Understanding the Universe requires a lot more than just knowing some advanced mathematics, and even more than knowing how to apply that math. It requires a knowledge of natural history, and an understanding of the requirements for allowing that history to happen.

In other words, the laws of nature must be such that the Universe can exist as it does. Seems like a very simple, innocuous, and self-evident statement. Yet in this simplicity, we can learn a few important things about the Universe. Reasoning, using this statement, is often referred to as the anthropic principle.

There is more matter than antimatter in the Universe, for example, and therefore there must be some physical law or process that allows us to produce more matter than antimatter. Although we don’t know what that process is exactly yet, the fact that the Universe is full of matter and relatively devoid of antimatter tells us that this process must exist. That’s one example of anthropic reasoning, and it’s valid and (marginally) useful. The anthropic principle has, exactly once in the course of human history, been used to make a scientific prediction: Fred Hoyle’s prediction of the existence of a Carbon-12 excited state that is equal to the mass of three Helium-4 nuclei. The Universe is full of carbon, and so we need some way to make it, he reasoned. The only conceivable way to make it from Hydrogen and/or Helium was to have three Helium nuclei combine into a Carbon nucleus, meaning that there had to be an excited state of Carbon-12 that allows this to happen. Fred Hoyle predicted it, and then Willie Fowler found it.

And that’s really it. That’s all the anthropic principle is useful for, and it upsets me when otherwise respectable scientists hijack it and make exaggerated claims using it. How? Instead of:

The laws of nature must be such that the Universe can exist as it does,

there are those who make the claim that the laws of nature exist as they do because human beings are here to observe it. This most recently came up because of my article on dark energy. Commenter jb paraphrased an argument often used by string theorists to validate their idea of the Landscape:

Well maybe our naive calculations for the cosmological constant give us a number too big by a factor of 10^120, but The Landscape(TM) gives us 10^500 possible universes, and at least some of those are going to have the right value, and the others don’t matter because there’s nobody there.”

This is problematic for one major reason: it isn’t a scientific prediction. It doesn’t teach us anything new. If we wanted to use the valid version of the anthropic principle, all we can learn is that the cosmological constant couldn’t be too large, or it wouldn’t allow for gravitational collapse to occur, creating stars, galaxies, heavy elements, and Scienceblogs readers. But there’s no reason, even with the anthropic principle, for dark energy or the cosmological constant to have the value that it does. It could be ten, or a hundred, or even a thousand times larger than its observed value, and the Universe would still be here, and similar to the way it is now. It could also be a tenth, or a millionth, or a googol-th of the value we observe, and the Universe (and you and I, for that matter) would be just fine. It could also be exactly zero, but it isn’t. The fact is, while we can measure the value that it has, we do not have any understanding of how it got to have that value. And the anthropic principle, no matter how you twist it, is of no help in that regard.

Any type of scientific reasoning is only useful when it tells you something you don’t already know. So beware of abuses of the anthropic principle. They may be logically consistent, and they may be possible, but unless they predict something, they’re not scientifically useful.

I await your hate mail, Lubos.

Comments

  1. #1 NewEnglandBob
    July 17, 2009

    Yes, that is exactly how I see it, Ethan.

    I call this the weak Anthropic principle which I accept:

    The laws of nature must be such that the Universe can exist as it does

    and I call this the strong Anthropic principle which I reject:

    the laws of nature exist as they do because human beings are here to observe it

    I have read books by Lee Smolen, Brian Greene, Phil Plait, Victor Stenger, Richard Feynman, Lawrence Krauss, Bruce Rosenblum & Fred Kuttner, Carl Sagan and Leonard Susskind. After synthesizing these books I accept the weak Anthropic principle and reject the strong Anthropic principle.

  2. #2 sean hogge
    July 17, 2009

    Am I being too simplistic when I say that the weak anthropic principle is not really a principle at all? It’s basically self-referential rhetoric declaring that reality exists.

    Even the weak anthropic principle assigns a purpose to reality, which seems unscientific.

    So I, in kind, would reject the strong anthropic, but call the weak anthropic “Je pense, donc je suis” in a lab coat. Which I don’t necessarily reject, but even its assistance to Hoyle seems sort of … superfluous? While it may certainly inspire discovery, it is not necessary for discovery. N’est-ce pas?

  3. #3 6EQUJ5
    July 17, 2009

    I have never been able to make any sense of the anthropic principle. Now I don’t feel bad, so thank you.

    I don’t get panspermia either. Why is it unacceptable for life to have arisen on Earth, but perfectly acceptable for it to have arisen somewhere else in the Universe and then spread to Earth?

  4. #4 Benjamin Franz
    July 17, 2009

    Re. panspermia: It isn’t that it is ‘unacceptable’ for life to have arisen on the Earth, it is just an odds game. Given 13.7 Billion years for the universe at large vs 4.5 Billion for the Earth in particular, and many billions of potential planets and other places where life might have arisen first it seems not unlikely that it might have arisen somewhere else first and spread.

    But right now panspermia is pure conjecture. Once we have a chance to explore the solar system (and particularly other potential places where life might exist like Europa) we may have a sample size larger than ‘one’ to work from.

  5. #5 Rook
    July 17, 2009

    I don’t think that’s the point. The strong anthropic principle isn’t a scientific principle in itself, it’s a refutation of unscientific mysticism, specifically of the claim that the ‘fine-tuning’ of natural law such that humans can exists is evidence of the existence of a higher power that tuned them for the sake of humans.

    The anthropic principle may not explain why the cosmological constant has the value it does, but it does explain why we can determine that it has that value.

  6. #6 Xanthippas
    July 17, 2009

    Thanks for this post! I’ve always been fairly suspicious of bold claims made in the name of the anthropic principle, and I’m glad to see someone more scientifically articulate than I who feels the same.

  7. #7 Dustin
    July 17, 2009

    An excellent and necessary post, thank you. On a related note, I think it’s also worth pointing out that there isn’t a great mystery over where the physical constants in nature come from — they’re what they are out of a combination of experimental results and logical necessity. It’s worth reminding everyone that the whole of physics is a model, and the question asked is “If this model is to describe this system/process/etc., what must the numerical values of these constants be?”

    No one, for example, mistakes the k in P(t)=P_0(exp(kt)) to be metaphysically real since that kind of population equation is transparently a model and breaks down very quickly. But we make that mistake with equations in physics very easily because they’re so accurate and so predictive and so general that we don’t have frequent departures between the theory and the observations to remind us that they are mere models. It’s in that sense that physics is a victim of its own success — it’s very easy to reify the equations because they work so well, even though we determine the values of the physical constants appearing in those equations in exactly the same way and for the same reasons we determine k in P(t) above.

  8. #8 Luboš Motl
    July 18, 2009

    Hi, I actually agree with some points of yours about the anthropic principle, and my readers know well which of them there are.

    But if you want hate mail, here it is. The goal of science is not to be useful, especially not useful for arrogant blogging crackpots. The goal of science is to find the right answers to scientific questions. Given the existing evidence, it is simply a fact that the laws of physics that allow our Universe to exist also allow googols of similar Universes to exist, which differ in some low-energy details. The existence of the landscape is a well-established fact, much like the existence of mirror images of your DNA molecules, even if they’re not explicitly find observationally. It doesn’t matter whether you call this fact a fact or a prediction. It doesn’t matter whether you like it or whether it is philosophically pleasing to you.

    What matters in science is whether it is true, and you have absolutely nothing to say about these things because you’re not using any scientific argument. There is no “beef” in your philosophy.

  9. #9 ABM
    July 18, 2009

    “I don’t think that’s the point. The strong anthropic principle isn’t a scientific principle in itself, it’s a refutation of unscientific mysticism, specifically of the claim that the ‘fine-tuning’ of natural law such that humans can exists is evidence of the existence of a higher power that tuned them for the sake of humans.”

    As they say on the Internets, “THIS”. It’s a metaphysical (?) argument for why everything seems so “right” for us, in the absence of any other evidence for a supreme being who set it up that way. It also ties into things like the supposed “directedness” of evolution etc.

  10. #10 söve
    July 18, 2009

    But right now söve panspermia is pure conjecture. Once we have a chance to explore the solar system söve (and particularly other potential söve places where life might exist like Europa) söve we may have a sample size larger than ‘one’ söve to work from.

  11. #11 island
    July 18, 2009

    The anthropic principle has, exactly once in the course of human history, been used to make a scientific prediction:

    I guess that you think that astrobiologists aren’t using the anthropic principle when they use the Goldilocks Enigma to predict exactly where life will and will not be found in the observed universe?

    http://evolutionarydesign.blogspot.com/2007/02/goldilocks-enigma-again.html

    Like, life, (past or present), will not be found on Mars nor Venus, but it will be found in other galaxy systems along the layer of spacetime that makes-up the habitable zone. Venus suffers from the runaway greenhouse effect, whereas Mars represents the cold stagnate proof of what will happen if extremist environmentalists get things all their way too, so heed the lesson of this anthropic coincidence.

    http://www.astronomynotes.com/solarsys/s9.htm

    Hey, no surprise… when you think that the freaking principle is about 10^500 fairy universes why should you have a clue what it’s actually about?

    Let’s see what else the theoretically righteous cutting edge doesn’t think…

    Oh, and this is for LubeMo the fanatic:

    http://thephysicistsblog.blogspot.com/

  12. #12 island
    July 18, 2009

    The anthropic principle has, exactly once in the course of human history, been used to make a scientific prediction:

    I guess that you think that astrobiologists aren’t using the anthropic principle when they use the Goldilocks Enigma to predict exactly where life will and will not be found in the observed universe?

    http://evolutionarydesign.blogspot.com/2007/02/goldilocks-enigma-again.html

    Like, life, (past or present), will not be found on Mars nor Venus, but it will be found in other galaxy systems along the layer of spacetime that makes-up the habitable zone. Venus suffers from the runaway greenhouse effect, whereas Mars represents the cold stagnate proof of what will happen if extremist environmentalists get things all their way too, so heed the lesson of this anthropic coincidence.

    Hey, no surprise… when you think that the freaking principle is about 10^500 unobservable fairy universes, why should you have a clue what it’s actually about?

    Let’s see what else the theoretically righteous cutting edge doesn’t think…

  13. #13 island
    July 18, 2009

    But there’s no reason, even with the anthropic principle, for dark energy or the cosmological constant to have the value that it does. It could be ten, or a hundred, or even a thousand times larger than its observed value, and the Universe would still be here, and similar to the way it is now.

    Say WHAT!?!

    Oh… you think that the universe is infinite… no wonder… lol

  14. #14 island
    July 18, 2009

    New England Bob writes:
    Yes, that is exactly how I see it, Ethan.

    I call this the weak Anthropic principle which I accept:

    hahaha… this dude thinks that the weak anthropic principle is valid without a multiverse, or without providing a cosmological structure principle that explains why we are just a consequence of otherwise highly pointed physics.

    The WAP isn’t observed, dude… LMAO!!!

    Oh, I could go on and on… ;)

  15. #15 island
    July 18, 2009

    Lubemo rants:
    The existence of the landscape is a well-established fact…

    WHOA!… my crackpot detector just blew a head gasket, and Lumo… this buds for you, buddy:

    http://thephysicistsblog.blogspot.com/

  16. #16 Ethan Siegel
    July 18, 2009

    Sean,

    Although you may think it’s completely self-evident, this line of reasoning wouldn’t have a name otherwise. NEBob is correct that many authors call the anthropic principle as I state it the “weak” anthropic principle, and that the strong one is, at best, extremely suspect.

    Dustin,

    I disagree with you; I think there is an incredible mystery over why the constants have the values that they do. For an analogy, we might ask why the mass of the proton and the neutron have the values that they do. Well, it turns out that people who work in the field of “Lattice QCD” can calculate what the mass ought to be based on the laws of the strong nuclear force. They not only get the mass of the proton and neutron, but of the other hadrons we’ve created and observed. We could assume that there are no dynamics governing our physical constants, but just because we don’t know or understand them doesn’t mean that there isn’t a compelling underlying reason that they have the value they do. That’s one of the open questions in science.

    Lubos,

    Thank you for your balanced, insightful comments. I hope you enjoy all of your pursuits of your version of science, as much as I enjoy mine. I will remind you, however, that mathematical existence does not imply physical existence, in the same way that correlation does not imply causation.

    Island,

    You are correct. I don’t think astrobiologists are using the anthropic principle when they do any of their work. As for the value of the CC, as long as it’s small enough that it allows stars and galaxies to form before it comes to dominate the expansion of the Universe, our local experience would not be much different at all. The only major observable differences would occur on extragalactic scales.

  17. #17 Dustin
    July 18, 2009

    I disagree with you; I think there is an incredible mystery over why the constants have the values that they do. For an analogy, we might ask why the mass of the proton and the neutron have the values that they do. Well, it turns out that people who work in the field of “Lattice QCD” can calculate what the mass ought to be based on the laws of the strong nuclear force. They not only get the mass of the proton and neutron, but of the other hadrons we’ve created and observed. We could assume that there are no dynamics governing our physical constants, but just because we don’t know or understand them doesn’t mean that there isn’t a compelling underlying reason that they have the value they do. That’s one of the open questions in science.

    I don’t mean to say that there aren’t perhaps more fundamental theories which lead to those physical constants because they are more general or capture dynamics more fundamentally. I may be making too much of it, but I’m regarding “Why is the cosmological constant what it is?” and “Why is the universe expanding in the way that it is?” as somewhat different questions. While physicists may mean those questions to be synonymous, the slight difference in language seems to burden the former question with metaphysical conjecture and unwarranted reification of mathematical models, while the latter is more transparently a question which deserves a proper scientific answer if it can be given.

    I don’t mean, in other words, to use instrumentalism as a kind of stop sign for further inquiry, just as a stop sign for metaphysical conjecture when a predictive theory, not a force-fit just-so story, is needed.

  18. #18 Dustin
    July 18, 2009

    Lubos — it does matter whether we call something a fact or a prediction. For any observed phenomenon, the number of possible theoretical descriptions is limited only by human imagination — and most of them will be wrong. The only way to know that we have captured some element of fact is for the theory to make a novel prediction about something observable. It is entirely insufficient to say that a theory seems to describe the data you already have, and then stop there. The goal of science is to get the right answers to scientific questions, not to construct interpolations and just-so stories and stop there.

  19. #19 Dustin
    July 18, 2009

    I’d also add that I think it’s probably a little generous to ascribe Hoyle’s work to the Anthropic Principle (even if that was exactly his starting point). That we wouldn’t be here if things didn’t work that way is really an implication of the what the abundance of carbon would be in any other case. It’s a bias of perception that causes us to invoke the Anthropic Principle — why don’t we invoke the equally good “There Wouldn’t Be As Much Carbon In The Universe Principle”?

    What Hoyle did was not in principle any different from determining the gravitational constant from a Cavendish apparatus (I don’t mean to suggest here that Hoyle’s work is anything less than impressive, of course). But the reasoning is “Assuming Newton’s Law of Gravitation, and what I see here in the deflection of these masses, the Gravitational constant must be…” and that’s not substantially different (in structure, anyway — again I’m not trying to trivialize Hoyle’s result) from saying “Assuming these results from nuclear physics, etc., and the abundance of carbon we see here, Carbon-12 must have such and such excited state…” But we don’t invoke anything like a “My Cavendish Apparatus Moved Principle”.

  20. #20 Leadhyena
    July 18, 2009

    There is a parallel for this in mathematics, although I don’t know if there’s a name for it. An example of it is where a geometry student is puzzling out a diagram of two concentric circles. There’s a point constructed on the inner circle, a tangent to the inner circle constructed from that point, a segment of which is a chord to the outer circle, and a measurement of that chord which is labeled x. The problem is to figure out the area of the torus from the value of x alone. The student reasons as follows, “I know there’s an answer to this, since it’s given as a problem. Therefore it doesn’t matter what the radius of the inner circle is, so I’ll make it 0. This degenerates the chord into the diameter of the outer circle, and the area is Pi*(x/2)^2.”

    The problem with the argument is that mathematically it’s not a valid argument because the problems’s existence in the greater context cannot be derived from the axioms of geometry. If there actually weren’t a formula for the area given only x, you could start the argument along the lines of “Assume for the sake of contradiction that there existed a formula for the area of this torus, and call it f(x)…”, and follow the rules of a classic proof by contradiction [(~p->F)=>p]. There is no inverse to the proof by contradiction.

    That being said, sometimes this “inverse proof by contradiction” tactic gives a really solid guess that leads to a proof (which is how most of differential calculus is established). The anthropic principle holds the same place for me. It’s like saying “I know there’s a valid model for this phenomenon because it’s observed in the universe, which exists for me…” Just like the math example, it only provides a guess, not a proof. But if this tactic provides a powerful set of guesses, regardless of whether or not any line of reasoning coming directly from it can be used in a final proof, it still deserves to be in the toolbox.

  21. #21 Thomas
    July 19, 2009

    I have to agree with Leadhyena. The antthropic principle is sometimes a useful tool in the toolbox a scientist need to come up with new ideas.

    Take for example the Earth. It is unusual in at least two respects: it contains abundant life and it has an extremely large moon. This makes you wonder if there may be some connection. Is a big moon necessary for life? Perhaps life originated in tidal pools repeatedly filled with water that evaporated and left a concentrated soup of chemicals? Perhaps the violent collision that created the moon has affected the distribution of elements or helped plate tectonics? This isn’t proof but a starting point for further research.

  22. #22 Mu
    July 20, 2009

    Sounds a bit like Einstein’s “God doesn’t play dice” argument taken to the grand scale.

  23. #23 Lotharloo
    July 20, 2009

    A very interesting post but I don’t get it. I can see how you can call anthropic principle unscientific but I don’t get how you can dismiss it.

    Here is how I see the anthropic principle: our location/time is not determined uniformly randomly. I completely agree that anthropic principle is sort of vague but I think it brings a good point. For example, in our vast universe, assign for each solar system the probability of “intelligent, curious, eventually physics loving life” evolving. Clearly it wont’ be a uniform distribution and if we happen to know everything about this distribution, it might eventually even explain something.

    So, my question is not whether this is a scientific argument (I know it is not). I simply want to know, do you believe there is a point in this principle or is it completely unrelated and must be ignored?

  24. #24 TheBear
    July 21, 2009

    There is more matter than antimatter in the Universe

    Slap me and call me Mr. Silly if I’m wrong – but is this something we know? Is this something we can know. For all we know the antimatter is somewhere else.

    From the (weak) antrophic principle we can tell that matter is a fact and that life is a fact. And then we can begin to speculate on the reason matter (and antimatter) is. In my view it is highly unlikely that the observable universe is the universe – but untill we know for sure everything becomes speculation.

  25. #25 stevefnp1
    April 6, 2011

    Science of course will most times not admit the possibility of something until it is proven and that’s where religion creationism comes in. No surprise there but I want to present a situation to you. When i email you this response you don’t actually hear or see me but you read my email in good faith and if you respond you respond in what I see are words the same as mine were on your screen. My point is that maybe there is both physical and a creationist aspects to our universe that work together and perhaps interact with us in certain ways physically and psychically for example by phenomenon described throughout history that we can take in faith if we choose like I am choosing to have faith that an email was sent by a person when I see an email. Its only speculation and perhaps a bad analogy but its possible that if such a force, entity, or whatever is outside of the physical laws that we know, it might only be able to communicate with us through certain experiences that are impossible to prove with the physical laws we now know. Perhaps studying people who claim to have had such experiences might help us learn about new ways to understand phenomenon but of course there are many problems with that too. I believe its a strong possibility there is some sort of anthropic principle or a version thereof that suggests designing especially if one considers how incredibly lucky we seem to be in such a privileged position on this planet given the rest of the solar system and all its lifelessness as we know it.