# What would it feel like to be orbiting near a merging black hole?

The recent 4th discovery of gravitational waves where the black hole masses were 31 and 25 times the mass of the sun, and they released 3 solar masses of energy as gravitational waves, made me wonder what would it feel like to be orbiting this binary black hole system on a planet at 1AU when the merger happened?

• This article addresses your question, written by a physicist. forbes.com/sites/briankoberlein/2016/02/13/… And here's a very similar question: physics.stackexchange.com/questions/235285/… – userLTK Sep 28 '17 at 5:23
• answers at physics are very different from @t.sand 's one... – J. Chomel Sep 28 '17 at 9:30
• The answers in the physic stackexchange is completely unsatisfactory. The ones dealing with energy don't even mention that gravity is so weakly interacting that the 3 solar mass of gravitational energy itself would do very little to a human because due to the size of a human and the size of the gravitational constant. The accepted answer is pretty useless given that spacetime distortions is nothing like normal compression and stretching and there is no proof that smooth gravitational waves distorting matter can be damaging to humans at 0.01% human size. – A. C. A. C. Sep 28 '17 at 15:54
• I'm amazed that no one picks up @A.C.A.C.'s point that the GWs are ripples in spacetime, and this distortion is fundamentally different to compression or stretching of objects in ordinary space. The other thing overlooked is that the GW travels at the speed of light: it's impossible to "feel" a ripple at that speed. – Chappo Hasn't Forgotten Monica Apr 6 '18 at 5:33
• similar question in Physics SE: How would a passing gravitational wave look or feel? – uhoh Sep 2 '19 at 0:10

It could do serious damage if things were just right.

Three solar masses of energy is about 5x1047 Joules. 1 AU is about 1.5x1013 cm. The area of the sphere is thus about 4x1027 cm2. That's a flux of 1020 joules cm-2. That's plenty of energy to do damage...if it can couple to you.

The strain of the GW measured at Earth is around 10-22. Strain drops off as the inverse distance, and the ratio of a gigaparsec to an AU is about 1014, so the strain at 1AU from the merger would be about 10-8. By itself, this would probably not harm you.

But it's not a simple one-time strain, it is oscillating with the orbital frequency of the merging black holes, and anything which had a physical resonance at that frequency could pick up energy, possibly significant energy, from the GW fields.

(Note that this depends on orientation. The mechanical resonance needs to align with the GW, and GWs are not emitted in all directions, so you'd want (or, perhaps, not want!) to be in the plane of the merger.)

The argument that GWs can't deposit energy is plainly wrong -- if it were true, LIGO couldn't detect them! -- but if you believed that, you were in superb company: It took Einstein decades before he became convinced that GWs actually carried energy!

• The strain of $10^{-8}$ is the key thing, so you would get about 20nm taller and 10nm narrower and then a tenth or so of a second later, the opposite, and that would be repeated for 10 or 20 cycles at increasing frequency before stopping. It's hard to see that that would do much to a human directly. A very rigid crystalline structure that happened to resonate with the signal might see something more dramatic. What's going to be much more noticeable is the effects on accretion disk(s) much closer to the BH. That would easily produce a very intense burst of X-rays which might not be so healthy. – Steve Linton Jul 19 '18 at 21:35

It is quite beautiful to imagine such a condition.

First thing is that we can't observe them with eyes, as no reflection of light will occur from black holes.

When the observer is in rest relative to merging black holes: now when the black holes are merging, the surrounding space-time's gravitational waves will start increasing, and their amplitude will also increasing. It affects the observer, because the waves start pushing away the observer from the black holes. But it doesn't effect the images of the observation, because light will also bend in the gravitational waves. Also time dilation will occur due to the gravitational waves, but what actually happening when they were merging, I can't imagine.

• I think you are right, that such strong gravitational waves have a pushing effect, however how could you imagine this without the deep knowledge of the General Relativity? It is a tricky question. :-) – peterh - Reinstate Monica Jul 19 '18 at 17:16
• Btw, I removed the Hawking radiation part, it is far too small for any visible effect (actually, it is around $10^{40} - 10^{50}$ times weaker). – peterh - Reinstate Monica Jul 19 '18 at 17:19
• sir thanks for your reply. i will try my best to complete the answer. and i hope next time i will not disappoint you. actually i am learning about relativity from 9th standard from you tube(Space-Time channel). – user23728 Jul 19 '18 at 17:28
• Ok. You don't need relativity for that, just consider impulse preservation what is true also in the General Relativity. – peterh - Reinstate Monica Jul 19 '18 at 17:29