Weyl points, first predicted in 1929, observed for the first time | Hacker News Reader

Weyl points, first predicted in 1929, observed for the first time

newsoffice.mit.edu

98 points_pius21 comments

For those interested (and with subscriptions or institutional access), here are the abstracts for the two papers, the first from the MIT group and the second from the Princeton group:

http://www.sciencemag.org/content/early/2015/07/15/science.a...

The MIT group's paper appears to be on arXiv: http://arxiv.org/abs/1502.03438 (albeit with a somewhat different abstract; perhaps a pre-acceptance version). Unfortunately the Princeton group's paper doesn't appear to be posted on arXiv.org as of right now.

The Princeton group's has a different title, and was posted March 9 (I downloaded it about a month later so pulled it off my hard drive when I saw this comment)

http://arxiv.org/abs/1503.02630v1

Thanks! I had only done quick searches on the titles.

I'm confused. It sounds like they though neutrinos were Weyl points, and neutrinos are real particles. But it also sounds like this experiment created quasiparticles. So are Weyl points still a proposed real particle? Or was it only ever theorized to be some kind of emergent phenomenon? Or did they really create a new type of real particle?

You mean Weyl _fermions_. Weyl fermions obey the Weyl equation, and Majorana fermions the Majorana equation. Majorana fermions are their own anti-particles, while Weyl fermions are not. They are cousins of the Dirac equation (think about how a Dirac point and a Weyl point are related). There is still some debate about whether neutrinos are Weyl or Majorana fermions.

"I have no doubt that in reality the future will be vastly more surprising than anything I can imagine. Now my own suspicion is that the Universe is not only queerer than we suppose, but queerer than we can suppose." - JBS Haldane

This is almost certainly true when you consider almost nobody understands what as humanity we actually know right now about the universe.

The chance that the smartest person in the world is just smart enough to understand exactly how the universe works is very unlikely considering understanding of the universe has never been under evolutional selection. We are going to have to wait for the Singularity to tell us how it really works :)

I misspoke here, I meant Dirac or Majorana fermions.

It's one of those cases where condensed matter physicists study a system which satisfies equations known from (more) fundamental theories. Other recurring examples are "black holes" in the lab [1] and "AdS/CFT" (of string theory fame) in solid state and quark-gluon plasma physics [2].

That aside, there is a mathematically well-defined sense in which Weyl fermions are the most "fundamental" fermions: the other kinds you hear about, Dirac and Majorana, can all be written as combinations of Weyl fermions. To make a Dirac, you take two Weyls of opposite helicity and couple them with a common mass term. To make a Majorana, you impose an additional algebraic condition on the two Weyls (take the complex conjugate of Weyl #2, reshuffle it a bit, and you get Weyl #1).

The most significant thing here is the mass term. When you look at the equations, it's immediately obvious that what the mass term does is mix the two Weyls in a Dirac: even if you prepare a pure state of one Weyl, as soon as you set the clock ticking, the mass term will turn it into a mix of two Weyls... unless the mass is exactly 0.

So a less mystifying way to describe the achievement of "making Weyl fermions" is simply "making massless fermions".

[1] http://www.nature.com/nphys/journal/v10/n11/full/nphys3104.h...

[2] https://en.wikipedia.org/wiki/AdS/CFT_correspondence#Applica...

>That issue of scalability in optical systems is “quite fundamental,” Lu says; this new approach offers a way to circumvent it. “We have other applications in mind,” he says, to take advantage of the device’s “optical selectivity in a 3-D bulk object.” For example, a block of material could allow only one precise angle and color of light to pass through, while all others would be blocked.

Putting my science fiction hat on, if that meant you could build a light-absorbing material you could potentially have (for instance) camouflage that reflected on a certain wavelength only known to your friendly forces. Or hidden assets that require a filter to see - hidden keypads, hidden instructions, that kind of thing.

It's already easy enough to make things that only reflect at specific angles. Filtering by exact wavelength doesn't add a lot; it leaves your message visible in sunlight and incandescent light, at the very least.

Plus camouflage isn't very useful in the first place if you have wide-spectrum cameras.

I don't understand what you mean about 'requiring a filter' to see something.

Sorry, I know you're trying to come up with fun ideas, but I think you're going in the wrong direction.

Lol. Duh. Multispectral light completely defeats the purpose. I'm an idiot.

Message could be written on a background of a slightly different frequency. Multispectral light would't differentiate. Similar to a color-blindness test.

I think single mode laser efficiency would be a better science fiction win, less loss so more power will less cooling. Perhaps a 100kW beam out of a backpack laser?

That's an excellent idea.

No way! I read about chaos theory butterflies. But now you're saying butterfly wings contain whales? Gnarly, man.

On a more serious note, it's good to see Zhejiang University from China in there. Their library also provides the only scanned copies online of some important ancient Chinese historical sources. Except for the big name unversities in China (Tsinghua, Fudan, etc.), this is a university to watch.

Link for the scanned copies?

https://archive.org/stream/06061161.cn#page/n0/mode/1up

Thanks

For a moment, I confused Weyl points with Ley lines and was shocked by disbelief.

Ley lines: https://en.wikipedia.org/wiki/Ley_line

I'm curious about the impact this has on lasing.

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