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# Quantum Physics Lesson 4.2 - Debroglie Waves, Cats in boxes and The Uncertainty Principle

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## Luke Bohni

on 6 June 2013

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#### Transcript of Quantum Physics Lesson 4.2 - Debroglie Waves, Cats in boxes and The Uncertainty Principle

Debroglie Waves, Cats in boxes and The Uncertainty Principle Lesson 4.2 Last lesson we spoke about light being both a particle and a wave at the same time (a photon).

We came to this conclusion because we had two experiments that demonstrated contradictory results:

Young's Double Slit Experiment showed light diffraction, suggesting it is a wave.
Lenard's Photoelectric Effect showed light behaving like a particle. So imagine our surprise when we discovered this... This is a diffraction pattern made by electrons Meet Louis De Broglie De Broglie asked the question:

"if light can behave as both a wave and a particle, could matter do so also?" The Double slit experiment with particles It was thought that conducting the double slit experiment with particles would result in a pattern like this Using De Brolies idea, physicists were able to determine that the slits being used weren't small enough.
They started using the crystal spacings between atoms as their slits and then were able to make electrons diffract. ...weird... the electrons seem to be interfering with themselves... So what would happen if only 1 electron was used? When one electron passes through the slit, it passes through like a particle and then hits the screen behind it at some random location BUT If we keep firing individual
electrons at the screen we begin to see a pattern... This raises an interesting idea, that everything can be modelled as a wave The Uncertainty Principle Schrodinger's Cat Schrodinger attempted to explain the Uncertainty Principle using a famous 'thought experiment' where he suggested placing a cat into a box with a vile of poison which would be broken by a trigger that is set off by the radioactive decay of a single atom.
The box would be completely sealed otherwise and so you would not know if the cat was alive or dead.
In Quantum Physics this is known as Quantum Superposition, we assume that both states are occuring simultaneously and that it is not until we observe the experiment (and thus end it) that a particular state ends up being chosen. So the Uncertainty Principle basically says that:

we cannot know an objects momentum and position simultaneously

the more accurately we know one, the less accurately we know the other.
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