argued that quantum mechanics was seriously flawed.
In 1927 at the 7th Solvay Conference in Brussels, he proposed
what we now call the Einstein Bubble Paradox.
It goes like this: An excited atom emits one photon of visible light.
The prevailing view of quantum mechanics says that this should produce a
spherical wave function, expanding at lightspeed, like an inflating
source-centered bubble. The wave
reaches a detector, and the photon is detected there.
Since the photon stops at the detector and deposits its energy there, it
is assumed that its wave-function bubble should "pop", disappearing
instantaneously over the entire spherical surface except at the detector.
how the bubble popped, i.e., how the remote parts of the wave function away from
the detector "knew" that they should disappear at the instant of detection,
and what mechanism governed their disappearance and prevented multiple photon
detections. (Note: Shahriar
Afshar points out that this scenario is even more paradoxical than Einstein
indicated. The original atom recoils
at the instant of photon emission in a direction opposite that of detection, and
this recoil occurs well before the detection event occurs.
From the bubble viewpoint, this is strange because photon detection retroactively
determines recoil direction, enforcing momentum conservation.)
Werner Heisenberg used his Copenhagen "knowledge" interpretation of quantum
mechanics to answer Einstein's question. The
wave function, he said, is not a real physical object moving through space.
Instead, it is an encoded mathematical representation of the knowledge
of some observer who is watching the process.
Until detection, the observer does not know the precise location of the
emitted photon, so the wave function must be spread out over the whole expanding
sphere to represent his ignorance. However,
at detection he learns the location of the photon, so the wave function "collapses" to the known location of the detector.
The other parts of the wave function must disappear because the
observer's knowledge changes when he learns that the photon reached the
detector and was absorbed.
Some of us find
Heisenberg's explanation of the Bubble Paradox unsatisfactory.
The photon's wave function is the
mathematical solution of the electromagnetic wave equation, a linear 2nd
order differential equation that relates space, time, energy, and momentum.
It has no discernable connection to "observers" or "knowledge".
transactional interpretation of quantum mechanics (TI) requires that the
photon's expanding wave function (i.e., its offer wave) must continue
to expand in all directions, even after a detection event, so that it can reach
other possible photon absorbers and give them a fair shot at receiving the
photon by the formation of a single advanced-retarded handshake connecting one
of them to the source. Further, the
emission end of that completed handshake, forming at the atom at the instant of
emission, implements momentum transfer and atom recoil.
Thus, we have two contrasting predictions: Heisenberg's knowledge interpretation requires the photon's expanding wave function to disappear when the observer learns the photon was detected elsewhere and absorbed. The TI requires the wave function to continue expanding after the detection event. We note that there are also many other interpretations of quantum mechanics, some predicting that Einstein's bubble pops, some predicting that it does not, and some so vague that there is no prediction. However, up to now the physics community (including myself) has believed that there was no way of resolving this issue with an experiment that might falsify one group.
situation changed when Shahriar Afshar, whose paradigm-breaking work on two-slit
interference was featured in my December-2004
Analog AV column, decided to find out experimentally whether
the photon's wave function really disappears following detection.
After nearly two decades of experimental research and theoretical
analysis, Afshar designed an experiment that may finally lift the veil on the behind-the-scenes workings of quantum
mechanics. Briefly, a single photon
is produced, detected, and then tested to see if its "dark" wave still
exists and can interfere with the overlapping coherent wave of a second photon
emitted earlier. This figure shows
Afshar's new experimental setup in more detail.
Afshar's Asymmetric Mach-Zehnder Interferometer
The source of
photons is a weak long-coherence-length laser.
Following the laser is a chopper, where the continuous laser beam becomes
a sequence of 10-50 ns pulses. The
laser's coherence length, several kilometers, ensures that the wave functions
in successive pulses will be coherent and can interfere.
The laser pulses pass into the optical arrangement shown, which Afshar
calls an "asymmetric Mach-Zehnder interferometer".
It is a rectangle
of optical paths, with the laser pulse entering horizontally at the lower left
corner of the rectangle, 50:50 beam splitters at the entrance and exit ports,
and 90o reflecting mirrors at the upper two corners.
In addition, there are "interrogator" beam splitters near the
entrance port on both paths. Single-photon
detectors are placed as shown, with D1 (short) and D4
(long) at the interrogator splitters detecting photons on the short lower and
paths, and D2 (bright) and D3 (dark) at the
interferometer's two exit-port outputs.
Since the upper
and lower interferometer paths have quite different lengths, the height of the
rectangle must be adjusted to sub-wavelength precision so that coherent light
waves on the two paths arrive precisely in phase at the bright port (D2)
and 180o out of phase at the dark port (D3).
A pulse on the
upper (longer) path requires a time Δt
(~50 ns) more than a pulse on the lower (shorter) path to reach the exit ports.
The pulser is adjusted so that the generated pulses are spaced Dt
apart, so that pulse #1 taking the upper path will arrive at the exit ports (D2,
D3) at the same time as pulse #2 taking the lower path.
Now Afshar looks for coincidences
between D1 detections of photon #2,
electronically delayed, and D2
or D3 detections.
John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.
SF Novels by John Cramer: Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106 . His new novel, Fermi's Question is coming soon from Baen Books.
Alternate View Columns Online: Electronic reprints of 219 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .
Einstein, in Electrons et Photons - Rapports et Discussions du Cinqui'ème
Conseil dePhysique tenu, Bruxelles du 24 au 29 Octobre 1927 sous les Auspices de
l'Institut International de Physique Solvay, Gauthier-Villars, Paris (1928).
Jammer, The Conceptual Development of Quantum Mechanics,
New York, (1966).
L. Zehnder, Z.
Instrumentenkunde 11, 275
L. Mach, Z.
Instrumentenkunde 12, 89 (1892).
The Afshar-2 Experiment
S. Afshar, "Experimental Test of No-Collapse Quantum Mechanics: Are there
Quantum 'Dark' States?", Bulletin
of the American Physical Society,
APS March Meeting (2022);
See also a video of the presentation at https://afsharlabs.org/afshar-2.
Transactional Interpretation of Quantum Mechanics:
G. Cramer, The Quantum Handshake - Entanglement, Nonlocality, and
Transactions, Springer: Berlin/Heidelberg, Germany (2016); ISBN
978-3-319-24640-6; Section 6.2 discusses the Einstein Bubble Paradox.
G. Cramer, "The
Transactional Interpretation of Quantum Mechanics",
Reviews of Modern Physics 58, pp. 647–687 (1986) LINK.
Note: The term "Dark
Wave" was coined by Donald W. Glazer, who along with Jeremy Grantham has
provided funding for Afshar's research.