Page 20 - Science Focus (Issue 016)

Introduction

Imagine if we were casted a spell and shrank
to the size of an atom, we would see a completely
different “quantum world” that cannot be – Your Eyes Can Kill
explained by common sense. How bizarre it could
be? Let’s discard our common sense and dive into
the realm of quantum mechanics. Schrödinger's Cat

Double-Slit Experiment & Wave-Particle This implies that the gunshots that passed through
Duality – Is It Wave or Particle? the double slit were waves instead of particles.
However, we clearly know that electron is a particle –
Whenever we talked about quantum mechanics, when one electron was shot, we could only see one
we must mention the iconic double-slit experiment signal on the wall. But when the signals accumulated,
because it directly demonstrates the strangeness of an interference pattern was shown – did the electrons
quantum mechanics. suddenly turn into wave when passing through the
double slit but become particles again before they hit
Let’s assume that we use an electron gun to shoot
electrons one at a time to a wall at random angles, the wall?
and the electrons have to pass through any of the Scientists at that time were confused by this
two narrow slits before arriving at the wall. On the ambiguous phenomenon because it could not
wall, there is a screen to accumulatively record the be explained by classical mechanics. Later, they
locations where the electrons hit it. If we let the electron realized that electrons can simultaneously possess
gun operate for a while, what pattern will form on the the properties of wave and particle. In other words,
screen? they show wave-particle duality . Electrons may
1
display the characteristics of wave and/or particle
According to our common sense, we should expect
the pattern shown in Figure 1: depending on the experimental setting, but both
kinds of characteristics
The electrons that pass can’t be fully shown in a
through the slits form two single experiment, e.g.
straight bands on the wall – the patterns in Figure 1
perhaps this is what we have and 2 can’t be shown
expected. This prediction is simultaneously.
still correct if we shoot objects
with the size of a tennis ball To further add mystery,
or a bullet. However, this is obser vation itself can
not what actually happens affect the experiment. The
if we shoot tiny particles like Figure 1 圖一 pattern shown in Figure
electrons and photons. 2 only appears when we
are not “observing”. If we
The pattern that actually monitor the path of each
forms (Figure 2): electron and know exactly
which slit they pass through, the pattern in Figure 1
The electrons seemed to have curved around the
two slits and arrived at the positions that we didn’t forms. On the other hand, if we are not “monitoring”
expect. An orderly stripes pattern formed. This pattern and don’t know which slit will the electrons pass
is super familiar to scientists – it is created when waves through, all the possibilities will superpose to form the
(e.g. water waves) pass through two narrow slits due to resulting pattern in Figure 2. This is like a classroom –
its property of interference, like in Figure 3. when the teacher is absent, the students can either be