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Al Stahler: Forever mystery

As we look out across the universe, time and again, we see things that are off-the-scale … things so hot, so large, so bright, so fast, they should not exist. Eventually, though – after centuries, sometimes – we figure out how to explain them.

There is, however, one thing that – in my humble opinion – we will never understand: What goes on inside a black hole?

Watching the moon tonight, we’re looking out across a quarter-million miles. Next Monday or Tuesday night, look a bit beyond the moon … another hundred-fifty million-billion miles. Look out that far, past the moon, Monday or Tuesday, and we’re looking at the center of our Milky Way Galaxy.

Switch your radio to the AM band, and doff a sweater. Pulling the sweater over your head generates sparks; sparks generate radio waves, picked up as static on the AM radio.

Back in the 1930s, telephone lines were picking up static … but from where? Assigned the problem, a physicist discovered radio waves coming from the sky, with an especially bright spot (bright if you “see” radio waves) in the constellation Sagittarius (in the direction of the moon, Monday and Tuesday). He denoted the spot as Sagittarius A (had he found a second such bright spot in Sagittarius, he would have called it Sagittarius B, and so on).

Astronomers abbreviated this to Sgr A, still pronounced “Sagittarius A” … or just “Sadge A.”

Looking more closely, in the 1970s, astronomers found an especially bright spot within Sgr A, which they denoted as Sgr A* (“Sagittarius A-star”).

In 2022 – last month – radio astronomers released a radio image of Sgr A* … what it would look like, if we could see radio.

Some years before, radio astronomers had already seen stars orbiting Sgr A* … stars moving so fast, they should fly off … unless Sgr A* had them on a super-strong leash … a super-strong gravitational leash.

To hold onto stars flying so fast, the gravitational leash of Sgr A* had to be as strong as a leash created by four million suns, pulling together.

Toss a ball upward at, say, a hundred miles an hour. As the ball flies upward, it loses energy, slows down. Losing all its energy, it slows to a stop … and falls.

Toss that ball faster, and it will fly higher … but will still eventually stop, and fall back.

Toss that ball upward at 25,000 miles an hour, and it will slow … but it will never slow to a stop – the ball will escape Earth forever. Twenty-five thousand miles an hour is escape velocity from Earth.

If we could weigh the sun, it would tip the scales at a third-of-a-million Earths, making its gravity way stronger than Earth’s. Escape velocity from the sun is over a million miles an hour.

Objects “heavier” than the sun would have escape velocities even greater.

Nothing flies faster than light – 186,000 miles a second, over 600 million miles an hour. Shine a flashlight straight up, and its light loses energy … but it does not slow down. Rather, it changes color (though not enough to see by eye).

Plunk a slice of bread into the toaster, and the wires begin to glow … first, red, then orange. It takes more heat – more energy – to make orange light than red. Orange light carries more energy than red.

The gravity of Sgr A* is so strong, orange light shining upward would lose energy and become red; that red light would lose yet more energy … eventually, losing all its energy, the light would go dark.

Escape velocity from Sgr A* is faster than the speed of light. Not even light – the fastest thing in the universe – can escape Sgr A*. Sgr A* is black.

Anything coming too close to Sgr A* falls in, and is gone forever. Sgr A* behaves like a hole … a black hole.

In the image of Sgr A*, the black hole is in the center … and is, indeed, black. It is surrounded by matter falling in, faster and faster, converting the energy of its fall into light … including radio waves, which we see here, surrounding the black hole.

Since nothing – not even light – can escape from a black hole, we can never get any information out of it. Amazing things might be happening inside, but we will never – in my humble opinion – know what’s happening inside a black hole.

Al Stahler enjoys sharing science and nature with friends and neighbors, in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and may be reached at a.a.stahler11@gmail.com

Sgr A* seen in radio.
Photo courtesy EHT Collaboration

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