Oooh, Dear, science blooper here:
During their brief time
in space, the experiments were weightless, allowing scientists to
observe physical processes when gravity is absent.
Gravity, absent in a rocket on a sounding trajectory? Err, why do the damn things fall down again then?
actually not a bad science blooper, in my opinion…
The fundamental idea of general relativity is that falling freely in a gravitational field is absolutely, identically indistinguishable from being in no gravity at all. (With that insight, and Einstein’s brain, and ten years to make up new math, or Hilbert’s brain, and a few months to deploy math you already know, you get GR.) So you *can* argue there’s still gravity there, just cancelled by acceleration, but that’s an arbitrary matter of how you choose to look at it. In the same sense, I *could* say that interstellar space isn’t empty, it’s just that all its particles have been cancelled by antiparticles; or that the earth is moving at 99% of the speed of light (relative to something). These things are true in some sense: individual choices from many arbitrary equally true ways of looking at the situation. But that kind of truth doesn’t justify contradicting someone else who picks another one of the equally true ways, e.g., that the earth is moving at only a few kilometers per second (in his favorite reference frame, probably relative to the sun).
Now, the GR principle only works for uniform gravitational fields, so there is a tiny tidal effect left over, because the gravitational field won’t be absolutely uniform over the inside of the capsule. And “absent” might not be the most felicitous choice of words, I dunno. (But “cancelled out” doesn’t seem to have quite the right connotations either, and I can’t think of a better construction.) Those don’t seem like serious problems, though.
Tim,
They’re right, I’m afraid. Freefall and zero gravity are different things – spacecraft in Low Earth Orbit are actually under 95% or more of sea-level gravity, which is why they hang around the neighbourhood 🙂
A sounding rocket is kind of in a very elongated orbit around the Earth’s centre which intersects the ground (as is anything that is lobbed off of the ground – a high jumper has a very abbreviated partial orbit, for example).
Think of being in a lift with the lift cable snapped – you’ll be weightless with respect to the lift floor – but very much subject to gravity still, as you’d find out in a couple of seconds). That’s the way that the experiments work.
Hmmm,
Zero gravity and free falling are indistinguishable – for instance
suppose we are all falling towards a local
black hole hole – how would we feel it?
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