What Are Black Holes?
Black holes refer to a region of space that exhibits such strong gravitational force that nothing (not even light) can escape from its grasp. But what exactly are black holes? Where do they come from? Finally, and perhaps most importantly, why are they important in understanding our overall universe? This article, through an analysis of current theories and research, explores the concept of black holes in an attempt to better understand not only their origins, but also their place and importance within the universe at large. Although theories pertaining to black holes remain limited, given the lack of data and empirical observation of these space entities, this article aims to provide its readers with a fundamental understanding of current hypotheses that dominate the scientific community today.
Black Holes Defined
Although the name “black hole” gives rise to the concept of “nothingness,” black holes are anything but empty. Scientists believe that the holes contain tremendous amounts of matter, and may result from the death of massive stars. Once a massive star dies, implodes, and undergoes a supernova explosion, it is believed that they sometimes leave behind a small, but dense remnant core that is approximately three times the mass of our Sun (science.nasa.gov). The result of such mass (in a relatively small space) is an overwhelming force of gravity that overcomes all objects that surround it (including light), creating the appearance of a black hole.
The concept of black holes is nothing new within the scientific community, as scientists and astronomers from the Eighteenth Century (most notably, John Michell) proposed that such objects may exist in our universe. In 1784, Michell argued that black holes were likely the result of Stars whose diameter exceeded the diameter of our Sun by a factor of 500. He also correctly observed that the holes could potentially be observed through an analysis of their gravitational pull on nearby celestial bodies. Michell remained perplexed, however, over how a supermassive object could effectively bend light. Albert Einstein’s theory of “general relativity” (1915) later helped demonstrate how this was possible. Expanding on Einstein’s theory, German physicist and astronomer, Karl Schwarzschild, helped develop the first modern version of what a black hole was in 1915, arguing that “it was possible for mass to be squeezed into an infinitely small point” that would not only bend spacetime (due to its incredible gravitational pull), but would also prevent “massless photons of light” from escaping its grasp as well (sciencealert.com). Despite his theories, however, credit for the term “Black Hole” lies with physicist John Wheeler, who first proposed the name in December of 1967.
Types of Black Holes
Currently, there are five types of black holes that have been identified by astronomers. These include miniature, stellar, intermediate, primordial, and supermassive black holes. No black hole, however, is alike as some (such as the supermassive black hole at the center of the Milky Way) contains masses that are equivalent to several billion Suns, while miniature black holes (which remain only theoretical at this time) are believed to be quite smaller in mass.
Scientists also believe that black holes change in size throughout their lifetime as well, growing with the absorption of gas, dust, and objects (including planets and stars) that pass by their event horizon (point where nothing can escape from the black hole’s pull). Scientists have also theorized that black holes can merge with other black holes. This merger would help to explain the size of supermassive black holes that exist throughout the universe.
- PRIMORDIAL BLACK HOLES
Primordial black holes are believed to be ancient (as the name implies) as they likely formed soon after the Big Bang occurred. It is likely that the first primordial black holes were extremely small, with many evaporating over time. Other primordial holes, with larger masses, may still exist today. However, such speculation remains only a theory at this time, as no primordial black hole has been detected or observed in the visible universe so far. Some scholars, such as the late Stephen Hawking, believe that primordial black holes may hold the key to understanding “dark matter” in the universe.
- STELLAR-MASS BLACK HOLES
The most common form of black holes are stellar-mass objects. It is believed that stellar-mass black holes result directly from supernova explosions, caused by the implosion of a supermassive star once it exhausts all of its internal fuel sources. For this reason, stellar-mass black holes are often found scattered throughout the galaxy. Stellar-mass black holes are approximately five to ten times the mass of our Sun. However, recent scientific research has indicated that some stellar-mass black holes may reach sizes up to 100 times the mass of our Sun.
- INTERMEDIATE-MASS BLACK HOLES
These black holes range in size from hundreds to several hundred-thousand times the overall mass of our Sun. Although none have ever been detected with a high-level of certainty, there is abundant evidence to support their existence in the universe. Astronomers and scientists, alike, believe that intermediate-mass black holes can form from three separate scenarios: A.) They are primordial black holes that formed from materials in the early cosmos, B.) They possibly formed in regions of space that contained a high-density of stars, or C.) They developed from the merger of two smaller black holes (stellar-mass) that collided with one another. For these reasons, intermediate-mass black holes are believed to exist at the center of globular clusters in the galaxy.
- SUPERMASSIVE BLACK HOLES
Supermassive black holes, as the name implies, are the largest forms of black holes in the universe, and often contain masses that are millions (and sometimes billions) of times larger than our own Sun. Currently, it is believed that supermassive black holes are at the center of almost every observable galaxy in the universe. Unlike stellar-mass black holes that form from the collapse of massive stars, it remains a mystery how supermassive black holes form. Powerful quasars, however, may hold the answer to their formation.
In 1974, Stephen Hawking revolutionized the study of black holes with his theory known as “Hawking Radiation.” In this theory, Hawking proposed that black holes were not entirely black, and argued that the holes “emit small amounts of thermal radiation” (Wikipedia.org). The theory was revolutionary in that Hawking’s analysis demonstrates that black holes are capable of shrinking and evaporating over time “as they lose mass by the emission of photons and other particles” (Wikipedia.org). Although the evaporation rate of supermassive black holes is incredibly long (approximately 2x10100 years for an average-sized supermassive black hole), the theory demonstrates that black holes are like the rest of the universe in that they are also in a state of decay.
Scientists have been unable to observe black holes with telescopes that detect forms of electromagnetic radiation. However, their presence has been inferred through the observation of their effect on matter within their general vicinities. For example, when distant objects are seen orbiting around seemingly invisible objects, or when objects move erratically, astronomers believe that black holes are likely to blame.
Black holes are sometimes more obvious, however, as their consumption of surrounding stars sometimes superheats gas and dust that surround the black hole, causing it to emit visible radiation. Occasionally, this radiation “envelops the black hole in a whirling region called an accretion disk” (nationalgeographic.com), making it partially visible to observers on Earth. Similarly, black holes can even eject stardust, giving a comparable radiation effect on the dust particles that are exiting.
Direct photos of black holes were largely considered impossible until earlier this year, when the “Event Horizon Telescope” (EHT), which consists of a large network of radio telescopes operating in unison, were able to construct the first image of a black hole at the center of Messier 87. Using complex algorithms and image reconstruction (known as CLEAN), astronomers have now developed a means for using radio frequencies (radio astronomy) to provide images of our distant neighbors.
What Happens To Objects That Fall Into Black Holes?
What happens to objects that fall into black holes? Although little is known about what transpires inside a black hole, scientists and astronomers believe that subjects that pass the hole’s event horizon are subjected to tremendous tidal stress. The object (or individual) would quickly find itself stretched and squeezed in all directions, before finally being torn completely apart. These tidal forces are the same phenomenon “responsible for ocean tides on Earth,” in relation to the Moon’s gravitational pull (Chaisson and McMillan, 599). The difference between a black hole and Earth’s tidal forces is that the black hole’s are incredibly stronger, and remain the strongest force known to exist within the universe at this time.
In addition to being stretched in all directions, matter entering the black hole is also squeezed and “accelerated to high speeds” (Chaisson and McMillan, 600). With countless objects being stretched, torn apart, and accelerated, violent collisions are believed to also occur between these particles, creating frictional heating. This, in turn, causes the matter to emit radiation as it plunges into the black hole through the form of x-rays. For this reason, some scientists believe that the region surrounding a black hole may be a potential source of energy.
Is Time Travel Possible Inside a Black Hole?
One popular element of science fiction and popular culture is the notion that black holes may hold the power for individuals to travel in time. Assuming an individual could pass beyond the event horizon of a black hole without being torn apart, and assuming that an object/individual could exit the black hole at their own choosing (which remains theoretically impossible at the present time), scholars believe that time travel is, indeed, possible with black holes. Due to the tremendous gravitational pull of a black hole, scientists believe that time slows down for objects approaching its event horizon. Clocks onboard a spacecraft entering a black hole would show “time dilation” in relation to clocks operating outside of the event horizon. As a result, scientists believe that once the spacecraft exited the black hole, it would appear days (even years) into the future, depending on how long it remained inside.
For the outside observer witnessing the spacecraft’s approach toward the event horizon, the journey would appear to take forever. For the space-crew onboard, however, scientists believe that time would appear completely normal; thus, making time travel into the future a real possibility.
Black Holes in Popular Culture
Black holes continue to play a prominent role in Hollywood and pop culture, alike. Although human understanding of black holes continues to remain miniscule, the human imagination (particularly in science fiction) has proven quite wild in more recent years with the portrayal of these deep-space objects. Here is a list of popular movies with references to black holes:
- Star Trek
- The Black Hole
- Event Horizon
“Black holes are where God divided by zero.”— Albert Einstein
Quotes About Black Holes
Quote #1: “Black holes are where God divided by zero.” – Albert Einstein
Quote #2: “The black holes of nature are the most perfect macroscopic objects there are in the universe. The only elements in their construction are our concepts of space and time.”
Quote #3: “The black hole teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as ‘sacred,’ as immutable, are anything but.” – John Wheeler
Quote #4: “Black holes are the seductive dragons of the universe, outwardly quiescent yet violent at the heart, uncanny, hostile, primeval, emitting a negative radiance that draws all toward them, gobbling up all who come too close. These strange galactic monsters, for whom creation is destruction, death life, chaos order.” – Robert Coover
Quote #5: “Consideration of particle emission from black holes would seem to suggest that God not only plays dice, but also sometimes throws them where they cannot be seen.” – Stephen Hawking
Quote #6: “We have this interesting problem with black holes. What is a black hole? It is a region of space where you have mass that’s confined to zero volume, which means that the density is infinitely large, which means we have no way of describing, really, what a black hole is!” – Andrea M. Ghez
Quote #7: “Do you realize that if you fall into a black hole, you will see the entire future of the Universe unfold in front of you in a matter of moments and you will emerge into another space-time created by the singularity of the black hole you just fell into?” – Neil deGrasse Tyson
Quote #8: “If you want to see a black hole tonight, tonight just look in the direction of Sagittarius, the constellation. That’s the center of the Milky Way Galaxy and there’s a raging black hole at the very center of that constellation that holds the galaxy together.” -- Michio Kaku
Quote #9: “Black holes provide theoreticians with an important theoretical laboratory to test ideas. Conditions within a black hole are so extreme, that by analyzing aspects of black holes we see space and time in an exotic environment, one that has shed important, and sometimes perplexing, new light on their fundamental nature.” – Brian Greene
Quote #10: “Data suggest that central black holes might play an important role in adjusting how many stars form in the galaxies they inhabit. For one thing, the energy produced when matter falls into the black hole may heat up the surrounding gas at the center of the galaxy, thus preventing cooling and halting star formation.” – Priyamvada Natarajan
Do you believe that black holes will make the prospect of time travel a reality in the future?
In closing, black holes continue to be one of the most fascinating (and strangest) objects to inhabit our vast universe at large. Although information about their existence and internal structure continues to be limited for the time being, it will be interesting to see what new forms of information can be gleaned about these fascinating deep-space objects in the near future. What can black holes tell us about our universe? How did they form? Finally, and perhaps most importantly, what can they us about the formation of our universe and the early cosmos? Only time will tell.
Chaisson, Eric and Steve McMillan. Astronomy Today, 6th Edition. New York, New York: Pearson, Addison Wesley, 2008.
NASA. Accessed May 04, 2019. https://science.nasa.gov/astrophysics/focus-areas/black-holes.
Wei-Haas, Maya. "Black Holes, Explained." What Is a Black Hole? December 17, 2018. Accessed May 04, 2019. https://www.nationalgeographic.com/science/space/universe/black-holes/.
Wikipedia contributors, "Black hole," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Black_hole&oldid=895496846 (accessed May 4, 2019).
Wikipedia contributors, "Event Horizon Telescope," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Event_Horizon_Telescope&oldid=895391386 (accessed May 4, 2019).
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© 2019 Larry Slawson