# Quantum Theory: Definition & Examples

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• 0:01 What Is Quantum Theory?
• 1:20 Examples and Key Theories
• 2:40 Schrodinger's Cat
• 3:35 The Elbow and the Table
• 4:15 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this lesson you will be able to explain what quantum theory is, including some key principles and scenarios, and list some applications of quantum theory. A short quiz will follow..

## What Is Quantum Theory?

Next time you're leaning your elbow on the table, what would you think if your arm fell right THROUGH the table? Well, you'd probably think that you'd gone crazy, but if you were a professor of quantum theory, you might be able to explain what happened because quantum theory tells us that it's actually POSSIBLE! Quantum theory is a body of work in physics that explains the way the universe works on the tiniest scales. The regular laws of physics, from Newton's laws to thermal physics, just don't help us when things are super small. They no longer work.

Quantum theory is... well, it's WEIRD. But it helps us explain a lot of things. Light behaves like a particle and a wave, depending on the exact scenario, and quantum theory helps us explain this. The behavior of elements in the periodic table can also be explained by quantum theory. And just observing a particle has an effect on that particle; the particle would be in a different state if you'd never bothered to look.

Even the weirdest conclusions you can make from quantum theory have led to real, practical applications. Quantum theory has been responsible for many inventions that we take for granted today, from flat-screen televisions to sensors in digital cameras to lasers in DVD and Blu-ray players.

## Examples and Key Theories

One of the basic ideas of quantum theory is that you can never know with absolute certainty the position of a particle. Instead of a particle having a position, it has a wavefunction. A wavefunction is an equation that tells you the probability of a quantity (like position) having various values. So, you'll know which is the most likely position the particle holds, not its actual position.

Once you measure the position of the particle, the wavefunction is said to 'collapse' into an actual, specific position. This is why, as mentioned previously, measuring a value actually affects the thing you're measuring. If this strikes you as odd, you're not alone. But this is the source of wave-particle duality; when you measure the particle's position, it goes from behaving like a wave (with a wavefunction) to behaving like a particle (with a specific position).

Another key idea in quantum theory is the Heisenberg Uncertainty Principle. This principle relates various pairs of quantities and says that the more precisely you know one value, the less precisely you know the other. For example, if you know really accurately the position of a particle, you'll hardly have any idea what its velocity is, and vice versa.

## Schrodinger's Cat

Schrodinger's Cat is such a famous thought experiment in quantum theory that it deserves its own section. The Schrodinger's Cat scenario shows what happens if you try to apply quantum physics to regular, everyday objects like a cat. So, the thought experiment goes, if you put a cat in a closed box containing a breakable vial of poison, the cat could be dead OR alive, depending on whether it knocked over and smashed the vial. The only way to know is to open the box.

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