Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

LIGO Detects Gravitational Waves for Third Time

caltech.edu

Post reply on HN
Print view View original

71–80 of 111 posts

Re: LIGO Detects Gravitational Waves for Third Time

#71

Post by greeneggs »

netcraft wrote: post #35

Great veritasium video about this latest wave: https://www.youtube.com/watch?v=NVKO7UCIlgs What is involved with increasing sensitivity I wonder? Is it purely lengthening the arms? or are there other advancements required? Hopefully one day we can have these things in space, isolated from noise and curvature of the earth and no need for vacuum equipment.

From that video:

"If we improve our detector sensitivity, by say a factor of two or three, the rates will go up from, you know, seeing one every month or every two months, to seeing one every day or every week."

- David Reitze, Executive director of LIGO

I don't know what their uptime is, but it sounds like they probably have a number of as-yet unreported observed events.

Re: LIGO Detects Gravitational Waves for Third Time

#72

Post by [deleted] »

lnx01 wrote: post #33
shortstuffsushi wrote: post #25

Earlier quoted context omitted.

I guess this was the biggest question answering piece to me. When I think of a black hole, I assume it has a gigantic mass, enough that it's always the most massive local object, and subsequently pulls in all other things. I didn't realize that might not be the case. As black holes "absorb" everything that "falls" into them, do they continue to build mass then?

Yes. Whether or not something is a black hole is dependent on its total mass and its radius. So anything can mathematically become a black hole if you compress it enough. The earth could be a black hole if it's total mass were compressed to something like 'less than the diameter of a grapefruit'. At that point space itself cannot contain the mass, physics breaks down and you get a singularity. The moon would continue…

[deleted]

Re: LIGO Detects Gravitational Waves for Third Time

#74

Post by aroberge »

marcosdumay wrote: post #63
castis wrote: post #57

When a gravitational wave hits the earth, does the planet oscillate in place for the duration, or is our position in the cosmos displaced, or something else altogether?

The planet gets shorter, then longer, then shorter again... All that in a single direction, while the size on the other two directions stays the same.

That is incorrect. What you describe is a dipolar oscillation. The simplest gravitational waves woudl have a quadrupole moment: stretch in one direction while contracting in the direction perpendicular to it. Have a look at https://en.wikipedia.org/wiki/Gravitational_wave where there is an animation illustrating the simplest case.

Re: LIGO Detects Gravitational Waves for Third Time

#75

Post by nthcolumn »

grigjd3 wrote: post #54
mudil wrote: post #40

Here's what I don't understand. In the first detection, they mentioned that two black holes collapsed, emitted gravitational waves, and the resulting combined mass was less than then sum of two previous masses because energy was spent on gravitational wave generation. Hence it means, that due to gravitational interactions, objects leak mass. Now, we know that every object in the universe is gravitationally related to…

It's important to note that gravitational waves themselves contain energy and angular momentum. Relativists often use the terms mass and energy interchangeably. When I worked in numerical relativity, we used units such that the speed of light was 1, so E=mc^2 (really E^2=m^2c^4+p^2) simplifies to E=m.

I have no idea what these people are talking about but I always thought as a small child that c should = 1. That square really bugged me - why squared, why not cubed or halved or more realistically some bally awkward number... Sounds like fun this numerical relativity!

Re: LIGO Detects Gravitational Waves for Third Time

#76

Post by jxramos »

japhyr wrote: post #64
smortaz wrote: post #48

FYI - if you want to check out the data, the code, even an audio of the wave checkout: https://notebooks.azure.com/roywilliams/libraries/LIGOOpenSc... It's a Jupyter notebook that anyone can clone and run. [edit: updated link]

It's just awesome to see these notebooks released. I'm not going to play with them, but I love that people are able to. It makes me much more confident in the results that are announced, and I hope this approach to doing science becomes the norm.

I second the wider adoption of notebooks. I'd really love to see government budgeting offices and what not begin to make their spending and analyses transparent through these documents. We need government on github and jupyter notebooks :D

Re: LIGO Detects Gravitational Waves for Third Time

#77

Post by Chronos »

nthcolumn wrote: post #75
grigjd3 wrote: post #54

Earlier quoted context omitted.

It's important to note that gravitational waves themselves contain energy and angular momentum. Relativists often use the terms mass and energy interchangeably. When I worked in numerical relativity, we used units such that the speed of light was 1, so E=mc^2 (really E^2=m^2c^4+p^2) simplifies to E=m.

I have no idea what these people are talking about but I always thought as a small child that c should = 1. That square really bugged me - why squared, why not cubed or halved or more realistically some bally awkward number... Sounds like fun this numerical relativity!

It's squared because energy is work, work is force times displacement (distance), force is mass times acceleration, acceleration is velocity per time, and velocity is vector distance per time. So you get:

mass × distance × ((distance ÷ time) ÷ time)

mass × distance^2 ÷ time^2

mass × (distance ÷ time)^2

Re: LIGO Detects Gravitational Waves for Third Time

#78

Post by johncolanduoni »

cyphar wrote: post #53
lnx01 wrote: post #33

Earlier quoted context omitted.

Yes. Whether or not something is a black hole is dependent on its total mass and its radius. So anything can mathematically become a black hole if you compress it enough. The earth could be a black hole if it's total mass were compressed to something like 'less than the diameter of a grapefruit'. At that point space itself cannot contain the mass, physics breaks down and you get a singularity. The moon would continue…

> So anything can mathematically become a black hole if you compress it enough. Not necessarily. If the Schwarzschild radius of the resulting black hole is on the order of a Planck Length you can't really say whether or not such an object is a black hole anymore (depending on your interpretation of the Planck Length). On my calculator the lower bound is [\frac{r c^2}{2G}] which is ~1.09e-9 kg (give or take a factor o…

It depends less on your interpretation of the Planck length and more on how quantum gravity actually works. We have some guesses and semi-supported theories (like Hawking radiation) but the jury is still out as to how micro black holes work, if they do in fact exist.

Re: LIGO Detects Gravitational Waves for Third Time

#79

Post by kstrauser »

nategri wrote: post #65
Florin_Andrei wrote: post #58

Earlier quoted context omitted.

It must be TNT equivalent. One solar mass is 1.99 × 10^30 kilograms, and we know that 2 solar masses were converted in total, so 4 x 10^30 kilograms, which is far less than the "megatons" mentioned, in terms of pure mass. I wish folks would avoid mixing military units and general relativity units like this, it's confusing.

I don't think it's that ambiguous. "Megaton" isn't really used anywhere except for explosive yields, where it always means TNT. As far as the 'native unit' astrophysicists will tend to use ergs for events like supernovae. https://en.wikipedia.org/wiki/Erg

I thought it was ambiguous in the context of an article talking about converting mass to energy. Without running the numbers, I couldn't tell if that was a unit of the input mass or the output energy.

Re: LIGO Detects Gravitational Waves for Third Time

#80

Post by mowenz »

cletus wrote: post #12

The numbers here are just staggering: - Black hole merger occurred 3 billion light years away - Two solar masses were converted to energy - Briefly 10^34 megatons of energy were released every second This is hard to intuitively wrap your head around because we think of space as constant. Something like this can distort space itself. Amazing stuff.

Anyone familiar enough in this area that can help me answer these questions:

-gravitational waves also propogate at c so how did they escape the event horizons from which they originated?

-what and where was/is the "more energy than all stars emit as light in the universe"--10^34 megatons--released from? From matter in the accretion disk orbiting the black holes?

-the article says these two were spinning non-uniformly. Can we know if the bh's are spinning or just the stuff around them?

Post reply on HN