Reeling From Wheeler It seems to me that the paradox of John Wheeler's cosmic-scale double-slit experiment ["Does the Universe Exist if We're Not Looking?" June issue] disappears if we view the problem from the perspective of the photon. As I understand it, time stops from the perspective of an observer traveling at the speed of light, like our photon. Also, distances are reduced to zero. So for the photon, its birth in the distant quasar and its observation on Earth occur simultaneously and at the same location. This concept of a history of a photon is a human perspective; the photon knows nothing of history. It is only our particular frame of reference that creates an apparent paradox.
Norman Horofker—Midland, Georgia
John Wheeler responds: Norman Horofker is absolutely right that for a photon, time stands still. However, in our frame of reference, a photon has a history and moves through time. Also, the cosmological double-slit experiment could be conducted, in principle, with electrons or protons or other massive particles, even slow ones.
Do scientists have any clues regarding the reason behind the double-slit mystery? There are several theories out there now that purport to bring us closer to a theory of everything. Probably the most fantastical is the multi-universe theory, which ponders the notion of an infinite number of parallel universes residing next to our own. Could this possibility be applied to the double-slit experiment? Could there be two different universes, one in which light acts as a particle and another alongside in which light acts as a wave?
Michael S. Bowen—Anderson, South Carolina
John Wheeler responds: Parallel universes were postulated by my student Hugh Everett in 1957 and have been the object of serious study ever since. This idea provides (so far) no new predictions for the outcomes of experiments, but it does offer mind-stretching insight into the nature of quantum theory. In this "many worlds" interpretation, all possible outcomes of such experiments as the delayed-choice experiment do occur, with the universe continually fractionating into other universes. A photon or other particle would exhibit both wave and particle aspects in every universe.
Your article mentions Wheeler's disagreement with Andrei Linde on whether observation must involve consciousness, but it does not explain why. I suspect it is because Linde's contention that "conscious observers are an essential component of the universe and cannot be replaced by inanimate objects" is rooted in mysticism and human chauvinism rather than science. Where is the line Linde imagines that separates animate from inanimate objects? The seat of consciousness, the human brain, is a three-pound lump of quarks and electrons that is subject to all physical laws. It receives its data from sense organs that also consist of matter and whose interaction with the rest of the universe is also governed by physical laws, such as those described by quantum mechanics.
David J. Schuller—Ithaca, New York
John Wheeler responds: Andrei Linde is a great physicist and a deep thinker, but he and I do disagree on the role of consciousness in observation. The process whereby the macroscopic world reacts to a quantum event—the process that makes reality—can, in my view, be accomplished with inanimate matter. Following Niels Bohr, I like to call this process "registration" rather than observation (which too strongly suggests human involvement). Like David Schuller, I find it hard to draw a line between the conscious observer and the inanimate one.
Andrei Linde responds: When I talk about conscious observers, I do not necessarily mean human beings; this indeed would be human chauvinism. If, following David Schuller, we assume that inanimate quarks and electrons have consciousness, then the universe does not exist without consciousness, just as I said. My main point, however, is that a universe consisting only of inanimate unconscious objects cannot be observed, so any statement about its existence or nonexistence cannot be tested. Any statements that cannot be tested belong not to the realm of science but to mysticism.
Erratum In "Does the Universe Exist if We're Not Looking?" we incorrectly stated that the recent John Wheeler symposium took place at Princeton University. It actually took place at the Harrison/Merrill Lynch Conference Center in Plainsboro, New Jersey, and was cosponsored by the John Templeton Foundation and the Peter Gruber Foundation Cosmology Prize.