An interesting phenomenon can be evoked in an actual practically feasible experiment with a Mach-Zehnder interferometer as proposed by Elitzur and Valtmann in 1993. With a Mach-Zehnder interferometer it can be investigated whether an object, for example a light-sensitive bomb as Elitzur and Valtmann suggested, is placed in the path of the interferometer without having to use any light to touch the object. For a more detailed description of a Mach-Zehnder interferometer, I refer to another page on this website. If necessary, read the description there first before continuing.
Shown above is a standard configured Mach-Zehnder interferometer. If no extra phase difference is introduced, using the ΔΦ block, and if the path lengths 1-2-4 (upper) and 1-3-4 (lower) are equal, then D1 receives all the light and D2 receives nothing. When we consider light as an EM wave, that phenomenon can be explained by constructive (D1) and destructive (D2) interference at beam splitter 4 by the waves traveling along the upper path and the lower path.
However, when we try to use the interpretation of light as traveling photons, little particles with a fixed amount of energy, their never reaching D2 becomes much more difficult to explain, because how can a photon, being a particle, travel simultaneously two paths in order to achieve interference?
But when we use the non-physical state function – the quantum wave – then the destructive interference at D2 means only that the chance that photons will be detected there is zero and that therefore all photons will manifest on D1. So far this is something I have already explained elsewhere – and more extensively.
Now it becomes interesting if we set up a blockade (for example an extremely light-sensitive bomb that goes off when hit by a single photon) in one of the two paths. See the figure below. Now assume the following circumstances:
- The experimenter knows that the paths in the interferometer are of equal length and that the configuration is closed.
- The experimenter cannot take a look in the interferometer whether or not a blockage has been placed there (the light-sensitive bomb definitely would explode).
- The experimenter only sees the output of D1 and D2,
- The experimenter does not know if and when photons are sent by the laser.
Does the experimenter now have a chance to find out if one of the two paths is blocked? Without setting of the eventual light-sensitive bomb? That’s indeed possible. He has a success chance of 1:4. Kind of reverse Russian roulette.
Explanation: a photon that chooses the upper path (50/50) will now hit the blockade. Neither D1 or D2 will then go off. The experimenter does not know whether photons have been fired and therefore does not yet know if a blockade exists. If the blockade is a ligth sensitive bomb it will explode upon being hit by the photon. However, if a photon takes the lower path, not setting off the bomb, either D1 or D2 goes off. If D1 goes off, the experimenter still knows nothing because that would be the normal expected behavior of an unblocked interferometer. However, if D2 is hit, the experimenter then knows with certainty that there is a blockage. But that photon that hit D2 has never been anywhere near the blockade!
The probability that a photon will reach D2, due to the two consecutive beam splitters, is now 50% x 50% = 25%. So after just a few laser shots the patient experimenter will find out by D2 beeping that there is a blockage and stay alive (or the bomb goes off ending the experiment). If he / she is lucky D2 will beep right at the first laser shot, chance 1:4. But this now raises the spooky behavior question: how does the photon that arrives at beam splitter 4 know that it now has a 50/50 chance of moving on to D2? It arrived there traveling the lower path and should therefore not be aware of any blockade in the upper path. Yet it is. In other words: for a photon traveling the lower path, there should be no difference between blocking the upper path or not, however, when that blockade is removed, the behavior of an identical photon traveling the unaffected lower path suddenly changes its behavior to 100% reflection at beamsplitter 4 towards D1.
Incidentally, the mystery is solved as soon as you use either wave model, EM wave or quantum state wave. The blockage eliminates the possibility of interference of the wave with itself at beam splitter 4. However, the quantum interpretation with physical photons that travel leads to undeniable contradictions.
The only other quantum interpretation known to me that also assumes that the photon is not traveling physically and that can explain this “spooky” phenomenon is John G. Cramer’s Transactional Interpretation that assumes advanced quantum waves that travel back in time from the detector to the laser. But there are serious objections to his interpretation. Among other things, the quantum state wave itself also contributes to the transferred energy in his interpretation and is therefore physical. For further reading about his idea I do recommend Cramers didactically excellent book “The Quantum Handshake.”
Other researchers have conducted a successful delayed-choice experiment using photons. Although these objects seem to move at the speed of light, the experimenters have managed to set up the experiment in such a way that the photon – assumed it moves actually – has to adapt in full flight to the changed measurement setup. The so-called two-photon quantum eraser experiment.