In raising the question “How Much Can We Know?” [The Biggest Questions in Science], Marcelo Gleiser focuses on human consciousness and the extent to which we can “make sense of the world.”

He misses the larger issue: our brains evolved to help us survive and reproduce, not to understand the cosmos. It may not be a question of whether the universe is stranger than we understand but whether it is stranger than we can understand.


Gleiser exposes the limits of knowledge in the physical sciences. Kurt Gödel settled this subject in mathematics with his incompleteness theorems in 1931. Because the sciences are rooted in mathematics, it is only natural to include his work in any such discussion of epistemology.

AVERY CARR Nesbit, Miss.

GLEISER REPLIES: Regarding Maletzky’s observation: It is indeed remarkable that brains that evolved to maximize our survival chances are able to write poetry, compose symphonies and prove theorems. Why this is so remains a mystery. It may well be that the universe is the puzzle we can’t solve. It’s hard to get out of the box when the box is everything that exists.

Gödel’s incompleteness theorems did expose the limitations of mathematics as a self-contained logical process. I agree with Carr that his work must be included in a longer piece, which I did in my book The Island of Knowledge. For this essay, space allowed me to focus only on the physical sciences. [Editors’ note: Read more about Gödel’s incompleteness theorems in “The Unsolvable Problem”.]


Erik Vance’s “Can You Supercharge Your Baby?” is a sensible article on the limitations of modern toys, videos and other paraphernalia in helping augment young children’s mental development. Yet there is another aspect of child play he overlooks: the substitution of social games with “passive” toys used mostly alone, typically via a television, computer or cell phone, without exercise.

Social games are vital for the mental and physical development of children. Perhaps most important, such games are based on rules that are accepted by all players, and they are fun only if everybody abides by those rules. Children who play with cell phones can cheat at will; they are the masters of their digital universe and thus become self-centered, without consideration for resolving social conflicts.

EDUARDO KAUSEL Massachusetts Institute of Technology


Christof Koch’s opening salvos against Daniel Dennett of Tufts University and like-minded philosophers in “What Is Consciousness?” [The Biggest Questions in Science] are misguided. Koch’s basic argument is: (1) Dennett, motivated by the belief that we live in a “meaningless universe of matter and the void,” denies that we have conscious experiences; (2) my toothache hurts; (3) ergo, Dennett is wrong.

Those who have read Dennett carefully should recognize the falsity of the initial premise. He understands fully the reality of pain. His goal is to encourage thinkers to exercise greater caution when theorizing about their own consciousness: given the human brain’s complexity, it is to be expected that some of our casual intuitions regarding its operation may be misguided.


KOCH REPLIES: Dennett argues in his 1991 book Consciousness Explained that people are terribly confused about consciousness. What they mean when they recount their experiences—for that is consciousness—is that they have certain beliefs about their mental states; each state has distinct functional properties with distinct behaviors. Once these outcomes are explained, there is nothing left to account for. Consciousness is all in the doing.

He and others who take his eliminative materialist view of conscious experiences deny the existence of anything above and beyond associated behavioral dispositions and function. I find this position bizarrely incongruous with my lived experience. How is my back pain a belief and not an excruciating subjective state? Having spent many a wonderful dinner with Dennett, one of the most eloquent and knowledgeable philosophers I have encountered, I know that outside business hours, he acts like he has experiences like everyone else.


“What Is Spacetime?” [The Biggest Questions in Science], George Musser’s article on quantum gravity, makes me wonder if there are differences we can observe between the cases of dark matter falling into a black hole and normal matter doing so.

WONTAEK YOO Pittsburgh

I have long wondered why the speed of light exists. What is it and why is it so fundamental to physics? Musser presents the idea that atoms of space might undergo “phase transitions” and that black holes could be places where space “melts.” It occurs to me that the speed of light could represent the melting point of spacetime.

ERIK EASON Oregon City, Ore.

MUSSER REPLIES: In answer to Yoo: Most physicists think that dark matter is a hitherto undetected but otherwise unexceptional type of particle, which would behave like ordinary matter, as far as black holes are concerned. Gravitation is a universal force that no matter is immune to. Although dark matter can fall into a black hole, it is less likely to do so because, if truly dark, it cannot lose energy by emitting light or dissipate momentum by friction and thus cannot readily spiral into a hole.

Regarding Eason’s question: If spacetime does emerge from deeper ingredients, as I speculate in my article, the speed of light can no longer be taken as a given and will have to be explained. The answer is not yet known. In some scenarios, the speed of light arises from the dynamics of the building blocks of spacetime. Like the rest of the structure of the spacetime we observe, the speed of light is a property of one of the phases that theorists hypothesize. It loses meaning in the others. Think of the speed of surface waves in liquid water: the waves cease to exist in the water’s solid and gaseous phases.


“A Painful Mystery,” by Jena Pincott, should had referred to nearly 11 hours a week as 27 percent of a 40-hour workweek rather than 7 percent.

“What Are the Limits of Manipulating Nature?” by Neil Savage [The Biggest Questions in Science], incorrectly said that David Hsieh of the California Institute of Technology creates photoinduced superconductivity in a material called a Mott insulator that becomes insulating at very cold temperatures. Andrea Cavalleri of the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany, and his colleagues found signs of photoinduced superconductivity in metals and insulators. Hsieh uses the same laser technique to induce unusual quantum effects in other materials.

Further, the article mistakenly referred to superconductors that must be cooled to within a few degrees of absolute zero as the only practical ones yet developed. While such superconductors have found more practical applications, those exhibiting superconductivity at much higher temperatures are widely used.