellipticals.
McConnell said that while Isaac Newton (1642–1727) figured out that the force (gravity) that makes things drop to the floor is the same force that keeps the Moon in orbit, he thought that light moves in straight lines. Albert Einstein (1879–1955) later realized that gravity affects light as well as matter. He introduced the concept of space-time with “space and time being intertwined in four dimensions.” He continued, “So one of the most important results from the General Theory of Relativity is that gravity not only changes the path of objects that I throw and deflects them downward so they land on the Earth [but also] that light itself will actually be influenced by objects with gravity.”
One verification of Einstein’s theories is called gravitational lensing: When one galaxy is behind another in our field of view, the gravity of the foreground galaxy bends the light coming from the background galaxy, resulting in “cool visual effects” such as an Einstein ring.
So what exactly is a black hole? If you cram enough matter into a small enough volume so that “space-time becomes so severely warped that even light is unable to escape and a normal deflection of light … turns into a well that traps the light permanently and never lets it get anywhere else,” you have a black hole.
There are three classes of black holes based on size. “Stellar-mass” black holes are believed to have evolved from individual stars. They may be up to about ten times the mass of our Sun. “Intermediate-mass” black holes are the ones scientists “know the least about, but we think black holes exist somewhere in the Universe with thousands to hundreds of thousands of solar masses.” Then there are “supermassive” black holes that exist in the hearts of galaxies. They have masses of at least 1 million to billions of times the mass of the Sun. They originated in a very different process from stellar-mass black holes, and have probably been around since the Universe was very young. These are the ones he spent most of the talk on.
Near a black hole, light shifts to longer wavelengths, time slows down (“the ultimate age reduction machine”), and the “force of gravity near a black hole is so severe that the difference in gravity over a short length is actually noticeable,” stretching out anything that comes near it, McConnell said. Anything that falls into a black hole is crushed by the extreme gravity. “However, there are some exotic ways you can play with theoretical physics” about what would happen if you went into a black hole. One of these ideas is that a black hole might be a wormhole that leads to “a different part of space and/or a different part of time.” This concept has been very useful for science fiction writers, but we have no way of knowing if it is true.
Since black holes are basically invisible—no light escapes from them—scientists learn about them by studying the behavior of objects around them. The stars close to the supermassive black hole at the center of our Milky Way galaxy have been studied with the Keck telescope on Mauna Kea and the Very Large Telescope in Chile. After watching the movement of these stars over many years, scientists have been able to determine that the black hole has a mass 4 million times that of our Sun. Using similar methods, they have calculated that the mass of the black hole at the center of the neighboring Andromeda galaxy is 140 million times that of the Sun. The largest black hole measured so far is estimated to be 20 billion solar masses.
So what would happen to you if you were to approach a black hole? If you were carrying a flashlight, the gravity of the black hole would cause the light from it to shift to longer wavelengths toward the color red and then into the infrared, which we cannot see with our eyes. As you got closer to the black hole, the light would go in a circle (“photon sphere”) rather than going outward. In fact, time near a black hole stretches out infinitely so that if your friends in a spaceship were watching from a safe distance, they would never see you fall into the black hole. Another effect, the one that would probably be fatal: You’d be stretched to death. If your feet are closer to the black hole, the gravity near your feet is so much stronger than near your head that you would get pulled apart. If that didn’t kill you, and you actually fell into the black hole, you would probably be crushed to death.
However, the idea that black holes are “cosmic vacuum cleaners” that suck everything up “is fortunately a little bit false.” To actually fall into a black hole, “you have to come very, very close to it.” Black holes are actually more like particle accelerators because matter that comes close to a black hole but does not actually fall into it is greatly speeded up. When gas orbits a black hole, some of it falls into the black hole, but the rest is heated to very hot temperatures causing the gas to glow so brightly that we can see it most of the way across the Universe. These objects, called quasars, are so far away that their light has been traveling toward Earth for billions of years. They therefore show us what the Universe was like when it was young.
As for whether black holes represent a threat to Earth, the answer is “no.” They are simply too far away.
To learn more about black holes, watch the entire talk at here. Talk (with sound) begins at 00:17:11.