Saturday, October 21, 2017

Thought Experiment

A long long time ago, in a university far far away, I was thinking about how the classic ring-shaped rotating Von Braun space station, like the one with the Hilton hotel in the movie 2001: A Space Odyssey, uses centrifugal forces to simulate gravity. I began to wonder: if you took that space station way way out into deep space, presumably its occupants would still feel pushed towards the floor that was the outer portion of the ring. But if they looked out the window, there is no way they could tell relative to what they were rotating. In fact, other than the fact they could still sip coffee out of open-topped mugs, they couldn't tell they were rotating at all, visually.

(image credit: Wikimedia)

When faced with such conundrums, I did what I always did at the time, I asked my friend and colleague David Hemmendinger. David and I were both graduate students in computer science at the time. But while I had a freshly minted bachelors degree in CS, David already had a Ph.D. in philosophy. At the time he was the smartest guy I knew. (He may still be the smartest guy I know. But in the more than three decades hence, I've continued my habit of working with people a lot smarter than I am, so it's not a given.)

Dave explained that this was a question that troubled lots of people. He told me about Mach's Principle - the same Mach as in Mach Numbers, for describing supersonic speeds. Mach proposed that the rotation that results in centripetal force is relative to absolute space (take that, Relativity!). It's the same principle that causes gimbaled gyroscopes to try to maintain their orientation as they spin no matter how you move and rotate them; they are trying to remain in the same position relative to the entire mass of the universe.


That sounded pretty darn fishy to me. I'd had several physics courses as an undergraduate and knew the math behind Relativity well enough. I understood that rotation was a form of acceleration, and that accelerated inertial reference frames were different from unaccelerated inertial frames of reference.

But how exactly does the distant mass in the universe affect the gyroscope on my desk? Apparently instantaneously? I didn't buy it. I could tell from Dave's clouded expression and half smile that he didn't quite buy it either. He made it clear that he was just reporting what he had read.

This question stayed with me for the next thirty-plus years. I just figured I was kinda stupid, and that someday I would come across the answer.

So this morning I read Tony Rothman's article in American Scientist, "The Forgotten Mystery of Inertia".
It's not long. And it's an easy read. Here's is what I learned.

No one understands this.

This same basic thought experiment troubled Newton, Einstein, Mach, Gödel, and lots of physicists and philosophers right up into the present day.

Everyone knows the what of inertia; the math we use to describe it is completely accessible. But no one can explain the how.

How do rotating space stations simulate gravity? How do gyroscopes strive maintain their original orientation - RELATIVE TO WHAT, FOR CROM'S SAKE?!?!?!? - once they start spinning, and do so with such reliability that we use them inside gyrocompasses as part of inertial navigation systems in everything from nuclear submarines to intercontinental ballistic missiles?

I'm not stupid. (Well, not because of this, anyway.)

The world we live in is. Just. That. Weird.
"If you don't find reality fascinating, you just aren't paying attention." - Me, often.

1 comment:

Anonymous said...

Absolutely LOVE this! MFleming