Tuesday, January 28, 2020

Frames of Reference III

Light travels about a foot in a nanosecond. That's a very rough approximation. But it makes it easy for users of the old Imperial system of measurements to grasp this idea.

I look across the table at my lovely wife of nearly thirty-six years, Mrs. Overclock. I think I'm seeing her in the present. But I'm not. She's about three feet away from me, so I'm really seeing her as she was three nanoseconds in the past.

Behind Mrs. Overclock, I see the far wall. It's about four feet behind her, or about seven feet from me. I'm seeing the far wall as it existed seven nanoseconds in the past.

Through the window in the far wall I see the Rocky Mountains in the distance. The closest mountain I can see, Table Mountain near Golden Colorado, is about three miles west of us, or in the neighborhood of 16,000 feet away. So I'm seeing a version of Table Mountain that's 16,000 nanoseconds, or sixteen microseconds, in the past.

In the night sky, I can see the Moon above the mountains. The Moon is an average of about 240,000 miles away. That's more than 1.2 billion feet. So I'm not seeing the Moon as it is in the present - which by now you've figured out is a highly abstract concept. I'm seeing the Moon as it was more than 1.2 seconds ago. It's even possible it no longer exists in this present, and the shock wave and debris field just haven't arrived yet. Unlikely. But possible.

Astronomers have recently speculated that the distant star Betelgeuse is about to end its life by going nova. Betelgeuse is seven hundred light years away. So when we observe Betelgeuse, we are not seeing it as it is today, but instead are looking seven hundred years into the past. Whatever happened to Betelgeuse, it happened a long long time ago, and the light from that event is just arriving here. This kind of time shift is true for all of the stars we see in the nighttime sky to varying degrees depending on their distance from us.

So the present that I have fooled myself into believing I see is not only not the present, it's not even the same time in the past. It's a mosaic of perceptions taken from a broad ensemble of times in the past. This is why Albert Einstein asserted that there was no such thing as absolute simultaneity in his Theory of Special Relativity. And none of this takes into account the latency of our nervous system, or the processing time in our brains.

Real-time systems work like this too, perceiving at best an approximation of the reality around them.

See Also

Chip Overclock, "Frames of Reference", 2018-03-14

Chip Overclock, "Frames of Reference II", 2019-04-19

Leslie Lamport, "Time, Clocks, and the Ordering of Events in a Distributed System", Communications of the ACM, 21.7, 1978-07

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