超光速的中微子？快速计算

译者 wizky

A number of people on twitter and elsewhere (including my grad student, Austen) alerted me to an interesting story making its way across the interwebs. A number of sources have reported on a press release put out by the OPERA collaboration.

Twitter和其他地方的许多人（包括我的研究生，Austen）提醒我一个有趣的故事正在网络上传播。许多媒体对OPERA（Oscillation Project with Emulsion-tRacking Apparatus）合作项目发出的通讯稿作出了报道。

Here’s the basic idea: OPERA has a detector in Gran Sasso Italy, about 730 km from the LHC. The LHC produces neutrinos in abundance, and neutrinos have a few important properties:

这里是基本理念：OPERA在意大利大萨索峰（Gran Sasso)有一个探测器，距LHC（Large Hadron Collider，大型强子对撞机）大约有730km。LHC产生大量中微子，而中微子有几条重要性质：

1. They are very nearly massless and thus, even at moderate energies, we’d expect them to travel essentially at the speed of light.

1、它们的质量非常接近零，因此，即使在中等能量下，我们也希望它们以光速传播。

2. They can “oscillate” or change identities, which means that the mu neutrinos produced at the LHC can turn into tau neutrinos which the OPERA detector is designed to measure.

2、它们能“振动”或改变形态，这意味着LHC产生的μ子中微子可以转变为OPERA探测器设计用来测量的τ子中微子。

3. They are very weakly interacting, which means that they can pass through solid earth unimpeded.

3、它们之间的相互作用非常弱，这意味着它们能畅通无阻地穿过坚固的土地。

Light should make the journey from the LHC to Gran Sasso in about 3 ms, but according to the OPERA collaboration press release, they are detecting the neutrinos as making the journey in about 60 ns less than expected.

从LHC到大萨索峰，光传播需要3ms，但根据OPERA合作项目通讯稿所述，他们探测到中微子比预期早到达目的地60ns。

In other words, they are claiming that, unless there is some hitherto undetected systematic, neutrinos are traveling faster than light.

换言之，他们声称，除非有迄今未被探测到的系统误差，中微子传播速度超过了光速。

A couple of caveats:

两个说明：

The actual paper doesn’t seem to be up on the arXiv yet, so I don’t know exactly what their measurement is.

实际论文好像还没有出现在arXiv上（译者注：arXiv是一个收集物理学、数学、计算机科学与生物学论文预印本的网站），所以我不知道究竟他们的测量是怎样的。

Even once it is, I’m a theorist, not an experimentalist, so I’m unlikely to be able to identify the systematics.

即使论文贴出来，因为我是一个理论物理学家，而不是一个实验物理学家，所以我也不可能识别出系统误差。

That said, I am pretty darn certain that this result is flawed. Neutrinos have mass, which is why they oscillate in the first place, so if it turned out that a massive particle could travel faster than light (and it wasn’t some sort of issue with not correcting for general relativistic effects or something like that), that would pretty much overturn special relativity.

也就是说，我相当确定这个结果是有缺陷的。中微子有质量，这即是他们起初振动的原因，所以如果结果表明大量粒子能够超光速（这不是那种未修正广义相对论效应或其他类似物的问题），这将在很大程度上推翻狭义相对论。

More to the point, I have a simple calculation that makes me extremely skeptical.

更重要的一点，我做了些简单计算，使得我产生了极度的怀疑。

Remember that the neutrinos are supposed to beat light by about 60 ns over a travel time of 3 ms. That’s

记住在一段3ms的传播时间中，中微子应当比光子快60ns。即：

Now consider a supernova explosion. In particular, consider Supernova 1987A.

现在考虑一次超新星爆发。特别是超新星1987A。

This was an explosion about 160,000 light years from earth. The thing is, the neutrinos and the photons from the explosion reached us at almost exactly the same time. In the cause of intellectual honestly, I need to point out that the neutrinos were detected first, by about 3 hours, but this is because the envelope of the explosion was optically thick and the photons had to bounce around a while, while the neutrinos just streamed right out.

在这里输入译文

But how much of a delay between neutrinos and photons would we expect if the OPERA result applied?

In other words, if the effect really were this large, we would have seen the neutrinos from SN 1987A way back in 1984. Yeah, we would have noticed that.

换言之，如果（OPERA得出的结果）效应真的如此之大，我们会早在1984年就观测到来自超新星1987A的中微子。是的，我们会注意到的。

I don’t want to be too glib, however. There are a couple of key differences:

无论如何，我不想太油嘴滑舌。有两个关键区别：

1. The neutrinos detected from 1987A were (anti) electron neutrinos, not tau neutrinos. However, since neutrinos oscillate from one flavor to another, I’d be surprised if this was the key difference.

1、  探测到的来自超新星1987A的中微子是（反）电子中微子，而不是τ子中微子。然而，因为中微子从一种类型振动到另一种类型，如果这是关键区别的话，我会对此感到惊讶。

2. The energies are quite different. In 1987A, neutrino energies were typically a few 10′s of MeV. The neutrinos measured by OPERA are a factor of 100 higher. It could very well be that this is a sensitive function of energy.

Of course, since the expectation is that neutrinos should NEVER travel faster than light, there’s no way to compute what we’d expect.

当然，因为预期是中微子绝不会超光速，所以没有办法计算出我们所期望的（结果）。

I, for one, am not going to hold my breath.

-Dave

-Dave

Edit:

编辑:

I posted this before getting my hands on the actual paper. In the cause of fairness, I need to mention that the authors of the paper talk about the constraints from 1987A themselves. They rightly refer to this result as a low-energy limit. As you can see from above, I find it unlikely that the differences between the high and low energy limit will have such a huge effect.

我在得到实际论文之前发表了这篇文章。为公平起见，我需要提出，论文作者本身谈到了超新星1987A的约束条件。他们正确地参考此结果作为一个低能量极限。正如你从上文中所看到的，我发现高能量和低能量极限之间的区别不可能产生如此巨大的效应。