On dewy mornings in mid-summer I'll pull up the window shade and see a dozen or more spider webs dotting the lawn. They resemble single-ply sheets of facial tissue but with circular shapes. Funnel weaver spiders make them.

Closer inspection reveals that each is slightly funnel-shaped with a hole in the center where the spider hides, waiting for its prey. If an unsuspecting cricket or grasshopper lands on the web, threads will catch on its body parts, entangling the creature. Movement alerts the spider, which rushes out at high speed, injects the victim with a paralyzing venom and carries it down the hole to feast on.

A profile view better shows the funnel shape of the funnel-weaver's web. 
Contributed / Bob King
A profile view better shows the funnel shape of the funnel-weaver's web. Contributed / Bob King

There's something hideous about this, I know. But spiders have as much right to a good meal and the means to procure it as we do. Evolution through mutation and natural selection spent a long time working out this food plan. Funnel weavers, also called grass spiders, also neatly illustrate the basics of black holes.

When a star at least eight times more massive than the sun runs out of nuclear fuel in its core (left), gravity crushes it, and the star explodes as a supernova (right). Sometimes a dense remnant is left over. If massive enough it can further collapse to form a black hole. 
Contributed / Left: NASA / right: ESO
When a star at least eight times more massive than the sun runs out of nuclear fuel in its core (left), gravity crushes it, and the star explodes as a supernova (right). Sometimes a dense remnant is left over. If massive enough it can further collapse to form a black hole. Contributed / Left: NASA / right: ESO

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A black hole is a place in space where gravity is so strong that not even light can escape. A black hole typically forms during a supernova, which occurs when a star's internal nuclear "furnace" shuts down. Without the heat and pressure to do battle against the crushing force of gravity, the star collapses and explodes.

When the dust clears, the blast may leave a tiny, compressed core behind. If it's three times or more the mass of the sun, it will further collapse into a single, infinitely dense point called a singularity and form a black hole.

Relatively speaking, there's lots of space between atoms and particles within atoms. If you got rid of that space by squeezing the Earth down to the size of a ping pong ball it would become a black hole. 
Public domain with Earth image from NASA, assembled by Bob King
Relatively speaking, there's lots of space between atoms and particles within atoms. If you got rid of that space by squeezing the Earth down to the size of a ping pong ball it would become a black hole. Public domain with Earth image from NASA, assembled by Bob King

We can't see it, but astronomers can detect a black hole's presence and even measure its mass by how much it tugs on other bodies that either orbit it or suffer an encounter with it. Making a black hole isn't easy. It takes A LOT of energy. If you crushed the sun into a ball just under 4 miles across it would further collapse and become a black hole. Likewise the Earth. Compact it to the size of a ping pong ball, and bingo —black hole!

This is a basic model of a black hole. The smeared starlight around the event horizon are background stars distorted by the black hole's strong gravity. (NASA, ESA and G. Bacon, STScI)
This is a basic model of a black hole. The smeared starlight around the event horizon are background stars distorted by the black hole's strong gravity. (NASA, ESA and G. Bacon, STScI)

Surrounding the singularity is a much larger funnel-shaped region of space called the event horizon which marks the edge of the black hole. Along this perimeter you'd have to travel at the speed of light to escape going down the hole. Since that's not possible, once you cross the horizon no one will ever hear from you again.

This simulation shows a black hole surrounded by a disk of glowing matter from shredded stars and gas clouds. The material spins rapidly around the hole, heats up and radiates light before crossing the event horizon. 
Contributed / ESO
This simulation shows a black hole surrounded by a disk of glowing matter from shredded stars and gas clouds. The material spins rapidly around the hole, heats up and radiates light before crossing the event horizon. Contributed / ESO

If you try to send an SOS via radio or laser from inside the black hole, forget it. It won't reach the outside world because the signal would have to travel faster than light, an impossibility according to our current understanding of physics. Instead, you'll drift inexorably toward the singularity and an unhappy end just like our cricket friend, who inadvertently crossed the funnel weaver's "event horizon" and now finds himself tugged toward the dark hole of the spider's lair, utterly helpless.

In this 3D interpretation you can see how gravity warps the fabric of 4D spacetime. The more massive the object the greater the warp. Shown here are the sun and Earth. 
Contributed / T. Pyle, Caltech, MIT, LIGO Lab
In this 3D interpretation you can see how gravity warps the fabric of 4D spacetime. The more massive the object the greater the warp. Shown here are the sun and Earth. Contributed / T. Pyle, Caltech, MIT, LIGO Lab

While funnel-weaver webs mimic the appearance of black holes with their webby event horizons and singularly deadly spiders, we have to remember they're only a convenient model. Black holes exist in space-time, a four-dimensional space that includes the three familiar dimensions alloyed with time. Funnels created by super-compacted matter are as real as the spider's. They truly warp space. But they'll remain invisible until we evolve the ability to visualize 4-dimensional space.

In the meantime, a 3D model helps. Thank you, spiders.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.