Anthropics and Biased Models

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I think that anthropic reasoning only works when you have a good model of how you could have gotten into the situation in question.

For the beginning of the universe kinds of questions, as I see it, the options boil down to:

1) Is something vaguely like String Theory correct, in which a near-infinite ensemble of universes with different laws is created at the dawn of time, or continuously across time?

2) Are the laws we observe actually perfectly fundamental, and they just happen to be right?

3) Did some entity pick out these laws?

Anthropic reasoning gives us no reason to go for 2, but it is perfectly happy with 1, since it lets us discard all of the parts of the universe with rules that don't produce life capable of considering the question.

One of the points that I was trying to make is that you can't apply anthropic reasoning like that. That is, you need to be comparative, to start with at least two models, then update on your anthropic data. As an analogy, I might be able to give you very good reasons for believing that theory A would explain a phenomena, but if theory B explains it better, then we should go with theory B. There are many cases where we can obscure this by talking exclusively about theory A.

So the question is not does 1) explain the situation well, but does 1) explain the situation better than 3), taking into account things such as prior probabilities.

*Update*: On second thought, multi-worlds is a pretty good answer when combined with the anthropic principle. I suppose that my argument then only shows that case 2) isn't a very good explanation.

I took it as too-obvious-to-mention that 2 & 3 explain the situation just fine, but have massive complexity penalties.

It is a well estabilished fact in probability that you cannot treat on the same footing a naive notion of surprise and the happening of an event of low probability.

The classical example is the extraction from a set of large cardinality with uniform distribution: one of the occurences is bound to happen, but each has a very low probability.

If you let naive surprise guide your model selection, and you do not have a sound base for model generation, you start falling into a slippery slope that culminates in solipsism.

Case in point: we have three universes, one is normal, one is magical (biased toward our existence), one is solipsistic (biased toward **my** existence). Clearly, since we exists, the magical is much more probable than the normal, but since *I* exist, the universe was probably born to allow me to write you in this exact moment (after all, what are the probability that I was born between the trillion of other possible beings?).

So if you generate a number randomly between one and one million, each number has a one in a million chance of being chosen. Like, if I get the number 5, I can say that it is unlikely that it is a coincidence, as there was only a one in a million chance of this happening. However, there is no reason why I wouldn't have said the same thing if I received a 6 or 335,687. So there isn't really a coincidence or a surprised, because regardless of result, we could have said something similar.

I don't believe in the magical universe theory either. My point was simply that the anthropic principle is not an effective counter-argument. If the maths suggests that a magical universe exists or that a sophistic universe exists, I suspect that you've probably set the prior probabilities to be too high.

I don't think there's actually a problem here.

Normal universe model: The universe has no bias towards supporting life

I think it only looks like there's a problem because you haven't separated this into a 'chance' hypothesis and an 'ensemble' hypothesis, such that we have three initial hypotheses.

Suppose that you are a gambler and a dealer wants you to determine whether or not zer dice are biased. The thing is, ze never lets you observe a dice roll; you're only allowed to see how the dice landed after they have been rolled. Every time ze goes to roll the dice, you have to leave the room. Over and over again, each time you enter the room, you observe that the dealer has rolled a double six. If our hypothesis space is really limited to these two hypotheses, then the probability of biased dice should skyrocket, which is, as far as I can tell, the point that you're making in the article above.

But there may be a hypothesis outside of your hypothesis space. Suppose the dealer secretly only lets you into the room if ze rolls a double six. The key here is that our observations of the dice are subject to a selection bias.

It would be really surprising to see a double six if it had been rolled after just one trial (because the probability of rolling at least one double six in one trial is ~0.027), but it would be expected if there had been many trials (because the probability of rolling at least one double six, in say, 200 trials, is ~0.996). And it seems like a sort of explanation to say to the gambler that even though the prior probability of a double six being rolled in any given trial is quite low, it's not quite so surprising to see it if there have actually been many rolls that you could not observe, since we would either see that outcome or see no outcome at all.

Does that make sense to you?

"Suppose the dealer secretly only lets you into the room if ze rolls a double six"

You seem to be proposing that we should have an alternate hypothesis:

"Our observations are filtered by the requirement of us being alive"

However, this isn't an alternate hypothesis as in both the Normal universe and the Magical universe it holds.

To make it clearer, if I an examining two hypothesises:

1) "Barrack Obama is human and he is president of the United States" 2) "Barrack Obama is human and he is not president of the United States"

And if your alternate hypothesis is:

3) "Barrack Obama is human"

Then you haven't actually created a new separate, hypothesis, just a hypothesis that is a superset of the other two.

Anyway, your post seems to just restate the anthropic argument. I explained in my post that this is can't be applied here because it is necessary to be comparative between the hypothesis, while the way the anthropic argument is being used there only considers a single hypothesis.

Not sure why you're thinking about these hypotheses as supersets and subsets of one another. If I wanted to get formal with it, I'd describe the hypotheses as programs. The design hypothesis would be an agent program that outputs our local universe by building our universe the hard way. The chance hypothesis would be a set of physical constants with rules determining the time evolution of reality that outputs our universe, and not an entire multiverse of which our universe is a small part. The ensemble hypothesis would be an even simpler and more fundamental set of rules than in the previous program, maybe with some constants as well, and it would output a multiverse, some parts of which are hospitable and even identical to the previous program's output. It confuses me to think about these hypotheses as subsets of one another, because it makes me think of substrings. These programs would not be substrings of one another. Their output would be though, because they all output us observing our universe. We're supposed to be talking about hypotheses, not output.

The single unbiased universe, multi-verse and biased universe are not subsets.

I was simply stating that the anthropic principle is a principle that applies to any of these models, not it is not its own separate model.

I think the delivery could be greatly improved by introducing symbols and clarifying the logical environment where the derivation is happening. Allow me to do this, without using LateX I'll assume /\ stands for logical conjunction and ~ for logical negation.

**Proposition symbols**

Our universe is normal: N

We exist: E

Our universe is magical: M

**Logical environment**

[1] The universe is either normal or magical: M = ~N

[2] The magical universe is strongly biased to support our existence: P(E|M) = 1

**Derivation**

By Bayes theorem:

P(N|E) = P(E|N) P(N) / P(E) <-->

P(N|E) = k P(E) /\ k = P(E|N)/P(E)

By partition of unity and [1]

P(E) = P(E|N)P(N) + P(E|M)P(M) => (by fact [2])

P(E|N)P(N) + P(~N) => (by law of probability)

P(E|N)P(N) + 1 - P(N)

If P(E|N) -> 0 with P(N) fixed (say c), we get

P(E) -> 0/c + 1 - c = 1 - c

From this

P(N|E) = k P(E) -> k (1-c) /\ k -> 0 / (1-c)

so that

P(N|E) -> 0

I believe you can use latex in less wrong. It says so under "show help". Let me try... $e^{i\pi}+1=0$

EDIT: Forget it. I just reread the help text. You have to use an external website to render the equation into an image...

Thanks for the incredible essay. I have to read it more thoroughly when I find the time. The point about the priors rejecting the hypothesis is a point I never considered and something that should be hammered to those of us working with data (and only consider hypothesis testing using null hypothesis). I wonder what other more mundane problem this might apply to...

Aren't you simply begging the question? If you have a universe where a fact happens almost surely and another where it almost never happens, but that fact indeed happens, simple logic dictates that the first universe should be favored.

However, I've seen no reason to justify that the probability of life should tend to zero given our normal universe nor that the magical universe should be the only other alternative.

How am I begging the question? That isn't clear.

I don't believe that the probability of life in our universe is almost zero, but you can read all about the Fine-Tuned Universe argument at Wikipedia which is how this claim is most often made. My post is not targeted at the question of whether life really is that unlikely (I suspect that the parameters needed for life are not as strict as often claimed, only for life as we know it). The question I am trying to examine is whether the anthropic principle is a sufficient counter to the Fine-Tuned Universe (it is not); I'll leave it to other people to discuss other objections to the Fine-Tuned Universe.

The Fine-tuned Universe Theory, according to Wikipedia is the belief that, "our universe is remarkably well suited for life, to a degree unlikely to happen by mere chance". It is typically used to argue that our universe must therefore be the result of Intelligent Design.

One of the most common counter-arguments to this view based on the Anthropic Principle. The argument is that if the conditions were not such that life would be possible, then we would not be able to observe this, as we would not be alive. Therefore, we shouldn't be surprised that the universe has favourable conditions.

I am going to argue that

this particular applicationof the anthropic principle is in fact an incorrect way to deal with this problem. I'll begin first by explaining one way to deal with this problem; afterwards I will explain why the other way is incorrect.Two model approachWe begin with two modes:

Standard anthropic argumentHowever, this is actually asking the wrong question. It is right to note that we shouldn't be surprised to observe that we survived given that it would be impossible to observe otherwise. However, if we were then informed that we lived in a normal, unbiased universe, rather than in an alternate biased universe, if the maths worked out a particular way such that it leaned heavily towards the alternate universe, then we would be surprised to learn we lived in a normal universe. In particular, we showed how this could work out above, when we examined the situation where p(we exist|normal universe) approached 0. The anthropic argument against the alternate hypothesis denies that surprise in a certain sense can occur, however, if fails to show that surprised in another, more meaningful sense can occur.

Coin flip argumentEdit - Extra Perspective: Null hypothesis testingSummaryEdit: After consideration, I have realised that the anthropic principle actually works when combined with the multiple worlds hypothesis as per Luke_A_Somers comment. My argument only works against the idea that there is a single universe with parameters that just happen to be right. If the hypotheses are: a multiverse as per string theory vs. a magical (single) universe, even though each individual universe may only have a small chance of life, the multiverse as a whole can have almost guaranteed life, meaning our beliefs would simply be based on priors. I suppose someone might complain that I should be comparing a Normal multiverse against a Magical multiverse, but the problem is that my priors for a Magical multiverse would be even lower than that of a Magical universe. It is also possible to use the multiple worlds argument without using the anthropic principle at all - you can just deny that the fine tuning argument applies to the multi-verse as a whole.Supplementary MaterialsLimit of p(normal universe|we exist)=p(we exist|normal universe)p(normal universe) + 1 - p(normal universe)

Performing Bayesian updatesAgain, we'll imagine that we have a biased universe where we have 100% chance of being alive.

We will use Bayes law:

p(a|b)=p(b|a)p(a)/p(b)

Where:

a = being in a normal universe

b = we are alive

We'll also use:

p(alive) = p(alive|normal universe)p(normal universe) + p(alive|biased universe)p(biased universe)

Example 1:Setting:

p(alive|normal universe) = 1/100

p(normal universe) = 1/2

The results are:

p(we are alive) = (1/100)*(1/2)+1*(1/2) = 101/200

p(normal universe|alive) = (1/100)*(1/2)*(200/101) = 1/101

Example 2:Setting:

p(normal universe)=100/101

p(alive|normal universe) = 1/100

p(normal universe) = 100/101

The results are:

p(we are alive) = 100/101*1/100+1/101*1 = 2/101

p(normal universe|alive) = (1/100)*(100/101)* (101/2) = 1/2