The “sensible anthropic principle” says that certain apparently unusual features of our environment might be explained by selection effects governing the viability of life within a plethora of diverse possibilities, rather than being derived uniquely from simple dynamical principles. Here are some examples of that principle at work.
- Most of the planetary mass in the Solar System is in the form of gas giants. And yet, we live on a rocky planet.
- Most of the total mass in the Solar System is in the Sun. And yet, we live on a planet.
- Most of the volume in the Solar System is in interplanetary space. And yet, we live in an atmosphere.
- Most of the volume in the universe is in intergalactic space. And yet, we live in a galaxy.
- Most of the ordinary matter in the universe (by mass) consists of hydrogen and helium. And yet, we are made mostly of heavier elements.
- Most of the particles of ordinary matter in the universe are photons. And yet, we are made of baryons and electrons.
- Most of the matter in the universe (by mass) is dark matter. And yet, we are made of ordinary matter.
- Most of the energy in the universe is dark energy. And yet, we are made of matter.
- The post-Big-Bang lifespan of the universe is very plausibly infinite. And yet, we find ourselves living within the first few tens of billions of years (a finite interval) after the Bang.
That last one deserves more attention, I think.
Peter,
As far as I know, only a small fraction of the landscape has been expored so far, so it seems to me to be premature to be making statistical predictions based on the totality of the landscape.
Peter Woit,
Sure it does! It makes one big, confirmed prediction: that Einstein’s equations should be satisfied in a space-time in which strings propagate. As Sean has pointed out on numerous occasions, this alone is a pretty good reason to pursue string theory. And one day, given a specific dynamic (such as the string theory landscape), perhaps we’ll be able to make other more specific, probabilistic predictions.
Of course, my opinion on the matter makes no difference at all, but there are a large number of theorists who believe that it is something worth pursuing.
Jason,
As Lisa Randall likes to point out, yes string theory does predict gravity: 10 dimensional gravity.
Eric,
If you know of a possible reason the landscape implies baryon number conservation, let’s hear it. If not, the argument that “OK, it looks like there’s a huge problem with this idea, and there’s not a smidgen of experimental evidence for it, but I want to believe it anyway, so maybe if we study the idea more its problems will go away” is just classic methodology of pseudo-science.
OK, folks, someone please explain what we’d get any idea of a “landscape” (other than the platonic everyworld idea) off the ground with? I mean, since every (?) attempt to construct with “first principles” is conditioned by what we have – isn’t it hard to say, something quite different couldn’t also be rationalized by its inhabitants to be naturally constructed etc.? (Or, did I get the idea of landscape wrong?) In any case, at the link above you can see a discussion of how three dimensions are special for conservation laws applied to electromagnetism. It’s just a summary, food for thought.
Neil B:
No, there doesn’t need to be an extrusion into the 3D world – the 2D beings would see the world in front of them as a line
I don’t see how they would. Say their 2D world is represented as a sheet of paper. In that case, yes, they see others as 1D lines. But they only see those lines because that paper has a smidgen of thickness in the 3rd dimension. Reduce that thickness to literally zero … from cardboard to bond to really skinny onion paper … keep compressing the sheet of paper until it is squeezed out of existence in the 3rd dimension, and that 1D line becomes undetectable. Heck their 2D existence ceases to be. It can only ‘be’ if that sheet of paper has some thickness.
So their space can exist if (1) it extrudes ever-so-slightly into the 3rd dimension, or (2) it is granular, not infinitely continuous
That must be some kind of identity. Granularity = Extrusion. Also a continuous space cannot exist; it must be ‘supported’ by some grain 🙂
WhatMeWorry: Most mathematicians and physicists would say that your intuition about planes is an unconscious 3-D chauvinism. Our three dimensions of space are not mathematically specially, although physically they may be (which is argued and arguable.) Euclid’s definitions of lines, planes, etc. are fully self-consistent. We can’t imagine the infinitesimal line that would be the presenting view for a 2-D being, but it is just as valid as a surface, or our 3-space, not needing further extrusion into a higher space. I mean, can’t you imagine “surfaces” without something outside them, without thickness? The specialness of space, thickness etc, is just an illusion deriving from our experience and familiarity with our own world. I know it’s hard to believe.
WhatMeWorry:
Though Neil B. provided a decent enough answer, imagine this: we see things because photons bounce off of them and hit us (in the eyes). Consider a two-dimensional being living in a two-dimensional world. It would observe things in much the same way: particles of some sort or other would come from the object it is observing and strike the two-dimensional being. If this two dimensional being has no thickness, is it possible for this particle to hit?
Most definitely! All that we need is for the particle to be itself constrained to two dimensions, and if it is traveling in the right direction, it cannot help but strike the two-dimensional creature. No thickness is required: the particle can’t go above or below because there is no above or below. So if it’s traveling towards are hypothetical 2D creature, it hits.
In Sean’s list would there be any logical difference if the words “we live” were replaced by “life exists” and “we are” with “life is” (and where in the last point one would say “we find life existing”)? Further, could “we” (or “life”) in this context be replaced by something different, some inanimate phenomenon just as (apparently) unique to our earth, perhaps “ice floating in water”, and still preserve the apparent probabilistic paradox? (I am guessing that oxygen is considered a heavier element in point #5 since “we” are mostly water by weight.)
Peter,
I do not argue for the string landscape or anthropic reasoning. I’d much prefer that there be one unique vacuum or at least some vacuum selection principle. However, as a scientist I cannot ignore the possibility that it may be true. If so, it simply means that there are questions that we cannot answer. I understand why you don’t like this. I don’t like it either. However, at the present we simply do not know enough. We won’t know either, if people do not work on these problems.
There already are questions that we cannot answer. Since the invention of Quantum Mechanics we gave up on predicting the precise outcome of experiments. But we did make a net gain. If a multiverse exists we will, in principle, be able to predict more things than can be predicted in single universe theories.
Eric,
I would like to present a very short argument for why the string landscape (or some similar theory) might be preferable to one unique vacuum. Consider, for a moment, the case where there is one unique vacuum. In this situation, we have set all of the current parameters for which we currently have no explanation by appeal to the fundamental laws of nature. And we have gotten a universe where life exists, and in fact must exist by the laws of nature. It’s seems to me absurd that the laws of nature should work out such that life must be possible, particularly given the tiny range of parameter space enclosed by those parameters which allow life to exist in the first place.
It seems much more natural to me to envision a universe where we have a theory of nearly anything at the heart of it all, such that life is inevitable not because the low-energy physics have to work out the way they have in our region of the universe, but instead because the universe produces so many regions with different low-energy physics that life is inevitable at some point. In this way, we no longer have to have special fundamental laws to describe us. We instead have very flexible fundamental laws where special things inevitably occur from time to time.
I think a lot of theorists dislike this idea because they seem to think it means that we stop working on theory, as if we’re going to explain everything by anthropics, why explain anything at all? Another argument to be had is that we only use the anthropic argument as a stand-in for a real explanation. Well, I claim that neither is the case. First, anthropic reasoning doesn’t remove theoretical work, it merely shifts it. Instead of looking for fundamental laws, we look for dynamic processes. Second, it doesn’t remove the need to look for fundamental laws, it merely adjusts our methodology in determining which theories that provide these fundamental laws are more or less likely to describe our universe.
Sean’s post misleads. Each example can be treated using simple physical principles without appeal to “selection bias” aka “selection effect”. Mentioning “selection effect” or using the term “anthropic” is something of a red herring.
This statement is just wrong: “…rather than being derived uniquely from simple dynamical principles. Here are some examples of that principle at work.”
To translate each example into practical operational terms one can ask if your job were to look for signs of extrasolar life where would you point your telescope. At wet rockies or at gas giants? At the surface of stars or at planets? At stuff in galaxies or at regions of intergalactic space? If you have a research grant to look for signs of life then presumably you have a working definition of what physical phenomena you are going to call life and what the physical signs are. And I think there are clear “simple dynamical principles” that explain why you and the funding agency would think it more likely to find signs of life (however you might reasonably define it for purposes of study) on wet rockies instead of on the surface of a star, a gas giant, or in nearly empty space.
The only one of Sean’s examples that doesn’t translate into a pragmatic telescope-pointing question is the last one. But this has been dealt with by for example Lineweaver and Egan in a recent (2007) paper available at arxiv. If you go by the standard LCDM cosmology, take into account the rigors of living in the cold dark old age of the universe and allow time for planetary system formation and biological evolution to elapse since bang, then now is a pretty ordinary time to be observing the universe. They explain the fact that the matter fraction and dark energy fraction are roughly comparable as something which is NOT an “unusual feature” but TO BE EXPECTED based on known physical principles. Indeed they estimate that 68 percent of all observers, during the effective lifespan of the universe, would be observing during this present era during which the matter and dark energy fractions are at least as roughly comparable as they are today. In other words, following Lineweaver and Egan’s argument, whatever reasonable definition of OUR ERA you adopt, it is NOT SURPRISING to be be an observer in this era. One should expect, on the contrary, that it is rather commonplace. If there are any other observers at all, they are not unlikely (e.g. 68 percent) to be, in a crude sense, contemporary.
It’s a great (and rather usual) time to be watching! 🙂
Jason Dick and Neil B:
Thanks yes now I see it might be 3D chauvinism. Seen through one 3D eye, we 3D beings see the equivalent of a 2D ‘surface’. A 4D being contemplating this in the same way I did, would say to herself ‘that hypothetical 3D guy cannot detect that 2D surface unless his 3D space extrudes into my 4th dimension.
The 2D photon hitting the 2D eye … from my 3D perspective, if both the photon and the eye have zero thickness, that space has no existence for me and I simply cannot visualize that 2D intersection. I was wrong to think ‘if that 2D space was granular, then I can visualize it’, because I was imputing a 3rd dimension to the 2D grains, making that space detectable to me.
As a 3D being, I am only able to visualize that 2D space if it is not continuous and the ‘atoms’ extrude ever so slightly into the third dimension. Ditto to the 4D being contemplating our 3D existence. Hey maybe that’s where the extra string dimensions hole up.
This provides an outline for a debate between myself, a student of engineering, a student of philosophy, and myself, a student of Business, a few weeks ago.
My philospher/scientist friend posited that though life on our planet exists in its present state, our composition (matter specifically), relative to that of the universe is more the exception than the rule; but that there could still possibly be environments other than our own, that could sustain us.
My engineer/CEO friend posited that life as we understand and experience it, likely exists uniquely in the universe; due to the unlikelyhood that there are other environments in our universe with the atmospheric and elemental properties neccesary to sustain life. ( e.g., We don’t know if there is oxygen there or an atmosphere to hold it in; what kind of radiation is there? etc.)
I (the meandering basement philospher, and student of life)posited that life as we experience it (and in general), is an alchemical inevitability of the elements of our environment (at the micro and macro levels-from our planet’s ecosystem to the overaching structure of the universe); and that our perception of “life” is limited to our perspective, and ultimately our understanding of what life is; therefore life as we know it, or otherwise, may possibly exist within environmental constructs unfamiliar to us. (Who says one needs a body to “live”?)
I eventually came to the realization, like with any argument, we were ultimately presenting different perspectives on the same fundamental dynamic.
Now onto this:
“…certain apparently unusual features of our environment might be explained by selection effects governing the viability of life within a plethora of diverse possibilities.”
Yeah… what Darwin said…
Or, depending on your interpretation…What Dr. Frankenstein said…. “It’s alive!” Gaia is alive!
Question: Wouldn’t it stand to reason that the “simple dynamical principles” that govern and order the existance, and behavior of all particles, and thier eventual arrangement into matter/antimatter or whatever, would be the very reason for the “..plethora of diverse possibilities” before us; upon which our development into what we currently are, is based? I think so.
Then’s there’s this:
Given that the subatomic particles of physical matter fall at the lower frequency end of the electromagnetic spectrum (as I understand it), this kind of begs the question:
Are we down here on the physical plane, riding the short bus on the highway of existance?
Sean brings up some interesting points, culminating (again, to me anyway) in a theory that we (at least in our present state) are very young, in our developmental journey through the universe-physically, at the very least.
Hell, maybe we are even still hauling ass down the sidewalk, trying to catch the short bus…
Or just maybe, just maybe….we’re right on time….
Aeon Flux,
I seriously seriously doubt that life that makes use of similar metabolism to ours is not out there somewhere. Now, I think that the specific building blocks of life may be incredibly different from what we have here on Earth, but I think we can be reasonably confident that there is other carbon-based life that makes use of the same sorts of basic molecules (DNA, RNA, proteins, etc.).
I claim this for the following reasons:
1. Life like our own doesn’t need terribly special conditions to arise. The carbon, nitrogen, and oxygen that such life requires are going to be reasonably plentiful around any population I stars. In such a situation, basically all you need is roughly the right mass planet at roughly the right distance from the host star (in the “Goldilocks zone”). We’ve already found one potential such planet within 30 light years of our own sun (though there are significant caveats there…), so it seems that such planets are probably pretty darned common.
2. From what little I know of abiogenesis, it appears that the basic building blocks of life (amino acids, nucleotides, and the like) are the way they are due to basic chemistry, and can’t change a whole lot (for carbon-based life, anyway).
So, from what I’ve read on the subject, I think the start of life is pretty much guaranteed to happen if the host planet has the right conditions, and it doesn’t seem that those conditions are that uncommon (compared to the 400,000,000,000 stars in our own galaxy, and the 100,000,000,000 visible galaxies). Once life starts, however, evolution takes over, and though I think we can expect convergent evolution to occur, I doubt there’s going to be much similarity at all in the specific DNA coding, the specific proteins used, or the various forms into which life organizes itself.
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Jason @ 63:
I think you’re missing the point. If we have a theory that actually predicts a certain value for X (be it the vacuum energy or whatever), then that’s all you need in order to evaluate the theory vis-a-vis our measured value of X. (The fact that certain arguments from our theories of low-energy physics predict values of the vacuum energy which are much too large compared with the measured value means that there are problems with those theories, or at least with the arguments made to estimate the vacuum energy from those theories.)
Knowing whether our measured value of X is “large” or “small” in terms of potentially allowing life is philosophically interesting, but irrelevant to the basic scientific issue of using theories to make predictions and testing those predictions against the observed data. A theory whose predicted value of X is 10 standard deviations too large is just as wrong as a theory whose predicted value is 10 standard deviations too small.
You wouldn’t apply this thinking in any other realm of physics or science. (“While it may be true that Ptolemy’s model for planetary motions badly mis-predicts the current position of the planets, we need to consider which planetary positions are consistent with our being here, or our continued survival. If Ptolemy’s theory predicts a planetary position that is consistent with our existence, then it’s just fine.”)
Peter Erwin,
There is a fundamental difference between predicting something like the motions of the planets and the properties of our observable universe. But this is a fair enough analogy to work with.
We expect that a proper theory of how the planets move relative to Earth-bound observers should be explicit: that there should be no anthropics involved. We know that general relativity is as yet the best description of the motions of the planets, as this theory predicts their motions to very high precision. The reason why there is no anthropic argument involved here is because the actual prediction is only one of time evolution. It doesn’t predict, for example, the distances of the planets from the Sun, the shapes of the orbits, the masses of the planets, the numbers of moons, etc.
Now imagine we don’t just want a theory that describes our solar system as it is today, but instead want to describe its origins. Why is the Earth habitable? Why is it just the right mass? Why is it just the right distance from the Sun? Why does it have just the right composition?
To answer these questions, we expect there to be some combination of anthropic reasoning and knowledge of dynamics. For example, we know today that our planet largely has the right chemical composition for life because our sun is a population I star. But we shouldn’t expect any theory to give a high probability of Earth itself forming around our sun, because we don’t have any reason to expect that the Earth itself is probable. It could have been an obscenely freak occurrence and still have happened, given that there are some 10^22 stars in the visible universe.
Now, given the properties of cosmic inflation alone, we can be reasonably confident that the universe as a whole is significantly larger than what we can observe. And it is entirely plausible that different regions of the universe have different low-energy physics. So we should not unduly bias our attempts to find a fundamental theory by claiming that all of the universe is like the small region that we can observe, just as we don’t try to bias our theories of solar system formation by claiming that all solar systems are like our own.
Jason Dick expounded forth (#86):
It seems much more natural to me to envision a universe where we have a theory of nearly anything at the heart of it all, such that life is inevitable not because the low-energy physics have to work out the way they have in our region of the universe, but instead because the universe produces so many regions with different low-energy physics that life is inevitable at some point. In this way, we no longer have to have special fundamental laws to describe us. We instead have very flexible fundamental laws where special things inevitably occur from time to time.
Well, first to remind folks in general that the constants (?) have to be very very narrowly within the current ranges or life can’t really take off. Also, this involves all kinds of things that need to be just right, like the carbon nuclear cycle inside stars, as explained well per the oft or even over-quoted classic The Anthropic Cosmological Principle. It goes down to properties of water, etc., really a lot of stuff needed to line up (so it’s more about whether the right values line up to within 1:10^100 or in 10^200 type of thing, and BTW not to be confused with what chance of life starting, given our laws as they are.)
Also, I’m not sure how we can imagine constants being different in other regions of our same (contiguous?) space. Are you really saying, that hydrogen atoms would not be the same as ones around here etc? If instead “matter is matter” then the “laws” and constants are intrinsic to the stuff, not to a boss medium they operate in. It would not be per what I consider the fallacy of the laws being a separable enforcer on otherwise property-free basal “vanilla stuff.” If matter does not boss itself in effect, you could then imagine different “legal classes” of particles being created out of the same event, instead of particles all acting the same anywhere unless subject to extreme stress.
Finally, the idea that matter can be/act differently elsewhere in the “same universe” brings up interesting perplexities, like: what if we brought some of it here? Would it act like our matter because our spatial region holds in it the guidelines for how to act, or would we have some oddly behaving (“self-bossing”) material that’s clearly not ordinary matter? Well, what if we could somehow import particles from “another universe” with different laws of physics instead, then what? Would they bring their funny laws with them, or be conditioned by existence in our space? Would say the latter, the properties of “vacua” invoked here? – but is the template similar enough to translate over? Great food for thought … and yes it’s very metaphysical I suppose, but let’s admit that’s what most of this subject is, OK?
What about the equality between tidal forces of Moon and Sun? Could it be anthropic?
(If you are tempted into the affirmative, think 30 seconds before reading the next remark) A couple years ago a lot of people was giving as an example of coincidence without further meaning the one of angular sizes of Moon and Sun. But, as you can read in the volume III of the Principia, the difference between tidal forces is the product of proportions between densities and between angular sizes, because the tidal force goes as the cube of the distance. So if tidal forces are anthropic-explained, then also the coincidence of solar eclipses is anthropic-explained.
To start a partial answer to my own question above: I suppose the virtual-particle repertoire, and what rules in principle apply to them, in a given space are what is “natural” to that space. That still doesn’t tell us what would happen to particles moved to regions of different laws, if such can be the case.
Neil B.,
One potential solution to the problem of envisioning different regions of space having different low-energy physical laws is that perhaps due to the way the physics works out, the different regions are separated by horizons. Thus there would be no problem of transferring mater from one region to another, as the horizons would prevent that from ever occurring.
Now, if this is not the case, then presumably understanding the most fundamental physics would tell us precisely what would happen when we try to move matter from a region with one set of low-energy physical laws to another. For example, if we imagine a string theory scenario where the different regions are separated by different compactifications of the higher dimensions, and we further imagine that they are not separated by a horizon, then we could easily ask the question of what would happen to a string from one region if it attempts to enter another. We might find that the differences in compactification prevent the string from entering the other region, or we might find some sort of scattering going on at the boundary, somewhat akin to what photons do when they cross the boundary between air and glass.
In either case, if we were capable of answering the question, “What is an electron, really?” then we would probably also be capable of answering the question as to what happens to it when it tries to pass to a region with different low-energy physical laws.
What about these guys?:
http://space.newscientist.com/article/dn12466-could-alien-life-exist-in-the-form-of-dnashaped-dust.html
…..source wikipedia
source.. Fecund Universes on Wikipedia