Date: Wed, 30 Dec 92 11:10:18 PST From: sichase@Csa2.LBL.Gov Message-Id: <921230111018.20c00e8f@csa2.lbl.gov> Subject: Sci.Physics Frequently Asked Questions - January 1993 - Part 1/2 To: distribution:@Csa2.LBL.Gov; (see end of body) X-St-Vmsmail-To: @[-]MAILING_LIST.FAQ Archive-name: physics-faq Last-modified: 1992/12/26 Editor's Note: The FAQ has expanded beyond the 100K limit set by my mailer. To facilitate distribution, as well as future expansion, it has been divided, abritrarily, into a two-part posting. -------------------------------------------------------------------------------- FREQUENTLY ASKED QUESTIONS ON SCI.PHYSICS - Part 1/2 -------------------------------------------------------------------------------- This Frequently Asked Questions List is posted monthly, at or near the first of the month, to the Usenet newsgroup sci.physics in an attempt to provide good answers to frequently asked questions and other reference material which is worth preserving. If you have corrections or answers to other frequently asked questions that you would like included in this posting, send E-mail to sichase@csa2.lbl.gov (Scott I. Chase). The FAQ is distributed to all interested parties whenever sufficient changes have accumulated to warrant such a mailing. To request that your address be added to the list, send mail to my address, above, and include the words "FAQ Mailing List" in the subject header of your message. To faciliate mailing, the FAQ is now being distributed as a multi-part posting. If you are a new reader of sci.physics, please read item #1, below. If you do not wish to read the FAQ at all, add "Frequently Asked Questions" to your .KILL file. A listing of new items can be found above the subject index, so that you can quickly identify new subjects of interest. To locate old items which have been updated since the last posting, look for the stars (*) in the subject index, which indicate new material. Items which have been submitted by a single individual are attributed to the original author. All other contributors have been thanked privately. New Items: 19. Gravity and the Radiation of Charged Particles Index of Subjects ----------------- 1. An Introduction to Sci.Physics 2. Gravitational Radiation 3. Energy Conservation in Cosmology and Red Shift 4. Effects Due to the Finite Speed of Light 5. The Top Quark 6. Tachyons 7. Special Relativistic Paradoxes (a) The Barn and the Pole (b) The Twin Paradox 8. The Particle Zoo 9. Olbers' Paradox 10. What is Dark Matter? 11. Hot Water Freezes Faster than Cold! 12. Which Way Will my Bathtub Drain? 13. Why are Golf Balls Dimpled? 14. Why do Mirrors Reverse Left and Right? 15. What is the Mass of a Photon? 16. How to Change Nuclear Decay Rates 17. Baryogenesis - Why Are There More Protons Than Antiprotons? 18. Time Travel - Fact or Fiction? 19.*Gravity and the Radiation of Charged Particles 20. The Nobel Prize for Physics 21. Open Questions 22. Accessing and Using Online Physics Resources ******************************************************************************** Item 1. updated 4-AUG-1992 by SIC An Introduction to Sci.Physics ------------------------------ Sci.Physics is an unmoderated newsgroup dedicated to the discussion of physics, news from the physics community, and physics-related social issues. People from a wide variety of non-physics backgrounds, as well as students and experts in all areas of physics participate in the ongoing discussions on sci.physics. Professors, industrial scientists, graduate students, etc., are all on hand to bring physics expertise to bear on almost any question. But the only requirement for participation is interest in physics, so feel free to post -- but before you do, please do the following: (1) Read this posting, a.k.a., the FAQ. It contains good answers, contributed by the readership, to some of the most frequently asked questions. (2) Understand "netiquette." If you are not sure what this means, subscribe to news.announce.newusers and read the excellent discussion of proper net behavior that is posted there periodically. (3) Be aware that there is another newsgroup dedicated to the discussion of "alternative" physics. It is alt.sci.physics.new-theories, and is the appropriate forum for discussion of physics ideas which are not widely accepted by the physics community. Sci.Physics is not the group for such discussions. A quick look at items posted to both groups will make the distinction apparent. (4) Read the responses already posted in the thread to which you want to contribute. If a good answer is already posted, or the point you wanted to make has already been made, let it be. Old questions have probably been thoroughly discussed by the time you get there - save bandwidth by posting only new information. Post to as narrow a geographic region as is appropriate. If your comments are directed at only one person, try E-mail. (5) Get the facts right! Opinions may differ, but facts should not. It is very tempting for new participants to jump in with quick answers to physics questions posed to the group. But it is very easy to end up feeling silly when people barrage you with corrections. So before you give us all a physics lesson you'll regret - look it up. (6) Be prepared for heated discussion. People have strong opinions about the issues, and discussions can get a little "loud" at times. Don't take it personally if someone seems to always jump all over everything you say. Everyone was jumping all over everybody long before you got there! You can keep the discussion at a low boil by trying to stick to the facts. Clearly separate facts from opinion - don't let people think you are confusing your opinions with scientific truth. And keep the focus of discussion on the ideas, not the people who post them. (7) Tolerate everyone. People of many different points of view, and widely varying educational backgrounds from around the world participate in this newsgroup. Respect for others will be returned in kind. Personal criticism is usually not welcome. ******************************************************************************** Item 2. Gravitational Radiation updated: 4-May-1992 by SIC ----------------------- Gravitational Radiation is to gravity what light is to electromagnetism. It is produced when massive bodies accelerate. You can accelerate any body so as to produce such radiation, but due to the feeble strength of gravity, it is entirely undetectable except when produced by intense astrophysical sources such as supernovae, collisions of black holes, etc. These are quite far from us, typically, but they are so intense that they dwarf all possible laboratory sources of such radiation. Gravitational waves have a polarization pattern that causes objects to expand in one direction, while contracting in the perpendicular direction. That is, they have spin two. This is because gravity waves are fluctuations in the tensorial metric of space-time. All oscillating radiation fields can be quantized, and in the case of gravity, the intermediate boson is called the "graviton" in analogy with the photon. But quantum gravity is hard, for several reasons: (1) The quantum field theory of gravity is hard, because gauge interactions of spin-two fields are not renormalizable. See Cheng and Li, Gauge Theory of Elementary Particle Physics (search for "power counting"). (2) There are conceptual problems - what does it mean to quantize geometry, or space-time? It is possible to quantize weak fluctuations in the gravitational field. This gives rise to the spin-2 graviton. But full quantum gravity has so far escaped formulation. It is not likely to look much like the other quantum field theories. In addition, there are models of gravity which include additional bosons with different spins. Some are the consequence of non-Einsteinian models, such as Brans-Dicke which has a spin-0 component. Others are included by hand, to give "fifth force" components to gravity. For example, if you want to add a weak repulsive short range component, you will need a massive spin-1 boson. (Even-spin bosons always attract. Odd-spin bosons can attract or repel.) If antigravity is real, then this has implications for the boson spectrum as well. The spin-two polarization provides the method of detection. All experiments to date use a "Weber bar." This is a cylindrical, very massive, bar suspended by fine wire, free to oscillate in response to a passing graviton. A high-sensitivity, low noise, capacitive transducer can turn the oscillations of the bar into an electric signal for analysis. So far such searches have failed. But they are expected to be insufficiently sensitive for typical radiation intensity from known types of sources. A more sensitive technique uses very long baseline laser interferometry. This is the principle of LIGO (Laser Interferometric Gravity wave Observatory). This is a two-armed detector, with perpendicular laser beams each travelling several km before meeting to produce an interference pattern which fluctuates if a gravity wave distorts the geometry of the detector. To eliminate noise from seismic effects as well as human noise sources, two detectors separated by hundreds to thousands of miles are necessary. A coincidence measurement then provides evidence of gravitational radiation. In order to determine the source of the signal, a third detector, far from either of the first two, would be necessary. Timing differences in the arrival of the signal to the three detectors would allow triangulation of the angular position in the sky of the signal. The first stage of LIGO, a two detector setup in the U.S., has been approved by Congress in 1992. LIGO researchers have started designing a prototype detector, and are hoping to enroll another nation, probably in Europe, to fund and be host to the third detector. The speed of gravitational radiation (C_gw) depends upon the specific model of Gravitation that you use. There are quite a few competing models (all consistent with all experiments to date) including of course Einstein's but also Brans-Dicke and several families of others. All metric models can support gravity waves. But not all predict radiation travelling at C_gw = C_em. (C_em is the speed of electromagnetic waves.) There is a class of theories with "prior geometry", in which, as I understand it, there is an additional metric which does not depend only on the local matter density. In such theories, C_gw != C_em in general. However, there is good evidence that C_gw is in fact at least almost C_em. We observe high energy cosmic rays in the 10^20-10^21 eV region. Such particles are travelling at up to (1-10^-18)*C_em. If C_gw < C_em, then particles with C_gw < v < C_em will radiate Cerenkov gravitational radiation into the vacuum, and decelerate from the back reaction. So evidence of these very fast cosmic rays good evidence that C_gw >= (1-10^-18)*C_em, very close indeed to C_em. Bottom line: in a purely Einsteinian universe, C_gw = C_em. However, a class of models not yet ruled out experimentally does make other predictions. A definitive test would be produced by LIGO in coincidence with optical measurements of some catastrophic event which generates enough gravitational radiation to be detected. Then the "time of flight" of both gravitons and photons from the source to the Earth could be measured, and strict direct limits could be set on C_gw. For more information, see Gravitational Radiation (NATO ASI - Les Houches 1982), specifically the introductory essay by Kip Thorne. ******************************************************************************** Item 3. ENERGY CONSERVATION IN COSMOLOGY AND RED SHIFT updated: 10-May-1992 by SIC ---------------------------------------------- IS ENERGY CONSERVED IN OUR UNIVERSE? NO Why? Every conserved quantity is the result of some symmetry of nature. This is known as Noether's theorem. For example, momentum conservation is the result of translation invariance, because position is the variable conjugate to momentum. Energy would be conserved due to time-translation invariance. However, in an expanding or contracting universe, there is no time-translation invariance. Hence energy is not conserved. If you want to learn more about this, read Goldstein's Classical Mechanics, and look up Noether's theorem. DOES RED-SHIFT LEAD TO ENERGY NON-CONSERVATION: SOMETIMES There are three basic cosmological sources of red-shifted light: (1) Very massive objects emitting light (2) Very fast objects emitting light (3) Expansion of the universe leading to CBR (Cosmic Background Radiation) red-shift About each: (1) Light has to climb out the gravitational well of a very massive object. It gets red-shifted as a result. As several people have commented, this does not lead to energy non-conservation, because the photon had negative gravitational potential energy when it was deep in the well. No problems here. If you want to learn more about this read Misner, Thorne, and Wheeler's Gravitation, if you dare. (2) Fast objects moving away from you emit Doppler shifted light. No problems here either. Energy is only one part a four-vector, so it changes from frame to frame. However, when looked at in a Lorentz invariant way, you can convince yourself that everything is OK here too. If you want to learn more about this, read Taylor and Wheeler's Spacetime Physics. (3) CBR has red-shifted over billions of years. Each photon gets redder and redder. And the energy is lost. This is the only case in which red-shift leads to energy non-conservation. Several people have speculated that radiation pressure "on the universe" causes it to expand more quickly, and attempt to identify the missing energy with the speed at which the universe is expanding due to radiation pressure. This argument is completely specious. If you add more radiation to the universe you add more energy, and the universe is now more closed than ever, and the expansion rate slows. If you really MUST construct a theory in which something like energy is conserved (which is dubious in a universe without time-translation invariance), it is possible to arbitrarily define things so that energy has an extra term which compensates for the loss. However, although the resultant quantity may be a constant, it is of questionable value, and certainly is not an integral associated with time-invariance, so it is not what everyone calls energy. ******************************************************************************** Item 4. EFFECTS DUE TO THE FINITE SPEED OF LIGHT updated 28-May-1992 by SIC ---------------------------------------- There are two well known phenomena which are due to the finite speed of electromagnetic radiation, but are essentially classical in nature, requiring no other facts of special relativity for their understanding. (1) Apparent Superluminal Velocity of Galaxies A distant object can appear to travel faster than the speed of light relative to us, provided that it has some component of motion towards us as well as perpendicular to our line of sight. Say that on Jan. 1 you make a position measurement of galaxy X. One month later, you measure it again. Assuming you know it's distance from us by some independent measurement, you derive its linear speed, and conclude that it is moving faster than the speed of light. What have you forgotten? Let's say that on Jan. 1, the object is D km from us, and that between Jan. 1 and Feb. 1, the object has moved d km closer to us. You have assumed that the light you measured on Jan. 1 and Feb. 1 were emitted exactly one month apart. Not so. The first light beam had further to travel, and was actually emitted (1 + d/c) months before the second measurement, if we measure c in km/month. The object has traveled the given angular distance in more time than you thought. Similarly, if the object is moving away from us, the apparent angular velocity will be too slow, if you do not correct for this effect, which becomes significant when the object is moving along a line close to our line of sight. Note that most extragalactic objects are moving away from us due to the Hubble expansion. So for most objects, you don't get superluminal apparent velocities. But the effect is still there, and you need to take it into account if you want to measure velocities by this technique. References: Considerations about the Apparent 'Superluminal Expansions' in Astrophysics, E. Recami, A. Castellino, G.D. Maccarrone, M. Rodono, Nuovo Cimento 93B, 119 (1986). Apparent Superluminal Sources, Comparative Cosmology and the Cosmic Distance Scale, Mon. Not. R. Astr. Soc. 242, 423-427 (1990). (2) Terrell Rotation Consider a cube moving across your field of view with speed near the speed of light. The trailing face of the cube is edge on to your line of sight as it passes you. However, the light from the back edge of that face (the edge of the square farthest from you) takes longer to get to your eye than the light from the front edge. At any given instant you are seeing light from the front edge at time t and the back edge at time t-(L/c), where L is the length of an edge. This means you see the back edge where it was some time earlier. This has the effect of *rotating* the *image* of the cube on your retina. This does not mean that the cube itself rotates. The *image* is rotated. And this depends only on the finite speed of light, not any other postulate or special relativity. You can calculate the rotation angle by noting that the side face of the cube is Lorentz contracted to L' = L/gamma. This will correspond to a rotation angle of arccos(1/gamma). It turns out, if you do the math for a sphere, that the amount of apparent rotation exactly cancels the Lorentz contraction. The object itself is flattened, but then you see *behind* it as it flies by just enough to restore it to its original size. So the image of a sphere is unaffected by the Lorentz flattening that it experiences. Another implication of this is that if the object is moving at nearly the speed of light, although it is contracted into an infinitesimally thin pancake, you see it rotated by almost a full 90 degrees, so you see the complete backside of the object, and it doesn't disappear from view. In the case of the sphere, you see the transverse cross-section (which suffers no contraction), so that it still appears to be exactly a sphere. That it took so long historically to realize this is undoubtedly due to the fact that although we were regularly accelerating particle beams in 1959 to relativistic speeds, we still do not have the technology to accelerate any macroscopic objects to speeds necessary to reveal the effect. References: J. Terrell, Phys Rev. _116_, 1041 (1959). For a textbook discussion, see Marion's _Classical Dynamics_, Section 10.5. ******************************************************************************** Item 5. TOP QUARK updated: 10-May-1992 by SIC --------- The top quark is the hypothetical sixth fundamental strongly interacting particle (quark). The known quarks are up (u), down (d), strange (s), charm (c) and bottom (b). The Standard Model requires quarks to come in pairs in order to prevent mathematical inconsistency due to certain "anomalous" Feynman diagrams, which cancel if and only if the quarks are paired. The pairs are (d,u),(s,c) and (b,?). The missing partner of the b is called "top". In addition, there is experimental evidence that the b quark has an "isodoublet" partner, which is so far unseen. The forward-backward asymmetry in the reaction e+ + e- -> b + b-bar and the absence of flavor-changing neutral currents in b decays imply the existence of the isodoublet partner of the b. ("b-bar", pronounced "bee bar", signifies the b antiquark.) The mass of the top quark is restricted by a variety of measurements. Due to radiative corrections which depend on the top quark circulating as a virtual particle inside the loop in the Feynman diagram, a number of experimentally accessible processes depend on the top quark mass. There are about a dozen such measurements which have been made so far, including the width of the Z, b-b-bar mixing (which historically gave the first hints that the top quark was very massive), and certain aspects of muon decay. These results collectively limit the top mass to roughly 140 +/- 30 GeV. This uncertainty is a "1-sigma" error bar. Direct searches for the top quark have been performed, looking for the expected decay products in both p-p-bar and e+e- collisions. The best current limits on the top mass are: (1) From the absence of Z -> t + t-bar, M(t) > M(Z)/2 = 45 GeV. This is a "model independent" result, depending only on the fact that the top quark should be weakly interacting, coupling to the Z with sufficient strength to have been detected at the current resolution of the LEP experiments which have cornered the market on Z physics in the last several years. (2) From the absence of top quark decay products in the reaction p + p-bar -> t + t-bar -> hard leptons + X at Fermilab's Tevatron collider, the CDF (Collider Detector at Fermilab) experiment. Each top quark is expect to decay into a W boson and a b quark. Each W subsequently decays into either a charged lepton and a neutrino or two quarks. The cleanest signature for the production and decay of the t-t-bar pair is the presence of two high-transverse-momentum (high Pt) leptons (electron or muon) in the final state. Other decay modes have higher branching ratios, but have serious experimental backgrounds from W bosons produced in association with jets. The current lower limit on M(t) from such measurements is 91 GeV (95% confidence), 95 GeV (90% confidence). However, these limits assume that the top quark has the expected decay products in the expected branching ratios, making these limits "model dependent," and consequently not as "hard" as the considerably lower LEP limit of ~45 GeV. The future is very bright for detecting the top quark. LEP II, the upgrade of CERN's e+e- collider to E >= 2*Mw = 160 GeV by 1994, will allow a hard lower limit of roughly 90 GeV to be set. Meanwhile, upgrades to CDF, start of a new experiment, D0, and upgrades to the accelerator complex at Fermilab have recently allowed higher event rates and better detector resolution, should allow production of standard model top quarks of mass < 150 GeV in the next two years, and even higher mass further in the future, at high enough event rate to identify the decays and give rough mass measurements. References: Phys. Rev. Lett. _68_, 447 (1992) and the references therein. ******************************************************************************** Item 6. Tachyons updated: 4-May-1992 by SIC -------- There was a young lady named Bright, Whose speed was far faster than light. She went out one day, In a relative way, And returned the previous night! -Reginald Buller It is a well known fact that nothing can travel faster than the speed of light. At best, a massless particle travels at the speed of light. But is this really true? In 1962, Bilaniuk, Deshpande, and Sudarshan, Am. J. Phys. _30_, 718 (1962), said "no". A very readable paper is Bilaniuk and Sudarshan, Phys. Today _22_,43 (1969). I give here a brief overview. Draw a graph, with momentum (p) on the x-axis, and energy (E) on the y-axis. Then draw the "light cone", two lines with the equations E = +/- p. This divides our 1+1 dimensional space-time into two regions. Above and below are the "timelike" quadrants, and to the left and right are the "spacelike" quadrants. Now the fundamental fact of relativity is that E^2 - p^2 = m^2. (Let's take c=1 for the rest of the discussion.) For any non-zero value of m (mass), this is an hyperbola with branches in the timelike regions. It passes through the point (p,E) = (0,m), where the particle is at rest. Any particle with mass m is constrained to move on the upper branch of this hyperbola. (Otherwise, it is "off-shell", a term you here in association with virtual particles - but that's another topic.) For massless particles, E^2 = p^2, and the particle moves on the light-cone. These two cases are given the names tardyon (or bradyon in more modern usage) and luxon, for "slow particle" and "light particle". Tachyon is the name given to the supposed "fast particle" which would move with v>c. Now another familiar relativistic equation is E = m*[1-(v/c)^2]^(-.5). Tachyons (if they exist) have v > c. This means that E is imaginary! Well, what if we take the rest mass m, and take it to be imaginary? Then E is negative real, and E^2 - p^2 = m^2 < 0. Or, p^2 - E^2 = M^2, where M is real. This is a hyperbola with branches in the spacelike region of spacetime. Tachyons are constrained to move on this hyperbola. You can now deduce many interesting properties of tachyons. For example, they accelerate (p goes up) if they lose energy (E goes down). Futhermore, a zero-energy tachyon is "transcendent," or infinitely fast. This has profound consequences. For example, let's say that there are electrically charged tachyons. Since they move faster than the speed of light in the vacuum, they produce Cerenkov radiation. This lowers their energy, and they accelerate. So any charged tachyon in the region of spacetime where you might choose to put a "charged tachyon detector" will quickly accelerate off to the edge of the universe, to be lost forever. You will never find a charged tachyon, whether they exist or not. However, tachyons are not entirely invisible. You can imagine that you might produce them in some exotic nuclear reaction. If they are charged, you could "see" them by detecting the Cerenkov light they produce as they speed away faster and faster. Such experiments have been done. So far, no tachyons have been found. Even neutral tachyons can scatter off normal matter with experimentally observable consequences. Again, no such tachyons have been found. Once you move away from relativistic kinematics and start talking about the quantum field theory or particle physics of tachyons, things get much more complicated. It is not easy to summarize results here. However, one reasonably modern reference is _Tachyons, Monopoles, and Related Topics_, E. Recami, ed. (North-Holland, Amsterdam, 1978). One little-publicized fact is that in the framework of field theory, one CANNOT transmit information faster than the speed of light with tachyons. Since this may be controversial let us be more precise. It's easiest to begin by looking at the wave equation for a free scalar particle, the so-called Klein-Gordon equation: (BOX + m^2)phi = 0 where BOX is the D'Alembertian, which in 1+1 dimensions is just BOX = (d/dt)^2 - (d/dx)^2. (For four-dimensional space-time just throw in -(d/dy)^2 -(d/dz)^2.) In field theory, noninteracting massive particles (tardyons) are described by this equation with the mass m being real. Non-interacting tachyons would be described by this equation with m imaginary. Regardless of m, any solution is a linear combination, or superposition, of solutions of the form exp(-iEt + ipx) where E^2 - p^2 = m^2. By actually solving the equation this way, one notices a strange thing. If the solution phi and its time derivative are zero outside the interval [-L,L] when t = 0, they will be zero outside the interval [-L-|t|, L+|t|] at any time t. In other words, disturbances do not spread with speed faster than 1 (the speed of light). However, there are lots of problems with tachyons in quantum field theory. A lot of mathematically rigorous work on quantum field theory uses the Garding-Wightman axioms for quantum fields. These rule out tachyons for other reasons because they require that all states satisfy E^2 - p^2 >= 0. This allows one to define the vacuum as the state minimizing E^2 - p^2 (required by these axioms to be unique). As described above, theories with tachyons violate this axiom. In fact, if one has a bunch of tachyons around, one can make E^2 - p^2 as negative as you like. Heuristically, this is bad because it means that the vacuum is unstable: spontaneous creation of tachyon-antitachyon pairs will tend to occur, reducing the total energy of the system. ******************************************************************************** Item 7. Special Relativistic Paradoxes - part (a) The Barn and the Pole updated 4-AUG-1992 by SIC --------------------- original by Robert Firth These are the props. You own a barn, 40m long, with automatic doors at either end, that can be opened and closed simultaneously by a switch. You also have a pole, 80m long, which of course won't fit in the barn. Now someone takes the pole and tries to run (at nearly the speed of light) through the barn with the pole horizontal. Special Relativity (SR) says that a moving object is contracted in the direction of motion: this is called the Lorentz Contraction. So, if the pole is set in motion lengthwise, then it will contract in the reference frame of a stationary observer. You are that observer, sitting on the barn roof. You see the pole coming towards you, and it has contracted to a bit less than 40m. So, as the pole passes through the barn, there is an instant when it is completely within the barn. At that instant, you close both doors. Of course, you open them again pretty quickly, but at least momentarily you had the contracted pole shut up in your barn. The runner emerges from the far door unscathed. But consider the problem from the point of view of the runner. She will regard the pole as stationary, and the barn as approaching at high speed. In this reference frame, the pole is still 80m long, and the barn is less than 20 meters long. Surely the runner is in trouble if the doors close while she is inside. The pole is sure to get caught. Well does the pole get caught in the door or doesn't it? You can't have it both ways. This is the "Barn-pole paradox." The answer is buried in the misuse of the word "simultaneously" back in the first sentence of the story. In SR, that events separated in space that appear simultaneous in one frame of reference need not appear simultaneous in another frame of reference. The closing doors are two such separate events. SR explains that the two doors are never closed at the same time in the runner's frame of reference. So there is always room for the pole. In fact, the Lorentz transformation for time is t'=(t-v*x/c^2)/sqrt(1-v^2/c^2). It's the v*x term in the numerator that causes the mischief here. In the runner's frame the further event (larger x) happens earlier. The far door is closed first. It opens before she gets there, and the near door closes behind her. Safe again - either way you look at it, provided you remember that simultaneity is not a constant of physics. References: Taylor and Wheeler's _Spacetime Physics_ is the classic. Feynman's _Lectures_ are interesting as well. ******************************************************************************** Item 7. Special Relativistic Paradoxes - part (b) The Twin Paradox updated 17-AUG-1992 by SIC ---------------- original by Kurt Sonnenmoser A Short Story about Space Travel: Two twins, conveniently named A and B, both know the rules of Special Relativity. One of them, B, decides to travel out into space with a velocity near the speed of light for a time T, after which she returns to Earth. Meanwhile, her boring sister A sits at home posting to Usenet all day. When A finally comes home, what do the two sisters find? Special Relativity (SR) tells A that time was slowed down for the relativistic sister, B, so that upon her return to Earth, she knows that B will be younger than she is, which she suspects was the the ulterior motive of the trip from the start. But B sees things differently. She took the trip just to get away from the conspiracy theorists on Usenet, knowing full well that from her point of view, sitting in the spaceship, it would be her sister, A, who was travelling ultrarelativistically for the whole time, so that she would arrive home to find that A was much younger than she was. Unfortunate, but worth it just to get away for a while. What are we to conclude? Which twin is really younger? How can SR give two answers to the same question? How do we avoid this apparent paradox? Maybe twinning is not allowed in SR? Read on. Paradox Resolved: Much of the confusion surrounding the so-called Twin Paradox originates from the attempts to put the two twins into different frames --- without the useful concept of the proper time of a moving body. SR offers a conceptually very clear treatment of this problem. First chose _one_ specific inertial frame of reference; let's call it S. Second define the paths that A and B take, their so-called world lines. As an example, take (ct,0,0,0) as representing the world line of A, and (ct,f(t),0,0) as representing the world line of B (assuming that the the rest frame of the Earth was inertial). The meaning of the above notation is that at time t, A is at the spatial location (x1,x2,x3)=(0,0,0) and B is at (x1,x2,x3)=(f(t),0,0) --- always with respect to S. Let us now assume that A and B are at the same place at the time t1 and again at a later time t2, and that they both carry high-quality clocks which indicate zero at time t1. High quality in this context means that the precision of the clock is independent of acceleration. [In principle, a bunch of muons provides such a device (unit of time: half-life of their decay).] The correct expression for the time T such a clock will indicate at time t2 is the following [the second form is slightly less general than the first, but it's the good one for actual calculations]: t2 t2 _______________ / / / 2 | T = | d\tau = | dt \/ 1 - [v(t)/c] (1) / / t1 t1 where d\tau is the so-called proper-time interval, defined by 2 2 2 2 2 (c d\tau) = (c dt) - dx1 - dx2 - dx3 . Furthermore, d d v(t) = -- (x1(t), x2(t), x3(t)) = -- x(t) dt dt is the velocity vector of the moving object. The physical interpretation of the proper-time interval, namely that it is the amount the clock time will advance if the clock moves by dx during dt, arises from considering the inertial frame in which the clock is at rest at time t --- its so-called momentary rest frame (see the literature cited below). [Notice that this argument is only of a heuristic value, since one has to assume that the absolute value of the acceleration has no effect. The ultimate justification of this interpretation must come from experiment.] The integral in (1) can be difficult to evaluate, but certain important facts are immediately obvious. If the object is at rest with respect to S, one trivially obtains T = t2-t1. In all other cases, T must be strictly smaller than t2-t1, since the integrand is always less than or equal to unity. Conclusion: the traveling twin is younger. Furthermore, if she moves with constant velocity v most of the time (periods of acceleration short compared to the duration of the whole trip), T will approximately be given by ____________ / 2 | (t2-t1) \/ 1 - [v/c] . (2) The last expression is exact for a round trip (e.g. a circle) with constant velocity v. [At the times t1 and t2, twin B flies past twin A and they compare their clocks.] Now the big deal with SR, in the present context, is that T (or d\tau, respectively) is a so-called Lorentz scalar. In other words, its value does not depend on the choice of S. If we Lorentz transform the coordinates of the world lines of the twins to another inertial frame S', we will get the same result for T in S' as in S. This is a mathematical fact. It shows that the situation of the traveling twins cannot possibly lead to a paradox _within_ the framework of SR. It could at most be in conflict with experimental results, which is also not the case. Of course the situation of the two twins is not symmetric, although one might be tempted by expression (2) to think the opposite. Twin A is at rest in one and the same inertial frame for all times, whereas twin B is not. [Formula (1) does not hold in an accelerated frame.] This breaks the apparent symmetry of the two situations, and provides the clearest nonmathematical hint that one twin will in fact be younger than the other at the end of the trip. To figure out *which* twin is the younger one, use the formulae above in a frame in which they are valid, and you will find that B is in fact younger, despite her expectations. It is sometimes claimed that one has to resort to General Relativity in order to "resolve" the Twin "Paradox". This is not true. In flat, or nearly flat space-time (no strong gravity), SR is completely sufficient, and it has also no problem with world lines corresponding to accelerated motion. References: Taylor and Wheeler, _Spacetime Physics_ (An *excellent* discussion) Goldstein, _Classical Mechanics_, 2nd edition, Chap.7 (for a good general discussion of Lorentz transformations and other SR basics.) ******************************************************************************** Item 8. The Particle Zoo updated 9-OCT-1992 by SIC ---------------- original by Matt Austern If you look in the Particle Data Book, you will find more than 150 particles listed there. It isn't quite as bad as that, though... The particles are in three categories: leptons, mesons, and baryons. Leptons are particle that are like the electron: they are spin-1/2, and they do not undergo the strong interaction. There are three charged leptons, the electron, muon, and tau, and three neutral leptons, or neutrinos. (The muon and the tau are both short-lived.) Mesons and baryons both undergo strong interactions. The difference is that mesons have integral spin (0, 1,...), while baryons have half-integral spin (1/2, 3/2,...). The most familiar baryons are the proton and the neutron; all others are short-lived. The most familiar meson is the pion; its lifetime is 26 nanoseconds, and all other mesons decay even faster. Most of those 150+ particles are mesons and baryons, or, collectively, hadrons. The situation was enormously simplified in the 1960s by the "quark model," which says that hadrons are made out of spin-1/2 particles called quarks. A meson, in this model, is made out of a quark and an anti-quark, and a baryon is made out of three quarks. We don't see free quarks (they are bound together too tightly), but only hadrons; nevertheless, the evidence for quarks is compelling. Quark masses are not very well defined, since they are not free particles, but we can give estimates. The masses below are in GeV; the first is current mass and the second constituent mass (which includes some of the effects of the binding energy): Generation: 1 2 3 U-like: u=.006/.311 c=1.50/1.65 t=91-200/91-200 D-like: d=.010/.315 s=.200/.500 b=5.10/5.10 In the quark model, there are only 12 elementary particles, which appear in three "generations." The first generation consists of the up quark, the down quark, the electron, and the electron neutrino. (Each of these also has an associated antiparticle.) These particle make up all of the ordinary matter we see around us. There are two other generations, which are essentially the same, but with heavier particles. The second consists of the charm quark, the strange quark, the muon, and the muon neutrino; and the third consists of the top quark, the bottom quark, the tau, and the tau neutrino. (The top has not been directly observed; see the "Top Quark" FAQ entry for details.) These three generations are sometimes called the "electron family", the "muon family", and the "tau family." Finally, according to quantum field theory, particles interact by exchanging "gauge bosons," which are also particles. The most familiar on is the photon, which is responsible for electromagnetic interactions. There are also eight gluons, which are responsible for strong interactions, and the W+, W-, and Z, which are responsible for weak interactions. The picture, then, is this: FUNDAMENTAL PARTICLES OF MATTER Charge ------------------------- -1 | e | mu | tau | 0 | nu(e) |nu(mu) |nu(tau)| ------------------------- + antiparticles -1/3 | down |strange|bottom | 2/3 | up | charm | top | ------------------------- GAUGE BOSONS Charge Force 0 photon electromagnetism 0 gluons (8 of them) strong force +-1 W+ and W- weak force 0 Z weak force The Standard Model of particle physics also predict the existence of a "Higgs boson," which has to do with breaking a symmetry involving these forces, and which is responsible for the masses of all the other particles. It has not yet been found. More complicated theories predict additional particles, including, for example, gauginos and sleptons and squarks (from supersymmetry), W' and Z' (additional weak bosons), X and Y bosons (from GUT theories), Majorons, familons, axions, paraleptons, ortholeptons, technipions (from technicolor models), B' (hadrons with fourth generation quarks), magnetic monopoles, e* (excited leptons), etc. None of these "exotica" have yet been seen. The search is on! REFERENCES: The best reference for information on which particles exist, their masses, etc., is the Particle Data Book. It is published every two years; the most recent edition is Physical Review D Vol.45 No.11 (1992). There are several good books that discuss particle physics on a level accessible to anyone who knows a bit of quantum mechanics. One is _Introduction to High Energy Physics_, by Perkins. Another, which takes a more historical approach and includes many original papers, is _Experimental Foundations of Particle Physics_, by Cahn and Goldhaber. For a book that is accessible to non-physicists, you could try _The Particle Explosion_ by Close, Sutton, and Marten. This book has fantastic photography. ******************************************************************************** Item 9. Olbers' Paradox updated: 2-JUL-1992 by SIC --------------- Why isn't the night sky as uniformly bright as the surface of the Sun? If the Universe has infinitely many stars, then it should be. After all, if you move the Sun twice as far away from us, we will intercept one-fourth as many photons, but the Sun will subtend one-fourth of the angular area. So the areal intensity remains constant. With infinitely many stars, every angular element of the sky should have a star, and the entire heavens should be a bright as the sun. We should have the impression that we live in the center of a hollow black body whose temperature is about 6000 degrees Centigrade. This is Olbers' paradox. It can be traced as far back as Kepler in 1610. It was rediscussed by Halley and Cheseaux in the eighteen century, but was not popularized as a paradox until Olbers took up the issue in the nineteenth century. There are many possible explanations which have been considered. Here are a few: (1) There's too much dust to see the distant stars. (2) The Universe has only a finite number of stars. (3) The distribution of stars is not uniform. So, for example, there could be an infinitely of stars, but they hide behind one another so that only a finite angular area is subtended by them. (4) The Universe is expanding, so distant stars are red-shifted into obscurity. (5) The Universe is young. Distant light hasn't even reached us yet. The first explanation is just plain wrong. In a black body, the dust will heat up too. It does act like a radiation shield, exponentially damping the distant starlight. But you can't put enough dust into the universe to get rid of enough starlight without also obscuring our own Sun. So this idea is bad. The second might have been correct, but estimates of the total matter in the universe are too large to allow this escape. The number of stars is close enough to infinite for the purpose of lighting up the sky. The third explanation might be partially correct. We just don't know. If the stars are distributed fractally, then there could be large patches of empty space, and the sky could appear dark except in small areas. But the final two possibilities are are surely each correct and partly responsible. There are numerical arguments that suggest that the effect of the finite age of the Universe is the larger effect. We live inside a spherical shell of "Observable Universe" which has radius equal to the lifetime of the Universe. Objects more than about 15 billions years old are too far away for their light ever to reach us. Historically, after Hubble discovered that the Universe was expanding, but before the Big Bang was firmly established by the discovery of the cosmic background radiation, Olbers' paradox was presented as proof of special relativity. You needed the red-shift (an SR effect) to get rid of the starlight. This effect certainly contributes. But the finite age of the Universe is the most important effect. References: Ap. J. _367_, 399 (1991). The author, Paul Wesson, is said to be on a personal crusade to end the confusion surrounding Olbers' paradox. _Darkness at Night: A Riddle of the Universe_, Edward Harrison, Harvard University Press, 1987 ******************************************************************************** Item 10. What is Dark Matter? updated 11-May-1991 by SIC -------------------- The story of dark matter is best divided into two parts. First we have the reasons that we know that it exists. Second is the collection of possible explanations as to what it is. Why the Universe Needs Dark Matter ---------------------------------- We believe that that the Universe is critically balanced between being open and closed. We derive this fact from the observation of the large scale structure of the Universe. It requires a certain amount of matter to accomplish this result. Call it M. You can estimate the total BARYONIC matter of the universe by studying big bang nucleosynthesis. The more matter in the universe, the more slowly the universe should have expanded shortly after the big bang. The longer the "cooking time" allowed, the higher the production of helium from primordial hydrogen. We know the He/H ratio of the universe, so we can estimate how much baryonic matter exists in the universe. It turns out that you need about 0.05 M total baryonic matter to account for the known ratio of light isotopes. So only 1/20 of the total mass of they Universe is baryonic matter. Unfortunately, the best estimates of the total mass of everything that we can see with our telescopes is roughly 0.01 M. Where is the other 99% of the stuff of the Universe? Dark Matter! So there are two conclusions. We only see 0.01 M out of 0.05 M baryonic matter in the Universe. The rest must be in baryonic dark matter halos surrounding galaxies. And there must be some non-baryonic dark matter to account for the remaining 95% of the matter required to give omega, the mass of universe, in units of critical mass, equal to unity. For those who distrust the conventional Big Bang models, and don't want to rely upon fancy cosmology to derive the presence of dark matter, there are other more direct means. It has been observed in clusters of galaxies that the motion of galaxies within a cluster suggests that they are bound by a total gravitational force due to about 5-10 times as much matter as can be accounted for from luminous matter in said galaxies. And within an individual galaxy, you can measure the rate of rotation of the stars about the galactic center of rotation. The resultant "rotation curve" is simply related to the distribution of matter in the galaxy. The outer stars in galaxies seem to rotate too fast for the amount of matter that we see in the galaxy. Again, we need about 5 times more matter than we can see via electromagnetic radiation. These results can be explained by assuming that there is a "dark matter halo" surrounding every galaxy. What is Dark Matter ------------------- This is the open question. There are many possibilities, and nobody really knows much about this yet. Here are a few of the many published suggestions, which are being currently hunted for by experimentalists all over the world: (1) Normal matter which has so far eluded our gaze, such as (a) dark galaxies (b) brown dwarfs (c) planetary material (rock, dust, etc.) (2) Massive Standard Model neutrinos. If any of the neutrinos are massive, then this could be the missing mass. Note that the possible 17 KeV tau neutrino would give far too much mass creating almost as many problems as it solves in this regard. (3) Exotica (See the "Particle Zoo" FAQ entry for some details) Massive exotica would provide the missing mass. For our purposes, these fall into two classes: those which have been proposed for other reasons but happen to solve the dark matter problem, and those which have been proposed specifically to provide the missing dark matter. Examples of objects in the first class are axions, additional neutrinos, supersymmetric particles, and a host of others. Their properties are constrained by the theory which predicts them, but by virtue of their mass, they solve the dark matter problem if they exist in the correct abundance. Particles in the second class are generally classed in loose groups. Their properties are not specified, but they are merely required to be massive and have other properties such that they would so far have eluded discovery in the many experiments which have looked for new particles. These include WIMPS (Weakly Interacting Massive Particles), CHAMPS, and a host of others. References: _Dark Matter in the Universe_ (Jerusalem Winter School for Theoretical Physics, 1986-7), J.N. Bahcall, T. Piran, & S. Weinberg editors. _Dark Matter_ (Proceedings of the XXIIIrd Recontre de Moriond) J. Audouze and J. Tran Thanh Van. editors. ******************************************************************************** Item 11. Hot Water Freezes Faster than Cold! updated 11-May-1992 by SIC ----------------------------------- original by Richard M. Mathews You put two pails of water outside on a freezing day. One has hot water (95 degrees C) and the other has an equal amount of colder water (50 degrees C). Which freezes first? The hot water freezes first! Why? It is commonly argued that the hot water will take some time to reach the initial temperature of the cold water, and then follow the same cooling curve. So it seems at first glance difficult to believe that the hot water freezes first. The answer lies mostly in evaporation. The effect is definitely real and can be duplicated in your own kitchen. Every "proof" that hot water can't freeze faster assumes that the state of the water can be described by a single number. Remember that temperature is a function of position. There are also other factors besides temperature, such as motion of the water, gas content, etc. With these multiple parameters, any argument based on the hot water having to pass through the initial state of the cold water before reaching the freezing point will fall apart. The most important factor is evaporation. The cooling of pails without lids is partly Newtonian and partly by evaporation of the contents. The proportions depend on the walls and on temperature. At sufficiently high temperatures evaporation is more important. If equal masses of water are taken at two starting temperatures, more rapid evaporation from the hotter one may diminish its mass enough to compensate for the greater temperature range it must cover to reach freezing. The mass lost when cooling is by evaporation is not negligible. In one experiment, water cooling from 100C lost 16% of its mass by 0C, and lost a further 12% on freezing, for a total loss of 26%. The cooling effect of evaporation is twofold. First, mass is carried off so that less needs to be cooled from then on. Also, evaporation carries off the hottest molecules, lowering considerably the average kinetic energy of the molecules remaining. This is why "blowing on your soup" cools it. It encourages evaporation by removing the water vapor above the soup. Thus experiment and theory agree that hot water freezes faster than cold for sufficiently high starting temperatures, if the cooling is by evaporation. Cooling in a wooden pail or barrel is mostly by evaporation. In fact, a wooden bucket of water starting at 100C would finish freezing in 90% of the time taken by an equal volume starting at room temperature. The folklore on this matter may well have started a century or more ago when wooden pails were usual. Considerable heat is transferred through the sides of metal pails, and evaporation no longer dominates the cooling, so the belief is unlikely to have started from correct observations after metal pails became common. References: "Hot water freezes faster than cold water. Why does it do so?", Jearl Walker in The Amateur Scientist, Scientific American, Vol. 237, No. 3, pp 246-257; September, 1977. "The Freezing of Hot and Cold Water", G.S. Kell in American Journal of Physics, Vol. 37, No. 5, pp 564-565; May, 1969. ******************* END OF FAQ PART 1/2 ******************** Date: Wed, 30 Dec 92 11:10:39 PST From: sichase@Csa2.LBL.Gov Message-Id: <921230111039.20c00e8f@csa2.lbl.gov> Subject: Sci.Physics Frequently Asked Questions - January 1993 - Part 2/2 To: distribution:@Csa2.LBL.Gov; (see end of body) X-St-Vmsmail-To: @[-]MAILING_LIST.FAQ Archive-name: physics-faq Last-modified: 1992/12/26 -------------------------------------------------------------------------------- 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