Exploring the Janus Universe: A Layperson's Guide

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In summary, the Janus universe is a model of space where there are two regions with their own arrow of time. The difference is that in the Janus universe, the arrow of time always points away from the Janus point, which means that there is no "bounce" in the direction of time.
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One idea I have been hearing about recently is the Janus universe
https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.113.181101
Im hoping someone might have a go at giving a layperson explanation of this paper. As I understand they are saying a system will eventually come to a point of low entropy naturally due to gravity and then bounce back at out so there is a reversal of the arrow of time at the bounce point. Is that right?
How is this different to Poincare recurrence?
 
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windy miller said:
One idea I have been hearing about recently is the Janus universe
https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.113.181101
Im hoping someone might have a go at giving a layperson explanation of this paper. As I understand they are saying a system will eventually come to a point of low entropy naturally due to gravity and then bounce back at out so there is a reversal of the arrow of time at the bounce point. Is that right?
How is this different to Poincare recurrence?
The difference is that Poincare recurrences arise repeatedly in confined systems as long as enough time passes while a Janus point is an unique point in an unconfined system which goes to infinity.

The idea is not so much that the arrow of time changes direction at the Janus point but that the emergence of the arrow of time always leads to the Janus point being in the past.

This paper by the same authors is a little more readable than the one you posted:-
Janus Points and Arrows of Time
https://arxiv.org/abs/1604.03956

Barbour also has a laymans level book just published about the subject.
 
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windy miller said:
As I understand they are saying a system will eventually come to a point of low entropy naturally due to gravity and then bounce back at out so there is a reversal of the arrow of time at the bounce point. Is that right?

Not quite. You are implicitly assuming an arrow of time that is not the one in the model, when you say it "bounces" and the arrow of time "reverses". But the whole point of the model is that there is no "time" external to the model. Instead, what you have is a spacetime model that is divided into two regions by the Janus point; each of the two regions has its own arrow of time, pointing away from the Janus point. Observers living in each of the two regions will perceive a "flow of time" in the direction of the arrow of time in their region, i.e., away from the Janus point. That means no observer will perceive a "bounce".
 
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PeterDonis said:
Not quite. You are implicitly assuming an arrow of time that is not the one in the model, when you say it "bounces" and the arrow of time "reverses". But the whole point of the model is that there is no "time" external to the model. Instead, what you have is a spacetime model that is divided into two regions by the Janus point; each of the two regions has its own arrow of time, pointing away from the Janus point. Observers living in each of the two regions will perceive a "flow of time" in the direction of the arrow of time in their region, i.e., away from the Janus point. That means no observer will perceive a "bounce".
Ah ok thanks for that.
 

1. What is the Janus Universe?

The Janus Universe is a theoretical concept that proposes the existence of two parallel universes that are mirror images of each other. It suggests that every particle and force in our universe has a counterpart in the other universe, making them perfectly symmetrical.

2. How is the Janus Universe different from the Multiverse theory?

The Multiverse theory suggests the existence of multiple universes that are vastly different from each other, while the Janus Universe proposes the existence of only two parallel universes that are identical in structure and composition.

3. What evidence supports the Janus Universe theory?

The Janus Universe theory is still a subject of debate and there is currently no concrete evidence to support it. Some scientists have proposed mathematical models and simulations that are consistent with the theory, but more research and evidence is needed for it to be widely accepted.

4. How does the Janus Universe impact our understanding of the universe?

If proven to be true, the Janus Universe would challenge our current understanding of the universe and the laws of physics. It would also open up new possibilities for exploring the relationship between our universe and the other parallel universe.

5. Can we ever observe or interact with the Janus Universe?

As of now, there is no way to observe or interact with the Janus Universe, as it is purely a theoretical concept. However, advancements in technology and further research may one day make it possible to detect any potential signatures or evidence of the parallel universe.

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