Quantum Astronomy: Information in the Universe
This is a short addition to the four-part series on Quantum Astronomy previously written for SPACE.com. Here, we add some details resulting from the process of submitting a paper to the scientific literature. If you?d like to read the technical paper it is entitled, ?Quantum Uncertainty Considerations for Gravitational Lens Interferometry? by Doyle and Carico, and can be downloaded at the Web site:? http://www.bentham.org/open/toaaj/openaccess2.htm.
Having written about four dozen articles now for SPACE.com and I can say that none have given me as much feedback as the series on quantum astronomy? I think people intuit that quantum physics is still redefining how we think of science and what we think the fundamental nature of reality may be, and thus enjoy participating in this amazing modern adventure.
To quickly summarize the preceding series on the quantum astronomy, in the first article 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 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 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, 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.
To review this experiment, John Wheeler (a colleague of Einstein?s) proposed that light from a quasar about 7 billion light years away is split by a gravitational lens, and so we have light traveling to us along two paths?A (the shorter path) and B (the longer path, that encounters more of the gravitational lensing galaxy and whose path is ?bent? toward us). If a fiber optics cable (trillions of miles long would be needed) could be used to make the distance along the shorter path A equal to the distance along path B, then one could get an interference pattern rather than just an image of A superimposed on an image of B at the detector. But, interestingly, at the detection rate of one photon at a time, that would mean one could decide to have the photon travel both paths at the last moment rather than just path A (or B) ? deciding this 7 billion years after the photon supposedly left the quasar! Thus this experiment really meant delayed-choice, to the point where John Wheeler could talk about his hypothetical experiment in terms of altering ?history.? But it could only be thought at the time (such thought-only experiments were dubbed ?gedanken? experiments by Albert Einstein).
Changing this experiment from a gedanken experiment to a performable experiment,? my colleague Dr. David Carico and I 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.
We did have to go through some mighty refereeing to get this paper in print, however. One of the biggest doubts about this experiment working was related to using it on extended objects in the sky. It was aid that one may measure a point source ?traveling? along two paths then, but what if the source is a whole extended galaxy? Well, even galaxies can be thought of as being made up of a lot of ?point? sources, so we argued that the technique would still nevertheless apply, as long as one could not tell what the extent of the actual galaxy (angular size on the sky) was. We did this by introducing what is called a ?Mach-Zehnder Interferometer?(MZI) which,? unlike a double-slit set-up, cannot tell the angular extent of a photon source because it does not produce an interference pattern ? it only indicates whether interference is taking place or not. (For those familiar with the MZI, the gravitational lens itself is the first beam splitter in the system and has an effective refractive index so can change the phase of the light. For those of you not familiar with the MZI, thanks for hanging in there so far!)?
We also talked with many physicists about this idea and all were encouraging. Freeman Dyson of Princeton Institute for Advanced Studies told us, ?I think you?re OK.? Andre Linde of Stanford University said, ?These things are tricky.? Daniel Greenberger of City College of New York said, ?I think it is worth a try.? And John Wheeler (at a scientific meeting on the occasion of his 90th birthday) said, ?That?s very interesting. I hope you succeed.? Of course the actual referees for our paper were more detailed and the process did drag on for a couple of years. Scientists are usually very friendly and happy to discuss new ideas, but when something is going into the refereed scientific literature, that is a whole ?nother story.
One referee wrote, ?The validity of the claim that interference would be observed between extended sources if observed through a sufficiently narrow band filter is absolutely critical?.if it is right the implications would be extremely profound, and extend far beyond the narrow confines of measuring time delays in lensed systems, as it would completely undermine the conventional understanding of how interferometry works.? I have to confess that as I read this at this point I thought ?Gulp.? But I also realized that?barring anything we and the referees and editors overlooked?on the other hand, if this experiment did not work it would be a more radical departure for physics than if it did. This is because it would imply that the quantum uncertainty principle itself did not apply in some circumstances?did not, for example, extend over macroscopic distances. So, with this argument, our paper was finally accepted.
The great quantum physicist, Richard Feynman, once said (to paraphrase); If you think you understand quantum physics then you don?t understand enough to understand that you don?t understand it! And Einstein himself once wrote, ?I have thought a hundred times as much about the quantum problems as I have about general relativity theory.? We can relate. And you are also most welcome to join Einstein?s ?hundred times? club. You, too, may begin thinking of the universe, not so much in terms of material objects, but rather in terms of information. And as quantum measurement begins to leave the laboratory and extend throughout space I think we?re all in for a lot of surprises. And a lot of fun too.
Note: A talk on this experiment can also be heard online as part of the SETI Institute lecture series at: http://archive.seti.org/Flash/csc-jan9-production/jan9-production.html.
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