9 ms·

> And then the ringing stopped as the two holes coalesced into a single black hole, a trapdoor in space with the equivalent mass of 62 suns. All in a fifth of a

> And then the ringing stopped as the two holes coalesced into a single black hole, a trapdoor in space with the equivalent mass of 62 suns. All in a fifth of a second, Earth time. Am I reading this correctly, that shortly after the detector came online we just happened to observe the exact moment a billion years ago that two black holes collided? Was that extremely coincidental? Or do these events happen all the time, and so if it wasn't those two black holes it would be two others?

hcrisp 11y ago
They state in the article that four events were detected during the engineering run. In the future tens of events could be detected per year.
dragonwriter 11y ago
> Was that extremely coincidental? Or do these events happen all the time, and so if it wasn't those two black holes it would be two others? From the article, no one knows: "Black holes, the even-more-extreme remains of dead stars, could be expected to do the same, but nobody knew if they existed in pairs or how often they might collide. If they did, however, the waves from the collision would be far louder and lower pitched than those from neutron stars."
antognini 11y ago
The predictions for the LIGO detection rate are very poor. They're based on a sample of just a handful of binary pulsars observed in our Galaxy, which would produce NS-NS mergers. The BH-BH merger rate is almost totally unconstrained, although it is generally thought to be less than the NS-NS merger rate. So the fact that a BH-BH merger was the first detection, and the fact that it was detected so soon after the sensitivity increases is evidence that the BH-BH merger rate is probably somewhat higher than expected. But we won't know for sure until LIGO detects more events and the rate can be better constrained. Sometimes you do just get lucky. I should add that there are lots of selection biases and educated guesses in all of this, too. The signal from BH-BH mergers is louder and easier to detect from larger distances. At the same time, NSs are probably more common than BHs, but it's not really clear whether there are more NS-NS binaries than BH-BH binaries because NSs receive kicks from the supernova when they are born but BHs (probably) do not. This may have the effect of blowing apart many nascent NS-NS binaries but leaving the BH-BH binaries intact.
- tomp 11y ago
  If this events are so rare (that we don't even know how rare they are), how is it possible that they achieved the required certainty (5 sigma)? I guess you could count one looong wave as a series of one-time events/measurements, but it could as well be a loooong interference.
  - cdelsolar 11y ago
    Both detectors detected the same event at slightly different times
    - 1024core 11y ago
      About 7ms apart, which ties in well with the distance between the two detectors.
  - davrosthedalek 11y ago
    It's not a counting experiment, which makes the calculation of a false positive rate somewhat harder. The key for LIGO is certainly that they saw the signal coincident at two stations, far apart.
  - antognini 11y ago
    The detection uncertainty is a separate matter from the predicted rate. Sure, if you had a strong prior that GWs should be detected once in a billion years, then you would want a better detection. But as it is the priors on the detection are pretty weak and this is totally consistent with what is expected.
  - dragonwriter 11y ago
    Others have answered other aspects of this, but as I understand it, it is not the case that we don't know how rare they (BH-BH events) are because they are so rare, we don't know how rare they are, because we don't have a really good model for them. So, we don't know how often we'd expect to detect them, once we had a detector.
deleted 11y ago
[deleted]
nathanielc 11y ago
Typically one would assume that it was not coincidental and then adjust the bounds for how often we expect the event occurs based on observations, or lack thereof.
misnome 11y ago
In the press conference Kip Thorne mentioned that the estimates were for several events per year, before sensitivity upgrades.
- davrosthedalek 11y ago
  But he neglected to mention the error bars on this, which AFAIK are huge at least for BH-BH mergers. Every time we built a new instrument, we saw something new, whether in astrophysics, or nuclear physics, or particle physics. Maybe the BH-BH rate is much higher than expected.
sandworm101 11y ago
>> that shortly after the detector came online we just happened to observe the exact moment a billion years ago that two black holes collided? Counterintuitive, but yes. Because it happened billions of years ago, it happened a long long way away. The sphere of objects billions of years away/ago is far larger than those closer to us. So such a detector should be detecting exponentially more very old objects than new ones. Given the rarity, I would expect nearly all detected events to have happened long long ago in galaxies far far away. Also, models point to such events being more common in the distant past where there were more black holes (primordials) floating around than there are now.
- idanoeman 11y ago
  *Polynomially more objects. The volume of a constant-thickness spherical shell is O(r^2).
  - tantalor 11y ago
    *Quadratically more objects.
    - TallGuyShort 11y ago
      *Pedantically
    - cdelsolar 11y ago
      less than quadratically because space is expanding in between
- pistle 11y ago
  If one event happens 1B years ago 1B light years away and another event happens at .5B years ago .5B light years away... how would we know there are two events?
  - explorer666 11y ago
    (not a physicist) The universe is expanding, which makes the wavelengths longer the farther away the source is.
tizzdogg 11y ago
I am not a physicist, and dont have a good mental model of gravitational waves (or general relativity at all), so maybe someone can answer my laymans question: do these waves behave like ripples in water, so a single event generates multiple repetitive, concentric waves permeating through space? That would make me think that a single massive event would be easier to detect because it would leave many repeated "echoes", ringing space for a long time after the actual event. The article makes it sound like the detection of these waves is just a quick one-time blip though. I'd expect something as big as black holes merging to generate more longer lasting waves than just a quick blip. What is the period of these waves?
- ScottBurson 11y ago
  An echo is a reflection. I don't know whether gravitational waves can be reflected even in principle, but even if they can, space is so empty that in practice there's nothing to reflect them. So even a single massive event, like the one described in the article, will just send out a single expanding spherical wavefront; if you're not listening at the right moment, you'll miss it.
  - tizzdogg 11y ago
    Sorry, "echo" wasnt the right term (hence my quotes). What I am trying to ask is if these behave like concentric water ripples, where from a single event you get first one peak of a wave, followed by many more repeated concentric peaks gradually getting smaller in amplitude? It sounds like there is just a single momentary wavefront without any residual secondary waves? Why is that?
    - jamesmaniscalco 11y ago
      So if you look at the waveform of the signal, there are in fact smaller ripples after the main event. However, how long these ripples take to settle afterwards to equilibrium is related to how quickly the waves propagate. In the case of ripples on a pond, those travel at about 1 m/s; these gravitational waves travel at the speed of light, roughly 300,000,000 m/s, so we should expect it to settle to equilibrium about 300,000,000 times faster. If it takes 60 seconds on a pond, we would expect the gravitational waves to settle in about 0.0000002 s, or 200 nanoseconds. Note that this is a _very_ rough estimate, but it should give you an idea of the order of magnitude for the settling time.
swayvil 11y ago
Excellent catch. That's damn suspicious.
- cdelsolar 11y ago
  I may have suffered permanent eye damage from the rolling.
  - macintux 11y ago
    Whatever you do, don't visit http://deepinsidetherabbithole.com/Is_the_earth_a_ball_.html http://deepinsidetherabbithole.com/Is_the_earth_a_ball_.html or you'll be chasing your eyes across the floor.
Florin_Andrei 11y ago
The coalescing and ring-down takes a fraction of a second. The fraction of a second refers to the duration of the event. It does not mean they caught an event a fraction of a second after they turned it on. Here's a better article: http://www.newyorker.com/tech/elements/gravitational-waves-exist-heres-how-scientists-finally-found-them http://www.newyorker.com/tech/elements/gravitational-waves-e...
deleted 11y ago
[deleted]
gammarator 11y ago
> Or do these events happen all the time, and so if it wasn't those two black holes it would be two others? This is right. Soon we'll have a much more precise value for "all the time!"

hcrisp 11y ago

They state in the article that four events were detected during the engineering run. In the future tens of events could be detected per year.

dragonwriter 11y ago

> Was that extremely coincidental? Or do these events happen all the time, and so if it wasn't those two black holes it would be two others? From the article, no one knows: "Black holes, the even-more-extreme remains of dead stars, could be expected to do the same, but nobody knew if they existed in pairs or how often they might collide. If they did, however, the waves from the collision would be far louder and lower pitched than those from neutron stars."

antognini 11y ago

The predictions for the LIGO detection rate are very poor. They're based on a sample of just a handful of binary pulsars observed in our Galaxy, which would produce NS-NS mergers. The BH-BH merger rate is almost totally unconstrained, although it is generally thought to be less than the NS-NS merger rate. So the fact that a BH-BH merger was the first detection, and the fact that it was detected so soon after the sensitivity increases is evidence that the BH-BH merger rate is probably somewhat higher than expected. But we won't know for sure until LIGO detects more events and the rate can be better constrained. Sometimes you do just get lucky. I should add that there are lots of selection biases and educated guesses in all of this, too. The signal from BH-BH mergers is louder and easier to detect from larger distances. At the same time, NSs are probably more common than BHs, but it's not really clear whether there are more NS-NS binaries than BH-BH binaries because NSs receive kicks from the supernova when they are born but BHs (probably) do not. This may have the effect of blowing apart many nascent NS-NS binaries but leaving the BH-BH binaries intact.

tomp 11y ago
If this events are so rare (that we don't even know how rare they are), how is it possible that they achieved the required certainty (5 sigma)? I guess you could count one looong wave as a series of one-time events/measurements, but it could as well be a loooong interference.
- cdelsolar 11y ago
  Both detectors detected the same event at slightly different times
  - 1024core 11y ago
    About 7ms apart, which ties in well with the distance between the two detectors.
- davrosthedalek 11y ago
  It's not a counting experiment, which makes the calculation of a false positive rate somewhat harder. The key for LIGO is certainly that they saw the signal coincident at two stations, far apart.
- antognini 11y ago
  The detection uncertainty is a separate matter from the predicted rate. Sure, if you had a strong prior that GWs should be detected once in a billion years, then you would want a better detection. But as it is the priors on the detection are pretty weak and this is totally consistent with what is expected.
- dragonwriter 11y ago
  Others have answered other aspects of this, but as I understand it, it is not the case that we don't know how rare they (BH-BH events) are because they are so rare, we don't know how rare they are, because we don't have a really good model for them. So, we don't know how often we'd expect to detect them, once we had a detector.

deleted 11y ago

[deleted]

nathanielc 11y ago

Typically one would assume that it was not coincidental and then adjust the bounds for how often we expect the event occurs based on observations, or lack thereof.

misnome 11y ago

In the press conference Kip Thorne mentioned that the estimates were for several events per year, before sensitivity upgrades.

davrosthedalek 11y ago
But he neglected to mention the error bars on this, which AFAIK are huge at least for BH-BH mergers. Every time we built a new instrument, we saw something new, whether in astrophysics, or nuclear physics, or particle physics. Maybe the BH-BH rate is much higher than expected.

sandworm101 11y ago

>> that shortly after the detector came online we just happened to observe the exact moment a billion years ago that two black holes collided? Counterintuitive, but yes. Because it happened billions of years ago, it happened a long long way away. The sphere of objects billions of years away/ago is far larger than those closer to us. So such a detector should be detecting exponentially more very old objects than new ones. Given the rarity, I would expect nearly all detected events to have happened long long ago in galaxies far far away. Also, models point to such events being more common in the distant past where there were more black holes (primordials) floating around than there are now.

idanoeman 11y ago
*Polynomially more objects. The volume of a constant-thickness spherical shell is O(r^2).
- tantalor 11y ago
  *Quadratically more objects.
  - TallGuyShort 11y ago
    *Pedantically
  - cdelsolar 11y ago
    less than quadratically because space is expanding in between
pistle 11y ago
If one event happens 1B years ago 1B light years away and another event happens at .5B years ago .5B light years away... how would we know there are two events?
- explorer666 11y ago
  (not a physicist) The universe is expanding, which makes the wavelengths longer the farther away the source is.

tizzdogg 11y ago

I am not a physicist, and dont have a good mental model of gravitational waves (or general relativity at all), so maybe someone can answer my laymans question: do these waves behave like ripples in water, so a single event generates multiple repetitive, concentric waves permeating through space? That would make me think that a single massive event would be easier to detect because it would leave many repeated "echoes", ringing space for a long time after the actual event. The article makes it sound like the detection of these waves is just a quick one-time blip though. I'd expect something as big as black holes merging to generate more longer lasting waves than just a quick blip. What is the period of these waves?

ScottBurson 11y ago
An echo is a reflection. I don't know whether gravitational waves can be reflected even in principle, but even if they can, space is so empty that in practice there's nothing to reflect them. So even a single massive event, like the one described in the article, will just send out a single expanding spherical wavefront; if you're not listening at the right moment, you'll miss it.
- tizzdogg 11y ago
  Sorry, "echo" wasnt the right term (hence my quotes). What I am trying to ask is if these behave like concentric water ripples, where from a single event you get first one peak of a wave, followed by many more repeated concentric peaks gradually getting smaller in amplitude? It sounds like there is just a single momentary wavefront without any residual secondary waves? Why is that?
  - jamesmaniscalco 11y ago
    So if you look at the waveform of the signal, there are in fact smaller ripples after the main event. However, how long these ripples take to settle afterwards to equilibrium is related to how quickly the waves propagate. In the case of ripples on a pond, those travel at about 1 m/s; these gravitational waves travel at the speed of light, roughly 300,000,000 m/s, so we should expect it to settle to equilibrium about 300,000,000 times faster. If it takes 60 seconds on a pond, we would expect the gravitational waves to settle in about 0.0000002 s, or 200 nanoseconds. Note that this is a _very_ rough estimate, but it should give you an idea of the order of magnitude for the settling time.

swayvil 11y ago

Excellent catch. That's damn suspicious.

cdelsolar 11y ago
I may have suffered permanent eye damage from the rolling.
- macintux 11y ago
  Whatever you do, don't visit http://deepinsidetherabbithole.com/Is_the_earth_a_ball_.html http://deepinsidetherabbithole.com/Is_the_earth_a_ball_.html or you'll be chasing your eyes across the floor.

Florin_Andrei 11y ago

The coalescing and ring-down takes a fraction of a second. The fraction of a second refers to the duration of the event. It does not mean they caught an event a fraction of a second after they turned it on. Here's a better article: http://www.newyorker.com/tech/elements/gravitational-waves-exist-heres-how-scientists-finally-found-them http://www.newyorker.com/tech/elements/gravitational-waves-e...

deleted 11y ago

[deleted]

gammarator 11y ago

> Or do these events happen all the time, and so if it wasn't those two black holes it would be two others? This is right. Soon we'll have a much more precise value for "all the time!"