What is gravity?

Gravity is a force which tries to pull two objects toward each other. Anything which has mass also has a gravitational pull. The more massive an object is, the stronger its gravitational pull is. Earth's gravity is what keeps you on the ground and what causes objects to fall. Gravity is what holds the planets in orbit around the Sun and what keeps the Moon in orbit around Earth. The closer you are to an object, the stronger its gravitational pull is. Gravity is what gives you weight. It is the force that pulls on all of the mass in your body.

How does gravity work?

Every time you jump, you experience gravity. It pulls you back down to the ground. Without gravity, you'd float off into the atmosphere -- along with all of the other matter on Earth.

You see gravity at work any time you drop a book, step on a scale or toss a ball up into the air. It's such a constant presence in our lives, we seldom marvel at the mystery of it -- but even with several well-received theories out there attempting to explain why a book falls to the ground (and at the same rate as a pebble or a couch, at that), they're still just theories. The mystery of gravity's pull is pretty much intact.

So what do we know about gravity? We know that it causes any two objects in the universe to be drawn to one another. We know that gravity assisted in forming the universe, that it keeps the moon in orbit around the Earth, and that it can be harnessed for more mundane applications like gravity-powered motors or gravity-powered lamps.

As for the science behind the action, we know that Isaac Newton defined gravity as a force -- one that attracts all objects to all other objects. We know that Albert Einstein said gravity is a result of the curvature of space-time. These two theories are the most common and widely held (if somewhat incomplete) explanations of gravity.

In this article, we'll look at Newton's theory of gravity, Einstein's theory of gravity and we'll touch on a more recent view of the phenomenon as well.

Although many people had already noted that gravity exists, Newton was the first to develop a cohesive explanation for gravity, so we'll start there.

Gaga for gravity waves

With a single chirp, scientists confirmed the existence of gravitational waves created by the collision of two black holes. Science News’ special report (SN: 3/5/16) and subsequent stories in the March 19 issue sparked a flurry of reader questions on the physics of gravitational waves.

Reader Peter Toot wondered if gravitational waves’ ability to bend spacetime stretches light waves. “It seems to me that the expansion and contraction effects of the waves on the [LIGO] detector and surroundings would also apply equally to the wavelength of the light beams used by the detector,” Toot wrote. “If so, it then seems like the impact on phase difference between the recombinant beams would net out to zero phase shift. That clearly didn’t happen. Why not?”

It’s true that gravitational waves affect both the LIGO observatory and the laser light. However, the interferrometer contains two perpendicular arms, says Tom Siegfried. While one arm is shortened by the wave, the other is lengthened. Since the speed of light remains the same in both arms, the change in arm lengths means the two laser beams will not arrive at the same time. Because of this setup, the change in the light’s wavelength is irrelevant.

It appears that the mass was turned directly into energy, Crockett says. LIGO researchers estimated the black holes’ masses before and after the collision. “The energy that came out is roughly what you would expect from that much mass transforming into wave energy,” he said. “The mind does, indeed, reel.In his article on the power generated by the black holes’ collision (SN: 3/19/16, p. 5),Christopher Crockett reported that three suns’ worth of mass transformed into gravitational wave energy as the black holes merged. “Was this mass converted to energy or was it momentum (already energy) transferred into a radiating spacetime wave?” asked Ron Blachman. “In any event, the mind reels…. Entropy’ll get us all.”