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Quantum Physics: Definition, Theories & Topics

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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.

Quantum physics or quantum mechanics is the study of the universe on a very small scale. Study the definition of quantum physics, key ideas and theories, and the topics associated with this branch of physics. Updated: 10/12/2021

What Is Quantum Physics?

Did you know that there is a tiny probability that the next time you walk into a wall you'll go through it? Or that light is both a wave and a particle? Or that a cat can be both alive and dead at the same time?

No? Well, those are just a few of the weird and wonderful conclusions people have made from studying quantum mechanics. Quantum physics is strange. There's just no denying it. But what exactly is quantum physics?

Quantum physics, more commonly known as quantum mechanics, is a body of work in physics that seems to explain the universe on tiny scales. The laws of physics that work on human scales just don't apply when things are extremely tiny.

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  • 0:00 What Is Quantum Physics?
  • 0:46 Quantum Mechanics…
  • 1:26 Key Ideas and Theories
  • 2:58 Weird Consequences
  • 4:27 Lesson Summary
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Quantum Mechanics Explains a Lot

Quantum mechanics might be weird, but it explains a lot about the universe. It explains why light seems to act like a particle in some circumstances and like a wave in others. Or why particles like electrons can also act like waves. It explains the properties of the elements in the periodic table and how chemical bonding works.

In fact, if you have a flat-screen television, you probably don't realize that those were invented using quantum mechanical principles. Some parts of computers and sensors in digital cameras are also a result of quantum mechanics. As hard as it might be to get your head around, quantum mechanics works.

Key Ideas and Theories

Examples of (traveling) wave functions

In quantum mechanics, a particle doesn't have an exact position but can be considered to be in multiple places at once. Instead of having things like position, mass, and velocity to describe a particle, you have a series of wave functions. A wave function is an equation that describes the probability of a quantity having a certain value. So, for example, it might tell you how likely it is that the particle is at point A vs. point B.

Examples of wave functions in 3D

When we actually go and measure a particle's position, that wave function 'collapses' into a simpler one that represents the specific position we measured. Before we measured it, the particle was considered to be in every possible location at once. After we measure it, it now does have a specific position. But by measuring its position, we have affected it - we have forced it into having a specific position. The act of measurement affects the outcome. It's as if we've changed its nature from a wave, or wave function, to a particle. This is the source of wave-particle duality, the idea that things like light can act like waves in some circumstances or particles in others.

Heisenberg Uncertainty Principle Between Momentum (and Therefore Velocity) and Position

This leads directly into something called the Heisenberg Uncertainty Principle. This tells us that if you know one thing about a particle with a lot of precision, you can only know another quantity with hardly any precision. For example, if you know where your particle is within a small margin of error, you'll have no idea how fast it's moving.

Weird Consequences

The Heisenberg Uncertainty Principle explains a lot, and the mathematics behind it allows us to do things we could never do before, like build flat-screen televisions and tiny computers. But it does have some weird implications, both technical and philosophical.

A classic example is Schrodinger's Cat. If you were to apply quantum mechanical principles to a larger object, like a cat, it wouldn't really work. Let's say you put a cat in a box, with a glass vial containing a poison that would kill it. Inside the box, the cat might have knocked over the vial and be dead, or the cat might be fine. The only way to know is to open the box and see.

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