Re: [Phys-L] gravitational waves --> smart kid penalty

[John Denker <jsd@av8n.com>]

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On 04/16/2016 12:39 PM, Bernard Cleyet wrote:

> http://www.aapt.org/Resources/GravitationalWavesResource.cfm
>
>
> bc thinks the second referred article is of great interest to LK.

Apparently that is referring to:
  Benjamin Farr, GionMatthias Schelbert and Laura Trouille
  "Gravitational wave science in the high school classroom"
  http://dx.doi.org/10.1119/1.4738365
  http://arxiv.org/pdf/1109.3720.pdf

> p.s.  One of the advantages of AAPT membership.

Actually you don't need to be an AAPT member to read the article:
  http://arxiv.org/pdf/1109.3720.pdf

Hint:  google scholar finds this article with approximately zero effort.
  https://scholar.google.com/scholar?q="Gravitational+wave+science+in+the+high+school+classroom";

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

I am underwhelmed by this passage in the article:

> > only about half of the students were able to deduce that the
> > amplitude of the wave will also increase with time, producing
> > qualitatively accurate models like the one shown in Fig. 4(a). The
> > most common misconception was that the amplitude of the wave would
> > remain constant with time, as illustrated in Fig. 4(b). Through
> > manipulating the computational model, students with the misconception
> > recognized that the amplitude of the wave does in fact increase 
> > with time. However, even after this stage, many could not explain the
> > physical cause for this amplitude increase. Only through a final full
> > class discussion, in which students with more accurate models
> > explained in their own words why the amplitude increases during the
> > inspiral phase, did the remainder of the students understand this
> > aspect of the system. More specifically, the students explained to
> > their peers that the curvature of spacetime becomes more extreme as 
> > the massive objects move closer together (as seen in the fabric
> > demonstration).

That's completely wrong.

In some hyper-technical sense it may be that the /maximum/ local curvature
becomes somewhat greater as the two sources approach each other;  however:
The fabric model predicts:
 1) This is at most a factor of 2.
 2) The maximum curvature near the source is a red herring.  The curvature
  in the far field is what we care about.  To first order, it doesn't change
  at all as the two sources approach each other.  The system is linear, so
  all the receiver sees is the curvature do to source A /plus/ the curvature
  due to source B.
 3) At any given orbital rate, it's even worse than that, because the maximum
  curvature occurs when the two sources sit right on top of each other.  So
  even though they create some curvature, the contribution to the
  /wavelike disturbance/ in the curvature is zero, because nothing is changing.
  There's no lever-arm.
 4) You know it's baloney by analogy to electromagnetism.  A radio with a short
  antenna does not get better if you make the antenna even shorter.

The correct physics goes like this:  At any given frequency, the wave amplitude
goes *down* as the two sources get closer together;  the lever-arm decreases.
On the other hand, the amplitude goes up with frequency, and the frequency 
increases as the sources get closer together, in accordance with Kepler's 1-2-3
law.

So there's a contest between the decreasing lever-arm and the increasing frequency.
The fabric model does not represent this correctly.  Evidently the computational
model doesn't represent it very well either, because none of the students and
none of the teachers figured out what's going on.  The frequency-dependence
is not mentioned or even hinted at in the article.  I'd be surprised if most
professional physicists could figure this out without looking up the formula
... so why are we asking high-school students to figure it out?

I suggest that the students didn't learn anything of value.  Instead they rote-
memorized the qualitative result.  They learned that it is OK to cook up a bunch
of words to "explain" the result, even if the words don't actually mean anything.
This follows a long-established pattern;  recall that Cain had an "explanation"
for why he slew Abel.  People are really adept at cooking up convenient a_posteriori
"explanations" of this kind.  It is, however, not science.  It is the opposite and
the enemy of science.

This is an example of what is known as the "smart kid penalty".  Anybody who
dares think about what's actually going on is just gonna get in trouble.

I'm skeptical that figure 4a is consistent with the actual equations of motion.
It looks to me like some sort of "artist's conception".  It's billed as being
"accurate" but later it's described as "qualitatively" accurate, whatever that
means.  In any case, we aren't told which aspects are faithful to reality and
which are not.

I'm even more skeptical of the central premise of the article, namely that it is
worthwhile to mess with this stuff at all.  Everything the students learn about
waves they could have learned in some more practical context, e.g. optics or
acoustics.  Playing around with Audacity is fine (although it might make more
sense at the undergraduate level).  At any level, it could be applied to something
much more relevant to the real world, e.g. human speech and/or musical acoustics.
I guarantee you there are more job openings for Speech/Language Pathologists than
for gravitational wave astronomers.

If you want to delve into signal detection issues, look at frequency-shift keying
as used in old-style 1200 baud modems and fax modems.  That stuff is mostly obsolete,
but it's still a thousand times more relevant to the real world than gravitational
waves are.  It involves signals you can actually send and receive in the lab.

I am especially skeptical of the final claim in the conclusion:
> > without losing focus on topics already in the curriculum.

As the saying goes, if it's not worth doing right, it's not worth doing.
General relativity is significantly more complicated than electromagnetism
or acoustics, and getting it right requires worrying about lots of stuff 
that would not otherwise be in the curriculum.  Explaining the quadrupole 
polarization is an obvious example.  The fabric model is at least two jumps
removed from representing this correctly.

If you want to add something to the curriculum, there are tons of things more
suitable than gravitational waves.  For example:
 ++ The laws of motion in an accelerated frame.  This is a big deal in the
  real world.  Everybody in the real world knows that centrifugal force is
  important, so when the HS physics teacher says it doesn't exist, it just
  makes people think that physicists are crazy and stupid.  There are direct
  practical applications to aviation and space flight.  Also, understanding
  accelerated frames is obviously a prerequisite for understanding the first
  thing about gravitation, even before you get to general relativity.
 ++ Geodesics have application to real-world navigation, surveying, physical 
  optics, classical mechanics, quantum mechanics, dressmaking, et cetera.
  And they lay the foundation for understanding general relativity if/when
  the time comes.
 ++ et cetera........

I'm glad we have gravitational wave astronomers.  However, we don't need very 
many of them ... and we should recruit them from upper-division physics majors
who volunteer.  We should not impose this stuff of unwilling and uninterested
high-school students.

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

Tangentially related:  If anybody wants to know how the LIGO machine actually 
works, here is "the" paper.
  LIGO Scientific Collaboration
  "Advanced LIGO"
  http://arxiv.org/pdf/1411.4547.pdf

The individual authors are listed here:
  https://dcc.ligo.org/public/0115/M1400319/008/LSC-Authors-Aug2014.pdf