It's crazy how violent these events are. I remember reading about one of the first where an estimated 5 Solar masses were converted into energy in about a second, which is why it distorted space a billion light years away. I have some questions.
1. What if you were 1 light year from such an event? What would you experience? What about 100 light years?
2. What does warping spacetime really mean? I know this happens around a black hole. But what would the macro effects of this be?
3. So the universe is expanding. Does that mean more "space" is being made? If so, how? Out of what?
Frustratingly, when I tried to ask on Physics Stack Exchange if you could hear a gravitational wave and if so how powerful it would have to be, I managed to simultaneously have the question closed as a duplicate with a link to someone saying “yes” without saying how powerful, and find the top answer on my question was “no”.
Hi Ben, as the person who initially flagged your question as a duplicate, I'm sorry. I can see why this was frustrating. I'll try to provide a more detailed answer here.
According to a 1948 paper by Bekesy [0], a human ear can detect sounds through several mechanisms. In a gravitational wave, since the whole head would vibrate, sound would reach the ear by bone conduction through the skull. Fortunately, this also happens to be the most sensitive mechanism. The smallest vibration amplitude detectable by bone conduction is about 4 x 10^-9 cm at around 3 kHz. If a typical skull is about 17.5 cm from front to back, that gives a minimum detectable wave amplitude h (the fractional expansion and contraction of spacetime) of about 2 x 10^-10.
The frequency of the gravitational waves produced in a collision between a neutron star and a small black hole turns out to be pretty close to an optimal match for the human ear. To estimate the amplitude, the first black hole collision detected at LIGO produced a strain of about 10^-21 at a distance of 410 Mpc. (The events in the article were smaller, but within the same order of magnitude.) Thus, since the amplitude scales as 1/r, the gravitational waves from the collision in the article would have been theoretically audible at a distance of about 400 AU or 40 billion miles.
Alas, attempting to test this calculation experimentally would incur certain practical difficulties. Notably, you would be vaporized by gamma radiation from the collision several seconds before the gravitational waves became audible.
The question is currently closed, so I can't post a new answer. I voted to reopen it, and I've edited the title to focus on the aspect of the question that is not adequately answered by the duplicate.
Be careful when you do this. It is easy to want to change a question you can't answer into one that you can, but you really have no idea what the original poster was asking. I had a question changed into something extremely idiotic by dilettantes who thought they knew what they were doing.
Fortunately, the person who asked the question is here (me), and it’s cool :)
What got removed from my original was effectively “could a human even hear this in principle?”, with my thoughts at the time being “or would you require an gravitational wave intensity so great that the energy of the gravitational waves would be enough to spontaneously collapse into a black hole?”
400 AU from a (stellar mass) black hole-neutron star merger isn’t going to be that intense, and is therefore implicitly gives me my answer (even if the environment itself would be deadly for other reasons).
You'd need a lot of shielding. A planet is about the right order of magnitude, though it's still not clear that a human could survive. You wouldn't die of acute radiation poisoning; a few thousand km of rock would be enough to absorb the gamma rays. At 400 AU, you wouldn't immediately boil to death, either. Part of the collision-facing surface of the planet would be converted into plasma and ablated into space, which would insulate the rest of the planet from the worst of the heat.
My guess is that you'd still die from shock waves propagating through either the atmosphere or the planet itself, but it's difficult to predict the exact effects.
Technically it’s possible to get a lethal dose of neutrino radiation, but in practice you’d have to be inside a giant star while it’s going supernova: https://what-if.xkcd.com/73/
In the recent Netflix documentary, "The Edge of All We Know" about the project to capture the first image of a black hole, there is a segment where they talk about what two black holes colliding might sound like, and they play a simulation of that sound. It was quite interesting.
i'm a former physicist but yeah, a few stellar masses worth of energy is beyond (my) imagination
1. always wondered the same. i'd suspect that for most astronomical BH you would first need to worry of X/gamma rays. you could possibly hear a loud chirp since it is audio frequency.
2. the warping of space time here on earth is actually mostly a warping of time, which is even weirder. i means that the time at your feet is flowing slower that where you're head is. about one second on your entire life. that's actually WHY you feel earth gravity.
3. yes, more space is created out of nothing! because there's a vacuum energy density that inflate the local patch of universe.
funnily why all of the above is hard to understand for our human brains, it all makes sense in the mathematics..
i highly recommend the YT channel PBS Space Time for more explanations
> funnily why all of the above is hard to understand for our human brains, it all makes sense in the mathematics..
Of the four fundamental forces, the higher order aspects of gravity effect humans the least. Satellite operators may need to correct their clocks every once in a while, but other than that, we just can't reach the speeds or distance from earth to notice time dilation, nor can we collide masses large enough to detect gravitational waves.
I was shocked in college when my General Relativity professor told me that very few scientists are studying GR, most likely because it's surprisingly more intuitive than quantum physics, and it's harder to experiment. I really wanted gravity to just be... cooler, I guess.
It's more intuitive in that it's deterministic. No rolling dice to figure out what happens. Yes, differential geometry is a beast and time being a spatial dimension is wacky, but at least things don't change when not observed.
>yes, more space is created out of nothing! because there's a vacuum energy density that inflate the local patch of universe.
I don't think you can say that with that kind of certainty. We don't have a theory of quantum gravity, we don't know what dark matter is, and we don't know what 'dark energy' is, so we don't actually know what is driving the expansion of the universe, so saying that 'space is created out of nothing' is too strong. Even something like the theory of cosmological inflation, which has some explanatory power, hasn't been experimentally verified, but rather tuned to match the observations.
It's something I've noticed in a lot of communicators of science (whether they have a background in science or not). They tend use terms that overstate our actual knowledge in some specific area of science.
you're absolutely correct, i was referring to models based on classical GR + QFT (standard cosmology + standard model). ofc those are wrong but yield the most accurate predictions any models has ever yielded.
i'm almost certain (no proof tho) that space time itself is just an illusion (and that there is only "information ").
in other term i know i know nothing (but still is able to give predictions up 10^-32 s after big bang)
I'm not sure if answers to those specific questions are there, but in general PBS Space Time deals with that kind of thing (and a lot more). Here's their YouTube channel [1] and here is their page at PBS.org [2]. It's also available via the PBS app on many streaming devices and smart TVs.
Most of their videos are in the 8-13 minute length, and they do a pretty good job at the start of each linking back to earlier episodes that cover prior material that the current video builds on, so if you see a video whose title/topic seems interesting it is fine to start their first.
5 solar masses is 1x10^34 g.
1 g of mass is roughly 0.9x10^14 J (mc^2).
So, roughly 1x10^48 J.
1 ly is roughly 1x10^16 m. Surface area of a sphere with r=1 ly, is 4πr^2 = 4π x 10^32 m^2 or roughly 1x10^33, so each m^2 of surface will receive 1x10^48/1x10^33 = 10^15 Watt, or about 10^12 x brighter than our Sun on Earth in sunny day, but for 1 second only.
2. Spacetime is 4d array: [x,y,z;t]. Warping of spacetime means that a scientist applies a mathematical formula to the array directly, like shader in OpenGL. It has no physical meaning. It just a quick way to simulate unknown physical process, to make a prediction.
3. No, it doesn't. Our local group of galaxies is expanding, but this is a coincidence. Actually, we are falling into a black hole. Nearest big black hole is in the center of our galaxy, then Great Attractor, then Shapley Attractor.
1. I suppose 10^12 brighter than the Sun for 1 second is like standing right next to a large nuclear blast, or a series of them to extend the irradiation phase to 1 second. It would take less than 1 second for an observer to evaporate, but the time is important to see if e.g. the surface of the planet would turn to cinder and glass.
2. I suppose it's still possible to illustrate. We can imagine that the spacetime gets "bulged" a bit, so light rays become slightly bent when passing it. It's likely impossible to notice with a naked eye from inside without being torn by tidal forces. But it's quite visible from some distance: pictures of gravitational lensing on astronomical scale are easy to find.
It would have that effect if the energy was in the form of photons, but it is gravity waves.
If you release 5 solar masses of energy in one second, electromagnetically and with the same spectrum as the sun, half way between the Milky Way and Andromeda, every star in both galaxies would see it only a little dimmer as Sol appears on Mars.
What determines whether the merger is non-radiative in the EM? Why was the collision of two neutron stars radiative?
It seems like the universe would be immeasurably more dangerous than it already is, if all of these events were primarily EM radiative in nature.
As a non-physicist, I'm also curious how gravity is simultaneously so much weaker than the other forces, but yet spacetime seems to be able to stably survive these immense releases of energy.
2. One macro effect would be (assuming the observer is at a safe distance) unexpected tidal waves in the oceans seeming to appear from nowhere right after the merger.
A better way to think of the speed of light is as the stiffness of spacetime. This allows for you to intuit what warping effects do to the trajectory of light and gravity.
1. What if you were 1 light year from such an event? What would you experience? What about 100 light years?
2. What does warping spacetime really mean? I know this happens around a black hole. But what would the macro effects of this be?
3. So the universe is expanding. Does that mean more "space" is being made? If so, how? Out of what?