There are several ways that surface gravity on Earth could change. We wouldn’t like most of them.
If a mini black hole collided with Earth and didn’t pass straight through, it would sink to the planet’s centre. Gravity would soar as the entire planet is pulled into the hole. It would probably get very hot too, so no one would survive to see the final spaghettification as we got within a few kilometres of the hole, when we are pulled apart by intense gravity gradients.
Alternatively, Earth’s gravity could dwindle to zero in a possible future event known as the big rip, when the universe has expanded to the point where everything, even subatomic particles, are trillions of kilometres apart.
Then there are scenarios in which an intensely energetic subatomic collision disturbs our cosmic “false vacuum” and resets everything to a lower-energy state. Or our universe could collide with another one, provoking a sudden change in physical properties. This could include a changed gravitational constant that rips outward from the initial event at the speed of light, so we would never see it coming.
Finally, and most benignly, gravity here changes by a minuscule amount every day as our planet gains some space dust and loses gases from the upper atmosphere. But as the mass of Earth is currently 6 × 1024 kilograms, you don’t need to reset your bathroom scales anytime soon.
Graham Smith Werribee, Australia
Over the next few billion years, Earth’s gravity will change by tiny amounts because of several events. As the sun expands, the oceans will boil off into space, reducing the planet’s mass and therefore cutting the force of its gravity.
But Earth’s core will also cool, so the planet will experience thermal contraction. As the radius of the planet decreases, the force of gravity on the surface will increase because, for a sphere, the force of gravity on the surface is inversely proportional to the radius squared. But if Isaac Newton and Albert Einstein were wrong about gravity and it turns out that the universal gravitational constant isn’t, in fact, constant but changes on a timescale of billions of years, then all bets are off.
Richard Glover London, UK
The only thing we know that affects a body’s gravity is the amount of mass it contains. So to change Earth’s gravity we would need to add or remove mass from our planet. But to make a noticeable change, we would need to move vast amounts of material.
Another factor is the rate at which our planet rotates. At the equator, where the speed is greatest, centrifugal force slightly counterbalances gravity, making you lighter than at the poles.
So there are a couple of ways to change Earth’s gravity, but not practical ones. Which is probably just as well!
Mike Follows Sutton Coldfield, West Midlands, UK
The close approach of a massive celestial body such as a neutron star would distort the pull of gravity on Earth’s surface.
If it were eight times as far from Earth as our moon is, people on the side nearest to the star would feel as if gravity had been reversed, acting upwards instead of downwards. They would be able to walk on the ceiling as if it were the floor, provided the building hadn’t already disintegrated. Meanwhile, people on the other side of Earth would feel heavier.
If the neutron star were to get much closer, its force of gravity would pull people and loose objects from the planet’s surface.