[HN Gopher] Physicists confirm two cases of "elusive" black hole...
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Physicists confirm two cases of "elusive" black hole/neutron star
mergers
Author : wglb
Score : 75 points
Date : 2021-06-29 14:16 UTC (8 hours ago)
(HTM) web link (arstechnica.com)
(TXT) w3m dump (arstechnica.com)
| ars wrote:
| Something I've wondered for a long time: Since time around a
| black hole is frozen, how can we detect a merger? Or when they
| say merger do they really mean close approach?
| drewrv wrote:
| I don't fully understand it, but I believe the way to think
| about it is that: we never "see" something fall into a black
| hole. But the photons from the "last moment we see" get so
| redshifted they're essentially invisible. Therefore from a
| reasonable distance it "looks" like the object disappeared and
| the event horizon grew in size.
|
| Here's a video, he talks about the redshift around the 8 minute
| mark. https://youtu.be/vNaEBbFbvcY
| cletus wrote:
| 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?
| bmitc wrote:
| I don't have answers for you, but you might be interested in
| the book _Black Holes and Time Warps: Einstein 's Outrageous
| Legacy_ by Kip Thorne.
|
| https://www.amazon.com/Black-Holes-Time-Warps-Commonwealth/d...
| immmmmm wrote:
| 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
| macspoofing wrote:
| >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.
| Iwan-Zotow wrote:
| > I don't think you can say that with that kind of
| certainty.
|
| well, any classic (in SM sense) QFT will produce positive
| vacuum energy density which translates into negative
| pressure. P = -E
| lapetitejort wrote:
| > 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.
| tzs wrote:
| 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.
|
| [1] https://www.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g
|
| [2] https://www.pbs.org/show/pbs-space-time/
| drran wrote:
| 1.
|
| 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 4pr^2 = 4p 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.
| nine_k wrote:
| 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.
| hypertele-Xii wrote:
| > about 100x brighter than our Sun on Earth in sunny day, but
| for 1 second only.
|
| Sounds hot. Flash burn everything not made of stone. Land
| life wouldn't survive?
| drran wrote:
| I made a mistake in the calculation (10^34 x 10^14 is
| 10^48, not 10^38), so it's 10^12 brighter. Nobody will
| survive on the lit surface of a planet.
| x3n0ph3n3 wrote:
| And it would likely ignite the atmosphere, which would
| travel at the speed of sound to the "dark" side of the
| planet, yes?
| phasetransition wrote:
| So 1e6 light year radius to match the solar irradiance
| from purely inverse square losses? Wow.
| diegoperini wrote:
| 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.
| ben_w wrote:
| 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".
|
| https://physics.stackexchange.com/questions/560833/could-a-h...
| Thorondor wrote:
| 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.
|
| [0]: https://asa.scitation.org/doi/10.1121/1.1906433
| ajross wrote:
| > I'll try to provide a more detailed answer here.
|
| Maybe you could provide a more detailed answer THERE
| instead?
| BitwiseFool wrote:
| >"Notably, you would be vaporized by gamma radiation from
| the collision several seconds before the gravitational
| waves became audible"
|
| How much lead shielding would you need to survive such a
| thing? Would standing on the far side of an Earthlike
| planet absorb the gamma rays?
| Thorondor wrote:
| 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.
| treeman79 wrote:
| In the right scenario the neutrinos would kill you. Aside
| from everything else. Read somewhere you would need a
| light year of lead to stop them
| ben_w wrote:
| Awesome, thanks for the detailed response!
| Topgamer7 wrote:
| Hacker News once again proving to be the king of social
| networks.
| jazzyjackson wrote:
| Or at least the royal court where you can bump into the
| squires and get your issue resolved without the plebeian
| bureaucracy.
| arthurcolle wrote:
| This is amazing. What are the chances you'd find this?
| java-man wrote:
| why not post _this_ answer to physics.stackexchange.com?
| Thorondor wrote:
| 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.
| R0b0t1 wrote:
| 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.
| quakeguy wrote:
| Great reply!
| varjag wrote:
| (400 AU is ~ 10x-15x the distance to Pluto)
| NegativeLatency wrote:
| Possibly your atoms, and probably your molecules would be
| ripped apart by the gamma rays and other particles emitted
| before a shockwave hit?
| AnimalMuppet wrote:
| _Before_? Gravitational waves travel at the speed of light
| too...
|
| (Or are you saying that the gamma rays are emitted earlier in
| the merger process than the gravitational waves?)
| goldenkey wrote:
| 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.
| triangleman wrote:
| Is there any chance these were somehow the same event, and the
| universe sort of loops back on itself?
| boringg wrote:
| How long ago did these events actually happen though? Is it a 1:1
| ratio therefore events happened 1.2B years in the past?
|
| All these merging events that we are picking up now - are there
| still on-going events or has the frequency dropped off as the
| cosmos have moved apart and relative, to early years, "settled"
| down?
| marcosdumay wrote:
| > How long ago did these events actually happen though?
|
| If you are asking if gravitational waves move at the speed of
| light, then yes, they do.
| rrauenza wrote:
| Having just finished Diaspora this summer based on a different HN
| posting recommending it... this is really cool.
|
| (I don't know that I could recommend Diaspora. It's dense and I
| think requires a level of familiarity with quantum physics to
| truly enjoy. It felt like a bit of a slog for me...)
| phasetransition wrote:
| FYI, primary source is Open Access:
| https://iopscience.iop.org/article/10.3847/2041-8213/ac082e
| m4r35n357 wrote:
| Impressive author list!
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