

Double Slit Experiment Explained
Name: Boii2
Status: other
Age: 18
Location: N/A
Country: N/A
Date: 9/30/2003
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).
Replies:
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
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
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
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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Update: June 2012

