Sunday, April 29, 2007

hey, Dad!

A trip like this-- probably with a crowd of other people, unlike Hawking's solo ride-- costs $3500 per person (the plane makes 15 parabolas, offering about 7-8 minutes of zero gee), but might be something to look into, eh?

Stephen Hawking recently took a trip on Zero-G Corp's 727 version of the military C-9 "Vomit Comet." The civilian version of the zero-gee flight is a full-day event lasting several hours, including an introduction and training session at the beginning. Read the FAQ writeup here for more information. It's unfortunate that weightlessness is only for a maximum of thirty seconds at a time, but still-- that's more weightlessness than most of us will ever experience.

Technically, it's not really weightlessness so much as either "free fall" or "microgravity": when the plane is descending rapidly, one's bodily inertia prevents one from descending quite as quickly as the plane for a few moments, thus granting the illusion that one is floating inside the jet. In actuality, one is falling relative to the ground. The term "microgravity" is more appropriate for what astronauts orbiting the earth experience: they are not completely free of the earth's gravity well, which still exerts some force on them.

For blog readers who don't know this: my father has long wanted to be either a pilot or an astronaut. Space travel fascinates him. It fascinates me, too, but I'm waiting for the advent of luxury spaceliners that shuttle people to the moon and back, and for Heinlein-style "ballistic" travel-- imagine going from one continent to another in less than an hour.



Malcolm Pollack said...

Hi Kevin,

Not to be too picky, but free fall really is weightlessness; it's not any different from being aboard the space shuttle, or being in a distant solar orbit, or floating in interstellar space. In all these cases bodies are still subject to gravitational influences, because nowhere is spacetime really perfectly "flat", due to the warping effect of the mass of nearby planets, stars, the galaxy itself, and even other galaxies, etc. The difference between these situations and what we experience standing on the Earth's surface is that in free fall we follow an inertial path through locally curved spacetime, while when we are standing on the ground we are prevented from doing so by the presence of the rigid Earth. We feel the force that causes this defelection on the soles of our feet, just as we would if we were in interstellar space in an accelerating rocket ship.

"Zero gravity" is rather a confusing term; The "principle of equivalence" tells us that free fall and "zero gravity" are really one and the same.

Kevin Kim said...

It never bodes well when a comment starts off with "Not to be picky."

I thought the difference between "mass" and "weight" had everything to do with the presence or absence of gravity. If, for example, I were floating out in interstellar space, my mass would be what it is now, but I would be weightless-- i.e., there would be almost no gravitational force acting upon me. On the earth's surface, though, I have both mass and weight-- i.e., my body is very much affected by the pull of earth's gravity.

By that reckoning, I was pretty sure that a person falling relative to the earth is not at all weightless. By extension, two bodies falling alongside each other would not be weightless, but would appear to be unmoving, each relative to the other. From this I concluded that Hawking's body, falling inside the civvie Vomit Comet, had the illusion of weightlessness because he and the plane were "falling" at roughly the same rate, but neither was unaffected by gravity.

My (mis)understanding, anyway.


Malcolm Pollack said...

Well, Kevin, you make a fair point. There is a conceptual distinction one can make between weight (the net force that is the sum of the gravitational influences on a body) and "apparent weight". To really have zero weight, one must be in a "flat" region of spacetime (not so easy to find!). From the observer's point of view, however, there's really no difference, and a spring scale, which is what measures weight (as opposed to mass) will indicate a state of weightlessness.

But I consider my previous comment to be wholly invalid anyway, because I see it contains the ghastly typo "defelection". This reminds me, somehow, of coprophilia, and ruins the whole thing.