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B What was the last step in coming up with general reletivity?

  1. Mar 18, 2016 #1
    Sorry if this seems like a stupid question but I cannot find an answer. I am asking this because to me, it seems like the hard part of coming up with general relativity would be discovering what gravity actually was ( the deformation of spacetime ), and the field equations and mathematics would simply follow, not the other way around. Can someone correct me if I'm wrong? The last step of general reletivity was discovering that gravity was the deformation of spacetime, true or false?
     
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  3. Mar 19, 2016 #2

    Simon Bridge

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    The last step was publishing the paper ... though, really, there is no last step because science is never finished.
    "Discovering what gravity actually was" has not happened yet ... we don't even know what "actually" would mean in this context.

    Starting with special relativity and newton ... I understand the first step was to realise that there is no way to distinguish gravitation from accelerating reference frames. The development of a geometrical description for gravity was, loosley, the middle step ... and the covariant mathematics finished it off.
    https://en.wikipedia.org/wiki/History_of_general_relativity

    You should realise that the "fabric of space time" is not what "actually" is there - that's a poetic metaphor for describing a mathematical model.
     
  4. Mar 19, 2016 #3

    PeterDonis

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    Discovering that gravity is spacetime curvature is not a single event. It took a number of years; the whole process of Einstein developing the correct field equation for gravity was the process of discovering that gravity is spacetime curvature. (More precisely, of discovering that all of the effects of gravity could be accounted for by modeling spacetime as curved instead of flat, as it is in special relativity.) And if you think experimental confirmation should be part of the process of discovery (which is certainly a defensible point of view), then the process didn't end with Einstein's publication of the correct field equation; it arguably is still going on, since we are still making new experimental discoveries that confirm predictions of GR that had not been confirmed before (the recent LIGO observation of gravitational waves, for example).
     
  5. Mar 19, 2016 #4
    I would say false. By using the equivalence principle, one can deduce that light will bend in a gravitational field. This implies a curvature of spacetime. The hard part was in the details - deriving the mathematics to describe such curvature and the specific way it is caused by mass/energy.
     
  6. Mar 19, 2016 #5

    stevendaryl

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    Roughly speaking, there are two parts to General Relativity, as it was put in Misner, Thorne and Wheeler's "Gravitation":
    1. Spacetime tells matter how to move.
    2. Matter tells spacetime how to curve.
    So once you have thought of the equivalence principle, and you've hit on the idea that freefall is just motion along geodesics of spacetime, then you've basically got the first part down. Einstein had this part figured out by about 1907, just a couple of years after he developed Special Relativity. It's the second part that took him another 8 years of work. He knew that he wanted an equation of the form "Something involving spacetime curvature = Something involving matter and energy", but exactly what the something on left was was a puzzle.
     
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