《那些古怪又讓人憂心的問題》第34期:牛排墜落(2)

Lastly, I had to figure out exactly how quickly heat spreads through a steak. I started by looking at some papers from industrial food production that simulated heat flow through various pieces of meat. It took me a while to realize there was a much easier way to learn what combinations of time and temperature will effectively heat the various layers of a steak: Check a cookbook. Jeff Potter's excellent book Cooking for Geeks provides a great introduction to the science of cooking meat, and explains what ranges of heat produce what effects in steak and why. Cook's The Science of Good Cooking was also helpful. Putting it all together, I found that the steak will accelerate quickly until it reaches an altitude of about 30–50 kilometers, at which point the air gets thick enough to start slowing it back down. The falling steak's speed would steadily drop as the air gets thicker. No matter how fast it was going when it reached the lower layers of the atmosphere, it would quickly slow down to terminal velocity. No matter the starting height, it always takes six or seven minutes to drop from 25 kilometers to the ground. For much of those 25 kilometers, the air temperature is below freezing-which means the steak will spend six or seven minutes subjected to a relentless blast of subzero, hurricane-force winds. Even if it's cooked by the fall, you'll probably have to defrost it when it lands.

最後我還需要找出熱量在牛排中的傳導速度。我一開始查閱的是在工業食物生產中模擬不同種類肉的熱傳遞的論文。後來我才意識到，要知道不同溫度和時間搭配下，牛排會被加熱到哪種程度的最簡單的方法是：去找一本烹飪書來看。在《極客廚房》這本很棒的書中，作者傑夫•波特很好地介紹了肉類烹製的科學道理，並解釋了不同溫度範圍下熱量對牛排的影響及其原因。庫克的《做好菜的科學》一書也很有幫助。把上面所有信息綜合到一起，我發現牛排會迅速加速墜落直到到達30～50千米的高度，在這個高度，空氣足夠稠密到能讓牛排開始減速下墜。隨着空氣密度越來越大，牛排下落的速度也會逐漸下降。不管牛排在下落到大氣層下部時速度有多快，它最終都將減速到終端速度。無論一開始起落點多高，牛排從25千米的高度墜落至地面總是需要6～7分鐘時間。在這段距離中，空氣溫度大部分時間都低於冰點，這意味着牛排會在溫度低於0℃、風力媲美颶風的狂風中待上6～7分鐘。就算在下落過程中牛排已經熟了，掉到地面後你還是需要解凍才能食用。

When the steak does finally hit the ground, it will be traveling at terminal velocity-about 30 meters per second. To get an idea of what this means, imagine a steak flung at the ground by a major-league pitcher. If the steak is even partially frozen, it could easily shatter. However, if it lands in the water, mud, or leaves, it will probably be fine. A steak dropped from 39 kilometers will, unlike Felix, probably stay below the sound barrier. It also won't be appreciably heated. This makes sense-after all, Felix's suit wasn't scorched when he landed. Steaks can probably survive breaking the sound barrier. In addition to Felix, pilots have ejected at supersonic speeds and lived to tell about it break the sound barrier, you'll need to drop the steak from about 50 kilometers. But this still isn't enough to cook it. We need to go higher. If dropped from 70 kilometers, the steak will go fast enough to be briefly blasted by 350°F air. Unfortunately, this blast of thin, wispy air barely lasts a minute-and anyone with some basic kitchen experience can tell you that a steak placed in the oven at 350 for 60 seconds isn't going to be cooked. From 100 kilometers-the formally defined edge of space-the picture's not much better. The steak spends a minute and a half over Mach 2, and the outer surface will likely be singed, but the heat is too quickly replaced by the icy stratospheric blast for it to actually be cooked. At supersonic and hypersonic speeds, a shockwave forms around the steak that helps protect it from the faster and faster winds. The exact characteristics of this shock front-and thus the mechanical stress on the steak-depend on how an uncooked 8-ounce filet tumbles at hypersonic speeds. I searched the literature, but was unable to find any research on this.

當牛排最終掉落到地面時，它的速度將會是約每秒30米的終端速度。如果想要對此有一個直觀的認識，想象一下一塊牛排被棒球大聯盟的投手砸在地上的樣子。就算牛排只有一部分被凍住了，這樣也很容易被摔碎。不過如果牛排最後掉到了水裏、泥裏或者樹葉堆裏，那應該沒事兒。從39千米高度掉下來的牛排應該不會像菲利克斯那樣突破音障，它甚至不會被顯著加熱。這也說得通，畢竟在落地時菲利克斯的保護服也沒被烤焦嘛。即使突破音障，牛排應該也不會支離破碎。因爲除了菲利克斯，許多曾在超音速狀態下彈射的飛行員都活了下來，並講述了他們自己的故事。要想突破音障，你需要讓牛排從50千米的高度落下來，但這仍然不足以把它烤熟。我們需要到更高的高度。如果從70千米高度落下，牛排的速度會快到把空氣加熱到177℃左右。不過這股纖弱稀薄的高溫空氣只會持續不到1分鐘，任何具有基本廚房經驗的人都會告訴你把牛排放到350°F的烤箱裏烤60秒並不能把它烤熟。從100千米——太空邊緣的正式定義——高度落下來並不會有太大區別。牛排超過2馬赫速度的時間會有1分半鐘，牛排的外表面很有可能會燒焦，但熱量很快就會被冰冷的平流層狂風消散掉，因而內部並不會被烤熟。在超音速和高超音速條件下，牛排周圍會形成激波，而激波會把牛排和越來越快的狂風隔離開來。激波前沿的準確性質，以及由此產生的對牛排的機械應力，取決於這塊8盎司的生牛排如何在高超音速狂風中翻滾。我嘗試着去找這方面的文獻，但沒有找到任何與此有關的研究。