Our bodies are surprisingly resilient in many situations, but rapid acceleration is not one of them. While the human body can withstand any constant speed—be it 20 miles per hour or 20 billion miles per hour—we can only change that rate of travel relatively slowly. Speed up or slow down too quickly and it's lights out for you, permanently.
Whether you're jumping out of an airplane or tripping over an ottoman, your fall to the ground is governed by the force of the Earth's gravity. This force causes falling objects to accelerate at a rate of 32 feet per second squared until they reach terminal velocity (which is the force of an object's drag equals and cancels out any further acceleration), or the plummeting object impacts another object that halts the fall.
Acceleration relative to gravity is quantified in "Gs", a nomenclature most commonly used in aviation, and one that you've surely heard before. 1 G is the equivalent to the pressure applied to the human body by the gravitational constant (9.80665 meters per second squared) at sea level. That is to say, just standing around. G-forces higher than this can't be produced by gravity alone; there has to be a mechanical force in effect as well.
When you're moving, G's are classified as either positive or negative. Positive G's (+Gx) push you back into your seat or causes all the blood to rush to your feet, negative G's (-Gx) pull you into the harness and puts your stomach into your throat as the blood rushes to your head.
With planes, things get even more complicated. Because planes fly within three dimensions (as opposed to cars which operate on a 2D plane), their pilots are subject to three forms of G-force, aligned with their x, y and z axis. Gx-forces push front to back, pressing the pilot back into his seat during takeoff and pulling him forward against the seatbelt when decelerating; Gy-forces take effect when spinning around the body's y-axis, such as during barrel rolls, but generally don't affect a pilot's ability to manage an aircraft; and Gz-forces come into play when rapidly changing vertical direction, such as when a plane pulls out of a steep dive. That's what makes your stomach lift into your throat when you go over that first hump on a roller coaster. This is also the kind of G that makes you pass out.
Under normal conditions, your body must maintain 22 millimeters of mercury blood pressure to get blood from your heart to your brain. Each additional +Gz (blood flows from the head to the feet) that a person experiences multiplies that requirement: The body has to muster double that at 2g, triple that at 3g, and so on until they hit around 4 or 5 G's, at which point most folks will pass out due to oxygen starvation because all the blood stays in their feet.
This condition is known as G-LOC (G-induced loss of consciousness). Fighter pilots, with the aid of flight suits packed with air bladders that force blood out of the lower extremities as well as specialized breathing and tension techniques, can be trained to withstand up to 9 +Gz.
Per the Federal Aviation Administration, the +Gz effects include:
(1) Grayout. There is graying of vision caused by diminished flow of blood to the eyes. Although there is no associated physical impairment, this condition should serve as a warning of a significant impairment of blood flow to the head.
(2) Blackout. Vision is completely lost. This condition results when the oxygen supply to the light sensitive retinal cells is severely reduced. Contrary to other common usages of the term, consciousness is maintained. In blackout, some mental activity and muscle function remains, thus the occurrence of blackout warns of seriously reduced blood flow to the head and of a high risk of loss of consciousness. Note: In some centrifuge studies, 50 percent of the pilots had simultaneous blackout and loss of consciousness. Therefore, a pilot cannot rely on blackout to precede loss of consciousness.
(3) Loss of Consciousness. When the blood flow through the brain is reduced to a certain level, the pilot will lose consciousness. He or she may have jerking, convulsive movements; these have been seen in many subjects of centrifuge studies and in some pilots during actual flight. The pilot will slump in his or her seat. Possibly, the pilot will fall against the controls, causing the aircraft to enter flight configurations from which it cannot recover even if consciousness is regained. In centrifuge studies, many pilots lost (and regained) consciousness without realizing they had done so.
Negative Gz-forces, however, are an entirely different matter. Nobody, literally no human—anti-g suit or not—can withstand more than 2 or 3 negative Gs before losing consciousness due to all the blood in their body pooling in their head. As the FAA continues:
b. Negative Gz Effects. Negative Gz is encountered when acceleration is in a foot to head direction, such as might be obtained during inverted flight, or during an outside loop or pushover maneuver (see Figure 2). Blood is then pushed toward the head, and the amount of blood returning from the head is diminished, so the blood tends to stagnate, particularly in the head. Under mild conditions of -Gz forces, the pilot will feel congestion, as when standing on his or her head. Engorgement of blood vessels causes a reddening or flushing of the facial skin. Blood vessels in the eyes will become dilated. Some persons may experience a headache. A condition termed "redout" may occur. This may be due in part to congestion but may also occur when the lower eyelid, reacting to -Gz, rises to cover the pupil, so that one sees light through the eyelid.
Planes, trains, and automobiles aren't the only places people experience the forces. Astronauts routinely endured 3g during shuttle launches, 8g atop a Mercury-era Atlas booster rocket, 7.25g aboard a Gemini-era Titan rocket, and around 4g for the Saturn 5s. Even reentry exposed astronauts to extreme forces: Mercury capsules hit 7.8g during reentry, Apollo capsules topped 6g.
However, the most extreme G-forces mankind has ever generated have actually been created here on Earth. For example, in the immediate post-WWII era, Air Force physician John Stapp set about researching how to improve cockpit designs to make them safer and better protect pilots against not just the G-forces experienced during a crash (which were thought to be the main cause of pilot deaths back in WWI) but also the mangling effects of the airplane as it disintegrated upon impact (which is what was really killing pilots).
To prove this was the case and that the human body could withstand much higher Gs than conventional wisdom dictated, Stapp developed the "Gee Whiz", a rocket-powered, track-mounted acceleration sled, to see just how many Gs the human body could really handle.
By 1948, Stapp had stopped using test dummies aboard the Gee Whiz and had begun using himself instead. Through these experiments—in which the sled was violently accelerated then stopped just as abruptly—Stapp showed that the body could withstand up to 35 Gs and survive.
In the 1950's, Stapp built and tested the Gee Whiz' successor, the Sonic Wind, which accelerated him to 632 mph in less than 5 seconds, then stopped in just one second. This generated a staggering 46.2 g (which means his 168 pound framed felt like it weighed just over 7,700 pounds) and exposed Stapp to 2 full tons of air pressure during the ride. Surprisingly, he walked away from the ride without a scratch—proving that the human body is fully capable of massive G loads, albeit only for a short time.
This rocket-sled record was then broken again in the 1970s aboard the Daisy Decelerator, which was built to test the effects of -Gx forces. Major John Beeding, an Air Force volunteer, endured a whopping 83g (albeit for .04 seconds) during the sled's nearly instantaneous stop. He too walked away from the experiments none the worse for wear.
Both of these experiments only focused on the effects of exceedingly large G-forces over extremely short time periods largely because that's what the human body can handle. This has important implications, not just here on Earth, but for our space exploration aspirations as well. As Bruce Thompson of NASA Quest explains:
The human body can tolerate violent accelerations for short periods, i ncluding the prolonged high-g acceleration necessary to reach Earth orbit. However very prolonged periods of high-g acceleration during travel between planets would be very harmful to the body, and therefore out of the question.
Imagine traveling to Mars, accelerating all the way at 3 gravities. You would weight three times your normal weight for the duration of the trip and would barely be able to move, but what would the unrelenting acceleration be doing to your body? Heavy acceleration is a speeded-up aging process. Tissues break down, capillaries break down and the heart has to do many times its proper work. You could not count on being in good shape when you arrived.
It's an interesting paradox. The closer to light we travel, the slower we age (relatively); yet the faster we accelerate to reach those speeds, the faster our bodies break down. Hopefully future advances in cryogenics, or at least fluid-filled pods which would help absorb the force of sustained high-G acceleration, will allow us to shorten that duration significantly.