Naturalness in the NYT

In the wake of the announcement of gravitational-wave signatures from inflation in the cosmic microwave background, I was invited to contribute a piece to The Stone section of the New York Times, on “naturalness” and how it’s used in physics. Mostly The Stone is devoted to philosophy, but occasionally they’ll let an outsider opine about a philosophical-sounding topic.

The hook is obviously the fact that inflation itself is motivated by naturalness:

Cosmic inflation is an extraordinary extrapolation. And it was motivated not by any direct contradiction between theory and experiment, but by the simple desire to have a more natural explanation for the conditions of the early universe. If these observations favoring inflation hold up — a big “if,” of course — it will represent an enormous triumph for reasoning based on the search for naturalness in physical explanations.

I conclude with:

Naturalness is a subtle criterion. In the case of inflationary cosmology, the drive to find a natural theory seems to have paid off handsomely, but perhaps other seemingly unnatural features of our world must simply be accepted. Ultimately it’s nature, not us, that decides what’s natural.

I like to capitalize “Nature,” but nobody agrees with me.

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30 Responses to Naturalness in the NYT

  1. John Gordon says:

    Your 2nd link is a repeat of your first link. Copy/paste error! You meant to link to your NYT post, but you linked back to your BICEP2 post.

  2. John Gordon says:

    Here’s Sean’s article btw if you’re looking for it (before his link gets fixed):

  3. John Grant says:

    I like to capitalize “Nature,” but nobody agrees with me.

    I agree with you! Just as happens to you, however, the copyeditors waspishly strike the capital down.

  4. Sean Carroll says:

    John Gordon– Thanks for the catch!

  5. Ben Goren says:

    Excellent essay, as expected.

    The way you described “the inflation” caused a somewhat significant shift in my own thinking about it, one that I think might make much more sense…but I’d appreciate a bit of confirmation if I’m following you properly.

    Would it be reasonable to suggest that cosmologists are currently thinking of “the inflation” as a new field / force / particle with a domain that only significantly applies at the insane energies of the Big Bang, and otherwise is vanishingly weak at other scales — as the nuclear forces are non-existent at human scales and gravity is (mostly) irrelevant at molecular scales?

    If so, would that imply that one could perhaps create a “the inflation” particle in a sufficiently big enough atom smasher? And in turn, what kinds of energies would that require and could that even be theoretically technologically possible?

    Thanks as always for helping me to better understand this unimaginably amazing Cosmos we’ve found ourselves in….



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  7. Sean Carroll says:

    Ben– Note it’s the “inflaton” (no second “i”) field that is responsible for “inflation,” which is a process.

    Basically your description is correct. But it’s (probably) not that the inflaton field is “weak,” just that it’s very heavy and hard to make. Much like the Higgs boson, but plausibly a trillion or more times heavier.

  8. Ben Goren says:


    Thanks for that clarification. I didn’t catch it reading the article originally…but, sure enough, it’s there. Perhaps you could ask the NYT to add a parenthetical note or a footnote or the like drawing attention to the distinction? I might not be the only one to miss it.

    I’m trying to warp my brain ’round a particle that massive. If the back of the envelope is right…that’s a single particle with as much energy as a spoonful of sugar, or a five-watt bulb burning for an entire hour. In a single particle! I’m gonna take a wild guess and suggest that I’m not going to live to see that accelerator built….

    Thanks again,


  9. Sean Carroll,

    I’m a contrarian, meaning that I’m not fond of current theories in modern physics. Although the Inflation hypothesis is not far from the original BB idea, the difference being the rate of the original expansion, it has had no observable basis in fact. Now such a claim is being made concerning primordial Inflation generated gravity waves, whereby evidence thereof supposedly can be seen in the polarization of microwaves.

    There are far too many assumptions being made behind this particular claim as well as the other BB hypothesis of Dark Matter and Dark Energy. It has been said by others that if you are paid to find something, sooner or later you will fool yourself into organizing a faction of reality as an explanation of what you have been looking for, hence gravity waves IMO.

    Nature as we see it on Earth is extremely interesting and can be broken down into simple explanations such as evolution and related Natural selection. On the other hand BB cosmology is not simple concerning Inflation, dark matter, dark energy, and other unobservable, and unNatural explanations IMO.

  10. Latverian Diplomat says:

    For the complexity of the ideas it aims to address, the word count of “The Stone” is a bit low. I think that worked against you here. Another example or two of how naturalness has influenced theory could have been helpful.

    Addressing the horizon and flatness problems through inflation seems to me an appeal to Occam’s Razor as much as anything else; it reduces the number of assumptions we have to make about the early universe. From your sole example it’s not clear to me how naturalness is different from this way of looking at it.

  11. Joan Hendricks says:

    I have to disagree with Latverian Diplomat. Sean does an excellent job of explaining scientific concepts to the general public without using complicated mathematical formulas. He is able to write things in a way that the average person can understand. And, as for writing a much longer article, again, this is for the general public and if it was too long and complicated, they would tune out.

    From now on, I will be capitalizing Nature when talking about how things work in this world (universe)!

  12. kashyap Vasavada says:

    I am not sure of your statement “And it was motivated not by any direct contradiction between theory and experiment” . If I understand correctly inflation model was proposed to explain many *experimentally* observed phenomena such as flatness, horizon problem, absence of monopoles etc. So in what sense it was guided by requirement of only naturalness?

  13. Brett says:

    I have a few questions which anyone can answer or mock as they please:

    Is there a viable model of inflation which supports a big crunch instead of a big bang as the current state of the universe? And why do we generally take a big bang or big bounce to be more valid than a big crunch? Is it for the sake of simplicity or is it because we would have to completely discard a relatively successful theory of inflation based on a 5% difference of interpretation stemming from the ‘assumption’ that it was a crunch instead of a bang?

    thank you.

  14. Otis Graf says:


    In the NYT article you wrote: “Elsewhere inflation continues, eventually producing other separate “universes,” eventually an infinite number.”

    Is it possible for a continuous physical process (chaotic inflation) to produce an infinite number of universes starting from a finite number? It would seem that finite physical entities cannot grow to become infinite. Is that possible in cosmology? It is difficult for me to conceive of an infinite multiverse without it being eternal. But then it would seem that an infinite eternal multiverse would be metaphysics, not something that science could establish.

    Thanks for your excellent explanations of science. I really appreciated your explanation of entropy and life on Earth at the Greer-Heard forum.

  15. Matt says:

    Sean, in your other popular work, you point out that the early universe had very low entropy, and explain why that fact is a fundamental problem in cosmology. Is inflation responsible for the low-entropy initial/early condition? In general, how are the two related?

    In the NYT article you say that there are many more ways to be lumpy than smooth, which makes sense. I have no intuition of one should define entropy when thinking about the universe before and after inflation — seems like there must be more to it than simply the matter distribution and multiplicity of states, because the amount of space is also increasing by many orders of magnitude. Maybe the Second Law doesn’t strictly hold in such strange circumstances? Some guidance on how to define entropy in the first second after the Big Bang would be much appreciated.

    I majored in physics as an undergraduate and loved it, but not everyone’s smart enough to be a professional theorist : ) You’re a lucky guy! Thank you very much for enabling me to continue enjoying physics and cosmology.

  16. Sameer says:

    If the energy of the inflaton field converted into ordinary particles and radiation, does this mean that during inflation there were only the inflaton field and the gravitational field? Are all the other fields of the standard model a result of the decay of the inflaton field? Also is there any connection between inflation and dark energy since both seem to work to accelerate the expansion of the universe?

  17. W. Curtis Jordan says:

    I believe with a few minutes at a chalk board, I could convince you that both time and gravity propogate at the speed of light as both are electromagnetic in nature. In the same discussion, I could likely convince you that the “Big-Bang” was the starting of time. where time propogated from the origin at the speed of light leaving mass and energy in balance just preceeding massive gravtational fields. Under this notion, the Universe is growing, with time just beginning at a remote boundary that we will never see. I just cannot prove it.

  18. Alan Graham says:

    We need to quantify Nature. We need to specify its distinction from perception, our interpritation. I agree with Nature as an overall umbrella for simplistic explanation, but Nature isn’t doing anything in this debate, like infinity it is just a word. There is no travelling faster than “c” and yet physicists have suddenly thrown a fundamental out the window to satisfy thier latest development. This is like trying to justify an unKnown with a Known – not correct science! We need to exhaust every avenue before jumping to conclusions. Lets hope we think a lot more about this before we come to any grand theories.

  19. Ben Goren says:

    I think the cosmologists deserve a wee bit more credit than you’re giving them.

    Nobody’s proposing faster-than-light travel; and, similarly, best I know, even Einstein himself never thought that the speed of light was a limit on the expansion of spacetime. For quite some time we’ve known that right now spacetime is expanding faster than light; inflation just has that expansion being more dramatic in the first infinitesimal fraction of a second. And inflation has been the best idea out there for a couple decades, already; it’s hardly a spur-of-the-moment flight of fancy.

    To top it all off, the BICEP2 team spent over a year poring over their results to make sure they weren’t being stupid. It may still turn out they they’ve missed something, of course, but the rumor mill is already pregnant with suggestions that everybody else is seeing the same thing. That’s not enough for formalized confidence, but it’s more than enough for informally justifying the hype.

    Modern physicists tend to be an extremely academically conservative lot, likely exactly because they spend most of their waking hours working with stuff that makes no intuitive sense in an effort to come up with explanations for other stuff that makes even less intuitive sense. They’ll spend almost as much time telling you how much or little confidence they have in a particular idea as they will telling you about the idea itself. It’s all about the error bars, and they not only have smaller error bars than anybody else but they’re much more obsessed about getting them in exactly the right places.

    So, yes, caution is warranted; it always is. And in any other profession we’d say they’re being excessively cautious. Physicists go out on limbs, yes, but only after performing insane safety inspections and over-engineering the safety support mechanisms. And that’s not a criticism — indeed, their almost incomprehensible success is due entirely to their unflinching commitment to not fooling anybody, especially themselves.



  20. Dave Hooke says:

    W. Curtis Jordan,

    I just cannot prove it.

    What is the chalkboard for then? Are you going to hit us over the head with it?

  21. Bill Bunting says:

    Wow, billions of Universes. Each with their own God, this would have to mean Creation by Committee, and would have to be compelling proof against Cosmological Inflation. Or else it is proof for Multiverse Inflation and the explanation for why the physical makeup of our universe is so damned complicated.

  22. Daniel Shawen says:

    Polarization of the cosmic background radiation (both mirror symmetric and not) appears to be “natural” alright. How could it not be? Even if the instruments to detect the different polarization states is a bit tricky to operate (possibly the reason it wasn’t noticed before!).

    I always liked the way Nikola Tesla seemed to believe that AC power was the way to go because it was a “natural” consequence of rotating a wire loop in a magnetic field. Anyway, commutators were pretty unnatural (and unforgiving) things that eventually wore out their contacts and so needed regular replacement. Not a very good attribute for a successful utility to replace gas lights, to say the least.

  23. John D says:

    A nice article Sean. Personally I’m very much against the multiverse. I think it’s turtles all the way down, unscientific, and bad for cosmology. Working back from that, I now feel rather unconvinced about inflation. You know how you said this: Faced with theories that fit all the data but seem unnatural, one can certainly shrug and say, “Maybe that’s just the way it is”. I think it’s the way it is for a reason, and the nature of black holes is key. If you read Kevin Brown’s formation and growth of black holes you can see him referring to the original frozen-star interpretation. I think that’s the one that’s right. There are of course parallels drawn between black holes and the early universe, see this. Imagine a “frozen star” early universe. You just don’t need inflation any more.