# 1.5 Categories of Physics | Summary and Q&A

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November 8, 2021
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MIT OpenCourseWare
1.5 Categories of Physics

## TL;DR

Special relativity is a theory that explains fast-moving objects and light behavior, while other physics theories like classical mechanics, quantum mechanics, and general relativity are needed to understand different aspects of the physical world.

## Key Insights

• 🙂 Special relativity has a specific range of validity, focusing on fast-moving objects and the behavior of light.
• 🦾 Classical mechanics, quantum mechanics, and general relativity are complementary theories that each have their specific applications.
• 🥺 Combining special relativity with quantum mechanics leads to the study of quantum field theories.

## Transcript

MARKUS KLUTE: Welcome back to 8.20 Special Relativity. In this short video, you're going to see how special relativity, here shown as SR, fits into the general landscape of physics theories. Very likely you have listened to 8.01 last semester or some previous time in your career at MIT. And once we enter the discussion of special relativity, it mig... Read More

### Q: What is the range of validity for classical mechanics?

Classical mechanics is valid for describing the motion, collision, and rotation of objects at small velocities compared to the speed of light.

### Q: What does special relativity explain?

Special relativity focuses on fast-moving phenomena, such as bodies with high speeds and the behavior of light. It complements 8.01 and 8.02.

### Q: When is quantum mechanics necessary?

Quantum mechanics is needed to properly describe objects on small scales, such as particles and their quantum behaviors.

### Q: What does general relativity encompass?

General relativity is a generalized view that incorporates special relativity and is necessary when dealing with objects of large masses or accelerations.

## Summary & Key Takeaways

• Classical mechanics describes the motion, collision, and rotation of objects, but it breaks down when dealing with very high velocities.

• Special relativity is specifically useful for understanding fast-moving phenomena and the behavior of light. It has its own range of validity.

• Quantum mechanics is necessary for studying objects on small scales, while general relativity is needed for objects with large masses or accelerations.

• When combining special relativity with quantum mechanics, it can describe quantum field theories. General relativity is a generalization of special relativity.