# A On the arrow of time

1. Nov 11, 2016

### MathematicalPhysicist

My question is, if somehow it were possible to change the change of entropy such that it were negative, i.e. $\Delta S <0$ wouldn't we go backwards in time, from the future to the past?

I have this paper:
https://arxiv.org/abs/gr-qc/0503077

I wonder if the stability problem was ever resolved.
I haven't yet read this paper (even if it's a little bit outdated).

2. Nov 11, 2016

### ObjectivelyRational

I think change in entropy defines the "direction" of time (past versus future) along the arbitrary parameter T.

By definition, going towards "-T" or "+T" is not what defines "backward" or "forward" in time. I.e. "Forward" in time will be that direction along T for which the total entropy increases... in a sense, in order for events to cause effects on your senses which cause effects on your brain and which cause your memories to form (generally speaking storage of information), and enable you to recall the past... defines it.

The past is thus also defined by the direction in T in which you are capable (because of the way entropy, information, causation, and memory work) of remembering.

3. Nov 11, 2016

### MathematicalPhysicist

"T" is time, right? (in thermodynamics it's temperature).

I am not concerned about my brain functioning of past or future but that if you can possibly change the enviornement but not you, s.t the enviornemnt will have a decrease in entropy while your brain will still have an increase in entropy, so to speak to isolate myself from the enviornment.

I think it's possible, the problem is how to implement it, toughy.
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4. Nov 11, 2016

### ObjectivelyRational

Why do you think it is "possible"?

5. Nov 11, 2016

### MathematicalPhysicist

It's a hunch from the fact that quantum mechanically you can have negative temperatures (I must admit that I haven't delved into this anomaly of negative tmperatures yet, but I am planning of doing it eventaully).

6. Nov 11, 2016

### ObjectivelyRational

Are you assuming you can define a system in a region A and a second system in separate (in space) region B such that according to a "time" defined for both systems, changes in entropy are positive in A but are negative in B?

Is there ANY interaction allowed between A and B while one system is reversing in time? If there IS, what is the change in entropy of the total system? Why? What is relationship of causation between A and B?

If there is no interaction (as long as they have change in entropy in opposite directions of time) what could cause one or other system to "flip" its direction of entropy? Could a system cause its own flip? How?

7. Nov 11, 2016

### MathematicalPhysicist

That's the trillion dollars question, how to implement this?

There are many papers that argue that it maybe possible through some interaction with black holes, I heard a conversation of Seth Lloyd in which he talks about such a model he has.

I don't know how to implement this but I am willing to keep on learning...

8. Nov 11, 2016

### rootone

Reverse time travel?, yes that's hard no matter how much dollars you throw at at.