[chiralSPO (OP)]

If there were aliens capable of using a "warp" drive (like an alcubierre drive, or star trek, etc.), how "big" would it have to be for LIGO to see (let's assume it's in our galaxy)? What about on the Earth? (we could see if anyone had successfully tested such a thing)

What would the signal look like?

[Colin2B]

I’m not sure whether it would create a gravitational wave, but NASA did say there had been a proposal for an interferometer which could detect the field distortion caused by an Alcubierre drive.
I also wonder whether the warp bubble would create a light cone similar to Cherenkov radiation?

[evan_au]

Oops! Overlap with Colin2B...
It's a bit hard to say, given that we know of no practical way to make such a warp drive.
- Some estimates have put the amount of mass/energy to create an Alcubierre drive at greater than the mas/energy of the universe
- That would certainly have an impact on the Earth if someone built one here.....

But if there were some practical way of creating such a drive
- And that it did radiate gravitational waves
          - Just like a supersonic plane radiates shock waves
          - This would be a continual power drain on the drive
          - I imagine designers would try to minimize wasted energy
- And the gravitational waves had a component in the right frequency range (about 50Hz to 1kHz for LIGO)
- Then it might be possible to detect it

I imagine that gravitational waves from a passing spaceship:
- Would be stronger, the closer the ship was
- Would be a single or double "blip" (like a supersonic shockwave), not the multiple cycles of oscillation that is currently being dredged out of the noise by the algorithms examining LIGO/VIRGO data.
- As a blip, it would have relatively little energy in the 50Hz-1kHz range
- But I imagine that different algorithms could search for coincident blips in the LIGO/VIRGO data

Future large gravitational wave detectors like eLISA (1 million km arms) would be even less sensitive to short "blips":
- A detector is most sensitive to frequencies whose arm length is about half a wavelength.
- Wavelengths much shorter than the arm length will have multiple cycles in the measurement arm (almost cancelling each other out)
- Wavelengths much longer than the arm length will have only a small part of a cycle in the measurement arm (dropping below the low-frequency response of the detector)

But if aliens had a warp drive, where are they? (Fermi's Paradox)