This is the very
famous and well-studied experiment that illustrates the wave/particle duality
of protons and electrons.
The double-slit
experiment, or sometimes called Young's interference experiment, is a simple
set-up of light source shining through two slits onto a receptor screen as
illustrated below.
What happens is that the two
streams of light on the receptor side of the double-slit set-up cause a pattern
on the screen of lighter and darker bands. This is because the light is coming
in waves: so picture two wave sources in water, two separate, waggling fingers
for example. The water waves will interfere with each other, with some parts
forming troughs and cancelling each other out, some parts combining to form
peaks, as illustrated below, which, you can imagine, would lead to an
interference pattern on a screen:
This
shows the wave nature of light; in fact, coloured light is measured in its
wavelength, or wave-frequency. Blue light is of shorter wavelength, changing
more frequently, while red light is of longer wavelength, changing less
frequently. The rapidity of change also denotes the energy of the light,
whereas the intensity is the amount of light. Blue light is therefore
more energetic than red light.
So, light is a wave.
But when the intensity of the
light source is turned down low enough, just one “piece” of light goes through
the system at a time, which instigates a single response at the screen. One
piece of light, one quanta, is the photon, the single particle of light. Which
suggests that light is actually a particle, not a wave. And the particles
appear on the screen randomly; no matter how the responses are analysed, there
is no predicting the next response position, no telling where the photon is
likely to land on the screen.
The strange thing is, though,
that if the system is left on like this, with single particles landing randomly
on the screen, the pattern they make will eventually build back into the
interference pattern, exactly as before.
So how do the random particles
“know” where they should and should not go? Somehow, they are aware of where
the other photons have landed, and are able to arrange themselves to finally
build the interference pattern back again. How?
And then, perhaps even stranger,
is what happens when physicists attempt to find out through which of the two
slits the photon has passed. By placing just one detector into the system, at
one slit or the other, so that the particle can be detected there or not,
whenever a photon makes it to the screen physicists can tell if it has either
passed through the slit with the detector or the other slit. The detector
doesn't stop the particle – the photon still gets to the screen. But, guess
what? The interference pattern has disappeared. The pattern appears as two separated
light patches directly behind the two slits on the screen, as if light was not
a wave at all.
Even if the detector is
positioned in the system after the slits, the photons seem to “know”
they are being examined and change their behaviour. In fact, it is possible to
leave the detector in place but stop taking readings and the interference
pattern returns. Examine the system and the pattern disappears again.
The system will simply not allow
itself to be examined. The American physicist Richard P. Feynman called it "a phenomenon which is impossible ... to
explain in any classical way, and which has in it the heart of quantum
mechanics. In reality, it contains the only mystery (of quantum
mechanics)". And he was fond of saying that all of quantum mechanics can
be gleaned from carefully thinking through the implications of this single
experiment.
Demystifying The Double Slit Experiment
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