Traveling to other universes in an infinite multiverse

[Jared409]

I'd just like to preface by saying that I'm not in any way an expert on high-level physics, but I would like to think that after reading a few books and taking a healthy interest in things like cosmology that I have a decent understanding of some concepts, so if you could please bear with me.

My question is that if we live in an infinite multiverse and that if inter-universe travel is possible, then wouldn't we have already had an infinite number of beings pass into our universe? If the probability of a plausible action occurring approaches one given an infinite set of universes, then wouldn't that imply that if inter-universe travel was possible, there would be an infinite number of beings that were not only capable of such travel, but also an infinite number that had decided to choose our particular universe to travel to at this very instant?

 

[DennisN]

This is a very (VERY) speculative question, but I will attempt a short answer:

This reasoning is similar to the Fermi Paradox. But - please note that all of the premises may be false; multiverse, infinite set of universes, infinite number of beings and possibility of inter-universe travel.

To complicate matters further, there are different types of multiverse hypotheses (and they are hypotheses). And together with the fact that there is only one Universe we know of at the moment (there is no observational support for anything else), inter-universe travel is pretty meaningless to even think about, IMHO.

n.b. My friendly advice is to take care if you elaborate on this topic, because the forum guidelines states
"Generally, in the forums we do not allow the following:
Personal theories or speculations that go beyond or counter to generally-accepted science."

 

[Chronos]

At this time, there is no persuasive evidence of other 'universes'.

 

[timmdeeg]

My question is that if we live in an infinite multiverse and that if inter-universe travel is possible, then wouldn't we have already had an infinite number of beings pass into our universe?

This is quite unlikely provided these beings travel with speeds < lightspeed, means provided Special Relativity holds. Not even light emitted beyond the Hubble sphere will ever reach us.

Your question excludes the case that there is no causal connection from one universe to the other. As you may have recognized from the books you mentioned the term 'multiverse' isn't clearly defined.

 

[phinds]

Although various multiverse ideas are taken seriously by non-crackpot physicists, it is basically "television science" (as shown on the History Channel, et al) with absolutely zero evidence to support it.

 

[PAllen]

For multi-verse ideas of all flavors explored in non-crackpot physics, neither travel between, nor any form of direct detection are considered possible.

While "on the edge" of serious physics, these ideas do not arise for frivolous reasons. I'll briefly describe what I think are the two major flavors of multi-verse idea:

1) You have a model of Big Bang/ inflation. A natural consequence of this is that what happened once can happen any number of times (universe genesis, inflationary episode). To posit that it only happens once, you actually have to add a restriction to the model. If you tie this mathematical feature to variation of properties in different universes (which is also a natural consequence of some of these models), you have a new hypothetical field of research - trying to explain features of the universe that seem accidentally well suited for life (for example) by [STRIKE]anthropomorphic[/STRIKE] anthropic principle; and for accidental features, seeing if they are in the 'likely range' of multiverse features, thus partially explaining the values we see. On the con side of these endeavors are fundamental difficulties with defining probability distributions, 'fitness for life', and any other measure in a provable way; and the simple fact that direct evidence will likely never be forthcoming.

2) Parallel universes arising in some interpretations of quantum theory. In some people's minds (but I see this group shrinking of late, after growing in popularity), this is a natural consequence of assuming the evolution of the whole universe is unitary. In a subset of this group, there is a possible testability - that is, several quantum computation theorists believe there are possible quantum computation algorithms that distinguish parallel universe interpretations from other interpretations.

Personlly, I expect (2) to fade out of mainstream theory, while (1) is going to stick around unless a good theory of birth of the universe has a natural limitation to only occurring once.

 

[phinds]

... (for example) by anthropomorphic principle ...

It's actually "the anthropic principle". I kept making that same mistake myself until I was corrected.

 

[Chalnoth]

I'd just like to preface by saying that I'm not in any way an expert on high-level physics, but I would like to think that after reading a few books and taking a healthy interest in things like cosmology that I have a decent understanding of some concepts, so if you could please bear with me.

My question is that if we live in an infinite multiverse and that if inter-universe travel is possible, then wouldn't we have already had an infinite number of beings pass into our universe? If the probability of a plausible action occurring approaches one given an infinite set of universes, then wouldn't that imply that if inter-universe travel was possible, there would be an infinite number of beings that were not only capable of such travel, but also an infinite number that had decided to choose our particular universe to travel to at this very instant?

They would, however, also have an infinite number of universes to choose from, so the result is indeterminate. Basically, what you'd be asking for is the ratio of the number of universes which support life over the typical number of universes they visit. For example, if only 1% of universes support life, and life within those universes tends to visit only one or two other universes on average, then we're unlikely to have visitors.

That said, there are very good reasons to believe that this isn't even remotely possible. But to see this, first we have to nail down what sort of universe we're talking about. Consider three basic multiverse types:

1. Different initial conditions. This is basically a corollary to the fact that our universe almost certainly extends beyond our horizon. The problem, then, is that traveling beyond our horizon requires faster-than-light travel. This is highly unlikely to be possible.
2. Different quantum wavefunction. In quantum mechanics, there are many branches of the wavefunction that all coexist, and we are generally constrained to see one branch at a time. The other branches are almost certainly there, but we cannot interact with them. This is largely down to the quantum phase of the particles that make us up. Traveling to another quantum multiverse would require fine-tuning the quantum phase of every individual particle of our bodies. I just don't think that's even remotely possible. In fact, I'd be willing to bet that there's a good quantum-mechanical argument that proves it is fundamentally impossible (though I don't know that argument offhand).
3. Different low-energy physics. It seems to be the case that the fundamental laws of physics allow for many possible realizations of low-energy physics. These amount to different vacuum states. And this is the least possible of all: different vacuum states have different energies. When vacuum states of different energies come into contact, the lower energy state spreads and destroys the higher-energy state. So if it were possible to visit a region with a lower-energy vacuum state than our own, the unfortunate explorer would pretty much instantly explode into a flash of high-energy particles. If, on the other hand, an explorer were to travel to a region with higher vacuum energy state, then our lower-energy state would rapidly expand and destroy that universe. The explorer would likely be vaporized by the radiation resulting from the universe's destruction.