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See Inside October 2010

Readers Respond to "Is Time an Illusion?" and Other Articles

Letters to the editor from the June 2010 issue of Scientific American


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

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