1158: "Rubber Sheet"

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Re: 1158: "Rubber Sheet"

Postby Wnderer » Wed Jan 09, 2013 8:21 pm UTC

I recall reading in Terman's Radio Electronics Handbook that rubber sheet models were used in vacuum tube design. They have Wikipedia article on it.

Prandtl's stretched-membrane concept was used extensively in the field of electron tube ("vacuum tube") design (1930's to 1960's) to model the trajectory of electrons within a device. The model is constructed by uniformly stretching a thin rubber sheet over a frame, and deforming the sheet upwards with physical models of electrodes, impressed into the sheet from below. The entire assembly is tilted, and steel balls (as electron analogs) rolled down the assembly and the trajectories noted. The curved surface surrounding the "electrodes" represents the complex increase in field strength as the electron-analog approaches the "electrode"; the upward distortion in the sheet is a close analogy to field strength.

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Re: 1158: "Rubber Sheet"

Postby dudiobugtron » Wed Jan 09, 2013 9:07 pm UTC

doinkisaac wrote:
Quicksilver wrote:Looks kind of dangerous. Why a bowling ball?

Because that way you think it's going to be a physics joke. Joke probably wasn't the word I meant, but It's 1:20 in the morning, and I can't think of the actual word.

The actual word is 'reference'.

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Re: 1158: "Rubber Sheet"

Postby orthogon » Thu Jan 10, 2013 1:50 pm UTC

keithl wrote:Also proportional to the ball weight, for small displacements.

I only ask because it looks to me like the force is quadratic in the (small) displacement for the 1D case of pushing on the middle of a rubber band tied on at both ends, if I assume the rubber band starts off unstretched, i.e. the distance between the fixing points is equal to the natural length and the band is light.

If anyone can explain how to get the math or imath tag to work I'll post my proof.
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Re: 1158: "Rubber Sheet"

Postby JJH » Thu Jan 10, 2013 1:55 pm UTC

Did the Beret Guy just invent a kind of warp drive?

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Re: 1158: "Rubber Sheet"

Postby Quizatzhaderac » Thu Jan 10, 2013 8:38 pm UTC

JJH wrote:Did the Beret Guy just invent a kind of warp drive?

Only if the height above level is greater than c2/g (white hole event horizon), which would require the robe pulling him below -c2/g (black hole event horizon).[citation needed]
The thing about recursion problems is that they tend to contain other recursion problems.

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Re: 1158: "Rubber Sheet"

Postby brenok » Thu Jan 10, 2013 8:40 pm UTC

I though you were going to link to xkcd. [citation needed]

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Re: 1158: "Rubber Sheet"

Postby ike » Fri Jan 11, 2013 5:38 am UTC

mathmannix wrote:
ysth wrote:Radical!

Well, Beret Guy is often irrational...

I don't think Beret Guy has been irrational; I think often, and esp here, he is an acknowledgement that not everything has to have a point.

[edited: changed "had" to"has"..... now sleep]

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Re: 1158: "Rubber Sheet"

Postby ijuin » Sat Jan 12, 2013 5:21 am UTC

No, it's a pun on "irrational numbers", in response to the exclamation "radical" (used also to refer to taking roots of numbers, which gives numerically irrational values--therefore if he is radical he is irrational).

Anyway, even if the numbers don't work out the same, the rubber sheet model does show that mass curves space in a hyperbolic form, rather than spherically (spherical curvature being the kind of curved-space that a student first being introduced to the concept of curved spacetime would be most familiar with, such as the surface of a globe).

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Re: 1158: "Rubber Sheet"

Postby J Thomas » Sat Jan 12, 2013 4:04 pm UTC

orthogon wrote:But on a 2D surface I'm less sure. Maybe most of the deformation is indeed local. I fear Bessel functions will come into it. I'm this close to googling it.

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Re: 1158: "Rubber Sheet"

Postby Kit. » Sat Jan 12, 2013 5:00 pm UTC

ijuin wrote:if he is radical he is irrational

And if he is irrational but not radical, he is transcendental.

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Re: 1158: "Rubber Sheet"

Postby mikrit » Sat Jan 12, 2013 8:36 pm UTC

Vroomfundel wrote:
AvatarIII wrote:This hypothetical always perplexed me, it's using the our ability to imagine the effects of gravity to describe how masses affect "space-time" to create gravity. so you need an understanding of one to comprehend the other. - - -

This reminds me of the famous Feynman lectures:

He uses the same argument to bash the flexible sheet visualization of gravity - you can't use gravity to illustrate gravity, he says, and he's right. - - -

Randall took up this dilemma in Teaching Physics, http://xkcd.com/895
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