Ask A Scientist Physics Archive

    
name         Terry D.
age          50s
   
   Question -   I am a lay person who likes to study science.  I am
currently reading Dr. Brian Greens book on the Fabric of the Cosmos.  I
am fascinated by his description of the double slit experiments that
demonstrate the dual nature of electrons and photons.  In the quantum
mechanic double slit experiments that reduce the electron or photon
emission to a very low number and record the accumulated strikes on the
detector in the famous interference pattern as would be indicated by the
probability wave, are all of the electrons or photons  emitted accounted
for or is the ratio of emissions to recorded strikes on par with the
ratio of the slit area to the illuminated area?
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All of the electrons / photons can be accounted for. Young's "simple"
experiment has been carried out in many variations at high levels of
sophistication. Even to the extent that the two slits are far enough apart
that light cannot make it from one slit to the other to "tell" the other
photon / electron whether the first slit is open or shut. Not withstanding,
the results are the same. The Quantum Mechanical particle "knows" whether one or both
slits are open. The experiment in principle is so simple, the only
conclusion is that our intuition utterly fails us when it comes to Q.M.
particles. This is why quantum mechanics (the people, not the subject) have
to let "the math" take them where it goes and not try to "explain" the
results in classical terms. As unsettling as that is, there does not seem to
be any other choice to align the predictions and the experimental results.

Vince Calder
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Well, Terry,
You are referring to the experiment wherein someone emits electrons at a pair of 
side-by-side slits.  The electrons that go thru the slits then hit the far wall in a 
slightly spread-out pattern, which is rippled, weirdly enough, instead of being just one
broad bump.  Even weirder,  the highest point in the rippled pattern is right in line 
with the blocked spot between the two slits.  So how can a straight-line-zinging particle 
get there?  Which slit did it go thru?  It looks like one electron split itself
and went thru both slits, and then rejoined itself!  And it must be one electron, because 
a single-strike detector ( something which "ticks" like a Geiger counter) was used.
The same ripples also show up when you use higher beam currents and average-current 
detectors, such as a glowing phosphor screen across the wall.

I believe that for most implementations of this experiment, a simple single wide beam of 
electrons is thrown in the general direction of the slits.
So it's true, most of the electrons in the beam miss the slits and hit one of the 3 metal 
barriers (left right, or between the two slits) and are stopped.
By narrowing the beamwidth to just wide enough to paint both slits, and making the slits 
fairly wide (each width being 1/4 or 1/2 of the separation between slits), one might tune 
up the efficiency to maybe 10%.  But most scientists have been happy to do
it with less than 0.1% efficiency;  pico-amp current meters and/or electron multipliers to 
detect single strikes are not too hard to come by.

If one tried to get higher than 10% by making two separate pin-point electron beams aimed 
only at the slits, I think the interference pattern would stop happening.
This is presuming you used two separate filaments to emit the two separate beams.  In that
 case the electrons approaching the left slit are quantum-mechanically distinct from the 
 electrons approaching the right slit, so they usually don't "mix" after they
go thru the slits.
If you take the beam from one filament and massage it with electric fields to make it one 
beam with two maxima, then if you were very careful to leave both beams having equal speed 
and distance, they would still interfere and make a rippled strike-density
pattern on the far side.  And maybe you would reach >10% efficiency.

This phenomenon depends on blurring across the one emitted electron beam, to make sure 
that there is "one electron" that might go thru either slit, which slit being unknowable 
in advance.

Jim Swenson
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