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| 阿秒（attosecond，10-18秒）科學家所能計量的最短時間落在此範圍。科學家利用先進的高速雷射，可製造出為時只有250阿秒的光脈衝。雖然這樣的時間長度似乎已經是想像不到的微小，但比起普朗克時間（Planck time，約10-43秒）來說，卻像永恆那麼長；據信普朗克時間可能是最短的間距。 飛秒（femtosecond，10-15秒）分子當中的原子通常在10~100飛秒內完成一次振動。就算快速的化學反應，也需要上百飛秒的時間才能完成。光線作用在視網膜的色素分子產生視覺的過程，約需200飛秒。 皮秒（picosecond，10-12秒，或稱微微秒）最快的電晶體是以皮秒為時間單位在運轉。由高能加速器所製造出的稀有的次原子粒子「底夸克」（bottom quark），只存在約1皮秒的時間就衰變了。室溫下，水分子之間形成的氫鍵，平均維持3皮秒的時間。 奈秒（nanosecond，10-9秒）穿越真空的光線，每奈秒進行的距離只有30公分。個人電腦裡微處理器執行一個指令，好比把兩個數字相加，要花2~4奈秒。另一種稀有的次原子粒子「K介子」（K meson），約存在12奈秒。 微秒（microsecond，10-6秒）在這個時間下，前文提到的光線就能跑上300公尺的距離；但在海平面的聲波，1微秒前進的距離只有1/3公釐。市面上的高速頻閃儀所發出的閃光時間，約在1微秒。一根炸藥從引信燒完到爆炸之間，約需24微秒。 毫秒（millisecond，10-3秒）是一般照相機最短的曝光時間；家蠅每3毫秒揮動一次翅膀，蜜蜂則是每5毫秒揮動一次。隨著月球運轉的軌道半徑逐漸變大，月球繞地球一圈的速度每年會慢上2毫秒。資訊科學裡，10毫秒的時間稱為一個「瞬間」（jiffy）。 十分之一秒（1/10 second）傳說中「一眨眼」的時間長度。人類的耳朵需要有這麼長的時間，才能分辨出原來的聲音與回聲。在這段時間內，目前正加速離開太陽系的太空船 「探險家一號」又遠離了太陽約兩公里的距離；蜂鳥擺動了翅膀七次；比中央C高4又1/2音階的A調音叉振動了四次。 秒 （second）健康人心跳一次的大約時間。這段時間內，美國人吃了350張披薩；地球繞太陽走了30公里，而太陽在銀河系裡的旅程中飛動了274公里； 但月光在1秒鐘內還到不了地球（需1.3秒）。按傳統定義，秒是一天的1/24的1/60的1/60，但科學上有更準確的定義：那是銫133原子所放出的 某種輻射，振盪9192631770次所花的時間。 分（minute）這段時間內，新生兒的大腦長了1~2毫克；鼩鼱的心臟跳了1000下；人平均可以說出150個字或閱讀250個字。光線從太陽抵達地球約需8分鐘；當火星最靠近地球時，從那顆紅色星球表面反射的太陽光，約需4分鐘可抵達地球。 時（hour）增生中的細胞一分為二，通常需要這麼長的時間。美國黃石國家公園的老忠實間歇泉約1小時16分鐘噴水一次。從太陽系最遠的行星冥王星發出的光線，需5小時20分鐘才能抵達地球。 日（day）對人類來說，這可能是最自然的時間單位，也就是地球自轉一圈的時間。目前該時間固定在23小時56分4.1秒，但由於月球的重力牽引以及其 他的影響，地球自轉的速度正在不斷減緩當中。人的心臟一天約跳10萬次，肺臟則吸入約1萬1000公升的空氣。在差不多相同的時間內，一頭新生藍鯨約增加 了90公斤的體重。 年（year）地球繞了太陽一圈，以及繞著地軸自轉了365.26圈。海平面平均上升了1~2.5公釐，北美 洲與歐洲的距離增加了三公分。光線從最靠近太陽系的比鄰星（Proxima Centauri）抵達地球，需4.3年；海洋表層的洋流也需要同樣的時間，才能繞地球一圈。 世紀（century）月球遠離了地球約3.8公尺；標準的雷射唱片及唯讀光碟經過這段時間，預計將損壞至不能使用。戰後嬰兒潮出生者，每26人中有一位可以活到100歲；不過大海龜可以活上177年。最先進的可錄式雷射唱片可能可以維持200年以上。 百萬年（million years）就算以光速前進的太空船，也還到不了前往仙女座星系（Andromeda galaxy，位於230萬光年的距離之外）的一半路程。比太陽還要亮100萬倍、質量最大的藍超巨星，也將在這段時間內燃燒殆盡。由於地球的板塊運動， 美國洛杉磯市將在100萬年內，從現在的位置朝北北西方向位移約40公里。 十億年（billion years）剛形成的地球約需這麼長的時間冷卻、形成海洋、產生第一個單細胞生物，並將大氣層裡豐富的二氧化碳轉變為氧。在此同時，太陽繞著銀河系中心繞 行了四圈。由於宇宙年齡在120~140億年左右，因此超過10億年的時間單位使用的機會不多。宇宙學家相信，宇宙可能會不斷擴張，直到最後一個恆星死去 （從現在算起100兆年）及最後一個黑洞消失（從現在算起100100年）後，也不會停止。我們的未來能夠延伸的時間，比我們過去所遺留的痕跡，要來得長遠許多。 |
ONE ATTOSECOND (a billionth of a billionth of a second) The most fleeting events that scientists can clock are measured in attoseconds. Researchers have created pulses of light lasting just 250 attoseconds using sophisticated high-speed lasers. Although the interval seems unimaginably brief, it is an aeon compared with the Planck time—about 10–43 second—which is believed to be the shortest possible duration. ONE FEMTOSECOND (a millionth of a billionth of a second) An atom in a molecule typically completes a single vibration in 10 to 100 femtoseconds. Even fast chemical reactions generally take hundreds of femtoseconds to complete. The interaction of light with pigments in the retina—the process that allows vision— takes about 200 femtoseconds. ONE PICOSECOND (a thousandth of a billionth of a second) The fastest transistors operate in picoseconds. The bottom quark, a rare subatomic particle created in high-energy accelerators, lasts for one picosecond before decaying. The average lifetime of a hydrogen bond between water molecules at room temperature is three picoseconds. ONE NANOSECOND (a billionth of a second) A beam of light shining through a vacuum will travel only 30 centimeters (not quite one foot) in this time. The microprocessor inside a personal computer will typically take two to four nanoseconds to execute a single instruction, such as adding two numbers. The K meson, another rare subatomic particle, has a lifetime of 12 nanoseconds. ONE MICROSECOND (a millionth of a second) That beam of light will now have traveled 300 meters, about the length of three football fields, but a sound wave at sea level will have propagated only one third of a millimeter. The flash of a high-speed commercial stroboscope lasts about one microsecond. It takes 24 microseconds for a stick of dynamite to explode after its fuse has burned down. ONE MILLISECOND (a thousandth of a second) The shortest exposure time in a typical camera. A housefly flaps its wings once every three milliseconds; a honeybee does the same once every five milliseconds. The moon travels around Earth two milliseconds more slowly each year as its orbit gradually widens. In computer science, an interval of 10 milliseconds is known as a jiffy. ONE TENTH OF A SECOND The duration of the fabled “blink of an eye.” The human ear needs this much time to discriminate an echo from the original sound. Voyager 1, a spacecraft speeding out of the solar system, travels about two kilometers farther away from the sun. A hummingbird can beat its wings seven times. A tuning fork pitched to A above middle C vibrates four times. ONE SECOND A healthy person's heartbeat lasts about this long. On average, Americans eat 350 slices of pizza during this time. Earth travels 30 kilometers around the sun, while the sun zips 274 kilometers on its trek through the galaxy. It is not quite enough time for moonlight to reach Earth (1.3 seconds). Traditionally, the second was the 60th part of the 60th part of the 24th part of a day, but science has given it a more precise definition: it is the duration of 9,192,631,770 cycles of one type of radiation produced by a cesium 133 atom. ONE MINUTE The brain of a newborn baby grows one to two milligrams in this time. A shrew's fluttering heart beats 1,000 times. The average person can speak about 150 words or read about 250 words. Light from the sun reaches Earth in about eight minutes; when Mars is closest to Earth, sunlight reflected off the Red Planet's surface reaches us in about four minutes. ONE HOUR Reproducing cells generally take about this long to divide into two. One hour and 16 minutes is the average time between eruptions of the Old Faithful geyser in Yellowstone National Park. Light from Pluto, the most distant planet in our solar system, reaches Earth in five hours and 20 minutes. ONE DAY For humans, this is perhaps the most natural unit of time, the duration of Earth's rotation. Currently clocked at 23 hours, 56 minutes and 4.1 seconds, our planet's rotation is constantly slowing because of gravitational drag from the moon and other influences. The human heart beats about 100,000 times in a day, while the lungs inhale about 11,000 liters of air. In the same amount of time, an infant blue whale adds another 200 pounds to its bulk. ONE YEAR Earth makes one circuit around the sun and spins on its axis 365.26 times. The mean level of the oceans rises between one and 2.5 millimeters, and North America moves about three centimeters away from Europe. It takes 4.3 years for light from Proxima Centauri, the closest star, to reach Earth—approximately the same amount of time that ocean surface currents take to circumnavigate the globe. ONE CENTURY The moon recedes from Earth by another 3.8 meters. Standard compact discs and CD-ROMs are expected to degrade in this time. Baby boomers have only a one-in-26 chance of living to the age of 100, but giant tortoises can live as long as 177 years. The most advanced recordable CDs may last more than 200 years. ONE MILLION YEARS A spaceship moving at the speed of light would not yet be at the halfway point on a journey to the Andromeda galaxy (2.3 million light-years away). The most massive stars, blue supergiants that are millions of times brighter than the sun, burn out in about this much time. Because of the movement of Earth's tectonic plates, Los Angeles will creep about 40 kilometers north-northwest of its present location in a million years. ONE BILLION YEARS It took approximately this long for the newly formed Earth to cool, develop oceans, give birth to single-celled life and exchange its carbon dioxide–rich early atmosphere for an oxygen-rich one. Meanwhile the sun orbited four times around the center of the galaxy. Because the universe is 12 billion to 14 billion years old, units of time beyond a billion years aren't used very often. But cosmologists believe that the universe will probably keep expanding indefinitely, until long after the last star dies (100 trillion years from now) and the last black hole evaporates (10100 years from now). Our future stretches ahead much farther than our past trails behind. |
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