A Force to Contend With
Tim Folger’s “Nailing Down Gravity” story in the October issue presents physicist Moti Milgrom’s Modified Newtonian Dynamics, a theory that may explain gravitational anomalies found in large-scale systems such as galaxies. The hypothesis suggests that when gravitational acceleration reaches minuscule levels, gravity ceases to be directly proportional to acceleration and instead becomes proportional to the square of acceleration, causing a slight strengthening in the force of gravity. If gravity’s strength increases at lower accelerations, could the opposite also be true? Could the enormous accelerations surrounding a black hole, for instance, cause gravity there to become weaker, thereby making impossible the formation of a singularity—a point in space-time at which gravitational forces cause matter to have infinite density and infinitesimal volume and cause space-time to become infinitely distorted?
GREGORY CHRISTIE
Sinking Spring, Pennsylvania
Milgrom:
Physics is replete with examples of theories needing amendment when a parameter attains extreme values. This does not mean that the theory will also fail when that parameter goes to the other extreme. For example, Newtonian dynamics has to give way to relativity when the velocity becomes very large and approaches the speed of light, but there is no indication that it fails in any way for very small speeds. It may well be that standard physics breaks down in the limit of very high accelerations, but this is not implied by, and if true, will most probably not be related to, the breakdown at low accelerations. So we cannot infer from my theory what the nature of such a putative breakdown will be, or at what accelerations it might appear.
I was intrigued by “Nailing Down Gravity,” particularly by a quote from Milgrom. Describing his transition acceleration, a0, Milgrom says: “It turns out that this value has a cosmological significance. It is the acceleration you get by dividing the speed of light by the lifetime of the universe.” It appears that his theory, or at least one coincidence used to justify it, is using the age of the universe, and thus our assumptions about properties of light, as part of its computation. In a universe defined by what we can measure, and with measurements defined by what we perceive the universe to be, how can we avoid circularity in our reasoning? Even such “constants” as the speed of light seem to be under assault. Can we escape this muddle of inconstant constants in which our assumptions, tools, and yardsticks are most accurately measuring only our assumptions, tools, and yardsticks themselves?
W. RON HESS
Temple Hills, Maryland
Milgrom: The acceleration constant, a0, was not introduced into the theory as the speed of light divided by the lifetime of the universe. It simply popped up in the studies of the internal dynamics within galaxies and just turned out, numerically, to be related to cosmology in this way. Now, galaxies are many orders of magnitude smaller than the universe and involve timescales much shorter than its age. There is no known reason (certainly not a circularity of argument) for this internal property of galaxies to have any connection with the universe at large. So it is definitely surprising, and hopefully will turn out to be revealing, to find this coincidence.
Hy-Wire Act
I enjoyed Brad Lemley’s article on General Motors’s Hy-wire car [“Stop Driving With Your Feet,” October]. Unfortunately, Lemley expresses a common fallacy regarding the use of hydrogen-powered vehicles. He says, “Establishing a hydrogen infrastructure depends on an unpredictable mix of technological innovation, consumer acceptance, and government incentives.” This statement is similar to arguments that the cost of developing a hydrogen infrastructure will prevent the fuel from gaining acceptance. The bottom line is that anywhere you have electricity and either natural gas or water, you can make hydrogen. There are many companies, such as Stuart Energy of Canada, selling equipment that allows people to make hydrogen themselves. Such a system could be sold with every vehicle. It is not the difficulty of developing a hydrogen infrastructure that will slow down acceptance of fuel cells but the desire to keep oil companies from losing customers and state and federal governments from losing gasoline tax revenues.
MATTHEW CUMMINGS
Arlington, Virginia
Diminishing Returns
We here at the Planktos Foundation read the story about sequestering carbon in the ocean [“Watery Grave,” October], and we thank you for mentioning our work. However, you omitted the most important fact about the world’s oceans: the diminishment of the oceans’ primary productivity, which corresponds to the amount of phytoplankton growth. Recent research reveals that there has been a 9 percent decline in primary productivity in the North Pacific Ocean, 7 percent in the North Atlantic, and about 6 percent diminishment worldwide. This decline is associated with a decrease in the amount of aerosol dust and the accompanying iron that reaches the oceans. The soil from deserts and dry lands is key to the health of the world’s oceans, as it is the most important source of iron and other nutrients. While the addition of iron to forest-size patches of ocean might help mitigate global warming, it is even more important that we use it to mitigate the decline in the ocean’s primary productivity. The problem is not whether we will damage the oceans by adding iron; it is the certainty that we are killing the oceans by not adding iron.
RUSS GEORGE
Founder, The Planktos Foundation
Half Moon Bay, California
Shoot for the Moon
October’s “Let’s Go to Mars” and “Or, Let’s Go to the Moon (Again)” lament the lack of vision and leadership by NASA. The key to this issue is found on page 58: “Without a moon race driven by a cold war, the goal has become the pursuit of scientific and technical knowledge.” Discover readers may agree that knowledge is worth a fortune, but national security is the only reason the general public and Congress would pay for another Apollo-scale effort. The threat to our security is in the form of asteroids too small to be detected at long range but large enough to cause major catastrophes; NASA is now searching for asteroids one kilometer or larger in diameter, the impact of which could have global consequences. We need a quick reaction capability to defend Earth from these asteroids. A periodically human-tended base on the moon of remotely operated interceptors armed with nuclear warheads (not capable of surviving atmospheric reentry) is what is needed.
ANDREW PROBERT
Troy, Ohio
Fred Guterl’s eloquent plea for manned exploration of Mars, or at least a return of humans to the moon, is bound to be frustrated by the high cost, the risk, and the lack of interest among the American public. But this interest is likely to be reignited when China lands its first taikonaut on the moon. Nothing excites the American public like a competition for control of that body, and China has just launched Shenzhou-5, its first manned spacecraft.
MIKE LIEBER
Professor, Department of Physics
University of Arkansas
Fayetteville, Arkansas
Countering Terrorism
Scott Atran [“The Surprises of Suicide Terrorism,” Discover Dialogue, October] is right when he says that what the media and the Bush administration said after September 11 about terrorists being crazed or despairing was “utter nonsense.” But his own characterization of them is not much better, as when he says that the government programs against poverty and illiteracy “seem completely wrong.” The typical terrorist sees the poverty of downtrodden people and wants to crush the supposed cause of that situation. So the best way to destroy terrorism—perhaps the only way—is to persuade people that the alleged enemy is really a friend. How? By helping them on a grand scale. The billions of dollars spent on finding and punishing the terrorists would be better spent on help for the poor.
TADEUSZ DEBSKI
Chicago, Illinois
Change Is Good
Upon reading “Brother, Can You Spare 18¢?” [The Mathematics of Pocket Change, October], I became exasperated and curious. Mathematician Jeffrey Shallit wants to eliminate the penny from the U.S. monetary system. But isn’t our system based on a factor of one? Getting rid of the penny would undermine the one-based system and reduce flexibility as well. All transactions would have to be based on a factor of five. How would this be done? By rounding prices—as gas stations do when they charge $1.569 for gas? Would you be charged $1.60 for something that costs $1.56? This would cheat consumers. The only thing wrong with a handful of change is that people are lazy.
KRISTEN JONES
New Hope, Minnesota
Raywatch
The article regarding cosmic rays [“They Came From Outer Space,” September] has a not-so-obvious error concerning the cosmic-ray tracks in the helmet of astronaut Jim Lovell. The photographs on page 46 do not represent the actual tracks but rather the process used to visualize the course of the cosmic-ray particle. A caustic solution was used to dissolve, or etch, the damaged plastic of the helmet along the path of the cosmic ray. Over time, the damaged plastic was etched deeper and deeper, while the undamaged plastic dissolved at a much slower rate. The result is a cone-shaped pit along the path of the particle (represented by the rubber mold shown in Figure A), which could have penetrated the helmet. Particles that stopped in the helmet left test-tube-shaped pits (Figure B). This research technique was pioneered by P. Buford Price, the preeminent authority on high-energy cosmic rays.
WALTER L. WAGNER
Pepeekeo, Hawaii
