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Path: sparky!uunet!spool.mu.edu!agate!dog.ee.lbl.gov!csa2.lbl.gov!sichase From: sichase@csa2.lbl.gov (SCOTT I CHASE) Newsgroups: sci.physics Subject: Sci.Physics Frequently Asked Questions - January 1993 - Part 2/2 Date: 30 Dec 1992 11:09 PST Organization: Lawrence Berkeley Laboratory - Berkeley, CA, USA Lines: 1102 Distribution: world Message-ID: <30DEC199211093967@csa2.lbl.gov> NNTP-Posting-Host: 128.3.254.197 Keywords: FAQ 2/2 Archive-name: physics-faq Last-modified: 1992/12/26 News-Software: VAX/VMS VNEWS 1.41 -------------------------------------------------------------------------------- FREQUENTLY ASKED QUESTIONS ON SCI.PHYSICS - Part 2/2 -------------------------------------------------------------------------------- Item 12. Which Way Will my Bathtub Drain? updated 11-May-1192 by SIC -------------------------------- original by Matthew R. Feinstein Question: Does my bathtub drain differently depending on whether I live in the northern or southern hemisphere? Answer: No. There is a real effect, but it is far too small to be relevant when you pull the plug in your bathtub. Because the earth rotates, a fluid that flows along the earth's surface feels a "Coriolis" acceleration perpendicular to its velocity. In the northern hemisphere high pressure storm systems spin clockwise. In the southern hemisphere, they spin counterclockwise because the direction of the Coriolis acceleration is reversed. This effect leads to the speculation that the bathtub vortex that you see when you pull the plug from the drain spins one way in the north and the other way in the south. But this acceleration is VERY weak for bathtub-scale fluid motions. The order of magnitude of the Coriolis acceleration can be estimated from size of the "Rossby number". Coriolis accelerations are significant when the Rossby number is SMALL. So, suppose we want a Rossby number of 0.1 and a bathtub-vortex length scale of 0.1 meter. Since the earth's rotation rate is about 10^(-4)/second, the fluid velocity should be less than or equal to 2*10^(-6) meters/second. This is a very small velocity. How small is it? Well, we can take the analysis a step further and calculate another, more famous dimensionless parameter, the Reynolds number. The Reynolds number is = L*U*density/viscosity Assuming that physicists bathe in hot water the viscosity will be about 0.005 poise and the density will be about 1.0, so the Reynolds Number is about 4*10^(-2). Now, life at low Reynolds numbers is different from life at high Reynolds numbers. In particular, at low Reynolds numbers, fluid physics is dominated by friction and diffusion, rather than by inertia: the time it would take for a particle of fluid to move a significant distance due to an acceleration is greater than the time it takes for the particle to break up due to diffusion. Therefore the effect of the Coriolis acceleration on your bathtub vortex is SMALL. To detect its effect on your bathtub, you would have to get out and wait until the motion in the water is far less than one rotation per day. This would require removing thermal currents, vibration, and any other sources of noise. Under such conditions, never occurring in the typical home, you WOULD see an effect. To see what trouble it takes to actually see the effect, see the reference below. Experiments have been done in both the northern and southern hemispheres to verify that under carefully controlled conditions, bathtubs drain in opposite directions due to the Coriolis acceleration from the Earth's rotation. The same effect has been accused of responsibility for the direction water circulates when you flush a toilet. This is surely nonsense. In this case, the water rotates in the direction which the pipe points which carries the water from the tank to the bowl. Reference: Trefethen, L.M. et al, Nature 207 1084-5 (1965). ******************************************************************************** Item 13. Why are Golf Balls Dimpled? updated 14-May-1992 by SIC --------------------------- original by Craig DeForest The dimples, paradoxically, *do* increase drag slightly. But they also increase `Magnus lift', that peculiar lifting force experienced by rotating bodies travelling through a medium. Contrary to Freshman physics, golf balls do not travel in inverted parabolas. They follow an 'impetus trajectory': * * * * (golfer) * * * * <-- trajectory \O/ * * | * * -/ \-T---------------------------------------------------------------ground This is because of the combination of drag (which reduces horizontal speed late in the trajectory) and Magnus lift, which supports the ball during the initial part of the trajectory, making it relatively straight. The trajectory can even curve upwards at first, depending on conditions! Here is a cheesy diagram of a golf ball in flight, with some relevant vectors: F(magnus) ^ | F(drag) <--- O -------> V \ \----> (sense of rotation) The Magnus force can be thought of as due to the relative drag on the air on the top and bottom portions of the golf ball: the top portion is moving slower relative to the air around it, so there is less drag on the air that goes over the ball. The boundary layer is relatively thin, and air in the not-too-near region moves rapidly relative to the ball. The bottom portion moves fast relative to the air around it; there is more drag on the air passing by the bottom, and the boundary (turbulent) layer is relatively thick; air in the not-too-near region moves more slowly relative to the ball. The Bernoulli force produces lift. (alternatively, one could say that `the flow lines past the ball are displaced down, so the ball is pushed up.') The difficulty comes near the transition region between laminar flow and turbulent flow. At low speeds, the flow around the ball is laminar. As speed is increased, the bottom part tends to go turbulent *first*. But turbulent flow can follow a surface much more easily than laminar flow. As a result, the (laminar) flow lines around the top break away from the surface sooner than otherwise, and there is a net displacement *up* of the flow lines. The magnus lift goes *negative*. The dimples aid the rapid formation of a turbulent boundary layer around the golf ball in flight, giving more lift. Without 'em, the ball would travel in more of a parabolic trajectory, hitting the ground sooner. (and not coming straight down.) References: Perhaps the best (and easy-to-read) reference on this effect is a paper in American Journal of Physics by one Lyman Briggs, c. 1947. Briggs was trying to explain the mechanism behind the `curve ball' in baseball, using specialized apparatus in a wind tunnel at the NBS. He stumbled on the reverse effect by accident, because his model `baseball' had no stitches on it. The stitches on a baseball create turbulence in flight in much the same way that the dimples on a golf ball do. ******************************************************************************** Item 14. Why do Mirrors Reverse Left and Right? updated 11-JUN-1992 by SIC -------------------------------------- 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. The further behind you an object is, the further in front of you it appears in the mirror. 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 Neutrino_, by Isaac Asimov, contains a very readable discussion of handedness and mirrors in physics. ******************************************************************************** Item 15. 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 16. updated 4-SEP-1992 by SIC Original by Bill Johnson How to Change Nuclear Decay Rates --------------------------------- "I've had this idea for making radioactive nuclei decay faster/slower than they normally do. You do [this, that, and the other thing]. Will this work?" Short Answer: Possibly, but probably not usefully. Long Answer: "One of the paradigms of nuclear science since the very early days of its study has been the general understanding that the half-life, or decay constant, of a radioactive substance is independent of extranuclear considerations." (Emery, cited below.) Like all paradigms, this one is subject to some interpretation. Normal decay of radioactive stuff proceeds via one of four mechanisms: * Emission of an alpha particle -- a helium-4 nucleus -- reducing the number of protons and neutrons present in the parent nucleus by two each; * "Beta decay," encompassing several related phenomena in which a neutron in the nucleus turns into a proton, or a proton turns into a neutron -- along with some other things including emission of a neutrino. The "other things", as we shall see, are at the bottom of several questions involving perturbation of decay rates; * Emission of one or more gamma rays -- energetic photons -- that take a nucleus from an excited state to some other (typically ground) state; some of these photons may be replaced by "conversion electrons," of which more shortly; or *Spontaneous fission, in which a sufficiently heavy nucleus simply breaks in half. Most of the discussion about alpha particles will also apply to spontaneous fission. Gamma emission often occurs from the daughter of one of the other decay modes. We neglect *very* exotic processes like C-14 emission or double beta decay in this analysis. "Beta decay" refers most often to a nucleus with a neutron excess, which decays by converting a neutron into a proton: n ----> p + e- + anti-nu(e), where n means neutron, p means proton, e- means electron, and anti-nu(e) means an antineutrino of the electron type. The type of beta decay which involves destruction of a proton is not familiar to many people, so deserves a little elaboration. Either of two processes may occur when this kind of decay happens: p ----> n + e+ + nu(e), where e+ means positron and nu(e) means electron neutrino; or p + e- ----> n + nu(e), where e- means a negatively charged electron, which is captured from the neighborhood of the nucleus undergoing decay. These processes are called "positron emission" and "electron capture," respectively. A given nucleus which has too many protons for stability may undergo beta decay through either, and typically both, of these reactions. "Conversion electrons" are produced by the process of "internal conversion," whereby the photon that would normally be emitted in gamma decay is *virtual* and its energy is absorbed by an atomic electron. The absorbed energy is sufficient to unbind the electron from the nucleus (ignoring a few exceptional cases), and it is ejected from the atom as a result. Now for the tie-in to decay rates. Both the electron-capture and internal conversion phenomena require an electron somewhere close to the decaying nucleus. In any normal atom, this requirement is satisfied in spades: the innermost electrons are in states such that their probability of being close to the nucleus is both large and insensitive to things in the environment. The decay rate depends on the electronic wavefunctions, i.e, how much of their time the inner electrons spend very near the nucleus -- but only very weakly. For most nuclides that decay by electron capture or internal conversion, most of the time, the probability of grabbing or converting an electron is also insensitive to the environment, as the innermost electrons are the ones most likely to get grabbed/converted. However, there are exceptions, the most notable being the the astrophysically important isotope beryllium-7. Be-7 decays purely by electron capture (positron emission being impossible because of inadequate decay energy) with a half-life of somewhat over 50 days. It has been shown that differences in chemical environment result in half-life variations of the order of 0.2%, and high pressures produce somewhat similar changes. Other cases where known changes in decay rate occur are Zr-89 and Sr-85, also electron capturers; Tc-99m ("m" implying an excited state), which decays by both beta and gamma emission; and various other "metastable" things that decay by gamma emission with internal conversion. With all of these other cases the magnitude of the effect is less than is typically the case with Be-7. What makes these cases special? The answer is that one or another of the usual starting assumptions -- insensitivity of electron wave function near the nucleus to external forces, or availability of the innermost electrons for capture/conversion -- are not completely valid. Atomic beryllium only has 4 electrons to begin with, so that the "innermost electrons" are also practically the *outermost* ones and therefore much more sensitive to chemical effects than usual. With most of the other cases, there is so little energy available from the decay (as little as a few electron volts; compare most radioactive decays, where hundreds or thousands of *kilo*volts are released), courtesy of accidents of nuclear structure, that the innermost electrons can't undergo internal conversion. Remember that converting an electron requires dumping enough energy into it to expel it from the atom (more or less); "enough energy," in context, is typically some tens of keV, so they don't get converted at all in these cases. Conversion therefore works only on some of the outer electrons, which again are more sensitive to the environment. A real anomaly is the beta emitter Re-187. Its decay energy is only about 2.6 keV, practically nothing by nuclear standards. "That this decay occurs at all is an example of the effects of the atomic environment on nuclear decay: the bare nucleus Re-187 [i.e., stripped of all orbital electrons -- MWJ] is stable against beta decay and it is the difference of 15 keV in the total electronic binding energy of osmium [to which it decays -- MWJ] and rhenium ... which makes the decay possible" (Emery). The practical significance of this little peculiarity, of course, is low, as Re-187 already has a half life of over 10^10 years. Alpha decay and spontaneous fission might also be affected by changes in the electron density near the nucleus, for a different reason. These processes occur as a result of penetration of the "Coulomb barrier" that inhibits emission of charged particles from the nucleus, and their rate is *very* sensitive to the height of the barrier. Changes in the electron density could, in principle, affect the barrier by some tiny amount. However, the magnitude of the effect is *very* small, according to theoretical calculations; for a few alpha emitters, the change has been estimated to be of the order of 1 part in 10^7 (!) or less, which would be unmeasurable in view of the fact that the alpha emitters' half lives aren't known to that degree of accuracy to begin with. All told, the existence of changes in radioactive decay rates due to the environment of the decaying nuclei is on solid grounds both experimentally and theoretically. But the magnitude of the changes is nothing to get very excited about. Reference: The best review article on this subject is now 20 years old: G. T. Emery, "Perturbation of Nuclear Decay Rates," Annual Review of Nuclear Science vol. 22, p. 165 (1972). Papers describing specific experiments are cited in that article, which contains considerable arcane math but also gives a reasonable qualitative "feel" for what is involved. ******************************************************************************** Item 17. 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 18. TIME TRAVEL - FACT OR FICTION? updated 25-Nov-1992 ------------------------------ 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. It 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 #1 - 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. 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 physical objects, whose 4-momentum must be timelike unless tachyons exist. Similar objections apply to the wormhole method. 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 # 6 - 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. ******************************************************************************** Item 19. Gravity and the Radiation of Charged Particles updated 26-DEC-1992 by SIC ---------------------------------------------- original by Kurt Sonnenmoser Three oft-asked questions about the Equivalence Principle and the radiation of charged particles in a gravitational field according to GR: A) DOES THE GRAVITATIONAL FIELD OF A STATIC MASSIVE BODY CAUSE RADIATION FROM A CHARGED PARTICLE AT REST ON ITS SURFACE? (Or: "According to the Equivalence Principle, the electron on my desk should radiate!") Answer: No, it doesn't. Reason: Static situation --> no magnetic fields --> vanishing field energy current, i.e. no radiation. The Equivalence Principle only leads you to the conclusion that if you put the particle on the bottom of an accelerated elevator in gravity free space, you will observe no radiation (in the reference frame of the elevator). B ) DOES A CHARGED STABLE PARTICLE IN FREE FALL IN THE GRAVITATIONAL FIELD OF A MASSIVE BODY RADIATE? (Or: "According to the Equivalence Principle, my electron should not radiate if it falls to the ground!") Answer: Yes, it does. Reason: It's like with any accelerated motion of a charged particle: The acceleration causes "kinks" in the field lines that propagate with the velocity of light and carry off energy. This energy comes from the orbital energy of the particle and not from its mass. As before, trying to apply the Equivalence Principle is misleading: the free falling particle is only _locally_ equivalent to one at rest in gravity free space, but in order to calculate the energy radiated off, you have to integrate the energy flux of the electromagnetic field over a sphere going to infinity (in a fixed reference frame), which is, of course, not a local procedure. The Equivalence Principle only tells you that if you go very close to the particle, you see no radiation. C) DOES A UNIFORMLY ACCELERATED CHARGE RADIATE? (Or: "Ok, let's forget about the Equivalence Principle! What happens globally?") Answer: David Boulware [Ann.Phys. 124, 169-188 (1980) ("Radiation from a Uniformly Accelerated Charge")] has shown that a uniformly accelerated charge in gravity-free space does in fact radiate (contrary to earlier beliefs, e.g. of Pauli), but also that it is _not_ globally equivalent to a charge at rest in a static gravitational field. More specifically, there are regions of space-time where there is no coordinate frame in which the accelerated charge is at rest and the gravitational field static. So there is no contradiction to the fact that charges at rest in a gravitational field do not radiate. ******************************************************************************** Item 20. The Nobel Prize for Physics (1901-1992) updated 29-Nov-1992 by SIC --------------------------------------- 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 metrological 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 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 Alven 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 ******************************************************************************** Item 21. Open Questions updated 13-OCT-1992 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? 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 to 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. ******************************************************************************** Item 22. updated 15-OCT-1992 by SIC Accessing and Using Online Physics Resources -------------------------------------------- (I) Particle Physics Databases The Full Listings of the Review of Particle Properties (RPP), as well as other particle physics databases, are accessible on-line. Here is a summary of the major ones, as described in the RPP: (A) SLAC Databases PARTICLES - Full listings of the RPP HEP - Guide to particle physics preprints, journal articles, reports, theses, conference papers, etc. CONF - Listing of past and future conferences in particle physics HEPNAMES - E-mail addresses of many HEP people INST - Addresses of HEP institutions DATAGUIDE - Adjunct to HEP, indexes papers REACTIONS - Numerical data on reactions (cross-sections, polarizations, etc) EXPERIMENTS - Guide to current and past experiments Anyone with a SLAC account can access these databases. Alternately, most of us can access them via QSPIRES. You can access QSPIRES via BITNET with the 'send' command ('tell','bsend', or other system-specific command) or by using E-mail. For example, send QSPIRES@SLACVM FIND TITLE Z0 will get you a search of HEP for all papers which reference the Z0 in the title. By E-mail, you would send the one line message "FIND TITLE Z0" with a blank subject line to QSPIRES@SLACVM.BITNET or QSPIRES@VM.SLAC.STANFORD.EDU. QSPIRES is free. Help can be obtained by mailing "HELP" to QSPIRES. For more detailed information, see the RPP, p.I.12, or contact: Louise Addis (ADDIS@SLACVM.BITNET) or Harvey Galic (GALIC@SLACVM.BITNET). (B) CERN Databases on ALICE LIB - Library catalogue of books, preprints, reports, etc. PREP - Subset of LIB containing preprints, CERN publications, and conference papers. CONF - Subset of LIB containing upcoming and past conferences since 1986 DIR - Directory of Research Institutes in HEP, with addresses, fax, telex, e-mail addresses, and info on research programs ALICE can be accessed via DECNET or INTERNET. It runs on the CERN library's VXLIB, alias ALICE.CERN.CH (IP# 128.141.201.44). Use Username ALICE (no password required.) Remote users with no access to the CERN Ethernet can use QALICE, similar to QSPIRES. Send E-mail to QALICE@VXLIB.CERN.CH, put the query in the subject field and leave the message field black. For more information, send the subject "HELP" to QALICE or contact CERN Scientific Information Service, CERN, CH-1211 Geneva 23, Switzerland, or E-mail MALICE@VXLIB.CERN.CH. Regular weekly or monthly searches of the CERN databases can be arranged according to a personal search profile. Contact David Dallman, CERN SIS (address above) or E-mail CALLMAN@CERNVM.CERN.CH. DIR is available in Filemaker PRO format for Macintosh. Contact Wolfgang Simon (ISI@CERNVM.CERN.CH). (C) Other Databases Durham-RAL and Serpukhov both maintain large databases containing Particle Properties, reaction data, experiments, E-mail ID's, cross-section compilations (CS), etc. Except for the Serpukhov CS, these databases overlap SPIRES at SLAC considerably, though they are not the same and may be more up-to-date. For details, see the RPP, p.I.14, or contact: For Durham-RAL, Mike Whalley (MRW@UKACRL.BITNET,MRW@CERNVM.BITNET) or Dick Roberts (RGR@UKACRL.BITNET). For Serpukhov, contact Sergey Alekhin (ALEKHIN@M9.IHEP.SU) or Vladimir Exhela (EZHELA@M9.IHEP.SU). (II) Online Preprint Sources There are a number of online sources of preprints: alg-geom@publications.math.duke.edu (algebraic geometry) astro-ph@babbage.sissa.it (astrophysics) cond-mat@babbage.sissa.it (condensed matter) funct-an@babbage.sissa.it (functional analysis) hep-lat@ftp.scri.fsu.edu (computational and lattice physics) hep-ph@xxx.lanl.gov (high energy physics phenomenological) hep-th@xxx.lanl.gov (high energy physics theoretical) lc-om@alcom-p.cwru.edu (liquid crystals, optical materials) gr-qc@xxx.lanl.gov (general relativity, quantum cosmology) To get things if you know the preprint number, send a message to the appropriate address with subject header "get (preprint number)" and no message body. If you *don't* know the preprint number, or want to get preprints regularly, or want other information, send a message with subject header "help" and no message body. ******************************************************************************** END OF FAQ