Thought Experiments


Scientific Approaches to the Fine-Tuning Problem

Embedded within the laws of physics are roughly 30 numbers—including the masses of the elementary particles and the strengths of the fundamental forces—that must be specified to describe the universe as we know it. Why do these numbers take the values that they do? We have not been able to derive them from any other laws of physics. Yet, it’s plausible that changing just a few of these parameters would have resulted in a starkly different universe: one without stars or galaxies and even without a diversity of stable atoms to combine into the fantastically complex molecules that compose our bodies and our world. Put another way, if these fundamental parameters had been different from the time of the Big Bang onward, our universe would be a far less complex universe. This is called the “fine tuning observation.” The fine-tuning problem is to find out why this is.

As someone whose thought is both fueled and constrained by the scientific tradition, I am only interested in explanations that are scientific. This means a candidate explanation must be three things: confirmable, falsifiable, and unique. By “confirmable,” I mean that the hypothesis must lead to further consequences, otherwise unexpected or surprising, which could be confirmed by novel but possible experiments. “Falsifiable” means that it is possible to specify a novel but doable experiment that would invalidate the hypothesis if the experimental result contradicted the predictions of the hypothesis. “Unique” means that there are no other simpler or more plausible hypotheses that make the same prediction.

Any explanation that fails these tests should be abandoned. After all, it is possible to imagine a multitude of possible non-scientific explanations for almost any observation. Unless we accept the stricture that hypotheses must be confirmable, falsifiable, and unique, no rational debate is possible; the proponents of the various explanations will never change their minds.

Yet several of the most popular explanations for the fine-tuning problem fail these tests. One such hypothesis is that there is a god who made the world and chose the values of the parameters so that intelligent life would arise. This is widely believed, but it fails the test for a scientific explanation.

Another hypothesis that fails the test is what we may call the “anthropic multiverse.” Though there are many variations on this theme, the essential idea is that our universe is one of a large or infinite set of worlds that exist simultaneously, each with different, random values for those 30-some physical parameters. Hence, our universe has the very rare property of having parameters that give rise to sufficient complexity to make it hospitable to intelligent life. To connect this hypothesis with observations we have to limit ourselves to the study of the subpopulation of universes in which we could live. This is called using the anthropic principle.

The anthropic multiverse cannot make any falsifiable predictions, though. Here is one proof: We can divide all the parameters that define each universe into two classes. First, there are those that matter to the existence of life—change one of those, and your universe is no longer hospitable to life. But since we already know, or could deduce from our existence, the values of these parameters, they can’t be used to falsify predictions of the anthropic multiverse. Second, there are parameters that don’t matter to the development of intelligent life. Those parameters can take any value and still yield up a universe teeming with life. These parameters are distributed randomly, so they might take any value in our universe. Because any and every value is allowed, this second set of parameters can’t be used to falsify predictions of the anthropic multiverse either.

Even if an anthropic multiverse is fundamentally unscientific, though, that does not mean we need to throw out all multiverse theories. One way to make a multiverse theory scientific is to suggest that complex universes like ours must be typical in the population of universes. Now we can make predictions without invoking the anthropic principle. For instance, in my first book, “Life of the Cosmos,” I proposed the “theory of cosmological natural selection,” which predicts that the parameters of physics are fine-tuned to produce many black holes, which is the case in our universe, as we see by its great chemical and astrophysical complexity. It turns out that a universe that makes many stars, and hence many black holes, is also filled with the oxygen and carbon needed for life. This theory made a few falsifiable predictions that have so far held up, despite several opportunities to contradict them with real observations in the last two decades. One of these predicts that no neutron star can have more than twice the mass of the sun. These predictions involve properties that are otherwise very surprising; were they to be confirmed there would be no other explanation on the table.

There is one property of the inflationary multiverse which just plausibly could be confirmed—were certain parameters very delicately tuned—which is the observation of patterns in the cosmic microwave background that could be explained by other bubble universes having collided with ours. But if this is not observed it doesn’t falsify the hypothesis. It just means those parameters are not so finely tuned.

Readers of popular science may have encountered claims that some predictions of the anthropic principle have been confirmed. However, I argue that those claims are based on at least three different kinds of fallacies. First, there is the assumption that the properties we observe around us—for example, the fact that carbon and galaxies are plentiful in the universe—are essential for life. However, we can seek an explanation for why galaxies are plentiful without needing to assert that galaxies are helpful for life. We can see by observation that galaxies are plentiful without having to be in one. Second, many of these anthropic arguments make untestable claims about properties of hypothetical universes that will remain forever unobserved. Finally, there is the “inverse gambler’s fallacy”: Observing a single trial with an improbable outcome and deducing that it must be one of a large number of trials.

Defenders of theistic explanations assert that it might be the case that there is a god who made the universe and tuned its parameters so that we could exist. It might. Similarly, defenders of anthropic multiverse scenarios assert that it might just be the case that our universe is one of a vast collection of worlds with random laws and parameters. This also might be true. But science is not about what might be true, it is about what can convincingly be argued for by rational argument from public evidence. If we weaken this standard to admit the anthropic multiverse, we open the door to equally unscientific theistic explanations. The proponents of each can (and do) argue with each other, but they will never convince each other, for they have given up the method and criteria that are necessary to make a convincing case for a claim in science. Meanwhile, the fine-tuning observation is a challenge that requires a scientific explanation.

Go Deeper
Editor’s picks for further reading

FQXi: Our Not-So-Special Universe
In this blog post, Zeeya Merali reviews recent papers questioning whether our universe is really as fine-tuned as we thought.

TED: Lee Smolin on Science and Democracy
In this video, blogger Lee Smolin explores the similarities between science and democracy.

Discover Magazine: Science’s Alternative to an Intelligent Creator: the Multiverse Theory
Tim Folger talks with proponents and critics of the anthropic multiverse.

Tell us what you think on Twitter, Facebook, or email.

Lee Smolin

    Lee was born in New York City in 1955 and raised there and in Cincinnati. In September of 2001 he moved to Canada to be a founding member of the Perimeter Institute for Theoretical Physics, where he has been ever since. Lee's main contributions to research are so far to the field of quantum gravity. He was, with Abhay Ashtekar and Carlo Rovelli, a founder of the approach known as loop quantum gravity, but he has contributed to other approaches including string theory and causal dynamical triangulations. He is also known for proposing the notion of the landscape of theories, based on his application of Darwinian methods to Cosmology. He has contributed also to the foundations of quantum mechanics, elementary particle physics and theoretical biology. He also has a strong interest in philosophy and his books "Life of the Cosmos," "Three Roads to Quantum Gravity" and "The Trouble with Physics," and "Time Reborn" are in part philosophical explorations of issues raised by contemporary physics. His latest book, "The Singular Universe and the Reality of Time," with Roberto Mangabeira Unger, was published by Cambridge University Press in November, 2014.

    • If we are willing to accept that the underlying characteristic of everything in the Universe is to self-organize and therefore allow the possibility of emergence, then that may offer an explanation that these 30 odd properties emerged.

      Here is one more arm chair theory to add to the clutter –

      BTW: Your book “Trouble with Physics” makes for provocative reading. At the very least it shoudl lead to a lot of introspection within the scientific community.

    • As a matter of’s true that the hypothesis of Parallel Universes are largely untestable..but don’t you think that labeling them as Unscientific is rather bit too harsh ??..

      I think you would agree to the fact Lee..that the untestability factor is more due to the limitations of our Scientific capabilities at this moment than anything else..but that does not mean to say that I am stating that the concept of Multiverse is absolutely true..but of course..they are not false either..

      And regarding the Fine-tuning Paradox !!!..I think that to solve it..the doctor would prescribe the hypothesis of Parallel Universes more than anything else..not Just here..I also claimed this in an article which Just yesterday..

    • Your narrow scope obviates faith and this is your loss. You are nothing without faith and with faith you can have everything. You can remove superstition and cherished beliefs from religion and science to get to the Truth, but to remove religion in favor of science makes you a blip in time. Thought achievements don’t exist apart from the original thinker and never encompass the entirety of his being. Each part reflects the whole, so you may think you own everything with each discovery but in the end you eat you shit and eventually you die. Have a little modesty while you pursue your game. Reality exists because it pleases God. Please continue sorting out the details as best you can. I am tired of scientists announcing the death of God with each small advancement in science.

      • Michael

        If science is so sacrilegious, why do you own a computer?

    • Anonymous

      The evidence for “fine tuning” is actually very strong. However, it in no way supports “Intelligent Design” or any other fictions arising from the superstitious myths of religions.

      Nor does it require the assumption of an infinity of universes each having different physical properties. A proposal for which there is not a skerrick of evidence and displays a gross lack of parsimony that is utterly contrary to the spirit of scientific enquiry.

      The physical parameters are but the tip of the iceberg. There is actually a much greater body of evidence to support fine tuning to be found in fields of science far better established than cosmology.

      After all, perhaps the earliest proponent of fine-tuning was the biochemist Lawrence Henderson. In “The Fitness of the Environment”, published in1913, he observed that “”the whole evolutionary process, both cosmic and organic, is one, and the biologist may now rightly regard the universe in its very essence as biocentric”

      Geology, biology and particularly chemistry provide many examples of “just right” prevailing conditions that enable and, indeed, make virtually inevitable, the strong directionality we observe in evolutionary processes.

      The most recent part of this evolutionary continuum is that most familiar to us and of which we have the best knowledge: The autonomous evolution of technology within the medium of the collective imagination of our species.

      But the commonly held assumption that IF fine tuning is a valid phenomenon THEN it favors theism is flawed.

      Because it predicated by the very common and entirely intuitive belief that it suggests a “designer”.

      But it can be very plausibly argued that, except in a very trivial sense, the concept of a “designer” is but an anthropocentric conceit for which there is no empirical basis.

      An objective examination of the history of science and technology bears this out.

      To quickly put this counter-intuitive view into focus, would you not agree that the following statement has a sound basis?

      We would have geometry without Euclid, calculus without Newton or Liebnitz, the camera without Johann Zahn, the cathode ray tube without JJ Thomson, relativity (and quantum mechanics) without Einstein, the digital computer without Turin, the Internet without Vinton Cerf.

      The list can. of course be extended indefinitely.

      This broad evolutionary model , extending well beyond the field of biology, is outlined, very informally, in “The Goldilocks Effect: What Has Serendipity Ever Done For Us?” which is a free download in e-book formats from the “Unusual Perspectives” website.

    • It is all about the Vibration aka Hertz! IMHO Kind Sir

    • Olias

      I do not believe that stating that deistic intervention is not a scientifically testable hypothesis is inherently anti-theistic. Most religions are predicated on the belief that limits exist on our ability to understand creation. And, actually, this position is based on sound logical principle…we are inherently unable to step outside the universe (or multi verse) to study it objectively. Even Einstein agreed with this…the universe is like a clock that we can never open to truly see what causes its behavior.

      But the irony here is that we will likely never know when we’ve reached the limit of human knowledge. So, instead of giving up on trying to understand the universe by stating that all else to know is in the ineffable deistic realm, we should continue to use our ‘God-given’ gifts of reason & curiosity to understand it and our place in it.

      When considering such fundamental aspects of the universe, it is difficult to predict what aspects may be testable in the future and which may not. In the end, it is possible that this fine-tuning is, in fact, the intent of a god that created us. It is also possible that we won the lottery of the multiverses by emerging in one …albeit likely not the only one…conducive to intelligent life. But we can’t suspend our scientific inquiry just because it is impossible to know the ultimate truth.

      Re: emergence…it would be interesting to read a compare & contrast of this principle with entropy. My first thought is that they would be opposing forces. They are both universal but emergence…defined as organizing principle…appears to be much more consistently observable in the universe, whereas it seems only useful to discuss entropy in terms of given limited system. Interesting to think about.

      • gnomish

        so, you can’t really know anything because you can’t know everything?
        and you know this how?

    • Jimmy53

      I’m neither a theist or necessarily believer in the anthropic principle but, having found these comments accidentally, I began to wonder if the it could make a little more sense if the anthropic principle were considered from another point of view; namely, if we exist simply because the universe is based on carbon based molecules and certain specific parameters that may not apply elsewhere

      In other words, if there is a multiverse, perhaps there are universes in which carbon is not the most versatile element and the local physics allow for other equally versatile elements that simply don’t exist here. Then other life could exist based on exotic elements that don’t exist or interact in the same way, here, because our physics don’t apply. A completely different mathematical and chemical existence. Thus, we exist, not because the universe is perfect for existence, but because existence is perfect for our local state.

      In that case, we exist because it was the only way to produce life in our local area, not because the universe is finely tuned to our existence. The same may apply for other universes in which our physics don’t exist at all.

      Maybe there is a universe composed entirely of a Argon, in which it’s the least dense element. Then all the other stuff would be unimaginably dense by our standards, but if that universe’s fundamental forces were much weaker it would believable (at least on paper). Life might be imaginable (Selenium based?) and they might be pondering the anthropic principle as well. In our universe, it’s ridiculous, but in their’s it might make perfect sense.

      Hell, we aren’t even sure about gravity, so maybe our universe leaks energy and their’s doesn’t

      I don’t really believe this, but, as a wacky thought experiment, I just got to thinking. At the very least, it might make a good SF story. Obviously, I have nothing better to do with my time over the Holidays.

    • Ujjal G.

      I a firm believer and proponent of “Intelligent Design”Without this our universe would be chaotic and life here or anywhere else in our universe would be unimaginable. Parallel Universe(s) is a hypothesis and its existence yet to be proved. Behind the Intelligent design is a “Megaforce” call it God or any other name.God is Supreme Physicist, Chemist, Biologist, Super Everything!!!

    • Manos

      Personally I don’t see what is the big mystery with the so called “fine tuning observation”. I do not really see any deeper meaning or further truth , scientific or otherwise, in asking the question :” Why do these numbers take the values that they do? “. Other than obtaining some sort of meta-understanding of our established scientific models of course -namely physics and math. We have developed models that describe our world and in the context of those models we end up somehow mapping parts of reality to specific values . I don’t see why this should be so mysterious and why should we even act surprised in realizing that perhaps had these values been different values then the world would have been a different world. Sure it would. But we started from this world so they can only be these specific values. Don’t know what I am missing in this question but it gives me a very remote sense of what must have felt for some people being members of the Pythagorean cult a couple of thousand years ago.

    • b.rand36

      We do seem to be incredibly lucky to live in a stable
      universe friendly to life. There is no
      reason why the forces of gravity, electromagnetism or the nuclear forces are
      the strengths they are. Why some
      subatomic particles are hundreds of times heavier than others. Stars or even atoms would have never formed if
      even one of these values where changed by one tenth of one percent. The cosmological constant cannot be varied
      even 0.000000000000000000001 percent without destroying the universe as we know
      it. Many people have argued this fine
      tuning of the laws and constants of our universe proves the existence of God.
      Surely our universe must have been designed by an intelligent mind!

      When a large star (one 20 to a million times the size of our
      sun) star stops producing energy, it can no longer maintain its volume. Gravity takes over and the star collapses
      under its immense weight. Gravity increases as the size decreases until all
      atoms and particles are crushed together into a singularity. This creates a black hole from which not even
      light can escape. Our universe will
      create countless trillions of black holes over its lifetime. Our universe will eventually die as all
      energy disperses evenly throughout the cosmos. The universe will then be completely dark,
      very cold and dead.

      Albert Einstein showed that gravity is actually the result
      of the warping of space/time by massive objects. The mathematics of a black hole describes the
      singularity at its center as being infinitely small and having infinite
      gravity. Infinite gravity would cause
      infinite warping of space/time at the point of the singularity. In reality, the fabric of space/time cannot stretch
      to infinity. Space/time will tear and
      the singularity must “drop out” of our universe into the nothingness “on the
      other side”. Without space/time to
      warp, gravity can no longer exist. The
      disappearance of gravity would then release the singularity to explode and
      inflate creating a new universe in a new Big Bang! Our Big Bang must have been the result of a
      singularity tearing through the space/time of a previous universe. Each black hole in our universe creates a
      singularity which drops out of our space/time and creates a new universe “on
      the other side”.

      This implies a very elegant theory of creation. Our universe will produce countless trillions
      of “offspring” during its life. It will
      die someday, but many of its children will live on. Each child will be a close copy of its
      parent. The physical properties of each child
      universe will be slightly different than its parent universe. These different physical properties will
      result in many unstable and “dead” universes which will not be able to produce
      any more offspring. Some of these random
      variations will result in the creation of more successful universes. Successful universes are defined as ones which
      have long lifespans and create more black holes which create more
      offspring. Universes which create more
      children will pass their positive traits down to more offspring universes. Universes with less positive changes will
      produce less or no offspring. This is
      the same process of natural selection which drives biological life to evolve here
      on earth. Our universe is so finely
      tuned because it is the result of countless generations of universes which
      existed before ours. Just as complex
      life on earth is the result of countless generations evolving through natural

    • DaveN

      The fine tuning argument is not a scientific claim, its a faith claim. It has its severe hurdles to overcome as a means of really explaining or positing what type of god (benevolent, sadistic, etc.) or what gods(could be more than 1 based on the argument.) may have created the universe. I do however take a dissagreement in your path to ascertaining the truth. Lee you say “As someone whose thought is both fueled and constrained by the
      scientific tradition, I am only interested in explanations that are
      scientific.” I don’t think this is the case at all. You want that to be true but if your wife or significant other said “I love you”, would you require a scientific explanation of their commitment? Would you test the hypothesis, etc?