Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Double Slit versus Overlapping Diffraction

Name: Julie
Status: other
Grade:
Location: Outside U.S.
Country: United Kingdom
Date: April 4, 2011


Question:
Why can't the double-slit experiment be explained by overlapping diffraction effects? A few weeks ago I watched a documentary called 'What Is Reality?' with my 15 year old daughter. After the show we discussed the double-slit experiment because the conclusions of this experiment, particles mysteriously splitting up and then interfering with themselves, sounded way too far-fetched for our liking. As I understand it, diffraction occurs at a single edge, and if you put two edges close to enough to each other (to form a slit) then the diffraction effects from the two edges will make the slit appear like a new light source. If two slits were close enough together, then there would be areas where the diffraction effects would overlap. It seems much more plausible to us that the interference is something related to the overlapping diffraction zones rather than some mystical wave-like nature of particles that nobody really understands. Thinking that many people would have come to this obvious conclusion, we searched the Internet (including your site) to find out why overlapping diffraction effects could not explain these results - but to no avail. There is an article on the Facebook Quantum Physics discussion page called that mentions a diffraction explanation, but the author then dismisses it without any good reason. Apparently, if a laser is aimed close to, but not above, a knife edge, some photons will bend over the top of the knife edge due to diffraction (demonstrating that diffraction starts at the front edge). Therefore if the gap between the two slits were small enough (close to the 'wavelength' of the oscillating particles in question), then presumably diffraction would pull some particles through one slit and some through the other, possibly leading to an interference pattern as opposed to a diffraction pattern? Furthermore, I have heard that experiments have been done as long ago as 1987 that prove you can detect which slit a particle has passed through without completely destroying the interference pattern - debunking the theory that particles mysteriously change their behaviour when we 'look at them'. To the layman, like me and my daughter, a diffraction-based explanation would make a lot more sense than the mystical (practically magical) explanation put forward by quantum physicists. It would explain why firing one particle at a time produces the same interference pattern, and why the pattern is different to the sum of two single-slit patterns. I realise that you may need to describe some advanced concepts in your answer, but I would be very grateful if you could try, as far as possible, to answer in layman's terms, that both me and my daughter might understand,



Replies:
You are addressing a problem that has been studied by the best minds in physics for a century (or so). By no means are they "Idiots propose an alternative explanation of the double-slit experiment". That assertion simply does not understand the problem.

Among other issues are: 1. The interference not only occurs in the case of "light", but also in the case of what we would call "light" in laymen's terms, the interference pattern is observed between two slits separated by a distance that exceeds the speed of light between the two slits. (2). The interference has also been observed between what we would call "particles". So how does the information contained in one slit "communicate that information" to the other slit if the distance is too great for that information about its interference is too "slow" to occur before the two beams reach the slits, before they can "communicate" with one another? And how does interference occur between "particles" that are larger than the "double slit". The "double slit" problem has received the attention of the best minds in physics for about 100 years (or so). People who trivialize the problem with simple minded "explanations" that the best minds in physics have grappled with, simply do not understand the problem. The "double slit" phenomenon has been studied from the sub-microscopic level to the astronomical level. As scientists (regardless of our level of study) we must accept the data. Our collective "job" is to understand the experimental results. You are on sound ground that has studied this weird behavior of our Universe, but rest (un) assured that the best minds in physics have not been "duped".

Vince Calder


Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory