THE TWO-SLIT EXPERIMENT–In Layman’s Terms

I would like to elaborate a bit on the TWO-SLIT EXPERIMENT which was the basis of my last posting.
It begins with Thomas Young's experiment in the 19th century in which he shown a beam of light through cardboard which had two vertical but parallel slits in it.  The pattern which appeared on the screen behind the two slits revealed something only seen with wave phenomena.  In other words, when any two waves [any substance including water]  intersect they can either cancel each other out, or compound their energies.  The resultant pattern is known as an interference pattern and appears as alternating bands of darkness and light–it characterizes wave forms.

It seemed quite clear that Young had determined that the nature of light was a wave.  However, Einstein's  work in quantum physics  in the first decade of the 20th century [ the photoelectric effect for which he won a Nobel Prize]  clearly showed that light in the form of photons was behaving as a particle.

The TWO-SLIGHT EXPERIMENT was then re-visited.  It seemed as if light could be slowed down so that only ONE photon at a time was emitted from the light source.  When both slits remained open, each photon could be noted appearing on the screen behind the slits  in succession.  The problem was this– although each photon exited its source and hit the screen as a unique particle, the pattern that emerged after time was that of an interference pattern!

In other words, somehow, although one particle/photon was emitted and one photon/particle hit the screen behind the slits, the cumulative effect showed that light was still behaving like a wave.  Yet what was interfering with what?  How could one photon interfere with itself?

Even stranger, if there was a way of noting which slit the photon passed through, by signaling a sound at each slit, the pattern on the screen lost its interference pattern.  Instead, two lines of photons appeared behind the two slits on the cardboard. 

Now taking note of which slit the photon passed through did not itself change anything.  Yet the observation which was made clearly did change the results of the experiment.

Making the observation seemed to change the nature of light from a wave to a particle. But how could this be possible?  How could the nature of something be so confusing.  Could light actually be of two natures, in a state of probability, before it was measured?
If you're confused by all of it–join the crowd.  No one is quite sure what it means.  Theories abound which add more confusion that clarity:  a 'virtual' photon may interfere with the visible one, perhaps from a parallel, invisible universe..[.? ]

Does this help anyone?  At least it gets us thinking.  Stranger than fiction?  Why not?

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