How Infrared Lasers Could Improve the Future of Gravitational Wave Detection

Welcome to the Quarkside Quantum Physics Podcast, an exploration of the fundamental structure of reality, where quantum laws govern matter, energy, and information. Here, uncertainty is a feature, not a flaw, and understanding begins at the smallest scales.

Imagine trying to feel a vibration coming from millions of light years away.

Which is just, I mean, it's almost impossible to even conceptualize. Right.

Not hearing a sound, not seeing a flash of light, but actually feeling a physical vibration from across the universe.

Yeah. From entirely across the cosmos.

The sheer audacity of even attempting that is wild, but humanity is actually doing it. We are building machines that measure changes in distance that are a thousand times smaller than the width of a single proton.

It really is one of the most absurd engineering feats we've ever attempted.

So today we are going to explore how we are upgrading our ability to listen to the universe's most massive collisions, like black holes, neutron stars, all of it.

Exactly.

And our mission is to figure out how solving this incredibly delicate, bizarre engineering puzzle right here on Earth is going to let us hear the oldest black holes out there.

It's a huge step forward for astrophysics.

Okay, let's unpack this scale first because, I mean, the human brain just isn't built to comprehend something a thousand times smaller than a proton.

No, not at all. We just don't have a frame of reference for it. Right.

So if you really think about it, measuring that kind of distance is like standing in your backyard, looking up at the night sky, and trying to measure the exact width of a microscopic dust mite that is just sitting.

On a rock on the moon.

That is, yeah, that is actually an incredibly accurate analogy. It is a staggering scale to try and wrap your head around.

It just sounds like sci-fi.

It does. But to ground your analogy in the physical reality of how we are actually pulling this off, you have to picture the hardware involved.

Okay.

We are talking about massive mirrors. They weigh 40 kilograms, which is about 88 pounds each.

So these are huge.

Huge. These aren't, you know, your standard bathroom mirrors. They are heavy, highly engineered cylinders of ultra pure glass. Wow. And they sit exactly four kilometers, so two and a half miles apart, from each other. inside these incredibly long vacuum tubes.

So you have these 88-pound glass cylinders, two and a half miles apart, just perfectly suspended in pitch darkness.

That's the setup. They act as the ultimate cosmic tripwire.

A tripwire?

Yeah.

I like that.

Yeah, they're suspended there in a complete vacuum, totally isolated from the outside world, just waiting for a ripple in space-time itself to pass through the Earth and jostle them.

And these ripples come from massive collisions, right?

Exactly. When massive objects in deep space, like two black holes, collide, they send out gravitational waves.

Right, which aren't sound waves.

No, not at all. They are actual physical ripples that stretch and squeeze the very fabric of space as they travel outward at the speed of light.

And by the time those ripples travel for like a billion years and finally reach Earth, that stretching is unimaginably tiny.

It is.

Which is why we need the tripwire.

Makes sense.

We bounce a main science laser back and forth between these 88-pound mirrors over that 2.5-mile stretch. Uh-huh. Because the speed of light is constant, we know exactly how long it should take the light to make that round trip.

Right. The timing is locked in.

Precisely. So, if a gravitational wave passes through the Earth, it stretches the space between the mirrors by that fraction of a proton.

And the laser catches that change.

Yes. The distance changes. The laser light's travel time shifts ever so slightly. And the tripwire is triggered. We've essentially heard the collision. Okay.

I have to admit, though, I'm lost on one thing right out of the gate.

Sure. What's that?

If these detectors are already built and they are already successfully acting as this cosmic tripwire measuring fractions of a proton, why are we talking about upgrading them? It sounds like they already do the impossible.

It's a fair question. It really comes down to reach.

Reach, like how far out we can hear.

Exactly.

Mm-hmm.

Right now, we can hear the loud, relatively recent collisions in our immediate cosmic neighborhood. Okay. But to listen further out into the universe, you know, to hear fainter, smaller, or much more ancient collisions from the very dawn of the cosmos, the detectors need to be exponentially more sensitive. Ah, I see. So the next evolution involves swapping out the current mirrors for new ones. They feature what is called a low-noise crystalline mirror coating.

No way.

When you say low noise, I assume you don't mean acoustic noise, right? Because they are in a vacuum tube. There is no sound.

You're right to clarify that.

Yeah.

When physicists talk about noise in this context, they mean thermal noise, heat.

Heat makes noise?

Well, at a microscopic level, the atoms inside any material are always jittering around.

Just constantly vibrating.

Exactly. That jittering is thermal energy. So if the atoms on the surface of your mirror are jittering, That creates microscopic fluctuations on the mirror's surface itself.

Oh, I get it. So to a laser trying to measure a proton-sized change in distance, the jittering atoms look exactly like a gravitational wave.

That's it. It completely blurs the signal.

So the mirror is basically vibrating itself and drowning out the universe.

That is a very good way to look at it. So the new crystalline coatings are fundamentally structured differently.

How so?

Their atomic structure is much more rigid and ordered. That drastically reduces the internal thermal jittering. It essentially quiets the mirror down so we can hear those fainter signals.

Okay, so we swap out the glass for the new crystalline mirrors, and boom, we can hear the edge of the universe.

If only it were that easy.

Right. I know it's not that simple, because there is this bizarre obstacle standing in the way of the upgrade.

There really is.

Because before we can even turn on the main science laser to listen to the cosmos... These 88-pound mirrors have to be completely at rest and perfectly locked in place, right?

They do. And this is where the engineering gets incredibly punishing.

Punishing is a good word for it.

I mean, you can't just hang an 88-pound mirror on a wire and expect it to stay still. The Earth is a terribly noisy, vibrating place.

Right, like a truck driving on a highway five miles away would shake the ground enough to ruin a measurement that small.

Oh, it is actually much worse than a truck. Really? Yeah, even the ocean waves crashing on a shoreline hundreds of miles away from the observatory create enough continuous seismic vibration to completely blind these mirrors.

Hundreds of miles away. That is insane.

It is. So we have to actively counteract that constant rumbling.

And how do we do that?

The system used to freeze these mirrors in place against the movement of the Earth is called arm-length stabilization, or we just call it the locking system.

Wait, so if the ground beneath it is always moving, how does that actually work? How do you lock something that heavy?

It relies on a hyper-precise feedback loop. We use a secondary laser, specifically a green laser light, that bounces off the mirrors to constantly measure their exact position.

Okay, a green laser.

Got it. And that green laser feeds data into a computer thousands of times a second.

Wow, so it's constantly monitoring.

Continuously. The computer calculates exactly how the mirror is swinging due to seismic noise. And then it sends signals to tiny magnetic actuators pushing against the mirror. Oh, wow.

So the computer is actively pushing back against the ocean waves in real time.

Exactly. The actuators nudge the mirror microscopically to perfectly cancel out the Earth's vibrations.

So it brings it to a standstill.

Right. Bringing the mirror to a complete dead stop and holding it at that exact four kilometer distance.

Which brings us to the ultimate paradox of this upgrade.

The big bottleneck.

Yeah, so we have this locking system that relies on a green laser to act as the sensor, right?

Correct.

And we have these incredible new super sensitive crystalline mirrors. But the problem is that these new crystalline coatings actually absorb the green laser light.

They do. The new coating is completely incompatible with green light.

Wait, so if the mirror absorbs the light instead of reflecting it, does the whole stabilization process just fall apart?

It completely falls apart.

Okay, let me try to visualize this. Is this like trying to balance a heavy spinning plate on a rigid stick, but suddenly your stick turns into a wet noodle because the plate just absorbed all its strength?

That's a great analogy, actually. What's fascinating here is that the stabilization system needs the green light to bounce back so the computer can feel where the mirror is.

Like the rigid stick. Right.

If the crystalline coating absorbs the green light, the system gets no feedback. The stick turns to mush.

The computer goes blind.

Yes. The computer goes blind. The actuators don't know when to push. and the mirror just starts swinging wildly from the seismic noise.

So the whole thing is just useless?

Well, it actually gets worse.

Worse? How can it be worse than useless?

Because when the mirror absorbs that laser light, the light doesn't just disappear. That energy is converted into heat. Oh, no.

And we literally just talked about how heat is the ultimate enemy here.

Exactly. The localized heat from absorbing the green laser causes the crystalline mirror coating to physically expand and warp.

So the laser's melting it?

Essentially, yes. It melts a microscopic dent into the surface of your perfect mirror, which completely ruins that proton-level precision.

Oh, wow.

So the pursuit of a better, quieter mirror accidentally blinded the machine's ability to hold itself still.

It did. We built a better ear. But in the process, we broke the neck that holds the ear steady.

That is incredibly frustrating.

It is, but... it's common in cutting-edge science. We often think of astrophysics as this purely theoretical, high-minded discipline where people are doing complex math on chalkboards to solve the mysteries of spacetime.

Right, like Einstein sitting in a room thinking about gravity.

Exactly. But in reality, it's not. Unlocking the cosmos almost always comes down to fixing a hyper-specific localized materials problem right here on Earth.

So our ability to map the universe is currently stalled by how a specific crystal reacts to green light.

That was exactly the bottleneck.

Okay, so since we can't use the green light with the new mirrors because they just eat it up and warp, how in the world do we keep these 88-pound mirrors perfectly still across two and a half miles?

Well, a team of researchers led by Dr. Jian Liu at the University of Western Australia, they're part of the OSGRAV team, they figured out a brilliant workaround.

What did they do?

They realized that if green light is the problem, they needed a color of light that the new crystalline coatings would completely ignore.

Something that just bounces right off.

Right. A light that would bounce off the surface without leaving any energy or heat behind.

And what color was that?

They found it in the infrared spectrum. They developed an entirely new locking scheme using infrared light.

Specifically, I think it was 1596 nanometers, right?

That's exactly right. 1596 nanometers. To the new crystalline mirrors, this specific infrared wavelength is perfectly fine. The coating doesn't absorb it at all. It reflects it beautifully.

Okay, so problem solved. The infrared laser bounces back perfectly, feeds the computer, and you get your arm length stabilization.

Well, not quite. Why not?

I know what people listening are thinking right now because I'm thinking it too.

Let's hear it.

If the new mirrors reflect infrared perfectly and the main science laser, the one that actually listens for the gravitational waves, is also an infrared laser, why is this complicated? Why not just have the main laser lock the mirrors itself?

The very logical question. The short answer is that the main science laser operates at a completely different infrared wavelength.

Oh, really?

Yeah. It operates at 1064 nanometers.

Okay, but it's still infrared.

It is. But you can't just use it to lock the mirrors from a dead stop. Because the main laser is incredibly powerful.

Like too strong to use for a delicate job.

Precisely. If the mirrors are swinging wildly and you hit them with the full power of the main science laser, it creates a chaotic optical resonance that actually pushes the mirrors around and makes the swinging worse.

Oh, wow. Is that like trying to stop a swinging pendulum by blasting it with a fire hose?

That is exactly what it's like. You need a gentler secondary laser, the locking laser. to grab the mirror, slow it down, hold it steady, and then hand control over to the powerful main laser once everything is perfectly still.

But wait, so we have two different wavelengths. The new locking laser is 1,596 nanometers, and the main science laser is 1,064 nanometers.

Correct.

If mismatched frequencies are the problem, why not just build a main science laser that also runs at 1,596 nanometers so they match?

Well, the main 1064 nanometer laser is a highly customized, multi-million dollar piece of hardware.

Ah, it's a money issue.

Money and infrastructure. The entire observatory's optical layout, all the sensors, the vacuum interfaces, it is all perfectly tuned to that specific 1064 nanometer light.

So you'd have to rebuild the whole place.

Exactly. Rebuilding the entire core of the billion dollar observatory just to match a new locking laser wasn't an option. we had to adapt the locking mechanism to fit the existing infrastructure.

Okay, so what does this all mean in practice? We have two different lasers running at two completely different frequencies, and they need to seamlessly pass control of an 88-pound mirror suspended two and a half miles away. I mean, this sounds like the most stressful relay race imaginable.

The relay race analogy is a really apt way to describe the handover process.

Let's walk through this race. So, The infrared locking laser runs the first leg, getting the mirror perfectly stabilized.

Right.

Then it has to pass the baton to the main measuring laser so the actual listening can begin.

Exactly.

But if they are running at different speeds, or in this case different frequencies, they are going to drop the baton. They can't communicate, right?

Right. The computer won't be able to read them both simultaneously to make the switch. The mirror will lose its lock and will start swinging all over again.

So how do they keep from dropping it? I read that the researchers used special crystals to somehow translate the frequencies.

They did.

Do these crystals act as the synchronization zone on the track? You know, that painted box where both runners have to be moving at the exact same speed so the handoff is perfectly smooth?

They absolutely serve as that synchronization zone. If we connect this to the bigger picture, the researchers used special nonlinear crystals to physically manipulate the light itself.

How does that work?

They take a small portion of the 1,596 nanometer locking light and a small portion of the 1064 nanometer main laser light, and they run both of those invisible beams through these special crystals. Okay. The crystals combine and convert those different frequencies to produce green light.

Wait, hold on. I need to push back on this.

Go ahead.

Converting frequencies. You mean taking two invisible infrared lasers and physically turning them into a visible galaxy?

Yes, that's exactly what happens.

That sounds like alchemy. How does a crystal just change the fundamental nature of light?

It does sound like magic, but it's an entire field of physics called nonlinear optics.

Okay, I'm listening.

Think of the crystal as an optical mixing board. Under normal conditions, light beams just pass right through each other.

Right.

But inside the very specific atomic lattice of a nonlinear crystal, the incoming photons of light are forced to interact.

Oh. Interesting.

The crystal essentially takes the energy of the incoming infrared photons, combines them, and emits new photons that have a higher energy.

And higher energy means?

Higher energy means a shorter wavelength, which shifts the invisible infrared light right up into the visible green spectrum.

Wait, wait, wait. Green light? But we just established like 10 minutes ago that the whole problem started because the new emitters hate green light. They absorb it and warp.

They do. But here's the genius of it. This conversion into green light isn't happening out in the main vacuum tubes where the precious crystalline mirrors are hanging.

Where is it happening?

This is happening entirely internally on a small optical bench safely inside the control room. Oh, whoa.

Okay, I see.

By converting samples of both infrared signals into the exact same frequency of green light inside the control room, the computer can directly compare them using standard, highly sensitive green light sensors.

So it forces the two lasers to speak the same language.

Yes. It forces them to synchronize perfectly.

That is brilliant.

And once they are synchronized, once both runners are perfectly matched in the handover zone, the computer smoothly dials down the power of the locking laser while dialing up the power of the main laser.

And the baton is passed.

Passed perfectly. The mirror never even knows what happened.

That is truly mind-bending. They used the very color of light that was causing the problem to solve the problem, just by moving where that color exists in the system.

It is an incredibly elegant workaround. And the operational reality here is that this handover is unimaginably delicate.

I can imagine.

Dr. Lee's team didn't just theorize this on a chalkboard. They built it and proved it works. They demonstrated they could achieve a fully stable handover from the locking laser to the main laser in about 60 seconds.

60 seconds. Wait, why is that specific time frame so important?

Because... The moment of transition, when both lasers are pushing on the mirror, is highly unstable.

So you want it over as fast as possible.

Exactly. The faster you can complete the handoff securely, the less chance there is for seismic noise to break the lock. A 60-second transition is actually much faster and more reliable than the previous green light system ever was.

Oh, wow. So they improved the speed, too.

Yes.

And they proved it wasn't just a fluke by repeating this rapid handover successfully over several days of rigorous testing.

Okay, so a 60-second baton pass using an infrared optical mixing board is undeniably cool. But let's talk about the results.

Sure.

What does this mean for the actual physical precision of the machine? Because we started this conversation talking about a thousandth of a proton.

Right. Well, the margin of error they maintained during this new infrared locking method is almost unfathomable.

Give it to me.

Based on the data from those days of testing, the measurement error between the lasers during the lock was estimated to be about one tenth the width of an atom.

I need to stop and just let that sink in. One tenth the width of a single atom.

Measured continuously.

Here's where it gets really interesting. Across an entire four kilometer, two and a half mile laser arm. That is the margin of error while pushing back against the vibration of ocean waves.

Yes. That is the level of stability they achieved.

Numbers like that, they stop feeling like math. If our error margin is a tenth of an atom over two and a half miles, how does that translate to what you and I will actually see in the news?

It's a huge shift.

Like, does this mean we'll find things we didn't even know existed?

It buys us a much deeper view of the universe. Think of it this way. The reason we want to install those new low-noise crystalline mirrors in the first place is because they are intrinsically quieter.

Right, less of that thermal atomic jittering.

Exactly. But we couldn't use them because we couldn't lock them. By solving the locking problem with this incredible one-tenth of an atom precision, we can finally install those mirrors.

We basically unlocked the hardware upgrade.

We did. And with those new mirrors successfully installed and locked, the detector's sensitivity will completely skyrocket.

And that lets us hear further.

This directly advances humanity's ability to discover and study cosmic events that are currently completely invisible to us.

Like what?

We will be able to detect the collision of ancient black holes that happened so incredibly far away and so early in the formation of the universe that their gravitational waves are just too faint for our current glass mirrors to pick up.

So we are essentially building a better time machine that can hear further back into the history of the cosmos?

That is exactly what it is. And we will also be able to listen to distant neutron stars colliding.

Why are neutron stars so important?

Those specific collisions are crucial because they are the cosmic forges that create the heaviest elements in the universe.

Like what kind of elements?

Like the gold in your jewelry or the platinum in your electronics.

Wait, really?

It does. But right now... Unless a neutron star collision happens relatively close by in cosmic terms. The tiny ripples it makes are completely lost in the thermal background noise of our own detectors.

Ah, the jittering.

Right. This new infrared locking scheme strips away that noise. It clears the path.

So, you know, every time you scroll through your news feed and you see a headline about some massive reality-altering cosmic discovery?

Like the sound of two black holes colliding billions of years ago?

Yeah, exactly. Or mapping where all the gold in the universe comes from. You have to remember what's really happening behind the scenes.

It's all engineering. It is.

You are reading about the triumph of tiny, incredibly grounded engineering fixes. We only get to understand the macro universe, the colossal, mind-breaking scale of deep space, by mastering the absolute micromechanics of things right here on Earth.

It is the ultimate synthesis of the large and the small.

It really is.

I mean, The universe is constantly broadcasting information in every conceivable way. It's vibrating. It's radiating. It's stretching the fabric of space-time itself.

But we just can't hear it.

Not without the tools. We are deaf and blind to it until we literally invent the right physical materials, the right glass, the right coatings, the right nonlinear crystals to actually catch those signals.

It all comes down to mastering the exact wavelength of an infrared light or figuring out how to use a crystal to synchronize two lasers in 60 seconds without dropping the baton.

Exactly right.

It is wild to think that our understanding of the infinite is entirely bottlenecked by the materials we can build with today.

It's humbling, honestly. It is.

Which leaves me with this final thought for you to just sort of mull over. If upgrading to better mirrors required a complete reimagining of the colors of light we use, and required us to invent new ways to use crystals just to keep our earthly instruments stable, what other hidden signals in the universe are currently washing right over us?

Oh, that's a profound thought.

Right. I mean, we are sitting here, so proud of our infrared lasers and perfectly quiet mirrors. But what if the next great cosmic secret is already here, pulsing through the room you are sitting in right now, just waiting for humanity to invent the right color of technology to finally tune into it?

It really makes you wonder.

It does. Something to think about the next time you look up at the moon and wonder just how small of a detail you're missing.

How Infrared Lasers Could Improve the Future of Gravitational Wave Detection

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