NATURE の物理ページにある今週の注目記事の一つ、タイトルが "Deep Thought"。もちろん銀河ヒッチハイクガイドに出て来たコンピュータのこと。紹介されてるこの論文では地球だけじゃなくて宇宙全体を量子コンピュータと見たときに、ビッグバンから今までどの程度の計算が出来たか、ということを試算してる。"Submitted to Nature" となってるけど、出たのは PRL らしい。で最後のパラグラフを引用：
`universal' universal computer ってのが笑える。前者は「宇宙の」、後者は「万能」の意味。しかし、なんか「塵理論」を彷彿とさせる話。ある運動が「計算」かどうかは結局は主観的な話になる。（関係ないけど上のプレプリ、LaTeXが通らないと思ったら 素のTeX だった。いまどきめずらしい。）
Is the universe a computer? The answer to this question depends both on themeaning of `computer' and on the meaning of `is.' On the one hand,the universe is certainly not a digital computer running Linux or Windows. (Or at any rate, not yet.) On the other hand, theuniverse certainly does represent and process information in a systematicfashion: as noted above, every spin can be taken to represent a bit,and every spin flip corresponds to a bit flip. Almost any interactionbetween degrees of freedom suffices to perform universal quantumlogic on those degrees of freedom. Such universal interactions include the fundamental interactions of quantum electrodynamics and oftopological quantum field theories. These resultsstrongly suggest that the universe as a whole is capableof universal quantum computation. In such a `universal' universalcomputer, every degreeof freedom in the universe would register information, andthe dynamics of those degrees of freedom would process that information:the amount of information and the amount of information processing possiblecan be calculated using the formulae derived above. Even if the universe is not a conventional computer, it can still compute.
