A Trick For Bending The Laws Of Physics | KUOW News and Information

# A Trick For Bending The Laws Of Physics

Mar 24, 2014

You see it on T.V. all the time: cops interrogating a suspect in a cramped room while prosecutors watch from the other side of a one-way mirror.

The prosecutors can see in, but the suspects can’t see out.

Those mirrors are specially coated and lighting is used to create the one-way illusion.

Now engineers at the University of Texas in Austin have figured out how to create a one-way illusion with sound.

From the Here & Now Contributors Network, Matt Largey of KUT in Austin explains.

#### Reporter

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JEREMY HOBSON, HOST:

Well, staying in Texas now. You see it on TV all the time: cops interrogating a suspect in a cramped room while prosecutors watch from the other side of a one-way mirror. The prosecutors can see in, but the suspects can't see out.

(SOUNDBITE OF TV SHOW, "LAW & ORDER: SPECIAL VICTIMS UNIT")

HOBSON: Well, those mirrors are especially coated and lighting is used to create the one-way illusion. But now engineers at the University of Texas, Austin have figured out how to create a one-way illusion with sound. From the HERE AND NOW Contributors Network, KUT's Matt Largey explains.

MATT LARGEY, BYLINE: So waves of energy behave in very specific ways. That's just the way it is. And the behavior we're talking about here, it has to do with how waves move between two points. It's called reciprocity.

ANDREA ALU: It actually is a general law of physics for which most materials have what is called time reversal symmetry.

LARGEY: That's Andrea Alu. He is an engineering professor UT Austin. Anyway, this time reversal symmetry.

ALU: That means that if I can send a wave down a path, then the wave should be able to come back.

LARGEY: Right. So I'm on one side of a space. You're on the other side. I shout to you. Hey. You hear it, and you can shout back. Hey. And I hear it. The sound waves can move through the air in both directions. It's reciprocal.

(SOUNDBITE OF MUSIC)

ALU: Most waves in most materials like sound, light, radio waves, they (unintelligible).

ROMAIN FLEURY: But there are a lot of situations in which you don't like this property.

LARGEY: That's Romain Fleury.

FLEURY: I am an international graduate student in electrical computer engineering.

LARGEY: And he wanted to find a way to eliminate reciprocity.

FLEURY: So what we want to do is isolate sound to create a one-way road for sound.

LARGEY: OK.

FLEURY: We want that when somebody talk, for instance, I talk to you. It's just a sound test, right?

LARGEY: Yeah. Yeah.

FLEURY: OK. You can hear me. So I don't know. But if you talk, I don't want to hear you.

LARGEY: Oh.

FLEURY: It sounds rude, but that's what we are trying to do.

LARGEY: Simple concept, but it means breaking a fundamental law of physics. And that is complicated.

FLEURY: It's a tough problem because physics doesn't allow that at all. It's against it.

LARGEY: It's hard to argue with physics too.

FLEURY: Yeah. It's very hard.

(LAUGHTER)

LARGEY: Fleury and his team worked on this problem for months. They tried all kinds of things. And then...

FLEURY: One night...

LARGEY: In the lab, Fleury tried something new.

FLEURY: And it just worked right away, like first simulation. I think the simulation took two minutes.

LARGEY: Here's how he did it. He built a ring.

FLEURY: Yeah, it's a ring cavity, so it's carved in an aluminum block.

LARGEY: So it's like the inside of a hollow aluminum donut.

FLEURY: And then this ring is connected to three pipes.

LARGEY: OK. Where are my pipes?

(SOUNDBITE OF BEEPING)

LARGEY: All right. There we go. Now, they're sticking off the ring in a kind of triangle shape. Three pipes.

UNIDENTIFIED MAN #2: (Singing) One.

UNIDENTIFIED MAN #3: (Singing) Two.

UNIDENTIFIED MAN #4: (Singing) Three.

UNIDENTIFIED MEN: (Singing) Three pipes.

LARGEY: Yes, exactly.

FLEURY: And you can send sound from one pipe, it goes into the ring and leaves by another pipe.

LARGEY: Now, by itself the sound would go in one pipe. Let's say pipe one.

UNIDENTIFIED MAN #2: (Singing) One.

LARGEY: It travels around the ring to come out pipes two and three.

UNIDENTIFIED MAN #2: (Singing) Two.

LARGEY: Right?

FLEURY: Yeah. It would be split.

LARGEY: But then Fleury did something that sounds super simple. He put a fan in the ring.

FLEURY: You put computer fans inside.

LARGEY: Just a regular, off-the-shelf fan you'd find in a computer.

FLEURY: And you rotate the air inside, along the ring.

LARGEY: At a very specific speed and angle. Now try pipe one again.

UNIDENTIFIED MAN #2: (Singing) One.

FLEURY: It goes into the ring and leaves by another pipe.

LARGEY: Out of pipe two.

UNIDENTIFIED MAN #2: (Singing) Two.

FLEURY: But if you send sound back from this other pipe...

LARGEY: Back into pipe two.

UNIDENTIFIED MAN #2: (Singing) Two.

FLEURY: ...it doesn't go back to the first one. It goes to the third one.

UNIDENTIFIED MAN #2: (Singing) Three.

LARGEY: And only the third one. The sound moves one way between the pipes now.

ALU: I have one, two, but not two, one. I have two, three, but not three, two. And I have three, one, but not one, three.

LARGEY: They broke the rules, or at least they bent the rules. A very specific way of moving the air with the fans - what they call angular momentum bias - allows sound to move one way, but not the other. In other words, nonreciprocally.

ALU: Essentially the signals always flow in the way contrary to the airflow spin.

LARGEY: Pretty cool stuff, but also it's like, how simple is that?

DR. STEVEN CUMMER: I know what you mean because you're right. What they built is deceptively simple looking.

LARGEY: That's Dr. Steven Cummer. He studies wave manipulation at Duke University.

CUMMER: But you have to give them a lot of credit that it's a very simple structure, but it also does exhibit some very fundamental physics in it. And nobody had demonstrated it before or thought to use it to demonstrate something as fundamental in wave physics as nonreciprocity.

LARGEY: As simple as it sounds, no one had ever figured it out before. So the question is, now that they've figured it out, what do you do with it?

FLEURY: So it could be like the acoustic equivalent of a one-way glass. So you can hear somebody through the glass, but they cannot hear you.

LARGEY: Better noise cancellation is one potential. But people who work on this stuff are pretty pumped about this discovery for another reason - because you might be able to apply this basic concept to all kinds of waves, not just sound. Maybe things like radio waves or...

ALU: We may be able to extend these same concepts to light.

LARGEY: And that would be a big deal because super fast computer chips that use light instead of electrical pulses are already being developed. And a cheap way to make a one-way path for light in those chips would be a huge leap for that technology. And it might help get them to market faster, eventually. For now, that sound that you aren't hearing could be the harbinger of big things to come. For HERE AND NOW, I'm Matt Largey.

HOBSON: The kinds of stories you can only really do on the radio. Transcript provided by NPR, Copyright NPR.