There are some quite interesting modifications that can be made to the simple double slit experiment from the last article.  It is possible to determine which slit each photon goes through as it makes its way from the light source to the recording surface without in any way changing the photon’s energy or direction.  If the information about which slit is traversed is recorded along with the impact point of each photon, it turns out that the individual photons will accumulate as two bright spots without any interference pattern.  If the information is not recorded, or even if it is recorded then erased before the researchers can view it, the photons will form the typical interference pattern.  Just like the photons that seem to “know” the configuration of the slits as in the last article, when there are two slits, they “know” whether the information about which path they followed is recorded to be viewed by the physicists running the experiment.  Somehow, they adjust their path according to the potential information available for the experimenters (or anyone else) to review.  Even though they have no known physical mechanisms to gather this information.

There are further modifications of this same experimental set-up that are quite elegant and provide other unexpected insights.  In place of the usual light source, it is possible to use a certain type of crystal that will produce two photons simultaneously that share certain characteristics and head off in different directions.  These photons are called “entangled” by physicists to indicate that when you measure certain parameters from one, you know the value of that parameter about the other.  One of the pair can be sent through the modified two slit experiment and the other can be sent to a recording device.  I won’t get into the specifics here, but the marker photon will let you know which slit the other entangled photon will be going through, if you measure the appropriate parameter on the marker.  This is one way to accomplish the recording of which slit a photon goes through mentioned in the last paragraph without any physical impact on that photon.

This way of marking the pathway using an entangled marking photon, though, lets you add the elegant twist to the experiment.  You can delay the measurement of the marker photon until after the other entangled photon has traveled all the way through the slits and has been recorded at the target surface.  Just as in the earlier experiment, whenever you fail to gather the which-slot information, you get an interference pattern after you allow enough of the single photons to accumulate.  Alternatively, if you gather the which-slot information, you just get two bright spots of photons.   

What you find is that, like before, when the which-slot information is recorded no interference is found and when the information is not available, you see the interference.   The decision to record or not, made after the photon’s trip is completed, changes the photon’s path.  It is as if the act of recording the which-way information, after the first photon’s travel is finished, goes back in time to adjust the photon’s path accordingly.  Not only does the act of measuring the which-slot information nonphysically change the photon’s path, it violates the rules of time and the rule of cause and effect to do so.  The effect of the changing path comes before the cause which brings it about!

Two more mysterious puzzle pieces are added, here, to our understanding of the way the world works.  First we find that inanimate pieces of matter adjust their behavior according to what humans can know about them.  Second, they do this is ways that violate widely accepted physical “laws” that govern the transfer of information.  The information can be transferred faster than the speed of light and makes use of no physical mediator to carry it.  The information transfer also violates the “laws” of causality and what is called “the arrow of time.”  These particles of matter do this automatically and they do it every time we carry out these procedures.  These insights are not based on esoteric theoretical conclusions from complicated quantum mechanics calculations.  They are undeniable, simple, observations of how our world works.  With the purchase of the necessary equipment and a little technical skill in setting it up, you could even verify these results yourself at home.

In the next article, we will look at anomalies from the experimental psychologist’s lab to learn how human beings can gather information about distant events in startling ways.