# Lecture 1 | Modern Physics: Classical Mechanics (Stanford) | Summary and Q&A

## Summary

In this video, the speaker discusses the principles of classical mechanics and the concept of the phase space. They explain that the laws of physics are based on principles of classical mechanics, such as the conservation of energy and momentum. They also introduce the concept of the phase space, which represents all possible states of a system and includes information about the position and velocity of particles. The speaker discusses different laws of physics that can be applied to simple systems, such as a coin (heads or tails) or a die (six states). They explain deterministic laws where knowing the current state allows you to predict the next state, as well as laws that violate determinism. They also highlight the importance of conservation laws and the need to know both position and velocity to accurately predict motion.

## Questions & Answers

### Q: What is the basis for all of physics?

Mechanics is the basis for all of physics. It describes the motion of objects and provides the basic framework for all other areas of physics, including concepts like conservation of energy and momentum.

### Q: What is the phase space?

The phase space is the space of all possible states of a system. In classical mechanics, it includes information about the position and velocity of particles. It represents everything you need to know to predict what happens next in the system.

### Q: What are the different kinds of laws that can govern a system with two states?

Two possible laws for a system with two states (heads and tails) are a law where the state stays the same (heads to heads, tails to tails), and a law where the state alternates (heads to tails, tails to heads). These laws are deterministic because knowing the current state allows you to predict the next state.

### Q: Can we think of laws of nature that only have one state?

Yes, but a system with only one state would be extremely limited and boring because nothing would ever change. In this case, there is only one possible law – the state stays the same.

### Q: Can we have more than two states in a system?

Yes, we can have systems with more than two states, such as a die with six states. In this case, there can be a variety of laws that govern the system, determining how it transitions between states.

### Q: Are all laws of physics deterministic?

Yes, all the known laws of physics in classical mechanics are deterministic. This means that if you know the initial conditions of a system precisely, you can accurately predict its future behavior. However, in practice, the limitations of precision and measurement lead to uncertainties and make long-term predictions impossible.

### Q: What are forbidden laws in classical mechanics?

Laws that violate the principles of classical mechanics are not allowed. For example, laws where a state goes back to itself (irreversible) or laws with branches that are not deterministically linked are not permitted.

### Q: Why is information conservation important in classical mechanics?

Information conservation is a fundamental law in classical physics. It ensures that no information is lost about the state of a system as it evolves over time. This allows us to accurately predict future states and trace back the past states of a system.

### Q: How does the concept of phase space change in continuous physics?

In continuous physics, such as the motion of a particle along a line, the phase space becomes two-dimensional. It includes the position of the particle on the line and its velocity. To accurately predict the motion, one needs to know both the position and velocity.

### Q: Can we predict the motion of classical systems indefinitely?

In theory, if we have infinitely precise initial conditions, we can predict the motion of classical systems for any length of time. However, in practice, the limitations of measurement and precision make long-term predictions impossible.

## Takeaways

The principles of classical mechanics form the basis for all of physics, describing the motion of objects and providing a framework for understanding the conservation of energy and momentum. The phase space represents all possible states of a system and includes information about the position and velocity of particles. Deterministic laws allow us to predict future states based on the current state, while violating determinism leads to ambiguity and loss of information. Information conservation is a fundamental law in classical mechanics, ensuring that no information is lost as a system evolves over time. In continuous physics, both position and velocity are necessary to accurately predict motion. While classical systems are theoretically predictable, practical limitations of precision and measurement make long-term predictions impossible.