How could you tell if someone was messing around with your gravity?

Question

I'm working on a story involving first contact with an alien species that bases their space travel on directly manipulating gravity fields. My question is not about how that would work, but rather how would you be able to DETECT it working?

More specifically: My intrepid human explorers are operating in at a technology level sufficiently advanced to allow interstellar travel, but not advanced enough to involve Faster-Than-Light technology of any kind. They are newly arrived in a previously unexplored system, and during the encounter with previously mentioned aliens, the aliens start moving the human's ship.

So: If you're in interplanetary space (e.g. not close to a planet), and something creates an artificial gravity well which alters the orbital trajectory of your vehicle, how would you know what had happened?

Obviously if you're paying close attention to your relative position with the planets and the star itself you'd notice that SOMETHING had altered your vector, but what other instrumentation would notice?

The ideal answer would involve something that generates a "Well of COURSE any reasonably well-equipped scientific spacecraft would have one of those." reaction from the reader, rather than a "Wow, they're lucky they had one of those that they probably never thought they'd need or use."

EDIT: You should be imagining the Endurance from the movie Interstellar, except mine isn't specifically exploring a black hole, so my ship would be even LESS likely to have specialized instrumentation to detect gravitational anomalies.

Answer 1

Measuring gravity to high precision is (relatively) easy, and doesn't need (much) high-tech equipment. An interstellar space ship -- even a warship -- will have enough equipment on board that this experiment could be performed.

https://www.nature.com/articles/s41586-018-0431-5

The Newtonian gravitational constant, G, is one of the most fundamental constants of nature, but we still do not have an accurate value for it. Despite two centuries of experimental effort, the value of G remains the least precisely known of the fundamental constants. A discrepancy of up to 0.05 per cent in recent determinations of G suggests that there may be undiscovered systematic errors in the various existing methods. One way to resolve this issue is to measure G using a number of methods that are unlikely to involve the same systematic effects. Here we report two independent determinations of G using torsion pendulum experiments with the time-of-swing method and the angular-acceleration-feedback method. We obtain G values of 6.674184 × 10⁻¹¹ and 6.674484 × 10⁻¹¹ cubic metres per kilogram per second squared, with relative standard uncertainties of 11.64 and 11.61 parts per million, respectively. These values have the smallest uncertainties reported until now, and both agree with the latest recommended value within two standard deviations.

If you think that They are fiddling with gravity, start taking measurements on a regular basis, and especially during a "gravitational anomaly event". Noticing any changes in G should tell you if They -- or Something -- are actually fiddling with gravity or you need to look somewhere else.

Answer 2

What about...

A human!

Humans are great at detecting changes in acceleration, which is what a gravity change would feel like. If your ship has been traveling in a straight line on inertia alone, as long-distance ships are probably doing, running into a gravity field will feel like you've taken a sharp turn. Everything in the spacecraft not-tied down will likely crash into a nearby wall. If you've got a human on board, they'll probably notice.

Even if the human is tied down or very distracted, they'll likely experience a sense of vertigo or confusion as the otoliths in their inner ear move about unexpectedly.

Answer 3

Equip your starship with sensors measuring the structural load at various points along its frame. In normal flight, this assures you that a) your engines are producing the thrust they're supposed to, and b) your spaceframe is still in one piece. It's especially valuable if your ship is supposed to perform any very-high-precision maneuvers or if you anticipate taking it into atmosphere at any point.

More importantly, though, the signature of gravity accelerating your ship will be different than conventional means of acceleration: gravity will affect your whole ship more or less evenly, whereas conventional thrust will produce a pattern of stresses depending on the shape of your frame. Another way to look at it is that thrust originates from one point (the thruster) and is spread to the rest of the ship by the frame, whereas gravity acts on every point in the ship at once.

Answer 4

Well of course they use radar...

If there is no FTL technology, then good old fashioned radar is still the best way to do range finding. Radar range finding off of multiple stars/planets should give you positional accuracy of less than one meter, easily, assuming enough computational power to handle the intricacies of Doppler effect and distance to target (minutes or more, in many cases).

Any ship at sea will use radar to make sure it doesn't hit something. Any ship in space would want to use a navigational radar both to be on the look out for various small objects that you might run in to and to keep an accurate position relative to whatever planets/stars/celestial objects are nearby.

I think that determination of position change is pretty trivial, and any navigational computer would detect an induced course change within a few minutes at most. For example, the navigation computer that I used 10 years ago in the US Navy would have told me about a ~1 degree course change within 5-10 minutes, as we started to deviate from our track towards a pre-set navigational waypoint. Also, I had a navigator on my bridge team whose job was specifically to tell me about such things. However, that was a military ship, a merchant ship would not have a full time navigation specialist on watch.

An exception could be if the ship is doing something that causes it to transfer momentum; then unexpected distance changes might be harder to notice. Examples might be launching a shuttle, or transferring cargo to a nearby ship or something.

...unless you are in battle

The only good reason to turn off your radar is if you are in some sort of wartime condition. Warships on Earth do this as well. There is some debate as to whether trying to hide is viable in space; I'm in the 'there is some stealth in space' camp so I think a military vessel would turn off its active sensors to try to be less obvious.

That being said, there are alternatives. Directed beams like lidar would be nearly undetectable unless you are in just the right direction from the offending vessel, so you could still calculate your position from them. I don't know what the protocols would be for military warships in space, but there has to be some accommodation for safe navigation.

Answer 5

The scientists are in a large space station using lasers to more precisely measure gravity waves.

One of the major functions of science is to reconcile all the forces which dictate the functions of the universe. Magnets and electricity were reconciled into electromagnetism. Your scientists will be in space helping to reconcile gravity and the other forces. Except out of no where large gravity waves appear, which is either the result of a major cosmic event, or a close by source of gravity.

Your aliens who have already reconciled gravity with some of the other forces are able to use it in their technology. Everything gets wrapped up in a nice little package.

Answer 6

It's all in the creaking

Generally speaking, gravity acts uniformly on all objects within its field, so existence within a gravity field feels exactly like freefall. So it almost seems like a perfect way to move a vessel without anyone detecting it.

That being said, there may be an inescapable flaw in using an artificial gravity well, especially if it is too close, due to the inverse square law. The acceleration caused by gravity is proportional to the square of the distance from the center of the well. So in theory, on Earth, you feel a different amount of acceleration affecting your head versus affecting your feet, because your feet are closer to the center of the Earth. But Earth is so large, this difference is very very small. But with an artificial gravity well, which presumably is smaller than the Earth, the difference could easily be detectable.

The acceleration due to gravity is computed as

$acceleration=(gravitational \ constant) \times (mass \ of \ the \ body)/(distance)^2$

So plugging in some basic numbers, if you were to feel acceleration of 1 gravity at 200 meters, you would only feel about 0.98 g at 202 meters. So the height of a man yields an accelerational difference of 0.2 m/sec^2, possibly enough a person to detect, although possibly not.

However, the ship itself is much longer than 6 feet (I hope). If the front end of the ship is closer to the gravity field and the tail is farther away, the ship would "stretch," i.e. the tip would be pulled harder than the tail. This may not cause any damage, but it may cause a certain amount of creaking or shuddering, and passengers may even be able to see the hull distort slightly, the same way you can detect the fuselage on an airplane changing shape if you pay careful attention.