Black holes and neutron stars merge unseen in dense star clusters | Hacker News Reader

Black holes and neutron stars merge unseen in dense star clusters

phys.org

33 pointsdnetesn9 comments

Interesting reading! But I take issue with the title. The article says

   "Laser Interferometer Gravitational Waves Observatory" (LIGO) in the USA, are able to detect these waves.

So not unseen, in as they've been seen :)

Curiously, its the fact that there was no visible light, that suggests it was this particular collision. Vs two black holes or two neutron stars. So I guess an indirect observation. Boy are astrophysicists clever!

The title is perfectly fine.

The article made it perfectly clear what is happening

> The fusion of a black hole with a neutron star was first observed by gravitational wave observatories in August 2019. Yet optical observatories around the world were unable to locate an electromagnetic counterpart in the region from which the gravitational wave signal originated, suggesting that the black hole had completely devoured the neutron star without first destroying it. If confirmed, this could be the first observed black hole-neutron star merger detected in a dense stellar environment, as described by Dr. Arca Sedda.

If we don't perceive this event using the electromagnetic spectrum, is it fair to state that it remains un'seen' as such?

We need a verb for gravitational perception!

I mean, events that are seen are a subset of events that are observed. So with that in mind, "observed" is probably sufficient.

Another example: you can observe a fire by smelling it.

Seeing is not the same as detecting. Can you call radioastronomy "seeing" when you can't see radio waves at all? Gravitational waves are even more "unseeable" in this respect.

Could gravitational waves be faster than Light/EM? Faster than c event?

They're not faster than C, but they could outrun a beam of light if there was some stuff hanging out in space and raising the index of refraction. In relativity the C limit is geometric, in the same way that "you can't make an apple bigger by rotating it around in your hands" is geometric.

Not only can, but do.

The intergalactic medium (IGM) is very diffuse, but not pure vacuum. The difference in arrival time of light and gravitational waves from an event billions of light years distant could help measure the density of the IGM.

Incidentally, while it is very cold, the IGM is plasma, so obeys plasma fluid-dynamic laws, which is why the clouds seen at the ends of Active Galactic Nucleus jets, millions of light years across, can be visibly turbulent.

Reply on news.ycombinator.com