Beyond the Boundary: Approaching and Exceeding the Light Barrier
By Kemi Ashing-Giwa
Space: the final frontier. If we ever want to explore this vast unknown in full, we’ll need to learn to navigate the cosmos effectively. One solution would be to build generation starships—a hypothetical spaceship that travels under the speed of the light. But traveling at this speed, it may indeed take generations to discover the far reaches of the universe. A more viable solution would be to use faster-than-light technologies. What are these technologies, and how exactly do they work?
Alcubierre Drives
The year is 1994, and Trekkies everywhere are celebrating. The key to unlocking the warp drive—the mystical propulsion system that allows Star Trek’s famous USS Enterprise to travel at several times the speed of light—has been discovered. Well, almost.
The Alcubierre warp drive, a speculative idea proposed by theoretical physicist Miguel Alcubierre, is based on a solution of Einstein's field equations in general relativity.1 These field equations include 10 calculations that seek to explain gravity using the curvature of spacetime by energy and mass.2 Simply put, spacetime can be thought of as a cosmic blanket; when an object with mass sits atop it, the “blanket” dips.
Alcubierre’s theory suggests that a spacecraft could achieve apparent faster-than-light travel by contracting space in front of itself and expanding space behind itself, all without breaking any physical laws.1,3 Therefore, the Alcubierre drive would shove space around a starship so that the ship would arrive at its destination before light would.
However, the proposed mechanism of the Alcubierre drive involves negative energy density,1,11 a quality of “exotic matter”. Exotic matter is matter that acts in ways that are so foreign to the known laws of physics that we don’t really have a good definition for it yet. Thus, until we can better characterize its inner workings, the warp drive remains a facet of science fiction.
Black Hole Engines
Another theoretical method of faster-than-light space travel is powering an interstellar ship with a black hole. Black holes are regions of space so massive that nothing—not even light—can escape the pull of their gravitational fields. The first serious proposal to create an artificial black hole was discussed in 2009.4 Black holes emit Hawking radiation, electromagnetic radiation that results from a black hole capturing one of a particle-antiparticle pair created spontaneously near the event horizon, the edge of the black hole.5 Hawking radiation produced by a tiny, non-rotating black hole could be used as a new energy source.4 Making this idea a reality is technically possible, but such an endeavor would require physics that is presently-unknown to the scientific community. For now, it’s reasonable to ask why you’d want to stick a cosmic death pit in the middle of starship in the first place.
Although black hole starships are far beyond our current technological grasp, they offer a number of advantages over other proposed methods, such as generating antimatter—particles that act as "partners" to ordinary matter.6 However, manufacturing antimatter from scratch would be extremely energetically inefficient, and the resulting antimatter would be very difficult to contain.
Although quite difficult to carry out, the process of generating a black hole doesn’t require new physics and is naturally efficient, so it would require millions of times less energy than producing our own antimatter. And, unlike antimatter, a black hole confines itself.4
Wormholes
Bending spacetime is only the beginning—you can tear and patch it together, too. Think of our universe is a huge flat sheet. Bend it in just the right way, and wormholes could connect two very distant locations. Such a bridge could be crossed instantaneously, allowing you to travel faster than the speed of light. If you looked at a wormhole, you’d see a sphere of stars; light from the other side flying through would give you a window into someplace far, far away. Once crossed, your previous location would fly back into a ring of concentric repeats of the same image, and the other side of the wormhole would come fully into view.7, 16
There are several kinds of wormholes, but what we want are traversable wormholes. Luckily, these wormholes are also described by Einstein’s field equations14, as well as a more recent line of work called string theory.
String theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings8. String theory suggests that our universe might contain a countless web of wormholes already, the result of quantum fluctuations occurring at the smallest scales after the Big Bang.9, 10, 16 These wormholes would be threaded through with cosmic strings, and during the Big Bang, the ends of these strings would have been stretched light years apart, scattering them throughout the universe.15, 16 Although finding one of these wormholes in space would likely take an unreasonable amount of time, there is good news: We might be able to make our own.
To be traversable, a wormhole would need three basic properties, besides the implicit property of connecting two distant points in spacetime. First, it shouldn’t have any event horizons, which would block two-way travel.13, 14, 16 Second, it should be large enough so the gravitational forces don’t smush anyone trying to get through.14, 16 Third, it should be stable14.
This third property is the most challenging because it is responsible for keeping our artificial wormhole open. This feature is crucial, since we wouldn’t want to crush any future explorers. Gravity would try to close any wormhole we make, pinching the cosmic tunnel closed and leaving only black holes at the ends.13, 16 So, we would need something to prop our wormholes open. Cosmic strings accomplish this job for wormholes described by string theory,15, 16 but for human-made wormholes, we’d likely have to turn to exotic matter. Exotic matter is quite different from antimatter in that the former has negative mass while the latter has positive mass.11 Thus, exotic matter would be repulsive, acting as a sort of antigravity to compete against the gravity in a wormhole. But how would we get our hands on such exotic matter?
As it turns out, the answer lies in the vacuum of space itself. Quantum fluctuations constantly create particles and antiparticles which are annihilated immediately after their formation.12 These fluctuations can be coerced into producing an effect similar to the wormhole-stabilizing negative mass we’re looking for.16 Still, wormholes are just a theory proposed by general relativity; they don’t necessarily exist.
While faster-than-light travel is still confined to the realm of science fiction, it is reassuring that it is supported by the laws of physics. With enough efforts to better characterize exotic matter and tame black holes, we may soon find ourselves traversing the universe at breakneck speed. The sky—or the event horizon—is truly just the beginning.
References
Space: the final frontier. If we ever want to explore this vast unknown in full, we’ll need to learn to navigate the cosmos effectively. One solution would be to build generation starships—a hypothetical spaceship that travels under the speed of the light. But traveling at this speed, it may indeed take generations to discover the far reaches of the universe. A more viable solution would be to use faster-than-light technologies. What are these technologies, and how exactly do they work?
Alcubierre Drives
The year is 1994, and Trekkies everywhere are celebrating. The key to unlocking the warp drive—the mystical propulsion system that allows Star Trek’s famous USS Enterprise to travel at several times the speed of light—has been discovered. Well, almost.
The Alcubierre warp drive, a speculative idea proposed by theoretical physicist Miguel Alcubierre, is based on a solution of Einstein's field equations in general relativity.1 These field equations include 10 calculations that seek to explain gravity using the curvature of spacetime by energy and mass.2 Simply put, spacetime can be thought of as a cosmic blanket; when an object with mass sits atop it, the “blanket” dips.
Alcubierre’s theory suggests that a spacecraft could achieve apparent faster-than-light travel by contracting space in front of itself and expanding space behind itself, all without breaking any physical laws.1,3 Therefore, the Alcubierre drive would shove space around a starship so that the ship would arrive at its destination before light would.
However, the proposed mechanism of the Alcubierre drive involves negative energy density,1,11 a quality of “exotic matter”. Exotic matter is matter that acts in ways that are so foreign to the known laws of physics that we don’t really have a good definition for it yet. Thus, until we can better characterize its inner workings, the warp drive remains a facet of science fiction.
Black Hole Engines
Another theoretical method of faster-than-light space travel is powering an interstellar ship with a black hole. Black holes are regions of space so massive that nothing—not even light—can escape the pull of their gravitational fields. The first serious proposal to create an artificial black hole was discussed in 2009.4 Black holes emit Hawking radiation, electromagnetic radiation that results from a black hole capturing one of a particle-antiparticle pair created spontaneously near the event horizon, the edge of the black hole.5 Hawking radiation produced by a tiny, non-rotating black hole could be used as a new energy source.4 Making this idea a reality is technically possible, but such an endeavor would require physics that is presently-unknown to the scientific community. For now, it’s reasonable to ask why you’d want to stick a cosmic death pit in the middle of starship in the first place.
Although black hole starships are far beyond our current technological grasp, they offer a number of advantages over other proposed methods, such as generating antimatter—particles that act as "partners" to ordinary matter.6 However, manufacturing antimatter from scratch would be extremely energetically inefficient, and the resulting antimatter would be very difficult to contain.
Although quite difficult to carry out, the process of generating a black hole doesn’t require new physics and is naturally efficient, so it would require millions of times less energy than producing our own antimatter. And, unlike antimatter, a black hole confines itself.4
Wormholes
Bending spacetime is only the beginning—you can tear and patch it together, too. Think of our universe is a huge flat sheet. Bend it in just the right way, and wormholes could connect two very distant locations. Such a bridge could be crossed instantaneously, allowing you to travel faster than the speed of light. If you looked at a wormhole, you’d see a sphere of stars; light from the other side flying through would give you a window into someplace far, far away. Once crossed, your previous location would fly back into a ring of concentric repeats of the same image, and the other side of the wormhole would come fully into view.7, 16
There are several kinds of wormholes, but what we want are traversable wormholes. Luckily, these wormholes are also described by Einstein’s field equations14, as well as a more recent line of work called string theory.
String theory is a theoretical framework in which the point-like particles of particle physics are replaced by one-dimensional objects called strings8. String theory suggests that our universe might contain a countless web of wormholes already, the result of quantum fluctuations occurring at the smallest scales after the Big Bang.9, 10, 16 These wormholes would be threaded through with cosmic strings, and during the Big Bang, the ends of these strings would have been stretched light years apart, scattering them throughout the universe.15, 16 Although finding one of these wormholes in space would likely take an unreasonable amount of time, there is good news: We might be able to make our own.
To be traversable, a wormhole would need three basic properties, besides the implicit property of connecting two distant points in spacetime. First, it shouldn’t have any event horizons, which would block two-way travel.13, 14, 16 Second, it should be large enough so the gravitational forces don’t smush anyone trying to get through.14, 16 Third, it should be stable14.
This third property is the most challenging because it is responsible for keeping our artificial wormhole open. This feature is crucial, since we wouldn’t want to crush any future explorers. Gravity would try to close any wormhole we make, pinching the cosmic tunnel closed and leaving only black holes at the ends.13, 16 So, we would need something to prop our wormholes open. Cosmic strings accomplish this job for wormholes described by string theory,15, 16 but for human-made wormholes, we’d likely have to turn to exotic matter. Exotic matter is quite different from antimatter in that the former has negative mass while the latter has positive mass.11 Thus, exotic matter would be repulsive, acting as a sort of antigravity to compete against the gravity in a wormhole. But how would we get our hands on such exotic matter?
As it turns out, the answer lies in the vacuum of space itself. Quantum fluctuations constantly create particles and antiparticles which are annihilated immediately after their formation.12 These fluctuations can be coerced into producing an effect similar to the wormhole-stabilizing negative mass we’re looking for.16 Still, wormholes are just a theory proposed by general relativity; they don’t necessarily exist.
While faster-than-light travel is still confined to the realm of science fiction, it is reassuring that it is supported by the laws of physics. With enough efforts to better characterize exotic matter and tame black holes, we may soon find ourselves traversing the universe at breakneck speed. The sky—or the event horizon—is truly just the beginning.
References
- Alcubierre, M. “The Warp Drive: Hyper-Fast Travel within General Relativity.” Classical and Quantum Gravity, vol. 11, no. 5, May 1994, pp. 73–77., doi:10.1088/0264-9381/11/5/001.
- Einstein, A. “The Foundations of General Relativity Theory.” Annalen Der Physik, vol. 354, no. 7, 1916, pp. 769–822., doi:10.1016/b978-0-08-017639-0.50011-x.
- Krasnikov, S. “Quantum Inequalities Do Not Forbid Spacetime Shortcuts.” Physical Review D, vol. 67, no. 10, 20 May 2003, doi:10.1103/physrevd.67.104013.
- Crane, L., and S. Westmoreland. “Are Black Hole Starships Possible.” 12 Aug. 2009. https://arxiv.org/pdf/0908.1803.pdf
- Hawking, S. W. “Particle Creation by Black Holes.” Communications in Mathematical Physics, vol. 43, 1975, pp. 199–220., doi:10.1142/9789814539395_0011.
- “What Is Antimatter?” Scientific American, Scientific American, a Division of Springer Nature America, Inc., 24 Jan. 2002, www.scientificamerican.com/article/what-is-antimatter-2002-01-24/.
- Bossinas, Les. “Wormhole Passage.” NASA, NASA, 1998, www.nasa.gov/centers/glenn/multimedia/artgallery/wormhole.html.
- Greene, Brian. “Why String Theory Still Offers Hope We Can Unify Physics.” Smithsonian.com, Smithsonian Institution, Jan. 2015, www.smithsonianmag.com/science-nature/string-theory-about-unravel-180953637/.
- Popov, Arkady. (2018). Wormholes Created by Vacuum Fluctuations. Proceedings. 2. 20. 10.3390/proceedings2010020.
- Preskill, John. “Wormholes In Spacetime And The Constants Of Nature.” Nuclear Physics, 1989, pp. 141–186., doi:10.1142/9789814539395_0028. http://www.theory.caltech.edu/~preskill/pubs/preskill-1989-wormholes.pdf
- Kluger, Jeffrey. “Nobel Prize in Physics: Why Exotic Matter Matters.” Time, Time, 4 Oct. 2016, time.com/4517897/nobel-prize-physics-2016/.
- Brown, Malcolm W. “New Direction in Physics: Back in Time.” The New York Times, The New York Times, 21 Aug. 1990, www.nytimes.com/1990/08/21/science/new-direction-in-physics-back-in-time.html?pagewanted=all.
- Morris, Michael S. and Thorne, Kip S. and Yurtsever, Ulvi (1988) Wormholes, time machines, and the weak energy condition. Physical Review Letters, 61 (13). pp. 1446-1449. ISSN 0031-9007. https://authors.library.caltech.edu/9262/1/MORprl88.pdf
- Morris, Michael S., and Kip S. Thorne. “Wormholes in Spacetime and Their Use for Interstellar Travel: A Tool for Teaching General Relativity.” American Journal of Physics, vol. 56, no. 5, 1988, pp. 395–412., doi:10.1119/1.15620. http://www.cmp.caltech.edu/refael/league/thorne-morris.pdf
- Cramer, John G., et al. “Natural Wormholes as Gravitational Lenses.” Physical Review D, vol. 51, no. 6, 20 Sept. 1994, pp. 3117–3120., doi:10.1103/physrevd.51.3117.
- “Wormholes Explained - Breaking Spacetime.” YouTube. Kurzgesagt - In a Nutshell, 12 Aug. 2018, https://m.youtube.com/watch?v=9P6rdqiybaw.