The Power Source for Future Space Travel?
University of Alaska Anchorage
Everyone knows that the spaceships in Star Trek that travel faster than the speed of light are mere science fiction. According to Einstein’s theory of relativity, if an object reached the speed of light, its mass would be immediately transformed into energy. Currently our spaceships can not even reach mars in less than five years. Now, with modern theories of black holes, trips to other solar systems may be possible at nearly the speed of light. Black holes were only proven to exist within the last twenty-five years and were only really considered to exist for the last seventy-five years. Yet these recently discovered objects could be the energy revolution of the future, much like nuclear power was in the sixties. Black holes generate tremendous amounts of energy in several different ways, and so can be harnessed in several ways to generate usable power.
In order to understand how energy can be created from black holes, one must first have an understanding of black holes themselves. Black holes are formed by matter being crushed within a certain radius (call the Shwarzchild radius or event horizon). This radius can be calculated by the equation r = 2GM / c2, were G is Newton’s gravitational constant, c is the speed of light, and M is the mass of the black hole. This shows that the density within the event horizon, which is equal to 3M / 4?r2 for a spherical object, will actually decrease as the mass increases. The gravitational field around a black hole will act same as an object of identical mass, so if the sun were to suddenly become a black hole would the earth go plummeting into it? No, it would continue on its orbit things just get interesting close to the black hole (Jebornak, 1998).
There are three types of black holes that scientist currently believe are capable of becoming future power sources: Schwarzschild black holes, Newmann black holes, and primordial black holes. Schwarzschild black holes are the simplest black holes because they do not rotate and have no charge. The Newmann black hole, on the other hand, rotates and has a charge, but like the Schwarzschild black hole can have varying masses from a couple times our sun’s mass to several billion times the mass of our sun. Primordial black holes were first theorized about In the year 1973 by Stephen W. Hawkings who postulated that in the early moments of the big bang, miniature black holes would have been created with masses around that of a small mountain, 2 * 10^13 kg (Wagner, 1999). These black holes could resemble a Schwarzschild black hole or Newmann black hole or even be a hybrid of their properties by rotating but possessing no charge.
Newmann black holes have an ellipsoid (three-dimensional oval) around them called the ergosphere, and it connects with the black hole’s outer event horizon at the poles of its axis of rotation (Jebornak, 1998). Once matter enters the ergosphere, it begins to rotate along with the rotation of the black hole; this forms what is known as an accretion disc. The closer that the matter comes to the event horizon, the faster the matter will be rotating. With the help of Hubble and other telescopes astronomers have found matter rotating around super massive black holes at speeds in excess of 1.9 million miles per hour at the centers of distant galaxies.
All of this rotating matter rubs together causing a build up of static electricity (similar to rubbing ones feet on a carpet) which, since the matter is rotating, makes an electric field with an extremely powerful magnetic field at its poles. This magnetic field operates much like the one here on earth: all matter with an opposite charge of the pole is attracted to it. Since these magnetic fields are so intense, matter is pull towards the poles and shot away from them at speeds of 90% the speed of light or higher.
Black holes with small mass have tremendous tidal forces around them. Tidal forces are caused by the difference in gravitational force between two points. In small black holes this difference is greatly enhanced over smaller and smaller distances causing the famed spaghettification affect on an object closing in on the black holes event horizon. Spaghettification is where an object approaching a black hole is stretched lengthwise and compressed widthwise. These forces are so powerful in small black holes that Stephen W. Hawking theorized the tidal forces of small black holes are strong enough to tear apart pairs of antiparticles and virtual particles producing enough energy to make the virtual particle materialize in space (Jebornak, 1998). This process is known as Hawking radiation and counterbalances a black holes ability to absorb matter and grow. When the black hole creates this virtual particle some of its energy gets used, thus lowering its mass due to Einstein’s famous equation E = Mc2. This causes a cascade effect since the smaller the black hole gets, the stronger the tidal forces get, which causes more particles to be formed. This process is called evaporation since it will lead to all of the black holes mass being changed into energy. William Kaufmann, a physicist, theorized that the total amount of energy released during the final second of evaporation is equivalent to a billion megaton hydrogen bombs (1989).
Black holes are capable of generating tremendous amounts of energy, in fact, black holes may be the force behind half the energy released since the universe began (Zabarenko, 1999). There are three ways physicists believe it is possible to harness energy from black holes: through accretion discs, by super radiance, and with hawking radiation. Using Hawking radiation is the only process that requires a primordial black hole because as a black hole increases in mass the amount of Hawking radiation decreases exponentially. The other two processes can be carried out on a black hole of any size, but the large the better.
Accretion discs around black holes generate large amounts of energy. All the matter swirling around these black holes heats up through friction with other matter. This hot, fast-moving gas emits lots of radiation, ranging from optical light to X-rays which could be collected much in the same way as solar panels collect power from our sun (Zabarenko, 1999). In the future it may be possible to build a sphere around a black hole and have the inside walls coated with this energy absorbing material which will convert it into electrical energy. This power can then be transferred to a collecting station where it can then be used or transferred for use elsewhere. Another advantage to this process is that the black hole can be used as a dumping site for our waste, which then will then actually be the matter that ends up producing energy. This process is known as the rotational vortex method of Roger Penrose (Controlling a Black Hole, 1999). Ibrahim Semiz wrote in 1995 that
the Penrose process has drawbacks: The black hole acts mostly as energy storage device, because the ‘rotational contribution’ of the total energy of the black hole is extracted. With each step, the rotational slows down, and when rotation finally stops, energy production stops.
The jets that are formed from the magnetic field of black holes could also be a possible future power source. The particles in these jets are propelled to incredible speeds and energy by the magnetic field. They pick up so much energy that they become x-rays and sometimes gamma-rays the highest form of energy (Jebornak 1998). Since large amounts of charged particles are shooting away from the black hole at speeds nearing that of light, if scientists figured out a way to collect that charge, it would be a power plant outputting large amounts of electrical energy. Furthermore, like with the accretion disc, a collector could also be set up to absorb the electromagnetic-spectrum radiation that is propelled through these jets.
Super radiance can only be performed on rotating black holes, preferably with large mass. When a light ray enters the ergosphere with a certain energy (say, a visible light ray), it comes out of the ergosphere with more energy (say, an x-ray) (Jebornak, 1998). This extra energy comes from the black holes rotational energy. When the ray enters the ergosphere it speeds up, causing the ray to gain more energy than when it had entered. This process has the same draw backs as the Penrose, though when the black hole stops rotating it can no longer generate power by this method. This is why massive black holes are preferred, the larger a black hole, the more rotational energy it has at the same rates of rotation.
Hawking radiation is the most efficient process of gathering energy from a black hole and the most feasible method. The ideal black hole for this method is one with a mass near that of primordial black holes since black holes whose masses are greater that the mass of the earth generate less energy than the background radiation of the universe (Kaufmann, 1989). The power output of a Schwarzschild black hole is equal to 4.8 * 10^33f(T) / M2, were f(T) is dependent on the particle degrees of freedom that can be radiated (Semiz, 1995). The f(T) stands for the amount of Hawking radiation that can escape the black holes event horizon that we can presently harness. This equation shows how as a black hole loses mass through changing its energy into matter: it will radiate even more energy. A Schwarzschild black hole with mass of 1012 kg will radiate 7.9 * 109 W but only 45.3% of the energy will be carried in channels we can intercept (Semiz, 1995). Semiz goes on to show that a black hole with one tenth the mass of the one above would radiate 2.2 * 1012 W, about three times the present power consumption of the earth, and have an 80.8% efficiency rating (1995). He also found that the energy can be kept constant by feeding mass into the black hole at a rate of 2.8 and 772 kg / yr respectively for the above examples.
Current sources of power like the sun and nuclear power have nowhere near the efficiency of power given off by a black hole due to Hawking radiation. The sun’s fusion of hydrogen atoms only has an efficiency rating of .71%, and nuclear power, like in power plants around the world has a rating of .09% (Semiz, 1995). In the future controllable fusion would be adequate for terrestrial energy needs. It is in space where high efficiency would become important (Semiz, 1995). As the world begins to look towards space for inhabitation and exploration the need for such power sources is becoming ever more prominent.
In Semiz’s power plant using Hawking radiation the mass of the black hole remains constant, the only effect of the process is conversion of fuel into energy (1995). Due to this, the process is amazingly efficient, and the smaller a black hole gets the high the efficiency rating will be. Being as how some physicists believe that mini black holes can be created from smashing a particle into its antiparticle at high velocities, this theory of mini black holes as power plants is a strong possibility for the future.
Black holes maybe the power plants of the future because of their ability to transform matter into energy. Since black holes needed for the Penrose process and super radiance require super massive Newmann black holes, they are in the distant future because the closest known black hole of sufficient mass is at the center of our galaxy. By harnessing Hawking radiation a power supply efficient enough to power future journeys out of our solar system are plausible. One of the most important facts is that particle accelerators here on earth could create the black holes needed. As Semiz wrote in 1995, if we hope to one day ‘go where no one has gone before’ the only feasible way seems to be to use the Hawking radiation of a mini black hole.