- "It's strange not pulling any Gs."
"Yeah. I kind of miss it. Although at this rate of acceleration we'd be dead by now."
"Oh, yeah. Without inertial dampening, we'd be hit by so many Gs, our eyes would pop, the skin would pull away from our faces, our brains would squish up to the back of our skulls, and our internal organs would be crushed into these chairs. What about that sandwich?"
- ―Major John Sheppard explaining the ramifications of no inertial dampeners to Lieutenant Aiden Ford[src]
Inertial dampeners, also known as inertial compensators, are devices used on spacecraft to counteract the effects of sudden movements on ships (including acceleration, deceleration, sudden turns, etc.) and decrease the amount of shudder from taking weapon hits. By creating an artificial gravity-field inside the spacecraft, a great amount of the otherwise occurring inertia is taken away. Inertial dampeners accelerate the molecules of everything inside the spacecraft at the same rate, just like gravity does. Without the dampeners, spaceflight would be instantly fatal at anything beyond the slowest of speeds.
Inertial dampeners by race
All Ancient ships are equipped with powerful inertial dampeners. They can be adjusted to negate or increase a vessel's weight (such as making a Puddle Jumper heavier to weight it during a storm). They use up a considerable amount of power if installed on a large ship. Atlantis' inertial dampeners are also capable of increasing the weight of the city for the purpose of sinking the city. The personal shield emitters are also equipped with inertial dampeners. (SGA: "Hide and Seek", "Suspicion", "The Eye", "Critical Mass", "Grace Under Pressure", "Travelers", "Ghost in the Machine")
All Goa'uld vessels are equipped with inertial dampeners. They are effective at any speeds, but cannot completely compensate for the activation or deactivation of the hyperdrive, causing the unprepared to be thrown across the room with a mild amount of force. (SG1: "Within the Serpent's Grasp")
Beginning with their F-302 fighter-interceptors, the Tau'ri have developed their own inertial dampeners, presumably based on Goa'uld technology, for use in their spacecraft. Those equipped in their 302s are only 90% effective, meaning pilots can still experience the effects of inertia when pulling high Gs. (SG1: "Fragile Balance", "Lost City, Part 2") (SGA: "The Intruder")