Why is there a difference between past and future?

In summary, there is an approximate symmetry of time in physics, but there is an asymmetry between past and future due to the expanding universe and increasing entropy. This can be traced back to the low initial entropy of the universe.
  • #1
ndung200790
519
0
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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  • #2
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.
 
  • #3
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.
 
  • #4
OK,I see,thank you very much.
 

1. Why does time only move forward, and not backwards?

One of the fundamental laws of physics is the Second Law of Thermodynamics, which states that the total entropy of a closed system will always increase over time. This means that disorder and randomness will always increase in a system, and it is impossible to reverse this process. Therefore, time can only move forward.

2. What causes the difference between past and future?

The difference between past and future is a result of the arrow of time, which is the direction in which time moves. This is thought to be caused by the initial conditions of the universe, particularly the low entropy state at the Big Bang. As time progresses, entropy increases and the universe becomes more disordered, leading to the difference between past and future.

3. How does the concept of time relate to the theory of relativity?

The theory of relativity, specifically Einstein's theory of general relativity, explains how gravity affects the passage of time. According to this theory, time is relative and can be influenced by factors such as gravitational pull and acceleration. This means that time can move at different rates for different observers, and the concept of a universal "now" may not exist.

4. Is time travel possible and would it affect the difference between past and future?

While time travel is a popular concept in science fiction, it is currently not possible according to our current understanding of physics. The laws of physics, particularly the Second Law of Thermodynamics, suggest that traveling backwards in time would create paradoxes and violate the principles of causality. Additionally, the concept of a fixed past and future is crucial for our understanding of the universe, so any changes in the past could have unpredictable consequences for the future.

5. Can the difference between past and future be explained by the concept of free will?

The concept of free will is a philosophical and psychological concept that is still heavily debated. While some argue that free will allows us to make choices that can affect the future, others argue that our choices are predetermined by the laws of physics and our past experiences. The difference between past and future is thought to be a result of both external factors, such as the laws of physics, and internal factors, such as our own choices and decisions.

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