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LIGO Detects Gravitational Waves for Third Time

caltech.edu

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Re: LIGO Detects Gravitational Waves for Third Time

#51

Post by grigjd3 »

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.

Sensitivity is about picking out a signal from the noise. The way you increase sensitivity is to decrease the noise. Effectively this means isolating the environment of the LIGO experiment and a great deal has been done on this. If you search arXiv for LIGO over the past two decades, you'll find plenty of articles on this, but I warn you, you may be reading about injecting null energy modes into the system in one paper while reading about mirror design and reflection results in the next paper.

Re: LIGO Detects Gravitational Waves for Third Time

#52

Post by grigjd3 »

eaq wrote: post #4
wolfram74 wrote: post #2

Anyone familiar with this branch of astronomy want to explain why one detection in a volume on the order of 27 billion cubic light years is reasonable? Are they still processing data and will find more events? Is the sensitivity highly anisotropic so the detection volume is significantly smaller? Or are events like this just really conveniently rare that we get about 1 every data gathering interval?

While there are many compact binary systems in the universe continuously emitting gravitational waves for a very long time, LIGO is only able to detect the most violent waves emitted by the final coalescence and merger. So, on the astrophysical side, there is some joint probability given by how common these systems are, and how likely they are to merge in a given time frame. (Space based observatories like LISA would…

It's not 100% accurate that "LIGO is only able to detect the most violent waves emitted" but that LIGO can only detect waves emitted in a certain frequency range (total mass) of binary mergers. For instance, LIGO pretty much cannot detect the merger of supermassive black holes.

Re: LIGO Detects Gravitational Waves for Third Time

#53

Post by cyphar »

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…

> 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 of two depending on whether you want to consider the diameter or radius). Which is small but not as small as you might think. I also believe there are some upper limits on black holes too (that come from the upper limits of stars).

Re: LIGO Detects Gravitational Waves for Third Time

#54

Post by grigjd3 »

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.

Re: LIGO Detects Gravitational Waves for Third Time

#55

Post by mturmon »

lnx01 wrote: post #42
CodeCube wrote: post #32

Earlier quoted context omitted.

This is a complete guess, as I don't have any firsthand knowledge ... but I can only imagine that they have a whole array of seismographs on the premises, which they can use to clear noise from the main LIGO readings they are interested in. Curious to know if this is the case from anyone who happens to know one way or the other :) edit: the more I think about it, the more I think that random vibrations from passing t…

Actually, I think it's measuring the intensity of light between two points. It has two perpendicular lasers of some wavelength which then interfere with one another. If they interfere perfectly you measure a zero, if the interference is off by some amount you measure a deviation in intensity from the i^ or the j^ direction. You can eliminate passing trucks, earth tremors, mining, asteroid impacts etc simply by applyi…

Take a look at the time series from the first detection:

http://www.caltech.edu/news/gravitational-waves-detected-100...

The frequency range varies (it's kind of a chirp) as the black holes spiral in towards each other.

But the frequency at the end is in the auditory range. The instrument is literally measuring a displacement in the same range as many of the things you mention (traffic, mining, ...). A band pass filter will not do.

Re: LIGO Detects Gravitational Waves for Third Time

#56

Post by YZF »

nurettin wrote: post #28

How does LIGO separate vibrations caused by a nearby truck from whatever reading that is required for gravitational waves?

The detector is isolated, see here: https://www.ligo.caltech.edu/LA/page/vibration-isolation

Essentially a combination of passive/active isolation that effectively completely decouples the system from the environment.

I went to a talk by one of the lead scientists in LIGO and they spent a lot of effort on this. I believe the sites are also fairly remote so they don't have trucks driving nearby.

Re: LIGO Detects Gravitational Waves for Third Time

#58

Post by Florin_Andrei »

kstrauser wrote: post #27
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.

> - Briefly 10^34 megatons of energy were released every second That quote caught my eye too. What's the full unit on that? Is that literally the "m" you'd plug into E=mc^2, or was there an elided "...of TNT", like we'd use to describe nuclear weapons?

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.

Re: LIGO Detects Gravitational Waves for Third Time

#59

Post by InclinedPlane »

wolfram74 wrote: post #2

Anyone familiar with this branch of astronomy want to explain why one detection in a volume on the order of 27 billion cubic light years is reasonable? Are they still processing data and will find more events? Is the sensitivity highly anisotropic so the detection volume is significantly smaller? Or are events like this just really conveniently rare that we get about 1 every data gathering interval?

These events are actually not rare. There are roughly billions of them in that volume over the current lifetime of the Universe. They are comparatively rarer than many other kinds of events, like supernovae, because they involve the extremely massive stars that are themselves very rare.

Re: LIGO Detects Gravitational Waves for Third Time

#60

Post by spyder »

YZF wrote: post #56
nurettin wrote: post #28

How does LIGO separate vibrations caused by a nearby truck from whatever reading that is required for gravitational waves?

The detector is isolated, see here: https://www.ligo.caltech.edu/LA/page/vibration-isolation Essentially a combination of passive/active isolation that effectively completely decouples the system from the environment. I went to a talk by one of the lead scientists in LIGO and they spent a lot of effort on this. I believe the sites are also fairly remote so they don't have trucks driving nearby.

Oh: "LIGO's mirrors must be so well shielded from vibration that the random motion of the atoms within the mirrors and their housings can be detected."
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