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Edit: Despite my vaguely worded question, I got some great responses! So here's what I want: a universe in which gravity/spacetime can be manipulated (perhaps by some form of exotic matter) to allow for starships to accelerate to near light speed in a narratively convenient amount of time (days or weeks instead of years). But now that this can of worms has been opened, what else must be permitted? Inertial dampening? Defensive shields? Personal force fields? I'd like to limit the space fantasy elements as much as possible. And I'd definitely want to exclude FTL in any form as well as any free energy machines.

Original question:

If gravity manipulation were possible (like in Star Trek or Mass Effect) for reactionless propulsion, anti-gravity levitation, etc. what unintended consequences or complications might result? What unexplored applications might be possible?

There is no FTL in this proposed setting, so the limits of Relativity would otherwise apply.

(I'm aware that perpetual motion machines would be implied. Anything else?)

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    $\begingroup$ Newton's third law is: For every action, there is an equal and opposite reaction $\endgroup$ – Traveler May 7 at 19:12
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    $\begingroup$ Perpetual motion is only implied if your antigravity device can be used to build a device that generates more energy than your antigravity device consumes. If the antigravity device uses energy proportional to the mass it is effecting, it's pretty easy to ensure this isn't possible. $\endgroup$ – Matthew May 7 at 19:40
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    $\begingroup$ As it stand now this question is open ended, asking for a potentially endless list, with no specified metric to evaluate the best answer. $\endgroup$ – L.Dutch - Reinstate Monica May 8 at 2:42
  • $\begingroup$ The edit doesn't seem to be helping to focus your question. You're asking for a list of unintended consequences; and then on top of that, you're now asking for a whole nother list of what else is going to happen. This is actually the opposite of focusing! If you think you've gotten some good responses thus far, I'd counsel you to cut your losses and abandon this question as is. Then, considering those responses and the large number of things you're asking about, hone in on one or two that you consider to be of paramount importance and ask a small number of actually focussed questions! $\endgroup$ – elemtilas May 8 at 21:33
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The important question to ask here is whether conservation of energy applies. If you use a gravity/anti-gravity field to accelerate a mass $m$ to velocity $v$, does it require $1/2mv^2$ energy from the gravity field generator?

If not, perpetual motion is the least of your worries. Free energy allows anyone with a gravity field generator to make a weapon of arbitrary power. If you don't have FTL, then everyone's trapped in what suddenly becomes a free-fire zone.

How far can the gravity fields be extended? Can you activate one inside a natural gravity well? How big do the generators have to be? Terrorist attacks that arbitrarily accelerate, say, skyscrapers would be almost inconceivably destructive, assuming that some nutcase doesn't just do something like apply a continent-spanning gravity neutralizing effect to tear the Earth apart.

What are the limits on the gravity being applied? How small a gravitational source can be created (that is, how quickly does the field fall off)? If it's sufficiently precise, fusion becomes an easy energy source (not that you'll need it if gravity manipulation doesn't obey conservation of energy). Conversely, any yutz with a gravity generator and a bottle of water can make themselves a hydrogen bomb.

So the unintended consequences depend on the limitations of gravity manipulation and prevalence of its technology.

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  • $\begingroup$ I am suddenly reminded of hydrofusers... $\endgroup$ – Matthew May 7 at 19:42
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Your biggest unintended consequence would be Alcubierre FTL drives

The ability to generate anti-gravity behind your ship and gravity in front of it would give you a movable spacetime gradient. This is the prerequisite to making an Alcubierre FTL drive. The reason movable spacetime gradients allow you to go faster than light is because of how tachion mechanics work. Basically as long as you are falling into a gravity well, causality and the average speed of light is conserved because of the added resistance of trying to then go uphill to get back to your point of origin. By bring the curvature with you (which is what your artificial gravity tech would do), you can stay on a steep slope indefinitely allowing faster than light travel.

Figure B below shows a ship interacting with gravity from a foreign body. As it falls down the slope, it might move at 2C compared to the flat line, but as any light it is emitting tries to turn around and come back, the light will max out a 0.5C because it is fighting the gradient meaning that any observer from the flat line will never see B move faster than 1.0C.

In Figure A however, you can use artificial anti-gravity to create slope where you never fall compared to flat space. This means you can move at 2C as far as you want, and then just turn the artificial gravity around to go back the way you came at 2 C.

enter image description here

In short, you don't actually need to violate any additional limits of Relativity to get to FTL. Since you said you are not planning on FLT, I'd call that a pretty big unintended consequence.

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  • $\begingroup$ This assumes that gravity/anti-gravity in the OP's question operates by modifying spacetime curvature, rather than some sort of point-by-point application of/counteraction of gravitational forces. Ultimately, it still comes down to "how do the gravity machines work?" $\endgroup$ – jdunlop May 7 at 19:16
  • $\begingroup$ It is worth saying that what you describe is an Alcubierre drive, in case interested readers might want to read more on this method of propulsion by artificially space curvature. $\endgroup$ – Willk May 7 at 19:47
  • $\begingroup$ @Willk They do say it, or did when I posted my comment. $\endgroup$ – jdunlop May 7 at 20:53
  • $\begingroup$ @jdunlop- you said it to people who understand how an Alcubierre drive works already, but not the word Alcubierre which might be a starting point for the people who don't. $\endgroup$ – Willk May 7 at 21:15
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    $\begingroup$ @Willk It's right there in the second sentence. You probably just skimmed over it but it's been there the whole time. I went ahead and put it in the title too so that people who already know what they are don't have to read through to get to the point of what I am trying to say though. $\endgroup$ – Nosajimiki May 7 at 21:28
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Positive unintended consequences relate to architecture. Normally when we think of gravity manipulation we think of using it on vehicles, either to make passengers feel normal gravity or to propel a vehicle without reaction mass. But you can use the same equipment to make impossibly huge structures. You don't need to rely on the strength of building materials if you can manipulate gravity; just have your gravity manipulation equipment hold the building up. This would also allow you to have colonies on planets with gravity significantly higher than Earth; just turn the gravity down in your habitats.

Any malfunctions in the gravity manipulation equipment will feel like a jolt, due to jerk, the change in acceleration. Somewhat like when you're on an old elevator and it suddenly starts or stops. If this is a big enough jerk, it could injure effected people. I'm fairly sure this is true even if the total change in acceleration is low; the problem is how quickly it changes.

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You might want to consider the effects of radiation from particles being pulled into the artificial gravity well. There would be two main types: primary radiation and secondary radiation.

  • Primary radiation: This would mostly be protons caught in the well and interacting with matter, similar to earths Van Allen Belts but replacing the magnetic field with a gravitational one.
  • Secondary radiation: Anytime a charged particle is accelerated it emits photon radiation. Depending on the strength of the acceleration the energy will be higher or lower. In most cases this is caused by magnetic fields rather than gravitational but, gravitational acceleration of charged particles is believed to be the source of some radiation in supermassive black holes. This radiation is typically in the radio wave frequency, but a steeper gradient caused by a artificial gravity well, might create higher energy radiation. On earth synchrotron light sources are used to create very high fluxes of X-rays for scientific imaging.
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Runaway motion

Anti-gravity necessitates the involvement of negative mass. Scientists speculating what the effects of negative mass would be came to some interesting conclusions. The funniest one is the runaway motion. It's kinda complex, but I'll give you the long story short of it through analogy and metaphor.

Look at the drawings below:

Troll Physics - the illustration shows magnets being used to generate perpetual motion machines.

What the image depicts does not happen because you can't violate Thermo's Second. However, if instead of magnets you used a portion of positive mass (regular, everyday mass we all deal with all the time) and a portion of negative mass, then you would get exactly the troll physics effects depicted above!

Don't trust me just because I am saying it. Check the Wikipedia entry on negative mass. The further you read, the more the troll nature of Nature itself becomes apparent (all emphasis are mine):

Runaway motion

Although no particles are known to have negative mass, physicists (primarily Hermann Bondi in 1957, William B. Bonnor in 1989, then Robert L. Forward) have been able to describe some of the anticipated properties such particles may have. Assuming that all three concepts of mass are equivalent the gravitational interactions between masses of arbitrary sign can be explored, based on the Einstein field equations and the equivalence principle:

  • Positive mass attracts both other positive masses and negative masses.

  • Negative mass repels both other negative masses and positive masses.

For two positive masses, nothing changes and there is a gravitational pull on each other causing an attraction. Two negative masses would repel because of their negative inertial masses. For different signs however, there is a push that repels the positive mass from the negative mass, and a pull that attracts the negative mass towards the positive one at the same time.

Hence Bondi pointed out that two objects of equal and opposite mass would produce a constant acceleration of the system towards the positive-mass object, an effect called "runaway motion" by Bonnor who disregarded its physical existence, stating:

“I regard the runaway (or self-accelerating) motion […] so preposterous that I prefer to rule it out by supposing that inertial mass is all positive or all negative.”

— William B. Bonnor, in Negative mass in general relativity.

Such a couple of objects would accelerate without limit (except relativistic one); however, the total mass, momentum and energy of the system would remain 0.

This behavior is completely inconsistent with a common-sense approach and the expected behaviour of 'normal' matter; but is completely mathematically consistent and introduces no violation of conservation of momentum or energy. If the masses are equal in magnitude but opposite in sign, then the momentum of the system remains zero if they both travel together and accelerate together, no matter what their speed.

(...)

Forward used the properties of negative-mass matter to create the concept of diametric drive, a design for spacecraft propulsion using negative mass that requires no energy input and no reaction mass to achieve arbitrarily high acceleration.

The existence of anti-gravity therefore causes a violation of the second law of thermodynamics, which will probably win you a visit from the auditors of reality!

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  • $\begingroup$ Thermo's Second? I'm assuming you meant the second law of thermodynamics? $\endgroup$ – NomadMaker May 8 at 21:21
  • $\begingroup$ @NomadMaker yeah $\endgroup$ – Renan May 9 at 1:37
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You need to state how it works before you can state the implications.

Give an obvious example: does it work something like magnets when you bring the same poles together, but the force repelling them gets stronger as they come closer together? The equivalent might be that in a steep gravitational gradient, say on a planet's surface, the anti-gravity force might be very strong, but as you move into lower gradients, the force is weaker. That would have different implications than other forms of gravity manipulation. As one example, it would mean you could impart a great deal of force and thus acceleration near a body than you can further away from it, and the size of the body you're pushing against would impart greater energy the higher gravity it is.

Does it work simply by isolating the body against gravity? In that case you're still going to need some kind of propulsion to actually move it.

So, specify how it works, and then the detailed implications can be extrapolated.

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