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Teaching relativity to a skeptic

by metiman
Tags: relativity, skeptic, teaching
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Jan5-12, 07:12 AM
P: 860
Quote Quote by metiman View Post
The leap from the math to a model of the world where massive objects create 'depressions' in a 4th (temporal) dimension needs its own proof.
Ultimately a physical theory is an assertion that if you go out into the real world and measure certain numbers, then some specific mathematical relationships between those numbers will hold. Imagery like "curved spacetime" is just words to help us intuit the behavior of the equations and can't be proved. In fact Steven Weinberg in his GR textbook deemphasizes the idea that general relativity is about "curved spacetime," writing

Quote Quote by Weinberg
...the geometric interpretation of the theory of gravitation [i.e., that gravity is really the curvature of spacetime] has dwindled to a mere analogy, which lingers in our language in terms like "metric," "affine connection," and "curvature," but is not otherwise very useful. The important thing is to be able to make predictions about images on the astronomers' photographic plates, frequencies of spectral lines, and so on, and it simply doesn't matter whether we ascribe these predictions to the physical effect of gravitational fields on the motion of planets and photons or to a curvature of space and time. [Weinberg adds the caveat:] (The reader should be warned that these views are heterodox and would meet with objections from many general relativists).
Quote Quote by metiman View Post
In this particular case that experimental proof may very well exist. In fact I'm assuming that it does and I'm trying to find it. Even if there is no direct proof, it may be possible to prove starting from the equations. The first thing I would do is examine the equations and try to prove that they are unique. That the same relationships between variables cannot be represented in any other form except through Minkowski's math. Once you've proven that then you just have to show spacetime is the only option. That without it the equations, with their great predictive value, just wouldn't work.
This seems off base. Mathematical relationships can often be expressed in many different forms, some amazingly different in appearance. The important thing, again, is the predictions that come out of the math, which must be the same if the different forms of the math are really equivalent.

To give an example that has gotten a lot of interest recently, the "AdS/CFT correspondence" is the statement that two very different specific mathematical theories actually (and very surprisingly) describe the same physics. But these two theories seem very different--for example, they posit different numbers of spacetime dimensions! So one theory might say that spacetime has 4 dimensions while the other says that spacetime has 5 dimensions. Since both predict the exact same results, it becomes clear that the mathematics of a physical theory doesn't tell you how you should picture the universe described by the theory. You can think of it as a 4-dimensional spacetime with one set of physical laws. You can think of it as a 5-dimensional spacetime with a different set of physical laws, and this turns out to be completely equivalent to the other description. You can think of it as a simulation proceeding in a cellular automaton computed by a sentient frog (though this is probably a less useful picture), so long as you posit that the simulation is programmed in a way that corresponds to the predictions of the theory.
Jan5-12, 07:38 AM
P: 17,213
Quote Quote by zonde View Post
This is very popular position but I am not sure I fully agree with that. Traditionally the core of the theory is explanation and then you use predictions (preferably quantitative) to test how good is that explanation.

Say we can have some empirical data and do some curve fitting using that data. Now we can make predictions using untested parts of that curve (we interpolate or extrapolate empirical data). This is not a theory as it lacks explanatory part, right?

For me explanation is the theory. If we have equations with very direct connection to physical observations I would probably call it something like "empirical theory".
The problem with this is that your idea of a scientific theory cannot be tested with the scientific method. The only part which can be investigated by the scientific method is what you call an "empirical theory".

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