Why is there a difference between past and future?

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Discussion Overview

The discussion revolves around the asymmetry between past and future in the context of time, exploring concepts such as entropy and the expansion of the universe. Participants examine the implications of these ideas on the nature of time and its directional flow.

Discussion Character

  • Exploratory, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions the apparent asymmetry of time despite the symmetry present in physics, particularly referencing CP symmetry.
  • Another participant introduces the concept of the "arrow of time," linking it to the expansion of space and the increase of entropy, suggesting that a low-entropy state is necessary for a well-defined arrow of time.
  • A third participant posits that the origin of this asymmetry is tied to initial conditions, arguing that while entropy increases over time, this is not a fundamental physical law but rather a characteristic of a specific solution describing our universe.

Areas of Agreement / Disagreement

Participants present multiple competing views regarding the nature of time asymmetry, with no consensus reached on the underlying reasons or implications of these concepts.

Contextual Notes

The discussion touches on complex ideas such as the relationship between entropy and the initial conditions of the universe, which may depend on specific definitions and interpretations that are not fully resolved within the thread.

ndung200790
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Please teach me this:
We know that there is a approximate symmetry of time T in physics(together with CP symmetry).Then why there is an asymmetry between past and future(we can not reverse the present world to past world)?
Thank you very much in advance.
 
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arrow of time

Two important concepts:
- space is expanding: "The past is a state with a 'smaller' universe"
- entropy is increasing: "The past is a state with less entropy"

In other words: If there is some reason to have a small, expanding low-entropy state, you'll get a universe with a well-defined arrow of time like the universe we are in.
If you begin with a high-entropy state, you won't get life as we know it.
 
The origin of asymmetry can be traced back to the initial conditions. Entropy increases with time, which means that initial entropy was very low. The fact that entropy increases with time is not really a physical law (even though it is sometimes referred to as the second "law" of thermodynamics), but merely a property of a particular solution which happens to describe the state of our universe.
 
OK,I see,thank you very much.
 

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