Sep 15, 2017 · 1h 17m · y-combinator
The Technical Challenges of Measuring Gravitational Waves - Rana Adhikari of LIGO · Y Combinator
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In this Y Combinator interview, Caltech physics professor Rana Adhikari explains the precision physics, laser technology, and data science required by LIGO to detect gravitational waves. He details the engineering hurdles behind measuring sub-atomic space-time warps, upcoming next-generation observatories like LISA, and the societal value of fundamental scientific research.
How this conversation actually went
Every chapter scored 0–10 on four independent dynamics. Hover any point for the reasoning behind the score. How this is scored →
The partners as informed peer 2.0 Guest teaching 4.9 Guest disagreement 0.3 The partners pushing back 0.1
speaking balance: gold is the partners, purple is the guest (3 minute bins)
Sharpest disagreement ▶ 21:05 Dismissing environmental trivia in favor of radiation pressure
Rana bluntly dismisses the host's focus on deer and environmental vibrations, stating those don't matter compared to the massive internal challenge of laser radiation pressure moving multi-kilogram mirrors.
Hardest push from the partners ▶ 14:51 Challenging laser amplification on noise scalingCraig interrupts to question whether increasing laser round trips in the Fabry-Perot cavity also multiplies background noise by 200 times, refusing to accept that power buildup is purely advantageous.
Biggest teaching moment ▶ 37:45 Clarifying thermal noise notch filteringWhen Craig suggests mirror resonance is the mechanism used to detect gravitational waves, Rana corrects the misconception by explaining they must deliberately ignore and filter out those specific frequencies.
The partners hold their own ▶ 14:51 Anticipating signal-to-noise limitationsCraig demonstrates sharp physical intuition by directly asking if repeated cavity bounces scale noise symmetrically with signal, prompting Rana to delve into quantum shot noise limits.
the scores for every segment, with the reasoning behind each
| Chapter | Topic | The partners as informed peer | Guest teaching | Guest disagreement | The partners pushing back | Why |
|---|---|---|---|---|---|---|
| How Laser Interferometry and Audio Conversion Work | 1 | 4 | 1 | 0 | Craig asks basic introductory questions regarding project cost and how gravitational wave signals are converted into sound. Rana gently clarifies that the laser itself is only around $100k and explains why gravitational wave frequencies naturally fall into the human audio band. | |
| The Scale of Gravitational Waves and Earth's Distortion | 2 | 5 | 0 | 0 | Craig asks about mirror development and confirms the fractional distortion scale on Earth. Rana explains wave attenuation and provides an intuitive comparison of Earth stretching by a fraction of a human hair width. | |
| The Fabry-Perot Cavity and Laser Power Sensitivity | 4 | 6 | 0 | 1 | Craig asks an incisive question about whether 200 bounces in a Fabry-Perot cavity also nets 200 times the noise. Rana validates the insight and explains the trade-off between quantum shot noise scaling and mirror motion limits. | |
| Laser Stability Challenges and Advanced LIGO Upgrades | 2 | 4 | 0 | 0 | Craig asks about the key upgrades between initial LIGO and Advanced LIGO. Rana uses the dancing meter stick analogy to describe frequency stabilization and credits graduate students for solving integration bugs. | |
| Radiation Pressure Controls and Feedback Systems | 2 | 5 | 1 | 0 | Craig brings up external noise like deer near beam tubes, which Rana dismisses as trivial compared to radiation pressure pushing 40kg mirrors. Rana details the multi-loop feedback controls needed to stabilize optomechanical oscillations. | |
| Data Processing, Machine Learning, and Ringing Black Holes | 2 | 5 | 0 | 0 | Craig asks about data processing and detection frequency. Rana explains linear Wiener filtering in analog hardware versus future nonlinear regression challenges for low-frequency black hole ringing. | |
| Waveform Templates and Cosmic Resonances | 3 | 4 | 0 | 0 | Craig asks whether detection relies on pre-calculated waveform guidebooks. Rana explains the multi-dimensional template catalog and matched filtering used to identify chirp signals. | |
| Thermal Noise, Hum Filters, and Bug Hunting | 2 | 6 | 1 | 1 | Craig assumes the laser measures during mirror resonance, but Rana corrects him, explaining that they notch out resonant frequencies and hum lines to avoid thermal vibration noise. | |
| Scattered Light Mitigation and Ultra-Black Materials | 1 | 5 | 1 | 0 | Craig asks about tracking bugs and clarifies what coating materials are being applied. Rana outlines how scattered light causes a disco ball effect and describes testing welder's glass and carbon nanotubes in vacuum chambers. | |
| 40-Kilometer Interferometers and Extra Dimensions | 2 | 6 | 0 | 0 | Craig asks if 40-kilometer arms clean up the signal. Rana explains that length directly boosts strain signal amplitude and discusses testing whether gravity leaks into extra dimensions. | |
| Paradigms in Physics and Space-Based Interferometry (LISA) | 2 | 5 | 0 | 0 | Craig asks about space-based interferometers. Rana explains low-frequency Newtonian gravitational noise from atmospheric and seismic motion on Earth, contrasting it with LISA's high-fidelity space measurements. | |
| The Value of Curiosity-Driven Basic Science | 2 | 4 | 0 | 0 | Craig asks how basic science of this scale is pitched and justified. Rana provides a passionate historical argument for curiosity-driven research and post-WWII scientific investments. | |
| Twitter Questions: Nearby Black Hole Mergers | 1 | 5 | 0 | 0 | Craig reads a Twitter question about a nearby black hole merger. Rana calculates distance scaling, noting that an Alpha Centauri merger would saturate electronics, while a solar system transit could acoustically excite Earth's 30 mHz resonant modes. | |
| Quantum Feedback and Future 40-Meter Prototypes | 2 | 5 | 0 | 0 | Craig asks whether current interferometry is the fundamental best approach. Rana highlights the massive SNR loss converting space-time strain to laser light and introduces coherent quantum feedback testing on Caltech's 40-meter prototype. |
Statements from this episode (18)
Adhikari: Decades of required effort would have deterred early gravitational wave researchers
“If they, I think if they had known how tough it would be, or that it was going to take fifties, five years to have success, probably no one would have started.”
Adhikari: LIGO laser only costs around $100,000
“The laser itself is cheaper. You can do, probably you could do the whole thing with a 100,000 dollar laser. That's about the laser cost.”
Adhikari: LIGO's detectable gravitational waves fall into human audio band
“It happens to be that the waves that we're detecting and the waves which are easiest to detect are exactly in the human audio band. So the waves that you and I can hear with our ears, that's the whole frequency range for gravitational waves that we can detect.”
Adhikari: LIGO has not seen another signal matching its first detection
“We've not seen anything of that size since then, since that first one.”
Adhikari: LIGO's first black hole detection measured a 10^-18 meter displacement
“About 10 to the -18 meters, which means one billionth of a, about the size of a atom.”
Adhikari: LIGO stabilizes lasers ten million times beyond commercial models
“So we take the best laser in the world that we can find, and then we stabilize it and make it about ten million times more stable than what you can buy, and then it's kind of just barely good enough, and we're gonna have to do better if we want to do better.”
Adhikari: Laser radiation pressure physically moves LIGO's 40-kilogram mirrors
“There's so much laser power that when you, it's weird to think about, but there's so much laser power when we hit the mirror, it moves the mirror. And the mirror, if you, to imagine it, is about this big. And it is 40 kilograms”
Adhikari: LIGO noise reduction has reached linear limits, needs nonlinear regression
“Now we've reached the limit of what you can do with linear noise subtraction, and we need some better ideas on how to do the next thing. And the next thing involves nonlinear regression.”
Adhikari: LIGO could probably double its detected signals from existing data
“I think in our data there must be, probably we could double the number of signals we have right now if we were to, I mean, that's my guess. It could be much more.”
Adhikari: LIGO cannot detect the largest black holes due to environmental noise
“So the biggest black holes we can't find right now, because This kind of technical noise feedback and the vibrations from the environment are bigger than the fundamental quantum physics limits of measurement.”
Adhikari: LIGO mirrors store vibrational energy 10,000 times longer than glass
“The mirrors we have are more like They store the energy better, something like. About 10,000 times longer. So if I were to ping one of those would last for, you know, hours. They would just keep ringing and ringing.”
Adhikari: Welder's glass effectively absorbs LIGO's one-micron infrared laser light
“It has a wavelength of one micron, and so for it some of the things that look black don't work, but if you have a special kind of welder's glass, it really works. Welder's glass is good as, good at absorbing pretty much everything with a longer wavelength than…”
Adhikari: Upgraded LIGO could detect signals from ten billion years ago
“With the current systems, you know, as big as they are, if we put in like our best technical hacks into them that we can imagine, we could maybe get to the place where the universe was about a one fifth or one sixth of its current age. So we could look back so…”
Adhikari: 40-kilometer interferometers could detect collapsing first stars
“We would be able to find signals from Basically, the, all the way back. I mean, it would have. We would find the first stars in the universe, and they were collapsing, if they exist, which I think they do.”
Adhikari: Earth's vibrations limit ground detectors to frequencies above 5-10 Hz
“On the ground, kind of, we're limited to measure things that are that have a signal frequency, which is more than five or 10 hertz or something like that, so we can go a little bit below the human audio band, but not much, and the reason for that is that the e…”
Adhikari: The LISA space interferometer aims to launch in 16-17 years
“There's a project called LISA, which is aimed to launch in 16 years, 1617 years from now, and that will put a system, a triangular interferometer in space, which is several interferometers, and that will measure gravitational waves at around a millihertz”
Adhikari: An Alpha Centauri black hole merger would overwhelm LIGO electronics
“Like the ones we detected were at, let's say several hundred million light years, and Alpha Centauri is only four light years, I believe, so it would be stronger by that factor of a hundred million. And, which sounds like a lot, it is a lot, but that means tha…”