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I am a complete neophyte regarding these topics. I did notice that the detected events (black holes merging, neutron stars merging) seem really exotic, while I

I am a complete neophyte regarding these topics. I did notice that the detected events (black holes merging, neutron stars merging) seem really exotic, while I assume gravitational waves are all around us all the time. Is our instrumentation for detecting gravitational waves simply very insensitive compared to the instrumentation used to detect electromagnetic waves? Or are they harder to detect for some more fundamental reason?

JohnBooty 7y ago
Gravity is super, super weak compared to the other forces. Every time you lift an object, you are overcoming the gravitational force of an entire planet. Gravity is that weak. Meanwhile, the incredible destruction caused by a nuclear bomb is the product of the energy stored in just a handful of atoms.
gibolt 7y ago
Gravitational waves are hard to detect because of their scale, speed and required precision. Some of the most precise lasers built are repeatedly bouncing light over miles in a tunnel to track changes in arrival significantly less than a trillionth of a second different. Doing this while filtering out noise from miscalibrated sensors or just small vibrations outside the chamber is HARD, but improving. It gets better as more come online around the world to confirm detections and aid in better directional targeting.
Retric 7y ago
They are hard to detect because you can only infer their existence from a mass of data vs directly detect them.
- jackcosgrove 7y ago
  So in other words these telescopes are measuring "the dog that didn't bark" rather than actually detecting gravitational waves? Much like we infer exoplanets from deviations of their stars' behaviors from what we would expect given no orbiting planets?
  - Retric 7y ago
    Yes. We actually detect gravitational waves because photons we created did not behave the way we expected.
  - bykhun 7y ago
    I see what you're aiming at, but technically it's not true. It can be similar though. The point about how we can only infer their existence from a mass of data is similar to how we talk about deep learning. The pretrained neural net works, but we can't pinpoint why, we can only test it on various data and receive results. There is no "dog that didn't bark", because we clearly can see the graphs and the correlations. The way we detect gravitational waves is not "the photon haven't arrived", but "the photon has arrived a few nanoseconds earlier than we expected it". (But multiplied by a few thousands of reflections and detected only by interference pattern, not the photon arrival itself). So, in a way, it's similar to deviations from paths by which we presume dark matter. But also, it's not that: we are almost certain here that these exact deviations are caused by the gravitational waves, because calculations and because multiple unconnected detectors, and because the form of the graph matches.
hardtke 7y ago
Gravitational waves are indeed everywhere. If the early universe underwent inflation, they should be everywhere. Any object orbiting another object emits gravitational waves. These exotic events, however, are the only gravitational waves that are observable using the terrestrial light interference technique (LIGO and Virgo). A space based mission would be sensitive in a different frequency range (satellites bouncing light off one another over millions of kilometers). The primordial gravitational waves from inflation might be observed indirectly -- there was much excitement about the BICEP telescope observing indirect evidence of these gravitational waves, but I believe that other explanations have come forth.
- tyscorp 7y ago
  I've been super excited about primordial gravitational waves for a while now. I have high hopes that they will lead to new physics. Just have to wait 15+ years for LISA...
chousuke 7y ago
As far as I understand it, the primary reason is that gravity is a much weaker force than any of the others, so detecting gravitational effects is difficult unless the scale of the event is colossal. These instruments are extremely sensitive; the effects they detect are just that small.
- kovrik 7y ago
  Yep, gravity is 39 orders of magnitude weaker than electromagnetic force. https://en.wikipedia.org/wiki/Hierarchy_problem https://en.wikipedia.org/wiki/Hierarchy_problem
- User23 7y ago
  It may be weak, but we can still measure it quite accurately: “...Concluding on a lighter topic, let me remind the GGP community of what I recall as probably the most memorable moment of the first campaign. It occurred at the GGP Workshop in Munsbach Castle, 1999, when Virtanen was describing the effect of snow cover on the residual gravity at Metsahovi. He showed a figure of gravity increasing by about 2 microgal over a 4-h period as men shoveled snow from the roof of the SG station, when a member of the audience asked why there was an interruption in the rise of gravity, Heikki said this was a 'tea break'...” D. Crossley, in Journal of Geodynamics 38/3-4 (2004), p. 234.
hansen 7y ago
Spacetime is extremely stiff or equivalently gravity extremely weak. Only these extreme events result in detectable gravitational waves.

JohnBooty 7y ago

Gravity is super, super weak compared to the other forces. Every time you lift an object, you are overcoming the gravitational force of an entire planet. Gravity is that weak. Meanwhile, the incredible destruction caused by a nuclear bomb is the product of the energy stored in just a handful of atoms.

gibolt 7y ago

Gravitational waves are hard to detect because of their scale, speed and required precision. Some of the most precise lasers built are repeatedly bouncing light over miles in a tunnel to track changes in arrival significantly less than a trillionth of a second different. Doing this while filtering out noise from miscalibrated sensors or just small vibrations outside the chamber is HARD, but improving. It gets better as more come online around the world to confirm detections and aid in better directional targeting.

Retric 7y ago

They are hard to detect because you can only infer their existence from a mass of data vs directly detect them.

jackcosgrove 7y ago
So in other words these telescopes are measuring "the dog that didn't bark" rather than actually detecting gravitational waves? Much like we infer exoplanets from deviations of their stars' behaviors from what we would expect given no orbiting planets?
- Retric 7y ago
  Yes. We actually detect gravitational waves because photons we created did not behave the way we expected.
- bykhun 7y ago
  I see what you're aiming at, but technically it's not true. It can be similar though. The point about how we can only infer their existence from a mass of data is similar to how we talk about deep learning. The pretrained neural net works, but we can't pinpoint why, we can only test it on various data and receive results. There is no "dog that didn't bark", because we clearly can see the graphs and the correlations. The way we detect gravitational waves is not "the photon haven't arrived", but "the photon has arrived a few nanoseconds earlier than we expected it". (But multiplied by a few thousands of reflections and detected only by interference pattern, not the photon arrival itself). So, in a way, it's similar to deviations from paths by which we presume dark matter. But also, it's not that: we are almost certain here that these exact deviations are caused by the gravitational waves, because calculations and because multiple unconnected detectors, and because the form of the graph matches.

hardtke 7y ago

Gravitational waves are indeed everywhere. If the early universe underwent inflation, they should be everywhere. Any object orbiting another object emits gravitational waves. These exotic events, however, are the only gravitational waves that are observable using the terrestrial light interference technique (LIGO and Virgo). A space based mission would be sensitive in a different frequency range (satellites bouncing light off one another over millions of kilometers). The primordial gravitational waves from inflation might be observed indirectly -- there was much excitement about the BICEP telescope observing indirect evidence of these gravitational waves, but I believe that other explanations have come forth.

tyscorp 7y ago
I've been super excited about primordial gravitational waves for a while now. I have high hopes that they will lead to new physics. Just have to wait 15+ years for LISA...

chousuke 7y ago

As far as I understand it, the primary reason is that gravity is a much weaker force than any of the others, so detecting gravitational effects is difficult unless the scale of the event is colossal. These instruments are extremely sensitive; the effects they detect are just that small.

kovrik 7y ago
Yep, gravity is 39 orders of magnitude weaker than electromagnetic force. https://en.wikipedia.org/wiki/Hierarchy_problem https://en.wikipedia.org/wiki/Hierarchy_problem
User23 7y ago
It may be weak, but we can still measure it quite accurately: “...Concluding on a lighter topic, let me remind the GGP community of what I recall as probably the most memorable moment of the first campaign. It occurred at the GGP Workshop in Munsbach Castle, 1999, when Virtanen was describing the effect of snow cover on the residual gravity at Metsahovi. He showed a figure of gravity increasing by about 2 microgal over a 4-h period as men shoveled snow from the roof of the SG station, when a member of the audience asked why there was an interruption in the rise of gravity, Heikki said this was a 'tea break'...” D. Crossley, in Journal of Geodynamics 38/3-4 (2004), p. 234.

hansen 7y ago

Spacetime is extremely stiff or equivalently gravity extremely weak. Only these extreme events result in detectable gravitational waves.