F.A.Q's: Particle 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 is there more matter than antimatter, at least around here?  Is there really more matter than antimatter throughout the universe?  This seems related to the previous question, since most attempts at explaining the prevalence of matter over antimatter make use of CP violation.
3. Are there really just three generations of leptons and quarks?  If so, why?  For example, the muon is a particle almost exactly like the electron except much heavier, and the tau particle is also almost the same, but heavier still.  Why do these three exist and no more?  Or, are these unanswerable questions?
   

   Besides the particles that carry forces (the photon, W and Z boson, and gluons), all elementary particles we have seen so far fit neatly into three "generations" of particles called leptons and quarks.  The first generation consists of:
   

   • the electron
   • the electron neutrino
   • the up quark
   • the down quark

   
   The second consists of:
   
   

   • the muon
   • the muon neutrino
   • the charmed quark
   • the strange quark

   
   and the third consists of:
   
   

   • the tau
   • the tau neutrino
   • the top quark
   • the bottom quark

   
   How do we know there aren't more?
   Ever since particle accelerators achieved the ability to create Z bosons in 1983, our best estimates on the number of generations have come from measuring the rate at which Z bosons decay into completely invisible stuff.  The underlying assumption is that when this happens, the Z boson is decaying into a neutrino-antineutrino pair as predicted by the Standard Model.  Each of the three known generations contains a neutrino which is very light.  If this pattern holds up, the total rate of "decay into invisible stuff" should be proportional to the number of generations!
   Experiments like this keep indicating there are three generations of this sort.  So, most physicists feel sure there are exactly three generations of quarks and leptons.  The question then becomes "why?"—and so far we haven't a clue!
   For details see:
   

   • D. Karlen, The number of neutrino types from collider experiments, revised August 2001.

   Honesty compels us to point out a slight amount of wiggle room in the remarks above.  Conservation of energy prevents the Z from decaying into a neutrino-antineutrino pair if the neutrino in question is of a sort that has more than half the mass of Z.  So, if there were a fourth generation with a very heavy neutrino, we couldn't detect it by studying the decay of Z bosons.  However, all three known neutrinos have a mass less than 1/3000 times the Z mass, so a fourth neutrino would have to be much heavier than the rest to escape detection this way.
   Another bit of wiggle room lurks in the phrase "decaying into a neutrino-antineutrino pair in the manner predicted by the Standard Model".  If there were a fourth generation with a neutrino that didn't act like the other three, or no neutrino at all, we might not see it.  However, in this case it would be stretching language a bit to speak of a "fourth generation", since the marvelous thing about the three known generations is how they're completely identical except for the values of certain constants like masses.

4. Why does each generation of particles have precisely this structure: two leptons and two quarks?
   

   If you're familiar with particle physics, you'll know it goes much deeper than this: the Standard Model says every generation of particles has precisely the same mathematical structure except for some numbers that describe Higgs couplings.  We don't know any reason for this structure, although the requirement of "anomaly cancellation" puts some limits on what it can be. If you're not an expert on particle physics, perhaps these introductions to the Standard Model will help explain things:
   

   • Wikipedia, The Standard Model.
   • M. J. Herrero, The Standard Model.

   The second is much more detailed and technical than the first.

5. Do the quarks or leptons have any substructure, or are they truly elementary particles?
6. Is there really a Higgs boson, as predicted by the Standard Model of particle physics?  If so, what is its mass?  If not, what breaks the symmetry between the electromagnetic and weak forces, and gives all the elementary particles their masses?
   

   The Standard Model predicts the existence of a spin-0 particle called the Higgs boson, which comes in two isospin states, one with charge +1 and one neutral.  (It also predicts that this particle has an antiparticle.)  According to the Standard Model, the interaction of the Higgs boson with the electroweak force is responsible for a "spontaneous symmetry breaking" process that makes this force act like two very different forces: the electromagnetic force and the weak force.  Moreover, it is primarily the interaction of the Higgs boson with the other particles in the Standard Model that endows them with their masses!  The Higgs boson is very mysterious, because in addition to doing all these important things, it stands alone, very different from all the other particles.  For example, it is the only spin-0 particle in the Standard Model.  To add to the mystery, it is the only particle in the Standard Model that has not yet been directly detected!On the 4th of July, 2012, two experimental teams looking for the Higgs boson at the Large Hadron Collider (LHC) announced the discovery of a previously unknown boson with mass of roughly 125-126 GeV/c². Using the combined analysis of two interaction types, these experiments reached a statistical significance of 5 sigma, meaning that if no such boson existed, the chance of seeing what they was less than 1 in a million.
   However, it has not yet been confirmed that this boson behaves as the Standard Model predicts of the Higgs. Some particle physicists hope that the Higgs boson, when seen, will work a bit differently than the Standard Model predicts.  For example, some variants of the Standard Model predict more than one type of Higgs boson.  LHC may also discover other new phenomena when it starts colliding particles at energies higher than ever before explored.  For example, it could find evidence for supersymmetry, providing indirect support for superstring theory.
   So, stay tuned.  But meanwhile, try these:
   

   • Wikipedia, Higgs boson.
   • Compact Muon Solenoid (CMS) Experiment, Observation of a new particle with a mass of 125 GeV.
   • ATLAS Experiment, Latest results from ATLAS Higgs search, 4 July 2012.

7. What is the correct theory of neutrinos?  Why are they almost but not quite massless?  Do all three known neutrinos—electron, muon, and tau—all have a mass?  Could any neutrinos be Majorana spinors?  Is there a fourth kind of neutrino, such as a "sterile" neutrino?
   

   Starting in the 1990s, our understanding of neutrinos has dramatically improved, and the puzzle of why we see about 1/3 as many electron neutrinos coming from the sun as naively expected has pretty much been answered: the different neutrinos can turn into each other via a process called "oscillation".  But, there are still lots of loose ends.  For details, try:
   

   • The Neutrino Oscillation Industry.
   • John Baez, Neutrinos and the Mysterious Maki-Nakagawa-Sakata Matrix.
   • Paul Langacker, Implications of Neutrino Mass.
   • Boris Kayser, Neutrino Mass: Where Do We Stand, and Where Are We Going?

   The first of these has lots of links to the web pages of research groups doing experiments on neutrinos.  It's indeed a big industry!

8. Is quantum chromodynamics (QCD) a precise description of the behavior of quarks and gluons?  Can we prove using QCD that quarks and gluons are confined at low temperatures?  Is it possible to calculate masses of hadrons (such as the proton, neutron, pion, etc.) correctly from the Standard Model, with the help of QCD?  Does QCD predict that quarks and gluons become deconfined and form plasma at high temperature?  If so, what is the nature of the deconfinement phase transition?  Does this really happen in Nature?
   

   Most physicists believe the answers to all these questions are "yes".  There are currently a number of experiments going on to produce and detect a quark-gluon plasma.  It's believed that producing such a plasma at low pressures requires a temperature of 2 million million kelvins.  Since this is 10,000 times hotter than the sun, and such extreme temperatures were last prevalent in our Universe only 1 microsecond after the Big Bang, these experiments are lots of fun.  The largest, the Relativistic Heavy Ion Collider on Long Island, New York, began operation in 2000.  It works by slamming gold nuclei together at outrageous speeds.  For details, see:

   • Relativistic Heavy Ion Collider, or RHIC

   But, in addition to such experimental work, a lot of high-powered theoretical work is needed to understand just what QCD predicts, both in extreme situations like these, and for ordinary matter.  In fact, it's a great challenge to use QCD to predict the masses of protons, neutrons, pions and the like to an accuracy greater than about 10%.  Doing so makes heavy use of supercomputers, but there are also fundamental obstacles to good numerical computations, like the "fermion doubling problem", where bright new ideas are needed.  See for example:

   • European Union IHP Network on Hadron Phenomenology from Lattice QCD.
   • Rajan Gupta, Introduction to Lattice QCD.

9. Is there a mathematically rigorous formulation of a 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?  Even the classical electrodynamics of point particles does not yet have a satisfactory mathematically rigorous formulation.  Does one exist or is this theory inconsistent?
   

   These are questions of mathematical physics rather than physics per se, but they are important.  At the turn of the millennium, the Clay Mathematics Institute offered a $1,000,000 prize for providing a mathematically rigorous foundation for the quantum version of SU(2) Yang-Mills theory in four spacetime dimensions, and proving that there's a "mass gap"—meaning that the lightest particle in this theory has nonzero mass.  For details see:
   

   • Clay Mathematics Institute, Yang-Mills and Mass Gap.

10. Is the proton really stable, or does it eventually decay?
    

    Most "grand unified theories" (GUTs) predict that the proton decays, but so far experiments have (for the most part) only put lower limits on the proton lifetime.  As of 2002, the lower limit on the mean life of the proton was somewhere between 10³¹ and 10³³ years, depending on the presumed mode of decay, or 1.6 x 10²⁵ years regardless of the mode of decay. Proton decay experiments are heroic undertakings, involving some truly huge apparatus.  Right now the biggest one is "Super-Kamiokande".  This was built in 1995, a kilometer underground in the Mozumi mine in Japan.  This experiment is mainly designed to study neutrinos, but it doubles as a proton decay detector.  It consists of a tank holding 50,000 tons of pure water, lined with 11,200 photomultiplier tubes which can detect very small flashes of light.  Usually these flashes are produced by neutrinos and various less interesting things (the tank is deep underground to minimize the effect of cosmic rays).  But, flashes of light would also be produced by certain modes of proton decay, if this ever happens.
    Super-Kamiokande was beginning to give much improved lower bounds on the proton lifetime, and excellent information on neutrino oscillations, when a strange disaster happened on November 12, 2001.  The tank was being refilled with water after some burnt-out photomultiplier tubes had been replaced.  Workmen standing on styrofoam pads on top of some of the bottom tubes made small cracks in the neck of one of the tubes, causing that tube to implode.  The resulting shock wave started a chain reaction in which about 7,000 of the photomultiplier tubes were destroyed!  Luckily, after lots of hard work the experiment was rebuilt by December 2002.
    In 2000, after about 20 years of operation, the Kolar Mine proton decay experiment claimed to have found proton decay, and their team of physicists gave an estimate of 10³¹ years for the proton lifetime.  Other teams are skeptical.
    For more details, try these:
    

    • Super-Kamiokande photo album and photographs of the accident.
    • Super-Kamiokande US collaboration home page.
    • The IMB Proton Decay Experiment.
    • H. Adarkar et al, Experimental Evidence for G.U.T. Proton Decay.
    • Jogesh C. Pati, With Grand Unification Signals in, Can Proton Decay be Far Behind?

11. Why do the particles have the precise masses they do?  Or is this an unanswerable question?
    

    Of course their mass in kilograms depends on an arbitrary human choice of units, but their mass ratios are fundamental constants of nature.  For example, the muon is about 206.76828 times as heavy as the electron.  We have no explanation of this sort of number!  We attribute the masses of the elementary particles to the strength of their interaction with the Higgs boson (see above), but we have no understanding of why these interactions are as strong as they are.

12. 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, that measures the strength of electromagnetism, about 1/137.036?  Where do such dimensionless constants come from?  Or is this an unanswerable question?
    

    Particle masses and strengths of the fundamental forces constitute most of the 26 fundamental dimensionless constants of nature.  Another one is the cosmological constant—assuming it's constant.  Others govern the oscillation of neutrinos (see below).  So, we can wrap a bunch of open questions into a bundle by asking: Why do these 26 dimensionless constants have the values they do?Perhaps the answer involves the Anthropic Principle, but perhaps not.  Right now, we have no way of knowing that this question has any answer at all!
    For a list of these 26 dimensionless constants, try:
    

    • John Baez, How Many Fundamental Constants Are There?

13. What is the explanation of the Pioneer anomaly?
    

    The Pioneer 10 and Pioneer 11 spacecraft are leaving the the Solar System.  Pioneer 10 sent back radio information about its location until January 2003, when it was about 80 times farther from the Sun than the Earth is.  Pioneer 11 sent back signals until September 1995, when its distance from the Sun was about 45 times the Earth's.The Pioneer missions have yielded the most precise information we have about navigation in deep space.  However, analysis of their radio tracking data indicates a small unexplained acceleration towards the Sun!  The magnitude of this acceleration is roughly 10⁻⁹ meters per second per second.  This is known as the "Pioneer anomaly".
    This anomaly has also been seen in the Ulysses spacecraft, and possibly also in the Galileo spacecraft, though the data is much more noisy, since these were Jupiter probes, hence much closer to the Sun, where there is a lot more pressure from solar radiation.  The Viking mission to Mars did not detect the Pioneer anomaly — and it would have, had an acceleration of this magnitude been present, because its radio tracking was accurate to about 12 meters.
    Many physicists and astronomers have tried to explain the Pioneer anomaly using conventional physics, but so far nobody seems to have succeeded.  There are many proposals that try to explain the anomaly using new physics — in particular, modified theories of gravity.  But there is no consensus that any of these explanations are right, either.  For example, explaining the Pioneer anomaly using dark matter would require more than 0.0003 solar masses of dark matter within 50 astronomical units of the Sun (an astronomical unit is the distance between Sun and Earth).  But, this is in conflict with our calculations of planetary orbits.
    For more information, see:
    

    • Wikipedia, Pioneer Anomaly.
    • Chris P. Duif, Pioneer Anomaly - Literature and Links.
    • The Pioneer Collaboration, A Mission to Explore the Pioneer Anomaly.

14. Are there important aspects of the Universe that can only be understood using the Anthropic Principle?  Or is this principle unnecessary, or perhaps inherently unscientific?
    

    Very roughly speaking, the Anthropic Principle says that our universe must be approximately the way it is for intelligent life to exist, so that the mere fact we are asking certain questions constrains their answers.  This might "explain" the values of fundamental constants of nature, and perhaps other aspects of the laws of physics as well.  Or, it might not.Different ways of making the Anthropic Principle precise, and a great deal of evidence concerning it, can be found in a book by Barrow and Tipler:
    

    • John D. Barrow and Frank J. Tipler, The Cosmological Anthropic Principle, Oxford U. Press, Oxford, 1988.

    This book started a heated debate on the merits of the Anthropic Principle, which continues to this day.  Some people have argued the principle is vacuous.  Others have argued that it distracts us from finding better explanations of the facts of nature, and is thus inherently unscientific.  For one interesting view, see:
    

    • Max Tegmark, Which Mathematical Structure is Isomorphic to Our Universe?

    In 1994 Lee Smolin advocated an alternative but equally mind-boggling idea, namely that the parameters of the Universe are tuned, not to permit intelligent life, but to maximize black hole production!  The mechanism he proposes for this is a kind of cosmic Darwinian evolution, based on the (unproven) theory that universes beget new baby universes via black holes.  For details, see:
    

    • Lee Smolin, The Life of the Cosmos, Crown Press, 1997.
    • Lee Smolin, The Fate of Black Hole Singularities and the Parameters of the Standard Models of Particle Physics and Cosmology.

    More recently, the string theorist Leonard Susskind has argued that the "string theory vacuum" which describes the laws of physics we see must be chosen using the Anthropic Principle:
    

    • Edge: The Landscape—a talk with Leonard Susskind.

15. Do the forces really become unified at sufficiently high energy?
16. Does some version of string theory or M-theory give specific predictions about the behavior of elementary particles?  If so, what are these predictions?  Can we test these predictions in the near future?  And: are they correct?
    

    Despite a huge amount of work on string theory over the last decades, it still has made no predictions that we can check with our particle accelerators, whose failure would falsify the theory.  The closest it comes so far is by predicting the existence of a "superpartner" for each of the observed types of particle.  None of these superpartners have ever been seen.  It is possible that the Large Hadron Collider will detect signs of the lightest superpartner.  It's also possible that dark matter is due to a superpartner!  But, these remain open questions.It's also interesting to see what string theorists regard as the biggest open questions in physics.  At the turn of the millennium, the participants of the conference Strings 2000 voted on the ten most important physics problems.  Here they are:
    

    1. Are all the (measurable) dimensionless parameters that characterize the physical universe calculable in principle or are some merely determined by historical or quantum mechanical accident and uncalculable?
    2. How can quantum gravity help explain the origin of the universe?
    3. What is the lifetime of the proton and how do we understand it?
    4. Is Nature supersymmetric, and if so, how is supersymmetry broken?
    5. Why does the universe appear to have one time and three space dimensions?
    6. Why does the cosmological constant have the value that it has, is it zero and is it really constant?
    7. What are the fundamental degrees of freedom of M-theory (the theory whose low-energy limit is eleven-dimensional supergravity and which subsumes the five consistent superstring theories) and does the theory describe Nature?
    8. What is the resolution of the black hole information paradox?
    9. What physics explains the enormous disparity between the gravitational scale and the typical mass scale of the elementary particles?
    10. Can we quantitatively understand quark and gluon confinement in Quantum Chromodynamics and the existence of a mass gap?

    For details see:
    

    • Strings 2000, Physics Problems for the Next Millennium.

F.A.Q's:Cosmology and Astrophysics

1. What happened at or before the Big Bang?  Was there really an initial singularity?  Does the history of the Universe go back in time forever, or only a finite amount?  Of course, these questions might not make sense, but they might.
2. Are there really three dimensions of space and one of time?  If so, why?  Or is spacetime higher-dimensional, or perhaps not really a manifold at all when examined on a short enough distance scale?  If so, why does it appear to have three dimensions of space and one of time?  Or are these unanswerable questions?
3. Is the Universe infinite in spatial extent?  More generally: what is the topology of space?
   

   We still don't know, but in 2003 some important work was done on this issue:

   • Neil J. Cornish, David N. Spergel, Glenn D. Starkman and Eiichiro Komatsu, Constraining the Topology of the Universe.

   Briefly, the Wilkinson Microwave Anisotropy Probe (WMAP) was used to rule out nontrivial topology within a distance of 78,000 million light years—at least for a large class of models.  For the precise details, you'll have to read the article!

4. Why is there an arrow of time; that is, why is the future so much different from the past?
   

   Here are two pieces of required reading for anyone interested in this tough question:

   • Huw Price, Time's Arrow and Archimedes' Point: New Directions for a Physics of Time, Oxford University Press, Oxford, 1996.
   • H. D. Zeh, The Physical Basis of the Direction of Time, second edition, Springer Verlag, Berlin, 1992.

5. Will the future of the Universe go on forever or not?  Will there be a "big crunch" at some future time, will the Universe keep on expanding forever, or what?
   

   There's been some progress on this one recently.  Starting in the late 1990s, a bunch of evidence has accumulated suggesting that the universe is not slowing down enough to recollapse in a so-called "big crunch".  In fact, it seems that some form of "dark energy" is making the expansion speed up!  We know very little about dark energy; it's really just a name for any invisible stuff that has enough negative pressure compared to its energy density that it tends to make the expansion of the universe tend to accelerate, rather than slow down.  (In general relativity, energy density tends to make the expansion slow down, but negative pressure has the opposite effect.)Einstein introduced dark energy to physics under the name of "the cosmological constant" when he was trying to explain how a static universe could fail to collapse.  This constant simply said what the density dark energy was supposed to be, without providing any explanation for its origin.  When Hubble observed the redshift of light from distant galaxies, and people concluded the universe was expanding, the idea of a cosmological constant fell out of fashion and Einstein called it his "greatest blunder".  But now that the expansion of the universe seems to be accelerating, a cosmological constant or some other form of dark energy seems plausible.
   For an examination of what an ever-accelerating expansion might mean for our universe, see:
   
   

   • John Baez, The End of the Universe.

   
   But, we still can't be sure the universe will expand forever, because the possibility remains that at some point the dark energy will go away, switch sign, or get bigger!  Here's a respectable paper suggesting that the dark energy will change sign and make the universe recollapse in a big crunch:
   

   • Renata Kallosh and Andrei Linde, Dark Energy and the Fate of the Universe.

   
   But here's a respectable paper suggesting the exact opposite: that the dark energy will get bigger and tear apart the universe in a "big rip":
   
   

   • Robert R. Caldwell, Marc Kamionkowski, and Nevin N. Weinberg, Phantom Energy and Cosmic Doomsday.

   
   In short, the ultimate fate of the universe remains an open question!
   But, before you launch into wild speculations, it's worth emphasizing that the late 1990s and early 2000s have seen a real revolution in experimental cosmology, which answered many open questions (for example: "how long ago was the Big Bang?") in shockingly precise ways (about 13,700 million years).  For good introduction to this material, try:
   

   • Ned Wright, Cosmology Tutorial and Frequently Asked Questions in Cosmology. 

   Our evidence concerning the expansion of the universe, dark energy, and dark matter now comes from a wide variety of sources, and what makes us confident we're on the right track is how nicely all this data agrees.  People are getting this data from various sources including:
   

   1. Distant Supernovae.  See especially these two experimental groups:
      • The High-z Supernova Search Team.
        See also their big paper.
      • The Supernova Cosmology Project.
        See also their big paper.
   2. The Cosmic Microwave Background (CMB).  There have been lots of great experiments measuring little ripples in the background radiation left over from the Big Bang.  For example:
      • Balloon Observations of Millimetric Extragalactic Radiation and Geophysics (BOOMERanG)
      • The Millimeter Anisotropy Experiment Imaging Array (MAXIMA)
      • The Wilkinson Microwave Anisotropy Probe (WMAP).
      • The Digital All-Sky Imager (DASI).
      • The Cosmic Background Imager (CBI).
   3. Large-Scale Structure.  Detailed studies of galactic clustering and how it changes with time give us lots of information about dark matter.  Here's the 800-pound gorilla in this field:
      
      • The Sloan Digital Sky Survey.

6. Is the universe really full of "dark energy"?  If so, what causes it?
   

   As mentioned above, evidence has been coming in that suggests the universe is full of some sort of "dark energy" with negative pressure.  For example, an analysis of data from the Wilkinson Microwave Anisotropy Probe in 2003 suggested that 73% of the energy density of the universe is in this form!  But even this is right and dark energy exists, we're still in the dark about what it is.The simplest model is a cosmological constant, meaning that so-called "empty" space actually has a negative pressure and positive energy density, with the pressure exactly equal to minus the energy density in units where the speed of light is 1.  However, nobody has had much luck explaining why empty space should be like this, especially with an energy density as small as what we seem to be observing: about 6 × 10⁻³⁰ grams per cubic centimeter if we use Einstein's E = mc² to convert it into a mass density.  Other widely studied possibilities for dark matter include various forms of "quintessence".  But, this term means little more than "some mysterious field with negative pressure", and there's little understanding of why such a field should exist.
   For more details, try these:
   

   • Ned Wright, Vacuum Energy Density, or: How Can Nothing Weigh Something?
   • John Baez, What's the Energy Density of the Vacuum?
   • Sean Carroll, The Cosmological Constant.

   The third is the most detailed, and it has lots of good references for further study.

7. Why does it seem like the gravitational mass of galaxies exceeds the mass of all the stuff we can see, even taking into account our best bets about invisible stuff like brown dwarfs, "Jupiters", and so on?  Is there some missing "dark matter"?  If so, is it ordinary matter, neutrinos, or something more exotic?  If not, is there some problem with our understanding of gravity, or what?
   

   Since the late 1990s, a consensus has emerged that some sort of "cold dark matter" is needed to explain all sorts of things we see.  For example, in 2003 an analysis of data from the Wilkinson Microwave Anisotropy Probe suggested that the energy density of the universe consists of about 23% cold dark matter, as compared to only 4% ordinary matter.  (The rest is dark energy.)Unfortunately nobody knows what this cold dark matter is!  It probably can't be ordinary matter we've neglected, or neutrinos, since these wouldn't have been sufficiently "cold" in the early universe to collapse into the lumps needed for galaxy formation.  There are many theories about what it might be.  There's also still a possibility that we are very confused about something, like our theory of gravity.
   For details, try these:
   

   • Jonathan Dursi, Dark Matter Tutorial.
   • Martin White, Dark Matter.
   • Timothy J. Sumner, Experimental Searches for Dark Matter.

   The last of these three is the most detailed, and it has lots of references for further study.

8. The Horizon Problem: why is the Universe almost, but not quite, homogeneous on the very largest distance scales?  Is this the result of an "inflationary epoch"—a period of rapid expansion in very early history of the universe, which could flatten out inhomogeneities?  If so, what caused this inflation?
   

   In 2003 the case for inflation was bolstered by the Wilkinson Microwave Anisotropy Probe, which made detailed measurements of "anisotropies" (slight deviations from perfect evenness) in the cosmic microwave background radiation.  The resulting "cosmic microwave background power spectrum" shows peaks and troughs whose precise features should be sensitive to many details of the very early history of the Universe.  Models that include inflation seem to fit this data very well, while those that don't, don't.However, the mechanism behind inflation remains somewhat mysterious.  Inflation can be nicely explained using quantum field theory by positing the existence of a special particle called the "inflaton", which gave rise to extremely high negative pressure before it decayed into other particles.  This may sound wacky, but it's really not.  The only problem is that nobody has any idea how this particle fits into known physics.  For example, it's not part of the Standard Model.
   For details, try:
   

   • Charles H. Lineweaver, Inflation and the Cosmic Microwave Background.
     Also available on the astrophysics arXiv.

9. Why are the galaxies distributed in clumps and filaments?
10. When were the first stars formed, and what were they like?
    

    As of 2004 this was quite a hot topic in astrophysics.  See for example:

    • Volker Bromm and Richard B. Larson, The First Stars.

11. What are Gamma Ray Bursters?
    

    Gamma ray bursters (GRBs) appear as bursts of gamma rays coming from points randomly scattered in the sky.  These bursts are very brief, lasting between a few milliseconds to a few hundred seconds.  For a long time there were hundreds of theories about what caused them, but very little evidence for any of these theories, since nothing was ever seen at the location where one of these bursts occurred.  Their random distribution eventually made a convincing case that they occurred not within our solar system or within our galaxy, but much farther away.  Given this, it was clear that they must be extraordinarily powerful.Starting in the late 1990s, astronomers made a concerted effort to catch gamma ray bursters in the act, focusing powerful telescopes to observe them in the visible and ultraviolet spectrum moments after a burst was detected.  These efforts paid off in 1999 when one was seen to emit visible light for as long as a day after the burst occurred.  A redshift measurement of z = 1.6 indicated that the gamma ray burster was about 10,000 million light years away.  If the burst of gamma rays was omnidirectional, this would mean that its power was about 10¹⁶ times that of our sun—for a very short time.  For details on this discovery, see:
    

    • Burst and Transient Source Experiment (BATSE), GOTCHA!  The Big One That Didn't Get Away, Gamma Ray Burst Headlines, January 27, 1999.

    A more detailed observation of a burst on March 3, 2003 convinced many astrophysicists that at least some gamma-ray bursters are so-called "hypernovae".  A hypernova is an exceptionally large supernova formed by the nearly instantaneous collapse of the core of a very large star, at least 10 times the mass of the sun, which has already blown off most of its hydrogen.  Such stars are called Wolf-Rayet stars.  The collapse of such a star need not be spherically symmetric, so the gamma ray burst could be directional, reducing the total power needed to explain the brightness we see here (if the burst happened to point towards us).  For more, try:
    

    • European Southern Observatory (ESO), Cosmological Gamma-Ray Bursts and Hypernovae Conclusively Linked, Press Release, June 18, 2003.

    It's hard to resist quoting the theory described here:
    
    

    Here is the complete story about GRB 030329, as the astronomers now read it.
    Thousands of years prior to this explosion, a very massive star, running out of hydrogen fuel, let loose much of its outer envelope, transforming itself into a bluish Wolf-Rayet star.  The remains of the star contained about 10 solar masses worth of helium, oxygen and heavier elements.
    In the years before the explosion, the Wolf-Rayet star rapidly depleted its remaining fuel.  At some moment, this suddenly triggered the hypernova/gamma-ray burst event.  The core collapsed, without the outer part of the star knowing.  A black hole formed inside, surrounded by a disk of accreting matter.  Within a few seconds, a jet of matter was launched away from that black hole.
    The jet passed through the outer shell of the star and, in conjunction with vigorous winds of newly formed radioactive nickel-56 blowing off the disk inside, shattered the star.  This shattering, the hypernova, shines brightly because of the presence of nickel.  Meanwhile, the jet plowed into material in the vicinity of the star, and created the gamma-ray burst which was recorded some 2,650 million years later by the astronomers on Earth.  The detailed mechanism for the production of gamma rays is still a matter of debate but it is either linked to interactions between the jet and matter previously ejected from the star, or to internal collisions inside the jet itself.
    This scenario represents the "collapsar" model, introduced by American astronomer Stan Woosley (University of California, Santa Cruz) in 1993 and a member of the current team, and best explains the observations of GRB 030329.
    "This does not mean that the gamma-ray burst mystery is now solved", says Woosley.  "We are confident now that long bursts involve a core collapse and a hypernova, likely creating a black hole.  We have convinced most skeptics.  We cannot reach any conclusion yet, however, on what causes the short gamma-ray bursts, those under two seconds long."

    Indeed, there seem to be at least two kinds of gamma-ray bursters, the "long" and "short" ones.  Nobody has caught the short ones in time to see their afterglows, so they are more mysterious.  For more information, try these:
    

    • NASA, Gamma Ray Bursts.
    • Edo Berger, Gamma-ray Burst FAQ.
    • Wikipedia, Gamma Ray Bursters.
    • Peter Mészáros, Gamma-ray Burst Physics.

    At the time this was written, NASA was scheduled to launch a satellite called "Swift", specially devoted to gamma-ray burst detection, in September 2004.  For details, see:
    

    • NASA, Swift: Catching Gamma-Ray Bursts on the Fly.

12. What is the origin and nature of ultra-high-energy cosmic rays?
    

    Cosmic rays are high-energy particles, mainly protons and alpha particles, which come from outer space and hit the Earth's atmosphere producing a shower of other particles.  Most of these are believed to have picked up their energy by interacting with shock waves in the interstellar medium.  But the highest-energy ones remain mysterious—nobody knows how they could have acquired such high energies.The record is a 1994 event detected by the Fly's Eye in Utah, which recorded a shower of particles produced by a cosmic ray of about 300 EeV.  A similar event has been detected by the Japanese scintillation array AGASA.  An EeV is an "exa-electron-volt", which is the energy an electron picks up going through a potential of 10¹⁸ volts.  300 EeV is about 50 joules—the energy of a one-kilogram mass moving at 10 meters/second, presumably all packed into one particle!
    Nobody knows how such high energies are attained—perhaps as a side effect of the shock made by a supernova or gamma-ray burster?  The puzzle is especially acute because because particles with energies like these are expected to interact with the cosmic microwave background radiation and lose energy after travelling only moderate extragalactic distances, say 100 mega light years.  This effect is called the Greisen-Zatsepin-Kuz'min (or GZK) cutoff.  So, either our understanding of the GZK cutoff is mistaken, or ultra-high-energy cosmic rays come from relatively nearby—in cosmological terms, that is.
    Right now the data is confusing, because two major experiments on ultra-high-energy cosmic rays have yielded conflicting results.  The Fly's Eye seems to see a sharp drop-off in the number of cosmic rays above 100 EeV, while the AGASA detector does not.  People hope that the Pierre Auger cosmic ray observatory, being built in western Argentina, will settle the question.
    For more information, try these:
    

    • HiRes—the High Resolution Fly's Eye experiment.
    • AGASA—the Akeno Giant Air Shower Array.
    • Pierre Auger Observatory.
    • A. A. Watson, Observations of Ultra-High Energy Cosmic Rays.
      
    • D. J. Bird et al., Detection of a cosmic ray with measured energy well beyond the expected spectral cutoff due to cosmic microwave radiation.

13. Do gravitational waves really exist?  If so, can we detect them?  If so, what will they teach us about the universe?  Will they mainly come from expected sources, or will they surprise us?
    

    Perhaps the most ambitious physics experiments of our age are the attempts to detect gravitational waves.  Right now the largest detector is LIGO—the the Laser Interferometer Gravitational-Wave Observatory.  This consists of two facilities: one in Livingston, Louisiana, and one in Hanford, Washington.  Each facility consists of laser beams bouncing back and forth along two 4-kilometer-long tubes arranged in an L shape.  As a gravitational wave passes by, the tubes should alternately stretch and squash—very slightly, but hopefully enough to be detected via changing interference patterns in the laser beam.LIGO is coming into operation in stages.  The first stage, called LIGO I, is supposed to allow detection of gravitational waves made by binary neutron stars within 65 mega light years of us.  These binaries emit lots of gravitational radiation, spiral into each other, and eventually merge.  In the last few minutes of this process you've got two objects heavier than the sun whipping around each other about 100 times a second, faster and faster, and they should emit a "chirp" of gravitational waves increasing in amplitude and frequency until the final merger.  It's these "chirps" that LIGO is optimized for detecting.  Later, in LIGO II, they'll try to boost the sensitivity to allow detection of in-spiralling binary neutron stars within 1000 mega light years of us.
    To give you an idea of what these distances are like: the radius of the Milky Way is about 50,000 light years.  The distance to the Andromeda galaxy is about 2.3 mega light years.  The radius of the "Local Group" consisting of three dozen nearby galaxies is about 6 mega light years.  The distance to the "Virgo Cluster", the nearest large cluster of galaxies, is about 50 mega light years.  The radius of the observable universe is roughly 10,000 mega light years.  So, if everything works as planned, we'll be able to see quite far with gravitational waves.
    However, binary neutron stars don't merge very often!  The current best guess is that with LIGO I we will be able to see such an event somewhere between once every 3000 years and once every 3 years.  I know, that's not a very precise estimate!  Luckily, the volume of space we survey grows as the cube of the distance we can see out to, so LIGO II should see between 1 and 1000 events per year.
    The really scary thing is how good LIGO needs to be to work as planned.  Roughly speaking, LIGO I aims to detect gravitational waves that distort distances by about 1 part in 10²¹.  Since the laser bounces back and forth between the mirrors about 50 times, the effective length of the detector is 200 kilometers.  Multiply this by 10⁻²¹ and you get 2 x 10⁻¹⁶ meters.  By comparison, the radius of a proton is 8 x 10⁻¹⁶ meters!  So, we're talking about measuring distances to within a quarter of a proton radius!  And that's just LIGO I.  LIGO II aims to detect waves that distort distances by a mere 2 parts in 10²³, so it needs to do 50 times better.
    Actually all this is a bit misleading.  The goal is not really to measure distances, but really vibrations with a given frequency.  However, it will still be an amazing feat... if it works.
    Getting LIGO to work has been a heroic endeavor: so far two earthquakes have caused damage to the equipment, and problems from tree logging in Livingston to wind-blown tumbleweeds in Hanford have made life more difficult than expected.  To keep up with the latest news, try the "LIGO Web Newsletter" here:
    

    • Laser Interferometer Gravitational Wave Observatory (LIGO) home page.

    LIGO is working in collaboration with the British/German GEO 600 detector in Hanover, Germany, a smaller detector that tests out lots of new technology.  Other gravitational wave detectors include the French/Italian collaboration VIRGO, the Japanese TAMA 300 project, and ACIGA in Australia.  For information on these and others try:
    

    • Other Gravitational Wave Detection Projects.

    But, the coolest gravitational wave detector of all—if it gets funded and gets off the ground—will be LISA, the Laser Interferometric Space Antenna:
    

    • NASA's LISA home page.
    • European Space Agency's LISA home page.

    The idea is to orbit 3 satellites in an equilateral triangle with sides 5 million kilometers long, and constantly measure the distance between them to an accuracy of a tenth of an angstrom (10⁻¹¹ meters) using laser interferometry.  The big distances would make it possible to detect gravitational waves with frequencies of 0.0001 to 0.1 hertz, much lower than the frequencies for which the ground-based detectors are optimized.  The plan involves a really neat technical trick to keep the satellites from being pushed around by solar wind and the like: each satellite will have a free-falling metal cube floating inside it, and if the satellite gets pushed to one side relative to this mass, sensors will detect this and thrusters will push the satellite back on course.
    For more details on what people hope to see with all these detectors, try this:
    
    

    • Curt Cutler and Kip Thorne, An Overview of Gravitational-Wave Sources.

14. Do black holes really exist?  (It sure seems like it.)  Do they really radiate energy and evaporate the way Hawking predicts?  If so, what happens when, after a finite amount of time, they radiate completely away?  What's left?  Do black holes really violate all conservation laws except conservation of energy, momentum, angular momentum and electric charge?  What happens to the information contained in an object that falls into a black hole?  Is it lost when the black hole evaporates?  Does this require a modification of quantum mechanics?
15. 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?
    

    Proving the Cosmic Censorship Hypothesis is a matter of mathematical physics rather than physics per se, but doing so would increase our understanding of general relativity.  There are actually at least two versions: Penrose formulated the "Strong Cosmic Censorship Conjecture" in 1986, and the "Weak Cosmic Censorship Hypothesis" in 1988.  A fairly precise mathematical version of the former one states:

    Every maximal Hausdorff development of generic initial data for Einstein's equations, compact or asymptotically flat, is globally hyperbolic.

    That's quite a mouthful, but roughly speaking, "globally hyperbolic" spacetimes are those for which causality is well-behaved, in the sense that there are no closed timelike curves or other pathologies.  Thus this conjecture states that for generic initial conditions, Einstein's equations lead to a spacetime in which causality is well-behaved.
    The conjecture has not been proved, but there are a lot of interesting partial results so far.  For a nice review of this work see:
    

    • Piotr Chrusciel, On the uniqueness in the large of solutions of Einstein's equations ("Strong cosmic censorship"), in Mathematical Aspects of Classical Field Theory, Contemp. Math. 132, American Mathematical Society, 1992.

F.A.Q's:Quantum Mechanics

1. How should we think about quantum mechanics?  For example, 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?
   

   Many physicists think these issues are settled, at least for most practical purposes.  However, some still think the last word has not been heard.  Asking about this topic in a roomful of physicists is the best way to start an argument, unless they all say "Oh no, not that again!".  There are many books to read on this subject, but most of them disagree.

2. Can we build a working quantum computer big enough to do things ordinary computers can't easily do?
   

   This question is to some extent impacted by the previous one, but it also has a strong engineering aspect to it.  Some physicists think quantum computers are impossible in principle; more think they are possible in principle, but are still unsure if they will ever be practical.Here are some ways to learn more about quantum computation:
   

   • Centre for Quantum Computation home page.
   • John Preskill, course notes on quantum computation.
   • Michael A. Nielsen and Isaac L. Chuang, Quantum Computation and Quantum Information, Cambridge U. Press, Cambridge, 2000.  Errata, table of contents and Chapter 1 available here.

F.A.Q's :Condensed Matter and Nonlinear Dynamics

1. What causes sonoluminescence?  Sonoluminescence is the generation of small light bursts in liquids caused by sound.  Bubbles form in the liquid at low pressure points of the sound wave, then collapse again as a high pressure wave passes.  At the point of collapse a small flash of light is produced.  The exact cause has been the subject of intense speculation and research.

   For more details, try this:
   

   • Sonoluminescence, Wikipedia.

2. What causes high temperature superconductivity?  Is it possible to make a material that is a superconductor at room temperature?  Superconductivity at very low temperatures has been understood since 1957 in terms of the BCS theory, but high temperature superconductors discovered in 1986 are still unexplained.

   To learn more about superconductivity, see this web page and its many links:

   • Superconductivity, Wikipedia.

3. How can turbulence be understood and its effects calculated?  One of the oldest problems of them all.  A vast amount is known about turbulence, and we can simulate it on a computer, but much about it remains mysterious.
4. The Navier-Stokes equations are the basic equations describing fluid flow.  Do these equations have solutions that last for all time, given arbitrary sufficiently nice initial data?  Or do singularities develop in the fluid flow, which prevent the solution from continuing?
   

   This is more of a question of mathematical physics than physics per se—but it's related to the previous question, since (one might argue) how can we deeply understand turbulence if we don't even know that the equations for fluid motion have solutions?  At the turn of the millennium, the Clay Mathematics Institute offered a $1,000,000 prize for solving this problem.  For details, see:
   

   • Clay Mathematics Institute, Navier-Stokes Equation.

Gravitation: Frequently Asked Questions (F.A.Qs)

62. What is gravitation?
The attraction that every particle of matter in the universe has for every other particle.

63. When the moon comes' between the sun and the earthy how does that affect the attraction between the sun and the earth ?
Such interposition of matter has no effect on the attract-
ive force originally existing.

64. What is the first law of gravitation?
The attractive force (gravitation) between two bodies varies as the product of the masses of the two bodies.

65. Illustrate this law.

If two bodies contain 5 and 10 pounds of matter respectively, the product may be represented by 5a \\ two
other bodies contain 4 and 25 pounds of matter respectively, the product may be similarly represented by
100. Acting at like distances, the attraction between the second two will be twice (=Vrf') as great as the at-
traction between the first two.

66. How would doubling the matter in one of the bodies affect their gravitation?
It would double one of the factors, and thus the product, and thus the gravitation.

67. How would doubling the weight of both of the bodies affect their gravitation ?
It would double each of the two factors, and thus increase product and gravitation fourfold.

68. What is the second law of gravitation ?
Gravitation varies inversely as the square of the distances between the centers of mass of the two bodies.

69. What is the center of mass of a body ?
The point about which all the matter of a body may be balanced.

7) Illustrate the second law of gravitation-.
Two bodies a foot apart (between centers) attract each other four times as much as do the same bodies when placed two feet apart.

71. How would doubling the product of the masses and doubling the distance affect gravitation ?
It would divide the attraction by two.

72. How would trebling the quantity of matter in each of the two bodies and doubling the distance between them affect the gravitation?
It would |3X3__^ \ increase it two and a quarter times.

73. How does gravitation pull?
It is generally conceived of as Kpull^ but it is more likely to be a push. At the present time, the mechanical
nature of the action is unknown.

74. What is gravity?
The attraction between the earth and a body on or near its surface.

75. How is gravity measured?
By the weight that it gives to a body.

76. In what direction does it act?
Vertically downward ; i, e. toward the center of the earth.

77. How is this direction easily illustrated?
By a plumb line.

78. Where does a body have the greatest weight ?
At the surface of the earth.

79. How does carrying a body below the surface of the earth affect its weight f

It decreases it as the distance from the center of the earth is decreased.

80. Why is this?
In descending, the matter left behind or above attracts it upward and neutralizes part of the attraction of the
matter still below it

81. How does carrying a body above the surface of the earth affect its weight?
It decreases it as the square of the distance from the center of the earth increases.

82. How much will a pound of iron weigh 4000 miles above the earth's surface?
The distance from the center of the earth has been multiplied by two, therefore its weight will be divided by
the square of two which is four. It will weigh a quarter of a pound.

83. Considering the earth as a hollow sphere.what would a body in its interior weigh ?
It has been mathematically demonstrated that a particle of matter within a spherical shell is equally attracted
in all directions by the matter of the shell. Hence, a body would have no weight anywhere within the shell
of such an earth.

84 Illustrate a method of finding the center of mass of a body.
Drive a tack in a slate frame. Tie the middle of a string around the tack and a weight (plumb-bob) to one end of the string. Suspend the slate by the free part of the string and mark the direction of the plumb-line across
the slate. Change the position of the tack and repeat the process. The intersection of the two lines thus
marked on the slate "will approximately indicate its center of mass.

85. How may this be proved?
Place the point thus found upon the finger-tip ; the slate will balance.

86. When is a body in stable equilibrium ?
When it is so supported that when it is slightly displaced it seeks to return to its original position.

87. Illustrate.
 A stick supported from its upper end; a pendulum; a hemispherical oil-can.

88. When is a body in unstable equilibrium ?

When it is so supported that when it is slightly displaced it seeks to fall further from its original position.

89. Illustrate.
A stick balanced on its lower end ; an ^^'g standing on its end.

90. When ts a body in neutral equilibrium?
When it is so supported that when slightly displaced, it seeks to move neither toward nor from its original
position.

91. Illustrate.
A ball resting on a table.

92. What is the line of direction f
The path of the center of mass of a body when the body falls.

93. Give another definition.
A line drawn vertically downward from the center of mass.

94. What is the base of a body ?
The side on which it rests, or the surface bounded by lines joining its points of support.

95. When will a body stand?
When its line of direction falls within its base.

96. When will a body fall?
When its line of direction falls without its base.

97. How may the stability of a body be increased?
By increasing its base or lowering its center of mass.

98. How do we determine the height that a body is raised?
By the distance its center of mass is raised.

99. Why is it easier to lift one end of a plank a yard than it is to lift the middle of the plank that distance f
• In the first case, the center of mass is lifted only half as high as in the second case.

100. What is the base of a sphere supported on a horizontal plane ?
A point.

loi. What is the base of a cylinder supported on a horizontal
plane?
A line.

102. Why is a sphere easily rolled on a horizontal plane ?
Because such motion does not raise the center of mass.

103. Why is a cylinder easily rolled on a horizontal plane?
For the same reason.

Work and Energy:Frequently Asked Questions (F.A.Qs)

38. What is work?

Work is the overcoming of resistance. The term implies
a change of position and is independent of the time
taken.

39. What may we take as a type of work?

The lifting of a bodv against the force of gravity, i, e,.
against the  pull  of the earth.

4a How may we measure such work ?

By considering both the weight of the body and the height
which it is raised.

41. How are work-units classified?

As gravitation units and absolute units, with two in each
class.

42. What are the gravitation units of work ?

The work expended in lifting one pound one foot against
the force of gravity is called a loot-pound. The work
expended in lifting one kilogram one meter against the
same force is called a kilogrammeter.

43. What are the absolute units of work ?

The work done by one poundal in producing a displace-
ment of one foot is called a foot-poundal. The work
done by one dyne in producing a displacement of one
centimeter is called an erg.

44. What is the numerical relation between these units ?

A foot-poundal is equivalent to 421,402 ergs ; a foot-pound
is equivalent to 32.16 times that many ergs. Since a
force of one kilogram is equivalent to 9^,000 d3mes,
and a meter to 100 centimeters, a kilogrammeter is
equivalent to 98,000,000 ergs. In any case, the work
done is numerically represented by the product of the
number of utiits of force into the number of units of
displacement Multiply the number of weight-units by
the number of height units.

45. A laborer with his hod of bricks weighs 300 pounds. How
much work does he perform in carrying his load to the top
of a building ^o feet high?
300X50=15,000, the number of foot-pounds. ,

46. What is CLctivity ?
The activity of an agent is the rate at which it can do
work.

47. Who/ is a horse-power?

It is the most common unit of activity, and represents the
ability to do 33,000 foot-pounds in a minute, or 550 foot-
pounds in a second.

48. How is horse- power computed ?

Multiply the number of pounds raised by the "number of
vertical feet through which it is raisea, and divide the
product by 33,000 times the number of minutes (or by
550 times the number of seconds) required to do the
work.

50. What is energy ?

The power of doing work.

51. Name the two great classes of energy.
Kinetic and potential.

52. What is kinetic energy ?

Euer^ of motion ; i, e.^ the power of doing work that a
body has by virtue of its motion.

53. What is potential energy ?

Energy of position ; 1. e.g the power of doing work that a
body has by virtue of its position.

54. Illustrate kinetic energy.

The energy of running water, a falling pile-driver, a re-
volving fly-wheel.

55. Illustrate potential energy, ,.

The energy of a head of water, a coiled spring, a drawn
bow.

56. How are these varieties of energy related ?

They are mutually convertible. Either may be converted
into an equivalent amount of the other.

57. Illustrate this statement.

It requires a certain amount of work to wind up a clock.
When the clock is wound up, it has a store of potential
energy. When the pendulum is Fet in vibration, the
energy stored in the coiled spring or raised weight will
perform an amount of work equal to that performed in
winding up the clock.

58. Give a further illustration,

A ball is thrown vertically upwards with a certain velocity.
Its kinetic energy lifts it to a certain height and, when
the ball is at that point, its velocity is zero. It, there-
fore, has no kinetic energy. All that it had at the start
has disappeared, having been converted into an equiva-
lent amount of potential energy. For, at this moment,
the ball has a position of advantage from which it de-
rives a power of doing work; it may be used as a
weight to run machinery or in other ways. This energy
of position maj be reconverted into its original form as
energy of motion, for, if the body is permitted to fall,
it will regain the velocity with which it started. At
the middle point, going up or down, the energy of the
t>a^l is half kinetic and half potential, and at every
point of the path, the sum of the two energies is a con-
stant quantity.

59. Hoiv is kinetic energy tneasured in gravitation units?

We have the formula K. E. =1/2 mv2, in which w represents
the weight and z/ the velocity of the moving body, and ^
the acceleration due to gravity {i.e.y 32.16 feet or 9.8
meters). Substituting in this formula values measured
in feet and pouuds, we have the value of the kinetic
energy in foot-pounds; using meters and kilograms,
we have the value in kilogrammeters.

6O How  kinetic energy mensured in absolute units ?

We have the formula, K. E.=>1/2 mv2. Measuring mass in
pounds, and velocity in feet per second, this g^ves the
energy in foot-poundals. Measuring mass in grams,
and velocity in centimeters per second, gives the energy
in ergs.

61. What is meant by the conservation of energy?

When the universe was hurled into space bjr the hand of
the Creator, it was endovred with a certain amount of
energy. Like matter, energy may appear in many
difiPerent shapes. The sum total of all these different
forms of energy in the universe taken as a whole is a
constant quantity, for energy, like matter, is indestruct-
ible.

Force and Motion : Frequently Asked Questions (F.A.Qs)

1. What is motion?
Change of position.

2. What is velocity ?

Rate of motion ; it may be uniform or variable.

3. What is acceleration ?

Rate of change of velocity ; i. e,, the change of velocity
per unit of time.

4. What is force ?

Any cause that tends to produce any change of motion.

5. What is momentum?
Quantity of motion.

6. How is it measured?

By the product of the number of units of mass into the
number of units of velocity.

7. What is the unit of momentum called ?

It has no specific name. We may compare the momenta
of two moving bodies by the ratio between the two
measuring products as above explained. The momentum
of a body having a mass of 40 pounds and a velocity of
15 feet per second is twice as ereat as that of a body
having a mass of 10 pounds and a velocity of 30 feet per
second.

8. What is the first law of motion ?

Every body continues in its state of rest or of uniform
motion, in a straight line^ unless compelled to change
that state by some external force.

9. From what does this law result ?
From the inertia of matter.

la What is centrifugal force?

So-called centrifagal force is simply a confasing name for
inertia, or the tendency of matter to obey the first law
of motion.

11. To what special act of obedience is the term applied?

When a body is compelled to move in a curve, it always
tends to pull away from the centre and to move in a
straight line, tangent to the cnrve.

12. Illustrate this tendency.

Mud flying from a carriage wheftl, or water from a grind-
stone.

13. Do the mud and the tvater^ after pulling away from their
circular paths^ move in straight lines?
They do not, because they are continually pulled there-
from by the force of gravity.

14. Give the second law of motion.

The effect of a force will be the same whether it acts alone
or jointly with others.

15. What name is given to the effect of two or more forces ^
acting jointly ?

Resultant motion, which will be different from the effect
of any one of the forces acting, and may be looked upon
as the result of a single force called the resultant force.

16. How is the resultant force determined?

By what is known as the composition of forces.

17. State one case of the composition of forces.

When the given forces act in the same direction, the re-
sultant equals their sum.

18. Give an illustration.

If a man rows a boat with a force that alone will produce
a velocity of 4 miles an hour, down a stream that has a
current of 3 miles an hour, the boat will move at the
rate of 7 miles an hour ; 4-1-3=7.

19. State another case under the composition of forces.

When the given forces act in opposite directions, the re-
sultant equals their difference, its direction will be that
of the greater force.

20. Give an illustralion,

Ifthe boat is rowed with the same force as before but
against the same current, it will move up stream at the
rate of one mile an hour ; 4 — 3=1.

21. State another case under the composition of forces.

When the given forces act at an angle with each other, the
resultant may be found by a process known as the
parallelogram of forces.

22. Give an illustration.

If the boat is rowed easterly with the same force as before,
and the same current is flowing northerly, these two
forces may be represented bv two lines, 4 inches and 3
inches long respectively, and meeting at a right angle.
Call the apex of this angle A. Consider these two lines
as two sides of a parallelogram, and draw the other two
sides. Draw a diagonal from A. By measurement,
or mathematically, we may find that this diagonal is 5
inches long. Its length and direction represent the
intensity and direction of the resultant force. The
boat will move in the direction thus indicated, and with
a velocity of 5 miles an hour.

23. What is the third laiv of motion f

Action and reaction are equal and opposite in direction.

24. Give an illustration.

When Columbus made the famous ^%% stand on end, the
action of the ^^'g may have made a dent in the table.
It is certain that the equal and opposite reaction of the
table broke the shell.

25. What is the law of reflected motion f

The angle of incidence equals the angle of reflection.

26. What is the angle of incidence?

The angle included between the path of the moving body
before reflection, and a line drawn perpendicular to the
int of reflection. reflecting surface at the point k

27. What is the angle of reflection f

The angle included between the path of the moving body
after reflection and the perpendicular drawn as above
described.

28. What very common error in this respect?

To think that these angles are included between the two
paths specified and the reflecting surface, instead of be-
tween the paths and the perpendicular to that surface.

29. How are forces measured?

By comparison with some standard called a unit of force.
There are two kinds of units of force, the gravity unit
and the absolute unit

3a What is the gravity unit of force?

It is the weight of any standard unit of mass, as the gram,
kilogram, or pound. When a force may be balanced by
a weight of 100 pounds, we call it a force of 100 pounds.
If a frictionless horizontal piston at the top of a steam
boiler must be loaded at the rate of 100 pounds to the
square inch to keep it in place against the force of the
confined steam, we say that there is a steam pressure of
100 pounds to the square inch.

31. What is the absolute unit of force ?

It is the force that, acting for unit of time upon unit of
mass, will produce unit of acceleration. There are two
such units m common use ; the poundal and the dyne.

32. What is the poundal ?

It is the force that, when applied for one second to one
pound of maiter, produces an acceleration of one foot
per second. It is called the F. P. S. (foot-pound-second)
unit of force.

33. What is the dyne ?

It is the force that, when applied for one second to one
gram of matter, produces an acceleration of one centi-
meter per second. It is called the C. G. S. (centimeter-
gram-second) unit of force.

34. What is the numerical relation between gravity units and ab-

solute units of force ?
At the sea-level at New York City, the force of gravity
gives to a falling (freely moving) body that weighs one

pound (or any other weight) an acceleration of 32.16
Feet; consequently, at New York, a force of one pound
equals 32.16 poundals. The same force produces an
acceleration of 980 centimeters; consequently, at New
York, a force of one gram equals 980 dynes ; a force of
one kilogram equals 980,000 dynes.

35. How may the acceleration be determined?

By dividing the total velocity that the force has produced
by the number of seconds that the force has acted.

36. How is a force measured in absolute units?

By multiplying the number of units of mass moved by the
number that represents the acceleration produced. For
poundals, the units used must be feet, pounds, and
seconds (F. P. S.); for dynes, the units used must be
centimeters, grams, and seconds (C. G. S.).

37. What force is necessary to give a body weighing jo grams
a velocity of 50 centimeters per second ^ by acting upon the
body for two seconds f
30X50-5-2=750, the number of dynes.

Properties of Matter : Frequently Asked Questions (F.A.Qs)

26. What is a property of matter ?
Some quality that pertains to matter.

27. What is a universal property of matter ?

A quality that pertains to all matter, a quality without
which matter, as we know it, could not exist.

28. What is a characteristic or accessory property of matter?

A quality that pertains to some kind or kinds of matter
and not to others, and that thus enables us to distinguish
one substance from another.

29. Name some of the universal properties of matter.
Extension, impenetrability, indestructibility, weight and
inertia.

30. Name some of the characteristic properties of matter.
Hardness, as of the diamond ; tenacity, as of steel ; brit-
tleness, as of glass; malleability, as of gold; ductility,
as of platinum.

31. What is extension ?

The property of matter by which matter takes up room,
i. e.y occupies space.

32. To what does it refer?

To length, breadth and thickness, a combination that is
essential to the existence of matter.

33. What is impenetrability ?

The property of matter by which one body excludes
another from the space in which it is. No two bodies
can be in the same place at the same time.

34. What is indestructibility?

The property of matter by which it defies annihilation.
God created matter; He alone can destroy a single
atom of it.

35. What caution should be observed in this connection ?

We should remember that there is a difference between
disappearance and destruction.

36. How can you illustrate this difference ?

Water " boils away," but we all know that thoufrh it thus
disappears, it does not cease to be. So a candle burns
away and disappears. The candle is destroyed, but not
an atom of the matter of which the candle was com-
posed is destroyed. The hydrogen, which was part of
the candle, burns [i, ^., unites with oxygen), and thus
forms watery vapor that will condense to the liquid
form, and perhaps help quench the thirst of a sheep
that may jrield tallow for another candle. The carbon
of the candle bums to carbon di-oxide (carbonic acid
gas), that may feed the plant on which feeds the
sheep that may yield more tallow for still another can-
dle. Matter goes through almost endless transforma-
tions, and appears in protean shapes.

37. What says Shelley' s poem on " The Cloud? "

' ' I pass through the pores of the ocean and shores ;
I change, but I cannot die."

38. What is weight ?

It is the downward pressure of a body on or near the
earth's surface, due to the attraction between that body
and the earth.

39. What does it measure ?

It measures the force of gravity upon the body weighed.

40. How is the term generally used ?

As above defined, referring especially to terrestrial ob-
jects. As a matter of fact, all matter has weight be-
cause the attraction of gravitation is of universal ap-
plication.

41. What is inertia ?

The property of matter whereby a body cannot change
its condition of rest or motion. It is a purely negative
property ; a quality of inability, and nothing more.

42. State a common erroneous notion concerning inertia.

It is sometimes (unconsciously) held that rest is the natu-
ral condition of matter, and that inertia means espe-
cially the tendency of matter to remain in such condi-
tion ; that t© overcome the inertia of a body means to
put it in motion.

43. What is the fact in the case ?

Matter has no " natural condition '* either of rest or of
motion. So far as we know, matter is nowhere at rest,
everywhere in motion. But this motion is due not to
any inherent tendency, but to the fact that the motion
was communicated to it by some agency outside itself.

44. State some of the consequences of inertia.

On account of inertia, a body at rest cannot put itself in
motion ; on account of inertia, a moving body cannot
bring itself to rest or even change its rate of motion
(velocity). Stopping a moving body is overcoming its
inertia as truly as in giving motion to a body at rest.

45. What is meant by the term, " body ? "

A body is some definite, separate, portion of matter. The
term refers only to matter, ** dead matter," or matter
considered independently of the living, willing, or
motive power of vegetable or animal existence. These
are forces, mysterious forces, distinct from the material
organism and acting upon it. With such vital forces,
physics (at least, elementary physics) has nothing to do.

46. What is hardness ?

The property of some kinds of matter whereby they are
able to resist being marked by scratching. The dia-
mond is harder than glass, and will, therefore, scratch
glass. Glass is harder than gold, and will, therefore,
scratch gold.

47. What is tenacity ?

The property of some kinds of matter whereby they are
able to resist being pulled asunder. Because an iron
bar will resist a greater pulling force than a similar
lead bar, we say that the former has the greater tenacity.

48. What is brittleness ?

The property of some kinds of matter whereby they are
easily broken by a blow.

49. What is malleability ?

The property of some kinds of matter whereby they may
be rolled or hammered into sheets.

50. What is ductility?

The property of some kinds of matter whereby they may
be drawn into wire.

51. In how many conditions does matter exist?

Three or more. The three universally recognized condi-
tions are the solid, liquid and aeriform (or gaseous).

52. What is a solid ?

A body that has a strong tendency to retain its given
form, like ice. It has little freedom of molecular motion.

53. What is a liquid?

A body, the molecules of which move easily among them-
selves, and yet tend to cling together. Water is the
most familiar illustration of a liquid.

54. What is an aeriform body ?

One in which the molecules move easily among them-
selves and tend to separate from each other, like steam.

55. How are aeriform bodies classified ?
As gases and vapors.

56. How do these differ ?

Gases retain their aeriform condition at ordinary temper-
atures and pressures, like oxygen or illuminating gas.
Vapors take the liquid or solid form at ordinary tem-
peratures and pressures, like steam.

57. What is a fluid ?

A body characterized by great freedom of molecular motion.

58. What does the term include ?
Liquids, gases, and vapors.

59. Is there any other form of matter ?

Mr. Crooke's experiments seem to show that there is a
form still more tenuous than the aeriform, for which he
has proposed the name "Radiant." The Inminiferous
ether, which pervades all space, is a form of matter
more nearly imponderable than the gaseous.

60. What would you call a fluid that is scarcely compressible?
A liquid.

61. What would you call a fluid that is easily compressible f
A gas or a vapor.

62. What would you call a body that has a definite form, of its
own?
A solid.

63. What would you call a body that can not, of itself maintain
a definite form f?
A fluid.

64. Why may larger' bubbles be blown zvith soap-suds than with

pure water?
Because of the greater surface viscosity of the soap solu-
tion. The 8U{)eriicial film of a liquid is highly viscous
as compared with the interior; 1. ^., it is comparatively
difficult to move or to break it.

65. Slate another fa£l concerning the superficial films of liquids,
A liquid surface is in a state of tension like that of a
stretched membrane.

66. What is capillary attraction ?

The tendency of water and other liquids to rise above theii
levels in fine tubes dipped into liquids that wet them.

67. What happens if the liquid does not wet the tube ?

The liquid, instead of being raised, will be depressed be-
low its level.

68. How can you illustrate your meaning ?

• By plunging a clean glass tube into mercurv, or a greased
tube into water, the liquid in the tube will be depressed
below its level outside the tube.

69. Give a familiar illustration of capillary attraction.
The ascent of oil in a lamp wick.

How to construct the Camera Obscura ( Pin Hole Camera)

Make a circular hole in the shutter of a window, from whence there is a prospect of some distance; in this hole place a magnifying glass,
either double or single, whose focus is at the distance of five or six
feet; no light must enter the room but through this glass. At a
distance from it, equal to its focus, place a very white pasteboard,
(what is called a Bristol board, if you can procure one large enough,
will answer extremely well;) this board must be two feet and a half
long, and eighteen or twenty inches high, with a black border round
it: bend the length of it inward to the form of part of a circle,
whose diameter is equal to double the focal distance of the glass. 

Fix it on a frame of the same figure, and put it on a moveable foot, thatit may be easily placed at that distance from the glass, where theobjects appear to the greatest perfection. When it is thus placed, allthe objects in front of the window will be painted on the paper in aninverted position, with the greatest regularity, and in the most
natural colours. If you place a swing looking-glass outside the
window, by turning it more or less, you will have on the paper all theobjects on each side the window.

If, instead of placing the looking-glass outside the window, you placeit in the room above the hole, (which must then be made near the topof the shutter,) you may have the representation on a paper placedhorizontally on a table, and draw at your leisure all the objects reflected.

Observe, the best situation is directly north; and the best time of
the day is noon.

Make Artificial Lightning and Artificial Thunder

Provide a tin tube that is larger at one end than it is at the other,
and in which there are several holes. Fill this tube with powdered
resin; and when it is shook over the flame of a torch, the reflection
will produce the exact appearance of lightning.


Mix two drachms of the filings of iron, with one ounce of concentrated
spirit of vitriol, in a strong bottle that holds about a quarter of a
pint; stop it close, and in a few minutes shake the bottle; then
taking out the cork, put a lighted candle near its mouth, which should
be a little inclined, and you will soon observe an inflammation arise
from the bottle, attended with a loud explosion.

To guard against the danger of the bottle bursting, the best way would
be to bury it in the ground, and apply the light to the mouth by means
of a taper fastened to the end of a long stick.


Another way.

Mix three ounces of saltpetre, two ounces of salt of tartar, and two
ounces of sulphur; roll the mixture up into a ball, of which take a
quantity, about the size of a hazel-nut, and, placing it in a ladle or
shovel over the fire, the explosion will resemble a loud clap of
thunder.

You will produce a much more violent commotion if you double or treble
the quantity of the last experiment; suppose you put two or three
ounces of the mixture into the shovel. For fear of accidents, it
should not be done in the house, but by placing the shovel over a
chafing-dish of very hot coals, in the open air, standing a great
distance off.

Common prudence will dictate the necessity of using great care in the
above experiments, as an accident will soon happen if a person does
not get out of the way before the composition explodes.

The Magical Picture

Take two level pieces of glass, (plate glass is the best,) about three
inches long and four wide, exactly of the same size; lay one on theother, and leave a space between them by pasting a piece of card, or two or three small pieces of thick paper, at each corner.

Join these glasses together at the edges by a composition of lime
slaked by exposure to the air, and white of an egg. Cover all theedges of these glasses with parchment or bladder, except at one end,which is to be left open to admit the following composition.
Dissolve, by a slow fire, six ounces of hogs'-lard, with half an ounce
of white wax; to which you may add an ounce of clear linseed oil.
This must be poured in a liquid state, and before a fire, between the
glasses, by the space left in the sides, and which you are then toclose up. Wipe the glasses clean, and hold them before the fire, to
see that the composition will not run out at any part.
Then fasten with gum a picture or print, painted on very thin paper,
with its face to one of the glasses, and, if you like, you may fix thewhole in a frame.
While the mixture between the glasses is cold, the picture will be
quite concealed, but become transparent when held to the fire; and, as the composition cools, it will gradually disappear.

How to make The Exploding Bubble

If you take up a small quantity of melted glass with a tube, (the bowl of a common tobacco-pipe will do,) and let a drop fall into a vesselof water, it will chill and condense with a fine spiral tail, which being broken, the whole substance will burst with a loud explosion, without injury either to the party that holds it, or him that breaks it; but if the _thick_ end be struck, even with a hammer, it will not break.

How to produce Fire by the Mixture of two cold Liquids

Take half a pound of pure dry nitrate, in powder; put it into a retort
that is quite dry; add an equal quantity of highly rectified oil of
vitriol, and, distilling the mixture in a moderate sand heat, it will
produce a liquor like a yellowish fume; this, when caught in a dry
receiver, is _Glauber's Spirits of Nitre_; probably the preparation,
under that name, may be obtained of the chemists, which will of course
save much time and trouble.

You then put a drachm of distilled oil of cloves, turpentine, or
carraways, in a glass vessel; and if you add an equal quantity, or
rather more, of the above spirit, though both are in themselves
perfectly cold, yet, on mixing them together, a great flame will arise
and destroy them both, leaving only a little resinous matter at the
bottom.

Matter : Frequently Asked Questions (F.A.Qs)

1. What is Natural Philosophy or Physics?

Natural Philosophy or Physics is the branch of science

that treats of matter, and of the forces operating upon

it, and of the physical changes thereby produced.

2. What is science?

Science is classified knowledge.

3. What is matter ?

Matter is anything that takes up room.

4. Is this a real definition of  Matter?

It is not, for it simply tells something about matter in-

stead of telling what matter really is.

5. Why then is it given ?

Because, in the present state of human knowledge, we

can tell nothing of the real nature of matter and this

does the next best thing ; it enables us to distinguish

matter from that which is not matter.

6. Give some other " definition " of matter.

Matter is anything that has weight

7. What are the divisions of matter?

Atoms, molecules, and masses.

8. What is an atom ?

The smallest quantity of matter that can enter into com-

bination. It is generally a part of a molecule.

9. How many kinds of atoms are there?

As many as there are " elements," of which we now know

more than seventy. Others will probably be found.

10. Can an atom be divided ?

Atoms are considered indivisible, but the form or consti-

tution of an atom is unknown. The atom is the chemi-

cal unit of matter.

11. What is a molecule?

It is the smallest quantity of matter that can have a sep-

arate existence.

12. How many kinds of molecules are there?

As many as there are kinds of matter. The nature of

the molecule determines the nature of the substance of

which it is a part.

13. Can a molecule be divided ?

Yes, but. only by a chemical process. A molecule is the

physical unit of matter.

14. Give an illustration of a molecule,

A particle of sugar so small that, if it is divided, none of

Its parts will be sugar.

15. What are the parts of a molecule called?

Atoms.

16. How many atoms are there in a molecule ?

Chemistry teaches that in the case of a very few of th«

elements, the molecule consists of a single atom, but

in almost every case, the molecule contains two or

more atoms. The common sugar molecule has 45

atoms ; the oxygen molecule has two atoms.

17. What is a mass?

A body of matter consisting of two or more molecules;

it is not necessarily very massive.

18. Give an illustration of a** mass " of matter,

A minute dust speck, a mountain or a planet

19. What is an element or an elementary substance ?

A substance, all of the atoms of which are alike, /. e,, one

composed of a single kind of atoms. Silver is an ele-

ment because all of its atoms are silver atoms ; it can-

not by any known or conceivable process be " analysed "

or decomposed.

20. What is a compound substance ?

One that contains more than one kind of atoms. One of

its molecules is like its other molecules, but each mole-

cule contains more than one kind of atoms. Thus, a

dew-drop is composed of many similar water molecules.

but each of the molecules is composed of one oxygen and

two hydrogen atoms. Hence, water in any quantity is

a compound substance. Compound substances are in-

numerable.

21. What is a physical change ?

One that does not change the structure of the molecule

of which the changed body is composed.

22. Illustrate the meaning of " physical change. "

A black iron nail may be made red-hot. The iron mole-

cules jostle one another more vigorously than before,

and the energy of their mutual thrusts we call heat. A

certain kind of motion produced by such increased

activity of the molecules we call light, but the mole-

cules themselves are simply iron molecules, just as

they were at first. The molecule is one thing; the

motion of the molecule is a wholly different thing.

By increasing the molecular motions^ we may even

melt the iron, but that will not change the mole-

cules ; it will simply be melted iron. Two such nails

may be united by welding. The identical molecules

are preserved, and with unchanged identity. With a

file, some of these may be torn away from others, but

each of the filings will be /V^w-filings, composed of un-

changed iron molecules. Hence all of these are physi-

cal changes.

23. With this idea of heat as a mode of motion  give another

 definition of molecule,

A molecule is an aggregation of material particles that

do not part company during the journey of a heat vi-

bration.

24. State and illustrate some change of matter that is not a

 physical change.

If the iron is treated with sulphuric acid, the iron atoms

in each iron molecule will be torn asunder. Each iron

atom will be joined to an atom of sulphur, and four

atoms of oxygen, to make a molecule of iron-sulphate

(green vitriol). We no longer have an iron molecule,

and hence, no iron. Iron and iron-sulphate are essen-

tially diflFerent. Changes that thus alter the constitu-

tion of the molecule, and hence, the nature of the sub-

stance, are chemical changes.

25. What objection is there to the term " Natural Philosophy ? "

There is a philosophy of mind as well as a philosophy of

matter, and one pertains as mostli to Nature as the

other. But the term "Natural Philosophy," with its

restricted significance, is so completely worked into

our language and literature that it would be difficult to

change the usage. The term ** Physics " is preferable.