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

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.

This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

• Is this actually a "scientific spacecraft? – RonJohn Sep 1 '18 at 1:10
• @RonJohn 100%. We're not talking Star Trek "heavily armed cruiser with a science lab" here. Imagine something much closer to the spaceship from Interstellar. – Morris The Cat Sep 1 '18 at 1:54
• I didn't see that movie. – RonJohn Sep 1 '18 at 1:59
• @RonJohn Buckaroo Banzai's RV? – Morris The Cat Sep 1 '18 at 2:10
• Saw that, but a long while ago and don't remember much except that it was... different. (Good, but different.) – RonJohn Sep 1 '18 at 2:22

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.

• Would anybody be likely to have an instrument like this operating if they didn't ALREADY think someone might be messing with gravity though? I'm specifically thinking of the scene that leads to the "Hey... they might be messing with gravity!" inspiration. – Morris The Cat Sep 1 '18 at 1:30
• @MorrisTheCat funny thing, I was just thinking of a response to that. I think that you can use very similar apparatus to measure weight (it may even be the same thing, idr). So I would assume they would have a torsion scale and just be measuring hyper-fine differences in weight between whatever they were normally sciencing. Seeing it shift all of a sudden with no loads would definitely raise suspicion. Which you could then more accurately test I presume. – Black Sep 1 '18 at 1:38
• Also (once again for today XD), Any local gravity worth noticing the effect of is going to have a higher gravitational gradient the smaller the source is. In other words, a scale is going to pick up the variation just fine I would assume (unless the whole point of a torsion scale is to negate that effect?.... research time). – Black Sep 1 '18 at 1:41
• Can't remember the name of the other apparatus... and a torsion balance/pendulum is very specific (although why not add it to your kit? Aids in determining G of foreign planets which is relevant to just about all branches of science... even Chemistry)...That said I found this specifically the small size leading to an array of sensors for redundancy, which would (part of noise filtering) basically take a picture of local gravity shape; and the last paragraph which gives you a reason to have them. – Black Sep 1 '18 at 1:53
• @MorrisTheCat maybe they'll have a G-measuring device as part of a suite of devices for testing whether or not the laws of physics that we've developed on Earth really are as universal as we think they are. Or they cobbled together the experiment from bits and bobs (small vacuum chamber, torsion bars, etc) in the ship's various equipment closets and laboratories. Remember: they're a long way from home and so must bring all sorts of stuff with them to meet many unusual contingencies. – RonJohn Sep 1 '18 at 1:58

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.

• This is useful, although it doesn't directly solve my problem. I'm envisioning a progression of "What the heck, we're moving?? Why are we moving?" leading to "hey, something MOVED us, how did they do that??" leading to "They must be messing around with our gravity...". This answer is very helpful for the first part of that, since I wasn't sure if a human inside a sealed spacecraft would even be able to TELL if the entire spacecraft started moving in a new direction due to gravitational pull. – Morris The Cat Sep 1 '18 at 1:33
• @MorrisTheCat Everything would stay stuck to the wall after they felt the acceleration stop, including non metallic objects. Since the objects will continue to move towards the wall even after being moved away, gravity would be the first assumption. – Clay Deitas Sep 1 '18 at 2:32
• @Clay Deitas that assumes the ship is applying its own thrust vector, doesn't it? If your ship isn't accelerating on its own, then the only acceleration ANYTHING would be feeling would be the gravitational field, and everything would react to it starting, stopping, or changing exactly the same way, wouldn't it? – Morris The Cat Sep 1 '18 at 4:05
• @ClayDeitas I'm missing something here... why would anything hit the wall? The same force is being applied to the wall that's being applied to everything else. If the entire ship is (effectively) stationary, and our hypothetical aliens create spacetime curvature equal to the mass of Luna at a distance of ~1000km. The entire ship, humans, objects, etc, would start accelerating towards that point. If you turn it off, everything stops accelerating simultaneously. The only way the wall does anything different from the pen (or whatever) is if the wall is attached to something creating thrust. – Morris The Cat Sep 1 '18 at 4:21
• There's your detection method. The crew knows their course changed. They would feel acceleration if the deviation was due to a conventional thrust method. Because they can't, it must be something that's accelerating the whole ship evenly, which is artificial gravity - or so near as to be indistinguishable. – Cadence Sep 1 '18 at 9:02

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.

• Ahh, so basically this would measure the tidal forces applied by different parts of the ship being close or further from the gravitational gradient... yeah... this might work. I'm not sure if I think something that a scientific probe would have though, unless they specifically had a reason to be looking for gravitational anomalies which I'm not sure they would. – Morris The Cat Sep 1 '18 at 1:37
• That's why I would suggest pitching it to the reader as being a structural integrity or maneuvering/thrust sensor - something that they would be monitoring during normal flight just in case something went wrong with e.g. the engines. – Cadence Sep 1 '18 at 1:49

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.

• This is useful, but a much more combative take than I was envisioning. The scenario is more like "A bunch of scientists in a mobile laboratory suddenly realize that someone is MOVING their lab and they don't know how.": – Morris The Cat Sep 1 '18 at 1:34
• @MorrisTheCat Radar and a nav computer is the way that they would first discover that someone was moving their lab. 99% sure on that; as long as you are pre-FLT, radar is the best thing for rangefinding. – kingledion Sep 1 '18 at 1:40
• How many years does one has to wait to complete a single radar measure? – L.Dutch Sep 1 '18 at 3:40
• @L.Dutch The OP says they are in a system, in some sort of orbit. So not that long. – kingledion Sep 1 '18 at 6:10
• But your answer says "radar range finding off of multiple stars / planets". Which, of course, takes decades, requires the power of a star, and still won't work because radio waves don't bounce off seething balls of fusion. I'm also dubious as to whether they'd bounce off of gas giants at any detectable signal strength beyond it's own moons. – RonJohn Sep 1 '18 at 7:17

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.

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.

• You can see what do what on an airplane? – Clay Deitas Sep 1 '18 at 6:17
• Have you never sat in the back seat and seen parts of the passenger compartment seem to sag? It is definitely detectable by the naked eye. – John Wu Sep 1 '18 at 6:27
• No I haven't, and I hope I never do. Some kind of damn nightmare fuel going to make it impossible to see planes as non flying deathtraps. – Clay Deitas Sep 1 '18 at 7:05
• How flexible is an aircraft fuselage? Sleep well. – John Wu Sep 1 '18 at 9:33
• Please use Mathjax to format formulas – L.Dutch Sep 1 '18 at 9:53