Unsolved Problems in Physics
(source: http://www.oglethorpe.edu/faculty/~m_rulison/top10.htm)
How can quantum gravity help explain the origin of the universe? Two of the great theories of modern physics are the standard model, which uses quantum mechanics to describe the subatomic particles and the forces they obey, and general relativity, the theory of gravity. Physicists have long hoped that merging the two into a "theory of everything" -- quantum gravity -- would yield a deeper understanding of the universe, including how it spontaneously popped into existence with the Big Bang. The leading candidate for this merger is superstring theory, or M theory, as the latest, souped-up version is called (with the M standing for "magic," "mystery," or "mother of all theories").
Why does the universe appear to have one time and three space dimensions? "Just because" is not considered an acceptable answer. And just because people can't imagine moving in extra directions, beyond up-and-down, left-and-right, and back-and-forth, doesn't mean that the universe had to be designed that way. According to superstring theory, in fact, there must be six more spatial dimensions, each one curled up too tiny to detect. If the theory is right, then why did only three of them unfurl, leaving us with this comparatively claustrophobic dominion?
Is nature supersymmetric, and if so, how is supersymmetry broken? Many physicists believe that unifying all the forces, including gravity, into a single theory would require showing that two very different kinds of particles are actually intimately related, a phenomenon called supersymmetry. The first, fermions, are loosely described as the building blocks of matter, like protons, electrons and neutrons. They clump together to make stuff. The others, the bosons, are the particles that carry forces, like photons, conveyors of light. With supersymmetry, every fermion would have a boson twin, and vice versa. Physicists, with their compulsion for coining funny names, call the so-called superpartners "sparticles": For the electron, there would be the selectron; for the photon, the photino. But since the sparticles have not been observed in nature, physicists would also have to explain why, in the jargon, the symmetry is "broken": the mathematical perfection that existed at the moment of creation was knocked out of kilter as the universe cooled and congealed into its present lopsided state.
What physics explains the enormous disparity between the gravitational scale and the typical mass scale of the elementary particles? In other words, why is gravity so much weaker than the other forces, like electromagnetism? A magnet can pick up a paper clip even though the gravity of the whole earth is pulling back on the other end. According to one recent proposal, gravity is actually much stronger. It just seems weak because most of it is trapped in one of those extra dimensions. If its full force could be tapped using high-powered particle accelerators, it might be possible to create miniature black holes. Though seemingly of interest to the solid waste disposal industry, the black holes would probably evaporate almost as soon as they were formed.
Can we quantitatively understand quark and gluon confinement in quantum chromodynamics and the existence of a mass gap? Quantum chromodynamics, or QCD, is the theory describing the strong nuclear force. Carried by gluons, it binds quarks into particles like protons and neutrons. According to the theory, the tiny subparticles are permanently confined. You can't pull a quark or a gluon from a proton because the strong force gets stronger with distance and snaps them right back inside. But physicists have yet to prove conclusively that quarks and gluons can never escape. When they try to do so, the calculations go haywire. And they cannot explain why all particles that feel the strong force must have at least a tiny amount of mass, why it cannot be zero. Some hope to find an answer in M theory, maybe one that would also throw more light on the nature of gravity.
Why is any of this important? In presenting his own list of mysteries, Hilbert put it this way: "It is by the solution of problems that the investigator tests the temper of his steel; he finds new methods and new outlooks, and gains a wider and freer horizon." And in physics, the horizon is no less than a theory that finally makes sense of the universe.
How can quantum gravity help explain the origin of the universe? Two of the great theories of modern physics are the standard model, which uses quantum mechanics to describe the subatomic particles and the forces they obey, and general relativity, the theory of gravity. Physicists have long hoped that merging the two into a "theory of everything" -- quantum gravity -- would yield a deeper understanding of the universe, including how it spontaneously popped into existence with the Big Bang. The leading candidate for this merger is superstring theory, or M theory, as the latest, souped-up version is called (with the M standing for "magic," "mystery," or "mother of all theories").
Why does the universe appear to have one time and three space dimensions? "Just because" is not considered an acceptable answer. And just because people can't imagine moving in extra directions, beyond up-and-down, left-and-right, and back-and-forth, doesn't mean that the universe had to be designed that way. According to superstring theory, in fact, there must be six more spatial dimensions, each one curled up too tiny to detect. If the theory is right, then why did only three of them unfurl, leaving us with this comparatively claustrophobic dominion?
Is nature supersymmetric, and if so, how is supersymmetry broken? Many physicists believe that unifying all the forces, including gravity, into a single theory would require showing that two very different kinds of particles are actually intimately related, a phenomenon called supersymmetry. The first, fermions, are loosely described as the building blocks of matter, like protons, electrons and neutrons. They clump together to make stuff. The others, the bosons, are the particles that carry forces, like photons, conveyors of light. With supersymmetry, every fermion would have a boson twin, and vice versa. Physicists, with their compulsion for coining funny names, call the so-called superpartners "sparticles": For the electron, there would be the selectron; for the photon, the photino. But since the sparticles have not been observed in nature, physicists would also have to explain why, in the jargon, the symmetry is "broken": the mathematical perfection that existed at the moment of creation was knocked out of kilter as the universe cooled and congealed into its present lopsided state.
What physics explains the enormous disparity between the gravitational scale and the typical mass scale of the elementary particles? In other words, why is gravity so much weaker than the other forces, like electromagnetism? A magnet can pick up a paper clip even though the gravity of the whole earth is pulling back on the other end. According to one recent proposal, gravity is actually much stronger. It just seems weak because most of it is trapped in one of those extra dimensions. If its full force could be tapped using high-powered particle accelerators, it might be possible to create miniature black holes. Though seemingly of interest to the solid waste disposal industry, the black holes would probably evaporate almost as soon as they were formed.
Can we quantitatively understand quark and gluon confinement in quantum chromodynamics and the existence of a mass gap? Quantum chromodynamics, or QCD, is the theory describing the strong nuclear force. Carried by gluons, it binds quarks into particles like protons and neutrons. According to the theory, the tiny subparticles are permanently confined. You can't pull a quark or a gluon from a proton because the strong force gets stronger with distance and snaps them right back inside. But physicists have yet to prove conclusively that quarks and gluons can never escape. When they try to do so, the calculations go haywire. And they cannot explain why all particles that feel the strong force must have at least a tiny amount of mass, why it cannot be zero. Some hope to find an answer in M theory, maybe one that would also throw more light on the nature of gravity.
Why is any of this important? In presenting his own list of mysteries, Hilbert put it this way: "It is by the solution of problems that the investigator tests the temper of his steel; he finds new methods and new outlooks, and gains a wider and freer horizon." And in physics, the horizon is no less than a theory that finally makes sense of the universe.
posted by Tyler @ 7:07 AM
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