Content Objective
Question #1: What Is Gravity and the Basic Laws It Follows?
So, what exactly is gravity? At the most basic level, gravity (gravitational force) is the universal attraction between all matter. Matter is anything that has mass and volume. Mass is the amount of substance. Volume is the amount of space that a substance occupies. A gravitational force is an attraction (a pull) acting upon an object as a result of its interaction with another object. For instance, the Earth’s gravity draws objects of smaller mass towards its center. Gravity keeps our Moon orbiting around the Earth, our Earth and other planets orbiting around the Sun, and all the stars in our Galaxy orbiting around its center.
The universal gravitational constant (denoted by the uppercase letter “G”) is the force of attraction between two bodies of 1 kilogram (kg) that are 1 meter (m) apart anywhere in the Universe, and its value is always 6.67430 (15) × 10−11 Nm2/kg2 (Newton-meter2 per kilogram2).19
Earth’s acceleration due to its surface gravity (denoted by the lowercase letter “g”) is the acceleration of a body experiencing free fall under Earth’s gravitational force and its value is approximately 9.8 m/s2, and it is sometimes denoted as 1g or mistakenly as 1G where G is not the gravitational constant, and should not to be confused with 1G, the first-generation wireless phone technology.20 Reminder: “G” and “g” are two separate and independent entities and, therefore they are not the same units of measurement. For example, since the Moon has a smaller mass than the Earth, its surface gravity is about 1/6 (16.6%) as powerful as Earth’s gravity or about 1.6 m/s2 or “0.166 g” where “g” acts as a unit of measurement that compares arbitrary forces to the force of Earth’s gravity.21 Moreover, Earth’s surface gravity is not a constant value because Earth is not a perfect sphere, and its mass is not distributed equally. The value of g-force on Earth varies depending on the location. For examples: 1) the gravity at the equator is weaker than the gravity at the poles; 2) the gravity at the top of Mount Everest is less than 9.8 m/s2 due to its greater distance from the Earth’s center.22
Four Fundamental Forces of Nature
There are four fundamental forces in nature. The four forces in the order from the strongest to the weakest are: 1) the strong nuclear force, 2) the electromagnetic force, 3) the weak nuclear force, and 4) gravity.23 The strong nuclear force holds protons and neutrons together in the nucleus of an atom.24 Electromagnetic force (emf) is the result of attraction or repulsion of charged particles (positive or negative charges) interacting together where the strength of the force is inversely proportional to the distance between the charges.25 In incredibly short distances that are less than the diameter of a proton, the weak nuclear causes the radioactive decaying of unstable nuclear particles through time.26
Imagine the force needed to lift your feet off the ground from Earth’s gravity. Easy, right? Now imagine the amount of physical force you would have to exert in order to pull the positive and negative charges of two magnets apart or push the positive and positive charges (or negative and negative) together. These mental pictures can help students understand why gravity is so weak. In fact, a drop of water can cling to the tip of your finger without falling to the ground. That’s because the strength of the hydrogen bonding in water (a weak electromagnetic force) can overpower the force of Earth’s gravity.27 Another example: Gravity wants to crush the Earth into denser mass and smaller volume, but the strength of the electromagnetic forces inside atoms is so much greater that it resists the crushing by gravity.28 That’s why static electricity can make your hair defies gravity. The strong force’s relative strength (based on a proton-proton pair) is about 100 times as strong as electromagnetism; 1013 times as strong as weak nuclear force; and 1038 times as strong as gravitational force.29 In fact, gravity is so weak at the atomic scale that scientists can ignore it without making significant errors in their calculations.30 For many years, physicists like Steven Weinberg (1933-2021), Abdus Salam (1926-1996), Sheldon Lee Glashow (1932, age 89), and recently Helen Quinn (1943, age 79) had attempted to prove that these four basic forces of nature are simply the “same universal force” guiding our Universe.31
However, at the astronomical scales, gravity does dominate over the other forces due to two reasons: 1) gravity has a long range (think distance between the Sun and Earth); 2) there is no such thing as negative mass.32 Of course, there are now theorists who are investigating if negative mass supports the existence of dark matter.33 Outside the short range of the atomic nucleus (< 10-15 meters), the strength of both strong and weak forces quickly drops to zero. In contrast, even though the Sun and the Earth are 94.208 million miles apart, the gravitational force between them keep the Earth rotates around the Sun. Electromagnetism also has a long range like gravity, but it cannot be the force that keeps the Earth rotates around the Sun because positive and negative electric charges tend to cancel out, making most objects electrically neutral.34 For objects more massive than one-fifth of the Earth, gravity (not electromagnetism) will dictate its shape, and most massive objects are likely to be spherical or elliptical.35
Gravity Observations from Galileo Galilei and Thomas Andrew Knight
During the early 1700s, Galileo Galilei (1564-1642) recorded how Earth’s force (gravity) influences life structures and processes. Galileo’s main contribution was to show that when objects fell, they picked up speed, in other words, he discovered acceleration is due to gravity.36 Galileo also concluded that the ratio of length to width from animal bones of different weights deviated greatly among light and heavy animals.37 In the late 1700s, the horticulturist Thomas Andrew Knight (1759-1838) was the first to record the effect of gravity on plants by simulating microgravity with the centripetal force of a rotating waterwheel, and observed that roots of plants grew parallel to the resultant gravity vector.38
Kepler’s Laws for Planetary Motion
Johannes Kepler (1571-1630) was a German astronomer who best known for the laws of planetary motion. Kepler’s 1st Law states: Each planet moves around the Sun in an elliptic orbit and the center of the Sun is one of two foci of this ellipse.39 Kepler’s 2nd Law explains that the orbital speed of a planet increases when near the Sun, and decreases when farther from the Sun where a line segment from the Sun to the planet sweeps out equal areas of the ellipse in equal intervals of time.40 Kepler’s 3rd Law (P2 ∝ a3) says that the squares of a planet’s orbital period (P2) is directly proportional to the cubes of the semi-major axes of its orbit (a3).41 Kepler's 3rd Law implies that if the planet’s orbit decreases in size, then it will take less time to orbit around the Sun. That is why Mercury (the innermost planet) takes only 88 days to rotate around the Sun, Venus takes 224 days, Earth takes 365 ¼ days, Mars takes 687 days, Jupiter takes 4,332 days, Saturn takes 10,759 days, Uranus takes 30,681 days, and Neptune takes 60,193 days.42
Isaac Newton’s Universal Law of Gravitation
Isaac Newton unified the work of Galileo, Kepler, and other modern scientists of his times into his Universal Law of Gravitation. In 1687 Isaac Newton (1642-1727) described how gravity acts between all objects of mass with this equation: F = G Mm/d2 where “F” is the force of gravity, “G” is the universal gravitational constant, “M” is the mass of an object, “m” is the mass of the second object, and “d” stands for the distance between the two objects; this equation is now known as Newton’s Universal Law of Gravitation.43 The universal gravitational constant (G) was first calculated in 1798 to be 6.6743 x 10-11 m3 kg-1s-2 by Henry Cavendish (1731-1810) who used a torsion balance designed by the physicist John Mitchell.44 This equation explains gravity on the scale of our Universe, stating that a force of gravity attracts one mass with another mass varies directly as the product of the two masses and inversely as the square of the distance between the two masses. Billions of stars including our Sun create the gravitational pull of our whole Milky Way Galaxy that can make other smaller galaxies orbit around our Galaxy. The equation also explains that the gravitational attraction between two bodies (for example: the Sun and Earth) must be proportional to their masses. The more mass an object has, the stronger the pull of its gravitational force; that is why the Sun with its greater mass pulls the Earth to orbit around it. Newton’s Universal Law of Gravitation also implies that the gravitational attraction between two masses can get weaker with greater distance, but never to zero because both masses will continue to act on each other no matter how far away they get.45
In addition to Newton’s Law of Universal Gravitation, there are three Newton’s laws of motion that state: 1) an object will not change its motion unless a force acts on it, also known as the law of inertia; 2) the force on an object is equal to its mass times its acceleration (F=ma where “F” stands for force of gravity, “m” for mass, and “a” for acceleration); 3) For every action, there is an equal (in size) and opposite (in direction) reaction, also known as the action-reaction law where forces come in pairs.46 Newton’s third law of motion is consistent with his Law of Gravitation where the magnitude of the force (F) on each object is the same, even when one object has a larger mass than the other object. For instance, when Earth exerts a force of 9.8 N on an object of mass of 1 kg, that object will exert an equal force of attraction of 9.8 N on Earth.47
Albert Einstein’s Theory of General Relativity
What does the world’s famous equation E=mc2 really mean? Why is the knowledge that “energy equals mass times the speed of light squared” so ground-breaking? Basically, the equation is saying: energy can turn into mass, and mass can turn into energy. Energy and mass are the same thing in different forms; they are interchangeable when the conditions are right. That’s how Nature works, even though humans can’t actually see energy such as gravity, heat, or light turning into mass of objects like an apple, our Moon, or our Sun. Albert Einstein (1879-1955) took 10 years of thinking and problem-solving to come up with the Theory of General Relativity; he first published an early version called the Theory of Special Relativity in 1905 (with the world’s famous question E=mc2), but he didn’t count gravity into his calculations.48 In 1907, Einstein had “the happiest thought of my life” while sitting on a chair: “If a person falls freely, he will not feel his own weight.” 49 This phenomenon speaks of the weightlessness feeling that is common to astronauts flying in orbit. Einstein expanded his Theory of Special Relativity to publish the Theory of General Relativity in 1915. Einstein’s theory explains how massive objects like stars and planets warp the fabric of space-time, a distortion that manifests as gravity.50 Later in 1922, Alexander Friedmann (1888-1925), a Russian mathematician and meteorologist published a paper that outlined the possibility that an expanding universe started from a singular point.51 On the largest scale where gravity has infinite range, our Universe is expanding rather than being pulled together by gravitational attraction. The renowned black hole physicist, John Wheeler (1911-2008) summed up the Theory of General Relativity this way: "spacetime tells matter how to move; matter tells spacetime how to curve"52
Today, most scientists still agree with Einstein’s General Theory of Relativity, but recognize that Newton’s Universal Law of Gravitation has limitations like it does not explain the existence of black holes.53 Einstein’s theory does, and black holes have been observed since 1964, including Sagittarius A*, a very massive black hole at the center of our own Galaxy.54
Question #2: How Does Gravity Affect Our Bodies and Space Travel?
Our human body is a living machine made up of systems with vital organs working together to keep us alive. Some of these systems and vital organs include: cardiovascular (heart), digestive (liver and stomach), endocrine (glands), immune (bone marrow, spleen and skin), muscular (muscle), nervous (brain), reproductive (ovaries and testicles), respiratory (lungs), and skeletal (bones).55 NASA performs space trainings on Earth with artificial gravity (hypo-gravity or hyper-gravity) ranging from orbital flight, parabolic flight, head down/head up tilt, head, body loading, centrifugation, to swimming in deep tanks.56 When astronauts travel into space, gravity on their bodies is altered, resulting in microgravity, a state of very low acceleration between two free-falling objects. Hypo-gravity is a related term that usually describes the change of acceleration that passengers experience inside an aircraft for a short time during a parabolic flight that usually produces about 25 seconds of free-fall (0 g) followed by 40 seconds of enhanced force (1.8 g) in a repeated cycle.57 Even though both the astronauts and the aircraft’s passengers experience weightlessness, microgravity happens in space and hypo-gravity happens artificially inside an aircraft. Another form of altered gravity is hyper-gravity which means gravity greater than the acceleration of 1g on the surface of the Earth.58 Examples of hyper-gravity include centrifuge training for astronauts, and animal centrifuge lab experiments. Since electromagnetism and gravity are linked phenomena, altering gravity may also alter electromagnetism.
Mass v. Weight
In science, mass and weight are not the same, even though weight is proportional to mass. When you step on a scale in a doctor’s office, you are actually measuring “your mass” and “not your weight. Another way to remember the difference is: mass is always the same (a constant value), but weight changes depending on the acceleration of Earth, or Moon, or in space. A person’s weight is six times greater on Earth than on the Moon. If you weigh 120 pounds (lb.) on Earth, you would weigh about 20 lb. on the Moon. Mass (m) is the amount of “matter” in an object, but weight (W) is the force exerted on an object by gravity. Here is a video explanation: https://www.youtube.com/watch?v=U78NOo-oxOY.
Figure 1: (Top) The colors of these globes represented how Earth’s surface gravity varies in value depending on the location; these images were captured by NASA’s Gravity Recovery and Climate Experiment (GRACE), visit https://grace.jpl.nasa.gov/resources/6/grace-global-gravity-animation/ to see a 3D animation.59 (Middle): This illustration from NASA’s Glenn Research Center explains how the Weight Equation “W=mg” is derived from Newton’s Universal Law of Gravitation F = G m1m2/d2 where W of earth is 9.8 m/s2 and the equation is also similar to Newton’s 2nd law of motion, F= ma.60 (Bottom): A comparison of how much a 100 pounds person would weigh on the Moon and other planets.61
Tides Due to the Moon’s Gravitational Force
Why do oceans have high and low tides? The answer has to do largely with gravitational forces exerted on the Earth’s oceans by the Moon, and to a lesser extent by the Sun. Since the Sun is 27 million times larger than the Moon, the Sun’s gravitational attraction to Earth is more than 177 times greater than that of the Moon’s pull on Earth.62 However, because tide-generating forces vary inversely as the cube of the distance from the object, and the Sun is 390 times further away to the Earth than our Moon is to the Earth, the Sun’s tidal force is reduced 3903 (about 59 million times).63 That’s why tides have more to due to the Moon’s pull on Earth than the Sun’s. In short, tides are caused by differences in gravitation between the Earth and the Moon. The part of the Earth facing the Moon experiences a larger gravitational force than that of the opposite side of the Earth; this difference is the tidal force which causes the Earth’s closest side to the Moon to bulge towards the Moon.64 High tides are made when the highest point in the wave (the crest) reaches a coast, while low tides are made when the lowest point (the trough) reaches a coast. There are also tides in other bodies of water like lakes, ponds, and rivers, but the gravity impact is so minimal that it is difficult for our naked eyes to detect.65 Does the Moon’s gravitational force impact human health? As of now, there is no reliable scientific proof that the Moon affects human’s mood, sleep, women’s menstrual cycle, and any other mental and physical health issues.66 However, some organisms like corals seem to time their reproduction cycle with the lunar cycle, but the conclusion seems to be that the Moon’s gravitational force is too small to adversely impact human’s health and behavior.67
Weightlessness, Microgravity and other Altered Gravity
In space, astronauts have to be concerned with the phenomenon called microgravity or weightlessness. The terms weightlessness does NOT mean “zero gravity” or “no gravity.” Plus, there is no such thing zero gravity. A common misconception about spacecraft orbiting the Earth is that they are operating in zero-gravity, an environment with no gravity which is misleading. The sensation of weightlessness experienced by astronauts is not the result of zero gravitational acceleration (remember Newton’s Universal Law of Gravitation), but that the astronauts cannot feel the g-force because there is no difference between the acceleration of the spacecraft and the acceleration of the astronauts. This is why when we go down in an elevator, for a few seconds we feel as though our weight has decreased. You can also think of it this way: the Earth is rotating very fast with one rotation in 24 hours, but we do not feel the Earth’s rotation since we are rotating at the same rate as the Earth. The feeling of weightlessness is exactly analogous to standing on Earth and doesn’t feel like we are rotating around the Sun.
Unlike the Sun which appears to rise and set, an object like a spacecraft appears motionless in the sky from Earth, looking as if it is defying gravity. In actuality, the spacecraft is moving in a circular orbit at about 35,785 km above Earth's equator, and its orbital period is equal to Earth's rotation period of 23 hours and 56 minutes.68 James Oberg (1944, age 77), a space journalist explains the phenomenon called geostationary this way: “The myth that satellites remain in orbit because they have "escaped Earth's gravity" is perpetuated further (and falsely) by almost universal misuse of the word "zero gravity" to describe the free-falling conditions aboard orbiting space vehicles.”69 Earth’s gravity keeps satellites from flying off into space. When our naked eyes see a satellite flowing in space, seemingly “weightless,” it’s because we are not able to see the satellite’s resistance of gravitational pulls from Earth. In other words, gravity exists in space, inside a spacecraft, around satellites, and is everywhere in our Universe, even inside the supermassive black hole at the center of our Milky Way Galaxy.
NASA’s Preventive Measures Against the Effects of Microgravity
Humans are used to Earth’s gravitational acceleration of 1g, but astronauts in space have to learn how to deal with microgravity. NASA has recorded that some astronauts who went to space for an extensive amount of time returned taller (in height) with reduced bone and muscle mass at the rate of 1% bone loss per month; some had memory loss and learning difficulties due to the increased pressure on the brain, and poorer immune system that weakens the function of T-cell.70 A list of other negative effects of microgravity include: 1) space adaptation syndrome (space sickness) with symptoms of nausea, vertigo, and lethargy; 2) loss of muscle mass (muscle atrophy); 3) loss of bone mass (osteoporosis); 4) eyesight problem; 5) “moon-face” due to blood rushing to the head; 7) plague buildup inside arteries; 8) and cancer.71 According to a NASA study (2012), spaceflights may also be harmful to the brains, and accelerate the onset of Alzheimer’s disease.72 Researches on the impact of gravity on human physiology may help scientists to figure out how to detect life in the space.
In microgravity, there is less weight load on the back and leg muscles, so these muscles and bones will weaken and shrink. Muscles degeneration and bone osteoporosis can be rapid for astronauts during a space mission, and without regular exercise astronauts may lose up to 20% of their muscle mass within 5 to 11 days, and 1.5% of bone density per month (compared to about 3% in 10 years for a healthy person in a healthy environment).73 The mass loss mainly affects the lower vertebrae of the spine, the hip joint, and the femur. Unlike patients with osteoporosis, astronauts who are in space for 3 to 4 months can later regain their normal bone density after a period of 2 to 3 years back on Earth.74 After returning to Earth, bone loss might not be completely corrected by rehabilitation; however, with proper diet and exercise, their risk for fracture is not higher than the risk of people who never experienced microgravity. On Earth, bones are destroyed and renewed regularly with a well-balanced system of bone destroyer cells and bone building cells; whenever bone tissue is destroyed, new layers take their place.75 When in space, the natural process of destruction and construction of bone cells is altered, and because an increasing number of bone building cells is lost, the bones decompose into minerals that are then absorbed into the body. The increase in calcium levels from the disintegrating bone causes a dangerous calcification of soft tissue and increases the potential of kidney stone formation; NASA is studying medicines like potassium citrate to help combat this risk.76
The best way to study the effects of microgravity is to create artificial gravity. To date, scientists have managed to create gravity only under laboratory conditions, using strong magnetic fields. Astronauts will encounter three basic gravity fields on a mission to Mars. On a 6-month trek from Earth to Mars, a space crew would feel weightlessness due to microgravity. While living and working on Mars, the crew will be living in a gravitational field about one-thirds of Earth’s gravity. In the movie “The Martian,” the spaceship had a rotating circular structure that has gravity equal to 40% of what would be on the Earth.77 Even though the Mars' gravity isn't extreme, living on the Mars with the gravity weaker than Earth’s could be dangerous to humans, causing long-term health issues, and may be even negative impacts on human fertility.78 When shifting from weightlessness back to Earth’s gravity, astronauts will have to re-adapt to Earth’s gravity, and the transition may be difficult. Astronauts may experience “post-flight orthostatic intolerance” where they may feel poor hand-eye coordination, spatial disorientation, lightheaded, unable to maintain their blood pressure and balance when standing up.79 For example, a full EVA (extravehicular activity) spacesuit weighs about 280 pounds on Earth during training, but weighs close to nothing in the microgravity environment of space.80 When astronauts are in their EVA spacesuits, they also wear anti-nausea patches because vomiting inside the suit can be a deadly choking hazard, and that’s why space suits are worn mostly during launching, landing, and doing activity outside the spacecraft.81 Sea sickness drugs can help to treat space sickness, but are rarely used because the natural adaptation is preferred during the first two days of space travel over side effects such as the drowsiness caused by the sea sickness drugs.82
Figure 2: (Top Left) This simulation shows how the ISS orbits around the Earth. The ISS circles the Earth about 15.5 times a day and each orbit takes about 93 minutes.83 (Top Right): Astronauts sleep inside sleeping beds in tight compartments and their bodies are strapped down to prevent flowing.84 (Bottom Left): NASA astronaut Karen Nyberg demonstrates how astronauts run in space on the COLBERT Treadmill. (Bottom Right): A spacesuit is more like a mini-spacecraft than a jumpsuit. This 1981 spacesuit named Extravehicular Mobility Unit (EMU) is still being used today, and was designed to be heavyweight to counteract microgravity.85
The weekly schedule of astronauts in the ISS is about 5.5 work days and 1.5 days off. Most work days are consisted of 8.5 hours of sleep, 6.5 hours of work, 2.5 hours of exercise, and an hour for lunch.86 The ISS has apparatus for astronauts to perform at least 2 hours of resistance training per day; the exercises include jogging on a treadmill while attached to a giant elastic bands, riding a stationary bicycle, and lifting weights.87 Aerobic and resistive exercises help astronauts keep their hearts, bones, and muscles strong, minds more alert, as well as a more positive outlook against the feelings of isolation, confined spaces, and possibly depressive thoughts. In microgravity, the fluids in the body can shift upward to the head putting pressure on the face, eyes, and ears which can cause headaches, vision and hearing problems. Astronauts wear compression cuffs on their thighs or pants that put pressure on their leg bones to keep blood in their lower extremities. Other ways to counteract microgravity include: using a pressure device to draw fluids from the head into the legs, performing spinal ultrasound screening to monitor back pain, and conducting MRI imaging to assess muscle size and bone density.88 Due to dehydration and calcium excretion, the body may develop kidney stones, therefore, urine is collected and measured daily to help astronauts to make modifications to their diet, water intake, and exercise routine.89 Since medications like bisphosphonate has been used successfully to reduce bone breakdown and prevent kidney stones in patients on Earth, NASA has cautiously extended the use of such medications for astronauts in space mission.90
Super Earths v. Earth-Like v. Mars-Like Exoplanets
As of August 2022, NASA’s website has acknowledged 5,063 confirmed exoplanets and 3,794 planetary systems.91 A habitable exoplanet is a planet that has liquid water, a stable star, the right mass and temperature, the ability to hold an atmosphere, and located in the Goldilocks Zone that is “not too close, no too far from its host star, but in just right conditions for life to exist.”92 Stars are classified according to their surface temperature into spectral types: (hottest) O-type, B-type, A-type, F-type, G-type, K-type, to M-type (coolest); these 7 spectral types are further divided into 10 subclasses (0 to 9, hottest to coolest).93 Under the Morgan-Keenan luminosity class system, stars are measured by its energy radiation (brightness) and each star was originally designates with Roman numerals from “I” (super giant star) and “V” (main sequence), but now further into subgroups: Ia-0, Ia, Ib, II, III, IV, V, VI or sd, and D.94 This order goes from: extremely luminous super giants, luminous super giants, less luminous super giants, bright giants, normal giants, sub-giants, main sequence dwarf stars, sub-dwarfs, to white dwarfs.95 The classification for our Sun is G2V, yellow-white in color with surface temperature about 6,000 degrees in Celsius, and a hydrogen-burning main-sequence star. The Hertzsprung-Russell (HR) diagram is a helpful tool that plots the star’s spectral type and luminosity on a graph. F, G, and K-type stars are more likely to host habitable exoplanets.96 G-type stars like our Sun would host exoplanets most similar to Earth, K-type stars would host Super-Earths, M-type stars may be able to host an exoplanet like Proxima Centauri b, and flare stars (mainly dim red dwarfs and a few brown dwarfs) could erode making their atmospheres inhabitable.97
The five basic types of exoplanets (from least to greatest mass and radius) are: 1) rocky planets (like Earth and Venus); 2) Super-Earths; 3) mini-Neptune; 4) ice giants and; 5) gas giants (like Jupiter and Saturn).98 A habitable exoplanet would have to have a mass between 0.1 and 10 of Earth masses, and a radius between 0.5 and 2.5 of the Earth’s radius.99 About 55 potentially habitable exoplanets have been found as of 2020; they are believed to comprised of one Sub-Terran (Mars-size), 20 Terran (Earth-size), and 34 Super-Terran (Super Earths).100 As of August 2022, there is no proof of life on Mars, but because Mars has weather, seasons, ice caps, volcanoes, and plate tectonics like Earth, scientists continue to entertain the idea of life on Mars, or at least the fact that there may have been life on Mars, and perhaps a future colonization by humans.101 In 2018, NASA detected a level of methane on Mars, and proposed that microorganism may have produced the methane.102
Question #3: Does Gravity Play a Role in Evolution?
If there were alien lifeforms, we would expect it to show traits of living things like those on Earth, mainly due to our bias and lack of knowledge of life outside of Earth. Some key traits of life on Earth are cellular organization, reproduction, growth, energy processing, homeostasis, and the ability to adapt.103 These key traits serve to define life on Earth, but the type of life found elsewhere in the cosmos might surprise us in ways that we have yet to imagine and understand. Gravitational biology is the study of how gravity impacts living things. Since the origin of life on Earth, gravity has served as a constant “parameter of confinement” experienced by living things from water, to air, to land, and unchanged for at least the last four billion years.104 One research study concluded that gravity may be an evolutionary factor to explain why the heart of a tree snake is located significantly closer to its head, making blood flow easier against gravity than the heart in both land and water snakes.105 When these snakes were centrifuged, it was found that the tree snake has the highest gravity tolerance, the land the intermediate, and the water snake the least.106 Gravity also serves as the way to support our body and how we move. When you spread your feet wide apart, you have better support and balance. The closer your center of gravity is to the ground, the more supported you are, and the less likely you would fall to the ground. According to the Guinness World Records, the human’s height-weight-age parameters may range from Robert Wadlow at 8 ft 11 in (272 cm) tall, weighed 485 lb. (220 kg), and aged 22 to Pauline Musters at 23 in. (58 cm), 3 lb. 5 oz. (1.5 kg), and 19.107 The heaviest animal is the blue whale at 160 tons (352,739 lb.), and the lightest of all vertebrates is the dwarf goby at 2 mg (14 oz.).108 Since gravity pulls all living things down, we can understand why plant’s roots grow downward (in the same direction as the force of gravity) and stems grow upward (in the direction of the Sun).109 When plants are grown upside down, gravity will force the roots to twist and turn until they grow downward, and stems will adapt and turn to grow toward the light source.
Experiments have shown that the size of single biological cells is inversely proportional to how strong the gravitational field is acting on the cell.110 A stronger gravitational field would have cells of smaller sizes. However, in hyper-gravity (> 1g), cell growth eventually decreased and diminished.111 When an aircraft followed a parabolic flight for 72 hours in hypo-gravity (zero < g < 1), the production of cells reduced.112 In a NASA study (2013) on the effects of microgravity (1 x 10-6 g), microbes were seen to have adapted to the spaceflight environment with increased growth, unique cellular changes and processes than on Earth, and the ability to survive in the vacuum of outer space.113 The food-poisoning bacteria called Salmonella Typhimurium became more virulent during a 2015 space shuttle experiment on microgravity.114
The effects of gravity on a many-celled organism have been shown to be more drastic than on a single-cell organism. Prior to animals living on land, most lifeforms were small and looked like a worm or a jellyfish, with no skeletal system. When animals evolved to survive on land, methods of locomotion and skeletal systems were formed to counteract the increased force of gravity. In larger animals, gravitational forces have shown to influence blood circulation, muscles, and bone structures. As a person ages, gravity will continue to compress the body and spine resulting in bad posture, damage organs causing them to shift downward, increase weight in a person’s mid-section, and may worsen blood circulation and other health issues. Scientists are making the parallels between the effects of microgravity on astronauts and that of aging on Earth; work from Donald Ingber based on his tensegrity theory has shown that “molecules, cells, tissues, organs, and our entire bodies use tensegrity architecture to mechanically stabilize their shape, and to seamlessly integrate structure and function at all size scales.”115
If there were lifeforms on exoplanets, or Mars, how would gravity impact them? In a lower gravity field like that of Mars, what would be the gravity’s parameter of confinement for living things to thrive? On Mars, would alien lifeforms evolve to be bigger or smaller than earthlings? Or have rounder mid-section, flatter feet, or shorter arms to lessen load? What about in a higher gravity field like that of Kepler 22b, a Super Earth with mass 36 times of Earth? Scientists know that astronauts are relatively adaptive to lower gravity and require a long time to recover, but they are less certain as to what would be the maximum gravitational field humans can survive long term. How gravity impacts life and how lifeforms react to different gravitational environments are still mysteries, under speculations, and in need of more research.
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