The idea that objects exist in the universe which cannot be seen but which greatly influence objects around them was proposed long before the 20th century. But this idea had to wait for credibility until after 1915, when Einstein completed his theory of general relativity, which connected gravity to a warp in spacetime, a concept difficult for many to understand including myself.
But numerous experiments have demonstrated that it is correct—most notably, the first demonstration in 1919 that light from a star does not reach the earth in a straight line if it has to pass by the sun’s gravitational force. The path of the light is distorted, or warped, by the sun’s gravity just as a ball rolling downhill follows the contours in the earth it is rolling over.
Now let’s turn to something called escape velocity. To get away from the earth’s gravitational field, objects have to accelerate to a speed of about 25,000 miles per hour. The escape velocity needed to overcome the gravitational field of any body depends on that body’s mass (how much matter it contains) and density (how much space that mass occupies). For example, escaping from the moon’s gravity requires reaching only about one-fifth of the speed necessary to escape from the earth’s gravity, a fact that had to be well understood by rocket scientists if the astronauts who landed on the moon were to lift off and come back.
Might there be an object in space whose mass is contained in a sufficiently small space that the escape velocity needed to leave its gravitational field would exceed 186,000 miles per second, the approximate speed of light in a vacuum? Such an object would not release any light, regardless of whether the light came from it, as with a light bulb, or came to it, like the light reflected from a mirror or the moon. We think of light as that which we see. But “light” is part of something much larger. In scientific terms, light is electromagnetic radiation, and there is a spectrum of this radiation. We see only a very tiny part of that spectrum in the colors we perceive—the visible region of the spectrum. We feel other parts of the spectrum as heat—the infrared region. Some of us tan our skin or synthesize vitamin D through exposure to the ultra-violet part of the spectrum. Dentists use X-rays, another part of the spectrum, to detect cavities, while we receive radio and television signals and talk on our cell phones using still other parts of the spectrum. The fixed speed of light applies to the whole spectrum. An object with such a strong gravitational force that no part of the electromagnetic spectrum can escape it is called a black hole.
Now that we’ve connected gravity with light, let’s turn to the life span of stars, including our star, the sun, which gives off light across much of the spectrum. Stars are formed from the “dust” of the universe, which tends to gather in “clouds” containing hydrogen. These clouds can become increasingly dense because the mass of the cloud creates a gravitational force, causing it to pull in more stellar dust, which increases the density further, which then pulls in more dust, and so on. This increasing density is the origin of a star. Energetic processes among these atoms become hotter and hotter as they come closer until nuclear fusion occurs—the source of the explosive power of the hydrogen bomb. During this fusion, which turns hydrogen into helium, some of the mass is converted to energy in accordance with another of Einstein’s insights: E=mc2, where c is the speed of light. This means that a very small change in mass leads to a very large production of energy, the very energy which is emitted from the sun as the electromagnetic radiation the earth receives. This nuclear process goes on and on as long as there are nuclei of hydrogen atoms to feed it. Eventually, the sun will consume all the nuclei available for this fusion, and the process will first slow and then stop. The sun will die. We are about half way there now, but don’t be concerned. We, on earth, have many billions of years to go.
Stars are massive, with enormous gravitational fields—large enough for our sun to keep the earth and all the other planets in our solar system in their orbits. The gravitational force is also acting to decrease the volume of the sun—to increase its density by making it take up less space. But this force is counteracted by the force of the energy produced from the nuclear fusion taking place—sort of like heating the air in a balloon. But cool that air enough and the balloon will grow smaller and eventually collapse. That’s the fate of a burned-out star. What form the collapsed star takes depends on its mass. Our star, the sun, is a bit too small to form a black hole. Its eventual collapsed state probably will be something else—what astronomers call a black dwarf. But other stars, especially those formed near the origin of the universe, the time of the Big Bang, are more than large enough to collapse into black holes, which, with their enormous gravity, will increase in their power to consume whatever is around until they run out of mass that is near enough. Anything that comes within a certain distance (called the event horizon) from the center of the black hole will be swallowed up. There are huge numbers of black holes in the universe, ranging in size from ten times to one million times the mass of our sun, with a large proportion of the galaxies in the universe each containing many black holes. There are estimated to be more than 100 billion galaxies in the universe, each with as many as 100 billion stars or more, many with the potential to become black holes!
We discover black holes by their effect on their surroundings. The concentrated mass of a black hole, with its huge gravitational force, affects the motions of large parts of whole galaxies, motion that would not be possible to understand if this invisible force were not present. (Think of the footprints appearing in the snow toward the end of the 1933 film “The Invisible Man,” adapted from H.G. Wells’s 1897 science-fiction novel.) This attractive gravitational force also pulls toward the black hole the same kind of stellar dust that stars are made of. This material swirls about the black hole, moving more and more quickly as it approaches the point of no return, like water going down a drain. This accelerating swirling process releases enormous amounts of energy in the form of quasars, sources of such powerful electromagnetic radiation that they give the impression of being “spotlights” in the universe, radiating with an intensity in the range of one trillion times—yes, one trillion—the luminosity of our sun. The light from some of the most distant quasars has taken almost 30 billion light years to reach us—30 billion years traveling at the speed of light. What we are seeing, in other words, occurred a very long time ago, near the origin of the universe, with the number of quasars now being detected giving scientists an idea of the dimensions of the universe we are part of and how ancient the black holes can be.
By Mark M. Green