Toward a Unified Epistemology of the Natural Sciences

By Sean Carroll | November 10, 2006 9:29 am

Donald Rumsfeld Dr. Free-Ride reminds us of the celebrated free-verse philosophizing of Donald Rumsfeld, from a 2002 Department of Defense news briefing.

As we know,
There are known knowns.
There are things we know we know.

We also know
There are known unknowns.
That is to say
We know there are some things
We do not know.

But there are also unknown unknowns,
The ones we don’t know
We don’t know.

We tease our erstwhile Defense Secretary, but beneath the whimsical parallelisms, the quote actually makes perfect sense. In fact, I’ll be using it in my talk later today on the nature of science. One of the distinguishing features of science, I will argue, is that we pretty much know which knowns are known. That is to say, it’s obviously true that there are plenty of questions to which science does not know the answer, as well as some to which it does. But the nice thing is that we have a pretty good idea of where the boundary is. Where people often go wrong — and I’ll use examples of astrology, Intelligent Design, perpetual-motion machines, and What the Bleep Do We Know? — is in attempting to squeeze remarkable and wholly implausible wonders into the tightly-delimited regimes where science doesn’t yet have it all figured out, or hasn’t done some explicit experiment. (For example, it may be true that we haven’t taken apart and understood your specific perpetual-motion device, but it pretty obviously violates not only conservation of energy, but also Maxwell’s equations and Newton’s laws of motion. We don’t need to spend time worrying about your particular gizmo; we already know it can’t work.)

Rumsfeld’s comprehensive classification system did, admittedly, leave out the crucial category of unknown knowns — the things you think you know, that aren’t true. Those had something to do with his ultimate downfall.

CATEGORIZED UNDER: Philosophy, Science, Words
  • Eclectic Floridian

    Sean, you are clearly knowledgeable about our universe and I am only one who “lurks” around physics sites.

    So, I have an honest question for you.

    The Zero Point Energy (ZPE)/Free Energy people theorize some things that seem reasonable. I’d like your reaction to the claims.

    ZPE people say that it has been proven there is energy still in existence in a vacuum at absolute zero, thus proving energy can be tapped from “the vacuum.” Is that true?

    ZPE people also say that, assuming the existence of “vacuum energy,” mechanisms that tap that energy do not violate the laws of thermodynamics. Instead, their energy is derived from outside the traditionally accepted energy available. Is that unreasonable for some reason?

    I’m not some wacko, just someone who wants to know.

  • Carlos

    Nice entry, However, language is not a commutation group and in your last line you got the “known unkowns” the wrong way around. Am I right?

  • onymous

    Your definition of unknown knowns doesn’t seem to respect the paralellism here. They should be things that you know, but that you don’t know that you know.

  • graviton383

    I’ve actually always liked this famous quote of our former Defense Secretary. It is actually the most intelligent thing that he ever said. See you Monday at SLAC, Sean!!

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  • Eclectic Floridian

    What am I, Helen Thomas at a Bush press conference?

    I admit I’m no physicist, but I think my question deserves a reasoned reply … or just tell me, “Go away kid, you’re bothering me.”

  • Joe

    Eclectic Floridian,

    While many systems have some minimum energy, even at 0 Kelvin, this energy can never be extracted, because there is no state of the system with a lower energy.

  • Antti Rasinen

    We don’t need to spend time worrying about your particular gizmo; we already know it can’t work.

    This is exactly what I have been thinking about computer science lately. Since it is such a new field, we do not yet know all the perpetuum mobiles that lurk just beneath the surface.

    A good theoretical background is important in every subject. It can guide you through difficult places and warn you against deceptive “shortcuts”.


  • Ken Muldrew

    I think Rummy was paraphrasing Lady Burton’s Arabian proverb:

    “Men are four: He who knows not and knows not he knows not, he is a fool–shun him; He who knows not and knows he knows not, he is simple–teach him; He who knows and knows not he knows, he is asleep–wake him; He who knows and knows he knows, hi is wise–follow him!”

  • Rob Knop

    Joe gets it right.

    In fact, we do have very good reason right now to believe that there *is* energy in the vacuum– what you might call “zero-point energy.” However, the only way that that interacts with anything is through gravity — it’s causing the expansion of the Universe to accelerate. Gravity depends only on total energy density. Indeed, gravity doesn’t really have a global conservation of energy law….

    To harness it for actual work on something small scaled (like, say, smaller than a cluster of galaxies), you would need to have transitions in the vacuum energy.

    Consider fusion. You can fuse Hydrogen up to Helium, because the mass of Helium is less than four times the mass of Hydrogen, and the extra energy comes out by E=mc^2. You can keep doing this all the way up to iron (56 nucleons). However, if you try to fuse iron to something heavier, it *costs* energy, you can’t get energy out.

    If you contemplate E=mc^2, you will realize that iron has a whole lot of energy available… much more so than was released by the fusion of hydrogen up (through intermediate stages) to iron. However, via fusion, there’s no way to get it out.

    The vacuum energy is analogous. Even though it’s there, we’d have to find a lower vacuum energy level which the vacuum could transition to in order to extract energy. Barring new physics, there isn’t any such thing available in our Universe. (And if there were, there’d have to be a good reason why the vacuum throughout most of observable Universe hadn’t *already* decayed to that lower energy– there would have to be some sort of serious “activation energy” (in the language of chemistry), similar to that which keeps stable elements heavier than iron from spontaneously decaying all the time.)


  • Rob Knop

    Addendum — lest anybody take something I said as a jumping-off point for wild and wrong speculations, gravity very much does have a local conservation of energy law. It basically says (colloquially) that the amount of energy that’s going into a region of space, minus the amount of energy going out, is equal to the time rate of change of the total amount of energy in that region of space.

    Thus, you only can see the effects of non-conservation-of-energy type things via gravity on the very largest scales — looking at the whole Universe.


  • Omri Ceren

    It’s a minor quibble, but I agree that your interpretation of unknown knowns is a little off. To complete the square, an unknown know would be those things that we know, but that we don’t know that we know. Slovenian philosopher Slavoj Zizek wrote an article about Iraq in which he suggested that unknown knowns are precisely the psychoanalytic unconscious.

  • Eclectic Floridian

    Thank you for these comments, Rob.

    I think I have a (general) grasp of what you’re saying. It certainly gives me a clearer perspective on why physicists are so dismissive of the ZPE claims.

    That’s what I was asking for, and I appreciate the time and thought you put into your post.

  • spyder

    Rummy, in his cups, also missed that theoretical consideration of how we know the things we don’t know. It is our capacity to ask questions that clarify further research and inquiry, opening up expanding fields of possible knowns for which we create experiments or theoretical formulas and such. Indeed, one of the Rum’s most significant problems was that he never found a question he liked. We know a great deal more about what he may have assumed to be unknowns, but in fact they are known probabilities and possibilities. Intuitive epistemological constructs are also significant, but it seems some inside that DC beltway and five-sided building, made intuitive assumptions based solely on theological views, and other unrealistic beliefs.

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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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