直到高一第一学期(1949年秋季),我的课余兴趣还停留在玩儿无线电上。我当时是北京四中的业余无线电小组的“头儿”。小组中有能人。北京的小报上报导过我们小组的活动,特别提到我们去清华大学求教专家,增长我们的无线电知识。恰恰是这个报导,和清华大学之行,使我从“玩儿无线电”上金盆洗手了。
对高中生来说,能看懂收音机的线路图,也能分辨出各个部分的功能:高放,外差变频,中放,检波,功放,电源整流等等,还能粗略说出为什麽有这些功能。根据这些浅近的理解,就可以换零件,改装,重组,试图达到自己的目的。这是高中生的玩儿法。它同初中生的玩儿法——按图装配听个响儿——已大不相同。
对于我和一些小组成员,最不明白的是天线。天线的功能当然是接受电磁波信号(收音机),或产生电磁波信号(发射机)。但是,天线为什麽能接受和发射? 通俗无线电书籍上就语焉不详了。语焉不详还不要紧,就怕误导。
有的书籍,在天线之上画上几条曲线。其形状就像小人书里武侠们身后的螺旋状曲线,表示武侠们翻几个筋斗, 驾着云就来了,或走了。用武侠的来去比喻电磁波的来去,当然是100% 误导。
有的书较认真,多说了几句:天线杆上有电荷,也有电流,可以产生电场和磁场,电磁波就是由电场和磁场构成的。这有道理。
好,那我们也来认真一下。杆状天线上的电流是沿杆方向的,所以磁力线应是围绕天线杆的圆环,即磁力线垂直于天线杆,这在许多书上都有。又,天线上如有电荷,应是沿天线杆方向分布的,它们的电力线应是是垂直于天线杆,沿径向方向的,这在许多书上也有。这样,天线杆外的电场与磁场相互垂直。很好,电磁波的电场和磁场就是处处相互垂直的。书上又说,电磁波的传播方向垂直于电场及磁场。这样,天线外电磁波的传播方向应是平行于天线杆的。亦即,沿杆向上,或沿杆向下。
这个推论只基于四个垂直关系:磁场与天线杆垂直;电场与天线杆垂直;电场与磁场相互垂直;电磁波传播方向与电场磁场二者垂直。这在很多普及读物上都有,不需要任何中学以上的知识。
如果电磁波真是沿天线杆方向发射,杆状天线就应指向接受者所在的方向。
这个结论显然不对。四中西边的官园里,就有高大的杆状天线,指向天空。难道它的目的是发射给天上的接受者?当时(1949年),还没有人上太空,连飞机都很少。
我业余无线电小组的成员决定去清华大学电机系去请教。当时,清华大学电机系的声望,在北京的中学生里,绝对是排名第一的,超过物理系。许多同学的大学目标是清华电机系。
无线电小组中,五六个有自行车的成员,找了一个好天,结伴直奔清华园。当时的北京城外(即如今二环以外)极少公共汽车,也没有像样的柏油路面。北京城里的学生上街游行,“反饥饿,反内战”,少有清华和燕京的学生参加,就因为进城交通不便。
电机系并不重视我们访问。也难怪,几个中学生嘛,懂什麽?接待我们的是一位年轻教师,也许只是个研究生。接待者的专长大概是电路。他总是用电路来解释电磁波,一直不提电场磁场。好像电磁波与电场磁场无关。他说,天线是个开放电路,所以能把电磁波发射出去。这种解释,甚至说不清“垂直-垂直-垂直-垂直”是否有错,错在那里。有点糊弄人,似是而非。不算100% 误导,就算99.9% 吧。
乘兴而来,败兴而归。
到了高一第二学期,无线电的玩儿兴顿然消失,我最终退出了无线电小组。
高二开始有物理课,讲课教师是四中的镇校之宝——张子谔老师。张老师的课,生动,有趣。说到雷电的时候,张老师特别提醒大家,不要学屈原。正在公演的郭沫若的话剧“屈原”里,有名的“雷电颂”一场,是屈原在雷电交加时指天发誓:“电啊!你这宇宙中最犀利的剑呀!……你劈吧,劈吧,劈吧!把这比铁还坚固的黑暗,劈开,劈开,劈开!”。很多同学喜欢用歇斯底里腔调朗诵。教室里一片“劈呀” “劈呀”。
幸好,屈原最终没有被劈到。张老师说那是因为屈原命大。命不够大的人,千万别在打雷时指天发誓,少跟雷电逗着玩儿。
雷击问题,很像天线问题。天线收到电磁波信号,就如同电磁波信号“劈”到了 天线。雷电交加时,收音机嘎嘎乱响,那就是闪电发射的电磁波在“劈” 天线。
那末,发射机呢?似乎应当反过来,是天线主动在“劈”。“劈”谁?谁被“劈”?如果屈原拿着一台无线电发射机指天发誓,在雷雨交加时,发射机天线是不是会多“劈”出点电磁波?我没敢去问。张老师是长辈,威望太高,有点令人望而生畏。
高二代数课老师是王景鹤,年轻,随和,风趣,不修边幅,安徽人,口音不重,和学生混得很熟。得知王老师是西南联大物理系毕业的,我就到他的单身宿舍问他天线如何发射,也谈到高一时的清华之行。王老师听后哈哈大笑:“你们是跑错了衙门,这种问题,你们应当去找物理系。”他还说,清华物理系的教授,很多是从西南联大回来的,他认识。
王景鹤老师告诉我:“你说的‘垂直-垂直-垂直-垂直’关系都对,天线周围是有你说的那种电磁场,不过,那种电磁场不是电磁波。电磁波里的电场是由振荡的磁场产生的,而磁场又是由振荡的电场产生的。‘垂直-垂直-垂直-垂直’电磁场是所谓的静场,或近场。不存在沿着天线杆向上或向下发射的电磁波。”这一段话,简单否定了“官园天线”的目标是太空人。
王老师又说:“杆状天线周围的电场不仅有你说的垂直于天线杆的电场,而且有平行于天线杆的电场。”第一次听到!
“如果电荷有加速度,电信号的传播速度有限(不是无限),就会产生平行于天线杆的电场。” ——为——什——麽?
王老师拿出一张纸,画上一条直线:“如果这是一条绳子,你握住一端,另一端拴在树干上,拉紧,绳子是直的,同你竖直的身体垂直。绳子相当于静场。
“如果你的手突然上提,或上下晃一晃,绳子上会有一段打弯。”王老师画了一个弯。
“打弯的的一段绳子,并不同你的身体垂直,它有平行于你身体的分量。打弯的一段不是静止的,它会从你的手的一端,一直跑到树干。绳子拉得愈紧,‘打弯’跑得愈快。”这不难懂,找一条长绳,再找一棵树,就容易验证。
“如果绳子很长,打弯的一段一直在向外传播,同你彻底脱离关系。这就是‘发射’
“再,如果绳子被拉得无限紧,绳子永远是直的,不会打弯,因为这种绳子上的信号传播速度是无限大。实际上,绳子不可能被拉得无限紧。拉得太紧,绳子早就断了。但是,物理就是要思考极端情况,才容易弄明白。”
“物理就是要思考极端情况,才容易弄明白。”有道理。
绳子相当于电力线,“手突然上提”或“上下晃一晃”(振荡一下)相当于天线中电荷的加速运动。如果电信号沿电力线传播速度有限(绳子不是被无限拉紧),就一定有一段电力线“打弯”,不再同你的身体垂直,而有平行分量,它向外传播。这就是“有加速度的电荷会发射电磁波”的原因。
“所以,发射电磁波是由于天线中(或闪电中)的加速运动电荷‘劈’到了天线周围的静场或近场,产生了电磁波的电场和磁场。”
后来知道,以绳子“打弯”(或称kink)说明电磁波的产生和传播,是汤姆孙(J.J.Thomson,英)首创的。他注意到电力线的一些性质类似于被拉紧(不是无限紧)的绳子。加速运动电荷会使电荷本身的静场(近场)的电力线打弯,产生了电磁波 [1]。电荷周围的静电场,永远跟着电荷走。所以,只要电荷有加速运动,就会“劈”到电场,发射电磁波。
插几段有关电磁波的历史。
1865年,麦克斯韦(J. Maxwell,苏格兰)建立了电磁学的基本方程后,预言有电磁波存在。
直到二十年后,1886年,才有赫兹(H. Hertz,德)证实电磁波的存在。赫兹证明,人造的放电(实验室里的的闪电),能发射电磁波。据传,当时有人问赫兹,你这个发明有什麽用?他说,没什麽用,除了在表演时,会令在场仕女们惊奇和尖叫。
又过了十年,马可尼(G. Marconi,意)开始建造天线,愈造愈大,逐步证实,电磁波信号可以传递数十,数百,上千公里。他创建了第一个商用无线电报公司。这是电磁波实用之始,其势头至今未减。
天线一词(Antenna,意为昆虫的触角)可能就是马可尼等首用的。
尽管如此,直到那时,为什麽放电(或闪电,或天线)能发射电磁波?还没有一个完整的理论。马可尼设计天线,还是用经验公式,如马可尼律:电磁波的有效传播距离正比于杆状天线高度的平方。
1898 - 1900年,A. Lienard(法)和E. Wiechart (德)前后独立地得到了完整的运动电荷的电磁场解。 Lienard - Wiechart (L-W) 公式严格分清了与加速度无关的近场(“垂直”场),和加速度产生的辐射场(电磁波)。
L-W 公式为电荷电流发射电磁波奠定了理论基础。马可尼律等不过是它的推论。 L - W 公式的计算过程很繁复,不易普及(不过,早在60年代,费曼在他称之为“普及水平”的物理中,已开始用L - W公式解释电磁波了 [2])。
1903年,汤姆孙在耶鲁大学作演讲时,用电力线的打弯说明加速电荷如何发射电磁波。“打弯”图像不但能正确地区分近场和辐射场,也能证明近场不参与能量传输,而辐射场携带能量。J.J. Thomson的“打弯”图像甚至还可以部分地得到L - W公式 [3]。所以,“打弯”解释虽然简洁易懂,但不失严谨有效 [3]。这远远不是“开放LC电路产生电磁波”的说法所能比的。
以下一项,也容易查实:1949年,在清华园里误导我们的“开放电路”说,迄今仍流传于一些中文的百科网站,甚至高中教材上。
后来,王景鹤老师还借给我一些通俗的量子论读物,其中介绍原子是如何产生辐射的,即如何发光,发X-射线等等,原子没有天线,也没有LC 回路,但与天线发射电磁波有相通的道理,都是由于电荷的加速运动。
上大学后,我还回四中看望过王景鹤老师。他同我在北大的第一位物理老师——黄昆教授——在西南联大是同学,他们讲起物理来,有一种类似的“味道”。
西南联大之后,黄先生远赴英伦留学,1950年回国。王老师则过海去台北教中学,1949年回大陆。
肃反运动后,再去四中,就没有见到王老师了。因台湾一段历史,王老师被当局怀疑为暗藏的反革命分子,离开了四中。数年之后,又听说,王老师已被调到北京师范学院教数学了。
不过,我再也没有见到过他。也不知他是否还保持着当年的风趣,潇洒和智睿……
从四中出来的人不少,回忆文章也很多,但很少提到王景鹤老师。我曾受惠于王景鹤老师,特别忘不掉他的一条绳子的启蒙。故草此文以记之。
[1] J.J.Thomson, Electricity and matter, (Charles Scribner’s Sons,1904).
[2] R. Feymann, Lectures on Physics (CIT,1963).
[3] J. Tessman,1967, American Journal of Physics,35, 523; H. Padmanabhan, ibid, 2009, 77, 151.
2012, Tucson
By ANDY KESSLER
I figured before heading out I would wait for the fog to lift to shine some light on the Occupy San Francisco annex of Occupy Wall Street. I'm not sure it ever did. What many refer to as the People's Republic of San Francisco is socially liberal and economically neutered. The heaviest traffic is leaving the city in the morning and coming back in the evening. It's a bedroom community that has effectively banned both McDonald's Happy Meals and foie gras and has sadly turned into the pretty but growth-deprived stepsister of San Jose.Still, who doesn't love a movement? Needing something to wear to an occupation and in bigger need of some protest mojo, my first stop was the corner of Haight and Ashbury, the mecca of movements, the center of the '60s hippie habitat. The streets were pretty empty—usually this area is filled with homeless, stumbling along the block lined with body piercing shops, vintage clothing stores and "smoke" shops.
I found the Jimi Hendrix shirt I was looking for, but no signs of any protests, although I was accosted by four aggressive pink-T-shirt-wearing folks from Planned Parenthood and a Hasidic Rabbi with a lulav and etrog. I dove into the only sign of civilization, the Ben & Jerry's ice cream on the corner, and ordered a small Stephen Colbert Americone Dream, which cost a small fortune.
OccupySF has taken up at Justin Herman Plaza, at the bottom of Market Street, an irony probably lost on most. It's nestled between the Financial District and the recently renovated Ferry Building by the bay. It's also a block away from the Federal Reserve of San Francisco. Both are filled with sleeping bags and dreadlocks and body piercings and the pungent odor that often accompanies this scene. And signs. And more signs. Like most, I'm confused by what Occupy Wall Street is all about. Some of it is a place to hang out for the unemployed and homeless. Some of it is a party. The Haight-Ashbury crowd moved 40 years into the heart of San Francisco.
But I've now become an aficionado of Occupy Wall Street signs, which I enjoyed in the flesh but also found all over Facebook and the Internet. They provide the missing narrative, the hard to discern story that was obvious at antiwar or civil rights or gay rights protests.
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Some of the message is certainly about anger. One aggressive sign warned, "Be Afraid Banks—99% to 1% is very bad odds." No wonder Warren Buffett is volunteering to pay higher taxes. Another prominent sign: "End Predatory Capitalism." Like Ben & Jerry's prices?
Maybe this is all really about disappointment. I spoke to a young woman who had clearly bathed more recently than most. I asked her why she was at OccupySF. She told me she'd done all the right things. Studied hard. Graduated college. (She was an art major.) And now she can't get a job. It didn't matter. It's all messed up. She was lied to.
Of course she was. She's a member of the Trophy Generation. Win or lose, you get a trophy. We embraced mediocrity to an entire generation of kids during good times who are now finding themselves mediocre in bad times. There still is that American dream: Go to college, get a job, buy a Prius. But like it or not, studying art or humanities or gender studies won't get you there. Marissa Mayer at Google complains she can't find enough computer-science majors. Civil engineers are getting hired sight unseen.
Educating the whole child was bad advice. So was follow your passion. California spends months teaching ninth-graders how to build a waste-treatment plant with only a day or two on natural selection. I think Occupy Wall Streeters are as much disappointed with the route they all took as they are with "fat cat" bankers.
An older gentleman grabs my elbow and launches. "There is a different set of rules," he tells me, "for the 1%, the thieves on Wall Street, than the rest of us 99%ers." I must have looked confused. "We need to end the reign of Wall Street. The status quo is over. Stick around until six and we've got a General Assembly meeting and . . ."
I couldn't stay. I had a meeting I was late for.
I drove out to the Mission, found a spot on Valencia, and raced into the Summit, a coffee shop/startup incubator/entrepreneur hangout. Oddly, there sat a similar collection of folks with shabby clothes and body piercings and dreadlocks, but to a person, they all were hunched over MacBook computers. My friend Aaron was showing off his new location-aware social app when I heard someone at the next table talking about the thieves on Wall Street and how he's found a way to end their reign. Can't get away, it seems.
I must have rolled my eyes because Aaron introduced me to the guy. He had long hair, a scruffy beard and was holding an iPhone in one hand and a 5-hour ENERGY drink in the other. All entrepreneurs are trained for the elevator pitch, the 30-second description of what they do in case they are ever on a short elevator ride with a venture capitalist.
"I've taken the best of social networking and high-frequency trading and built a system that beats those Wall Street thieves at their own game. Users input their portfolio, it could be stocks or bonds or even derivatives and then we log each trade and anonymously share the spreads so everyone is on an even keel. First it's just about information, but then we can start matching trades away from Wall Street. Its over for those guys, the status quo is toast."
Apparently there's more than one way to Occupy Wall Street.
Mr. Kessler, a former hedge-fund manager, is the author most recently of "Eat People: And Other Unapologetic Rules for Game-Changing Entrepreneurs" (Portfolio, 2011).