Meeting Pearls 1

home *** CD-ROM | disk | FTP | other *** search

/ Meeting Pearls 1 / Meeting Pearls Vol 1 (1994).iso / installed_progs / text / faqs / physics-faq.part4 < prev next >

Wrap

Internet Message Format | 1994-05-01 | 51.4 KB

Subject: Sci.physics Frequently Asked Questions - May 1994 - Part 4/4 Newsgroups: sci.physics,sci.physics.particle,alt.sci.physics.new-theories,news.answers,sci.answers,alt.answers From: sichase@csa2.lbl.gov (SCOTT I CHASE) Date: 30 Apr 1994 17:02 PST Archive-name: physics-faq/part4 Last-modified: 26-APR-1994 -------------------------------------------------------------------------------- FREQUENTLY ASKED QUESTIONS ON SCI.PHYSICS - Part 4/4 -------------------------------------------------------------------------------- Item 22. Why do Mirrors Reverse Left and Right? updated 04-MAR-1994 by SIC -------------------------------------- original by Scott I. Chase The simple answer is that they don't. Look in a mirror and wave your right hand. On which side of the mirror is the hand that waved? The right side, of course. Mirrors DO reverse In/Out. Imaging holding an arrow in your hand. If you point it up, it will point up in the mirror. If you point it to the left, it will point to the left in the mirror. But if you point it toward the mirror, it will point right back at you. In and Out are reversed. If you take a three-dimensional, rectangular, coordinate system, (X,Y,Z), and point the Z axis such that the vector equation X x Y = Z is satisfied, then the coordinate system is said to be right-handed. Imagine Z pointing toward the mirror. X and Y are unchanged (remember the arrows?) but Z will point back at you. In the mirror, X x Y = - Z. The image contains a left-handed coordinate system. This has an important effect, familiar mostly to chemists and physicists. It changes the chirality, or handedness of objects viewed in the mirror. Your left hand looks like a right hand, while your right hand looks like a left hand. Molecules often come in pairs called stereoisomers, which differ not in the sequence or number of atoms, but only in that one is the mirror image of the other, so that no rotation or stretching can turn one into the other. Your hands make a good laboratory for this effect. They are distinct, even though they both have the same components connected in the same way. They are a stereo pair, identical except for "handedness". People sometimes think that mirrors *do* reverse left/right, and that the effect is due to the fact that our eyes are aligned horizontally on our faces. This can be easily shown to be untrue by looking in any mirror with one eye closed! Reference: _The Left Hand of the Electron_, by Isaac Asimov, contains a very readable discussion of handedness and mirrors in physics. ******************************************************************************** Item 23. What is the Mass of a Photon? updated 24-JUL-1992 by SIC original by Matt Austern Or, "Does the mass of an object depend on its velocity?" This question usually comes up in the context of wondering whether photons are really "massless," since, after all, they have nonzero energy. The problem is simply that people are using two different definitions of mass. The overwhelming consensus among physicists today is to say that photons are massless. However, it is possible to assign a "relativistic mass" to a photon which depends upon its wavelength. This is based upon an old usage of the word "mass" which, though not strictly wrong, is not used much today. The old definition of mass, called "relativistic mass," assigns a mass to a particle proportional to its total energy E, and involved the speed of light, c, in the proportionality constant: m = E / c^2. (1) This definition gives every object a velocity-dependent mass. The modern definition assigns every object just one mass, an invariant quantity that does not depend on velocity. This is given by m = E_0 / c^2, (2) where E_0 is the total energy of that object at rest. The first definition is often used in popularizations, and in some elementary textbooks. It was once used by practicing physicists, but for the last few decades, the vast majority of physicists have instead used the second definition. Sometimes people will use the phrase "rest mass," or "invariant mass," but this is just for emphasis: mass is mass. The "relativistic mass" is never used at all. (If you see "relativistic mass" in your first-year physics textbook, complain! There is no reason for books to teach obsolete terminology.) Note, by the way, that using the standard definition of mass, the one given by Eq. (2), the equation "E = m c^2" is *not* correct. Using the standard definition, the relation between the mass and energy of an object can be written as E = m c^2 / sqrt(1 -v^2/c^2), (3) or as E^2 = m^2 c^4 + p^2 c^2, (4) where v is the object's velocity, and p is its momentum. In one sense, any definition is just a matter of convention. In practice, though, physicists now use this definition because it is much more convenient. The "relativistic mass" of an object is really just the same as its energy, and there isn't any reason to have another word for energy: "energy" is a perfectly good word. The mass of an object, though, is a fundamental and invariant property, and one for which we do need a word. The "relativistic mass" is also sometimes confusing because it mistakenly leads people to think that they can just use it in the Newtonian relations F = m a (5) and F = G m1 m2 / r^2. (6) In fact, though, there is no definition of mass for which these equations are true relativistically: they must be generalized. The generalizations are more straightforward using the standard definition of mass than using "relativistic mass." Oh, and back to photons: people sometimes wonder whether it makes sense to talk about the "rest mass" of a particle that can never be at rest. The answer, again, is that "rest mass" is really a misnomer, and it is not necessary for a particle to be at rest for the concept of mass to make sense. Technically, it is the invariant length of the particle's four-momentum. (You can see this from Eq. (4).) For all photons this is zero. On the other hand, the "relativistic mass" of photons is frequency dependent. UV photons are more energetic than visible photons, and so are more "massive" in this sense, a statement which obscures more than it elucidates. Reference: Lev Okun wrote a nice article on this subject in the June 1989 issue of Physics Today, which includes a historical discussion of the concept of mass in relativistic physics. ******************************************************************************** Item 24. updated 16-MAR-1992 by SIC Original by John Blanton Why Do Stars Twinkle While Planets Do Not? ----------------------------------------- Stars, except for the Sun, although they may be millions of miles in diameter, are very far away. They appear as point sources even when viewed by telescopes. The planets in our solar system, much smaller than stars, are closer and can be resolved as disks with a little bit of magnification (field binoculars, for example). Since the Earth's atmosphere is turbulent, all images viewed up through it tend to "swim." The result of this is that sometimes a single point in object space gets mapped to two or more points in image space, and also sometimes a single point in object space does not get mapped into any point in image space. When a star's single point in object space fails to map to at least one point in image space, the star seems to disappear temporarily. This does not mean the star's light is lost for that moment. It just means that it didn't get to your eye, it went somewhere else. Since planets represent several points in object space, it is highly likely that one or more points in the planet's object space get mapped to a points in image space, and the planet's image never winks out. Each individual ray is twinkling away as badly as any star, but when all of those individual rays are viewed together, the next effect is averaged out to something considerably steadier. The result is that stars tend to twinkle, and planets do not. Other extended objects in space, even very far ones like nebulae, do not twinkle if they are sufficiently large that they have non-zero apparent diameter when viewed from the Earth. ******************************************************************************** Item 25. original by David Brahm Baryogenesis - Why Are There More Protons Than Antiprotons? ----------------------------------------------------------- (I) How do we really *know* that the universe is not matter-antimatter symmetric? (a) The Moon: Neil Armstrong did not annihilate, therefore the moon is made of matter. (b) The Sun: Solar cosmic rays are matter, not antimatter. (c) The other Planets: We have sent probes to almost all. Their survival demonstrates that the solar system is made of matter. (d) The Milky Way: Cosmic rays sample material from the entire galaxy. In cosmic rays, protons outnumber antiprotons 10^4 to 1. (e) The Universe at large: This is tougher. If there were antimatter galaxies then we should see gamma emissions from annihilation. Its absence is strong evidence that at least the nearby clusters of galaxies (e.g., Virgo) are matter-dominated. At larger scales there is little proof. However, there is a problem, called the "annihilation catastrophe" which probably eliminates the possibility of a matter-antimatter symmetric universe. Essentially, causality prevents the separation of large chucks of antimatter from matter fast enough to prevent their mutual annihilation in in the early universe. So the Universe is most likely matter dominated. (II) How did it get that way? Annihilation has made the asymmetry much greater today than in the early universe. At the high temperature of the first microsecond, there were large numbers of thermal quark-antiquark pairs. K&T estimate 30 million antiquarks for every 30 million and 1 quarks during this epoch. That's a tiny asymmetry. Over time most of the antimatter has annihilated with matter, leaving the very small initial excess of matter to dominate the Universe. Here are a few possibilities for why we are matter dominated today: a) The Universe just started that way. Not only is this a rather sterile hypothesis, but it doesn't work under the popular "inflation" theories, which dilute any initial abundances. b) Baryogenesis occurred around the Grand Unified (GUT) scale (very early). Long thought to be the only viable candidate, GUT's generically have baryon-violating reactions, such as proton decay (not yet observed). c) Baryogenesis occurred at the Electroweak Phase Transition (EWPT). This is the era when the Higgs first acquired a vacuum expectation value (vev), so other particles acquired masses. Pure Standard Model physics. Sakharov enumerated 3 necessary conditions for baryogenesis: (1) Baryon number violation. If baryon number is conserved in all reactions, then the present baryon asymmetry can only reflect asymmetric initial conditions, and we are back to case (a), above. (2) C and CP violation. Even in the presence of B-violating reactions, without a preference for matter over antimatter the B-violation will take place at the same rate in both directions, leaving no excess. (3) Thermodynamic Nonequilibrium. Because CPT guarantees equal masses for baryons and antibaryons, chemical equilibrium would drive the necessary reactions to correct for any developing asymmetry. It turns out the Standard Model satisfies all 3 conditions: (1) Though the Standard Model conserves B classically (no terms in the Lagrangian violate B), quantum effects allow the universe to tunnel between vacua with different values of B. This tunneling is _very_ suppressed at energies/temperatures below 10 TeV (the "sphaleron mass"), _may_ occur at e.g. SSC energies (controversial), and _certainly_ occurs at higher temperatures. (2) C-violation is commonplace. CP-violation (that's "charge conjugation" and "parity") has been experimentally observed in kaon decays, though strictly speaking the Standard Model probably has insufficient CP-violation to give the observed baryon asymmetry. (3) Thermal nonequilibrium is achieved during first-order phase transitions in the cooling early universe, such as the EWPT (at T = 100 GeV or so). As bubbles of the "true vacuum" (with a nonzero Higgs vev) percolate and grow, baryogenesis can occur at or near the bubble walls. A major theoretical problem, in fact, is that there may be _too_ _much_ B-violation in the Standard Model, so that after the EWPT is complete (and condition 3 above is no longer satisfied) any previously generated baryon asymmetry would be washed out. References: Kolb and Turner, _The Early Universe_; Dine, Huet, Singleton & Susskind, Phys.Lett.B257:351 (1991); Dine, Leigh, Huet, Linde & Linde, Phys.Rev.D46:550 (1992). ******************************************************************************** Item 26. TIME TRAVEL - FACT OR FICTION? updated 07-MAR-1994 ------------------------------ original by Jon J. Thaler We define time travel to mean departure from a certain place and time followed (from the traveller's point of view) by arrival at the same place at an earlier (from the sedentary observer's point of view) time. Time travel paradoxes arise from the fact that departure occurs after arrival according to one observer and before arrival according to another. In the terminology of special relativity time travel implies that the timelike ordering of events is not invariant. This violates our intuitive notions of causality. However, intuition is not an infallible guide, so we must be careful. Is time travel really impossible, or is it merely another phenomenon where "impossible" means "nature is weirder than we think?" The answer is more interesting than you might think. THE SCIENCE FICTION PARADIGM: The B-movie image of the intrepid chrononaut climbing into his time machine and watching the clock outside spin backwards while those outside the time machine watch the him revert to callow youth is, according to current theory, impossible. In current theory, the arrow of time flows in only one direction at any particular place. If this were not true, then one could not impose a 4-dimensional coordinate system on space-time, and many nasty consequences would result. Nevertheless, there is a scenario which is not ruled out by present knowledge. This usually requires an unusual spacetime topology (due to wormholes or strings in general relativity) which has not not yet seen, but which may be possible. In this scenario the universe is well behaved in every local region; only by exploring the global properties does one discover time travel. CONSERVATION LAWS: It is sometimes argued that time travel violates conservation laws. For example, sending mass back in time increases the amount of energy that exists at that time. Doesn't this violate conservation of energy? This argument uses the concept of a global conservation law, whereas relativistically invariant formulations of the equations of physics only imply local conservation. A local conservation law tells us that the amount of stuff inside a small volume changes only when stuff flows in or out through the surface. A global conservation law is derived from this by integrating over all space and assuming that there is no flow in or out at infinity. If this integral cannot be performed, then global conservation does not follow. So, sending mass back in time might be alright, but it implies that something strange is happening. (Why shouldn't we be able to do the integral?) GENERAL RELATIVITY: One case where global conservation breaks down is in general relativity. It is well known that global conservation of energy does not make sense in an expanding universe. For example, the universe cools as it expands; where does the energy go? See FAQ article #4 - Energy Conservation in Cosmology, for details. It is interesting to note that the possibility of time travel in GR has been known at least since 1949 (by Kurt Godel, discussed in [1], page 168). The GR spacetime found by Godel has what are now called "closed timelike curves" (CTCs). A CTC is a worldline that a particle or a person can follow which ends at the same spacetime point (the same position and time) as it started. A solution to GR which contains CTCs cannot have a spacelike embedding - space must have "holes" (as in donut holes, not holes punched in a sheet of paper). A would-be time traveller must go around or through the holes in a clever way. The Godel solution is a curiosity, not useful for constructing a time machine. Two recent proposals, one by Morris, et al. [2] and one by Gott [3], have the possibility of actually leading to practical devices (if you believe this, I have a bridge to sell you). As with Godel, in these schemes nothing is locally strange; time travel results from the unusual topology of spacetime. The first uses a wormhole (the inner part of a black hole, see fig. 1 of [2]) which is held open and manipulated by electromagnetic forces. The second uses the conical geometry generated by an infinitely long string of mass. If two strings pass by each other, a clever person can go into the past by traveling a figure-eight path around the strings. In this scenario, if the string has non-zero diameter and finite mass density, there is a CTC without any unusual topology. GRANDFATHER PARADOXES: With the demonstration that general relativity contains CTCs, people began studying the problem of self-consistency. Basically, the problem is that of the "grandfather paradox:" What happens if our time traveller kills her grandmother before her mother was born? In more readily analyzable terms, one can ask what are the implications of the quantum mechanical interference of the particle with its future self. Boulware [5] shows that there is a problem - unitarity is violated. This is related to the question of when one can do the global conservation integral discussed above. It is an example of the "Cauchy problem" [1, chapter 7]. OTHER PROBLEMS (and an escape hatch?): How does one avoid the paradox that a simple solution to GR has CTCs which QM does not like? This is not a matter of applying a theory in a domain where it is expected to fail. One relevant issue is the construction of the time machine. After all, infinite strings aren't easily obtained. In fact, it has been shown [4] that Gott's scenario implies that the total 4-momentum of spacetime must be spacelike. This seems to imply that one cannot build a time machine from any collection of non-tachyonic objects, whose 4-momentum must be timelike. There are implementation problems with the wormhole method as well. TACHYONS: Finally, a diversion on a possibly related topic. If tachyons exist as physical objects, causality is no longer invariant. Different observers will see different causal sequences. This effect requires only special relativity (not GR), and follows from the fact that for any spacelike trajectory, reference frames can be found in which the particle moves backward or forward in time. This is illustrated by the pair of spacetime diagrams below. One must be careful about what is actually observed; a particle moving backward in time is observed to be a forward moving anti-particle, so no observer interprets this as time travel. t One reference | Events A and C are at the same frame: | place. C occurs first. | | Event B lies outside the causal | B domain of events A and C. -----------A----------- x (The intervals are spacelike). | C In this frame, tachyon signals | travel from A-->B and from C-->B. | That is, A and C are possible causes of event B. Another t reference | Events A and C are not at the same frame: | place. C occurs first. | | Event B lies outside the causal -----------A----------- x domain of events A and C. (The | intervals are spacelike) | | C In this frame, signals travel from | B-->A and from B-->C. B is the cause | B of both of the other two events. The unusual situation here arises because conventional causality assumes no superluminal motion. This tachyon example is presented to demonstrate that our intuitive notion of causality may be flawed, so one must be careful when appealing to common sense. See FAQ article # 7 - Tachyons, for more about these weird hypothetical particles. CONCLUSION: The possible existence of time machines remains an open question. None of the papers criticizing the two proposals are willing to categorically rule out the possibility. Nevertheless, the notion of time machines seems to carry with it a serious set of problems. REFERENCES: 1: S.W. Hawking, and G.F.R. Ellis, "The Large Scale Structure of Space-Time," Cambridge University Press, 1973. 2: M.S. Morris, K.S. Thorne, and U. Yurtsever, PRL, v.61, p.1446 (1989). --> How wormholes can act as time machines. 3: J.R. Gott, III, PRL, v.66, p.1126 (1991). --> How pairs of cosmic strings can act as time machines. 4: S. Deser, R. Jackiw, and G. 't Hooft, PRL, v.66, p.267 (1992). --> A critique of Gott. You can't construct his machine. 5: D.G. Boulware, University of Washington preprint UW/PT-92-04. Available on the hep-th@xxx.lanl.gov bulletin board: item number 9207054. --> Unitarity problems in QM with closed timelike curves. 6: "Nature", May 7, 1992 --> Contains a very well written review with some nice figures. ******************************************************************************** Item 27. The EPR Paradox and Bell's Inequality Principle updated 31-AUG-1993 by SIC ----------------------------------------------- original by John Blanton In 1935 Albert Einstein and two colleagues, Boris Podolsky and Nathan Rosen (EPR) developed a thought experiment to demonstrate what they felt was a lack of completeness in quantum mechanics. This so-called "EPR paradox" has lead to much subsequent, and still on-going, research. This article is an introduction to EPR, Bell's inequality, and the real experiments which have attempted to address the interesting issues raised by this discussion. One of the principle features of quantum mechanics is that not all the classical physical observables of a system can be simultaneously known, either in practice or in principle. Instead, there may be several sets of observables which give qualitatively different, but nonetheless complete (maximal possible) descriptions of a quantum mechanical system. These sets are sets of "good quantum numbers," and are also known as "maximal sets of commuting observables." Observables from different sets are "noncommuting observables." A well known example of noncommuting observables are position and momentum. You can put a subatomic particle into a state of well-defined momentum, but then you cannot know where it is - it is, in fact, everywhere at once. It's not just a matter of your inability to measure, but rather, an intrinsic property of the particle. Conversely, you can put a particle in a definite position, but then it's momentum is completely ill-defined. You can also create states of intermediate knowledge of both observables: If you confine the particle to some arbitrarily large region of space, you can define the momentum more and more precisely. But you can never know both, exactly, at the same time. Position and momentum are continuous observables. But the same situation can arise for discrete observables such as spin. The quantum mechanical spin of a particle along each of the three space axes are a set of mutually noncommuting observables. You can only know the spin along one axis at a time. A proton with spin "up" along the x-axis has undefined spin along the y and z axes. You cannot simultaneously measure the x and y spin projections of a proton. EPR sought to demonstrate that this phenomenon could be exploited to construct an experiment which would demonstrate a paradox which they believed was inherent in the quantum-mechanical description of the world. They imagined two physical systems that are allowed to interact initially so that they subsequently will be defined by a single Schrodinger wave equation (SWE). [For simplicity, imagine a simple physical realization of this idea - a neutral pion at rest in your lab, which decays into a pair of back-to-back photons. The pair of photons is described by a single two-particle wave function.] Once separated, the two systems [read: photons] are still described by the same SWE, and a measurement of one observable of the first system will determine the measurement of the corresponding observable of the second system. [Example: The neutral pion is a scalar particle - it has zero angular momentum. So the two photons must speed off in opposite directions with opposite spin. If photon 1 is found to have spin up along the x-axis, then photon 2 *must* have spin down along the x-axis, since the total angular momentum of the final-state, two-photon, system must be the same as the angular momentum of the intial state, a single neutral pion. You know the spin of photon 2 even without measuring it.] Likewise, the measurement of another observable of the first system will determine the measurement of the corresponding observable of the second system, even though the systems are no longer physically linked in the traditional sense of local coupling. However, QM prohibits the simultaneous knowledge of more than one mutually noncommuting observable of either system. The paradox of EPR is the following contradiction: For our coupled systems, we can measure observable A of system I [for example, photon 1 has spin up along the x-axis; photon 2 must therefore have x-spin down.] and observable B of system II [for example, photon 2 has spin down along the y-axis; therefore the y-spin of photon 1 must be up.] thereby revealing both observables for both systems, contrary to QM. QM dictates that this should be impossible, creating the paradoxical implication that measuring one system should "poison" any measurement of the other system, no matter what the distance between them. [In one commonly studied interpretation, the mechanism by which this proceeds is 'instantaneous collapse of the wavefunction'. But the rules of QM do not require this interpretation, and several other perfectly valid interpretations exist.] The second system would instantaneously be put into a state of well-defined observable A, and, consequently, ill-defined observable B, spoiling the measurement. Yet, one could imagine the two measurements were so far apart in space that special relativity would prohibit any influence of one measurement over the other. [After the neutral-pion decay, we can wait until the two photons are a light-year apart, and then "simultaneously" measure the x-spin of photon 1 and the y-spin of photon 2. QM suggests that if, for example, the measurement of the photon 1 x-spin happens first, this measurement must instantaneously force photon 2 into a state of ill-defined y-spin, even though it is light-years away from photon 1. How do we reconcile the fact that photon 2 "knows" that the x-spin of photon 1 has been measured, even though they are separated by light-years of space and far too little time has passed for information to have travelled to it according to the rules of Special Relativity? There are basically two choices. You can accept the postulates of QM" as a fact of life, in spite of its seemingly uncomfortable coexistence with special relativity, or you can postulate that QM is not complete, that there *was* more information available for the description of the two-particle system at the time it was created, carried away by both photons, and that you just didn't know it because QM does not properly account for it. So, EPR postulated the existence of hidden variables, some so-far unknown properties, of the systems should account for the discrepancy. Their claim was that QM theory is incomplete; it does not completely describe the physical reality. System II knows all about System I long before the scientist measures any of the observables, and thereby supposedly consigning the other noncommuting observables to obscurity. No instantaneous action-at-a-distance is necessary in this picture, which postulates that each System has more parameters than are accounted by QM. Niels Bohr, one of the founders of QM, held the opposite view and defended a strict interpretation, the Copenhagen Interpretation, of QM. In 1964 John S. Bell proposed a mechanism to test for the existence of these hidden parameters, and he developed his inequality principle as the basis for such a test. Use the example of two photons configured in the singlet state, consider this: After separation, each photon will have spin values for each of the three axes of space, and each spin can have one of two values; call them up and down. Call the axes A, B and C and call the spin in the A axis A+ if it is up in that axis, otherwise call it A-. Use similar definitions for the other two axes. Now perform the experiment. Measure the spin in one axis of one particle and the spin in another axis of the other photon. If EPR were correct, each photon will simultaneously have properties for spin in each of axes A, B and C. Look at the statistics. Perform the measurements with a number of sets of photons. Use the symbol N(A+, B-) to designate the words "the number of photons with A+ and B-." Similarly for N(A+, B+), N(B-, C+), etc. Also use the designation N(A+, B-, C+) to mean "the number of photons with A+, B- and C+," and so on. It's easy to demonstrate that for a set of photons (1) N(A+, B-) = N(A+, B-, C+) + N(A+, B-, C-) because all of the (A+, B-, C+) and all of the (A+, B-, C-) photons are included in the designation (A+, B-), and nothing else is included in N(A+, B-). You can make this claim if these measurements are connected to some real properties of the photons. Let n[A+, B+] be the designation for "the number of measurements of pairs of photons in which the first photon measured A+, and the second photon measured B+." Use a similar designation for the other possible results. This is necessary because this is all it is possible to measure. You can't measure both A and B of the same photon. Bell demonstrated that in an actual experiment, if (1) is true (indicating real properties), then the following must be true: (2) n[A+, B+] <= n[A+, C+] + n[B+, C-]. Additional inequality relations can be written by just making the appropriate permutations of the letters A, B and C and the two signs. This is Bell's inequality principle, and it is proved to be true if there are real (perhaps hidden) parameters to account for the measurements. At the time Bell's result first became known, the experimental record was reviewed to see if any known results provided evidence against locality. None did. Thus an effort began to develop tests of Bell's inequality. A series of experiments was conducted by Aspect ending with one in which polarizer angles were changed while the photons were `in flight'. This was widely regarded at the time as being a reasonably conclusive experiment confirming the predictions of QM. Three years later Franson published a paper showing that the timing constraints in this experiment were not adequate to confirm that locality was violated. Aspect measured the time delays between detections of photon pairs. The critical time delay is that between when a polarizer angle is changed and when this affects the statistics of detecting photon pairs. Aspect estimated this time based on the speed of a photon and the distance between the polarizers and the detectors. Quantum mechanics does not allow making assumptions about *where* a particle is between detections. We cannot know *when* a particle traverses a polarizer unless we detect the particle *at* the polarizer. Experimental tests of Bell's inequality are ongoing but none has yet fully addressed the issue raised by Franson. In addition there is an issue of detector efficiency. By postulating new laws of physics one can get the expected correlations without any nonlocal effects unless the detectors are close to 90% efficient. The importance of these issues is a matter of judgement. The subject is alive theoretically as well. In the 1970's Eberhard derived Bell's result without reference to local hidden variable theories; it applies to all local theories. Eberhard also showed that the nonlocal effects that QM predicts cannot be used for superluminal communication. The subject is not yet closed, and may yet provide more interesting insights into the subtleties of quantum mechanics. REFERENCES: 1. A. Einstein, B. Podolsky, N. Rosen: "Can quantum-mechanical description of physical reality be considered complete?" Physical Review 41, 777 (15 May 1935). (The original EPR paper) 2. D. Bohm: Quantum Theory, Dover, New York (1957). (Bohm discusses some of his ideas concerning hidden variables.) 3. N. Herbert: Quantum Reality, Doubleday. (A very good popular treatment of EPR and related issues) 4. M. Gardner: Science - Good, Bad and Bogus, Prometheus Books. (Martin Gardner gives a skeptics view of the fringe science associated with EPR.) 5. J. Gribbin: In Search of Schrodinger's Cat, Bantam Books. (A popular treatment of EPR and the paradox of "Schrodinger's cat" that results from the Copenhagen interpretation) 6. N. Bohr: "Can quantum-mechanical description of physical reality be considered complete?" Physical Review 48, 696 (15 Oct 1935). (Niels Bohr's response to EPR) 7. J. Bell: "On the Einstein Podolsky Rosen paradox" Physics 1 #3, 195 (1964). 8. J. Bell: "On the problem of hidden variables in quantum mechanics" Reviews of Modern Physics 38 #3, 447 (July 1966). 9. D. Bohm, J. Bub: "A proposed solution of the measurement problem in quantum mechanics by a hidden variable theory" Reviews of Modern Physics 38 #3, 453 (July 1966). 10. B. DeWitt: "Quantum mechanics and reality" Physics Today p. 30 (Sept 1970). 11. J. Clauser, A. Shimony: "Bell's theorem: experimental tests and implications" Rep. Prog. Phys. 41, 1881 (1978). 12. A. Aspect, Dalibard, Roger: "Experimental test of Bell's inequalities using time- varying analyzers" Physical Review Letters 49 #25, 1804 (20 Dec 1982). 13. A. Aspect, P. Grangier, G. Roger: "Experimental realization of Einstein-Podolsky-Rosen-Bohm gedankenexperiment; a new violation of Bell's inequalities" Physical Review Letters 49 #2, 91 (12 July 1982). 14. A. Robinson: "Loophole closed in quantum mechanics test" Science 219, 40 (7 Jan 1983). 15. B. d'Espagnat: "The quantum theory and reality" Scientific American 241 #5 (November 1979). 16. "Bell's Theorem and Delayed Determinism", Franson, Physical Review D, pgs. 2529-2532, Vol. 31, No. 10, May 1985. 17. "Bell's Theorem without Hidden Variables", P. H. Eberhard, Il Nuovo Cimento, 38 B 1, pgs. 75-80, (1977). 18. "Bell's Theorem and the Different Concepts of Locality", P. H. Eberhard, Il Nuovo Cimento 46 B, pgs. 392-419, (1978). ******************************************************************************** Item 28. The Nobel Prize for Physics (1901-1993) updated 15-OCT-1993 by SIC --------------------------------------- original by Scott I. Chase The following is a complete listing of Nobel Prize awards, from the first award in 1901. Prizes were not awarded in every year. The description following the names is an abbreviation of the official citation. 1901 Wilhelm Konrad Rontgen X-rays 1902 Hendrik Antoon Lorentz Magnetism in radiation phenomena Pieter Zeeman 1903 Antoine Henri Bequerel Spontaneous radioactivity Pierre Curie Marie Sklowdowska-Curie 1904 Lord Rayleigh Density of gases and (a.k.a. John William Strutt) discovery of argon 1905 Pilipp Eduard Anton von Lenard Cathode rays 1906 Joseph John Thomson Conduction of electricity by gases 1907 Albert Abraham Michelson Precision meteorological investigations 1908 Gabriel Lippman Reproducing colors photographically based on the phenomenon of interference 1909 Guglielmo Marconi Wireless telegraphy Carl Ferdinand Braun 1910 Johannes Diderik van der Waals Equation of state of fluids 1911 Wilhelm Wien Laws of radiation of heat 1912 Nils Gustaf Dalen Automatic gas flow regulators 1913 Heike Kamerlingh Onnes Matter at low temperature 1914 Max von Laue Crystal diffraction of X-rays 1915 William Henry Bragg X-ray analysis of crystal structure William Lawrence Bragg 1917 Charles Glover Barkla Characteristic X-ray spectra of elements 1918 Max Planck Energy quanta 1919 Johannes Stark Splitting of spectral lines in E fields 1920 Charles-Edouard Guillaume Anomalies in nickel steel alloys 1921 Albert Einstein Photoelectric Effect 1922 Niels Bohr Structure of atoms 1923 Robert Andrew Millikan Elementary charge of electricity 1924 Karl Manne Georg Siegbahn X-ray spectroscopy 1925 James Franck Impact of an electron upon an atom Gustav Hertz 1926 Jean Baptiste Perrin Sedimentation equilibrium 1927 Arthur Holly Compton Compton effect Charles Thomson Rees Wilson Invention of the Cloud chamber 1928 Owen Willans Richardson Thermionic phenomena, Richardson's Law 1929 Prince Louis-Victor de Broglie Wave nature of electrons 1930 Sir Chandrasekhara Venkata Raman Scattering of light, Raman effect 1932 Werner Heisenberg Quantum Mechanics 1933 Erwin Schrodinger Atomic theory Paul Adrien Maurice Dirac 1935 James Chadwick The neutron 1936 Victor Franz Hess Cosmic rays Carl D. Anderson The positron 1937 Clinton Joseph Davisson Crystal diffraction of electrons George Paget Thomson 1938 Enrico Fermi New radioactive elements 1939 Ernest Orlando Lawrence Invention of the Cyclotron 1943 Otto Stern Proton magnetic moment 1944 Isador Isaac Rabi Magnetic resonance in atomic nuclei 1945 Wolfgang Pauli The Exclusion principle 1946 Percy Williams Bridgman Production of extremely high pressures 1947 Sir Edward Victor Appleton Physics of the upper atmosphere 1948 Patrick Maynard Stuart Blackett Cosmic ray showers in cloud chambers 1949 Hideki Yukawa Prediction of Mesons 1950 Cecil Frank Powell Photographic emulsion for meson studies 1951 Sir John Douglas Cockroft Artificial acceleration of atomic Ernest Thomas Sinton Walton particles and transmutation of nuclei 1952 Felix Bloch Nuclear magnetic precision methods Edward Mills Purcell 1953 Frits Zernike Phase-contrast microscope 1954 Max Born Fundamental research in QM Walther Bothe Coincidence counters 1955 Willis Eugene Lamb Hydrogen fine structure Polykarp Kusch Electron magnetic moment 1956 William Shockley Transistors John Bardeen Walter Houser Brattain 1957 Chen Ning Yang Parity violation Tsung Dao Lee 1958 Pavel Aleksejevic Cerenkov Interpretation of the Cerenkov effect Il'ja Mickajlovic Frank Igor' Evgen'evic Tamm 1959 Emilio Gino Segre The Antiproton Owen Chamberlain 1960 Donald Arthur Glaser The Bubble Chamber 1961 Robert Hofstadter Electron scattering on nucleons Rudolf Ludwig Mossbauer Resonant absorption of photons 1962 Lev Davidovic Landau Theory of liquid helium 1963 Eugene P. Wigner Fundamental symmetry principles Maria Goeppert Mayer Nuclear shell structure J. Hans D. Jensen 1964 Charles H. Townes Maser-Laser principle Nikolai G. Basov Alexander M. Prochorov 1965 Sin-Itiro Tomonaga Quantum electrodynamics Julian Schwinger Richard P. Feynman 1966 Alfred Kastler Study of Hertzian resonance in atoms 1967 Hans Albrecht Bethe Energy production in stars 1968 Luis W. Alvarez Discovery of many particle resonances 1969 Murray Gell-Mann Quark model for particle classification 1970 Hannes Alfven Magneto-hydrodynamics in plasma physics Louis Neel Antiferromagnetism and ferromagnetism 1971 Dennis Gabor Principles of holography 1972 John Bardeen Superconductivity Leon N. Cooper J. Robert Schrieffer 1973 Leo Esaki Tunneling in superconductors Ivar Giaever Brian D. Josephson Super-current through tunnel barriers 1974 Antony Hewish Discovery of pulsars Sir Martin Ryle Pioneering radioastronomy work 1975 Aage Bohr Structure of the atomic nucleus Ben Mottelson James Rainwater 1976 Burton Richter Discovery of the J/Psi particle Samual Chao Chung Ting 1977 Philip Warren Anderson Electronic structure of magnetic and Nevill Francis Mott disordered solids John Hasbrouck Van Vleck 1978 Pyotr Kapitsa Liquifaction of helium Arno A. Penzias Cosmic Microwave Background Radiation Robert W. Wilson 1979 Sheldon Glashow Electroweak Theory, especially Steven Weinberg weak neutral currents Abdus Salam 1980 James Cronin Discovery of CP violation in the Val Fitch asymmetric decay of neutral K-mesons 1981 Kai M. Seigbahn High resolution electron spectroscopy Nicolaas Bleombergen Laser spectroscopy Arthur L. Schawlow 1982 Kenneth G. Wilson Critical phenomena in phase transitions 1983 Subrahmanyan Chandrasekhar Evolution of stars William A. Fowler 1984 Carlo Rubbia Discovery of W,Z Simon van der Meer Stochastic cooling for colliders 1985 Klaus von Klitzing Discovery of quantum Hall effect 1986 Gerd Binning Scanning Tunneling Microscopy Heinrich Rohrer Ernst August Friedrich Ruska Electron microscopy 1987 Georg Bednorz High-temperature superconductivity Alex K. Muller 1988 Leon Max Lederman Discovery of the muon neutrino leading Melvin Schwartz to classification of particles in Jack Steinberger families 1989 Hans Georg Dehmelt Penning Trap for charged particles Wolfgang Paul Paul Trap for charged particles Norman F. Ramsey Control of atomic transitions by the separated oscillatory fields method 1990 Jerome Isaac Friedman Deep inelastic scattering experiments Henry Way Kendall leading to the discovery of quarks Richard Edward Taylor 1991 Pierre-Gilles de Gennes Order-disorder transitions in liquid crystals and polymers 1992 Georges Charpak Multiwire Proportional Chamber 1993 Russell A. Hulse Discovery of the first binary pulsar Joseph H. Taylor and subsequent tests of GR ******************************************************************************** Item 29. Open Questions updated 01-JUN-1993 by SIC -------------- original by John Baez While for the most part a FAQ covers the answers to frequently asked questions whose answers are known, in physics there are also plenty of simple and interesting questions whose answers are not known. Before you set about answering these questions on your own, it's worth noting that while nobody knows what the answers are, there has been at least a little, and sometimes a great deal, of work already done on these subjects. People have said a lot of very intelligent things about many of these questions. So do plenty of research and ask around before you try to cook up a theory that'll answer one of these and win you the Nobel prize! You can expect to really know physics inside and out before you make any progress on these. The following partial list of "open" questions is divided into two groups, Cosmology and Astrophysics, and Particle and Quantum Physics. However, given the implications of particle physics on cosmology, the division is somewhat artificial, and, consequently, the categorization is somewhat arbitrary. (There are many other interesting and fundamental questions in fields such as condensed matter physics, nonlinear dynamics, etc., which are not part of the set of related questions in cosmology and quantum physics which are discussed below. Their omission is not a judgement about importance, but merely a decision about the scope of this article.) Cosmology and Astrophysics -------------------------- 1. What happened at, or before the Big Bang? Was there really an initial singularity? Of course, this question might not make sense, but it might. Does the history of universe go back in time forever, or only a finite amount? 2. Will the future of the universe go on forever or not? Will there be a "big crunch" in the future? Is the Universe infinite in spatial extent? 3. Why is there an arrow of time; that is, why is the future so much different from the past? 4. Is spacetime really four-dimensional? If so, why - or is that just a silly question? Or is spacetime not really a manifold at all if examined on a short enough distance scale? 5. Do black holes really exist? (It sure seems like it.) Do they really radiate energy and evaporate the way Hawking predicts? If so, what happens when, after a finite amount of time, they radiate completely away? What's left? Do black holes really violate all conservation laws except conservation of energy, momentum, angular momentum and electric charge? What happens to the information contained in an object that falls into a black hole? Is it lost when the black hole evaporates? Does this require a modification of quantum mechanics? 6. Is the Cosmic Censorship Hypothesis true? Roughly, for generic collapsing isolated gravitational systems are the singularities that might develop guaranteed to be hidden beyond a smooth event horizon? If Cosmic Censorship fails, what are these naked singularities like? That is, what weird physical consequences would they have? 7. Why are the galaxies distributed in clumps and filaments? Is most of the matter in the universe baryonic? Is this a matter to be resolved by new physics? 8. What is the nature of the missing "Dark Matter"? Is it baryonic, neutrinos, or something more exotic? Particle and Quantum Physics ---------------------------- 1. Why are the laws of physics not symmetrical between left and right, future and past, and between matter and antimatter? I.e., what is the mechanism of CP violation, and what is the origin of parity violation in Weak interactions? Are there right-handed Weak currents too weak to have been detected so far? If so, what broke the symmetry? Is CP violation explicable entirely within the Standard Model, or is some new force or mechanism required? 2. Why are the strengths of the fundamental forces (electromagnetism, weak and strong forces, and gravity) what they are? For example, why is the fine structure constant, which measures the strength of electromagnetism, about 1/137.036? Where did this dimensionless constant of nature come from? Do the forces really become Grand Unified at sufficiently high energy? 3. Why are there 3 generations of leptons and quarks? Why are there mass ratios what they are? For example, the muon is a particle almost exactly like the electron except about 207 times heavier. Why does it exist and why precisely that much heavier? Do the quarks or leptons have any substructure? 4. Is there a consistent and acceptable relativistic quantum field theory describing interacting (not free) fields in four spacetime dimensions? For example, is the Standard Model mathematically consistent? How about Quantum Electrodynamics? 5. Is QCD a true description of quark dynamics? Is it possible to calculate masses of hadrons (such as the proton, neutron, pion, etc.) correctly from the Standard Model? Does QCD predict a quark/gluon deconfinement phase transition at high temperature? What is the nature of the transition? Does this really happen in Nature? 6. Why is there more matter than antimatter, at least around here? Is there really more matter than antimatter throughout the universe? 7. What is meant by a "measurement" in quantum mechanics? Does "wavefunction collapse" actually happen as a physical process? If so, how, and under what conditions? If not, what happens instead? 8. What are the gravitational effects, if any, of the immense (possibly infinite) vacuum energy density seemingly predicted by quantum field theory? Is it really that huge? If so, why doesn't it act like an enormous cosmological constant? 9. Why doesn't the flux of solar neutrinos agree with predictions? Is the disagreement really significant? If so, is the discrepancy in models of the sun, theories of nuclear physics, or theories of neutrinos? Are neutrinos really massless? The Big Question (TM) --------------------- This last question sits on the fence between the two categories above: How do you merge Quantum Mechanics and General Relativity to create a quantum theory of gravity? Is Einstein's theory of gravity (classical GR) also correct in the microscopic limit, or are there modifications possible/required which coincide in the observed limit(s)? Is gravity really curvature, or what else -- and why does it then look like curvature? An answer to this question will necessarily rely upon, and at the same time likely be a large part of, the answers to many of the other questions above. ******************************************************************************** END OF FAQ