[syhprum (OP)]

Colliding black holes emit a vast amount of energy in the form of gravitational waves for a few milliseconds  ,What would be the effect of this on one or would other forms of radiation pose a greater threat

[evan_au]

If you were up close with a single stellar-mass black hole (say 1 to 3 times the mass of the Sun), the tidal forces would tear you apart - the scientific term is spaghettification. 

If  the black hole were actively accreting matter, the hot plasma of the accretion disk emits considerable X-Rays, which would require considerable lead shielding. This is apart from the risk of actually colliding with the accretion disk at relativistic orbital velocities, which would instantly turn you into a plasma as well.

If you were up close with closely orbiting black holes, the rapidly varying tidal forces could cause you harm in several other ways, including being swallowed by one black hole or the other, or being flung out at high speed due to the gravitational slingshot effect.

I imagine that being close to colliding black holes could do you some damage, as the compression of space would be quite intense close to the epicenter. But gravitational waves interact fairly weakly with matter.

There was a hint that the recently observed black hole merger may have been accompanied by a burst of gamma rays. Gamma rays interact strongly with matter, so I suspect that these would prove lethal at a greater distance than the gravitational waves.

[Bored chemist]

In principle, we can shield against the other radiation, but not the gravitational waves.
In practice, if you are close enough to notice the gravitational waves then the other things will kill you.

[syhprum (OP)]

Although at present it does not seem possible to shield against gravity does it not depend on whether or not the Graviton has mass ?.
The graviton is postulated to have a mass vastly smaller than that of the Neutrino and its hard enough to shield against that but if the Graviton has non zero mass there is at least a theoretical possibility that it can be screened against

[JoeBrown]

If you were up close with a single stellar-mass black hole (say 1 to 3 times the mass of the Sun), the tidal forces would tear you apart - the scientific term is spaghettification. 

That's not a scientific term.  It would be science if it could be observed, but it's never been so, AFAIK.  It is a potential possibility.  Science is kinda funny, it likes things that can be verified by observation, before being termed as "based on scientific principles" or considered as fact.

[JoeBrown]

All things considered, we're ~fairly~ close to the super massive blackhole in the center of the galaxy.  The Earth is roughly 4 billion years old.  Now if Lambda-CDM is right, the Universe is only 14 billion years old.

Personally I find those numbers a little short, but that's my opinion. The supermassive black hole in the center of the milky way galaxy is more than 4 billion suns worth of mass.

Our view of the Universe in these measures of time and mass are really quite short.  Type 1a supernovas are believed to be smallest black holes at ~1.4 solar masses.   There's about 2 supernova events per century in the Milky Way galaxy.


Those numbers don't add up right for me.  14 billion year old universe has our galaxy weighing somewhere near 4 billion solar masses.   It seems reasonable that a black hole would need to merge with the central super massive hole fairly often, for it to gain so much weight, over time.

But, it is possible that it was a really huge star.  Stars billions of times the size of the sun have been witnessed by cosmologists.  But I don't know where...  Obviously they're not a central super massive black holes, if they've been seen as a star.

I suspect holes merge often, universally speaking.  We've only begun detecting them.  Maybe the next one we detect will ruin our chances of detecting more in the future, but I doubt it.

[evan_au]

The supermassive black hole in the center of the milky way galaxy is more than 4 billion suns worth of mass.

The stellar orbits in the Galactic Centre show that the central mass concentration of four million solar masses must be a black hole

I think you are off by about 3 orders of magnitude. See: https://en.wikipedia.org/wiki/Sagittarius_A*

Stars billions of times the size of the sun have been witnessed by cosmologists.  But I don't know where...

There are several stars which have been estimated at over 1500 times the radius of the Sun. As you say, this puts them at billions of times the volume of the Sun. But this does not mean that they will collapse into a black hole weighing a billion times the mass of the Sun. See: https://en.wikipedia.org/wiki/List_of_largest_stars#List

The most massive known stars are thought to be around 300 times the mass of the Sun. These could easily form a black hole weighing more than 100 times the mass of the Sun, after they eject a huge amount of gas and dust as a nebula. See: https://en.wikipedia.org/wiki/List_of_most_massive_stars#List_of_the_most_massive_stars

[JoeBrown]

The stellar orbits in the Galactic Centre show that the central mass concentration of four million solar masses must be a black hole

I think you are off by about 3 orders of magnitude. See: https://en.wikipedia.org/wiki/Sagittarius_A*

Yeah, I was off.  Most central black holes are comparable to equal mass of their host galaxy.  Millions is off by about 3 orders.  I read billions.  Don't know if that was spellcheck or my DOH.

Stars billions of times the size of the sun have been witnessed by cosmologists.  But I don't know where...

There are several stars which have been estimated at over 1500 times the radius of the Sun. As you say, this puts them at billions of times the volume of the Sun. But this does not mean that they will collapse into a black hole weighing a billion times the mass of the Sun. See: https://en.wikipedia.org/wiki/List_of_largest_stars#List

The most massive known stars are thought to be around 300 times the mass of the Sun. These could easily form a black hole weighing more than 100 times the mass of the Sun, after they eject a huge amount of gas and dust as a nebula. See: https://en.wikipedia.org/wiki/List_of_most_massive_stars#List_of_the_most_massive_stars

We know very little about black holes.  Mostly because they are "black" or non visible.  We can only guess at their weight.  General Relativity puts them past infinity on time/speed/gravity scale, but we've been able to estimate their masses based star/speed analysis orbiting their mass concentration.

General Relativity is a poor scale to use.  It's simplified formula for point vectors through fields of gravity, which isn't exactly simple...  But certainly doesn't tell the (w)hole story.

[jeffreyH]

An interesting point to note. The photon sphere surrounding a non-rotating black hole is at a radius 1.5 times the radius of the event horizon. If we take rest energy as E=mc^2 and the maximum kinetic energy as 1/2mc^2 then kinetic energy plus rest energy is 1.5 times rest energy. In both cases, photon orbit and maximum kinetic energy, it is impossible for particles with rest mass to participate.