# Lecture 1 | Quantum Entanglements, Part 1 (Stanford) | Summary and Q&A

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April 23, 2008
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Stanford
Lecture 1 | Quantum Entanglements, Part 1 (Stanford)

## Summary

In this video, the speaker discusses the intuitive ways of thinking about the physical world that humans have inherited through evolution. He explains how our intuitions about physics are based on our biological origins and how these intuitions are limited to the range of parameters we have experienced in the ordinary world. He also introduces the concept of classical bits and how classical physics can be represented in terms of bits.

### Q: How do our intuitive ways of thinking about the physical world come from our biological origins?

Our intuitive ways of thinking about the physical world are a result of the process of evolution. As animals, we have inherited certain intuitive ways of thinking about the physical world that have helped us survive and navigate our environment. For example, the way a lion stops dead when the relative velocity between the lion and its prey changes sign is an example of our intuitive understanding of velocity and direction.

### Q: Why do our intuitions about physics only apply to parameters within the range of our experience?

Our intuitions about physics are based on the parameters and situations we have experienced in our everyday lives. For example, we have never experienced velocities approaching the speed of light, so our intuitions about adding velocities are incorrect in the realm of relativity. In order to understand and work with parameters that are outside the range of our experience, such as in modern physics, we have had to develop new mathematical concepts and ways of thinking.

### Q: What is the difference between classical bits and quantum bits?

Classical bits, or c-bits, are used in classical physics to represent information in the form of a binary digit - either a 0 or a 1. They are used to represent discrete information, such as yes or no answers or up or down states. On the other hand, quantum bits, or qubits, are used in quantum mechanics to represent information in a superposition of states. They can exist in multiple states simultaneously, allowing for more complex and versatile information processing.

### Q: How can classical physics be represented in terms of bits?

Classical physics can be represented in terms of classical bits, which are used to represent discrete information. For example, the temperature in a room can be represented by a series of bits, where each bit represents a specific temperature value. By breaking up physical systems into smaller cells and assigning bit values to each cell, we can represent the information of the system using bits.

### Q: What does it mean for a physical system to be reversible?

Reversibility in the context of physics means that a physical process can unfold in both the forward and backward directions of time. In classical physics, the laws of motion are reversible, meaning that if you know the current state of a system, you can uniquely determine its past and future states. This is known as the uniqueness of the future and past points. However, in certain cases, such as irreversible processes, information may be lost, making it impossible to determine the past state from the future state.

### Q: Is time symmetry the same as time reversibility?

While time symmetry and time reversibility are related concepts, they are not exactly the same. Time symmetry refers to the invariance of physical laws under time translations, meaning that the laws of physics should remain unchanged when time is reversed. On the other hand, time reversibility specifically refers to the ability to reverse the direction of time in a physical process without changing its outcome. Time reversibility is a property of reversible systems, where knowing the present state of the system uniquely determines its past and future states.

### Q: Why is it that some physical systems suppress quantum mechanics and appear classical?

The reason why some physical systems appear classical and suppress quantum mechanics is still not fully understood. However, it is believed to be due to a phenomenon known as decoherence, where interactions with the environment cause the delicate quantum superpositions to "collapse" into classical states. This collapse leads to the emergence of classical behavior and the loss of quantum effects. The study of decoherence and the transition from quantum to classical behavior is an active area of research in quantum mechanics.

### Q: Can irreversible processes occur in classical physics?

In classical physics, irreversible processes can occur, but they are generally not considered fundamental or fundamental laws of physics. Irreversible processes are typically macroscopic phenomena that arise due to complexity, dissipative effects, or interactions with the environment. At the most fundamental level, the laws of classical physics are reversible and do not allow for the loss of information. Irreversibility emerges when considering systems at larger scales or when incorporating additional factors like friction, heat transfer, or energy dissipation.

### Q: Can reversible laws of motion occur in classical mechanics?

Yes, reversible laws of motion can occur in classical mechanics. Reversibility in classical mechanics refers to the fact that starting from a given state, the laws of motion allow for the determination of both the past and future states of a system. Reversible laws of motion ensure that information is not lost or destroyed during the evolution of the system. However, it is worth noting that in practice, certain factors such as energy dissipation, friction, or irreversibilities introduced by the macroscopic properties of the system may limit the practical reversibility of a process.

### Q: What is the difference between quantum mechanics and classical mechanics?

Quantum mechanics and classical mechanics are two different frameworks for understanding and describing the behavior of physical systems. Classical mechanics, also known as Newtonian mechanics, is the branch of physics that deals with the motion and interaction of macroscopic objects, such as planets, billiard balls, or everyday objects. It is based on principles like Newton's laws of motion and the conservation of energy and momentum. On the other hand, quantum mechanics is the branch of physics that deals with the behavior of matter and energy at the atomic and subatomic scales. It introduces the concept of wave-particle duality, probabilistic behavior, and superposition, which are not present in classical mechanics.