Milky Way Time
In “Is Time an Illusion?” Craig Callender discusses the difficulty of telling if two events are simultaneous or not and thus of establishing a universal, standard measure of time. This argument always seems unconvincing to me. We know how fast our galaxy is rotating, we know our sun’s position and velocity, and we know Earth’s position and velocity. It seems to me that we could easily define a “Milky Way Standard Time” much as was done when we agreed on Greenwich Mean Time way back in the late 1800s, which made it easy to decide what time it was in California when something happened at a certain time in Chicago. By the same token, but with more to calculate than just a difference in longitude, it should be possible to compute the Milky Way Standard Time when two events occurred and determine if they were really simultaneous or not. Does this make the problem go away?

When discussing past-to-future slicing of spacetime, Callender also writes that “the data you need ... are fairly easy to ob­­tain. For instance, you measure the velocities of all particles.” But Heisenberg’s un­certainty principle puts definite limits on how accurately one can measure the position and velocity (or momentum) of a particle. It is a very important limitation, and it seems to me that the entire ar­gu­ment falls apart at this point.
Crawford L. Sachs
Oxnard, Calif.

CALLENDER REPLIES: Saying that events are “really simultaneous” suggests that physics, or nature, prefers one foliation of spacetime—and thus one convention for what events are simultaneous or not—over others. But that preference in this case is really yours, not nature’s. A “Milky Way Standard Time” might be a good choice locally, if only as an approximation (the galaxy is not a rigid body). But because according to general relativity spacetime is curved, there is no standard way to extend a local foliation to the entire universe.

A better choice might be to take the cosmic microwave background to be the definition of what is “at rest” and use that frame of reference to synchronize clocks. Either way, the theoretical problem of time does not go away. There always exist coordinate systems that will make two spacelike-related events happen “at the same time.” Relativity states that no such system is the “right” one.

The point about the uncertainty principle is one that deserves some further study. Indeed, in quantum mechanics we have not only practical but also in-principle limitations on what information we can gather across space at a moment.

Callender lucidly writes that as money is one way to describe the relation between disparate objects, so is time. That eminent experimenter Benjamin Franklin would be pleased to learn that in the physical, as well as financial world, “time is money.”
Doug McKenzie
Palo Alto, Calif.

Cosmetic Changes
Fake Botox, Real Threat,” by Ken Coleman and Raymond A. Zilinskas, should perhaps have been headed “Fake Threat, Real Botox.” Botox is sold in vials containing 100 units, or 4.8 billionths of a gram, of toxin. To accumulate the deadly dose of 70 micrograms of botulinum neurotoxin (BoNT) from cosmetic sources would re­-quire purchasing more than 14,000 vials. For the toxin to be lethal, a person would have to drink more than 145 liters of the liquid. And even a highly discounted price would likely be excessive. Why would a terrorist even consider buying cosmetic Botox when the authors suggest they could make it themselves?
Sydney Bird
Guelph, Ontario

THE AUTHORS REPLY: Our article fo­­cused solely on current, and probable future, illicit manufacturers of counterfeit BoNT products. Nowhere in the article do we even hint that legitimate man­u­fac­turers, or their products, pose a security threat. As we wrote, referring specifically to the market for illegal BoNT and to its makers and distributors: “From a security perspective, this booming market is troubling because for man­u­fac­turer-dist­rib­u­tors it is only a small step from selling coun­terfeit BoNT products for cosmetic uses to selling the botulinum toxin itself in bulk quantities directly to subversive interests.” To reiterate, our concern is that anyone with a credit card and access to the Internet, including criminals and terrorists, can contact illicit manufacturers of BoNT, purchase gram quantities of it and have the purchase delivered to an address of their choice. This is a new proliferation development that we have found is not being addressed by security agencies or international law and therefore needs to be publicized.

One Body, Many Problems
In “Asteroid Collision” [“12 Events That Will Change Everything”], Robin Lloyd discusses how to realistically prevent an asteroid or comet from impacting a high-value target on Earth. She cites the idea of slightly altering the path of the incoming object, using either a kinetic impactor or a nuclear charge. The menace might thus be diverted from, say, a megalopolis, or made to miss Earth altogether. But there’s a catch: the farther the object, the smaller the necessary nudge, yet the greater the uncertainty in predicting the point of impact. Given the notorious chaotic nature of the long-term gravitational many-body problem, a far enough slight nudge calculated to save a city might in­ad­vert­ently end up turning a would-be comfortable miss into an actual bull’s-eye, might it not?
Dov Elyada
Haifa, Israel

LLOYD REPLIES: In the general many-body problem, it is indeed hard to make predictions. Here, however, the near-Earth object is too small to affect the orbits of the planets. Thus, it is just one body moving in a predictable environment. The hard part is to know how much of a nudge to give an object, because its properties, such as its mass, are difficult to measure from afar.

Gossip vs. Science
I am a longtime subscriber to Scientific American and enjoyed your June issue. But political satire as exemplified by Steve Mirsky’s “Presidential Harrisment” [Anti Gravity] seems out of place in the magazine. Mirsky usually makes an effort to have at least a tenuous tie-in to science, but even that is missing this month. You might as legitimately have a gossip column, sports results, wine comparison or travel log. Perhaps Mirsky should seek a position in one of the many fine publications devoted to politics.
William S. Haney
Onancock, Va.

MIRSKY REPLIES: The column was not about politics. It was about rationality, a necessary part of scientific thinking.