# Inference from fine-tuning to a multiverse

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• haushofer
In summary: So, in order to observe a planet with life, you must exist first. And since you already exist, it's not surprising that you observe a planet with life. So the question is whether this anthropic selection should affect your probability for the existence of life in the multiverse.
haushofer
TL;DR Summary
Given fine-tuning and our existence, can we make the hypothesis plausible that there are more universes?
Dear all,

I'm reading up on Bayesian inference, and recently read some papers about the fine-tuning argument (FTA) again. I'm not that interested in the details of this fine-tuning, or details of the multiverse. My question concerns Bayesian inference. I'll make an analogy about habitable planets. The scenario is the following.

Say, you are a Middle-Aged monk with revolutionary ideas (say, Giordano Bruno), and you understand that in order to get life as we know it on earth the conditions must be very special. Of all the imaginable planets which could form, it is a priori quite improbable that a life-sustaining planet like Earth will form. Our existence does surprise you. But you haven't observed other planetary systems yet. Given that you exist, and given the fine tuning, can you infer that there probably are a lot more planets outside our solar system such that the existence of Earth becomes more probable?

The first naive answer would be: yes, because given more planets the expectation value of number of habitable planets increases.

At a second thought, you think about the inverse gamber's fallacy (https://en.wikipedia.org/wiki/Inverse_gambler's_fallacy): if you see an improbable event like 5 dices which are thrown, given you 5 sixes, you cannot statistically infer that there have been many throwns before your arrival. This can be easily shown by Bayes' formula.

At a third thought, you realize that there is no way you could observe a planet which is not habitable. This condition "selects observers", and is also known as the anthropic principle. How does this influence the probability? Shouldn't we make a distinction between the events ''there is a habitable planet'' and '' there is a habitable planet and I happen to live on it?''

So what do you think? Can our hypothetical Bruno make the inference? If so, why and under which conditions? If not, why not?

For those who are interested, here are some papers:

https://arxiv.org/abs/1505.05359?context=physics

http://philsci-archive.pitt.edu/16785/

https://philpapers.org/rec/WHIFAM

haushofer said:
At a third thought, you realize that there is no way you could observe a planet which is not habitable.
Missing "from"? Or some other meaning ?

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BvU said:
Missing "from"? Or some other meaning ?

##\ ##
Yes, "observe from a planet..." ;)

Maybe this topic suits better in the probability subforum...?

Even then: why wouldn't one be able to observe from the moon or saturn or ..

BvU said:
Even then: why wouldn't one be able to observe from the moon or saturn or ..
Well, yes, but the analogy is about the existence of life in the multiverse. The point is that the formation of life seems improbable.

## 1. What is the concept of fine-tuning in relation to the multiverse theory?

Fine-tuning refers to the idea that the fundamental constants and laws of the universe are finely tuned to allow for the existence of life. The multiverse theory suggests that there may be an infinite number of universes with varying physical laws, and ours happens to be one that is suitable for life.

## 2. How does the concept of fine-tuning support the idea of a multiverse?

If our universe was not finely tuned for life, it is unlikely that we would exist to observe it. The concept of a multiverse allows for the possibility that there are other universes with different physical laws, and we happen to exist in one that is suitable for life.

## 3. What evidence is there for the idea of a multiverse?

Currently, there is no direct evidence for the existence of a multiverse. The idea is mainly based on theoretical models and mathematical calculations. However, some scientists argue that the fine-tuning of our universe could be seen as indirect evidence for the existence of a multiverse.

## 4. Can the concept of a multiverse be tested or proven?

At this point, the concept of a multiverse remains a theoretical idea and cannot be directly tested or proven. However, as our understanding of the universe and its fundamental laws continues to advance, it is possible that we may find evidence that supports the existence of a multiverse.

## 5. How does the idea of a multiverse impact our understanding of the universe and our place in it?

The concept of a multiverse challenges our traditional understanding of the universe and our place in it. It suggests that our universe may just be one of an infinite number of universes, each with its own set of physical laws and possibilities. This can lead to philosophical and existential questions about the nature of reality and our significance in the grand scheme of things.

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