Re: [Phys-l] Relativistic Time Dilation and the Bernoulli Effect

[John Denker <jsd@av8n.com>]

On 03/25/2007 03:29 PM, Tibor G Molnar wrote:
> can you please tell me how to 
> post a reply to a list message such that it appears in the correct place in 
> the list?  

Email is email.  Email to the list is email to the list.

You have control over the "To:" field, the "Subject:" field, etc. in
any mail you send.  The list does not recognize any "right place" or
"wrong place" beyond that.

> And, if you feel so inclined, can you please elaborate on what is wrong 
> with the jostling demo? 

You first.  You tell us what's right about it.  The guy who comes
up with the model has the burden of explaining in what way(s) the
model is apt, i.e. explaining what the model is good for.

Wrong models are a dime a dozen.  Why should we pay any attention
to this model?

As the saying goes,
  "Learning proceeds from the known to the unknown."

In that spirit, please tell us:  What is there about jostling that
we /already know/ that sheds light on what we /don't already know/
about fluids?

Did you check the model for correctness?  What checks did you make?
If the model were completely preposterous, would you have noticed?

What does the model predict for the pressure gradient when a horde of
students is standing on a merry-go-round as it rotates?
What does the model predict for the pressure gradient when a horde of
students walking on a non-rotating merry-go-round, walking around
in big circles?
What does the model predict for the pressure gradient when one horde
of students is streaming southbound in the west half of the corridor,
adjacent to another horde of students streaming northbound in the
east half of the corridor?

These are not meant to be tricky questions.  The goal is to figure
out whether or not the model tells us how fluids behave.  If you
think the model is informative, please explain.

>  Is not kinetic energy conserved in the demo?

You tell us.  Is it conserved, in your favorite version of the
demo?  What about other versions?

Specifically:  When a horde of kids expands, does it expand
isentropically or isothermally?  How do you know?  How are our
students supposed to know?  If they don't know, what good does
anybody get out of the model?

It seems to me that if (!) kinetic energy is conserved in the demo,
it is only because it was built in as a /premise/ of the demo;
it cannot be /predicted/ as a consequence of the ordinary natural
behavior of kids jostling in a hallway.

If you look at actual traffic-engineering data for cars approaching
and passing through a narrow section of road, you find that the data
does not conform to any Bernoulli-like description.  Not even close.

Ditto for people queuing at (say) a checkout counter.