Foundations of the anthropic principle

[QuArK21343]

What role does the anthropic principle play in modern cosmology? How can we justify it? (Is it merely good old common sense or does it have a mathematical formulation?) What does it succeed in explaining? Is it scientific? Is it necessary?

 

[Chronos]

The anthropic principle is based on the premise the universe cannot possesses properties that forbid our existence. This is obviously logical and does allow us to constrain a large number of things that would be inimical with life in general. It is not logical to infer the anthropic principle is somehow evidence for the existence of an indeterminate number of other universes with properties hostile to life, thus explaining our universe as merely a statistical anomaly that just happens to have the right properties to be habitable.

 

[Dotini]

The anthropic principle is based on the premise the universe cannot possesses properties that forbid our existence. This is obviously logical and does allow us to constrain a large number of things that would be inimical with life in general. It is not logical to infer the anthropic principle is somehow evidence for the existence of an indeterminate number of other universes with properties hostile to life, thus explaining our universe as merely a statistical anomaly that just happens to have the right properties to be habitable.

http://en.wikipedia.org/wiki/Anthropic_principle
Paul Davies's book The Goldilocks Enigma (2006) reviews the current state of the fine tuning debate in detail, and concludes by enumerating the following responses to that debate:

1. The absurd universe

Our universe just happens to be the way it is.

2. The unique universe

There is a deep underlying unity in physics which necessitates the universe being the way it is. Some Theory of Everything will explain why the various features of the Universe must have exactly the values that we see.

3. The multiverse

Multiple Universes exist, having all possible combinations of characteristics, and we inevitably find ourselves within a Universe that allows us to exist.

4. Creationism

A creator designed the Universe with the purpose of supporting complexity and the emergence of Intelligence.

5. The life principle

There is an underlying principle that constrains the universe to evolve towards life and mind.

6. The self-explaining universe

A closed explanatory or causal loop: "perhaps only universes with a capacity for consciousness can exist." This is Wheeler's Participatory Anthropic Principle (PAP).

7. The fake universe

We live inside a virtual reality simulation.

 

[Chalnoth]

It is not logical to infer the anthropic principle is somehow evidence for the existence of an indeterminate number of other universes with properties hostile to life, thus explaining our universe as merely a statistical anomaly that just happens to have the right properties to be habitable.

Yes, well, there's more than enough evidence for that without resorting to anthropic arguments.

 

[Chalnoth]

What role does the anthropic principle play in modern cosmology?

Very little at present. It mostly plays a role at the edges of theory, when people contemplate potential theories for the birth of our universe. The vast majority of cosmology is focused on how our universe has changed over time, not the actual moment of birth.

Basically, whenever you write down a theory for the birth of our universe, you have to consider that the only outcomes which anybody is ever going to observe are outcomes where there can be observers. Failure to consider this selection effect may lead to premature rejection of ideas that are actually true.

How can we justify it? (Is it merely good old common sense or does it have a mathematical formulation?) What does it succeed in explaining? Is it scientific? Is it necessary?

It's a simple logical tautology: observers can only observe conditions that allow them to exist. One has to be a bit careful in applying it to physical theories, but it is a tautology that is necessarily true no matter what.

 

[Chronos]

Or an unnecessary waste of time pondering theories that cannot possibly be correct.

 

[zacuum]

The strong version of the principal suggests that life is a fundamental outcome of the laws of physics(often thought to be a result of intelligent design).
The weak version states that the laws of physics simply allow for life to exist, which is obviously true.

 

[mrspeedybob]

4. Creationism

A creator designed the Universe with the purpose of supporting complexity and the emergence of Intelligence.
7. The fake universe

We live inside a virtual reality simulation.

I don't see the difference.

 

[rbj]

Yes, well, there's more than enough evidence for that without resorting to anthropic arguments.

what, precisely, is there more than enough evidence for?

 

[rbj]

my understanding for what the weak anthropic principle (WAP) is good for, is for turning some questions around.

before it was applied to the issue of the ostensible fine-tuning of the dimensionless fundamental constants, it was applied to the issue of the age of the universe. so rather than say "Isn't it remarkable that the universe is about 10¹⁰ years old, enough for beings like us to evolve and exist and note that the age of the universe is 'just right' for that?"

The WAP turns that around and asks the question: "In a universe that once was 10⁴ years old and someday might be 10²⁰ years old, at what age will this universe be most compatible with life of sufficient sophistication to evolve and exist and note that the age of the universe is 'just right' for this life?"

so it makes perfect sense because the Universe gets to experiment and try out all of these different ages. but, as far as we know, it does not get to try out all of these different combinations of values of fundamental constants, and only when it hits the constants "just right" will there be someone like us around to observe that fact and say "how remarkable". so the remarkability of the fine-tuned universe remains.

unless someone cooks up a scenario in which the other values (that are less conducive to matter and life) of the fundamental constants do have occasion to be in use and one of those scenarios is that of the multiverse. if multiple universes really do exist (in some other space and time) and if the universe-generating mechanism spits out universes with all sorts of values for these (or comparable) fundamental constants, then it would not be remarkable that we find ourselves in the universe that happened to get those values 'just right'. all of the other universes (at least those with life-unfriendly fundamental constants) go utterly unbeheld, just as our universe did when it was 10⁴ years old, despite what the Young-Earth Creationists would have you believe.

but without such a reality, if this happens to be the only Universe there is, it remains remarkable that this Universe, given a single opportunity to get it 'just right', actually did.

the FTU is the observation and the WAP (with an additional assumption of various other universes or pocket universes or something that can create repeated trials with different such fundamental constants) is the explanation or an attempt to.

i think that the SAP basically says that the Universe is big enough that eventually somewhere intelligent life would have to appear (given the life-friendly fundamental constants we have). sort of like saying that if you continually repeat shuffling an honest deck of cards and draw the card off of the top, eventually you will draw the Ace of Spades (a.k.a. the "life-giving card" or outcome).

 

[rbj]

Or an unnecessary waste of time pondering theories that cannot possibly be correct.

what are the theories that cannot possibly be correct? the WAP? the FTU?

 

[Chalnoth]

what, precisely, is there more than enough evidence for?

1. The low-energy laws of physics are dependent upon accidents in the past (spontaneous symmetry breaking events). Right now this is limited to the electroweak symmetry breaking, but it is likely that this extends to quite a lot about how physics behaves.
2. Our universe is very, very big.

Put points one and two together, and you get a situation where it is very likely that there are regions far enough away that have very different low-energy laws of physics, which is a multiverse by most reasonable definitions. You can also plug in the nature of quantum mechanics into point 2 to make it even more concrete that different low-energy laws will be realized.

 

[rbj]

1. The low-energy laws of physics are dependent upon accidents in the past (spontaneous symmetry breaking events). Right now this is limited to the electroweak symmetry breaking, but it is likely that this extends to quite a lot about how physics behaves.

i know we have models of symmetry-breaking epochs in the very early universe. but do we actually have evidence of such?

2. Our universe is very, very big.

have we seen any radiation from anything in our very, very big universe that suggests that the source of the radiation comes from something with different physics? or even the same physics, qualitatively, but different fundamental constants? is there anything in the CMBR that indicates it came about from a symmetry-breaking epoch?

Put points one and two together, and you get a situation where it is very likely that there are regions far enough away that have very different low-energy laws of physics, which is a multiverse by most reasonable definitions.

sounds more like Guth's "pocket universes". I've thought that the different universes in the multiverse (if they really do exist) were supposed to come from different solutions to the M-theory of everything (the string or brane landscape thing that i don't understand well). these universes exist for the same reason ours exists (as a solution to some kinda brane equation).

You can also plug in the nature of quantum mechanics into point 2 to make it even more concrete that different low-energy laws will be realized.

i still don't see evidence from putting points one and two together. i see a hypothesis, even a theory. but not evidence.

BTW, i don't see the AP as evidence for such either. but if other universes exist, i see the WAP as a good explanation for why we have life-friendly parameters in our universe (among other universes with not-so-friendly parameters).

 

[bcrowell]

IMO there are serious problems with the anthropic principle. If we imagine an n-dimensional parameter space with the axes being things like the fine-structure constant, etc., then there may be some regions of this landscape that allow intelligent life and some that don't. The problem is that it's far beyond the reach of science to be able to map out which regions are which. For any given point in the parameter space, you would have to do some incredibly difficult theoretical calculations to infer whether it would allow intelligent observers. We can't even do that for one such point, much less an infinite number of them. It's not so hard when you consider small perturbations on our own universe's physical constants, but for radically different sets of physical constants, we can't possibly anticipate the radically different forms of life that might arise.

There's also a measure problem similar to the one that plagues inflationary theories.We can't assign any prior probability distribution to this parameter space. If intelligent observers are possible but extremely improbable in a certain type of universe, how do we know that there aren't just so many of those universes that a few of them just get lucky, despite their unfavorable physical constants? There is even more silliness added on top of that, because many of these universes are probably spatially infinite, so that if there is any finite probability of intellient life at all, they're guaranteed to have an infinite number of them. Do we somehow say that a particular set of physical constants is more acceptable if it results in an infinite number of intelligent observers that's a relatively large infinity compared to the infinite number of observers that would occur with some other set of physical constants? It gets pretty silly.

IMO when physicists invoke the anthropic principle what it means is that their research program is a failure, but they still want to keep getting funding for it.

There was a very interesting debate on this between Lee Smolin and Leonard Susskind: http://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html

4. Creationism

A creator designed the Universe with the purpose of supporting complexity and the emergence of Intelligence.
7. The fake universe

We live inside a virtual reality simulation.

I don't see the difference.

One is natural, the other supernatural.

 

[Chalnoth]

IMO there are serious problems with the anthropic principle. If we imagine an n-dimensional parameter space with the axes being things like the fine-structure constant, etc., then there may be some regions of this landscape that allow intelligent life and some that don't.

Well, it's difficult to do it in detail. It is not difficult to do it in general. There are some pretty basic requirements for life to exist, such as a universe that doesn't recollapse on itself almost instantly, or a universe where there is structure formation. You also need stable matter that forms complex structures.

It is much more difficult to map up exactly the sorts of complex structures that various forms of matter might potential produce. But you can apply the anthropic principle quite easily for the more general requirements without worrying so much about this one.

 

[rbj]

Well, it's difficult to do it in detail. It is not difficult to do it in general. There are some pretty basic requirements for life to exist, such as a universe that doesn't recollapse on itself almost instantly, or a universe where there is structure formation. You also need stable matter that forms complex structures.

and, i have read, there are parameters dependent on some of the fundamental constants that control (for lack of a better word) the "burn rate" of nuclear fusion in stars. you could even have matter, astronomical structures, some kind of elemental diversity, but if the stars burn out before life can emerge or complex life has a chance to evolve, you don't have observers who observe that conditions observed in the universe allow observers to exist.

just as the WAP is sort of useful in explaining why the universe is around 13.7 billion years old (and our planet is about 4.5 billion years) rather than 10,000 years or 10 trillion years, it could have some use in explaining why we are in this particular universe that is life-friendly (at least on planet Earth, perhaps we are alone in the Milky Way, perhaps we are alone in the Universe) among other universes if they exist. but it does nothing to support that premise in the first place. i know that materialists don't like to concede this, but believing in the multiverse is just as much a matter of faith is it is to believe in a supernatural creator. (we are not going to create a measuring device that will indicate the existence or absence of either.)