Space.com - 2009 August 20
- To quickly summarize the preceding series on the quantum astronomy, in the first article (Quantum Astronomy: The Double Slit Experiment) we looked at the double-slit experiment and how it appears to indicate that a single particle of light (a photon) travels through two slits (apertures) to make an interference pattern, apparently being in two places at once, and yet still be detected as a small particle when it registers on a detector screen.
In the second article (Quantum Astronomy: The Heisenberg Uncertainty Principle) we looked at the uncertainty principle which requires that certain pairs of measurable quantities (position and momentum, for example) cannot both be measured accurately simultaneously. Time and energy are another such set of "complimentary pairs" so that if one measures the energy of a particle really well, one cannot tell very accurately at what time the particle had that energy. This uncertainty principle can be manipulated—one might say that one can trade off one kind of information for another, as long as ignorance is conserved.
In the third article (Quantum Astronomy: Knowability and Unknowability in the Universe) we noted that waves associated with particles in quantum physics are waves of probability (not waves like ocean waves, although they do share many characteristics). So what one can know or cannot know about, for example, which path a photon took to a detector, actually determines what one will detect—for example whether an interference pattern is detected or not. If one cannot tell which path a photon took to a detector, one can get interference, but not otherwise.
And finally, in the fourth article (Quantum Astronomy: A Cosmic-Scale Double-Slit Experiment), we discussed doing a cosmic-scale double-slit experiment, first proposed by John Wheeler of Princeton University, where a decision about which path a photon takes around a gravitational lens (a galaxy aligned so it can bend light from a more distance quasar) can be decided long after—even billions of years after—the photon had supposedly already left the source and traveled along one path or the other. This was called the "cosmic-scale delayed-choice" experiment.
Changing this experiment from a gedanken experiment to a performable experiment, (We) proposed that one might actually utilize the uncertainty principle itself to replace the trillions-of-miles-long fiber optics cable. This notion was based on the idea that, since knowability or unknowability is the important consideration (rather than actual distances involved), we proposed not so much to make the two paths a photon traveled equal, but rather to just render any difference in the length of the two paths unmeasureable (i.e., unknowable). We proposed that by knowing the energy of the photon very well (by using a narrow band radio filter, for example) that the time that the photon actually had that energy would be unknowable (since time is the complimentary pair of energy). So, if the unknowability in the time is unmeasureably longer than the delay time between the light paths of the gravitational lens itself, then the two paths are, essentially, "unmeasureably equal," and one cannot tell which path the photon took. If one persists in thinking classically, the photon can then be said to have taken both paths then. To put it in physics-ese, we have used the uncertainty principle as a quantum eraser — it erases the quantum nature of a photon, making it a probability wave again, which can "exist" (if probability wave can be said to exist) along both possible paths again.
The Open Astronomy Journal
ISSN: 1874-3811; Vol. 2 pp. 63-71
Authors: Laurance R. Doyle, David P. Carico
doi: 10.2174/1874381100902010063
Abstract: The measurement of the gravitational lens delay time between light paths has relied, to date, on the source having sufficient variability to allow photometric variations from each path to be compared. However, the delay times of many gravitational lenses cannot be measured because the intrinsic source amplitude variations are too small to be detectable. At the fundamental quantum mechanical level, such photometric “time stamps” allow which-path knowledge, removing the ability to obtain an interference pattern. However, if the two paths can be made effectively equal (zero time delay) then interference can occur. We describe an interferometric approach to measuring gravitational lens delay times using a “quantum-eraser/restorer” approach, whereby the light travel time along the two paths may be rendered unmeasurable. Energy and time being non-commuting observables, constraints on the photon energy in the energy-time uncertainty principle — via adjustments of the width of the radio bandpass — dictate the uncertainty of the time delay and therefore whether the “path taken” along one or the other gravitational lens geodesic is “knowable.” If one starts with interference, for example, which-path information returns when the bandpass is broadened (constraints on the energy are relaxed) to the point where the uncertainty principle allows a knowledge of the arrival time to better than the gravitational lens delay time itself, at which point the interference will disappear. We discuss the near-term feasibility of such measurements in light of current narrow-band radio detectors and known short time-delay gravitational lenses.
