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Spacetime and Black Holes, by Scientic American

Scientific American has an article in its 12 June 2017 issue titled "Black Holes, Cosmic Collisions and the Rippling of Spacetime." My apology to the author, the astrophysicist Katie Mack, for trying, here, to put her descriptions in my own words.

She writes:

The most revolutionary idea Einstein proposed was that physics only makes sense when space and time are inextricably linked together ...

Okay. Time is something in motion. Without something in motion there is no time. That something has mass and energy, and without this mix — that includes time — there is no space. And astrophysicists call it spacetime.

I'd like to add another ingredient to Mack's statement about spacetime — sequence. The astrophysicist Neil deGrasse Tyson has mentioned the impossibility of going back in time, the impossibility of disrupting what has already occurred. I buy this and dismiss anyone who goes wild with complexity in claiming the possibility of future events affecting past events. Katie Mac doesn't appear to be among them.

Katie Mack's article is also about gravitation. Gravitation pulls on substances, including the electromagnetic radiation we call "light." Gravitation bends light.

Gravitation produces black holes. A black hole is a region in spacetime where gravitation is great enough that particles and electromagnetic radiation (such as light) cannot escape from inside it.

Here for the sake of clarity the issue of distance needs to be considered. The closest black hole yet discovered is several thousand light-years away — a light-year being the distance that light travels in one year. Sunlight takes about 8 minutes 17 seconds to travel the average distance from the surface of the Sun to the Earth. In other words, in one year light travels 66,000 times farther than the average distance between the sun and earth. Multiply that by the several thousand light years that make up the distance to the nearest black hole and you have a long long distance.

Mack writes of our inability to travel to a black hole. But what she is aiming at in her article is our vision of things in outer space, events that occurred millions of years ago just reaching us now. (On May 24 this year Science Daily wrote of astronomers finding a star that exploded 970 million years ago.)

She writes that gravitation increases as you approach a black hole. If you approached near enough to a black hole the gravitational pull would be so great as to tear you apart "limb from limb."

She writes that as you approach a black hole "every other aspect of your existence, will have slowed to a standstill." I must confess that I do not understand this, and I wonder whether she understands it. I know that she understands what astrophysicists call "time dilation": An observer on a rocketship moving faster than another observer, measures time as passing more slowly than does a slower moving observer. But this is about perception, not physical fact outside the head of either of the observers.

Katie Mack goes on, using the metaphor "dent" to describe what "any gravitating body" does when it accelerates through the universe. The "dent" moves with it, she writes, and creates a disturbance in the space around it. This disturbance she describes as "ripples," and these ripples are "gravitational waves." And "last year, for the first time, scientists had a machine sensitive enough to detect them." This is at the Laser Interferometer Gravitational-Wave Observatory.

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Copyright © 2016 by Frank E. Smitha. All rights reserved.