One Model of Multiverse Structure
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Wired.com: In this multiverse theory you have a static universe in the middle. From that, smaller universes pop off and travel in different directions, or arrows of time. So does that mean that the universe at the center has no time?
Carroll: So thatâ€™s a distinction that is worth drawing. Thereâ€™s different moments in the history of the universe and time tells you which moment youâ€™re talking about. And then thereâ€™s the arrow of time, which give us the feeling of progress, the feeling of flowing or moving through time. So that static universe in the middle has time as a coordinate but thereâ€™s no arrow of time. Thereâ€™s no future versus past, everything is equal to each other.
Wired.com: So itâ€™s a time that we donâ€™t understand and canâ€™t perceive?
Carroll: We can measure it, but you wouldnâ€™t feel it. You wouldnâ€™t experience it. Because objects like us wouldnâ€™t exist in that environment. Because we depend on the arrow of time just for our existence.
Wired.com: So then what is time in that universe?
Carroll: Even in empty space, time and space still exist. Physicists have no problem answering the question of â€œIf a tree falls in the woods and no oneâ€™s there to hear it, does it make a sound?â€ They say, â€œYes! Of course it makes a sound!â€ Likewise, if time flows without entropy and thereâ€™s no one there to experience it, is there still time? Yes. Thereâ€™s still time. Itâ€™s still part of the fundamental laws of nature even in that part of the universe. Itâ€™s just that events that happen in that empty universe donâ€™t have causality, donâ€™t have memory, donâ€™t have progress, and donâ€™t have aging or metabolism or anything like that. Itâ€™s just random fluctuations.
Wired.com: So if this universe in the middle is just sitting and nothingâ€™s happening there, then how exactly are these universes with arrows of time popping off of it? Because that seems like a measurable event.
Carroll: Right. Thatâ€™s an excellent point. And the answer is, almost nothing happens there. So the whole point of this idea that Iâ€™m trying to develop is that the answer to the question, â€œWhy do we see the universe around us changing?â€ is that there is no way for the universe to truly be static once and for all. There is no state the universe could be in that would just stay put for ever and ever and ever. If there were, we should settle into that state and sit there forever.
Itâ€™s like a ball rolling down the hill, but thereâ€™s no bottom to the hill. The ball will always be rolling both in the future and in the past. So, that center part is locally static â€” that little region there where there seems to be nothing happening. But, according to quantum mechanics, things can happen occasionally. Things can fluctuate into existence. Thereâ€™s a probability of change occurring.
So, what Iâ€™m thinking of is the universe is kind of like an atomic nucleus. Itâ€™s not completely stable. It has a half-life. It will decay. If you look at it, it looks perfectly stable, thereâ€™s nothing happeningâ€¦ thereâ€™s nothing happeningâ€¦ and then, boom! Suddenly thereâ€™s an alpha particle coming out of it, except the alpha particle is another universe.
Wired.com: So inside those new universes, which move forward with the arrow of time, there are places where the laws of physics are different â€” anomalies in spacetime. Does the arrow of time still exist there?
Carroll: It could. The weird thing about the arrow of time is that itâ€™s not to be found in the underlying laws of physics. Itâ€™s not there. So itâ€™s a feature of the universe we see, but not a feature of the laws of the individual particles. So the arrow of time is built on top of whatever local laws of physics applyâ€¦