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Low energy, fantastic range

From HDE in another topic, I learned gravitational waves have an intensity which is inverse proportional to distance, not square distance, like in electromagnetic fields..

$$h\sim\frac{1}{R}\frac{GM}{c^2}\left(\frac{v}{c}\right)^2$$

Because of this, the gravitational wave energy becomes spread over a giant distances and their amplitude is extremely small.

Now suppose future detectors of gravitational waves will be very sensitive. I was thinking of using an array of highly sensitive gravitational wave detectors in orbit around a planet, as a long range sensor for ships that make use of artificial gravity, or Alcubierre drives.

https://en.wikipedia.org/wiki/Alcubierre_drive

An approximate direction to the ships is known

Important thing to keep in mind for this question: we know where to look, the direction: we know in what region of space these aliens are to be expected. So far, they were not particularly hostile to Earth, but we would like to know their capabilities. Distance to earth of the alien ships can be anywhere between 26 and 200 light years.

The source of the gravitational wave used for this

For our detector array, there is a suitable and regular gravitational wave, coming from that direction. The source of this GW is an imminent neutron star collision, which is in progress ca 600,000 light years behind our observation region, as seen from Earth. We expect to be able to use this constellation for about 100 years, before they collide.

https://marvelcinematicuniverse.fandom.com/wiki/Gravity_Field_Generator

Interference patterns

I wonder if an "obstacle" such as an Alcubierre drive, could cause a ripple in the gravitational waves, which could be found invoking interference patterns, like it happens in the water, when you put 2 sticks and disturb the surface.

enter image description here enter image description here

Alternative method: the bending of light

(thanks @ChristopherJamesHuff for the feedback..)

The linear artificial gravity and the Alcumbiere drive introduce excessive gradients in space time, which could be detected using other means. When the field required for the drive has enough range, it could be detected using the same methods applied for exo-planets: find a disturbance in the starlight characteristic for sudden ST-gradients, like like gravitational lensing. We've chosen to invest in the GW array, because 1) we don't know if these ships emit EM radiation, and 2) whether these ships will be be parked anywhere near a star.

Question: is it plausible that a ship invoking a space time distortion may form characteristic interference patterns in gravitational waves, that can be detected by my array ?

https://en.wikipedia.org/wiki/Shack%E2%80%93Hartmann_wavefront_sensor

Answers I'll vote for would explain how artificial gravity or other space-time irregularities would disturb gravitational waves, or not. The best answer would make me replace a tag.. I'd like this question to become science-based, now it is not.

Note 1: the goal of my detector is to find out if FTL or Alcubierre drives or artificial gravity can exist, if it is in use anywhere within our Milky Way, say 100 thousand light years. My sensor has no space-war or defensive purpose. EDIT: You may assume we know in what direction to look.

Note 2: science based frame challenges are very welcome ! When gravitational waves e.g. can't do interference patterns of any kind, please correct me.

Goodies
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    Remember, this will tell you where they WERE, not where they ARE. – Justin Thyme the Second Feb 12 '22 at 03:53
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    I love this idea, but aren't you still constrained to information traveling no faster than the speed of light? If so, the only practical detection might be within one's own solar system (unless you're SETI, they'd take this in a heartbeat). – JBH Feb 12 '22 at 06:34
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    @JoinJBHonCodidact clear, this can't be used for defense, they would be here when we see them, when their Alcubierre drive works properly. – Goodies Feb 12 '22 at 11:16
  • @JustinThymetheSecond yes that is relevant, but we know - about, only a direction - where they are, we want to find out more about their technology, the above is related to an earlier question https://worldbuilding.stackexchange.com/questions/220922/how-to-keep-the-peace-after-an-attempt-to-colonize-an-inhabited-planet – Goodies Feb 12 '22 at 11:17
  • It is remarkably difficult to find out how much energy would be lost by the ship in the creation of a gravitational wave... discussions of the subject tend to involve things like "Ricci tensors" which is my cue to give up and find an easier question to answer. The underlying problem though... could a warp-drive ship generate gravity waves powerful enough to be detected? Would it require an unreasonable power source in order to overcome these wave losses? I can't say, but I suspect that it puts sharp limits on your sensor array. – Starfish Prime Feb 12 '22 at 15:24
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    The biggest problem that gravity wave detectors have is the ambient noise level. There is so much going on with gravity waves that LIGO needed special signal processing to even detect anything. When I looked at the raw data, I was surprised that they could even find any signal in that much noise. There have been other teams processing the data and suggesting that other events were hidden in the noise. A ship would easily disappear into the noise. – David R Feb 12 '22 at 15:42
  • @StarfishPrime I don't need the Alcubierre ship itself to emit any gravitational waves. I wonder if the distortion of gravitational waves as a result of its presence of the ship would be detectable.. The Alcubierre drive forms a (steep!) disturbance in the space time gradiient, changing the waves? – Goodies Feb 13 '22 at 02:25
  • @DavidR you may assume for this question, we'll have a level of technology that allows a better S/N ratio than we see today, with much more precision in determining amplitude and frequency of the gravitational wave. My story world is 1-2 centuries in the future, the LISA architecture will be optimized to the bone. – Goodies Feb 13 '22 at 02:29
  • @Goodies it'd also distort electromagnetic waves, which are vastly easier to work with, and you have a highly-uniform microwave background as well as scattered high-intensity point sources of shorter wavelengths. Why try to use gravitational waves? – Christopher James Huff Feb 13 '22 at 14:10
  • @ChristopherJamesHuff it's range. What we're looking for is a "view" from a distance of at least 30 light years, we don't know how much more we need. My story characters hope to use their GW detector array as an hourglass, or telescope to find the home base of these ships. We hope the GW array will be sensitive enough to find anomalies like an Alcubierre drive. – Goodies Feb 13 '22 at 15:44
  • @Goodies why would gravitational wave detectors have any sort of range advantage? All the physics is in favor of optical/radio detectors. They have higher resolution, higher sensitivity, a brighter background, etc. You'll probably have detailed optical maps of the warp bubble before you can even detect it by its effect on ambient gravitational radiation. – Christopher James Huff Feb 13 '22 at 15:59
  • @ChristopherJamesHuff electromagnetic methods could work, but I wonder where I will find the photons.. our colony is 26 light years away and the adversary may be anywhere near that spot. What the effect of an Alcubierre drive, or a linear artificial gravity would be in terms of electromagnetics ? Can it be detected at 26 light years or more ? You need a star occlusion to see it ? does it emit light ? I thought of gravitational waves, because the phenomenon may be especially sensitive to Alcumbierre drives and artificial gravity. – Goodies Feb 13 '22 at 18:39
  • @ChristopherJamesHuff I've added some remarks to my answer about methods of detection using other means.. and focused some more (explain direction is known, and there is a GW available) – Goodies Feb 13 '22 at 19:13
  • @Goodies low-intensity ambient gravitational radiation would react to gravitational fields essentially identically to electromagnetic radiation. Both are just waves traveling at c through curved space-time. The latter itself involves a change in that curvature, but by a minuscule amount in this situation. I expect you could directly replicate any gravitational wave observation with ELF-VLF antennas. – Christopher James Huff Feb 13 '22 at 19:17
  • @ChristopherJamesHuff comment sections are too fragmented for this.. I have just finished some remarks in my opening text. Please put further remarks in a frame challenge answer. I could accept a frame challenge as best answer.. (see above) – Goodies Feb 13 '22 at 19:19
  • @Goodies While you assume that technology will be able to pick it out, I have my doubts. It would be like trying to pick out a specific car's horn through 5 miles of rush hour traffic. There are a large number of local gravitation wave sources such as earthquakes, tides, etc. which all contribute to the noise. Picking out a faint signal as you postulate would require knowing what that signal looks like first. Secondly, it would have to be large enough of a wave to be picked out against the background noise. I still think that it will disappear in the noise. – David R Feb 17 '22 at 15:19
  • @David R Satelites can pick up a particular cell phone from the millions of active cell phones in the area. – Justin Thyme the Second Feb 20 '22 at 02:34
  • @JustinThymetheSecond It helps that cell phones broadcast an id on a very specific frequency using a very specific protocol. Without that id, satellites would not be able to pick out a specific phone. Contrast that with how difficult it is to identify underground structures using seismic tomography which takes super computers, long time, difficult calculations and the results are still quite vague and open to different interpretation. – David R Feb 21 '22 at 14:49
  • This is a really great question, probably top 5% of questions – wokopa Jan 20 '23 at 13:06
  • Wow thanks for the compliment, @wokopa.. I'm used to answering things, not putting questions here.. sometimes I try, but most attempts have failed. In fact this question is my only successfull one on WB sofar (grin) – Goodies Jan 21 '23 at 09:48

3 Answers3

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Not practically.

Need many sensors to determine if patterns

To reconstruct a pattern you need a matrix/array of sensors. The example given, the 'Hartmann wavefront sensor' is backed by sensing devices that are millions of 1d sensors packed into a 2d array.

So to be able to create an image that makes it clear the patterns that may exist many sensors are needed ideally in 2d or 3d configurations. Each sensor to be one pixel of an image.

How many pixels do you need to clearly determine there is a pattern? Depends on what is attempted to be observed. What frequency, what resolution. It can be assumed that spaceships have less impact then stellar object mergers. My guess Is that millions of LIGO detectors scattered throughout the solar system would be needed to get good resolution of patterns.

Gravity-Radar (gravdar?)

Same idea as radar just with gravity. Much more practical then checking interference patterns.

To merely detect signal source magnitude and direction would require far fewer devices. Perhaps on the order of a dozen sensors. A few on earth plus a few at the L4, L5 points. This would assume they are large enough to be sensitive enough to detect smaller signals. Bare minimum would be three but large sensors in space with separation would allow for much better resolution both in detecting weak signals and for determining angle.

This is already being done. The array just needs to have bigger sensors that are not earthbound to be able to detect something like ships.

Military applications:

Sensor array. High resolution. Enough said.

Gault Drakkor
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  • "My guess Is that millions of LIGO detectors scattered throughout the solar system would be needed to get good resolution of patterns" No problem, my sensor array could be upscaled to let the gravitational wave detectors reside in solar orbit.. Technology level is science fiction, say 23th century. The array could even be launched into a galactic orbit, if needed. I just wonder if the principle would work. – Goodies Feb 12 '22 at 03:11
  • Will you take a check? – Mazura Feb 12 '22 at 08:41
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Maybe

The best warp drive paper I've seen lately comes from the Heisenberg lab at the Swiss Federal Institute of Technology. She says a warp drive could be constructed with just 1/10000 the energy obtainable by converting the sun to pure energy, though this might be improvable by a factor of, say, 10³⁰.

The energy requirement as described, if converted to gravitational waves, is not all that much less than is dissipated in a black hole merger. So if the aliens can't tune their drive well, you might see it. If, of course, they do better than that ... well, keep watching the skies!

Community
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Mike Serfas
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    Alien ships may roam around consuming stars (or DM), no problem. Thanks for your answer & PDF – Goodies Feb 11 '22 at 23:49
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Gravitational waves and electromagnetic waves traveling through a gravitational field would behave basically identically in terms of lensing, Shapiro delay, etc. Both are just waves traveling at c through curved spacetime. The gravitational waves are themselves curved spacetime, but to a minuscule degree when speaking of ambient gravitational radiation from sources at astronomical distances.

So, given similar sources, any detection of warp drives or artificial gravity by lensing or other effects could likely be replicated exactly with electromagnetic antennas.

Worse, there are very few gravitational wave sources, and they are all very low frequency, so any instruments will have to be similarly huge to match them, and so will any disturbances you wish to detect. In electromagnetic radiation, there is a thoroughly-studied cosmic microwave background that reaches peak intensity at a wavelength of 1.9 mm, and a sky full of near-black-body stars, interstellar clouds of gas absorbing or emitting with sharp spectral lines, etc. There is far more signal to work with, and the shorter wavelengths will reveal smaller targets or provide a more detailed view of those targets, as well as improving sensitivity to small disturbances.

Gravitational sensors would be more useful for detecting the emissions produced by such ships, which are likely to be quite distinct from any natural ones. They could also provide information on objects that can't be seen electromagnetically due to obstructing dust clouds, though this seems unlikely to be relevant.

Christopher James Huff
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  • Aha +1 thx for the answer.. "Gravitational sensors would be more useful for detecting the emissions produced by such ships, which are likely to be quite distinct from any natural ones" I am also looking for these! if the ship would produce gravity waves itself, that would be very nice, because the GW from behind would not be needed. Btw I still don't understand how light or radio waves from 26 light years distance would be affected in such a way, that the result can be observed. The ship must be huge, and you'd still need a light source behind the ship, to see distortion, or lensing. – Goodies Feb 13 '22 at 22:07
  • @Goodies you have precisely the same illumination problem with gravitational radiation, except the available sources are dim and few and your detectors are less sensitive. As unlikely as it is with EM, it's even less likely with gravitational radiation. – Christopher James Huff Feb 14 '22 at 00:24
  • I have changed the opening question yesterday to clarify, also what the source is and how long it will last. Comments were taken into account, also yours. – Goodies Feb 14 '22 at 07:18
  • I acknowledged this answer, it does not make my topic scientific, but it yielded an idea for my story... deviating gravitational waves - or pulses - could origin from the ship's Alcubierre drive itself. I think I can use that idea. It also solves the source-problem. In my original plan I need a steady field of gravitational waves, above answer circumvents that. – Goodies Feb 20 '22 at 09:47