Re: [Phys-L] Fwd: [PTSOS] spacetime simulator: flexible conduit?

[Ken Caviness <caviness@southern.edu>]

That is indeed an important distinction that geodesics in GR depend on curvature, while the force on the rolling marble on a warped trampoline depends on the slope, not on the curvature.

Also it is a good point that the tape demo still works in interstellar space, whereas the trampoline demo requires a constant gravitational field.

Thanks.  

But I really doubt whether even 1% of the people notice or learn incorrect physics from the non-fitting parts of the demo.  It should be taken only as an analogy:  the heavy mass deforms the surface, it's no longer flat, and marbles on the no-longer flat surface behave differently!  That much is shared by the demo and the actual theory.  As good as many analogies, I'd say.

KC

-----Original Message-----
From: phys-l-bounces@mail.phys-l.org [mailto:phys-l-bounces@mail.phys-l.org] On Behalf Of John Denker
Sent: Tuesday, 17 July 2012 7:24 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Fwd: [PTSOS] spacetime simulator: flexible conduit?

On 07/17/2012 02:38 PM, Ken Caviness wrote in part:
> JD, could you explain the statement the prediction would be the same  
> if the trampoline were deformed upward instead of downward?

OK ...

> A large mass on the trampoline curves the surface around it, but (I
> think) not the right amount, not with the correct r-dependence.

My point is that a profoundly important concept -- namely geodesics -- is represented in the model, but represented quite wrongly.  Categorically, conceptually, and qualitatively wrongly.

  In contrast, I am not interested in the possibility that the trampoline
  might be quantitatively not-quite-correct as to the details of the r-
  dependence.  I am not concerned that this-or-that advanced concept
  might be missing.

Geodesics follow the straightest available path on the curved surface.
A geodesic responds the same way to upward curvature as to downward curvature, as you can demonstrate by applying masking tape to the inside and outside of a thin, curved bowl.  I have a thin steel mixing bowl that came with a Kitchenaid mixer that works nicely.  If you match the initial conditions, the two tape trajectories will track rather nicely.

It should be obvious that the masking tape demo works in the weightless environment of the space station, since it depends only on the *curvature* of the bowl ... whereas the trampoline trick depends not on the curvature of the trampoline but rather the /slope/ and indeed the projection of the earth's /gravity/ along the slope.

My point is that everything you learn using the trampoline demo will have to be unlearned before you can understand anything about the real mechanism of curved spacetime.  A simple model that is *much* more faithful to the actual physics is here:
  http://www.av8n.com/physics/geodesics.htm

On 07/17/2012 03:36 PM, Anthony Lapinski wrote:

> > Nice informative video of curved spacetime!

No.  The interaction depends on the slope, not the curvature.

The trampoline model is a profoundly wrong explanation of curved space.
It is roughly about as wrong as the "filled Lewis octets" that are used to explain chemical bonding ... given that there are *not* any filled octets in real molecules ... as is well known based on 100+ years of spectroscopy data, based on the paramagnetism of the O2 molecule, based on Hund's rules, based on modern molecular-orbital theory, et cetera.
Everything you know about "filled Lewis octets" will have to be unlearned before you can understand the first thing about how chemical bonding actually works.

Again, I am not interested in not-quite-right quantitative details.
I'm talking about conceptually, categorically, and qualitatively wrong.
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