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So in (a lot of) sci-fi we have magic gravity that gives up and down and layer-cake decks, and nice and easy set design for the TV studios. I’m actually pretty much okay with this, but what rubs me the wrong way is the fact that if we can manipulate gravity like this, why don’t we have relativistic rail-guns or gravity drives when we move around?

Assume the tech level is around (but not specifically) Battlestar Galatica or Star Wars (Star Trek gets way to broken when you start examining specific techs [warp wave cannon anyone?])

Is there an internally consistent explanation as to why I can have gravity inside my spaceship that doesn’t mean other examples of shown technology are silly? Why would I ever have guns or nukes if I can make a gravity gun? Why would I have a reaction drive or atmospheric jets or a RCS system as my main maneuvering drive when I could have a gravity drive take care of it?

My goal is more to have a reasonable explanation for why I can have rule-of-cool looking spaceships, guns that go bang, thrusters that go whoosh, and still have a “down” for my crew on the spaceship. The idea is to have an explanation that makes sense in and of itself (inside its own universe). Handwavium is fine as long as you can explain to me why I can’t use handwavium to break some OTHER aspect of the world (a little breakage is OK)/invalidate all the ‘cool’ guns/thrusters/missiles stuff, or have the universe self-destruct into something unrecognizable.

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  • $\begingroup$ Well, one thing that strikes me as a possibility is energy requirements. Either it requires a specific type of energy carrier that is difficult to work with, or it requires so much energy that sure you could make, say, a weapon out of it, but you'd only be able to fire it once and only within 20 seconds of unplugging it from the wall after it's been charging for a day... (hey, sounds a bit like modern cell phones) $\endgroup$ – a CVn Sep 26 '16 at 21:22
  • $\begingroup$ @MichaelKjörling lol @ cell phones for being too true. As far as the 20 seconds after unplug.. that works great actually for personal weapons and devices, but what about ship scale weapons/drives? $\endgroup$ – Marky Sep 26 '16 at 21:31
  • $\begingroup$ My answer here applies exactly to this question as well. I would posit that this question is a dup while appearing different because the Q line is a single example and the question is about the thing in general. $\endgroup$ – JDługosz Sep 26 '16 at 22:28
  • $\begingroup$ How about this: gravity technology doesn't allow for more than 1-2g maximum at the moment, rendering the technology pretty much useless for weapons? $\endgroup$ – Andreas Heese Sep 27 '16 at 6:23
  • $\begingroup$ @JDługosz while your answer may be a perfectly applicable one here, I don't believe the questions are the same. The question by MadGab asks about the creation of complex objects based on 'force field' technology, while mine asks for a plausible explanation for the implementation of gravity on a ship. Two different questions; on the surface they appear to be similar because they are asking about "future technology" but in review are, in my opinion, different. The devil is in the details. $\endgroup$ – Marky Sep 27 '16 at 21:16
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Artificial gravity is a "charge"

In order to generate artificial gravity, you require a pair of charged surfaces, one pushing gravitons1 to the other (and/or one pulling them from the other). This allows anything between the two plates to experience the force of gravity (which can of course be tuned by adjusting the amount of energy used by the system), but it also means that without another plate to interact with, you can't use artificial gravity to steer your ship. In this sense, you effectively have "positive" and "negative" gravity plates, one above your crew's head and the other under their feet. Depending on how you want to work this, you could have independent plates on each deck (maybe artificially generated gravitons can't penetrate the decking, or lose a considerable amount of their force as they do), or just a single pair sandwiching the entire ship.

Gravity thus behaves very similarly to electricity: Without a supply and a ground, electricity doesn't do anything; once you have that completed circuit, however, it instantly goes to work. It's also very much akin to magnetism: Even with monopoles (if they exist), magnetism does nothing until and unless it has something to interact with. This makes this theory of artificial gravity particularly compelling (IMHO, anyway) because of the marked similarity to other forces in the universe.

It is worth noting that you probably could design a weapon that uses gravity to accelerate a projectile. You can easily wave this away by simply asserting that other mechanisms -- e.g. magnetic rail guns or laser cannons -- are more effective at destroying things than these "gravity guns" are. Maybe it takes more energy to effect the necessary acceleration than a comparable magnetic rail gun requires, for instance. You have that today even: Sure, you can use simple elastic torsion to launch a projectile with lethal effect, but magazine-fed firearms are clearly more effective weapons than bows.


1 It's worth pointing out that gravitons are, of course, a real (albeit hypothetical) particle, but for our purposes this is your handwavium, a particle or force with properties you get to dictate. (Therefore it's probably worthwhile to introduce and name an entirely new particle, though I leave that to you.) Note that here, while we're talking about it in terms of artificial (versus natural) gravity, it could very well be that gravitons really just simulate gravity, rather than are the source of all gravitational forces in the galaxy; for your average crewman walking across the deck, this distinction is irrelevant, but for explaining why gravitons enable crewmembers to walk around your ships and yet can't drive your ships directly, this distinction is absolutely critical. It's akin to using centrifugal force to simulate gravity: Walking around you really don't need to know the difference, but simply spinning a tube or donut obviously can't make a ship "go" all on its own.

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  • $\begingroup$ Great for a handwave but is not consistent with real physics. You need to introduce a different particle that acts well enough like gravity to normal matter but comes in two charges etc. $\endgroup$ – JDługosz Sep 26 '16 at 22:31
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    $\begingroup$ Liked your answer. It has similar ideas to ones I've entertained about to conceptualize science-fictional artificial gravity. Gave you a plus one. $\endgroup$ – a4android Sep 27 '16 at 9:36
  • $\begingroup$ @JDługosz That's why I emphasized that while the graviton is (hypothetically) real, in our case it's our handwavium. I tend to leave naming things to the world's creator. $\endgroup$ – Kromey Sep 27 '16 at 16:28
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There are a couple of reasons why apparent anti-gravity technology is only used in a relatively pedestrian way (ship design) with little evidence of other applications.

  1. Deliberate choice. Perhaps gravity weapons are too destructive, unpredictable, or clumsy compared to current weapons. So ancient treaties limit research into this area to the point where folks just take it as a given. Plus they revere the "old ways", so despite all the wild things liberal use of anti-grav can make happen, folks prefer things to look and feel as much like pre-anti-grav society as possible. Ships are the exception because they are just so damned convenient.

  2. Difficult application. The ship design (and presumably things like Cloud City) application is, for whatever reason, really cheap and easy. Direct military use is really energy intensive, so outside of specific uses (like the hyperspace limiting gravity well Interdictor ships from Star Wars) you just don't see it much.

  3. Anti-grav IS everywhere. We see that Star Wars has tractor beams, force fields, force shields, and repulsor arrays for common things like cargo sleds. Perhaps the "anti-gravity" effect we see on ships is just an application of the same science as tractor beam/shield technology. The common blaster (and perhaps the famed lightsaber) may also be using an application of this tech to contain energy in various ways. Thus the basic principles of "gravity" manipulation are ALL OVER THE PLACE, it is just that the end result looks so different it is hard to realize that the core tech is the same.

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First, I think it's worth noting that there are models of gravity nullification in science fiction that do not require energy, like the Cavorite in H.G. Wells' The First Men in the Moon. Cavorite is simply opaque to the force of gravity. In fact, this seems to be how the antigravity repulsorlifts work in the Star Wars films, as we see various speeders and spacecraft "parked" in a planetary gravity well still hovering a few inches to a foot off the ground. This is at least plausible since gravity is force, and so is not necessarily doing any work.

This allows some possibilities for restrictions beyond that of simple power requirements. For starters, the gravity manipulating tech might not be able to project itself at great distances, the way (for example) a projectile or blaster beam would, because the effects on the gravitational field are restricted to the volume of space close to this exotic, gravity-nullifying matter. Another possibility is that while a relatively inexpensive and non-bulky quantity of this exotic matter can be used to produce or negate 1G, it is prohibitively expensive or prohibitively massive to produce effects much larger than that. Remember that, even if a spacecraft is made out of anti-grav material, it may still be massive, and that would limit its acceleration and likely its logistical situation as well.

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  • $\begingroup$ Cavorite unfortunately has some serious conservation of energy problems. (Take a flywheel and put cavorite under one half of it.) $\endgroup$ – TLW Oct 15 '17 at 2:56
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There exist better ways of firing projectiles and flying ships.

Gravity control exists, and it's certainly possible to build a gravity-powered rail gun or gravity-based engine, but nobody does. Why? Because there's better guns and better engines out there. Gravity rail guns take several times the energy to accelerate a projectile to the same speed as an electric rail gun, and nuclear thrusters put out far more thrust than gravity based ones.

One possible explanation for this based on modern science could be that gravity leaks into other dimensions, so for a given amount of energy, you generate less force with gravity than you can with forces like electricity. If this were the case, it would be fundamentally impossible for a gravity generator to be as efficient as an electricity generator, and that inefficiency would make gravity a bad choice for guns and engines. Gravity would be useful for applying low, steady forces to large volumes, and hence would be useful in starship design and in tractor beam technology, but for other applications of force, other technologies would be superior.

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This answer does have similarities to Kromey's proposal for artificial gravity by positive and negative gravitons. Congratulations, Kromey, for thinking of something good. However, our mechanisms are different.

Assume there is a field that can be generated between two plates. This field has the property of causing the curvature of local space to increase proportional to the amount of energy pumped into it. The increased curvature of space create what is effectively an artificial gravity field and with the preferred direction being downwards to what has been designated as the floor.

This means spacecraft will have internal gravity fields anywhere on the vessel where pairs of gravity plates are installed. The strength of the artificial gravity will be determined by the amount of power stored in the curvature making field. For convenience this field will be called a barygenic field. "Barygenic" has the literal meaning of "heavy-making" which seems appropriate.

If power fails to the barygenic field the strength of artificial gravity will fade away as more energy is drained from it every time someone drops a tea cup or jumps up and down. This assumes the barygenic field has zero effect when turned on, and it is only the energy stored in the field that creates the "action" of gravity.

Gravity guns cannot work with a barygenic field system. Any munition accelerated to a lethal velocity would collide with the opposite barygenic field plate.

However, a form of gravity drive is possible. This would consist of an extremely powerful barygenic field system built around a major structural member of the spacecraft. This structural component is subject to high acceleration which will require pumping extremely large amounts of power into the barygenic field.

The maximum amount of acceleration this type of gravity drive can generate will limited by two factors. One, by the amount of power a spacecraft can generate safely. Two, by the amount of acceleration passengers and crew can endure safely and in comfort.

If barygenic field gravity drives, here this stretches the already rubbery limits of speculative physics possibly beyond any reasonable bounds, can embrace the entire fabric of a spacecraft and provide acceleration for its motion, there might be one interesting side-effect.

Spacecraft as shown in movies and TV, with artificial gravity, always configure their artificial gravity systems such that the floors are at right angles to the axis of acceleration and motion of the vessel as if spaceships were marine vessels or aircraft. So the nose of a spaceship is located in the front instead of overhead which is the realistic configuration for a vessel under continuous acceleration.

Possibly this can be explained if the barygenic fields producing artificial gravity inside the spacecraft interfered with the main barygenic field responsible the spacecraft's gravity drive. Then the front of the vessel will be in its direction of motion.

No real physics was harmed in the making of this answer.

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  • $\begingroup$ > "Gravity guns cannot work with a barygenic field system. Any munition accelerated to a lethal velocity would collide with the opposite barygenic field plate." This is false. Look at electron guns. $\endgroup$ – TLW Oct 15 '17 at 3:02
  • $\begingroup$ @TLW Probably not as well explained as it should be. The acceleration of munition in barygenic field version of a gravity gun is assumed to be between two plates. This suggests it can only be fired once & is unworkable thereafter. I also assumed that there were no holes in the two plates. Perhaps if the opposite was curved & had a hole a barygenic field gravity gun might be feasible. Is that what you were suggesting? Nice idea if it is. "Look at electron guns" isn't the fullest of explanations & leaves me otherwise puzzled. $\endgroup$ – a4android Oct 15 '17 at 3:27
  • $\begingroup$ Yes. Sorry, I started with a comment, ran out of characters, and deleted it all in favor of something that was, in retrospect, too short. $\endgroup$ – TLW Oct 15 '17 at 16:23
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    $\begingroup$ Imagine, for instance, taking two sets of plates in an X shape. In the center diamond, things will be accelerated up. Alternatively, imagine a cross shape. Things get accelerated up along the long axis, then get pushed enough sideways by the crossbar to miss the top. $\endgroup$ – TLW Oct 15 '17 at 16:25
  • $\begingroup$ @TLW That's great! Now I see. Two pairs of plates. A longer acceleration path to get a round up to speed, while a lateral acceleration moves it into position to emerge from an exit hole -- or the muzzle of a gravity gun. Wonderfully simple, and it would work too. Thank you very much for coming up with such beautiful idea. I hope you don't mind if I edit my answer to incorporate this concept? $\endgroup$ – a4android Oct 16 '17 at 1:44

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