How far would you have to go to escape gravity? - Rene Laufer

More than six thousand light years

from the surface of the earth,

a rapidly spinning neutron star

called the Black Widow pulsar

blasts its companion brown dwarf star with radiation

as the two orbit each other every 9 hours.

Standing on our own planet,

you might think you’re just an observer of this violent ballet.

But in fact, both stars are pulling you towards them.

And you’re pulling back,

connected across trillions of kilometers

by gravity.

Gravity is the attractive force between two objects with mass—

any two objects with mass.

Which means that every object in the universe attracts every other object:

every star, black hole,

human being, smartphone, and atom

are all constantly pulling on each other.

So why don’t we feel pulled in billions of different directions?

Two reasons: mass and distance.

The original equation describing the gravitational force between two objects

was written by Isaac Newton in 1687.

Scientists’ understanding of gravity has evolved since then,

but Newton’s Law of Universal Gravitation

is still a good approximation in most situations.

It goes like this:

the gravitational force between two objects

is equal to the mass of one

times the mass of the other,

multiplied by a very small number

called the gravitational constant,

and divided by the distance between them, squared.

If you doubled the mass of one of the objects,

the force between them would double, too.

If the distance between them doubled,

the force would be one-fourth as strong.

The gravitational force between you and the Earth pulls you towards its center,

a force you experience as your weight.

Let’s say this force is about 800 Newtons

when you’re standing at sea level.

If you traveled to the Dead Sea,

the force would increase by a tiny fraction of a percent.

And if you climbed to the top of Mount Everest, the force would decrease—

but again, by a minuscule amount.

Traveling higher would make a bigger dent in gravity’s influence,

but you won’t escape it.

Gravity is generated by variations in the curvature of spacetime—

the three dimensions of space plus time—

which bend around any object that has mass.

Gravity from Earth reaches the International Space Station,

400 kilometers above the earth,

with almost its original intensity.

If the space station was stationary on top of a giant column,

you’d still experience ninety percent

of the gravitational force there that you do on the ground.

Astronauts just experience weightlessness

because the space station is constantly falling towards earth.

Fortunately, it’s orbiting the planet fast enough that it never hits the ground.

By the time you made it to the surface of the moon,

around 400,000 kilometers away,

Earth’s gravitational pull would be

less than 0.03 percent of what you feel on earth.

The only gravity you’d be aware of would be the moon’s,

which is about one sixth as strong as the earth’s.

Travel farther still

and Earth’s gravitational pull on you will continue to decrease,

but never drop to zero.

Even safely tethered to the Earth,

we’re subject to the faint tug of distant celestial bodies and nearby earthly ones.

The Sun exerts a force of about half a Newton on you.

If you’re a few meters away from a smartphone, you'll experience

a mutual force of a few piconewtons.

That’s about the same as the gravitational pull

between you and the Andromeda Galaxy,

which is 2.5 million light years away

but about a trillion times as massive as the sun.

But when it comes to escaping gravity,

there’s a loophole.

If all the mass around us is pulling on us all the time,

how would Earth’s gravity change

if you tunneled deep below the surface,

assuming you could do so without being cooked or crushed?

If you hollowed out the center of a perfectly spherical Earth—

which it isn’t, but let’s just say it were—

you’d experience an identical pull from all sides.

And you’d be suspended, weightless,

only encountering the tiny pulls from other celestial bodies.

So you could escape the Earth’s gravity in such a thought experiment—

but only by heading straight into it.