“A Cosmic Crisis,” by Richard Panek, discusses possible reasons why the two methods used to measure the universe's rate of expansion find conflicting values—a discrepancy known as the Hubble tension. I am puzzled that the article does not mention forces from outside our universe acting on it. Can't we expect that there are other universes with mass like ours that will have gravitational, and possibly other, effects on us? I don't mean quantum parallel stuff but other big bang results beyond our own. We live in a galaxy that is part of a cluster of galaxies in a universe. Why not a cluster of universes? How do we know we are not in collision with one that is pulling our universe in ways that massively mess with our measurements?

via e-mail

PANEK REPLIES: Theorists have indeed been investigating the gravitational (or other) influence of parallel universes as a possible source of seeming anomalies in our universe. A multiverse, in fact, is what inflation theory would imply: one quantum pop creating our universe would almost certainly lead to other universes. But falsifying such a hypothesis would be difficult, if not impossible, so attempts to address the Hubble tension have focused on theories and observations involving effects we can measure.


In “What Is Killing the Monarchs?” Gabriel Popkin seems to overlook a possible contributor to the drastic drops seen in the butterflies' numbers. Could insecticide use across the South and along migration pathways contribute significantly to the discrepancy between summer counts and winter populations?

Local yard signs here in the South seem to indicate an expanded use of mosquito-control and lawn-maintenance companies that apply sprayed insecticides on a regular and recurring basis. And particularly after one neighbor had his yard sprayed for cutworms and another initiated mosquito-control spraying, I saw a great increase in the mortality of the monarch caterpillars I raise each summer. Friends elsewhere in town have commented about a similar experience, especially with regard to the August-September generation of caterpillars.

Hickory, N.C.

We have been cultivating milkweed plants around our house in San Diego for the past six years in an effort to help preserve these amazing butterflies. In the past two summers we have been alarmed to see that the number of caterpillars, and thus pupae, parasitized by flies (probably Lespesia archippivora) seems to have jumped from around 10 to 80 percent. I wonder if this increase might be related to climate change.

San Diego


In “Dollars for Dikes,” Wade Roush advocates building infrastructure to accommodate a base sea-level rise of up to 2.8 feet (plus a margin for surges and so on)—the top end of the Intergovernmental Panel on Climate Change's estimate for 2100. That estimate, however, is what scientists consider “conservative,” meaning that they are careful not to overestimate the rise.

Sea-level rise will almost surely be higher. When engineers design things such as jet aircraft, they use worst-case estimates of critical parameters. That is the only way of assuring the success of the project. If we are to solve the problem of building infrastructure to accommodate sea-level rise, we need to take an engineering approach and use worst-case estimates of how much rise to design for.

West Palm Beach, Fla.


I was initially thrilled to see that “The Enigma of Aerodynamic Lift,” by Ed Regis [February 2020], concerned a field to which I have devoted 35 years of my life. But I reread its title and summary—“No one can completely explain why planes stay in the air”—with some alarm. Was this article going to expose some major flaw in our understanding of fluid dynamics? I found it said no such thing.

You just can't explain aerodynamic lift adequately with high school physics. (I would argue that even a four-year bachelor's degree in aerospace engineering will not equip you to do so.) Daniel Bernoulli's and Isaac Newton's laws, though essential, are not sufficient. No one working in the field would design or analyze a machine with only these equations.

These problems are hard. They are complex. But they are not an enigma.

Sterling, Va.

I was an aeronautical engineer for 38 years. My colleagues and I encountered no ambiguity in explaining the flow-induced pressure distribution around lifting surfaces. I would respectfully disagree with Regis's claim that the Navier-Stokes equations or their solutions do not offer interpretable explanations of fluid physics. These equations are the mathematical language that engineers have used to understand fluid mechanics for more than a century. They describe the simultaneous conditions of mass, momentum and energy conservation that govern fluid flows. These conditions determine, for example, velocity and pressure distributions around a lifting surface and its attendant lift. In general, when understood and properly applied, the Navier-Stokes equations provide a rich explanation of fluid dynamic phenomena.

These phenomena are often complex and cannot necessarily be explained in simple, one- or two-sentence reductions. I suspect this is what John D. Anderson, Jr. (whom I had the good fortune to study under when I was a graduate student), meant in the statement, attributed to him in the article, that “there is no simple one-liner answer” regarding aerodynamic lift.

Fairfax Station, Va.

As a retired aeronautical engineer, I find Regis's article to be lacking in basic knowledge that every student of aerodynamics learns early on in his or her studies about how wings generate their lift forces. The operative words are “circulation theory.” His failure to include the theory in his explanations should be embarrassing to your technical staff.

via e-mail

REGIS REPLIES: The diversity of opinions in the 67 letters sent in response to my article aptly illustrates its theme: namely, there is no one simple, nontechnical explanation of aerodynamic lift that is universally acceptable. Several readers proposed alternative explanations of lift. And most of the letters introduced technical matters. I chose not to address such topics in the piece, which was intentionally confined to arguments and principles accessible to nonspecialists. Thus, I did not include a discussion of phenomena such as the Kutta-Joukowski theorem, circulation theory, Ludwig Prandtl's boundary layer theory, the Reynolds-averaged Navier-Stokes equations, the Coanda effect or other staples commonly found in advanced accounts of aerodynamics.

I also avoided touching on even as elementary an issue as whether the same forces that act on a wing moving through stationary air act on a stationary wing around which air moves. That seemingly simple question is a controversial and undecided one among aerodynamicists. Anderson really did say it best in the quote Slomski refers to.