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(Medium)   Experiments reveal that a deformed rubber sheet is not like spacetime. Who knew?   ( divider line
    More: Obvious, space-time, general relativity, analogy, calculus, physicists, non-Euclidean geometry, experiments, motions  
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3260 clicks; posted to Geek » on 06 Jan 2014 at 10:21 AM (3 years ago)   |   Favorite    |   share:  Share on Twitter share via Email Share on Facebook   more»

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2014-01-06 10:23:50 AM  
3 votes:
imgs.xkcd.comView Full Size
2014-01-06 10:15:57 AM  
3 votes:
a comparison between two things, typically on the basis of their structure and for the purpose of explanation or clarification.

Now simply explain the differences when demonstrating the analogy and ta-da -still germane.
2014-01-06 12:30:09 PM  
2 votes:

BolloxReader: I would still like to know how gravity works, if changes in gravity are instantly felt everywhere or if they are propagated.

They propagate at the speed of light (in a vacuum, anyway, and since nearly all of the universe is vacuum...). So if the sun blinked out of existence right now, roughly eight minutes later you would see it disappear from the sky at the same instant that the Earth stopped experiencing its gravitational pull and careened off into space.

(Incidentally, if this weren't the case, highly advanced civilizations would be able to send Faster Than Light messages by annihilating large chunks of matter and antimatter, which would be enormous fun.)
2014-01-06 09:48:07 AM  
2 votes:
ORLY? You mean to tell me that trampolines and pieces of rubber are fundamentally different from the fabric of the universe?
2014-01-07 12:41:17 AM  
1 vote:

Myria: My question is why is gravity special?

You're probably smarter than 95% of physics majors anyways, for asking a question in the first place, and for thinking about it, but there is a fundamental difference. Inertial mass and gravitational mass are the same. In other words, the m in F=ma is the same m as the m in F = mg. If you double the mass of the object in the elevator, it will experience twice as much force if the force is gravitational, but the force would be unchanged if it had been an electric force.
2014-01-06 03:15:05 PM  
1 vote:
2014-01-06 02:17:37 PM  
1 vote:
The problem with the analogy is that it uses gravity to explain gravity.
2014-01-06 10:36:54 AM  
1 vote:

indarwinsshadow: You mean that space time isn't some two dimensional plane, and that the resulting gravity isn't a product of mass acting on the surface of that plane.

Who would'a thunk.

...I hate modern explainations of science. It's as bad the "it's turtles all the way down man".

What time period do you prefer for explanations?
2014-01-06 10:35:31 AM  
1 vote:
But the truth is that this work cannot diminish the extraordinary utility of this analogy. And so the public love affair with general relativity is safe. Long may it continue!

Then don't write a farking useless article about it because I'm pretty sure most people that use the analogy are smart enough to understand that it's not entirely accurate...
2014-01-06 10:06:37 AM  
1 vote:
In other news, fog does not have little cat feet... or any feet at all!
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