Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] earth's rotation



On 01/05/2008 02:44 PM, Larry Woolf wrote:
" The notion that the Gulf Stream is responsible for keeping Europe
anomalously warm turns out to be a myth."
<http://www.americanscientist.org/template/AssetDetail/assetid/51963?ful
ltext=true>

OK! Point taken. Interesting article. A "keeper".

Still, the curiously warm British climate remains a good answer
to the original question. I fell victim to mythical physics, but
the argument can easily be repaired by replacing incorrect water
physics with correct air physics. Reversing the prevailing winds
would probably give Britain the climate that Kamchatka now has.



On 01/04/2008 02:39 PM, Ken Caviness wrote:

... anything dependent on the Coriolis force ...

Yes, that's the key idea.

=========

Taking a step backwards, students were not born knowing the
connection between Coriolis and climate (or more generally,
the connection between physics and climate) ... so if you're
going to discuss this in class, you need to lay some groundwork.

An introduction to this, including some useful pictures, can
be found at
http://www.av8n.com/how/htm/atmo.html

For present purposes, you can skip the section on altimetry.

At the next level of detail, Encyclopedia Britannica has a
wonderful article on climate. Long, easily readable, and
informative.

===============================================

Taking two steps backwards, I am surprised to see some people
apparently not understanding what it means to reverse the
rotation of the earth.

Rotation is physics. I don't think the original question in
this thread was poorly worded ... or if it was, I don't see
where the bug is, except for some details that are so nit-picky
that nobody has even mentioned yet.

Rotation is best represented by a bivector. If we want to
reverse the direction of rotation of the earth, we can put
up two big billboards and paint a bivector on each one. We
label one of them "before" and the other "after".

----------<-----------
| |
| |
| before |
| |
| |
---------->-----------


---------->-----------
| |
| |
| after |
| |
| |
----------<-----------


The billboards allow us to bypass any discussion of the definition
of east/west, the definition of CW/CCW, et cetera. It suffices
to speak of physical rotation "thataway" or "thisaway". In this
case we are reversing the rotation of the earth relative to the
earth itself, i.e. relative to the geographical facts, billboards,
et cetera.

================

Reversing the earth's rotation remains a Gedankenexperiment. Carrying
out the experiment would not be practical. If you look closely, you
also see some conceptual questions. For starters, we must ask what
should be included in the reversal. Obviously macroscopic classical
angular momentum should be reversed, but what about the nonclassical
spin angular momentum? That's tricky, because it would reverse the
N and S markings on typical bar magnets and compass needles. The
needles would no longer agree with the N and S on electromagnets.

Similarly, what about the direction of rotation of hard disk drives?
I would prefer that we not reverse those, because it would break
them.

=====================================================

A much deeper set of questions arises if we flip the parity of the
entire earth. In addition to reversing the rotation, this would
mirror-image the geography and the billboards (and the people).
The point is that if you do it right, parity inversion is a
symmetry of the basic laws of mechanics, so mechanical phenomena
would be unaffected by the inversion.

If you parity-invert the earth but not the moon and sun, then
there would be some small mechanical changes due to the tides
et cetera. This would be a minor correction term, unless you
live along the Bay of Fundy or the like.

The killer would be the electromagnets. Parity inversion (P)
is *not* a symmetry of the Maxwell equations. To fix this, you
would need to flip the sign of the charge on each fundamental
particle. This is called charge conjugation (C). Basically it
converts matter into antimatter and vice versa. Until 1957 it
was thought that CP (i.e. parity inversion and charge conjugation
together) was an exact symmetry of all of physics, but it turns
out weak nuclear decays are not quite CP symmetric.

Feynman devotes an entire chapter to this ("symmetry in physical
laws"). Well worth reading.