Every nanosecond really counts
Changing the clocks in this digital age can be done with the touch of a button. But to tell the time with the accuracy required by today's electronic technologies requires a remarkable mixture of cutting-edge physical sciences.
The super clocks, which keep the rest of our alarm clocks, wall clocks, speaking clocks and wrist watches ticking in time, combine lasers that spit out pulses a quadrillionth of a second long with chambers that chill atoms to a few millionth of a degree above absolute zero.
“Measuring time is the place where the highly abstract theories of relativity, gravity and quantum mechanics directly interact with such everyday objects as mobile phones and fax machines,” says John Laverty, one of the “time lords” at the National Physical Laboratory (NPL) in Teddington, south-west London. It is one of a small number of atomic clocks based at centres around the world, including the US, Germany and Japan.
At Teddington, as Britons this week adjusted to the clocks going back to mark the end of summer time, the accuracy of British time is both painstakingly maintained and cut into ever smaller slices. Some of the apparently bizarre predictions of Einstein's theory of relativity that, for instance, speed and gravity can slow time down are fulfilled here every day. “The atomic clocks we have developed are so accurate that if you had started one running 32m years ago,” says Mr Laverty, “it would only be about a second adrift today.” This sensitivity makes them vulnerable to relativity just placing one on a floor that is 10 metres higher than another puts the two out of synch. An atomic clock at the top of Mount Everest would pull ahead of those at sea level by about 30 microseconds a year.
Such subtleties, however, were not involved in a landmark court case this year when the NPL played a key part in the conviction of a driver for causing death by dangerous driving because he was using his mobile phone at the time of an accident. Testimony from NPL that the time signals from Vodafone, the defendant's phone provider, synchronised with those logged by a T-mobile 19 seconds later when the accident was reported, clinched the prosecution's case.
Atomic clocks were first developed in the 1950s when a second was defined as a fraction of the mean solar day, based on the earth's rotation. Because they proved so much more accurate than conventional clocks, the definition of the second was changed in 1967 when it became linked to the number of oscillations in the caesium atom. Atomic clocks work because atoms oscillate between energy states if they are hit with radiation, usually microwaves or light, of the right frequency. They are so reliable because these oscillations are very regular.
The constant drive to refine them will get a significant boost next year when several miniaturised atomic clocks will be launched into space the earth-bound one at Teddington is 2 metres tall seeking to benefit from zero gravity. At the moment, the best clocks are off by 86 picoseconds a day for comparison, during one picosecond, light travels the diameter of a full stop. As it is, the quest for accuracy means that, every few years, the NPL adds a “leap second” to Greenwich Mean Time, which is based on the length of a solar day, to keep it in synch with International Atomic Time based on the atom. If this adjustment was not made, fluctuations of a few milliseconds in the rotation of the earth would mean the two got out of step.
These other-worldly facts are all very interesting, but they beg the question: so what? Do we really need ever more accurate time machines? Surely even the most punctual of us could tolerate a drift of a few picoseconds. But the fact is that time does not just rule our lives, it rules the technology that makes the modern world work. The relaying of information via the internet and telephone networks, the signals from global positioning satellites (GPS) and the complex grids governing power supplies these rely on accurate synchronisation. As these networks become ever more overloaded, they risk breaking down unless there is improved synchronisation and this requires more accurate clocks.
“GPS could benefit from greater accuracy,” says Mr Laverty. “The system works by measuring how long radio signals from several satellites take to reach one point. The miniaturised clocks the satellites use are synchronised to within a few nanoseconds, allowing you to get within 10 metres. To be accurate to three centimetres, you would have to reduce your error to well under 100 picoseconds and that is some way off commercially.”
The practical spin-off that is likely to have the biggest effect on our lives is the production of small and highly accurate commercial atomic clocks. The most advanced of these was unveiled in August by the US defence department, which has developed one that fits on to a computer chip. It has workings the size of a grain of rice and needs less than 75 thousandths of a watt to run. Such a device could transform the effectiveness of networks used by power grids or GPS. In the case of GPS, this would make possible the precise targeting of underground pipes and telecommunications cables thereby reducing the cost and inconvenience of repairs and the creation of new services for people linked to GPS via their mobile phone.
Meanwhile, the scientists at Teddington are concentrating on the “optical ion clock”, a totally new system using an ultraviolet laser to cause a single electron one orbiting a nearly motionless atom of mercury or strontium to change orbits a million billion times a second. Research, like time itself, never stands still.
超级时钟 “纳”秒必争
在我们这个数码时代，碰个按钮就可以调整钟表的时间。但要以当今电子科技所要求的精确度来报时，则需出色地融汇各门尖端自然科学。
超级时钟使我们日常用到的闹钟、挂壁钟、报时钟和手表保持准确无误。超级时钟将脉冲长度为千万亿分之一秒的激光和可将原子冷却至绝对零度以上几百万分之一度的腔体结合在一起。
“在时间测量领域，相对论、重力和量子力学等极为深奥的理论与移动电话和传真机等日常物体直接发生关系，” 约翰?莱弗蒂（John Laverty）说，他是伦敦西南部特丁顿国家物理实验室（NPL）的 “时间之王”之一。该实验室拥有全世界为数不多的一台原子钟，其它原子钟则座落在美国、德国和日本等研究中心。
上周英国的时钟回拨，标志着夏令时结束。而在特丁顿，英国时间得到了一丝不苟的维护，其精确度能以越来越小的单位计算。爱因斯坦相对论中一些看似古怪的预言天天都在这里得到证实，例如，速度和重力能导致时间放慢。莱弗蒂先生说，“我们开发的原子钟非常精确，假如你在3200万年前开动一台原子钟，到今天只有大约1秒钟的偏差。”这么高的灵敏度使它们很容易受相对位置的影响，只要把一台原子钟放在地板上，而另一台放在比它高10米的地方，那么两台钟的走时就会不同步。如果把一台原子钟放在珠穆朗玛峰顶峰，那一年下来，它会比放在海平面高度上的原子钟快30微秒左右。
但原子钟的这些细微之处，还未用到今年一件具有里程碑意义的法院案例。在这一案件中，国家物理实验室对法庭最终定罪扮演了关键的角色，被告的控罪是危险驾驶导致死亡，因为事发时他正在使用手机。国家物理实验室的证供表面，来自被告手机供应商沃达丰（Vodafone）的时间信号，与19秒后事故报告时T-mobile记录的时间一致，从而让控方赢得了该案。
原子钟最初问世是在上世纪50年代，当时是以地球自转为基础，把一个平均太阳日的若干分之一定义为一秒。由于原子钟证明比传统钟表精确得多，因此秒的定义于1967年被修改，开始与铯原子的振荡次数联系在一起。原子钟的工作原理是，如果被适当频率的射线（通常是微波或光线）击中，那么原子就会发生振荡，从一种能量状态变为另一种能量状态。由于振荡很有规律，因此原子钟非常可靠。
不断提高原子钟精确度的努将在明年取得重大进展，届时几台微型原子钟将被发射入太空，以求从零重力中获益。特丁顿的那台落地原子钟有2米高。目前，最准确的钟每天都会偏差86微微秒，而一微微秒就是光线行走一个句号直径距离的时间。照目前的情形来看，对精确度的追求意味着，每隔几年，国家物理实验室就要为基于太阳日长度的格林威治标准时间增加一“闰秒”，使之与基于原子的国际原子时间保持同步。如果不作这样的调整，地球自转中几毫秒的波动就将使这两个时间步调不一致。
这些天方夜谭式的事实都很有趣，但它们都回复到最根本的问题：那又怎样呢？我们真的需要越来越精确的时间机器吗？就算我们之中最守时的人也可以容忍几微微秒的偏差吧？然而事实上，时间不仅支配着我们的生活，还支配了使现代世界运转的技术。经互联网和电话网络传播信息、全球定位系统（GPS）发送信号，以及控制电力供应的复杂电网，都有赖于精确的时间同步。随着这些网络变得越来越超负荷，除非改进时间同步，否则它们就有瘫痪的危险；而要改进时间同步，则需要更精确的时钟。
“GPS会因更高的精确度而受益，” 莱弗蒂先生说，“这个系统是靠测定几个卫星的无线电波信号到达某一点所需要的时间来运作。卫星使用同步的微型钟，其精确度只差若干纳秒，这可让你能精确定位到10米以内。若要精确定位到3厘米之内，你就必须把误差减小到大大低于100微微秒，而这种精确度尚未应用到商业。”
也许对我们生活产生最大影响的实用场合，就是生产小型、高精度的商业原子钟。美国国防部在8月份向公众展示了这类最先进的原子钟，该机构已开发出可装在电脑芯片上的原子钟，其尺寸为一粒米大小，运转所需电力不到75毫瓦。这种装置能改变输电网或GPS所使用的网络功效。以GPS为例，这种原子钟使得对地下管道和电讯电缆的精确定位成为可能，从而降低了维修和开通新服务的成本及不便，方便人们通过手机与GPS连网。
目前，特丁顿的科学家们正致力于研究“光学离子钟”，这是一个全新的系统，它使用一束紫外激光使一个电子进行每秒一千兆次的轨道变换，这个电子原本围绕几乎静止不动的汞原子或锶原子旋转。看来研究就如同时间本身一样，是永无止境的。
