You'd be forgiven for imagining that one of the benefits of e-books is that, unlike heavy hardbacks, a digital best-seller is entirely weightless。

　　However, this is not quite correct, according to a computer scientist - a Kindle full of downloads weighs more than an empty one。

　　But even the keenest readers are unlikely to suffer too much of a burden, as a full library of e-books weighs no more than a single molecule of DNA。

　　According to John Kubiatowicz, from the University of California, Berkeley, the difference between an empty e-reader and a full one is just one attogram, or 10-18 of a gram。

　　This weight is so tiny as to be "effectively unmeasurable" by any scales that currently exist, he says。

　　The difference in weight is far less than the difference produced by charging the battery, or wiping dust off the screen。

　　And electronic devices tend to grow heavier in sunlight and in warm weather, a change which massively outstrips the effect of downloading more data。

　　By comparison, 3,500 paperbacks - equivalent to the capacity of the Kindle - would weight around two tonnes。

　　Prof Kubiatowicz's findings are based on the fact that, while downloading an e-book does not change the number of electrons in an e-reader, those electrons holding data have a higher level of energy。

　　The relationship between energy and mass - famously summarised by Einstein as E=mc2 - means that those with a higher energy also have a higher mass。

　　The total extra mass of a full Kindle, Prof Kubiatowicz wrote in the New York Times, is an attogram - or 0.000000000000000001g。

　　据英国《每日邮报》10月30日报道，你可能会理所当然地认为，电子书的好处之一就是，不像精装书那么厚重，电子版的畅销书几乎是完全没有重量的。

　　然而，这并不一定完全正确，根据电脑科学家所说——下载满容的Kindle阅读器比没有下载任何东西的Kindle重。

　　不过即使是感觉最敏锐的读者也不会为这一负担感到承受过多，因为即使是相当于整个图书馆容量的电子书，重量也不会超过一个DNA单分子。

　　根据加州大学伯克利分校的教授约翰·库比亚托维兹所说，下载满容的电子书阅览器与空的阅览器相比，差距仅有1微微微克，也就是10-18 克(十的负十八次方克)。

　　他说，这个重量太过渺小，以致现有的任何天平都无法进行“有效测量”。

　　这种重量上的差异要远远小于由充电或是擦拭屏幕灰尘引起的差异。

　　而且电子设备在日照和温暖的天气影响下往往会变得重一些，这种变化也远远超过了下载更多的数据所造成的影响。

　　相较之下，3500本平装书——相当于Kindle阅读器的下载容量——却要重达大约两吨。

　　库比亚托维兹教授的研究结果是基于这样的事实：当你下载一本电子书时，电子阅读器中的电子数量并未发生改变，因此那些承载数据的电子就会产生更高水平的能量。(电子设备通过捕获电子并将其转化为0和1信息进行数据处理，你的Kindle上装的电子书越多，数据处理量越大，电子运动也越频繁)

　　能量与质量之间的关系——著名的爱因斯坦质能守恒定律E=mc2 ——意味着更高的能量就会产生更大的质量。

　　库比亚托维兹教授在《纽约时报》的一篇文章中写到，一个下载满容的Kindle阅读器的全部超额重量，是1微微微克——或者说0.000000000000000001克。