研究：電子閱讀器會隨下載量的增加而加重？

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。據英國《每日郵報》10月30日報道，你可能會理所當然地認爲，電子書的好處之一就是，不像精裝書那麼厚重，電子版的暢銷書幾乎是完全沒有重量的。

然而，這並不一定完全正確，根據電腦科學家所說——下載滿容的Kindle閱讀器比沒有下載任何東西的Kindle重。

不過即使是感覺最敏銳的讀者也不會爲這一負擔感到承受過多，因爲即使是相當於整個圖書館容量的電子書，重量也不會超過一個DNA單分子。

根據加州大學伯克利分校的教授約翰·庫比亞托維茲所說，下載滿容的電子書閱覽器與空的閱覽器相比，差距僅有1微微微克，也就是10-18 克(十的負十八次方克)。

他說，這個重量太過渺小，以致現有的任何天平都無法進行“有效測量”。

這種重量上的差異要遠遠小於由充電或是擦拭屏幕灰塵引起的差異。

而且電子設備在日照和溫暖的天氣影響下往往會變得重一些，這種變化也遠遠超過了下載更多的數據所造成的影響。

相較之下，3500本平裝書——相當於Kindle閱讀器的下載容量——卻要重達大約兩噸。

庫比亞托維茲教授的研究結果是基於這樣的事實：當你下載一本電子書時，電子閱讀器中的電子數量並未發生改變，因此那些承載數據的電子就會產生更高水平的能量。(電子設備通過捕獲電子並將其轉化爲0和1信息進行數據處理，你的Kindle上裝的電子書越多，數據處理量越大，電子運動也越頻繁)

能量與質量之間的關係——著名的愛因斯坦質能守恆定律E=mc2 ——意味着更高的能量就會產生更大的質量。

庫比亞托維茲教授在《紐約時報》的一篇文章中寫到，一個下載滿容的Kindle閱讀器的全部超額重量，是1微微微克——或者說0.000000000000000001克。