An astronaut floating above Earth.
There's nothing like a nasty cold to make you appreciate good health. The same goes for the state of the universe: Tweaking just one of the fundamental physical laws or constants, normally perfectly "fine-tuned" at the right values to allow stars, planets, atoms and life as we know it to flourish, could turn things very different — quite unpleasantly so. Imagining such a "bizarro" universe may heighten your appreciation for the norm.
Consider, for example, how horrifically unrecognizable the universe would be if it had formed with just three fundamental forces instead of four — if electromagnetism, the strong interaction and the weak interaction were all exactly as we know them, but that fourth force, the one that pulled together a bunch of rocks to form Earth and still keeps your feet firmly planted on the planet, never existed. What if there was no force of gravity?
Picture a barren wasteland. According to James Overduin, a physicist at Towson University in Maryland who specializes in gravitation, a universe without gravity would be "completely flat and featureless." Overduin explained that gravity is just another term for the curvature of space-time — how steep or shallow the fabric of the universe is in a given place (and thus how likely objects are to fall toward the source of curvature). Just as a bowling ball placed on a trampoline curves its surface, it is the presence of matter and energy that cause space-time to curve. So, if the universe can't curve (because gravity doesn't exist), then there can be no matter or energy within it.
"This would be a boring universe," Overduin told Life's Little Mysteries, a sister site to LiveScience. [6 Everyday Things that Happen Strangely In Space]
In the interest of not being boring, let's consider an alternative scenario: What if the universe formed with gravity and developed normally up until a certain moment in space-time, and what if then, suddenly, gravity switched off?
According to Overduin, Einstein proved that you can't change the value of the gravitational constant known as "G" — doing so simply doesn't work mathematically, so there's no way for physicists to do it and still make sense of what would happen next. However, there are two alternative ways to switch gravity off, while letting all the other physical laws of the universe keep working.
One way to scoot around the constant is to manipulate a lesser-used physics model (but one that nonetheless produces an equivalent picture of the universe as Einstein’s), which considers G to be a field that permeates space-time, rather than a constant. Called a "scalar field," it works just as well as G in describing the way the universe works, except that mathematically, unlike the constant, its strength is allowed to vary in time and space.
Dialing down the scalar field to zero everywhere would essentially flatten the universe. Objects would no longer be drawn toward each other, because there would be no sloping surface for them to fall down. Instead, they would fly off in whatever direction gravity was keeping them from going. Back to the trampoline analogy, deleting gravity would cause the trampoline to suddenly flatten, and the bowling balls would roll every which way.
Alternatively, you could get rid of the Higgs field, a field permeating all space that is generated by the infamous Higgs particle. "All elementary particles get their masses from their interactions with this field, kind of like being 'slowed down' by passing through a thick syrup," Overduin said. "If the Higgs field instead dropped to zero, the syrup would have no thickness and all elementary particles … would zip around freely and become massless." Having no mass, they'd be unable to curve space-time, so there would be no gravity. On top of that, they'd start moving at the speed of light, ditching the other particles they used to hang out with inside atoms.
If gravity suddenly disappeared in either of the dramatic fashions described above, what would happen here on Earth? "I would expect that [Earth's] constituents, including its atmosphere, and oceans, and us, etc., would drift apart or even fly apart, assuming that the Earth would still be spinning," he said.
We'd be gasping for air, but that would be the least of our problems: we'd also be grasping for the very atoms in our bodies, and even the rapidly scattering particles in those atoms, all the while lamenting the way times had changed!