So the Virgo gravity wave detector, recently got an upgrade. That upgrade has been pretty much disastrous to the performance, and they had to scramble to barely cludge the detector together for the last run. These detector upgrades are planned DECADES in advance, with year-long commissioning times. You don't build something hoping it'll hold. But they did. And I think the way this happened is one of the most frustrating moments in science.
- So, it's 2014. Advanced LIGO is about to come online. And Advanced Virgo is preparing to join in too, with a delay.
- Between the input laser and the beamsplitter sits a power recycling unit. In its simplest form, its a single mirror, about 97% reflective.
- Laser enters on one side, beamsplitter is on the other side, and the mirror is positioned perfectly so that there's a resonance forming inside the cavity, and extremely powerful (hundreds of kilowatts) laser is formed in the cavity.
- All of in vacuum.
- They upgraded the beam size from 2cm to 5cm, replacing mirrors.
- For Virgo, due to not having much budget (this is before any detections were even made), they did not have the money to replace the vacuum vessel to put a more complex system free of harmful resonances in the detector bandwidth. So they just put a single larger mirror in what space they could, which has bad resonances.
- A few years later, Virgo is in the spotlight, along with LIGO. It's time to design upgrades.
- Rather than *fix the vacuum vessel and replace the mirror, they yolo it in favor of adding a signal recycling mirror in front of the beamsplitter and the sensor. Its 2 decade old tech. All detectors except Virgo have it.
- ...knowing full well that a bit of constant signal leaks out of the beamsplitter even when nothing is detected.
- ...knowing that feeding signal back INTO the detector will make the power recycling mirror amplify them in a loop.
- Fast forward to 2 years ago. They turn it on. Awful.
- Resonances galore. In the most sensitive part of the detector.
- Signal recycling should have improved performance by 2x. Instead, they get utter junk.
- They decide to decrease the laser beam energy to reduce resonances to keep the detector running, somewhat.
- Power recycling mirror was designed for 125 W. Dropping the power to 17 W removes the bad resonances in the detector bandwidth.
- But Because the mirrors were designed for 125 W, it means power recycling no longer works properly, leading to much lower actual laser energy.
- Which leads to bad detection.
- So they fudge together 2 telescopes in the signal mirror to try to change the signal up to avoid the new resonances.
- It works, but all they can do is have a bit less performance than they did just before their upgrade.
- Now they're very behind on upgrades, because they had no choice but to tear apart the building, the old vacuum vessel, and replace it with a new one with enough space to have more stable but complex power recycling mirrors.
