3 ms·

"First, [gravitational waves] have really long wavelengths." Why?

speff 3y ago
λ = v / f The velocity (v) is the speed of light for gravitational waves, which is already a really big number. If the frequency (f) is based on the period of the black holes which circle around each other, then I assume one rotation happens over a long period of time. Long period -> low frequency -> small denominator which makes the wavelength (λ) even longer.
- geon 3y ago
  Some rotations are fast. LIGO detects gravitational waves at 200-10000 Hz. That’s kind of fast. Just imagine a couple of black holes orbiting each other that fast.
  - walnutclosefarm 3y ago
    But the higher frequency waves detected by LIGO are not caused by two bodies orbiting their common center of mass at a distance, but rather by two much smaller masses - a few to a few tens times the mass of our sun - than the ones described in the cartoon. These masses orbits have decayed and they are spiraling into each other merging. The Short waves we detect only occur in the final bit of the spiral and merge - we see only the final milliseconds of the merger, and just a few wave crests.
    - ooterness 3y ago
      The idea of something with ten solar masses moving so fast still terrifies me. It's just mind-boggling to think about something at that scale completing an orbit in the blink of an eye. I am reminded of Randall Monroe taking about the sheer energy of a supernova: > Which of the following would be brighter, in terms of the amount of energy delivered to your retina: > A supernova, seen from as far away as the Sun is from the Earth, or > The detonation of a hydrogen bomb pressed against your eyeball? > Applying the physicist rule of thumb suggests that the supernova is brighter. And indeed, it is ... by nine orders of magnitude. https://what-if.xkcd.com/73/ https://what-if.xkcd.com/73/
defrost 3y ago
Not a bad question at all really. In principle, gravitational waves could exist at any frequency. The speed, wavelength, and frequency of a gravitational wave are related by the equation c = λf, just like the equation for a light wave. For example, the animations shown here oscillate roughly once every two seconds. This would correspond to a frequency of 0.5 Hz, and a wavelength of about 600 000 km, or 47 times the diameter of the Earth. https://en.wikipedia.org/wiki/Gravitational_wave https://en.wikipedia.org/wiki/Gravitational_wave To get a short wavelength requires a high frequency, to get an observable gravity wave requires a very large mass. We haven't yet seen a Super Massive Black Hole orbiting with (say) an Mercury orbit radius at a thousand times a second. There is another quote from that wikipedia article: Stephen Hawking and Werner Israel list different frequency bands for gravitational waves that could plausibly be detected, ranging from 10^−7 Hz (very slow) up to 10^11 Hz (very very fast). The faster the wave the shorter the wavelength and the greater the difficulty in detection (as the amplitude likely lessens and with distance falls below our current direct means). You'd have to chase the Hawking-Israel paper for their thoughts on short wavelength high frequency gravitational wave sources and how they believe they might plausibly be detected .. I anticipate some devil in the detail. http://library.lol/main/F92F35CD83F13A6021FC2385BBA171B0 http://library.lol/main/F92F35CD83F13A6021FC2385BBA171B0 ( perhaps wikipedia misquoted that high frequency )
- tzs 3y ago
  >> The speed, wavelength, and frequency of a gravitational wave are related by the equation c = λf, just like the equation for a light wave [...] > To get a short wavelength requires a high frequency For light that is assuming a vacuum. The more general equation is c' = λf where c' is the speed of light in the medium the light is traveling through. c' <= c. Hence, for light, you can get a shorter wavelength without having to raise the frequency if you work with the light in a medium with a lower c'. Is there anything similar with gravitational waves?
  - dotnet00 3y ago
    Probably not because the medium is the same spacetime everywhere and there isn't anything we know of that slows down or blocks gravity.
deleted 3y ago
[deleted]

λ = v / f The velocity (v) is the speed of light for gravitational waves, which is already a really big number. If the frequency (f) is based on the period of the black holes which circle around each other, then I assume one rotation happens over a long period of time. Long period -> low frequency -> small denominator which makes the wavelength (λ) even longer.

geon 3y ago
Some rotations are fast. LIGO detects gravitational waves at 200-10000 Hz. That’s kind of fast. Just imagine a couple of black holes orbiting each other that fast.
- walnutclosefarm 3y ago
  But the higher frequency waves detected by LIGO are not caused by two bodies orbiting their common center of mass at a distance, but rather by two much smaller masses - a few to a few tens times the mass of our sun - than the ones described in the cartoon. These masses orbits have decayed and they are spiraling into each other merging. The Short waves we detect only occur in the final bit of the spiral and merge - we see only the final milliseconds of the merger, and just a few wave crests.
  - ooterness 3y ago
    The idea of something with ten solar masses moving so fast still terrifies me. It's just mind-boggling to think about something at that scale completing an orbit in the blink of an eye. I am reminded of Randall Monroe taking about the sheer energy of a supernova: > Which of the following would be brighter, in terms of the amount of energy delivered to your retina: > A supernova, seen from as far away as the Sun is from the Earth, or > The detonation of a hydrogen bomb pressed against your eyeball? > Applying the physicist rule of thumb suggests that the supernova is brighter. And indeed, it is ... by nine orders of magnitude. https://what-if.xkcd.com/73/ https://what-if.xkcd.com/73/

defrost 3y ago

Not a bad question at all really. In principle, gravitational waves could exist at any frequency. The speed, wavelength, and frequency of a gravitational wave are related by the equation c = λf, just like the equation for a light wave. For example, the animations shown here oscillate roughly once every two seconds. This would correspond to a frequency of 0.5 Hz, and a wavelength of about 600 000 km, or 47 times the diameter of the Earth. https://en.wikipedia.org/wiki/Gravitational_wave https://en.wikipedia.org/wiki/Gravitational_wave To get a short wavelength requires a high frequency, to get an observable gravity wave requires a very large mass. We haven't yet seen a Super Massive Black Hole orbiting with (say) an Mercury orbit radius at a thousand times a second. There is another quote from that wikipedia article: Stephen Hawking and Werner Israel list different frequency bands for gravitational waves that could plausibly be detected, ranging from 10^−7 Hz (very slow) up to 10^11 Hz (very very fast). The faster the wave the shorter the wavelength and the greater the difficulty in detection (as the amplitude likely lessens and with distance falls below our current direct means). You'd have to chase the Hawking-Israel paper for their thoughts on short wavelength high frequency gravitational wave sources and how they believe they might plausibly be detected .. I anticipate some devil in the detail. http://library.lol/main/F92F35CD83F13A6021FC2385BBA171B0 http://library.lol/main/F92F35CD83F13A6021FC2385BBA171B0 ( perhaps wikipedia misquoted that high frequency )

tzs 3y ago
>> The speed, wavelength, and frequency of a gravitational wave are related by the equation c = λf, just like the equation for a light wave [...] > To get a short wavelength requires a high frequency For light that is assuming a vacuum. The more general equation is c' = λf where c' is the speed of light in the medium the light is traveling through. c' <= c. Hence, for light, you can get a shorter wavelength without having to raise the frequency if you work with the light in a medium with a lower c'. Is there anything similar with gravitational waves?
- dotnet00 3y ago
  Probably not because the medium is the same spacetime everywhere and there isn't anything we know of that slows down or blocks gravity.

deleted 3y ago

[deleted]