Testing String Theory
Michio Kaku has a great way of bringing ideas shrouded in physics mysticism to the layperson -- and he's done it again in the latest issue of Discover magazine, this time taking on String Theory. What is String Theory? In very simple terms, String Theory states that at the elementary level, the universe is made up of tiny vibrating strings of energy. The elementary particles -- electrons, photons, quarks, gluons, bosons, the anti-particles, etc. -- are therefore strings of energy vibrating at specific frequencies. As these strings vibrate, time and space are forced to curl around them, giving rise to gravity -- in fact, the vibrations result in all the known forces that govern the universe: gravity, electromagnetism, and the strong and weak nuclear forces.
The big problem with String Theory is the fact that it is just that -- a theory. There is no proof that it is true -- and to complicate things, there are different versions of String Theory. The prevailing theory -- the Supersymmetric String Theory -- calls for the universe to exist in 11 dimensions (or sometimes 10 or 26). The outcome of this theory is the unification of the macro with the micro -- a single theory that unifies the theory of general relativity with quantum mechanics -- a theory of everything. One theory, one equation, that explains the entire universe. Cool, and it's only a theory -- a theory that has so far been extremely successful at matching the predictions of other accepted theories -- such as Einstein's theory of relativity -- as well as predicting some of the things we've seen in the universe. The only way to verify String Theory however is to test it completely, by creating a universe in the lab and observing it. Since we're not about to do that, we have to find other creative and indirect ways of verifying String Theory.
Gravity-Wave Test: The vibration of strings in the early universe should have created ripples in gravity -- gravitational waves -- that spread across the early universe. String Theory predicts the frequencies of gravity waves. None have been observed as yet. Like the recent Wilkinson Microwave Anisotropy Probe that looked at the microwave energy left over from the Big Bang -- going back to an early universe 300,000 years old -- two observatories are being readied that will peer back to a time around one-trillionth of a second after the Big Bang -- the Laser Interferometer Gravitational Wave Observatory and the Laser Interferometer Space Antenna.
Particle-Accelerator Test: Particle accelerators make things go boom. They smash particles together, such as electrons, and observe the pieces that come flying out from the collision. If String Theory is correct, and the elementary particles are vibrations of strings of energy at specific frequencies, then we should see particles with other vibration modes that String Theory predicts. Specifically, String Theory predicts that all subatomic particles have equivalents belonging to a more massive family, called superparticles. Further, particle accelerators may be able to miniature black holes, as predicted by one version of String Theory -- or confirm that there really are more dimensions to our universe than those we can observe, by knocking some particles and energy into another dimension. Physicists are looking forward to the Large Hadron Collider coming online in 2007, as it will be the world's largest particle accelerator, capable of smashing protons together.
Dark-Matter Searches: Studies show that a good deal of our universe doesn't emit any light or interact with ordinary matter, other than through gravity. Our galaxy for instance is surrounded by dark matter -- in abundance more massive than our galaxy itself. String Theory has an explanation for dark matter. It's superparticles. If superparticles are in such abundance, we should be able to observe them with sensitive enough detectors, as the Earth should be constantly moving through this material. All we would have to do is detect the superparticles colliding with ordinary matter. String Theory predicts a leading candidate for dark matter in the superparticle called a neutralino -- in fact, it states that there should be 10 times more neutralinos than there are atoms in the universe. If only we could see some. Another possible explanation for dark matter is the muti-dimensions predicted by String Theory. There is a possibility that the dark matter whose gravitational influence we observe, resides in dimensions outside our reach -- perhaps even in dimensions that belong to another universe altogether, as predicted by multiverse versions of String Theory. It's interesting to note that Einstein's theory of general relativity does predict that gravity from matter in another universe would leak into ours.
Laboratory Gravity Tests: If the additional dimensions or universes predicted by String Theory exists, then gravity should leak into those dimensions, and we should be able to measure the effects as deviations in Newton's law of gravity. To date, there has been no luck with this route. Newton's law shows no deviations at sub-millimetre scales -- so experiments are now being designed to look how Newton's law holds up at the atomic scale.
Pure Mathematics: This is Michio Kaku's preferred way of vindicating String Theory -- putting pencil to paper, and completing the theory. If, or when, the theory is completed, it will provide a single equation to explain the entire universe. From first principles, it would be able to predict the properties of everything in the universe. That would be the ultimate test -- when everything could be explained, without exception -- and the answer to everything would come from the human brain. How neat would that be?
My summary doesn't do Michio Kaku's article justice. It needs to be read to be appreciated -- as it also includes some neat graphical representations of the proposed tests of String Theory. The Discover site for the article also provides the following additional recommended reading:
The big problem with String Theory is the fact that it is just that -- a theory. There is no proof that it is true -- and to complicate things, there are different versions of String Theory. The prevailing theory -- the Supersymmetric String Theory -- calls for the universe to exist in 11 dimensions (or sometimes 10 or 26). The outcome of this theory is the unification of the macro with the micro -- a single theory that unifies the theory of general relativity with quantum mechanics -- a theory of everything. One theory, one equation, that explains the entire universe. Cool, and it's only a theory -- a theory that has so far been extremely successful at matching the predictions of other accepted theories -- such as Einstein's theory of relativity -- as well as predicting some of the things we've seen in the universe. The only way to verify String Theory however is to test it completely, by creating a universe in the lab and observing it. Since we're not about to do that, we have to find other creative and indirect ways of verifying String Theory.
Gravity-Wave Test: The vibration of strings in the early universe should have created ripples in gravity -- gravitational waves -- that spread across the early universe. String Theory predicts the frequencies of gravity waves. None have been observed as yet. Like the recent Wilkinson Microwave Anisotropy Probe that looked at the microwave energy left over from the Big Bang -- going back to an early universe 300,000 years old -- two observatories are being readied that will peer back to a time around one-trillionth of a second after the Big Bang -- the Laser Interferometer Gravitational Wave Observatory and the Laser Interferometer Space Antenna.
Particle-Accelerator Test: Particle accelerators make things go boom. They smash particles together, such as electrons, and observe the pieces that come flying out from the collision. If String Theory is correct, and the elementary particles are vibrations of strings of energy at specific frequencies, then we should see particles with other vibration modes that String Theory predicts. Specifically, String Theory predicts that all subatomic particles have equivalents belonging to a more massive family, called superparticles. Further, particle accelerators may be able to miniature black holes, as predicted by one version of String Theory -- or confirm that there really are more dimensions to our universe than those we can observe, by knocking some particles and energy into another dimension. Physicists are looking forward to the Large Hadron Collider coming online in 2007, as it will be the world's largest particle accelerator, capable of smashing protons together.
Dark-Matter Searches: Studies show that a good deal of our universe doesn't emit any light or interact with ordinary matter, other than through gravity. Our galaxy for instance is surrounded by dark matter -- in abundance more massive than our galaxy itself. String Theory has an explanation for dark matter. It's superparticles. If superparticles are in such abundance, we should be able to observe them with sensitive enough detectors, as the Earth should be constantly moving through this material. All we would have to do is detect the superparticles colliding with ordinary matter. String Theory predicts a leading candidate for dark matter in the superparticle called a neutralino -- in fact, it states that there should be 10 times more neutralinos than there are atoms in the universe. If only we could see some. Another possible explanation for dark matter is the muti-dimensions predicted by String Theory. There is a possibility that the dark matter whose gravitational influence we observe, resides in dimensions outside our reach -- perhaps even in dimensions that belong to another universe altogether, as predicted by multiverse versions of String Theory. It's interesting to note that Einstein's theory of general relativity does predict that gravity from matter in another universe would leak into ours.
Laboratory Gravity Tests: If the additional dimensions or universes predicted by String Theory exists, then gravity should leak into those dimensions, and we should be able to measure the effects as deviations in Newton's law of gravity. To date, there has been no luck with this route. Newton's law shows no deviations at sub-millimetre scales -- so experiments are now being designed to look how Newton's law holds up at the atomic scale.
Pure Mathematics: This is Michio Kaku's preferred way of vindicating String Theory -- putting pencil to paper, and completing the theory. If, or when, the theory is completed, it will provide a single equation to explain the entire universe. From first principles, it would be able to predict the properties of everything in the universe. That would be the ultimate test -- when everything could be explained, without exception -- and the answer to everything would come from the human brain. How neat would that be?
My summary doesn't do Michio Kaku's article justice. It needs to be read to be appreciated -- as it also includes some neat graphical representations of the proposed tests of String Theory. The Discover site for the article also provides the following additional recommended reading:
Official String Theory Web Site Not Even Wrong -- a skeptical look at String Theory, specifically, the entry titled Is String Theory About to Snap? that responds to the Discover article. Parallel Worlds: A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos -- by Michio Kaku. The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory -- by Brian Greene. Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps, and the 10th Dimension -- by Michio Kaku. The Fabric of the Cosmos: Space, Time, and the Texture of Reality -- by Brian Greene. A First Course in String Theory -- by Barton Zwiebach. Warped Passages: Unraveling the Mysteries of the Universe's Hidden Dimensions -- by Lisa Randall.
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