The Double Slit Experiment is a famous lab experiment that was stumbled upon by physicists quite accidentally. It has led some physicists to speculate over the existence of multiple parallel worlds, in which every outcome of every event is played out in an infinite multiverse.
The Double Slit Experiment
Voted by Physics World readers as “the most beautiful experiment in physics”, the classic two slit experiment was first conducted by Sir Geoffrey Ingram Taylor in 1909. It involves firing tiny individual particles at a thin plate with two parallel slits in the centre, and watching the pattern they make on the wall behind it.
The outcome of this simple experiment is quite shocking. But to understand the true significance of the Double Slit Experiment, first take a look at the way particles and waves behave in different circumstances.
Particle Behaviors
Firing these tiny balls of matter at a plate with a single slit in the middle always leads to the same result: a vertical line of particles on the back wall. This is because some particles bounce off the plate, while some go directly through the slit and land on the wall predictably. Adding a second slit should therefore create the same result: two vertical lines on the back wall.
Wave Behaviors
Waves are a little more complex; so think of them as like ripples in a pond. When the ripple collides with the plate, only the apex passes through the slit and radiates out. It strikes the back wall with the most intensity in the middle – directly in line with the single slit. This is similar to the single vertical line created by firing particles.
But, sending waves through two slits has a completely different effect. As the top of one wave meets the bottom of another wave, they cancel each other out. This creates an interference pattern on the back wall. There are now lots of vertical lines across the wall where the many little waves hit it with the highest intensity.
The Quantum Effect
In the Double Slit Experiment, electrons are fired at the slit in much the same way. A single slit causes a single vertical line of electrons along the back wall. But when the electrons are fired at two slits – and here comes the kicker – the result is an interference pattern. Why are these tiny bits of matter, fired individually at the wall, suddenly behaving like waves? What are they interacting with?
The Observer Effect
For some physicists, the conclusion was inescapable. The tiny electron arrives at the plate as a single particle, becomes a wave of potentials, goes through both slits, and interferes with itself. Mathematically, this theory is even more bizarre. The electron goes through both slits – and neither. It goes through just one – and just the other. By this reckoning, the Double Slit Experiment suggests that every possibility actually occurs in parallel worlds.
But this was just a theory, so physicists put up a measuring device – an observer – next to the plate to see which one the electron really went through. Amazingly, the electrons returned to behaving like particles again, creating two vertical lines on the back wall. The simple act of observing the quantum world actually changed the outcome!
In short, the electron "decided" to act differently, as if it were "aware" it was being watched. The observer collapsed the wave function simply by observing. And this is the strange world of quantum mechanics.
Source: "The Feynman Double Slit", David M Harrison, Dept of Physics, University of Toronto, 15/03/2006
Further Reading
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