Ask A Scientist Physics Archive

name         Boii2
status       other
age          18

Question -   Hi, I have a question to ask regarding the double slit experiment. In the double 
slit experiment, when we allow just one electron (or in that case, even a photon) to pass 
through the slits (both open), we consider the probability functions of the electron and say 
that the this probability function splits up into 2 and after passing through the slits 
reinforce. That is to say that the electrons pass through both the slits simultaneously 
(provided that no one is observing). I find this pretty hard to comprehend especially when 
I think about energy conservation. How is that one electron now appears as two violating 
the Einstein equation E=m*c*c (I have never found any mention of this anywhere else, so I 
understand that my question is inherently absurd but please clear my doubts).
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Boii2,
You have hit on one of the most confusing things in quantum mechanics.  When you are measuring 
properties of an electron, the electron behaves as a particle.  It has standard electron mass 
and other standard properties. This only holds at the instant you do the measurement.  The 
more time a particle spends without being measured, the more random the electron becomes.  
If you measure which hole the electron passes through, then it will only pass through one of 
the holes.  If you do not measure which hole,
then it will pass through both holes as a wave.  This fact is known by measuring the angle of 
the electron's motion after passing through.  I do not think any person truly understands why 
this happens.  Richard Feynman, perhaps the greatest quantum physicists, once said  that anyone 
who claims to understand quantum physics must be telling a lie.  Accept the fact that nothing 
tiny is completely a particle or completely a wave.  Every individual particle or wave has both
properties.  A photon of light tends more toward wave properties.  An electron tends more 
toward particle properties.  Still, both are somewhere between particle and wave.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College
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It *is* pretty hard to comprehend, but you do not want to be thinking about one electron 
appearing as two electrons.  The fact is that you do not know where the single electron is 
with enough accuracy to say which slit it goes through.  (If you do a measurement to 
determine which slit the electron goes through, the measurement will destroy the interference 
pattern.)  Note that physicists did not make this up, and they do not make it happen.  All 
they did was to notice that this is the way electrons behave, and then they built a 
mathematical formalism consistent with this behavior, and other known behaviors, so that 
they could guess what an electron might do in different circumstances.  The formalism 
describes what electrons do with fantastic precision, but -- as you have noticed -- it 
does not make much intuitive sense, and it is pretty hard to put into words.

Tim Mooney
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If you have difficulty understanding the "double slit" experiment, you join a large audience 
of some of the best minds in physics. What makes the experiment so frustrating and infuriating 
is that it is so simple, in principle. It applies to electrons as well as photons and 
presumably to other "particles" as well. To the best of my knowledge, there is no 
interpretation that is generally accepted by scientists who know a lot more quantum 
mechanics than I. All sorts of very "strained" explanations have been offered. Here is a 
recent web page that will let you observe the fray, but I do not think it can be called 
an "explanation".

http://www.sciscoop.com/story/2003/7/21/1715/66159

Vince Calder
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It is, of course, impossible to really comprehend as quantum mechanics is only significant 
for objects much smaller that we have experience with.  It is, after all, quite mysterious 
that when you release a ball, it falls without anything visible doing anything to it.  However, 
we have observed that so many times and in so many ways that it seems reasonable and even 
obvious to us.

It might help if remember (as you correctly and with considerable insight point out) that an 
electron passes through both slits PROVIDED THAT NO ONE IS OBSERVING.  Actually, the wave 
function of the electron always spreads out throughout the universe, although with very 
little amplitude where it is very unlikely to be.  Then when one observes it, the wave 
function instantaneously collapses to the point where the electron is observed.  The 
probability of it appearing at any point is proportional to the square of the wave function 
at that point which will, of course, depend on whether the electron wave function "went 
through" two slits.  This is the "Copenhagen" interpretation of quantum mechanics and how 
that really works is not terribly well understood and, in fact, there is disagreement about 
it.

The crucial point is that the electron never "appears as two".  When observed, it is just 
one.

Best, Dick Plano...
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