Straight Dope Message Board

[Wesley Clark]

Is there a limit on potential physical laws? If we live in a multiverse and if different universes have different physical laws (hypothetical), only a microscopic percentage can harbor life. Has anyone done a guesstimation on what % could harbor life? Is there a limit on physical laws, meaning, can lambda have infinite values or are the values within a set range? I realize that you can have infinite values within a set range, but there are going to be values that allow chemistry and life within that range.

I don't understand physics well so I'm not sure if this is even a valid question. Point is, if we live in a multiverse of endless variations of physical laws, and assuming there are finite limits on physical laws, what percentage of universes relative to the whole could harbor life? Or is the answer 'an infinite minority of an infinite majority' or something?

[Tranquilis]

If we live in a multiverse, the question is essentially meaningless - How can you take a percentage of infinity?

[Just Asking Questions]

An infinite number. And there's an infinite number without life. And an infinite number where almost everything else is the same but you spell your SD name with an e at the end.

I hate the concept of the multiverse. Not only does it take away free will, it takes away any point to existence.

[Czarcasm]

Originally Posted by Just Asking Questions

An infinite number. And there's an infinite number without life. And an infinite number where almost everything else is the same but you spell your SD name with an e at the end.

I hate the concept of the multiverse. Not only does it take away free will, it takes away any point to existence.

It doesn't matter what you do, because in some other universe you will always do the right thing.
This idea was covered very well in a science fiction short story by Larry Niven-"All The Myriad Ways"

[Tranquilis]

Originally Posted by Czarcasm

It doesn't matter what you do, because in some other universe you will always do the right thing.
This idea was covered very well in a science fiction short story by Larry Niven-"All The Myriad Ways"

One might suggest that without connectivity and communication *between* the various incarnations of a multiverse, that what you do DOES matter - because you will never see the alternative acts, nor their consequences. Thus, morality and free will are preserved.

[TonySinclair]

18.36%.

That's the right answer in at least one universe.

[Stranger On A Train]

Originally Posted by Wesley Clark

I don't understand physics well so I'm not sure if this is even a valid question. Point is, if we live in a multiverse of endless variations of physical laws, and assuming there are finite limits on physical laws, what percentage of universes relative to the whole could harbor life? Or is the answer 'an infinite minority of an infinite majority' or something?

I do know a bit about physics and a few things about biology, and not only do I think this is a question that informed speculation cannot begin to address, I'm not confident that it is a even meaningful question.

All life that we know--which, to date, is exclusively limited to life on Earth which as we observe has a single origin--is chemical in nature, and furthermore based on the six of the most abundant chemicals in the universe (hydrogen, carbon, nitrogen, oxygen, phosphorus, sulphur). Although living forms at even the monocellular level are formed of incredibly complex systems, the basic structures of those systems are formed from twenty amino acids, lipids, and bound and powered by carbohydrates, most of which we can synthesize in the lab (with relatively crude and inefficient methods). It seems likely to the point of statistical certainty that life based on organic chemistry will be found elsewhere and probably frequently in our universe and in any other universe which has similar chemical laws. (Those bemoaning that we have not yet found signs of extraterrestrial life and rationalizing that it must not exist should recognize that we have landed probes on only four extraterrestrial bodies in our own solar system (Luna, Venus, Mars, and the saturnian moon of Titan), only one of which could possibly support a biotic environment with a liquid medium and available hydrocarbons, and have about as much sense of the wider possibilities for life even in neighboring star systems much less the larger universe as a gnat does of Shakespeare.)

We naively assume that life would be chemical in nature and similar to our own fundamental construction of proteins and cellular organization which is reasonable given the lack of other examples but exclusionary to other possibilities of self-reproducing regulated net thermodynamic systems with large scale organization which could, depending on the qualifications you apply, be considered as life. As to universes with other laws of physics, perturbing basic parameters which appear to be arbitrary (to our current limited understanding of fundamental particle physics and quantum field theory) would certainly make chemistry that we know untenable but could be replaced by other organizing systems using different combinations of forces and an analogue to chemical elements, or perhaps even some more exotic combination of organized structures and interactions. It is entirely possible that there are additional forces beyond the four that we know that are suppressed or unmanifested in higher dimensional organizations which could come into play in a universe organized along different physical principles, and we honestly don't know enough to even begin to speculate on what that might look like.

So, in summary, we have no real evidence to base an estimate of the propensity of life to emerge even within our own solar system, even less to evaluate the vast expanse of the universe that is and will remain beyond our immediate reach for the foreseeable future, and not even a fundamental basis to speculate about life in hypothetical universes with alternative laws of physics and some kind of chemistry-like system of interactions.

Stranger

[Czarcasm]

Originally Posted by Tranquilis

One might suggest that without connectivity and communication *between* the various incarnations of a multiverse, that what you do DOES matter - because you will never see the alternative acts, nor their consequences. Thus, morality and free will are preserved.

And one might suggest that this attitude works even better of a smaller scale, where people feel that it doesn't matter if they do a good job, because someone else further down the line will fix any problems that slip by, and even if they don't it doesn't really matter in the long run.

[Czarcasm]

It's like trying to guess "How many beans are in all my jars?" without knowing what kinds of beans, what size jar and how many jars there are total.

[Riemann]

Originally Posted by Tranquilis

If we live in a multiverse, the question is essentially meaningless - How can you take a percentage of infinity?

There are infinitely many positive integers. Yet we know what proportion are divisible by 2.

Originally Posted by Czarcasm

It doesn't matter what you do, because in some other universe you will always do the right thing.
This idea was covered very well in a science fiction short story by Larry Niven-"All The Myriad Ways"

But an infinite number of things does not imply infinite variety. There may be infinitely many universes, but with rather tight constraints on what kind of universe is possible.

I think that there is often a mistaken conflation of the multiverse with Everett's many-worlds interpretation of QM. The latter does indeed seem to imply that (in some poorly defined sense) all possible outcomes exist. But most versions of the multiverse are based on completely different ideas.

[Riemann]

ETA, having said that:

http://www.preposterousuniverse.com/...the-same-idea/

But, I think it's important to first understand how multiverse and many-worlds are completely different ideas before contemplating the deeper (and controversial) idea that they may be the same!

[Stranger On A Train]

Originally Posted by Just Asking Questions

I hate the concept of the multiverse. Not only does it take away free will, it takes away any point to existence.

Far from it; not only does it not matter what may happen in other universes, but you have the opportunity to make your universe the best of all possible existences.

As to the concept of other universes, it should be recognized that there are two separate concepts here. One comes from the quantum mechanical notion of simultaneous superposition of possibilities at the quantum level, derived from Hugh Everett's "relative state" interpretation (note that Everett did not promote the idea that there was a literal "multiverse"; just that we're experiencing one of a multitude of simultaneous possibilities). In this multiverse, all the laws of physics are the same but the resolutions of individual states fill all possible options, and share the same mass-energy matrix in a state of superposition.

The other is from brane cosmology that there are separate, unconnected physical universes formed from an initial singularity of energy and which deconstructed into different balances of dimensions and physical parameters. In this case, each universe has its own separate mass and energy (which may or may not be infinite in extent) and are in no way causally connected with one another. In this case, the laws of physics may vary wildly and perhaps randomly, and only a very small number of them would have enough of a structure of chemistry or some analog to develop a stable order. This is what people refer to when trying to justify some "anthropomorphic principle" of higher intelligence even though there is no evidence to suggest that our universe is specially crafted for us as much as that we simple have evolved to fit into a universe and world where relatively stable self-organizing systems are possible.

Stranger

[Senegoid]

I see articles and essays from time to time, speculating on what a universe might be like if some of the fundamental facts were just a little bit different from what they are in this universe, and they often suggest that with slight variations in those facts, life couldn't exist. Basically, the "anthropic principle" (AIUI) says that all sentient observers must necessarily see a universe where life can exist because those are only kind of universes in which the sentient observers themselves can exist. (Duh.)

What I see mentioned less often is that, in a universe with slightly different fundamentals, there's a good chance that nothing could exist at all (meaning, at least, nothing like "matter" as we know it). Quantum physics tells us that the universe, at some fundamental level, is really nothing more than "complex probability density distribution fields", all vibrating and interacting in various ways, and that "matter" is nothing more than spots where it gets a little more dense than elsewhere, and becomes somewhat stable and persistent.

Change some basic constants, and that doesn't happen. You could end up with a universe full of fields that don't "congeal" to create anything.

What a waste of a universe!

See:
The Known (Apparently-) Elementary Particles

The Known Particles — If The Higgs Field Were Zero

by Professor Matt Strassler. (He has a whole series of on-line essays about particle physics, starting here. -- Very clearly written, at a good level for lay audiences.)

[Tranquilis]

Originally Posted by Czarcasm

And one might suggest that this attitude works even better of a smaller scale, where people feel that it doesn't matter if they do a good job, because someone else further down the line will fix any problems that slip by, and even if they don't it doesn't really matter in the long run.

*shrug*
Sure. Fatalism is a thing. So is denial of responsibility. But I cannot presume my mistakes will be corrected - because I have no way to communicate with any timeframe/worldframe where some other actor has intervened - thus leaving me no choice but to muddle through as best I am able.

[Tranquilis]

Originally Posted by Riemann

There are infinitely many positive integers. Yet we know what proportion are divisible by 2.

Which still leaves us with infinity.

[TonySinclair]

Originally Posted by Riemann

There are infinitely many positive integers. Yet we know what proportion are divisible by 2.

But there are orders of infinity. The number of positive integers is countable. The number of possible universes is not.

[Riemann]

Originally Posted by TonySinclair

But there are orders of infinity. The number of positive integers is countable. The number of possible universes is not.

The number of reals between 0 and 1 is uncountable. But (in decimal representation) we know what proportion start with the digit 3.

We really have no firm idea of the properties of the multiverse - in fact, the term encompasses a numnber of different ideas. I was just making the point that the idea that something is infinite does not imply that we can say nothing about its properties.

[SamuelA]

Originally Posted by Just Asking Questions

An infinite number. And there's an infinite number without life. And an infinite number where almost everything else is the same but you spell your SD name with an e at the end.

I hate the concept of the multiverse. Not only does it take away free will, it takes away any point to existence.

The sad thing is, even if this particular universe is semi unique (as in, our neighbors are different enough that the earth doesn't have life or the same planetary structure, etc), there is no empirical evidence for free will or any physical theories to support it.

The most likely explanation given current knowledge is that the particular atoms in your brain determine what you will do and how you will learn from what you did and thus any future changes, and those atoms are governed by physical laws.

So whether or not you read this post and realize it's true, or dismiss it because it disagrees with your pre-existing false beliefs is only determined by physics. You didn't decide anything.

[Tranquilis]

Originally Posted by SamuelA

You didn't decide anything.

Heisenbergian logic. Except that, it's demonstrable that decision trees are influenced by experience, so pure randomness is ruled out.

[Riemann]

Originally Posted by Tranquilis

...it's demonstrable that decision trees are influenced by experience, so pure randomness is ruled out.

Nobody claims that decisions are random. Just that decisions are nothing more than computation. But if we're going to go off on a free will tangent, I'd suggest that a separate thread is in order.

[Tranquilis]

Originally Posted by Riemann

I'd suggest that a separate thread is in order.

Seems reasonable.

Of course, I've just about tapped out my science/philosophy of thought and consciouness depths. So if you do it, it'll be the other brains computing the course of that thread.

[boffking]

What percentage of numbers are prime?
You can't take a percent of infinity.

[SamuelA]

Originally Posted by boffking

What percentage of numbers are prime?
You can't take a percent of infinity.

You can't? I would swear that you in fact can. Isn't that what we do in math when we take the limit, for example, the value of an asset over infinite time? (aka an infinite series of a number that is shrinking) Primes are funny because they aren't smooth mathematically so while we can observe that they get rarer and rarer the bigger the numbers get, we can't prove exact properties of them over infinity.

[OldGuy]

Originally Posted by boffking

What percentage of numbers are prime?
You can't take a percent of infinity.

Actually this calculation is the idea behind the Prime Number Theorem.
https://en.wikipedia.org/wiki/Prime_number_theorem

And to a first approximation the number of primes less than x is asymptotically x/ln(x). This means that the fraction of numbers that are prime goes to zero in the limit, but it does so in a well defined way.

[Chronos]

Some probability problems are well-defined when dealing with infinities, and some are not. Some seem well-defined, but turn out not to be once one looks at them with enough rigor. And some problems which lack sufficient rigor can have that rigor applied in multiple different ways, which lead to different answers. Which one applies here depends on just exactly how the multiverse is structured.

Quoth Stranger on a Train:

As to the concept of other universes, it should be recognized that there are two separate concepts here. One comes from the quantum mechanical notion of simultaneous superposition of possibilities at the quantum level, derived from Hugh Everett's "relative state" interpretation (note that Everett did not promote the idea that there was a literal "multiverse"; just that we're experiencing one of a multitude of simultaneous possibilities). In this multiverse, all the laws of physics are the same but the resolutions of individual states fill all possible options, and share the same mass-energy matrix in a state of superposition.

There's a third possibility, also: Bubble universes in the eternal inflation model. In these, you could in principle point with your finger in the direction towards some other universe, but the bubbles are receding from each other at such great speed that causal connection between them is impossible. Such bubbles might or might not show variation in fundamental constants: I don't think that question is actually resolved.

The other is from brane cosmology that there are separate, unconnected physical universes formed from an initial singularity of energy and which deconstructed into different balances of dimensions and physical parameters. In this case, each universe has its own separate mass and energy (which may or may not be infinite in extent) and are in no way causally connected with one another.

In most braneworld models, the branes are in fact causally connected, just not easily. Electromagnetism, the strong force, and the weak force are confined to the branes, but gravitational effects can still bridge the gap.

[Chronos]

Quoth me:

Some probability problems are well-defined when dealing with infinities, and some are not. Some seem well-defined, but turn out not to be once one looks at them with enough rigor. And some problems which lack sufficient rigor can have that rigor applied in multiple different ways, which lead to different answers. Which one applies here depends on just exactly how the multiverse is structured.

It occurs to me that I ought to clarify what I mean here. By way of example, here's an argument that exactly 1/3 of all natural numbers are even. List out all numbers, like so: 1 3 2 5 7 4 9 11 6 13 15 8 17 19 10 21 23 12.... That's a list of all natural numbers, right? And if I look at the first n numbers on that list, and find the proportion that are even, that proportion is definitely approaching a limit of 1/3 as n increases.

Now, most people would say that this argument is wrong, because I'm taking the numbers out of order. And this is perhaps a reasonable thing to say, because the natural numbers have an obvious simple order to them. But when I'm looking at the set of all universes, is there a natural order to place them in? I suppose it makes sense to put our own Universe first on the list, but is there one particular universe that ought to be second, or third? Maybe I could list the universes in order of distance from our own... but then, how are we measuring the distance? Some sort of measure of how different they are from our own, perhaps? But then, the first universes I count will always be the ones most like our own, and if there are an infinite number that are enough like our own to have life, then I might never reach any of the non-life-bearing ones on my list.

[eburacum45]

Originally Posted by Chronos

In most braneworld models, the branes are in fact causally connected, just not easily. Electromagnetism, the strong force, and the weak force are confined to the branes, but gravitational effects can still bridge the gap.

This opens the possibility of signalling. If we could create gravity waves at will, then we could send messages to nearby branes, and maybe even get messages back.


I can call spirits from the vasty deep.
... Why, so can I, or so can any man; But will they come when you do call for them?

[Mijin]

Originally Posted by Riemann

But an infinite number of things does not imply infinite variety. There may be infinitely many universes, but with rather tight constraints on what kind of universe is possible.

I think that there is often a mistaken conflation of the multiverse with Everett's many-worlds interpretation of QM. The latter does indeed seem to imply that (in some poorly defined sense) all possible outcomes exist. But most versions of the multiverse are based on completely different ideas.

Well I've never been clear on that aspect. But if it's as you say then ISTM the multiverse doesn't really help with the issue of why all the constants in our universe appear fine-tuned. Because it just kicks it further down the road: considering the infinity of multiverses that could not harbour life, what good fortune this is the one that exists.

NB: This isn't a religious argument. And I'm on the fence of whether I think it's even a meaningful question

[Stranger On A Train]

Originally Posted by Mijin

Well I've never been clear on that aspect. But if it's as you say then ISTM the multiverse doesn't really help with the issue of why all the constants in our universe appear fine-tuned. Because it just kicks it further down the road: considering the infinity of multiverses that could not harbour life, what good fortune this is the one that exists.

The "fine tuning" argument is really solipsistic nonsense. If the universe were not configured as it is, we would not be here to ask the question and the issue would be resolved. The need for the universe to be finely tuned to permit our existence is predicated on some essential necessity of that existence, rather than that we just happened to emerge from the existing mechanics of physics and chemisty of a convenient universe. Could those mechanics be altered by an arbitrary change in universal constants making any order impossible? Sure, that is concievable, but there would be no one in that universe to ask questions.

The question of multiple universes is purely speculative and not possible of being frames as even a hypothesis without some fundamental breakthoughs in the physics of cosmology, e.g. some ability to finely measure and control gravity (which, as Chronos adroitly corrected me, may allow causal connections between seperate branes), so any question predicated on their existence is also speculative. Speculation can be quite fun, of course, and lead to enjoyable fiction, but we cannot draw any useful conclusions without observable phenomena and measureable data.

Stranger

[Hari Seldon]

Originally Posted by Riemann

There are infinitely many positive integers. Yet we know what proportion are divisible by 2.

This is true only if you look at the integers as an ordered set and then only in the usual order. What proportion is even if you put them in the order:
1,2,3,5,4,7,9,11,6,13,15,17,19,8,...
putting n odd numbers between 2n-2 and 2n? As far as anyone knows, there is no way of ordering the universes and Stranger's point is well taken. Anyway, I'll take a flier and say the odds are 0. Suppose there is a universe out there just like ours except the gravitational constant differs in the 10th decimal. Maybe life would still be possible, maybe not. How could we know? I think the question is essentially meaningless.

[Mijin]

Originally Posted by Stranger On A Train

The "fine tuning" argument is really solipsistic nonsense. If the universe were not configured as it is, we would not be here to ask the question and the issue would be resolved. The need for the universe to be finely tuned to permit our existence is predicated on some essential necessity of that existence, rather than that we just happened to emerge from the existing mechanics of physics and chemisty of a convenient universe. Could those mechanics be altered by an arbitrary change in universal constants making any order impossible? Sure, that is concievable, but there would be no one in that universe to ask questions.

As I mentioned in my previous post, I'm not sure that the fine-tuning "problem" is really a problem that needs or could be solved. However, I also don't find the anthropic principle compelling either.

A pretty sizeable chunk of science is investigating and understanding phenomena that human life depends on.
Now it's true that why this constant is 4.3 instead of 4.2 is a different kind of question to "How does water dissolve so many things?", say. It's a Why question vs a How question, for a start.
But for me, I don't see why that makes the anthropic principle "work" or be a satisfying answer.

[Riemann]

Originally Posted by Hari Seldon

This is true only if you look at the integers as an ordered set and then only in the usual order. What proportion is even if you put them in the order:
1,2,3,5,4,7,9,11,6,13,15,17,19,8,...
putting n odd numbers between 2n-2 and 2n? As far as anyone knows, there is no way of ordering the universes and Stranger's point is well taken. Anyway, I'll take a flier and say the odds are 0. Suppose there is a universe out there just like ours except the gravitational constant differs in the 10th decimal. Maybe life would still be possible, maybe not. How could we know? I think the question is essentially meaningless.

Yes, but please take my point in the context of what it was replying to. I was not claiming that we actually do know anything about the properties of the multiverse, if it exists. Of course we do not. I was countering the false assertion that it is impossible, in principle, to determine any properties of something that is infinite.

[Lemur866]

Originally Posted by Mijin

As I mentioned in my previous post, I'm not sure that the fine-tuning "problem" is really a problem that needs or could be solved. However, I also don't find the anthropic principle compelling either.

A pretty sizeable chunk of science is investigating and understanding phenomena that human life depends on.
Now it's true that why this constant is 4.3 instead of 4.2 is a different kind of question to "How does water dissolve so many things?", say. It's a Why question vs a How question, for a start.
But for me, I don't see why that makes the anthropic principle "work" or be a satisfying answer.

Isn't it amazing that human beings just happen to live on Earth, where there is liquid water, an oxygen atmosphere and photosynthesis, which can support us, instead of Mars, where we'd freeze, suffocate, and starve to death?

Of course there aren't any human beings on Mars, and it's not just random chance that humans evolved on Earth, a planet fine-tuned to support human life, rather than on Mars, a planet without any of the things required to support human life.

Our lungs aren't perfectly adapted to breathe Earth's air by chance, rather, the Earth's atmosphere existed and our ancestors evolved to breathe it.

So since we exist, we must exist in a universe and planet where human life could exist.

Of course, our universe isn't exactly finely tuned to support the presence of life, since even here in our solar system life apparently only exists on one small planet. The other bodies in the solar system are implacably hostile to life. Most of the matter in the universe is collected into starts, which can't support life, or frozen iceballs that can't support life, or finely dispersed gas and dust, which can't support life.

If I were going to design a universe capable of supporting life as we know it, Jim, I could do a much better job. Like that giant sun and tiny Earth, that's ridiculous, it should be the other way around. And those other big planets are just a waste of space, so replace them with tiny moving lights. And other stars? Silly. And let's put a dome over the Earth to hold in the atmosphere rather than use gravity. So something more like this: http://i.onionstatic.com/avclub/6235/81/16x9/1200.jpg

[SamuelA]

Originally Posted by Stranger On A Train

The "fine tuning" argument is really solipsistic nonsense. If the universe were not configured as it is, we would not be here to ask the question and the issue would be resolved. The need for the universe to be finely tuned to permit our existence is predicated on some essential necessity of that existence, rather than that we just happened to emerge from the existing mechanics of physics and chemisty of a convenient universe. Could those mechanics be altered by an arbitrary change in universal constants making any order impossible? Sure, that is concievable, but there would be no one in that universe to ask questions.
Stranger

Stranger, it isn't, and it in fact supports the multiple universe hypothesis.

Observation : the various parameters of this universe must be set an exact way or we would not be here.

Observation : no coherent model of time and cause and effect as observed by the nature of this universe allows an event to spontaneously, from nothing, create just this universe and no other.

Resulting Hypothesis : some type of eternal mechanism has created this universe and others with different parameters and will do so forever.

If this hypothesized mechanism didn't create all possible parameters somehow (whether those extra universes will exist before/after this one, or they all exist right now simultaneously), it would be extraordinarily unlikely for us to be here.

I know this hypothesis is thin. There are many other explanations for our existence, but this one has the benefit of being as simple as possible.

The simplest possible explanation that explains all observations is the model you should use.

[SamuelA]

Originally Posted by Lemur866

Of course, our universe isn't exactly finely tuned to support the presence of life, since even here in our solar system life apparently only exists on one small planet. [/url]

No, but it is thought that slight changes in the parameters and the universe would just be an ever expanding ball of dark gas, or just light expanding forever and no matter, or other such possibilities where no life at all can be possible. The possibility of self replicating complex matter spontaneously evolving requires an awful lot of characteristics specific to this universe.

That specificity is information, and the simplest model of universe formation posits a simple, eternal mechanism that creates them. Therefore, that mechanism could not have the information to create this specific universe alone, and thus, there must be other universes - or this one must repeat with different parameters.

Everything beyond that is mostly speculation, but the edge possibility that the other universes are visible from telescopes here allows at least a possibility of gathering the evidence to investigate it one way or another.

[Riemann]

Originally Posted by SamuelA

...the simplest model of universe formation...

I don't think I accept your invocation of Occam's Razor here.

First, here's Andre Linde with a good account of the fine-tuning problem, the anthropic principle and the multiverse:
https://www.edge.org/response-detail/25535

From the early days of string theory, physicists knew that there are exponentially many different ways to compactly the extra 6 dimensions, but we did not know what can prevent the compactified dimensions from blowing up. This problem was solved about 10 years ago, and the solution validated the earlier expectations of the exponentially large number of possibilities. Some estimates of the number of different options are as large as 10^500. And each of these options describes a part of the universe with a different vacuum energy and different types of matter.

In the context of the inflationary theory, this means that our world may consist of incredibly large number of exponentially large "universes" with 10^500 different types of matter inside them.

A pessimist would argue that since we do not see other parts of the universe, we cannot prove that this picture is correct. An optimist, on the other hand, may counter that we can never disprove this picture either, because its main assumption is that other "universes" are far away from us. And since we know that the best of the theories developed so far allow about 10^500 different universes, anybody who argues that the universe must have same properties everywhere would have to prove that only one of these 10^500 universes is possible.

And then there is something else. There are many strange coincidences in our world. The mass of the electron is 2000 times smaller than the mass of the proton. Why? The only known reason is that if it would change few times, life as we know it would be impossible. The masses of the proton and neutron almost coincide. Why? If one of their masses would change just a little, life as we know it would be impossible. The energy of empty space in our part of the universe is not zero, but a tiny number, more than a hundred orders of magnitude below the naive theoretical expectations. Why? The only known explanation is that we would be unable to live in the world with a much larger energy of vacuum.

The relation between our properties and the properties of the world is called the anthropic principle. But if the universe were given to us in one copy, this relation would not help....

But it seems to me that there are three possible resolutions to the apparent fine-tuning problem, and (given the lack of evidence to support string theory) it would be extremely difficult to defend the position that any one of these is a more economical explanation than the others.

(1) Multiverse + anthropic principle. No constraint on parameter space, a vast variety of universes DO all exist, the vast majority inconsistent with life. The anthropic principle explains why we see ours.

(2) There is a better model for how universes are built, that does not have all the free parameters of current string theory. In other words, the parameter values found in our universe arise from the model somehow as the only valid solution. The only real basis to claim that this is unlikely is that we have been looking for such a model for decades and have failed. But that seems a weak argument to me.

(3) We are wrong about our interpretation of the parameter space and "life" is not as unlikely as it appears. This would probably need to be combined with a weaker form of (2) above, where a better model that current string theory might constrain parameter space allowing a much narrower variety of possible universes.

[Mijin]

Originally Posted by Lemur866

Of course, our universe isn't exactly finely tuned to support the presence of life, since even here in our solar system life apparently only exists on one small planet. The other bodies in the solar system are implacably hostile to life. Most of the matter in the universe is collected into starts, which can't support life, or frozen iceballs that can't support life, or finely dispersed gas and dust, which can't support life.

Of course, but the issue with the fine-tuning argument is that even our universe, even if earth is the only planet with life, would look lush compared to one where the strong force was a smidge stronger, say.

I'm no proponent of the fine-tuning argument, I am just saying I don't think these standard handwaves quite work.
(although, as I say, I'm not sure it's an issue that needs addressing anyway)

[Riemann]

Originally Posted by Mijin

Of course, but the issue with the fine-tuning argument is that even our universe, even if earth is the only planet with life, would look lush compared to one where the strong force was a smidge stronger, say....

Yes. If the current model of string theory is correct, it appears the vast majority of allowable parameter values are inconsistent with the existence of any form of stable matter, let alone "life" under even the broadest of definitions. This may be wrong, but that's what the only viable current models seem to say. And we need a little better than a hand-wavey "maybe life is less unlikely than we think" to resolve it.

[Chronos]

Keep in mind, we're all talking about "life as we know it", here. I can envision, for instance, a form of life not dependent on any form of matter at all, and composed entirely of gravitational waves. Such life would exist far more slowly than the sort we're familiar with, but that only matters if you're measuring using life-as-we-know-it time units. By the same token, I can also envision a form of life based on strong-force "chemistry" instead of electromagnetic, which could only have existed in the first few minutes of our Universe's history, and which would consider absurd the notion that anything could live 10^28 picoseconds past the heat death of the Universe.

[Riemann]

Originally Posted by Chronos

Keep in mind, we're all talking about "life as we know it", here. I can envision, for instance...

But I think the argument is that even forms of "life" that are beyond our wildest imagination surely have a requirement for some degree of stability, and that under the current unconstrained models the vast majority of the parameter space is unstable. (I'm not quite sure if stable/unstable is the best terminology here, but something like that.)

[SamuelA]

Originally Posted by Chronos

I can envision, for instance, a form of life not dependent on any form of matter at all, and composed entirely of gravitational waves.

Only if you are stupendously lazy*. Gravity waves, like all light waves, pass through each other without interacting. Life actually requires a number of far more specific characteristics to work :

a. It requires something able to catalyze something else into a piece of the same something. Gravity waves are right out because of the lack of interaction.

b. Complex life requires a pool of these self-catalyzing devices to be present in the same place and time such that they can compete with one another. A cloud of gas or small specs of nanometer scale dust expanding forever into the void cannot become complex life.

c. Complex life requires the environment to be stable enough that the information - the catalyzing "something" - is not destroyed faster than it can be created. This also means there must be an energy gradient to create information. Thus, a ball of matter at a smidgen over zero kelvin can't create life, no energy gradient. The hot gas cloud of our sun doesn't support life starting there, too much disruption for it to ever evolve.

And like 10 others, point is, gravity waves fail the first test and many other tests will fail out a great many other possibilities.

*Ok, I apologize for being harsh, but I don't see how you could have "envisioned" this working. You would have needed to think of a way for a gravity wave - which requires mass to be created - to catalyze the creation of an identical gravity wave to itself, or a sub-component of itself that can be assembled into itself. That's the first step towards life. The very first one.

[Asympotically fat]

Originally Posted by SamuelA

Only if you are stupendously lazy*. Gravity waves, like all light waves, pass through each other without interacting. Life actually requires a number of far more specific characteristics to work :

a. It requires something able to catalyze something else into a piece of the same something. Gravity waves are right out because of the lack of interaction.

b. Complex life requires a pool of these self-catalyzing devices to be present in the same place and time such that they can compete with one another. A cloud of gas or small specs of nanometer scale dust expanding forever into the void cannot become complex life.

c. Complex life requires the environment to be stable enough that the information - the catalyzing "something" - is not destroyed faster than it can be created. This also means there must be an energy gradient to create information. Thus, a ball of matter at a smidgen over zero kelvin can't create life, no energy gradient. The hot gas cloud of our sun doesn't support life starting there, too much disruption for it to ever evolve.

And like 10 others, point is, gravity waves fail the first test and many other tests will fail out a great many other possibilities.

*Ok, I apologize for being harsh, but I don't see how you could have "envisioned" this working. You would have needed to think of a way for a gravity wave - which requires mass to be created - to catalyze the creation of an identical gravity wave to itself, or a sub-component of itself that can be assembled into itself. That's the first step towards life. The very first one.

I have to admit it would be difficult to imagine how gravitational waves could form the necessary structures that we might recognize as being something akin to light, but they are non-linear (i.e. gravitational waves interact with gravitational waves unlike em waves)

[SamuelA]

Originally Posted by Asympotically fat

I have to admit it would be difficult to imagine how gravitational waves could form the necessary structures that we might recognize as being something akin to light, but they are non-linear (i.e. gravitational waves interact with gravitational waves unlike em waves)

Ok, I stand corrected on that point. You then do have to posit how they can catalyze creation of similar waves, and how the system deals with perpetual energy loss. If waves are ever expanding and ever growing weaker, you cannot have "something" creating itself - the fundamental definition of what life is.

[Riemann]

Is there any conceivable way that a self-aware being could arise in any universe except through natural selection?

Thus, to restate SamuelA's requirements slightly differently: in addition to suffiient basic stability, we need a mechanism for reproduction and heredity (variation is also required, but that's almost inevitable).

The actual mechanisms for reproduction and heredity could be beyond our wildest imagination, but I think they must exist in some form. What alternative is there?

[Chronos]

Quoth SamuelA:

*Ok, I apologize for being harsh, but I don't see how you could have "envisioned" this working. You would have needed to think of a way for a gravity wave - which requires mass to be created - to catalyze the creation of an identical gravity wave to itself, or a sub-component of itself that can be assembled into itself. That's the first step towards life. The very first one.

Life as we know it is based on atoms. Is there any way for an atom to catalyze the creation of an identical atom? It's not the gravitational waves themselves which need to be reproduced; it's the patterns in them.

[Sage Rat]

The safest guess would be that our universe is "average". That might be wrong, but it's the safest assumption for a sample size of 1.

So the best guess would be something over 50%.

[Riemann]

Originally Posted by Chronos

...I can also envision a form of life based on strong-force "chemistry" instead of electromagnetic, which could only have existed in the first few minutes of our Universe's history...

The hard SF novel "Dragon's Egg" by Robert L Forward describes the evolution of life (in our universe) in a strong-force environment on the surface of a neutron star.

[MacLir]

Originally Posted by Stranger On A Train

We naively assume that life would be chemical in nature and similar to our own fundamental construction of proteins and cellular organization which is reasonable given the lack of other examples but exclusionary to other possibilities of self-reproducing regulated net thermodynamic systems with large scale organization which could, depending on the qualifications you apply, be considered as life.
Stranger

"Meat. They're made of meat."

[Irishman]

Originally Posted by Mijin

Well I've never been clear on that aspect. But if it's as you say then ISTM the multiverse doesn't really help with the issue of why all the constants in our universe appear fine-tuned. Because it just kicks it further down the road: considering the infinity of multiverses that could not harbour life, what good fortune this is the one that exists.

It's amazing that we exist in a universe that is able for us to exist and not in a universe that we can't exist in.

So there are a bajillion universes where any kind of life like us could exist. Well, we don't exist there, so speculating why we exist where we exist is like speculating why fish swim in water rather than fly through the air. (Except for flying fish - shush, don't bring up silly exceptions.)

Originally Posted by Mijin

As I mentioned in my previous post, I'm not sure that the fine-tuning "problem" is really a problem that needs or could be solved. However, I also don't find the anthropic principle compelling either.

The fine-tuning "problem" is that we developed in a universe in which we could develop. That seems hardly something that needs explaining. Without knowing what other possibilities there are, it's meaningless to try to evaluate the likelihood that we have arisen at all. The likelihood is now 1 - we're here.

Asking why the universe exists at all is one thing. Asking why our universe is the way it is requires mostly speculation and very little actual science. It's a guessing game with no answer book to check and see who is right.

A pretty sizeable chunk of science is investigating and understanding phenomena that human life depends on.
Now it's true that why this constant is 4.3 instead of 4.2 is a different kind of question to "How does water dissolve so many things?", say. It's a Why question vs a How question, for a start.

Maybe it's a why question instead of a how question because we don't yet have enough foundational information to make it a how question. Suppose there are some constraints that we haven't yet identified. Those would be like the chemistry rules that make water a dissolver.

Originally Posted by Riemann

The actual mechanisms for reproduction and heredity could be beyond our wildest imagination, but I think they must exist in some form. What alternative is there?

Well, proponents of the fine-tuning argument say "God", not that I think that's a scientific explanation.

[Riemann]

Originally Posted by Irishman

...The fine-tuning "problem" is that we developed in a universe in which we could develop. That seems hardly something that needs explaining...

That is not the fine-tuning problem. See e.g. the account from Andre Linde that I linked at post#36, or read the rest of this thread. The fine-tuning problem arises from the fact that the only viable models that we have for the universe do not constrain parameter values (and we can't find one that does, despite decades of trying); all parameter values seem to be possible, and the vast majority are not consistent with life.

Originally Posted by Irishman

...Well, proponents of the fine-tuning argument say "God", not that I think that's a scientific explanation.

The fine-tuning argument for God is little more than "we don't understand X, therefore Jeebus".

But don't confuse this with the real fine-tuning problem in physics. When physicists call this a "problem" it does not imply that it's paradoxical or insoluble of that we must turn to God. It's just a problem in the sense that we don't know which of several possible naturalistic explanations is the correct resolution. One of the possible resolutions is the multiverse and the anthropic principle.