Dr Hawking's bright idea
霍金博士的闪亮思想
A long-predicted phenomenon has turned up in an unexpected place
一个被长久期待的现象出现于一个出人意料的地方
Sep 30th 2010
英国《经济学人》杂志 9月30日文章
IN 1974 Stephen Hawking, pictured, had a startling theoretical insight about black holes—those voracious eaters of matter and energy from whose gravitational clutches not even light can escape. He predicted that black holes should not actually be black. Instead, because of the quirks of quantum mechanics, they should glow ever so faintly, like smouldering embers in a dying fire. The implications were huge. By emitting this so-called Hawking radiation, a black hole would gradually lose energy and mass. If it failed to replenish itself it would eventually evaporate completely, like a puddle of water on a hot summer's day.
1974年，斯蒂芬•霍金（上图）提出了一个关于黑洞的理论性理解——这种黑洞贪婪地吞噬一切物质与能量，即使光线也无法逃脱其强大的吸力。霍金博士预测到，黑洞实际上不应该就是"黑色"。相反，根据量子力学的怪异说法，黑洞就像行将熄灭的火焰里那飘摇的灰烬，其光亮度应该是微乎其微的。释放出这种所谓的霍金辐射，黑洞会逐渐失去能量与质量。如果黑洞无法充实自己，那它就会像炎热夏日的一个水洼。最终完全被蒸发。
Unfortunately for physicists, Dr Hawking also predicted that the typical temperature at which a black hole radiates should be about a billionth of that of the background radiation left over from the Big Bang itself. Proving his theory by observing actual Hawking radiation from a black hole in outer space has therefore remained a practical impossibility.
对物理学家来说不幸的是，霍金博士同样预测到一个黑洞辐射的正常温度应该是"大爆炸"遗留下的背景辐射温度的十亿分之一。因此，通过从外空的一个黑洞来观察实际的霍金辐射来证明其理论的正确性，就成了在实践上一个不可能完成的任务。
In a paper just accepted by Physical Review Letters, however, a team of researchers led by Daniele Faccio from the University of Insubria, in Italy, report that they have observed Hawking radiation in the laboratory. They managed this trick not by creating an Earth-gobbling black hole on a benchtop but by firing pulses of laser light into a block of glass. This created a region from which light could not escape (analogous to a black hole) and also its polar opposite, a region which light could not enter. When the team focused a sensitive camera on to the block, they saw the faint glow of Hawking radiation.
然而，在一份刚刚被《物理评论快报》接受的论文中，由意大利伊苏布利亚大学的丹尼尔勒•法西奥领导的一组研究人员报告说，他们在实验室观察到了霍金辐射。他们完成的这个绝招，不是通过在工作台上制造一个"吸土"的黑洞，而是把激光脉冲打入一块玻璃中。这样就制造了一块光线无法逃脱的区域（好比黑洞），而在玻璃的对立面也制造了一块光线无法进入的区域。当科研小组将一部高敏摄像机的光线对准玻璃块时，他们看到了霍金辐射的微光。
Black and light
黑色的，很轻的
If a dying star is massive enough, it can collapse to form a region of infinite density, called a singularity. The gravity of such an object is so strong that nothing, not even light itself, can break free if it strays too close. Once something has passed through the so-called event horizon that surrounds this region, it is doomed to a one-way trip.
如果行将消逝的行星有足够的质量，那么它就能瓦解并形成一个有着无限密度的区域，我们称之为单一体。这种物质的引力很强，如果它们脱离的距离不大，那么就没有任何东西能冲破它们，甚至是光线。一旦某些东西穿越了环绕这个区域的所谓"视界"，那这个东西就注定无法返回。
Dr Hawking's insight came from considering what happens in the empty space just outside the event horizon. According to quantum mechanics, empty space is anything but empty. Rather, it is a roiling, seething cauldron of evanescent particles. For brief periods of time, these particles pop into existence from pure nothingness, leaving behind holes in the nothingness—or antiparticles, as physicists label them. A short time later, particle and hole recombine, and the nothingness resumes.
霍金博士的思想源于对视界之外的真空发生的事情进行的深思熟虑。根据量子力学的原理，真空里绝非空无一物，它更有点像是一个滚动的、沸腾的以及由逐渐消散的粒子组成的大气锅。在短时间里，这些粒子从纯粹的虚无中突然存在，便在虚无中留下了很多洞——或者如物理学家定义的，称之为"反粒子"。即刻之后，粒子与洞再次融合，一切再次回到虚无状态。
If, however, the pair appears on the edge of an event horizon, either particle or hole may wander across the horizon, never to return. Deprived of its partner, the particle (or the hole) has no "choice" but to become real. These particles, the bulk of which are photons (the particles of light), make up Hawking radiation—and because photons and antiphotons are identical, the holes contribute equally. The energy that goes into these now-real photons has to come from somewhere; that somewhere is the black hole itself, which thus gradually shrinks. By linking the disparate fields of gravitational science, quantum mechanics and thermodynamics, Hawking radiation has become a crucial concept in theoretical physics over the past quarter of a century.
然而，如果粒子与洞出现于视界的边际上，那么二者之一就会穿越视界流走，永远不能返回。在搭档失踪的情况下，粒子（或洞）没有任何"选择"，只能成为真实物质。能最终组成光子（光粒子）的这些粒子就形成了霍金辐射——又因光子与反光子比较相似，洞发挥了同样的功能。于是，进入这些如今真实的光子的能量就必须来自于某个实质的地方；这个地方便是黑洞，并通过释放能量这种方式逐渐萎缩。霍金辐射将引力学、量子力学与热力学三种不同的学科综合起来，已经在过去的二十五年里成为物理理论界中一个关键的概念。
In 1981, that concept was extended. William Unruh of the University of British Columbia pointed out that black holes are actually extreme examples of a broader class of physical systems that can form event horizons. Consider a river approaching a waterfall. As the water nears the edge, the current moves faster and faster. In theory, it can move so fast that ripples on the surface are no longer able to escape back upstream. In effect, an event horizon has formed in the river, preventing waves from making their way out. Since then, other researchers have come up with other quotidian examples of event horizons.
1981年，这个概念延伸出了新的意义。英国哥伦比亚大学的威廉姆•安鲁指出，黑洞实际上是能够形成视界的物理体系更为广泛意义上的极端例子。我们来看看流向瀑布的一条小溪。随着溪水流向边缘地带，溪流的速度就会变得越来越猛烈。从理论上讲，溪流的速度如此之快，表面上的波纹根本不可能再往上游流去。结果，小溪就形成了一个视界，阻止了波浪逃离整个溪流的逆流。而之后，其他研究人员也提出了关于视界的其它司空见惯的种种例子。
Dr Faccio and his team were able to create their version because, as the laser pulse moves through the glass block, it changes the glass's refractive index (the speed at which light travels through a material). Light in the vicinity of the pulse is slowed more and more when the refractive index changes as the pulse passes by.
法西奥博士与他的科研小组之所以能创造他们的证明方式，是因为随着激光脉冲在玻璃块中穿越的过程中，它改变了玻璃的折射率，即光穿越物质的速度。随着脉冲的穿越，折射率出现改变，临近脉冲的光的速度就会越来越慢。
To see how the pulse can act like a black hole, imagine that it is sent chasing after a slower, weaker pulse. It will gradually catch up with this slow pulse, reducing the speed of light in the slow pulse's vicinity. That will slow the slow pulse down still more until eventually it is slowed so much that it is stuck. Essentially, the leading edge of the chasing pulse sucks it in, acting like the event horizon of a black hole.
为了弄清楚脉冲如何发挥黑洞的功能，我们可以想象这个脉冲摄入是为了追赶另一个更慢、更弱的脉冲。它将逐渐赶上这个速度慢的脉冲，于是在慢脉冲的附近降低了光的穿越速度。这将进一步减慢之前慢脉冲的速度，直到最终其移动出现停滞。从本质上来讲，后来追逐的脉冲的主导边缘将慢脉冲吞噬了，就像一个黑洞的视界那样。
Now imagine that the chasing pulse is itself being chased, but again by a much weaker pulse. As this third pulse approaches the tail of the second one it will also slow down (because the speed of light in the glass it is passing through has been reduced by the second pulse's passage). The closer it gets, the slower it travels, and it can never quite catch up. The trailing edge of the second pulse, therefore, also acts as an event horizon. This time, though, it stops things getting in rather than stopping them getting out. It resembles the opposite of a black hole—a white hole, if you like.
现在我们想象一下，追逐的脉冲反过来被追逐，但还是被比它弱很多的脉冲追逐。随着第三个脉冲接触到第二个脉冲的尾部，它的速度也同样被减慢（因为其穿越的玻璃中的光速已经被第二个脉冲的穿越而减慢）。第三个脉冲接触得更近，其穿越速度就会更慢，这样它就永远都追不上上述更快的脉冲。因此，第二个脉冲的后缘地带也发挥了视界的作用。不过这一次，这个视界是将物质吸引进来，而不是将它们驱赶出去。这就像黑洞的对立面——如果你愿意，可以称之为"白洞"。
In the actual experiment, there were no leading and trailing pulses. Instead, their role was played by evanescent photons continually popping into existence around the strong pulse. As the pulse passed through the glass, its event horizons should have swept some of these photons up, producing Hawking radiation from the partners they left behind.
在实际的试验中不存在主导脉冲与后缘脉冲。相反，由易消散的光子发挥的这些作用在强脉冲附近持续存在。而随着脉冲穿越玻璃的过程中，其视界就会把这样的一些光子扫起来，这样就通过它们留在身后的搭档制造出霍金辐射。
Sure enough, when Dr Faccio and his team focused a suitable camera on the block and fired 3,600 pulses from their laser, they recorded a faint glow at precisely the range of frequencies which the theory of Hawking radiation predicts. After carefully considering and rejecting other possible sources for this light, they conclude that they have indeed observed Hawking radiation for the first time.
当法西奥与他的科研团队将一个合适的摄像机对准玻璃块，并从他们的激光射出3600道脉冲时，他们果然记录下了一道微弱的闪光，其频率范围与霍金辐射预测的保持着精确的一致。在仔细考虑并排除了这道光的其它可能的来源之后，他们总结道，他们的确第一次观测到了霍金辐射。
Because of its tabletop nature, other groups will certainly attempt to replicate and extend Dr Faccio's experiment. Although such studies cannot prove that real black holes radiate and evaporate, they lend strong support to the ideas that went into Dr Hawking's line of reasoning. Unless a tiny black hole turns up in the collisions of a powerful particle accelerator, that may be the best physicists can hope for. It may even be enough to convince Sweden's Royal Academy of Science to give Dr Hawking the Nobel prize that many think he deserves, but which a lack of experimental evidence has hitherto caused it to withhold.
由于试验在工作台上完成，其它科研团队也将肯定去重复并延伸法西奥博士的试验。尽管这些研究无法证明真正的黑洞的辐射与蒸发行为，他们却为霍金博士的论证提供了有力的支持。这也许是最拔萃的物质学家所希望看到的，除非我们使用强大的粒子加速器的冲撞实验制造出一个小黑洞。这甚至可以说服瑞典皇家科学院为霍金博士颁发许多人都赞同的诺贝尔奖，但实验证据的缺乏也不得不暂时对此采取保留的做法。
霍金博士的闪亮思想
A long-predicted phenomenon has turned up in an unexpected place
一个被长久期待的现象出现于一个出人意料的地方
Sep 30th 2010
英国《经济学人》杂志 9月30日文章
IN 1974 Stephen Hawking, pictured, had a startling theoretical insight about black holes—those voracious eaters of matter and energy from whose gravitational clutches not even light can escape. He predicted that black holes should not actually be black. Instead, because of the quirks of quantum mechanics, they should glow ever so faintly, like smouldering embers in a dying fire. The implications were huge. By emitting this so-called Hawking radiation, a black hole would gradually lose energy and mass. If it failed to replenish itself it would eventually evaporate completely, like a puddle of water on a hot summer's day.
1974年，斯蒂芬•霍金（上图）提出了一个关于黑洞的理论性理解——这种黑洞贪婪地吞噬一切物质与能量，即使光线也无法逃脱其强大的吸力。霍金博士预测到，黑洞实际上不应该就是"黑色"。相反，根据量子力学的怪异说法，黑洞就像行将熄灭的火焰里那飘摇的灰烬，其光亮度应该是微乎其微的。释放出这种所谓的霍金辐射，黑洞会逐渐失去能量与质量。如果黑洞无法充实自己，那它就会像炎热夏日的一个水洼。最终完全被蒸发。
Unfortunately for physicists, Dr Hawking also predicted that the typical temperature at which a black hole radiates should be about a billionth of that of the background radiation left over from the Big Bang itself. Proving his theory by observing actual Hawking radiation from a black hole in outer space has therefore remained a practical impossibility.
对物理学家来说不幸的是，霍金博士同样预测到一个黑洞辐射的正常温度应该是"大爆炸"遗留下的背景辐射温度的十亿分之一。因此，通过从外空的一个黑洞来观察实际的霍金辐射来证明其理论的正确性，就成了在实践上一个不可能完成的任务。
In a paper just accepted by Physical Review Letters, however, a team of researchers led by Daniele Faccio from the University of Insubria, in Italy, report that they have observed Hawking radiation in the laboratory. They managed this trick not by creating an Earth-gobbling black hole on a benchtop but by firing pulses of laser light into a block of glass. This created a region from which light could not escape (analogous to a black hole) and also its polar opposite, a region which light could not enter. When the team focused a sensitive camera on to the block, they saw the faint glow of Hawking radiation.
然而，在一份刚刚被《物理评论快报》接受的论文中，由意大利伊苏布利亚大学的丹尼尔勒•法西奥领导的一组研究人员报告说，他们在实验室观察到了霍金辐射。他们完成的这个绝招，不是通过在工作台上制造一个"吸土"的黑洞，而是把激光脉冲打入一块玻璃中。这样就制造了一块光线无法逃脱的区域（好比黑洞），而在玻璃的对立面也制造了一块光线无法进入的区域。当科研小组将一部高敏摄像机的光线对准玻璃块时，他们看到了霍金辐射的微光。
Black and light
黑色的，很轻的
If a dying star is massive enough, it can collapse to form a region of infinite density, called a singularity. The gravity of such an object is so strong that nothing, not even light itself, can break free if it strays too close. Once something has passed through the so-called event horizon that surrounds this region, it is doomed to a one-way trip.
如果行将消逝的行星有足够的质量，那么它就能瓦解并形成一个有着无限密度的区域，我们称之为单一体。这种物质的引力很强，如果它们脱离的距离不大，那么就没有任何东西能冲破它们，甚至是光线。一旦某些东西穿越了环绕这个区域的所谓"视界"，那这个东西就注定无法返回。
Dr Hawking's insight came from considering what happens in the empty space just outside the event horizon. According to quantum mechanics, empty space is anything but empty. Rather, it is a roiling, seething cauldron of evanescent particles. For brief periods of time, these particles pop into existence from pure nothingness, leaving behind holes in the nothingness—or antiparticles, as physicists label them. A short time later, particle and hole recombine, and the nothingness resumes.
霍金博士的思想源于对视界之外的真空发生的事情进行的深思熟虑。根据量子力学的原理，真空里绝非空无一物，它更有点像是一个滚动的、沸腾的以及由逐渐消散的粒子组成的大气锅。在短时间里，这些粒子从纯粹的虚无中突然存在，便在虚无中留下了很多洞——或者如物理学家定义的，称之为"反粒子"。即刻之后，粒子与洞再次融合，一切再次回到虚无状态。
If, however, the pair appears on the edge of an event horizon, either particle or hole may wander across the horizon, never to return. Deprived of its partner, the particle (or the hole) has no "choice" but to become real. These particles, the bulk of which are photons (the particles of light), make up Hawking radiation—and because photons and antiphotons are identical, the holes contribute equally. The energy that goes into these now-real photons has to come from somewhere; that somewhere is the black hole itself, which thus gradually shrinks. By linking the disparate fields of gravitational science, quantum mechanics and thermodynamics, Hawking radiation has become a crucial concept in theoretical physics over the past quarter of a century.
然而，如果粒子与洞出现于视界的边际上，那么二者之一就会穿越视界流走，永远不能返回。在搭档失踪的情况下，粒子（或洞）没有任何"选择"，只能成为真实物质。能最终组成光子（光粒子）的这些粒子就形成了霍金辐射——又因光子与反光子比较相似，洞发挥了同样的功能。于是，进入这些如今真实的光子的能量就必须来自于某个实质的地方；这个地方便是黑洞，并通过释放能量这种方式逐渐萎缩。霍金辐射将引力学、量子力学与热力学三种不同的学科综合起来，已经在过去的二十五年里成为物理理论界中一个关键的概念。
In 1981, that concept was extended. William Unruh of the University of British Columbia pointed out that black holes are actually extreme examples of a broader class of physical systems that can form event horizons. Consider a river approaching a waterfall. As the water nears the edge, the current moves faster and faster. In theory, it can move so fast that ripples on the surface are no longer able to escape back upstream. In effect, an event horizon has formed in the river, preventing waves from making their way out. Since then, other researchers have come up with other quotidian examples of event horizons.
1981年，这个概念延伸出了新的意义。英国哥伦比亚大学的威廉姆•安鲁指出，黑洞实际上是能够形成视界的物理体系更为广泛意义上的极端例子。我们来看看流向瀑布的一条小溪。随着溪水流向边缘地带，溪流的速度就会变得越来越猛烈。从理论上讲，溪流的速度如此之快，表面上的波纹根本不可能再往上游流去。结果，小溪就形成了一个视界，阻止了波浪逃离整个溪流的逆流。而之后，其他研究人员也提出了关于视界的其它司空见惯的种种例子。
Dr Faccio and his team were able to create their version because, as the laser pulse moves through the glass block, it changes the glass's refractive index (the speed at which light travels through a material). Light in the vicinity of the pulse is slowed more and more when the refractive index changes as the pulse passes by.
法西奥博士与他的科研小组之所以能创造他们的证明方式，是因为随着激光脉冲在玻璃块中穿越的过程中，它改变了玻璃的折射率，即光穿越物质的速度。随着脉冲的穿越，折射率出现改变，临近脉冲的光的速度就会越来越慢。
To see how the pulse can act like a black hole, imagine that it is sent chasing after a slower, weaker pulse. It will gradually catch up with this slow pulse, reducing the speed of light in the slow pulse's vicinity. That will slow the slow pulse down still more until eventually it is slowed so much that it is stuck. Essentially, the leading edge of the chasing pulse sucks it in, acting like the event horizon of a black hole.
为了弄清楚脉冲如何发挥黑洞的功能，我们可以想象这个脉冲摄入是为了追赶另一个更慢、更弱的脉冲。它将逐渐赶上这个速度慢的脉冲，于是在慢脉冲的附近降低了光的穿越速度。这将进一步减慢之前慢脉冲的速度，直到最终其移动出现停滞。从本质上来讲，后来追逐的脉冲的主导边缘将慢脉冲吞噬了，就像一个黑洞的视界那样。
Now imagine that the chasing pulse is itself being chased, but again by a much weaker pulse. As this third pulse approaches the tail of the second one it will also slow down (because the speed of light in the glass it is passing through has been reduced by the second pulse's passage). The closer it gets, the slower it travels, and it can never quite catch up. The trailing edge of the second pulse, therefore, also acts as an event horizon. This time, though, it stops things getting in rather than stopping them getting out. It resembles the opposite of a black hole—a white hole, if you like.
现在我们想象一下，追逐的脉冲反过来被追逐，但还是被比它弱很多的脉冲追逐。随着第三个脉冲接触到第二个脉冲的尾部，它的速度也同样被减慢（因为其穿越的玻璃中的光速已经被第二个脉冲的穿越而减慢）。第三个脉冲接触得更近，其穿越速度就会更慢，这样它就永远都追不上上述更快的脉冲。因此，第二个脉冲的后缘地带也发挥了视界的作用。不过这一次，这个视界是将物质吸引进来，而不是将它们驱赶出去。这就像黑洞的对立面——如果你愿意，可以称之为"白洞"。
In the actual experiment, there were no leading and trailing pulses. Instead, their role was played by evanescent photons continually popping into existence around the strong pulse. As the pulse passed through the glass, its event horizons should have swept some of these photons up, producing Hawking radiation from the partners they left behind.
在实际的试验中不存在主导脉冲与后缘脉冲。相反，由易消散的光子发挥的这些作用在强脉冲附近持续存在。而随着脉冲穿越玻璃的过程中，其视界就会把这样的一些光子扫起来，这样就通过它们留在身后的搭档制造出霍金辐射。
Sure enough, when Dr Faccio and his team focused a suitable camera on the block and fired 3,600 pulses from their laser, they recorded a faint glow at precisely the range of frequencies which the theory of Hawking radiation predicts. After carefully considering and rejecting other possible sources for this light, they conclude that they have indeed observed Hawking radiation for the first time.
当法西奥与他的科研团队将一个合适的摄像机对准玻璃块，并从他们的激光射出3600道脉冲时，他们果然记录下了一道微弱的闪光，其频率范围与霍金辐射预测的保持着精确的一致。在仔细考虑并排除了这道光的其它可能的来源之后，他们总结道，他们的确第一次观测到了霍金辐射。
Because of its tabletop nature, other groups will certainly attempt to replicate and extend Dr Faccio's experiment. Although such studies cannot prove that real black holes radiate and evaporate, they lend strong support to the ideas that went into Dr Hawking's line of reasoning. Unless a tiny black hole turns up in the collisions of a powerful particle accelerator, that may be the best physicists can hope for. It may even be enough to convince Sweden's Royal Academy of Science to give Dr Hawking the Nobel prize that many think he deserves, but which a lack of experimental evidence has hitherto caused it to withhold.
由于试验在工作台上完成，其它科研团队也将肯定去重复并延伸法西奥博士的试验。尽管这些研究无法证明真正的黑洞的辐射与蒸发行为，他们却为霍金博士的论证提供了有力的支持。这也许是最拔萃的物质学家所希望看到的，除非我们使用强大的粒子加速器的冲撞实验制造出一个小黑洞。这甚至可以说服瑞典皇家科学院为霍金博士颁发许多人都赞同的诺贝尔奖，但实验证据的缺乏也不得不暂时对此采取保留的做法。
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