Re: REPLY TO ALL (about textbook misconceptions)

[Leigh Palmer <palmer@sfu.ca>]

> Date: Wed, 11 Feb 1998 20:47:01 -0800
> From: Leigh Palmer <palmer@sfu.ca>
> Subject: Re: A list of textbook misconceptions
>
> > > 5) Gravity in space is zero (or very small, e.g. NASA's 'microgravity')
>
> > Gravity inside an orbiting shuttle is very small compared with gravity
> > at the same point at rest with respect to Earth. The misconception is
> > that one can ascribe a unique "gravity" to a point in space. The
> > gravitational field is relative (just as are the electric and magnetic
> > fields). It depends upon the frame of reference of the observer.
>
> > > 13) Laser coherence is caused by atoms' in-phase emission
>
> > Why is this a misconception?
>
> I do not understand the comment next to the item (5). The distance from
> the center of Earth to the orbit is nearly the same as to the sea level.
> Why are you saying g is very small in comparison with 9.8?

I merely refer (and not at all obliquely) to the principle of equivalence.
If you claim that there exists something called "gravity" you should be
prepared to offer an operational definition by means of which it can be
measured. According to the principle of equivalence there is no experiment
which can be performed which will didtinguish locally between the presence
of a gravitational field and acceleration of the frame in which the
measurement is carried out. The shuttle environment is an excellent
example of a frame in which the net effect of these terms is small.

Calling the shuttle a microgravity (better would be migro-gee) is entirely
appropriate. The gravitational field of the Earth varies by one part in a
million over a radial distance of three meters from the center of the
Earth if my quick mental calculation does not err*. That would be a
microgee over a scale parameter appropriate to shuttle experiments.

Scratch misconception 5. Gravity in the shuttle is small, as one might
well expect seeing those folks floating around in there. Anyone wanting
to pursue this further should see the old "Science" article "Apples in a
Spacecraft" by Alfv=E9n and Arrhenius - the exact citation of which I have
lost, but it is in 1971, I believe.

While the list Ludwik is compiling is in principle a nice idea, it won't
be of much use as an intellectual tool beyond the intellectual level of
David Letterman. Too much explanation must be added to the item "Entropy
is the measure of the disorder of a system" to justify its being called a
misconception, which it certainly is. A list item like this will do no
real good. The present example is another, and my mention of the picture
leading to a misconception is yet another. We don't need a list; we need
a book, with explanations ranging all the way from the level of mine to
the level of David Bowman's. Every item that appears on the list seems to
be in the odious category of something that "isn't even wrong".

Leigh

* OK. Mental calculation redone below, partly to show the value of math
in doing conceptual problems. Conventional symbols (g is grav. field):

           G M
      g =3D -----
             2
           r


       -6    dg        dr
     10   =3D ---- =3D -2 ----  ;  r =3D 6600 km (or so)
             g         r


             r   dg
      dr  =3D --- ---- =3D (-) 3.3 m
             2   g