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The Double-Slit Experiment

The double-slit experiment is a physical experiment that was originally developed in 1801 by Thomas Young to study the wave behavior of light. In the experiment, particles like light or electrons are directed through two closely spaced slits onto a detection screen behind them.

According to classical physics, you would expect to see two bright bands behind the slits, corresponding to the slits themselves. However, what actually appears is an interference pattern of several bright and dark stripes. This means that the waves (or particles) interfere with each other, either amplifying or canceling out, similar to water waves. This result suggests that light is not made of particles alone but also exhibits wave-like properties.

Now here's the interesting part: if we try to measure which slit the light actually passes through by placing a detector at one of the slits, it suddenly behaves like a particle. The interference pattern disappears entirely, just because we performed the measurement.

To summarize: As long as no measurement is made, the particles behave as if they pass through both slits simultaneously, existing in multiple states at once. Only when a measurement occurs does this superposition collapse, leaving a single observable state.

The intriguing question is what a measurement actually means. According to the Copenhagen Interpretation, it is the act of observation that determines which state the system ends up in.

And even if this may still sound a bit unspectacular to some, let's bring in the thought experiment of the Austrian physicist Erwin Schrödinger, which I already mentioned in an older blog post. In this scenario, a cat is placed inside a box that contains a mechanism connected to a single radioactive atom. If the atom decays, a Geiger counter is triggered, which in turn activates a device that releases poison and kills the cat.

Now let's recall the double-slit experiment. As long as the atom is not observed, it exists, according to the Copenhagen Interpretation of quantum mechanics, in a superposition of both decayed and not decayed states. This means that the cat would be alive and dead at the same time.

Apart from the Copenhagen Interpretation, there is also the Many-Worlds Interpretation (There is also an old blog post about this here). This one suggests that in every quantum event, not just one possible outcome becomes reality, but all of them do. The universe branches into different parallel realities where every possibility actually takes place.

In the case of Schrödinger's cat, this means that in one universe the cat is dead, while in another it is still alive. Both versions exist simultaneously, but in separate worlds that can no longer influence each other.

Closing Words

The double-slit experiment leaves us with more questions than answers. I'm curious whether we'll ever find out what it's all about.