If you’ve watched the very good science fiction series The Expanse, you will see that the concept of “g” is frequently mentioned. The accelerations/decelerations and maneuvers of spacecraft are always given in units of “g”. This is also true in real life. The load exerted on astronauts due to the acceleration of the spacecraft is given in terms of “g”. For example, if a spacecraft accelerates or decelerates at 2g, the astronaut will feel twice their own weight.
If we’re talking about an 80-kilogram astronaut, for example, the effect of the acceleration/deceleration (which are essentially equivalent, as both are referred to as “acceleration” in physics and exert the same load on the astronauts in both cases) will make the astronaut feel as if they weigh 160 kg. If the acceleration is 3g, they will feel 240 kg, at 4g they will feel 320 kg, and so on. As you can imagine, this places an excessive amount of extra load on the skeletal and muscular systems. So, have you ever wondered, what is the maximum “g” a human can withstand? In other words, what is the maximum gravity humans can survive?
What is g?
To understand gravitational acceleration (g), we must first understand acceleration.
Acceleration is the rate of change of velocity of an object. It is a vector quantity, meaning it has both magnitude and direction. When an object speeds up, slows down, or changes direction, it is accelerating. The basic formula for acceleration is:
a = Δv/Δt
Where a is acceleration, Δv is the change in velocity. Δt is the change in time.
Since the unit of velocity (v) is m/s (meter/seconds), the unit of acceleration (a) is m/s2
Gravitational acceleration (g) specifically refers to the acceleration due to gravity at the Earth’s surface, which is approximately 9.8 m/s2. This means that an object in free fall will increase its velocity by 9.8 m/s every second, assuming no air resistance.
In the context of space travel, g is used to describe the forces experienced by astronauts due to the acceleration of their spacecraft. As explained above, when a spacecraft accelerates at 2g, the astronauts inside feel a force equivalent to twice their weight on Earth. This is because the spacecraft’s acceleration generates an inertial force that mimics the effect of gravity.
Einstein’s Equivalence Principle, which is a cornerstone of general relativity, states that the effects of gravity are indistinguishable from the effects of acceleration. In other words, locally (in a small enough region of space and time), the force felt due to acceleration is the same as the force felt due to gravity.
For example, if you are in an elevator in deep space accelerating upwards at 1g, you would feel the same force as if you were standing on the surface of the Earth, experiencing Earth’s gravity. Similarly, an astronaut on the surface of a planet with a gravitational pull of 2g would feel the same force as an astronaut in a spacecraft accelerating at 2g. Both would feel twice their normal weight.
In other words, gravitational forces and acceleration are essentially equivalent. Therefore, an astronaut on the surface of a planet with a gravitational force of 2g would feel the same effect as an astronaut in a spacecraft accelerating at 2g.
In space travel, it is crucial to consider the g-forces experienced by astronauts during launch, maneuvers, and re-entry. The human body can tolerate varying levels of g-force for different durations.
Prolonged exposure to high g-forces can cause significant strain on the skeletal and muscular systems, leading to potential injury. Typically, humans can withstand up to about 5g to 9g for a few seconds without losing consciousness, but sustained g-forces above this can be harmful or even fatal.
So, what is the maximum gravity (or maximum g-force) that humans can survive for extended periods?
There is a recent study on this topic titled “Effects of Exoplanetary Gravity on Human Locomotion Ability” by Nikola Poljak, Dora Klindzic, and Mateo Kruljac, published in The Physics Teacher’s September 2019 issue. The research considered ultimate limits, including the breaking point of the human skeleton and the maximum gravitational force that human muscles can overcome to lift the body.
In their research, the scientists set out to understand the limitations of the human body under varying gravitational conditions. One area of focus was determining the gravitational threshold at which our skeleton would break under its own weight. Simultaneously, they studied muscle capacity, particularly when it comes to lifting the body from the ground in heightened gravitational conditions.
Their findings suggest that the human body exhibits remarkable resilience and adaptability. According to the research, even under a gravitational pull four times stronger than that of Earth (4g), humans could potentially maintain normal locomotion given adequate and rigorous training.
So, 4g is probably the maximum gravity a healthy human can survive for long durations. However, higher g loading probably leads to shorter lifetimes.
It’s important to remember that these findings apply to healthy, trained individuals. For those with existing health conditions, the situation may be different. Individuals with knee problems, back pain, obesity, and other health issues might already struggle with Earth’s gravity of 1g. Increased gravitational force could further exacerbate their discomfort and mobility challenges.
Conversely, the absence of gravity brings its own set of problems. Extended periods in zero-gravity (actually microgravity) environments, such as those experienced by astronauts, can lead to muscle atrophy, weakened immunity, bone density loss, and various other health issues.
What is the highest g-force a human has endured and survived?
The human body can withstand higher g-forces for short durations. For example, astronauts and fighter pilots might experience several g’s of force during takeoff, which can feel like several times their weight pressing down on them.
So, what is the maximum g-force that any human has been exposed to and survived?
According to the Guinness World Records book, the Swedish Indycar driver Kenny Bräck survived a split-second deceleration of 214 g during a 220 mph (354 km/h) crash on lap 188 of the Chevy 500 at Texas Motor Speedway, USA, on October 12, 2003. This is according to data registered in Bräck’s in-car “crash violence recording system”.
Bräck suffered fractures of his right femur, sternum, lumbar vertebrae, and ankles when his Dallara-Honda made wheel-to-wheel contact with Tomas Scheckter’s car, sending his vehicle high into the air and into a steel fence post. The in-car information system registered a peak value of 214 g for a fraction of a second.
Sources
- Study: Effects of Exoplanetary Gravity on Human Locomotion Ability on pubs.aip.org
- “Highest g force endured – non-voluntary” on the Guinness World Records website
- Acceleration on Wikipedia
- Gravity on Wikipedia
- Equivalence Principle on The Eöt-Wash Group [Laboratory Tests of Gravitational and sub-Gravitational Physics] website
- Kenny Bräck on Wikipedia
- g-force on Wikipedia
