LIGO detects probable black hole merger event

https://twitter.com/ligo/status/1145405632553656326

by apaprocki • 7 years ago

196 42 7 years ago

astro123• 7 years ago

With the caveat that I have nothing to do with ligo and so don't know much...

I think that during this ligo run (it just turned back on in April-ish) they expect to find roughly 1 event per week. So this will be fairly common! [1] is a nice summary of what they have found so far.

For people here interested in the engineering side of LIGO, it is absolutely mind blowing. The effect of the gravitational waves is tiny (fractions of the size of a proton change in length) and so there are so many things that need to be incredibly tightly controlled for. I went to a talk on this a year or so ago, but can't find the slides... Here's a summary [2] and the wikipedia page also has some info.

1 - https://www.sciencenews.org/article/ligo-virgo-made-5-likely... 2 - https://www.engineering.com/Education/EducationArticles/Arti...

dreamcompiler• 7 years ago

Veritasium has a good episode on the LIGO engineering challenges.

https://youtu.be/iphcyNWFD10

chrischen• 7 years ago

fractions of the size of a proton change in length

...less than a ten-thousandth the charge diameter of a proton.

badosu• 7 years ago

I wonder if, as we get to smaller scales, do things start quantizing? Are we far from it?

civilian• 7 years ago

Can anyone explain how I should be reading that graph?

I'm surprised that the gravity anomaly is coming from a large region of space. And it's a curve. Is that expected? Why is it happening?

A black hole merge is coming from a single point in space, so I would have expected there to be a single point. But I'm definitely missing something.

petschge3• 7 years ago

It does come from a single point but they can not tell where inside the shaded area of space the single point was.

As far as I can tell from their website the event was seen by Virgo and only one of the Ligo detectors (Livingston), which makes localization in the sky rather uncertain. No clue at the moment why the second Ligo detector (Hanford) did not see it.

turndown• 7 years ago

From what I understand a lack of detection can also tell you something about where the point of origin is.

SiempreViernes2• 7 years ago

If not all of them see it and they have this shitty sky location, the one missing was either offline or its data was unusable for other reasons (ie someone left their phone on one of the mirrors and got a call).

gus_massa2• 7 years ago

It is easier to understand if you imagine that they are "hearing" the gravitational waves instead of "seeing" the gravitational waves.

Analyzing how the waves affect both detectors and the delay, they can get a rough idea of the direction. The origin was in a single point that is probably in the dark zone in the bottom left.

I'm not sure of the reason of the shape, but resolving equations in sphere usually give curved solutions, and then projecting it to a planisphere add more distortion. [I'm interested to read a more detailed explanation. Also, why the longitude is in hours?]

petschge3• 7 years ago

That's probably not longitude, but right ascension which is typically given in hours (and minutes and seconds).

SiempreViernes2• 7 years ago

The short version is "we heard something, but we don't know were from, somewhere over there probably waves at the sky"

pfdietz2• 7 years ago

The localization was particularly poor because LIGO Hanford apparently wasn't up(?), and didn't contribute data for this event.

jeanlucas• 7 years ago

Ok, their twitter humor is on point: https://twitter.com/LIGO/status/1145427537629208577

edit: actually all their replies are hilarious.

mehrdadn• 7 years ago

Wow yeah, these are hilarious.

DiogenesKynikos2• 7 years ago

This will sound cynical, but at this point, it's no longer news unless it's a binary neutron star merger. If there's an electromagnetic counterpart, then it's big news.

If you have a gravitational wave detection with an EM counterpart, you can get a redshift and luminosity distance, which means that you can measure the Hubble constant (in a way that is completely independent from Type Ia Supernovae). Binary black hole mergers are not expected to give off any EM radiation.

There should be a few binary neutron star mergers this run, and with some luck, there may be one with an EM counterpart.

cperciva• 7 years ago

Presumably if the location could be determined accurately enough and it was close to a galaxy, we could conclude that it was most likely in that galaxy, and thereby get a useful measurement though?

DiogenesKynikos2• 7 years ago

Not with gravitational waves alone.

People do want to correlate large numbers of binary black hole mergers with galaxy positions and come up with a statistical measurement of the Hubble constant, but no single BBH merger will be clearly identified with one galaxy under that scheme. You need lots of mergers, and the Hubble-constant determination is more model-dependent.

Binary neutron stars with EM counterparts are much more straightforward.

pfdietz2• 7 years ago

I'm waiting for a neutron star + black hole merger.

idlewords• 7 years ago

This is so incredibly cool.

Odenwaelder• 7 years ago

I'm fascinated about the degree of automation. It seems that all the necessary computations are made and presented on that website. I would love to learn more about this and their tech stack.

choeger• 7 years ago

Was there not some severe doubt on the whole signal inference? I recall an article that critized them for some changes in the protocol and also questioned the statistical significance of their findings. How is the current state of that discussion?

KnightOfWords• 7 years ago

Pretty much dismissed by later analysis I believe. Also, please bear in mind LIGO/VIRGO pinned down a neutron star merger with an optical counterpart, so there is independent confirmation the detectors can lift signal out of the noise.

perlgeek• 7 years ago

I love how that data is public, and viewable by anybody.

Geenirvana• 7 years ago

Is there anywhere that I can read up to understand the graphs?

gnufx• 7 years ago

Yes, the experimental details are stunning, particularly with an experimental physics background.

As someone who had a narrow escape from a PhD working on resonant bar-type detection attempts, I've never understood how those were ever thought to be sensitive enough -- whether decent calculations just weren't available or what. Unfortunately, you're typically not in a good position to evaluate such things before starting the work.

effie• 7 years ago

Weber's initial calculations and experiments with bar detectors were a reasonable effort, because gravitational waves were something that nobody believed in. Bar detector has an advantage - it is cheap. A strong enough wave could excite it and it would have been great and cheap discovery. But later when nothing was being reliably detected, people turned to more expensive interferometric detector. Today it is believed the bar type detector could only detect extreme events nearby, which are very unlikely.

73737373732• 7 years ago

How can black holes merge from our perspective, if something needs an infinite time to reach the event horizon from our perspective?

delecti• 7 years ago

The event horizon itself isn't really a "thing". The event horizons can merge without any "somethings" reaching any event horizon.

fsh• 7 years ago

Because it doesn't take infinite time from our perspective. The object falling into the black hole experiences strong time dilation due to the extreme gravitational field, but that doesn't affect a remote observer.

73737373732• 7 years ago

In the twin experiment, the observer sees his twin age slower, so it _does_ have a remote effect. From the perspective of the object falling into the black hole, the local conditions do not change, time is progressing normally etc.

peter303• 7 years ago

19 candidate events past 92 days since the improved system was turned on- about every five days.

I am not sure what elevates a candidate to a confirmed event.

Candidate events are posted quickly in hopes of observing associated photons.

LIGO detects probable black hole merger event

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