开始翻译 Einstein@Home 主题站

碧城仙

引用 Youth 在 2006-1-18 18:35 时的帖子:
这个多了五个页面，大仙在一楼更新一下？

好的，第一楼帖子我已经修改过了。这边可以先开始翻译，页面我随后就做。

Youth

刚发现专家问答这个部分又有改版：
http://www.einsteinathome.org/ask/archive/index.html

相关的文件也改成了 类别+qaxxx.html 的方式

Youth

前面6个Q&A都翻译过的，改一下文件名就可以了

LIGO

Q: How can one know that the length of one arm is exactly 4km (or so)?
A: Actually, what the interferometer measures is the difference between the lengths of the two arms…

Q: I understand that the gravitational waves scientists believe we can detect are from extremely massive events in space such as binary systems, pulsars, and supernovae. Is Einstein@Home searching for gravitational waves from a particular one of these source types?
A: The LIGO Science Collaboration is implementing searches for all those sources and more, but the one Einstein@Home is running is the "all-sky pulsar search"…

Q: Is the baseline between LIGO and the German detector long enough to be able to pinpoint the source of the waves? The Einstein@home screensaver displays the location on the sky of where it is searching, so how is this directionality achieved?
A: Physically, LIGO is passive. It sits where it is, and that's it. The “pointing” is done in the processing of the data…

Sources of Gravitational Waves

Q: Would so-called solid quark stars that undergo "glitches" produce gravity waves due to the changing moment of inertia. Could such a detection be used to prove or rule out the existence of strange quark stars?
A: A "glitch" is what radio astronomers call it when they see a pulsar's frequency suddenly jump up a little bit…

Q: In order to detect gravitational waves from a pulsar, wouldn't that pulsar have to precess about its axis since symmetrical configurations would not produce gravity waves?
A: Precessing stars do radiate gravitational waves, but they're not the only ones…


General Theory of Relativity

Q: I had always thought that there is no up or down in the outer spaces of the Universe. This now seems not to be correct if matter causes curvature and other objects fall towards the curves. What am I forgetting here?
A: Matter does indeed cause curvature, and this curvature is present even out in space. But that's not the same as the notion of up or down…

Youth

Which would be considered more fundamental - the constant "c" (speed of light in vacuum), or the constant "epsilon_0" (permittivity of free space), or the constant "alpha" (fine structure)? Are they equally fundamental?

常量c（光速）、epsilon_0（真空介电常数）和alpha（精细结构常数）中哪一个更基本？还是同样基本？

Submitted by Chipper Q from the USA

由美国的Chipper Q提交

These constants all include the same information - they can be expressed in terms of each other (and some other constants). But they have different dimensions: "c" is a speed or distance per unit time, "epsilon_0" is some funny electrical thing, and "alpha" has none - it's just a number.

这些常量均包含同样的信息－可以互相转换（借助其它常量）。但它们的量纲并不相同：c是速度或者说单位时间的位移，epsion_0是电学常量，而alpha没有量纲－它仅仅是一个数字。

Here we need to distinguish between dimensions (like speed) and units, the specific things we measure with (like miles per hour or meters per second).

我们需要分清楚量纲（比如速度）和单位（我们用来衡量事物的尺度，比如每小时英里数或者每秒米数）。

Most physicists would say that alpha is a more fundamental constant than c or something else because dimensionless numbers are more fundamental than anything with dimensions. Really what we measure are numbers. When I say it's fifty yards to the street corner, what I mean is that the ratio of that distance to the stride of some long-dead English king is fifty. That ratio would not change if I measured both lengths in meters. It's only forty-five meters to the corner, but the king's stride is a little less than a meter, so it's still fifty of them.

大部分的物理学家会说alpha比c或其它常量更为基本，因为无量纲的数字比有量纲的任何事物更为基本。实际上我们测量的就是数字。当我说离街角五十码，我是想说的是这个距离和那位早已过世的英国国王的步长的比例是50。这个比例是不会改变的，即使我是用米来测量这两个距离。虽然到街角只有45米，但那位国王的步长也少于1米，因此除下来仍然是50。

What good are dimensions then, if we really measure dimensionless numbers?

既然我们测量的是无量纲的数字，那量纲有什么用呢？

Dimensions let us distinguish between things that cannot be compared without reference to some other factor. For instance, I can't define a distance as a time without using some outside information: I can say that my house is four hours from Philadelphia if I use my average driving speed as a conversion. But if I used light speed as a conversion, I would say that Philadelphia is a millisecond away. Contrast this to saying Philly is two hundred miles away. At first that looks just as arbitrary as quoting a time, but really I am saying the ratio of that distance to that English king's stride is 350,000. And that ratio doesn't change no matter what units I use, and I don't need to bring anyone or anything into it except me and the king.

量纲可以帮助我们区分那些只能通过引用其它一些参数来进行比较的事物。如果我要将距离定义为时间，我必须使用一些额外的信息：以我的平均驾车速度为参照，我可以说我的房子离费城四小时。但如果我使用光速作为参照，我会说费城在一毫秒之外。于之对比，我还可以说费城在两百英里外。看上去，这似乎就像随便说一个时间，但实际上我是在说到费城的距离和那位英国国王的步长比例是350,000。无论我使用什么单位，这个比例都不会变，我也不需要借助我自己和那位国王之外的任何人或任何事物。

Youth

Do you think that Albert Einstein had a big influence on the way people live their lives today? Do you think German people have a different opinion of him than English?

在你看来，阿尔伯特·爱因斯坦对今天人们的生活方式有大影响吗？德国人对他的看法是否和英国人一样？

Submitted by Rebecca from Liverpool, England

由英国利物浦的Rebecca提交

I don't know how opinions differ from one country to another, but Einstein has a lot of influence on how we live today, even if you only look at the more practical things.

我不清楚各个国家的人们对爱因斯坦的看法是否不同，但即使是从应用的角度来看他对我们现在的生活的影响仍是相当多的。

GPS, the Global Positioning System, is a good example. It determines the location of your car, a missile, or whatever by comparing clock signals with several satellites. The curvature of spacetime may sound like a very abstract thing, but it has very concrete consequences. Basically, relativity predicts that the clocks up in orbit on the satellites tick faster than clocks on the ground. It's only forty microseconds per day, but if the effect weren't correctly calculated from Einstein's equations and compensated for, GPS would accumulate position errors of seven miles per day. GPS would very quickly become useless.

GPS即全球定位系统就是一个很好的例子。它可以通过比较几颗卫星的时钟信号而给出你的汽车、一枚导弹或其它任何东西的位置。时空扭曲也许听上去很抽象，但却有着相当实际的影响。相对论预言在卫星轨道上的时钟要比地面上的时钟走得快。虽然就算一天下来也只有40毫秒的差别，但如果不借助爱因斯坦的理论对其进行修正，一天下来GPS将会产生七英里的位置偏差，不用很长时间这个GPS就将变得毫无用处。

Einstein did much more than relativity. He predicted all sorts of properties of atoms and light. Arguably he was one of the founding fathers of quantum mechanics, though he never was very happy with it. Einstein worked out how the right sort of microscopic particles could clump up in ways that would be visible at human scales, which is basically how superconductors work. He also worked out that atoms making light could make other atoms make the same kind of light. It took decades to develop the technology to make this practical, but it has a huge influence. It's called the laser, and it's used for everything from reading prices at supermarkets to wowing the fans at Pink Floyd concerts to searching with LIGO for the gravitational waves predicted by relativity - nicely tying together what are arguably Einstein's biggest results.

爱因斯坦除了相对论还有很多其它的贡献。他预言了原子和光的所有属性。虽然他一直不赞同量子力学，但他却是量子力学的创立人之一。他发现了全同微观粒子如何以人们可观测到的方式进行凝聚，这可以用来解释超导体的工作原理。他还发现了发光原子可以让其它原子也发出同样的光。虽然几十年后技术的进步才将这个理论应用到实际中，但它的作用却非常之大。这就是激光，从超市的读价器到Pink Floyd演唱会震撼的灯光效果再到用LIGO来搜寻相对论所预言的引力波－所有这些都有爱因斯坦的功劳。

And Einstein explained why rivers meander - that is, why they don't flow straight over flat land but rather grow big loops which pinch off or move downstream. I found out about that achievement of his when I was growing up in Louisiana, where that sort of thing has a big effect on the landscape. So it made a big impression on me, but it doesn't seem very widely known. It ought to be, since it shows the breadth of Einstein's achievements, and that he thought about very down-to-earth things as well as the very far out.

爱因斯坦还解释了为什么河流是弯曲的－也就是说，为什么不是在平地上直直地流而是东弯西绕的。我在路易斯安那长大的时候才知道这个，虽然它并不太广为人知，但却给我留下了深刻的印象。从中我们可以看到爱因斯坦成就的范围之广，无论是现实的还是抽象的。


说明：红色部分或者不太确定或者完全是随意翻的，欢迎大家更正：）

Youth

http://www.einsteinathome.org/ask/archive/misc-qa1.html

Is there any truth to the idea that dark colors attract heat and light colors do not?

暗色真的比亮色吸引更多的热量吗？

Submitted by Mary Lyman from New Windsor, New York

由来自纽约New Windsor的Mary Lyman提交

You could say something like that, though "attract" isn't really the right word. If you leave a dark car and a light car out in sunlight, you'll notice after a while that the dark one feels hotter. But it's not that the dark car is actually attracting any more heat. They're both getting the same amount of light and heat from the Sun, but the dark one absorbs most of it while the light one reflects most of it. That's why the light one looks light - the photons are bounced to your eyes instead of disappearing into the material and heating it up.

你可以那样说，但“吸引”并不太确切。如果你把一辆暗色车和一辆亮色车放在阳光下，你很快就会注意到暗色车要感觉更热些。但这并不是因为暗色车吸引了更多的热量。两辆车从太阳得到了同样的光和热，但暗色车吸收了其中的大部分，而亮色车反射了太部分。这也是亮色车看上去亮的原因－光子被反射回你的眼睛而不是消失在材料中并使其升温。

It's different if you talk about things that are emitting light (and heat) of their own. You often hear physicists and astronomers talking about the Sun itself as a "blackbody," which might sound very strange since you can get blinded by looking at it. But that refers to the fact that the Sun reflects very little compared to how much it emits. And it turns out a good emitter is also a good absorber, so in the sense of absorbing light rather than reflecting it the Sun could actually be called very dark.

但对于自身发光或发热的物体来说就是另外一回事了。你可能经常听到物理学家或天文学家们将太阳称之为“黑体”，这听上去很奇怪，因为直接看太阳甚至会你失明。其实这指的是相对太阳发射出的大量光和热，它几乎不反射什么光和热。一个好的发射体也是一个好的吸收体，从吸收而不是反射光的角度来看，太阳确实可以称得上很黑。

It turns out everything emits light, or electromagnetic radiation, at various wavelengths. The hotter it is, the more it emits, and the more of what it emits is at shorter wavelengths. Even cold space is full of microwaves, warmer things like humans emit mostly infrared (which we feel as heat), medium-hot stars like the Sun emit mostly visible light, very hot neutron stars emit x-rays. If you leave that dark car out in the sunlight long enough, it will heat up enough that you can feel the warmth without quite touching it. That's from its emission. It'll never get hot enough to emit visible light, like what it absorbed from the Sun - it has to lose a little energy in the turnaround, which means longer wavelength. But when it's hot it emits infrared, which is the next longer wavelength after visible light.

实际上任何物体都会发射出某个波长的光或电磁辐射。物体越热，发射的就越多，而且更集中于短波长范围。即使是冰冷的空间也充满了微波，稍热些的物体比如人类主要发射红外线（我们可以从中感觉到热），中等热的星体比如太阳主要发射可见光，非常热的中子星则发射X射线。如果你把暗色车停在阳光下足够长时间，你甚至不需要用手摸就能感觉到它的热，这就来源于车子的辐射。车子永远不会热到像太阳一样可以发出可见光，光从车子上出来时已经丢失了一些能量，这意味着更长的波长。因此车子热了后它会发射波长比可见光更长的红外线。

So you could say that dark materials are better at absorbing light, which heats them up - and they're also better at sending the heat back out.

因此，你可以说暗色的材料更易于吸收光并被其加热－也更易于将吸收到的热再送出来。

[ Last edited by Youth on 2006-2-13 at 16:25 ]

碧城仙

谢谢 Youth 斑竹了，我晚上就转到我们服务器上去。

Youth

If you were traveling at the speed of light and turned on the lights what would appear to happen? Also --- Does darkness have a traveling speed?

如果我们以光速行进并打开灯将会发生什么？黑暗有传播速度吗？

Submitted by Sherry Ocasio from Reading, Pennsylvania

由宾夕法尼亚州Reading的Sherry Ocasio提交

My experiments indicate that darkness leaves the refrigerator at least as fast as I can open the door. But darkness leaving is just light moving in, so it's not a surprise that I can't catch it. Light moves very fast.

我的实验表明当我打开冰箱门时黑暗立即离开了冰箱。其实黑暗的离开和光线的进入就是一回事，因此我不能看到它的离开过程再正常不对了，光的传播速度可是非常之快的。

And light always moves at the same speed (if it's not bouncing around in matter), a speed you can never catch up with. If you try, maybe by strapping yourself to a rocket, your friend standing back at the starting line sees you accelerating away and gaining speed. But your friend also sees you getting heavier, in fact infinitely heavy as you approach the speed of light relative to the starting line, so you would need an infinite amount of rocket fuel to get up to light speed with respect to your friend.

光总是以同样的速度进行传播（如果不是在物质内部跳来跳去的话），一个你永远追不上的速度。如果你试图把你自己绑在一枚火箭上，你站在起点的朋友会看到你不断地加速离开。但你的朋友也会看到你变得越来越重，实际上当你无限接近光速时你也将变得无限重，因此你会需要无限的火箭燃油来使自己接近光速（相对于你的朋友）。

But you don't feel yourself getting heavier. In fact, as you see your friend moving backwards very fast relative to you, you see your friend getting heavier. For that matter, you and your friend see each other's watches ticking more slowly; and you see each other shrinking along the direction of travel. This is may seem inconsistent (why don't you see your friend getting lighter?) but it's actually got a simple explanation. It's basically a geometric effect, like perspective. If you walk away from your friend at a more normal speed, you see your friend getting smaller. Your friend sees you getting smaller too rather than bigger.

但你自己并不会感觉到自己变重。实际上，在你离你的朋友越来越远时，你也会看到朋友的体重在增加。你和你的朋友都觉得对方的手表走得更慢，而且双方都在行进方向上被压扁了。这看上去也许有点矛盾（为什么你不会看到你的朋友变轻呢？），但这实际上有个简单的解释。基本上这可以看作一个类似于透视的几何效应。如果你用一般的速度从你朋友边走开，你会看到朋友变得越来越小，而你的朋友也会看到你变得越来越小而不是变大。

Since you can't quite get to light speed, let's ask: What would you see if you were traveling through a room very close to light speed and flipped the switch on your way by? Actually, you would outrun the electric signal on its way from the switch to the bulb. That signal is a propagating electric field like light, and would run at light speed through a perfect conductor. But even the best wires would slow it down significantly on its way to the bulb.

既然你不可能达到光速，我们再来看这个问题：如果你以非常接近光速的速度走进一个房间，然后顺手打开电灯开关，你会看到什么？实际上，你将会比电信号从开关到灯泡还要快。传播的电场就像光一样，如果是在理想的导体中它将以光速传播。但即便是最好的导线也会大大减慢它的传播速度。

So you wouldn't see the lights go on in the room until you were long gone. Looks like darkness wins this one.

因此直到你离开很远了，灯才被点亮。看起来这回合是黑暗赢了。

Youth

It appears that the speed of light isn't constant. At least it can be demonstrated that light can be manipulated by humans at room temperature (up to c times 1.4).

好像光速并不是恒定的。有人证明光能在室温下以1.4倍光速（c）的速度进行传播。

Submitted by Tatheg

由Tatheg提交

You hear a lot of claims like this in the news, and they often come with a claim of breaking the rule that nothing can move faster than the speed of light, "c". But they're not really breaking the speed limit.

你会在新闻中听到很多类似的言论，并且往往都宣称突破了光速c的限制。但实际上他们并没有真正做到。

With any signal you can talk about two speeds called the phase velocity and the group velocity. It's the "phase velocity" that can be faster than c. The phase velocity basically tells you how fast a pure sine wave (with a single frequency) moves. But there's no such thing as a pure sine wave, because it extends infinitely far ahead and behind and lasts forever too.

对于任何信号都有两个速度即相速度和群速度。能够超过光速c的是“相速度”。相速度可以告诉我们一个纯正弦波的传播速度。但现在中并没有这样的波，因为它是无限延伸并且永远存在的。

Realistic signals are made by summing sine waves of lots of frequencies, so that after some finite number of wavelengths they interfere destructively and you get a signal of finite width and duration. When you figure out how fast a real signal (group of sine waves with different frequencies) goes, that's the "group velocity." And that's always less than c.

现实中的信号由各种频率的正弦波组成，因此，在有限的波长数后，它们会干涉相消，你就得到一个有限宽度和有限延续的信号。当你描述一个真实信号（由不同频率的正弦波组成的波群）的传播速度时，你用的是“群速度”，而这个速度是永远小于光速c的。

Then there's the fact that light slows down in matter as compared to vacuum. So if the speed of light in some medium is 0.5c, you can make matter particles go 0.6c, and you can call them faster than light if you want to impress your friends. You do get neat effects like Cerenkov radiation (that lovely nuclear reactor glow), but you're not violating causality or any of the other features of the light-speed limit because they're tied to the vacuum speed c.

另外，光在介质中的传播速度要比在真空中慢。因此如果光在某种介质中的传播速度是0.5倍光速c，你再让这个介质以0.6倍光速c运动，然后你就可以向你的朋友们宣称这个介质比光走得还要快了。类似于契伦科夫辐射（核反应堆所发出的漂亮蓝光就由其产生）的奇妙效应确实存在，但你仍未打破光速的限制，因为所有这些速度最终都不能超过真空中的光速c。

And there's the searchlight effect, where you shine a light on a screen far away and move your arm in a circle. You can technically make the spot "move" faster than c across the screen, but no real physical information is actually carried that fast from one point on the screen to the other.

还有就是探照灯效应，你将一束光照在远处的一个屏幕上然后用你的手画圈。你会看到屏幕上的光斑在屏幕上的移动速度要超过光速c，但其实光斑在屏幕上一点到另一点的过程中并没有传递任何真实的物理信息。

Since Einstein proposed the limit a hundred years ago, we've been looking hard for ways to break it. But every test has made it seem more universal.

既然爱因斯坦在一百年前就提出了这个速度的限制，我们一直在努力地寻找突破它的方法。但每一次实验都更好地证明了这个限制。

[ Last edited by Youth on 2006-2-13 at 20:58 ]

Youth

Why does combining the principle of relativity with the invariance of the speed of light lead one to conclude that no medium is required for light to propagate?

为什么由相对论原理以及光速的不变性可以推导出光不需要介质即可传播？

Submitted by Chipper Q from the USA

由来自美国的Chipper Q提交

First, why do we get light propagating at all?

James Clerk Maxwell's equations for the electric and magnetic fields imply special relativity, though it wasn't obvious at the time (Einstein wasn't born). They led him to predict that there could be waves in the fields, and that these waves were in fact light. Up to then, people thought that electricity, magnetism, and light were unrelated.

麦克斯韦关于电磁场的方程组即暗示了狭义相对论，虽然在那个时候（爱因斯坦还没有出生）还不明显。他由此推测可以在场中产生波，而这些波实际上就是光。直到那时，人们仍然认为电、磁和光是互不相关的。

Qualitatively, the equations mean this: Electric fields point out from positive charges and into negative charges, and magnetic fields curl around currents (moving charges). Those are the main things you notice in a lab, which is full of matter, and historically they were the first to be discovered by playing with bits of wire and amber and so on. If that was the whole story, you wouldn't see much in a vacuum because there aren't any charges or currents.

定性地看，这个方程组意味着：电场是由正电荷指向负电荷，磁场围绕着电流即移动的电荷。这些你在实验室都可以观察到，也是很早就在人们摆弄电线或琥珀时被发现了。如果仅仅是这些，你将不会在真空中看到什么，因为那里没有任何电荷或电流。

But electric fields also curl around magnetic fields that are changing with time, and magnetic fields curl around electric fields that are changing with time, never mind any charges or currents. Those effects were harder to detect, especially the latter which is very weak (but you might know the former as "induction"). Maxwell put them into his equations and noticed that if you set up electric and magnetic fields curling around each other and changing with time just right, they can make waves that will propagate off and sustain themselves even in vacuum where there aren't any charges or currents. And those waves have to travel at exactly the speed of light, which was too suspicious to be a coincidence.

但在随时间变化的磁场周围也会产生电场，在随时间变化的电场周围也会产生磁场，而不需要任何电荷或电流的介入。这些效应更难探测到，特别是后一种更是非常微弱（你也许已经知道前者也被称为“感应”）。麦克斯韦把这些加入他的方程组后注意到如果合理地放置电场和磁场，使它们互相围绕并且适当地随时间变化，就能在没有电荷或电流的真空中产生可以自由传播的波。而且这些波的传播速度正是光的速度，这并不是巧合.

Because you get this mutual curling even in the absence of matter, it doesn't rely on a medium to exist and can be perfectly happy in vacuum. In fact, light and other electromagnetic waves slow down when going through matter (unless it's opaque and they can't go at all).

就算没有任何物质的存在，也能产生这种效应，它并不依赖于任何一种介质，能够在真空中产生。实际上，光和其它电磁波在穿越物质时速度会减慢（除非是不透明物体，那就完全走不动了）。


说明：红色的句子不太好翻译，大家帮忙改正：）

Youth

I have heard that there are particles that travel faster than light emitted in nuclear reactors. Is this true? How can things travel faster than the speed of light?

我听说核反应堆里的粒子可以走得比光还要快。这是真的吗？怎么会有物体的速度比光还要快？

Submitted by Vinayak from India

由来自印度的Vinayak提交

Yes, it's true. In fact it's why radioactive stuff glows.

嗯，这是真的。实际上放射性材料会发光也是因为同样的原因。

But it doesn't violate special relativity, which famously states that nothing can travel faster than the speed of light. Actually the words "in vacuum" should be added to the end of that statement.

但这并不和狭义相对论冲突，众所周知，狭义相对论认为没有任何物体的行进速度可以超过光速。实际上，应该给这个认为加上“在真空中”的限定条件。

Light travels through vacuum at a certain speed, usually called c, and that speed is the absolute limit. But light slows down when it travels through other things. For example, in water the speed is about 3/4 of c. In glass the speed can be a lot less, and in the last few years people have come up with materials that slow light down to almost nothing. Basically light involves a vibrating electric field, and when that field has to push around the electrons in a material its life gets more complicated. It may even die out altogether, as in opaque materials.

光在真空中的传播速度是一定的，一般用c来表示，这个速度是自然界最快的速度。但如果光在介质内传播就会慢下来。比如在水中，光的传播速度大概是c的3/4。在玻璃中就更慢一些，近年来人们已经找到了一些可以让光几乎停下来的材料。光从本质上看就是一个振荡的电场，当这个电场碰到介质中的电子，情况变得更加复杂。它甚至会完全消失，就像在不透明的物体里。

Many nuclear reactor cores consist of radioactive fuel rods dunked in water. The fuel rods emit electrons, all of which have to obey the ultimate speed limit of c. But in water they don't have to obey the local speed limit of 3/4 of c, and some don't. Those fast electrons cause the water to emit light, which comes out more or less along the electron's direction of travel. This effect is called Cherenkov radiation, and it's what makes that eerie blue glow you see in a reactor core (preferably from a safe distance). The electrons eventually slow down, which is why the glow isn't seen too far from the fuel rods. The glow occurs in air, too, but it's a lot fainter because the speed of light in air is almost c and fewer electrons are emitted that fast.

许多核反应堆的核心部分就是由浸在水中的放射性燃料棒组成。这些燃料棒会放射电子，所有的电子都必须遵循c这个最终的速度限制。但在水中它们的速度可以却超过c的3/4。那些快速的电子会导致水沿着电子的行进方向发出光。这个效应被称之为契伦科夫辐射，就是它产生了你从核反应堆核心处看到的奇异的蓝光（最好站在安全距离以外）。电子会逐渐慢下来，这就是蓝光看上去离燃料棒不太远的原因。这种发光现象在空气中也会产生，但是要暗淡得多，因为光在空气中的速度和c非常接近，很少有电子的速度能到这么快。

Cherenkov radiation isn't just pretty, it's useful: Many physics experiments involve looking for charged particles traveling almost as fast as c. It can be hard to track those particles directly, but you can set up a tank of water and look for the Cherenkov radiation. You can even use it to work out how energetic the particles are and which direction they are traveling.

契伦科夫辐射并不只是美丽而已，它还很有用：许多物理实验都试图寻找接近光速c的带电粒子。直接追踪这些粒子比较困难，但你可以架设一个水箱然后寻找契伦科夫辐射。你甚至可以使用它来研究粒子的能量高低以及它们的行进方向。

Of course, the glow is also useful as a "hands off" warning.

当然，这种蓝光还是一个有用的“别动！”警示牌：）

Youth

Do white holes exist? If so, how are they formed? Does time exist in white holes, since they are inverse black holes?

白洞存在吗？如果存在的话它们都怎么形成的？时间在白洞里面是否仍然有效，既然它们和黑洞是相反的？

Submitted by Chanuka from Srilanka

由来自斯里兰卡的Chanuka提交

A white hole is indeed the inverse of a black hole, but we don't expect to find one in real life.

白洞确实是和黑洞相反的，但很可能在现实中是不存在白洞的。

Within a year of Einstein developing his general theory of relativity - and saying the equations were too hard to be solved - Karl Schwarzschild found the first solution describing what we now call a black hole. This was the simplest version of a black hole: very symmetric, sitting in isolation for all eternity, no matter falling in, not rotating or changing with time. It became apparent that it had a surface (which got named an event horizon) which you could get into but not out of. Light couldn't get out either, which is why the whole thing eventually got named a black hole.

在爱因斯坦提出广义相对论后的一年内－那些方程式还很难解开的时候－卡尔·施瓦西给出了第一个解，描述了现在人们所说的黑洞。这是一个最简单的黑洞版本：高度对称，永远孤立地存在着，没有物质的进入，既不旋转也不随时间改变。很显然它有一个只能进去却无法出来的表面（后来称其为视界）。即便是光也无法逃脱，这也就是它得到黑洞这个名称的原因。

Later, when people developed more sophisticated mathematical tools to look at such solutions, they realized that wasn't the whole story. The structure of space and time in this simple case had to have what is called time reversal symmetry, which means if you let time run backwards everything should look the same. Therefore if there is a horizon extending into the future which light can enter but not leave, there has to be another horizon extending into the past which light can leave but not enter. That would look like the inverse of a black hole, so they called it a white hole although it's really the extension of the black hole into the past. (And it gets even weirder: There is what looks like another universe inside the horizon, although the meaning of "inside" becomes distorted.) Time would exist inside a white hole, although since you couldn't get in you'd have to be born in there to measure it.

后来，当人们有了很复杂的数学工具来分析这些方程式，他们发现了更多。在这个简单的情形下时空结构必须具备时间反演对称性，这意味着如果你让时间倒流，所有一切都应该没什么两样。因此如果在未来某个时刻光只能进不能出，那过去一定有个时刻光只能出不能进。这看上去就像是黑洞的反转，因此人们称之为白洞，虽然它只是黑洞在过去的一个延伸。（更奇怪的是：在视界里面似乎应该还有一个宇宙，虽然这里用“里面”可能不太确切。）时间在白洞里面是存在的，但既然你不能进去，那你只有出生在里面才能知道了。

But in real life we don't expect to see white holes, because real black holes are more complicated than this simple solution to the equations of general relativity. They didn't exist infinitely far back in the past, but rather formed some finite time ago by collapsing stars. This ruins the time reversal symmetry, so if you look at past history you don't get the white hole part of the solution but rather a black hole forming towards the end of the stellar collapse.

但在现实中，白洞可能并不存在，因为真实的黑洞要比这个广义相对论的简单解所描述的要复杂得多。他们并不是在过去就一直存在，而是在某个时间恒星坍塌后所形成的。这就破坏了时间反演对称性，因此如果你顺着倒流的时光往前看，你将看不到这个解中所描述的白洞，而是看到黑洞变回坍塌中的恒星。

Youth

archive里面的应该都翻译好了，大家帮忙修改一下：）

碧城仙

请看 http://boinc.equn.com/einstein/ask/archive/index.htm ，131 楼前的都已添加。老版本的几个文件均保留(文件名及文件内容)，老版本归档问题首页修改为 http://boinc.equn.com/einstein/ask/archive/index2.htm ，相同的六个问题页面是直接“另存为”的。http://boinc.equn.com/einstein/ask/index.htm 页面的我随后更新。

另外，官方网站上的右下角的图片都换了，因为 2005 年世界物理年已经过去了。这个不急，我有空慢慢改，一页一页的改。

Youth

http://boinc.equn.com/einstein/ask/archive/relativity-qa2.html就是上面的132楼

才发现ask physicis首页的q&a是没有在archive里的，这个我先认领了：）