BLACK HOLES: SHINING SOME LIGHT ON INTO THE DARKNESS
Black holes are an astronomical phenomenon made popular through science fiction based entertainment. Due to its complex nature we often resort to our imagination when deciding upon what they are and what they can do. Instead of jumping into the topic of black holes directly, let’s start with the basics and examine gravity, the fundamental force behind them.
The presence of gravity is obviously easy to observe. People knew that apples would fall from trees, catapulted rocks would eventually hit the ground and that something was keeping us relatively tethered to the earth. Sir Isaac Newton was the first person to accurately describe the fundamental force which attracts all objects of mass to one another – gravity. Newton expanded the traditionally isolated view of gravity and realized that the earth wasn’t the only gravity exerting object; he applied his theory of gravity to all objects of mass. He actually hypothesized that the force of gravity was responsible for orbital motions which he proved by calculating the moon’s time to orbit the earth![1]
The force of gravity between two objects is dependant on two things, their mass and distance between each other. The greater the mass of the objects, the more strongly they are attracted to one another. The fact that the earth is so much more massive than anything else around us makes it easy for us to understand why it’s usually the only one that we notice. The second factor contributing to gravity is the distance between the two objects. The force of gravity increases exponentially (gravity is proportional to the inverse squared distance for you math-type folks!) as the two objects come closer and closer together. For example, the force of gravity between Bob and Suzy will be 10,000 times greater when they are sitting 1 meter away as opposed to 100 meters away (1002 = 10,000).
One final concept that is crucial to understanding the theory behind black holes (save extremely complicated physics equations and new systems of mathematics) is the idea of density. Density is the amount of mass per unit of volume. Objects will become denser as you increase their mass and decrease the amount of space that they occupy. A solid wooden box for example will have a far greater density than an identical box made of Styrofoam. To put everything into proportion and to get an idea of scale, let’s examine the two extremes of the density spectrum. It’s easy to imagine something with low density; by examining any volume in a vacuum we can determine that it has a density very close to zero. But what about the densest object? How dense can an object become? Scientists believe that the densest object in the history of all time was the universe before the Big Bang when all matter and energy was contained within a single point known as a singularity which occupied a near zero volume.
Black holes are supremely dense regions of space with mind-boggling masses yet occupy a tiny space. Their incredible density allows their massive singularity to come extremely close to other objects which skyrockets the force of gravity exerted. Their gravity is so intense that once any object crosses a certain proximity to it known as the event horizon, it will be attracted to and inevitably integrated with the singularity. At this distance there is no hope of escape from the pull of gravity. You could be a beam of light and it wouldn’t matter, you’d still be drawn into the singularity.[2]
Just how massive are black holes? Black holes are categorized based on mass relative to our sun. An object that is 2 solar masses is twice the mass of our sun. To put things into perspective yet again, one solar mass is approximately 1.9891×10^30 kg or 1,989,100,000,000,000,000,000,000,000,000 kg (yes that is a big number) which is equivalent to 332,946 times the mass of the earth or 27,068,510 times the mass of our moon! Black holes categorized as supermassive which are billions of times the mass of the sun, intermediate-mass which are thousands of solar masses, or stellar mass which are 1.5-15 solar masses. A final category of black holes, micro black holes [3] have masses less than our sun are and it is thought to be possible to create them artificially in particle accelerators. Most black holes are predicted to be in the stellar mass category or larger originating from the collapse of large stars near the end of their lives. Stars normally burn tons of hydrogen as fuel in their core and exert an impressive outwards force that maintains their temperature and size, however when stars don’t have enough fuel left to maintain this outwards push, the star forcefully collapses on itself condensing it’s mass into a very small volume and becomes a black hole. [4]
The existence of black holes was proposed back in 1783 by a man named John Mitchell who predicted that gravity could affect light and that a massive enough star could have a gravitational pull large enough to prevent light from escaping it. This idea was furthered by Albert Einstein in 1916 when his paper on the General Theory of Relativity was published whose equations made it possible to predict the existence of black holes. Using Einstein’s equations, Karl Schwarzschild was able to describe a black hole by defining the distance from any spherical mass at which light cannot escape. The term “black hole” wasn’t actually coined until 1969 by the American scientist John Wheeler which brings us to present day.
Unfortunately due to the fact that black holes prevent light from escaping, it’s impossible to actually directly observe them. Knowing this, how is it possible to find these things and how do we know that they actually even exist? In order to “see” black holes, we have to examine the matter and energy that the black hole interacts with. For example, in 1973 astronomers observed that a blue supergiant star was orbiting around an invisible object that emitted x-rays leading them to believe that this invisible object was a black hole. Using some of the theory behind Newton’s calculation of the moon’s orbit, modern scientists can calculate the velocity of gas and dust orbiting any mass using radio telescopes and use this information to determine the mass of that object. This technique was applied to gas and dust in the centre of the galaxy known as NGC4258, and it was determined that the mass that they were orbiting was 40 million times the mass of the sun – strongly implicating the existence of a black hole.[5]
Black holes make regular appearances on any science fiction based television show or book. I’m sure that Captain Kirk or Picard and all other captains from the entire Star Trek franchise have had to deal with their fair share of black holes throughout their voyages through outer space. But how accurate are these depictions of the ever-mysterious black hole? Let’s test your assumptions of black holes as the vacuum cleaners of the universe by asking what you believe would happen if the sun was suddenly transformed into a black hole? Would our solar system immediately get sucked into oblivion, would we slowly spiral to our doom or end up in some alternate dimension? If you had thought that any of these fates was true, you might be a victim of Hollywood fantasy. Despite being in perpetual darkness and freezing, if our sun had become a black hole, it would still have the same mass as before and we would still be the same distance away meaning that we would continue to orbit this black hole as we had when it was still our sun. The event horizon of our sun is predicted to be extremely small (a few kilometers) and as long as we didn’t tow our planet to within this range we wouldn’t be “sucked” in.
For interests sake what would happen to something that entered the event horizon of a black hole? The object would be attracted to the singularity in the middle of the black hole and would continue to gravitate towards it. Depending on the size of the black hole, the object would experience something known as a tidal force. As mentioned earlier the force of gravity between two objects is dependent how apart they are; the force increases exponentially as the objects get closer to one another. When an object gets close to a black hole, one side of it is actually closer to the singularity and thus has a stronger attraction to the black hole than the other. This gravity near a black hole is so strong that the tidal forces can cause objects to get ripped apart atom by atom – a process known as spaghettification.
What we observe as an object is being drawn into a black hole depends on whose point of view we’re looking from. If we pretend that Bob the astronaut is being drawn into a black hole, the amount of time that it takes to be smash his atoms into the singularity is finite and takes place within an observable time. However to Suzy watching Bob drift towards the black hole from a safe distance away, he seems to slow down the closer he got to the even horizon until he was at the boundary and would appear to be frozen in time. The reason for this is that as Bob got closer and closer to the event horizon, the light that he emits is also being affected by the same gravity and would take longer and longer to reach Suzy. As the light slows down its wavelength (which defines the colour of light) would progressively increase making Bob appear more red until the wavelength was no longer in the visible spectrum where he would then disappear from sight. The wavelength would continue to increase until the waves entered the range of infrared, followed by radio waves and would eventually approach infinity.
A popular concept in science fiction is the existence of worm holes – gateways through which space-faring adventurers can traverse vast distances instantaneously or even through time. In order to understand this, you’re going to have to imagine that it’s possible to not only move forwards through time, but backwards as well. When you assume that time is going backwards, Einstein’s equations of general relativity (which work both forwards and backwards through time) predict the existence of what is known as a white hole. White holes are the complete opposite of a black hole, constantly spewing matter out into space. If a black hole is paired up with a white hole, it would be theoretically possible to enter the black hole and emerge through the white hole a very far distance away.[6] Although theoretically possible, the existence of white holes and worm holes is unlikely as they violate the laws of thermodynamics, don’t take cause and effect into account and even if they did exist would be extremely unstable and prone to collapse.
Our understanding of the universe and the laws that govern it is scant at best. Despite the countless more questions than answers, there is hope that some of them will be answered when the world’s largest and most powerful particle accelerator, the Large Hadron Collider in Geneva, Switzerland opens for use in May of 2008. Experiments are planned to investigate the leading theory known as the “Grand Unified Theory” that integrates all fundamental particles that make up the universe as well as incorporating 3 of the 4 forces that govern their interactions. In fact, the one force that hasn’t been explained in this theory is gravity! Assuming that this particle accelerator doesn’t create a micro black hole that devours the earth, we should all be excited to see what new corners of the universe we can shine some light upon.
References
1. Newton, Isaac, Philosophiae Naturalis Principia Mathematica, July 5, 1687.
2. Hawking, Stephen (1988). A Brief History of Time. Bantam Books. ISBN 0-553-38016-8.
3. Giddings, Steven and Thomas, Scott. (2002). High energy colliders as black hole factories: The end of short distance physics. Phys. Rev. D. 65:056010.
4. McClintock, Jeffrey E. “Black hole.” World Book Online Reference Center. 2004. World Book, Inc. link
5. Miyoshi M., Moran J., Herrnstein J., Greenhill L., Nakai N., Diamond P., and Inoue M. (1995). Evidence for a black hole from high rotation velocities in a sub-parsec region of NGC4258. Nature. 373:127.
6. Einstein A. and Rosen N. (1935). The particle problem in the general theory of relativity. Phys. Rev. 48:73 – 77.