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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