4 ms·

Scott Aaronson estimated the strain if you were 1 AU away from one of the previous BH mergers detected by LIGO and found you would only be stretched by about 50

Scott Aaronson estimated the strain if you were 1 AU away from one of the previous BH mergers detected by LIGO and found you would only be stretched by about 50 nm (about a thousandth the width of a human hair). So you'd need to be very close indeed to feel something. http://www.businessinsider.com/what-gravitational-wave-feels-like-2016-2 http://www.businessinsider.com/what-gravitational-wave-feels... http://www.scottaaronson.com/blog/?p=2651 http://www.scottaaronson.com/blog/?p=2651 The alternative take is that LIGO's detectors are unfathomably sensitive, detecting when its ~4km arms stretch by just 10^-19 meters. That's a 10,000th the width of a proton, where the proton is just a 100,000th the width of an atom, and the atom is about a 1,000,000th the width of a hair.

Pyxl101 9y ago
> The alternative take is that LIGO's detectors are unfathomably sensitive, detecting when its ~4km arms stretch by just 10^-19 meters. That's a 10,000th the width of a proton, where the proton is just a 100,000th the width of an atom, and the atom is about a 1,000,000th the width of a hair. Wow. Hearing the sensitivity described in those terms is absolutely mind-blowing.
- pedro_hab 9y ago
  I know right? There is a video on veritasium on how absurd this is: https://www.youtube.com/watch?v=iphcyNWFD10 https://www.youtube.com/watch?v=iphcyNWFD10
  - StavrosK 9y ago
    I don't know, I can never process numbers this big. "To understand how sensitive the LIGO is, imagine each arm stretched out as far as the sun. Even if we had the resources to build an object this big, we still would not be able to get people to understand this intuitively at all."
FeepingCreature 9y ago
For comparison, the Planck length is 10^-35, so another 10^16 smaller again.
- dTal 9y ago
  That's amazing. Well over half the zeros to the Planck length. And they said we'd never measure that small, but I sure wouldn't bet the farm on it now.
mirimir 9y ago
That's exactly what I was wondering about, and here's the answer. I love HN! So from the article: > Black holes [merging] produce gravitational waves but not light. So really, if you were orbiting such a pair of about-to-merge black holes, the gravitational waves produced by the merger would cause just nm-scale fluctuations? Also, 1 AU seems awfully close to a pair of orbiting ~25 solar-mass black holes. I skimmed the paper, and didn't see anything about how the orbital distance evolved, leading up to the merger. One could probably calculate that from the mass and spin estimates, it's over my head. So maybe, at 1 AU, the about-to-merge black holes would look effectively like a point mass. Anyone want to venture a guess?
- jessriedel 9y ago
  In principle you could have a two black holes orbiting each other in a region otherwise devoid of matter, but in practice there is usually a large amount of matter being heated to extreme temperatures in their vicinity. So, optically, it would look very dramatic even if the bare BHs don't emit light. https://en.wikipedia.org/wiki/Accretion_disk https://en.wikipedia.org/wiki/Accretion_disk A 25 solar-mass BH is about 150km in diameter, and the gravitational waves emitted won't be appreciable until the two BHs had spiraled inward to a separation of a few hundred km. You would need to be roughly 1,000 km away to experience a 1% strain during the merger https://physics.stackexchange.com/questions/235285/how-strong-were-the-gravitational-waves-that-ligo-detected-at-the-source https://physics.stackexchange.com/questions/235285/how-stron... I don't think spin is important at the order-of-magnitude level (for naturally occuring BHs).
  - mirimir 9y ago
    Thanks. Would the radiation at 1 AU from such an accretion disk be harder on an Earth-like planet than that from our Sun? I'm guessing that it would at least be far more variable, given dependence on mass flow. And perhaps skewed toward gamma and other bad stuff.
    - jessriedel 9y ago
      Way outside my field so I can't tell you about variability, but BH accretion disks are definitely hotter and emit intense x-rays and other nasty stuff.

Pyxl101 9y ago

> The alternative take is that LIGO's detectors are unfathomably sensitive, detecting when its ~4km arms stretch by just 10^-19 meters. That's a 10,000th the width of a proton, where the proton is just a 100,000th the width of an atom, and the atom is about a 1,000,000th the width of a hair. Wow. Hearing the sensitivity described in those terms is absolutely mind-blowing.

pedro_hab 9y ago
I know right? There is a video on veritasium on how absurd this is: https://www.youtube.com/watch?v=iphcyNWFD10 https://www.youtube.com/watch?v=iphcyNWFD10
- StavrosK 9y ago
  I don't know, I can never process numbers this big. "To understand how sensitive the LIGO is, imagine each arm stretched out as far as the sun. Even if we had the resources to build an object this big, we still would not be able to get people to understand this intuitively at all."

FeepingCreature 9y ago

For comparison, the Planck length is 10^-35, so another 10^16 smaller again.

dTal 9y ago
That's amazing. Well over half the zeros to the Planck length. And they said we'd never measure that small, but I sure wouldn't bet the farm on it now.

mirimir 9y ago

That's exactly what I was wondering about, and here's the answer. I love HN! So from the article: > Black holes [merging] produce gravitational waves but not light. So really, if you were orbiting such a pair of about-to-merge black holes, the gravitational waves produced by the merger would cause just nm-scale fluctuations? Also, 1 AU seems awfully close to a pair of orbiting ~25 solar-mass black holes. I skimmed the paper, and didn't see anything about how the orbital distance evolved, leading up to the merger. One could probably calculate that from the mass and spin estimates, it's over my head. So maybe, at 1 AU, the about-to-merge black holes would look effectively like a point mass. Anyone want to venture a guess?

jessriedel 9y ago
In principle you could have a two black holes orbiting each other in a region otherwise devoid of matter, but in practice there is usually a large amount of matter being heated to extreme temperatures in their vicinity. So, optically, it would look very dramatic even if the bare BHs don't emit light. https://en.wikipedia.org/wiki/Accretion_disk https://en.wikipedia.org/wiki/Accretion_disk A 25 solar-mass BH is about 150km in diameter, and the gravitational waves emitted won't be appreciable until the two BHs had spiraled inward to a separation of a few hundred km. You would need to be roughly 1,000 km away to experience a 1% strain during the merger https://physics.stackexchange.com/questions/235285/how-strong-were-the-gravitational-waves-that-ligo-detected-at-the-source https://physics.stackexchange.com/questions/235285/how-stron... I don't think spin is important at the order-of-magnitude level (for naturally occuring BHs).
- mirimir 9y ago
  Thanks. Would the radiation at 1 AU from such an accretion disk be harder on an Earth-like planet than that from our Sun? I'm guessing that it would at least be far more variable, given dependence on mass flow. And perhaps skewed toward gamma and other bad stuff.
  - jessriedel 9y ago
    Way outside my field so I can't tell you about variability, but BH accretion disks are definitely hotter and emit intense x-rays and other nasty stuff.