One staple of science fiction (particularly if you look at Star Trek in all of its incarnations) is that spaceships are equipped with artificial gravity; a second one is that each time a ship is hit in a battle, the crew is exposed to the violent movement of the ship that is a byproduct of that hit. Yet, if the ship is moved even violently, the gravitational field should be moved in the same manner, since it's directly attached to the ship. Wouldn't the result be that the crew experiences no such bumps and tremors as long as the gadget responsible for the creation and the maintenance of the gravitational field remains intact?

Isn't that situation similar to why we, living within the gravitational field of Earth, feel neither the spin of the planet nor its movement through space?

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    This would be off-topic for a real world explanation, or dependent on the settings particular variants of physics. Frankly, you're wrong, if the planet ran into something that would give it a significant jolt, we would feel it. It's called preservation of momentum, and it has nothing to do with how well gravity is working. In fact, it's a good reason why any ship exposed to a violent movement isn't just going to effect the crew, but actively splatter crew as they impact the insides of the ship, in crunchier settings. – Radhil May 19 at 10:15
  • @Radhil Thank you very much for you explanation. I haven't had a physics lessen in two decades, so I'm an absolute layman in that regard. If this question is off-topic, what am I supposed to do with it? Scrap it altogether? – glahn May 19 at 10:18
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    This is too broad to sensibly answer. You need to narrow it down to a single canon universe – Valorum May 19 at 10:19
  • @Valorum okay - done. – glahn May 19 at 10:34
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    The problem is with Inertial Dampeners which aren't perfect. – Endgame May 19 at 14:21
up vote 30 down vote accepted

This is addressed in the TNG Technical Manual. The Enterprise has artificial gravity (of 1g) pulling the inhabitants downwards. When the ship experiences sudden unexpected movement, the ship is able to compensate to prevent the crew from fatally slamming into the walls, but there's sufficient lag in the system that still allows unpleasant lateral movement.

As acceleration effects are anticipated, this [Structural Integrity] field is distorted along a vector diametrically opposed to the velocity change. The IDF [Inertial Dampening Field] thereby absorbs the inertial potential, which would otherwise have acted upon the crew. There is a characteristic lag time for the shifting of IDF direction and intensity. This lag varies with the net acceleration involved, but averages 295 milliseconds for normal impulse maneuvers.

Because IDF control is generally derived from Flight Controller data, normal course corrections can be anticipated so there is rarely any noticeable acceleration to the crew. Exceptions to this sometimes occur when power for IDF operations is restricted or when sudden maneuvers or other externally caused accelerations occur more rapidly than the system can respond.

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    @Politank-Z - Cheers for the edit, but the Inertial Dampening Field (IDF) is a function of the Structural Integrity Field (SIF), not the other way around. – Valorum May 19 at 13:27
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    It's probably worth noting that this is a retrospect explanation, even if it's canon. The original, real world explanation is that it makes for better drama, and thus better storytelling. – Austin Hemmelgarn May 19 at 18:43
  • Isn't the IDF separate from the artificial gravity? – David Z May 19 at 20:39
  • @DavidZ - yes. Each deck plate has an artificial gravity generator that provide a constant 1g pull. – Valorum May 19 at 20:46
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    What's awesome about this clip is that the empty chair doesn't move, while Riker's chair is going mental. – Jontia Jun 20 at 9:44

Whatever means of generating artificial gravity you have, it can't react to changing acceleration quickly. If it could react quickly, it could conceivably start oscillating (gravity going up and down quickly) in response to minor changes in acceleration. Explosions and collisions are sudden things that happen faster than you want your artificial gravity to be able to respond to.

For normal operation (accelerating from full stop to 0.5c in a few seconds,) your ship's systems can coordinate with the artificial gravity to keep you from being smashed into ketchup from the acceleration. But, that isn't a spontaneous reaction to an outside event. That is a planned and coordinated effort of your ship's systems. So, the restriction on speed of reaction doesn't apply.


This also explains the crew reaction during extreme maneuvers. The artificial gravity must be modulated to neutralize the acceleration in various directions caused by the maneuvering. If the coordination isn't perfect, the crew will be tossed around by the imperfections.

"Perfect" would mean compensating thousands of Gs of acceleration to a tiny fraction of 1G. So, you need perfect matching to better than 1/10000 between acceleration and artificial gravity.

Judging by the effects seen in movies and TV shows, the compensation gets in the neighborhood of 1 or 2 Gs. Better than being mashed flat, still enough to toss around anyone who isn't expecting it.

  • In the second-to-last sentence, do you mean "1 or 2 G" or "1 or 2 G/s"? Numeral + plural-unit is throwing me off, so I can't quite tell. (the earlier "thousands of Gs" doesn't throw me off, because that's word + plural-unit) – Ethan Kaminski May 19 at 18:32
  • Multiples for G forces. 1G or 2Gs – JRE May 19 at 18:59
  • The unit for "gravities" (plural) is just G. Gs is a unit for delta-V, acceleration maintained for a period of time. – Ben Voigt May 20 at 17:22

IMHO there is a difference between the artificial gravity and the Inertial Dampening Field (IDF) that reduces the effects of the ship acceleration, deceleration, and shaking.

Suppose that you were standing on the ground on Earth under one gravity acceleration due to Earth's gravity, and a one thousand mile per hour wind struck you. The force of the wind would blow you away at a speed great enough that hitting the ground occasionally would really hurt, despite the fact that the acceleration of one gravity from Earth's gravity was still working.

And suppose that you were standing on the surface of Earth under one gravity acceleration due to Earth's gravity, and you were struck by a large moving vehicle at a speed of 200 miles per hour. You would be sent flying, suddenly accelerated by the vehicle, and probably at an acceleration of several Earth gravitys, despite the fact that the acceleration of one gravity from Earth's gravity was still working.

And suppose that you were riding in an airplane at a speed of hundreds of miles per hour near the surface of the Earth, under a steady one gravity of acceleration due to Earth's gravity, and the airplane suddenly hit an obstacle, like the World Trade Center, at a speed of hundreds of miles per hour. Clearly the sudden deceleration would kill you and destroy the airplane despite Earth's gravity remaining constant at one gravity.

So clearly the IDF is necessary to prevent the crew from being tossed around in all directions and injured or killed whenever a starship rapidly accelerates, decelerates, or is shaken.

In the Sector General stories by James White there are weapons called rattlers that shoot alternating tractor and pressor beams at their targets to shake them to pieces.

I once read that powerful enough laser weapons could be used to destroy targets without vaporizing them. Instead the laser weapons would be turned on and off and would hit the targets so hard that it would be like hammering them with giant hammers. Successive blows form the laser beams would be enough to smash in the parts of the target they hit, or if the target was solid enough to shake it until it fell to pieces.

So possibly Star Trek weapons do have a component that hits and pushes their targets hard.

If such a beam hit an unprotected starship the beam would push in the hull section hit by the beam hard enough to detach it and push it back into the starship, smashing through bulkheads and decks and coming out the other side, leaving the starship with a tunnel through it leaking air out into space as well as damaged and destroyed equipment and crew members.

But if a starship has its energy shields up the shields will intercept the beam and transfer it's kinetic energy to the shields as a whole, which in turn will transfer the kinetic energy to their generators which in turn will transfer the kinetic energy to the ship as a whole. Thus the amount of kinetic energy sufficient to punch a narrow tunnel though the starship will instead push the entire starship back.

And since the entire starship has many times the mass of the section that would have been punched out of it by the energy beam the entire starship will be pushed back with much less force than the section would have been pushed out of the starship.

But the acceleration that the energy beam gives to the entire starship as it pushes it back might still equal tens, hundreds, or thousands of gravities, enough to smash up equipment and kill crew members as they fly into bulkheads.

So the job of the IDF is to reduce the acceleration the energy beam gives to the starship down to zero so the crew doesn't feel anything.

When such an energy beam hits a star ship, there are two possibilities that are highly probable.

The first possibility is that the starship's IDF will be able to compensate for the acceleration the beam gives to the ship perfectly, so that no force is felt by the equipment or the crew.

The second possibility is that the staship's IDF will be able to compensate for the acceleration the beam gives to the ship almost perfectly, so that only a tiny fraction of that acceleration force will be felt, and that tiny fraction will be enough to give every part of the ship an acceleration of at least several gravities and all equipment will be smashed and all crew members splattered against bulkheads and killed instantly.

And in the history of space war in Star Trek weapons designers constantly seek to increase the power of their beams so that IDFs will fail and the second possibility will happen to the target starships, while designers of force shields and IDFs work to improve them so that no force will be felt in a starship when hit by such a beam, the first possibility.

And in TOS space battles starships fired their weapons when the targets were tens of thousands or hundred of thousands of kilometers away, indicating that it would be suicidal to fight at closer distances where the first hit would destroy a starship.

But in space battles in the TOS movies starships got really close to each other to fire their weapons, implying that defenses had improved vastly and weapons not as much, so that starships had to get very close to have any chance of damaging their enemies.

So in a Star Trek space battle, either the weapons should be pretty much useless against their enemies, as in the first possibility, or else the weapons should usually destroy the enemy ships with the first shot, as in the second possibility.

There is a third possibility, one that exists on a "knife's edge" between the other two, that the IDF will be almost totally successful in neutralizing the forces of acceleration when a starship is hit by an energy beam, but enough force will be felt to shake the ship and throw people around without killing them.

And it seems that every Star Trek movie and series with a space battle has been set in an era of weapons development where the third possibility is in effect, despite the fact that it should be an extremely rare situation, on an extremely improbable "knife's edge" between the two main possibilities.

But perhaps the commanders in Star Trek space battles seek to fight at distances where their weapons have a chance to damage the enemy, but their own ships have a chance to survive being hit by enemy weapons. Maybe they always plan to fight at distances were the third possibility, the extremely narrow "knife's edge" between the first and the second possibility, is in effect.

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