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Comment by pavel_lishin | Hacker News Reader

> (The million- and billion-solar-mass supermassive black holes that anchor galaxies’ centers formed differently, and rather mysteriously, in the early universe. LIGO and Virgo are not mechanically capable of detecting the collisions of supermassive black holes.)

Is this because those collisions would be too "loud", in the same way that I wouldn't expect my microphone to be able to pick up an earthquake?

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Supermassive black holes are larger, so the characteristic timescale of their dynamics is longer, and the gravitational waves generated by their merger are correspondingly of lower frequency. More specifically, supermassive black holes binary (SMBHB) mergers usually ring at 0.1-1 millihertz, whereas LIGO's sensitivity is concentrated around more like 0.1-1 kilohertz, a typical frequency of stellar-mass black holes binary mergers. The ~6 orders of magnitude difference comes directly from the fact that supermassive black holes typically weigh in at millions of stellar masses.

See the plot here:

http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.ht...

(To convert from wavelength (meters) to frequency (hertz), take the speed of light and divide it by the wavelength.)

So it's like how you can easily see a small swell from a small boat but a larger swell can be almost undetectable.

Is this also why we don't just get ripped apart during one of these ripple events which are probably all around us as gravitational noise?

I wonder if this is why seemingly tiny vessels can cross large oceans just as easily as larger ones.

The reason we don't get "ripped apart" during these ripple events is simply that spacetime is so stiff that even absolutely monstrous amounts of gravitational energy lead to only the tiniest distortions in spacetime geometry. A black-hole collision may put out more power in gravitational waves than the entire optical luminosity of the visible universe, and the result when it reaches us will result in a distortion of less than the diameter of a proton.

http://www.astronomy.com/news/2019/07/colliding-supermassive...

> If the theories are wrong and there is no problem, supermassive black hole pairs should approach each other and merge frequently enough to create a background “hum” of gravitational waves. “This noise is called the gravitational wave background, and it's a bit like a chaotic chorus of crickets chirping in the night,” Goulding said. “You can't discern one cricket from another, but the volume of the noise helps you estimate how many crickets are out there.” But this hum is outside the hearing range of LIGO and VIRGO, though astronomers are looking forward to projects like LISA, a space-based gravitational wave detector that will “hear” at frequencies current instruments cannot.

I would think it is more like an antenna tuned to a different frequency.

I would expect they would need to be larger to detect the significantly lower frequency.

I'm a total illiterate in this context, so forgive the possibly stupid question, but the antenna analogy brought this to mind: is there any effect similar to heterodyning with respect to gravitational waves like we have in the electromagnetic ones? If yes, could some signals have been already recorded but elsewhere in the spectrum because they're the result of sum and/or subtraction with other known signals?

As far as I understand, that effect is just a result of the superposition characteristic of waves in general, so yeah it should also apply to gravitational waves. I think the difference is that there aren't a lot of sources of "carriers" in the gravitational wave universe.

The heterodyne effect comes from multiplication of signals. But you really just need to pass their superposition through a nonlinearity, because the Taylor series expansion will typically have an early term with the multiplication of the two signals' frequencies with a high coefficient. Of course, you'll have lot of artifacts from all the other terms of the expansion, but you can use a bandpass filter to drop all the unwanted terms.

Or so I think I remember. It's been a while.

There are no other devices capable of detecting spatial vibrations at the level of sensitivity required (strain of 1 part in 10^21).

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