Alan Stahler: Extreme gravity |

Alan Stahler: Extreme gravity

The black hole in the core of our galaxy is cocooned within the bright-white cluster of stars to the right of center
Hubble: NASA, ESA, & D. Q. Wang (U. Mass, Amherst); Spitzer: NASA, JPL, & S. Stolovy (SSC/Caltech) |

Throw a ball straight up, at a hundred miles an hour. The ball rises, slows to a stop, and falls back down.

Throw it upward at a thousand miles an hour. The ball flies higher, but, again, slows to a stop and falls.

Now fling that ball up at 25,000 miles an hour. The ball slows, but never stops. The ball drifts slowly into space, leaving Earth behind.

Twenty-five thousand miles an hour is the velocity of escape from Earth. To leave Earth for another planet, a spacecraft must fly at least that fast.

The rock beneath our feet exerts the gravitational tug that pulls the ball downward. Earth’s core pulls on the ball, but, thousands of miles beneath our feet, the core’s pull is not as strong.

On the far side of the world, the floor and the waters of the Indian Ocean pull on the ball, but the distant ocean’s pull is even weaker than that of the core.

If we could bring core and ocean closer, their pull would be stronger.

Imagine squeezing the Earth, down to the size of a beach ball, a foot across. Much closer, core and Ocean pull much harder; we’d have to fling that ball 150 million miles an hour to get it into interplanetary space.

Squeeze the Earth yet more, down to a large marble, two-thirds of an inch across.

Core and ocean are now so close, and pull so hard, escape velocity is just over 186,000 miles a second — the speed of light.

Light never slows, but it does lose energy. Climbing out from a marble-size Earth, light would lose all its energy; not even light can escape. The marble is pitch-black.

Anything coming anywhere near this marble-Earth would fall in, as into a hole — a black hole.

Look to the east before dawn.

Draped in morning twilight, the constellation Sagittarius hangs over the eastern horizon. Facing Sagittarius, you’re looking into the center of our galaxy. At the very center — 26,000 light years off — lurks a black hole: Sgr A* (“sadj-A-star”).

If we could weigh it, marble-Earth would weigh as much as the Earth — it would weigh “one earth”.

SgrA* would weigh-in at over a trillion Earths. Its gravity is similarly stupendous. Gravity distorts space and time. In a strong gravitational field, time slows down.

Orbiting thousands of miles above the Earth, GPS (Global Positioning System) satellites feel weaker gravity than you and I. Their clocks would run fast, were they not tweaked to run slow, to cancel the effect.

As we approach a black hole, feeling its gravity grow stronger, our time – relative to the rest of the universe — runs slower and slower.

Not just clocks, but everything: Your heartbeat, your breathing, your life — all would slow down, relative to the outside world. The larger your telescope, the finer the detail it reveals.

The Event Horizon Telescope (EHT) will be a world-spanning assemblage of radio dishes, to study Sgr A* and the spacetime around it. The most extreme effects will manifest as we approach the event horizon — the very edge of the hole – where (relative to us) time slows, and then stops.

Al Stahler’s science programs can be heard on KVMR (89.5 FM). He brings an enjoyment of science and nature to students of all ages — kid to adult — and may be reached at

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