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Riddle Me This In "The 11 Greatest Unanswered Questions of Physics" [February], Eric Haseltine left out one question: What caused the Big Bang? I think the answer is unknowable and outside the realm of science. I remember a Russian scientist saying about 30 years ago that the Big Bang could have been caused by the high-speed collision of two particles, neutrons for example. So is the answer truly unknowable? Does it make any sense to speculate on the cause of the most important event in the history of the universe? If so, why was it not on the list of the 11 most important questions?

Tom Hammer—Hockessin, Delaware

Eric Haseltine responds: There are many ideas about what caused the Big Bang. One provocative theory postulates that the universe is eternal and simply goes through oscillations that periodically produce big bangs. Imagine that in the distant future, gravity wins out over the forces currently driving the universe apart, collapsing the universe into a dense state that gets so hot it explodes in another big bang, only to eventually run out of steam, collapse, and repeat the cycle ad infinitum. Although the rate of expansion of the universe is increasing, there is no theoretical reason why this expansion couldn't slow in the future, as many inflation theories argue it has slowed in the past. Another theory is that the universe simply popped out of the vacuum in the same way that virtual particle-antiparticle pairs emerge from empty space, only to recombine and vanish with a burst of energy. In this scenario there would be an "anti-universe" somewhere out there with which we may ultimately reconnect. A lot of scientists are uncomfortable speculating about the causes of the Big Bang because no one has come up with a way to test these theories. Any hypothesis that cannot be tested and proved false falls more into the realm of philosophy than physics.

I read Eric Haseltine's article with much interest. However, I was disappointed that there were only 11 questions. It seems the National Research Council is looking at the trees (particles) and not the forest (purpose). A unified theory of the universe must include the most important theory of all—the nature of life itself. Could some of the unknown forces, particles, and energies described in the article be responsible for life? Or does life spring up spontaneously, fueled by some energy or particle yet undiscovered? Who cares if we understand all the physics behind the universe if we can't answer the question of how life starts?

Mark Mittereder—via the Internet

I have some questions to be added to your list of the greatest unanswered questions of physics. As I sit here typing this, I ask myself, "Can modern physics explain the space-time motion of the atoms and electrons that make up my computer? Can they explain why they just happen to be here today? Can physics explain the travels of the metal atoms that make up the wires and those peculiar arrangements of silicon in the glass and on the chips?" If the ultimate goal of physics is to be able to explain the space-time motion of everything at all scales, then it seems most inadequate at explaining the space-time motion of objects on the human scale. Physics is very good at describing the space-time movements of the very large, like super-galaxies, and the very small, like quarks and atoms, but at the scale where I work and breathe, in my day-to-day interactions with my fellow man and woman, physics does not explain much about the space-time movements of the objects swirling about me.

Jeffrey Roseman, M.D., Ph.D., M.P.H. Birmingham, Alabama

"The 11 Greatest Unanswered Questions" was very enlightening to me, a geological engineer with a passion for astronomical history. I have a problem, however, with the answer to where the heavy elements came from in conjunction with the dark-matter quandary. Earth and other rocky planets are rich in heavy elements. These planets are huge accretions of matter and sometimes have massive iron concentrations. The article indicates that these elements traveled across space from supernova events. Could the dark matter that is being sought be heavy elements or the precursors of heavy elements? Since supernova events still appear to occur, why isn't space still full of heavy elements? If supernovas in fact don't still occur, why isn't space chock-full of unaccreted heavy elements, like a fog?

Mike Whims—Wixom, Michigan

Eric Haseltine responds: Dark matter may include both heavy and light ordinary elements bound up in brown dwarfs, white dwarfs, or clouds of gas. Supernovas still occur occasionally—about once a century in the Milky Way—spewing all kinds of detritus into space, including heavy elements. I'm not certain how much heavy-element "fog" these events might create, but I doubt that they would fill space chock-full because stars are typically pretty far apart from each other (separated by millions of star diameters), and most of them do not die in supernovas.

Regarding question 4 (Do neutrinos have mass?): Some 13 years ago, the Kamiokande and Irvine Michigan Brookhaven neutrino detectors measured the time-of-flight difference between the arrival of light and the arrival of neutrinos from a new supernova. Some scientists announced that the potential error in measurement of this difference was small enough to preclude neutrinos having a mass of more than 10 electron volts. I understood this to mean that oscillating neutrinos would not be possible. That would present a big problem for cosmologists, since they need oscillating neutrinos to explain the electron-neutrino shortage from the sun. Later pronouncements said that neutrinos do have mass, enough to permit oscillations. Has anyone reconciled the differences between the two announcements?

C. Norman Winningstad—Newport, Oregon

Eric Haseltine responds: As you suggest, supernovas are extremely useful in studying neutrino masses because these cataclysmic events emit copious amounts of both light and neutrinos. If neutrinos are massive, then they will travel more slowly than photons of light, which are massless. So any observed time-of-flight differences between neutrinos and photons from a single supernova strongly suggest that neutrinos do indeed possess mass. Data from the 1987 supernova you mentioned place an upper limit on this mass at 30 electron volts (mass and energy are equivalent according to E=mc2, so very small masses are often conveniently described as energy in terms of electron volts). There is no theoretical reason why this value—or even values much lower than 30 electron volts—should preclude neutrino oscillations (neutrinos changing from one type to another, such as a mu neutrino to an electron neutrino). A survey of the literature reveals a number of plausible models in which neutrinos oscillate with masses smaller than .1 electron volt. The bottom line is that there is no contradiction between the supernova data and more recent data on neutrino oscillation coming from the Super-K neutrino detector in Japan.

"The 11 Greatest Unanswered Questions of Physics" boggles my mind and takes it to a philosophical conundrum. Since we don't expect our cousins the chimpanzees ever to comprehend calculus, doesn't it follow that there may be a super-set of knowledge that lies beyond the comprehension of mere human minds? A point of near-infinite energy expanding much faster than the speed of light with random quantum fluctuations in density makes me wonder: Just how would one chimp explain a differential equation to another?

Russ Agreen—Denton, Maryland

Erratum The article "Photography, Old & New Again" [February] was fascinating, but I must correct the caption of the insect pictured on page 51. The insect is identified as a New Guinea beetle, but it is actually an adult female of the stick insect Heteropteryx dilatata, which as I recall is native to Malaysia.

Carl Moxey, Ph.D., Senior Lecturer, Biology Northeastern University, Boston, Massachusetts

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