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Let`s suppose there is an alien supercivilization whose interstellar vessel has «exited hyperspace» in a close proximity of the Earth (i.e. a certain volume of physical space only filled with vacuum suddenly gets occupied by a macroscopic object).

Let`s also suppose there is no way to detect it by conventional means.

The question is: can we, in theory, use our existing gravitational wave detectors to tell where the vessel exited, if exited at all? Which are the probable restrictions on its linear size, or mass, or distance, or direction to the exit spot?

Clarification: the only noticeable effect of «exiting the hyperspace» is the ship popping up where there used to be nothing. So, where the spacetime metric used to be more or less flat, a sudden curvature appears, just like when a guitar string gets tapped.

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The answer to this question resolves itself into two parts. First, whether a spaceship exiting hyperspace will be a source of gravitational waves. Second, whether gravitational wave detectors can detect these gravitational waves.

Any object with mass that accelerates (which in science means changes position at a variable rate, and includes spinning and orbiting objects) produces gravitational waves, including humans and cars and airplanes etc. But the gravitational waves made by us here on Earth are much too small to detect. In fact, it isn’t even remotely possible to build a machine that can spin an object fast enough to produce a detectible gravitational wave – even the world’s strongest materials would fly apart at the rotation speeds such a machine would require.

Since we can’t generate detectable gravitational waves on Earth, the only way to study them is to look to the places in the Universe where they are generated by nature. The Universe is filled with incredibly massive objects that undergo rapid accelerations (things like black holes, neutron stars, and stars at the ends of their lives).

Source: https://www.ligo.caltech.edu/page/gw-sources

For a spaceship to be a source of gravitational waves as it exits hyperspace this spaceship must physically act as if it is an object with mass undergoing an acceleration. Effectively, the spaceship might be apparently decelerating, but this is still an acceleration.

Presumably the rate of acceleration will be determined by the time it takes the spaceship to exit hyperspace. The shorter the exit time, the higher the acceleration will be. Also, the more massive the spaceship is, the stronger its signature as a gravitational wave will be.

The strength of a gravitational wave signature is the more detectable it will be. Now existing LIGO systems can detect black holes merging, binary neutron stars, and supernovae. Therefore, the parameters that need to be manipulated to ensure the probability of a spaceship exiting hyperspace are its mass, the time to exit hyperspace, its proximity to the detector systems, and the sensitivity of gravitational wave detectors.

Since the OP wants the spaceship to be detected only by its gravitational waves, this sets an upper bound to the mass of the spaceship. For example, a Jupiter-mass spacecraft exiting hyperspace in a fraction of an attosecond might be the source of gravitational waves of sufficient strength to be detected. However, if a Jupiter-mass object arrived near Earth it would be readily detected because it would change the orbits of Earth and the Moon plus the satellites orbiting the Earth. Also, it likely to be big enough to be seen in the sky from Earth. So this example fails the only detectable by gravitational waves test.

However, what this example does is set out the parameters that need to be meet. However, there are factors that cannot be readily determined. Assume a spacecraft that is sufficiently massive, which exits hyperspace in a manner that is effectively an acceleration and acts a source of gravitational waves, the key question is can it generate a sufficiently strong enough burst of gravitational waves to be detected?

Fortunately, the spaceship will be exiting hyperspace close to Earth, so proximity of the source to the detectors is in their favour. Now gravitational wave detectors are incredibly sensitive instruments. On the other hand, gravitational waves themselves are incredibly weak which is why LIGO and other GW detectors are so mind-bogglingly sensitive.

This puts us in the realm of speculation, because we don't know exactly how strong any gravitational waves generated by a spaceship exiting hyperspace will be. In a work of fiction, the author can optimize the factors to ensure the alien spaceship could be detected. But if the author of this answer had to guess, he would guess against its detection. The gravitational waves wouldn't be strong enough.

The mass of the spaceship would have be absolutely massive. Its exit time from hyperspace has to be close to Planck intervals. Both the proximity and sensitivity of gravitational wave detectors need be better than existing devices. So, possibly future detectors will be good enough. if all these factors are in the right proportions, then yes it can be done, theoretically, at least, but guaranteeing requires dramatic licence and every fiction writer should have one.

Plausible, yes, Possible, uncertain.

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Well this isn't really a question anyone can answer, as we don't know what "hyperspace" is and hence we have no idea what the gravitational effect of something suddenly popping into real space from hyperspace would be.

But we could not in all probability detect it.

Why say that if I say I don't know all that much about the scenario ?

Well first things first : gravity is an incredibly weak force. So it's effects are tiny.

If I popped a new moon near Earth (*what does "near" mean ? :-)) the effects will be quite small. All we get from the moon we have (which is a large moon) is a tiny tidal effect on our ocean. So a ship won't deliver much of a gravitational field.

And the gravitational waves are way, way, way weaker than the gravitational forces.

So we wouldn't notice them.

We notice the huge events that LIGO detects simply because they are powered by the almost instant conversion of multiple solar masses into gravitational energy - an exceptionally energetic event. You might think of it this way : a star going nova can be seen a really long way away, but a grenade ? Not so much. :-)

It's worth reading this section of Wikipedia's article on Gravitational Waves to get an idea of the relative strength and practicality of detecting waves from small objects.

Our existing detectors would need a huge object (like a large planet) to pop into existence "close" to Earth to be detected, I'd say. But, again, that's not knowing or having any basis for the effect of something switching from "hyperspace" to real space and just making rough guesses.

Finally LIGO can't work out location or direction as you wish. This article explains what its limitations are in that regard. So locating alien space ships is way beyond it.

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  • $\begingroup$ Undoubtedly, gravity is weak. But tides, for example, are still macroscopic. LIGO and VIRGO, in their turn, can detect spacetime metric oscillations on zetta- to yotta- range. Even LHC events are affected by the moon: home.cern/about/updates/2012/06/full-moon-pulls-lhc-its-protons $\endgroup$ – hidefromkgb Jul 3 '17 at 0:36
  • $\begingroup$ @hidefromkgb yes, by the moon. You can see the moon clearly, right? But you can't see any of the rovers on it, I guess? So we can gravitationally detect moon. We need a kilometers to planet size detector to do that (lhc vs tides) but we can. With much less detail than we can see it with naked eye. What does it tell you? $\endgroup$ – Mołot Jul 3 '17 at 6:29
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In the service of the story being told by the OP: Don't worry about the gravitational wave, just detect the gravity. See this map of the highly inconsistent gravity field on the surface of the Earth.

As the article says, it can map gravitational pull on the Earth with a 7.2 arc-second resolution: That is 0.14 miles, or 738 feet across. smaller than a typical stadium.

One of the satellites being used for this is called GRACE (Gravity Recovery And Climate Experiment), here is the NASA technology page describing how gravitational pull is detected (it requires multiple satellites).

The other data incorporated into the model comes from GOCE (Gravity-field and steady-state Ocean Circulation Explorer, here is the Wiki page).

Gravitational waves are momentary, weak, and difficult to detect. Gravitational pull is not!

In fact, for the purpose of fiction, Satellites similar to these (GOCE is over, I think Grace is over too) can act as your detector, if the ship has the mass of a mountain. These satellites were mapping the Earth, but the gravitational pull of the spacecraft will affect these satellites (and our GPS satellites too, for that matter).

Say we launch several new mapping missions, to cover the poles and ice caps, to increase our resolution by a factor of 10, or for some other excuse. Military reasons, perhaps. We launch several because we want continuous mapping, updated daily, able to track ships and planes, tectonic movement, underground facility construction, new buildings, and so on. Finer gravitational mapping could easily discover tunnels used for drugs and smuggling, for example (the absence of Earth reduces gravitational pull above the tunnel, if it is not extremely deep). It is another lens on the Earth.

The alien ship gravitational mass will distort the map of the Earth the satellites are producing in a very consistent way; since the aliens are not moving. A clever mathematician at NASA or ESA (there are boatloads of them), learns that the distortion is the same in all the mapping satellites (so is not a glitch or bug). But she computes the distortion is completely accounted for by a single new gravitational source, and given a few weeks of super-computer time she can compute the location and mass of the object.

This may not be the gravitational waves the OP was hoping for, but it is at least gravity and real life tech already used and in the history books. A little fictional liberty, and the OP could say we have secret military satellites already up there and engaged in precisely this activity, from multiple nations (e.g. US, Russia, China, Japan, EU). I would not find that implausible at all.

P.S. Added: With a continuous series of such maps; she could tell which day it appeared, that it did not move into place but appeared more or less instantaneously, and also whether it is moving and in what direction and at what speed.

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