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(I could be wrong about this, I haven't thought about it in great detail) I think invoking the Anthropic principle only makes sense if we can demonstrate that

(I could be wrong about this, I haven't thought about it in great detail) I think invoking the Anthropic principle only makes sense if we can demonstrate that there are a number of other "places" with varying "conditions". For such a selection bias to occur there needs to be a population to actually select from. For example, the Anthropic principle makes sense regarding the composition of the earth because we know that there are a large number of planets with a distribution of orbits and masses around a distribution of stars. Of course we will be on a planet that supports life as we know it. To invoke the anthropic principle regarding a constant would imply — to me, at least — that there would need to be a number of universes with different constants. Or different regions of this universe where the constant would vary. If there truly is only a single universe, and the constant is not changing in time or space, then it would seem we did actually just happen to get lucky. Edit: of course, at this point we can't say definitely either way about our universe. We don't know if ours is one among many, or if its the solitary universe in existence. We also, clearly, have uncertainty about how constant this constant actually is.

jessaustin 11y ago
ISTM the different measured values of α described in TFA constitute evidence of "other places with varying conditions"?
- ncallaway 11y ago
  Yes, if we were able to consistently measure different values of α over space and/or time it would be significant and help to strengthen the anthropic principle as it could be applied here. There are a couple challenges to these measurements, though. Sean Carrol presents a theoretical challenge in his blog post "The Fine Structure Constant is Probably Constant"[1]. Chad Orzel has a more observational critique of the results in "Why I’m Skeptical About the Changing Fine-Structure Constant" [2]. To be fair, these critiques are from 2010 and Webb, et al. has published several papers since these critiques were made. I haven't seen updated debate or discussion over Webb, et al's findings. Still, I would think we have some potential evidence that the constant may vary, but I don't think this evidence is near conclusive yet. [1] http://www.preposterousuniverse.com/blog/2010/10/18/the-fine-structure-constant-is-probably-constant/ http://www.preposterousuniverse.com/blog/2010/10/18/the-fine... [2] http://scienceblogs.com/principles/2010/09/14/httpksjtrackermitedu20100907e/ http://scienceblogs.com/principles/2010/09/14/httpksjtracker...
raattgift 11y ago
Another alternative was proposed by Hawking & Hertog (Phys Rev D, 2006, see below): walk the present values surface back to a superposition of possible universes and study the mechanisms that generate probabilities near one. That then is their way of asking about the degree of fine-tuning, with the anthropic principle being recasta as a constraint on the set of possible consistent histories. H&H spend a lot of time comparing themselves to the approach of selecting an initial values surface in the early universe and marching it forward; they more or less do the same but flip the direction of the arrow of time and arrive at a superposed state. Re-flipping the arrow of time leads to relic fields like the CMB carrying evidence for the superposition. Essentially this is just taking time-reversibility of local microscopic physics seriously (great), and absorbing the difference in degrees of freedom in macroscopic physics into a superposition (uh, ok). The problem of setting up a values surface remains pretty much the same. "Fine tuning" is just a way of saying that the system is sensitive to the values it evolves from (or to). I'm not really convinced that that a present-day spacelike hypersurface is easier to write down than an early-time initial values surface. ArXiv version: http://arxiv.org/abs/hep-th/0602091 http://arxiv.org/abs/hep-th/0602091
- ncallaway 11y ago
  This paper looks very interesting, but it uses concepts a bit above my current understanding so I'll be studying it over the next week. Thanks for the link, though! I'll get studying.
baddox 11y ago
I don't see why there needs to be other universes that actually exist in order for the anthropic principle to apply. It seems to make perfect sense to apply it to a set of conceivable universes, even if such universes don't actually exist and even if it's not physically possible for them to exist.
- ncallaway 11y ago
  Again, I could easily be wrong about all this. I think this entire discussion is sensitive to exactly what one means by "The Anthropic Principle". If one's definition of the Anthropic Principle in this context simply means that an observed universe will be consistent with a universe that can be observed, then I find it to be trivially true and applicable to a single universe scenario. I subscribe to a definition of the anthropic principle that invokes selection bias as its core mechanism. I think that, in order to have a selection bias, one needs to actually be selecting from a population. Hence my assertion that you need a multi-verse (with different constants in each universe) or a constant that is variable in space or time [2]. The difference, to me, is this: If you are selecting from a population the Anthropic Principle can actually help explain why a value is what it is. Under our understanding of planetary bodies, we often invoke the Anthropic Principle to explain why the earth falls in the habitable-zone. Earth falls in the habitable-zone because — we think — life is most likely to arise on a planet in that zone. However, if you are not selecting from a population, I don't find that the Anthropic Principle explains why a value is what it is. In the scenario where there is only a single value, I think the Anthropic Principle simply demonstrates that that particular value must support life. Put another way, if all planets we observed were the exact same distance from their star as Earth [1], then we couldn't use the Anthropic Principle to explain why Earth is that particular distance from the sun. We would have to find other astronomical reasons to explain why planets form at that specific distance from a star. In this single-distance scenario the only thing the Anthropic Principle actually does for us is confirm that such a distance supports life. [1] Well, not distance per se, but placed such that they received the same amount of stellar energy in their orbits. [2] This final sentence of the introduction of the Anthropic Principle also makes some reference to this saying: "Most often such arguments draw upon some notion of the multiverse for there to be a statistical population of universes to select from and from which selection bias (our observance of only this universe, compatible with life) could occur."

jessaustin 11y ago

ISTM the different measured values of α described in TFA constitute evidence of "other places with varying conditions"?

ncallaway 11y ago
Yes, if we were able to consistently measure different values of α over space and/or time it would be significant and help to strengthen the anthropic principle as it could be applied here. There are a couple challenges to these measurements, though. Sean Carrol presents a theoretical challenge in his blog post "The Fine Structure Constant is Probably Constant"[1]. Chad Orzel has a more observational critique of the results in "Why I’m Skeptical About the Changing Fine-Structure Constant" [2]. To be fair, these critiques are from 2010 and Webb, et al. has published several papers since these critiques were made. I haven't seen updated debate or discussion over Webb, et al's findings. Still, I would think we have some potential evidence that the constant may vary, but I don't think this evidence is near conclusive yet. [1] http://www.preposterousuniverse.com/blog/2010/10/18/the-fine-structure-constant-is-probably-constant/ http://www.preposterousuniverse.com/blog/2010/10/18/the-fine... [2] http://scienceblogs.com/principles/2010/09/14/httpksjtrackermitedu20100907e/ http://scienceblogs.com/principles/2010/09/14/httpksjtracker...

raattgift 11y ago

Another alternative was proposed by Hawking & Hertog (Phys Rev D, 2006, see below): walk the present values surface back to a superposition of possible universes and study the mechanisms that generate probabilities near one. That then is their way of asking about the degree of fine-tuning, with the anthropic principle being recasta as a constraint on the set of possible consistent histories. H&H spend a lot of time comparing themselves to the approach of selecting an initial values surface in the early universe and marching it forward; they more or less do the same but flip the direction of the arrow of time and arrive at a superposed state. Re-flipping the arrow of time leads to relic fields like the CMB carrying evidence for the superposition. Essentially this is just taking time-reversibility of local microscopic physics seriously (great), and absorbing the difference in degrees of freedom in macroscopic physics into a superposition (uh, ok). The problem of setting up a values surface remains pretty much the same. "Fine tuning" is just a way of saying that the system is sensitive to the values it evolves from (or to). I'm not really convinced that that a present-day spacelike hypersurface is easier to write down than an early-time initial values surface. ArXiv version: http://arxiv.org/abs/hep-th/0602091 http://arxiv.org/abs/hep-th/0602091

ncallaway 11y ago
This paper looks very interesting, but it uses concepts a bit above my current understanding so I'll be studying it over the next week. Thanks for the link, though! I'll get studying.

baddox 11y ago

I don't see why there needs to be other universes that actually exist in order for the anthropic principle to apply. It seems to make perfect sense to apply it to a set of conceivable universes, even if such universes don't actually exist and even if it's not physically possible for them to exist.

ncallaway 11y ago
Again, I could easily be wrong about all this. I think this entire discussion is sensitive to exactly what one means by "The Anthropic Principle". If one's definition of the Anthropic Principle in this context simply means that an observed universe will be consistent with a universe that can be observed, then I find it to be trivially true and applicable to a single universe scenario. I subscribe to a definition of the anthropic principle that invokes selection bias as its core mechanism. I think that, in order to have a selection bias, one needs to actually be selecting from a population. Hence my assertion that you need a multi-verse (with different constants in each universe) or a constant that is variable in space or time [2]. The difference, to me, is this: If you are selecting from a population the Anthropic Principle can actually help explain why a value is what it is. Under our understanding of planetary bodies, we often invoke the Anthropic Principle to explain why the earth falls in the habitable-zone. Earth falls in the habitable-zone because — we think — life is most likely to arise on a planet in that zone. However, if you are not selecting from a population, I don't find that the Anthropic Principle explains why a value is what it is. In the scenario where there is only a single value, I think the Anthropic Principle simply demonstrates that that particular value must support life. Put another way, if all planets we observed were the exact same distance from their star as Earth [1], then we couldn't use the Anthropic Principle to explain why Earth is that particular distance from the sun. We would have to find other astronomical reasons to explain why planets form at that specific distance from a star. In this single-distance scenario the only thing the Anthropic Principle actually does for us is confirm that such a distance supports life. [1] Well, not distance per se, but placed such that they received the same amount of stellar energy in their orbits. [2] This final sentence of the introduction of the Anthropic Principle also makes some reference to this saying: "Most often such arguments draw upon some notion of the multiverse for there to be a statistical population of universes to select from and from which selection bias (our observance of only this universe, compatible with life) could occur."