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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:

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.
lutorm 11y ago
https://www.black-holes.org/gw150914 https://www.black-holes.org/gw150914 has some visualization of the event. There is the initial inspiral, and then there is a ringing afterwards. However, the entire event is over in a fraction of a second, which may be a "blip" to humans, but is very long when things happen at the speed of light.
- tizzdogg 11y ago
  Thanks, that's helpful. It's hard to get my head around the idea that an event so massive can be over so "quickly", without any residual longer-lasting effects.
  - lutorm 11y ago
    You're left with a big-ass black hole...
  - valarauca1 11y ago
    >without any residual longer-lasting effects A massive ripple in the very fabric of reality?
    - tizzdogg 11y ago
      Well it sounds like a massive ripple in the fabric of reality which passes by us in a fraction of a second, never to be seen again. So from my non-physicist point of view, no it doesnt seem like a very long lasting effect, relative to us at least. Thanks for the snark though.
  - lmm 11y ago
    There's a theorem that black holes have "no hair": two black holes of the same mass, charge and angular momentum are indistinguishable. So the merge must happen instantaneously: if the combined black hole were "sloshing" afterwards that would violate that theorem.
    - spacehome 11y ago
itaibn 11y ago
"do these waves behave like ripples in water, so a single event generates multiple repetitive, concentric waves permeating through space?" No they don't. There is a law known as [Huygens' principle](https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_princi...) which says that when a disturbance at a particular point creates a wave, that wave only propagates on an outwards-expanding sphere that is centered at that point of disturbance, and does not produce any effect on the interior of that sphere. This was originally formulated for light waves, but it also holds for other kinds of waves, such as sound waves or, in this case, gravitational waves. What this means is that when you look at something that's far away you see a sharp image of exactly what happened there a short time ago (the time it had taken the light to reach you), whereas if the principle did not hold, each light source would have a small "echo" after it which would blur the image. However, one of the reasons Huygens' principle holds is that the waves are propagating over three dimensions. In contrast, water waves only propagate over two dimensions, so Huygens' principle fails. That is why ripples continue to emanate from a spot even long after the disturbance there is over. More generally, Huygens' principle holds whenever the number of dimensions is odd and fails whenever the number of dimensions is even. [Note: I may be wrong on why Huygens' principle fails for water waves. Water waves are actually pretty complicated compared to other kinds of waves and I am not knowledgeable in all the subtleties.]

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.

lutorm 11y ago

https://www.black-holes.org/gw150914 https://www.black-holes.org/gw150914 has some visualization of the event. There is the initial inspiral, and then there is a ringing afterwards. However, the entire event is over in a fraction of a second, which may be a "blip" to humans, but is very long when things happen at the speed of light.

tizzdogg 11y ago
Thanks, that's helpful. It's hard to get my head around the idea that an event so massive can be over so "quickly", without any residual longer-lasting effects.
- lutorm 11y ago
  You're left with a big-ass black hole...
- valarauca1 11y ago
  >without any residual longer-lasting effects A massive ripple in the very fabric of reality?
  - tizzdogg 11y ago
    Well it sounds like a massive ripple in the fabric of reality which passes by us in a fraction of a second, never to be seen again. So from my non-physicist point of view, no it doesnt seem like a very long lasting effect, relative to us at least. Thanks for the snark though.
- lmm 11y ago
  There's a theorem that black holes have "no hair": two black holes of the same mass, charge and angular momentum are indistinguishable. So the merge must happen instantaneously: if the combined black hole were "sloshing" afterwards that would violate that theorem.
  - spacehome 11y ago

itaibn 11y ago

"do these waves behave like ripples in water, so a single event generates multiple repetitive, concentric waves permeating through space?" No they don't. There is a law known as [Huygens' principle](https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_princi...) which says that when a disturbance at a particular point creates a wave, that wave only propagates on an outwards-expanding sphere that is centered at that point of disturbance, and does not produce any effect on the interior of that sphere. This was originally formulated for light waves, but it also holds for other kinds of waves, such as sound waves or, in this case, gravitational waves. What this means is that when you look at something that's far away you see a sharp image of exactly what happened there a short time ago (the time it had taken the light to reach you), whereas if the principle did not hold, each light source would have a small "echo" after it which would blur the image. However, one of the reasons Huygens' principle holds is that the waves are propagating over three dimensions. In contrast, water waves only propagate over two dimensions, so Huygens' principle fails. That is why ripples continue to emanate from a spot even long after the disturbance there is over. More generally, Huygens' principle holds whenever the number of dimensions is odd and fails whenever the number of dimensions is even. [Note: I may be wrong on why Huygens' principle fails for water waves. Water waves are actually pretty complicated compared to other kinds of waves and I am not knowledgeable in all the subtleties.]