4 ms·

> No, it wasn't. There was a huge body of experimental evidence already about gravity. It was true that all of that evidence, except for the anomalous precessio

> No, it wasn't. There was a huge body of experimental evidence already about gravity. It was true that all of that evidence, except for the anomalous precession of Mercury's perihelion, could be explained by Newtonian gravity; but general relativity, as a proposed new theory of gravity, had to also account for all of that experimental evidence. In other words, in the appropriate limiting case (weak fields and low speeds compared to the speed of light), general relativity had to reproduce all of the correct predictions of Newtonian gravity. That was a huge, and very important, experimental constraint. Yes. I assumed that reproducing Newtonian gravity in the classical limit would be considered theoretical, as we had a solid theoretical understanding of Newtonian physics. Implicit in that would be a bunch of empirical evidence; but Einstein wouldn't need to explicitly think about the details of what evidence supported Newtonian mechanics (beyond the general understanding that it was all in the classical limit). > Not if the differences between its predictions and the predictions of the previous theory (in this case Newtonian gravity) are only measurable in astronomical observations at the time the new theory is proposed. Such a theory is not "useful", for many definitions of useful. Astronomical observations allowed us to verify Relativity earlier than we otherwise would be able to; but eventually technology advanced to the point where we could conduct satellite based experiments to directly confirm it. The theory was not practically useful until we had this technology. > Technology can of course be developed on a purely empirical basis, if you're prepared to make a lot of mistakes along the way that you wouldn't make without a good theoretical foundation. If it hadn't been for the knowledge of relativity on the part of the scientists involved, the GPS satellites probably would have been launched without any capability for adjusting their clock rates. Its not like we launched the entire GPS constellation then just hoped it would work. Before launching the GPS constellation, we launched the experimental Timation satellites to act as a proof of concept. Difficulties with maintaining accurate clocks in orbit would have been noticed then (if not sooner).

pdonis 5y ago
> I assumed that reproducing Newtonian gravity in the classical limit GR itself is a classical (i.e., non-quantum) theory. The Newtonian limit of GR is not usually considered a "classical" limit, just a weak field, slow motion limit. > would be considered theoretical Ok, then we just have different definitions of "theoretical". To me, having to account for a mass of experimental data is an experimental constraint. It might be easier for a theory to meet it if the theory can be shown to reduce in an appropriate limit to a previous theory that accounts for all that data. By contrast, for researchers investigating possible quantum theories of gravity, any quantum constraints on their theories can only be theoretical, since we have no experimental data at all on quantum aspects of gravity. But to me, the fact that a quantum theory of gravity has to account for all of the classical experimental data on gravity is an experimental constraint (which any such theory would presumably meet by reducing to GR in the classical limit). > Such a theory is not "useful", for many definitions of useful. No theory is useful for all definitions of useful. > Before launching the GPS constellation, we launched the experimental Timation satellites to act as a proof of concept. As I understand it, the Timation satellites did not keep independent time using clocks aboard the satellites. They just re-broadcast time signals from the ground. One of the major difficulties with getting GPS operational was the additional complexity involved in having independent clocks on the satellites that would keep their own time, with only periodic updates from the ground. And one of the major issues with that was that many non-scientists involved in the GPS project did not understand relativity and did not believe that the "natural" clock rates of the clocks on the satellites would be different from those of ground clocks, so some kind of adjustment would have to be made. And there was no data from previous projects to convince them since the relative clock rates of previous satellites vs. ground clocks had not been measured. If the non-scientists in the GPS project had not given in to the scientists who wanted to include the capability for adjusting the clock frequency in the initial set of GPS satellites, those satellites would indeed have been useless.
- heavenlyblue 5y ago
  Citation needed for the whole paragraph you wrote here :)

pdonis 5y ago

> I assumed that reproducing Newtonian gravity in the classical limit GR itself is a classical (i.e., non-quantum) theory. The Newtonian limit of GR is not usually considered a "classical" limit, just a weak field, slow motion limit. > would be considered theoretical Ok, then we just have different definitions of "theoretical". To me, having to account for a mass of experimental data is an experimental constraint. It might be easier for a theory to meet it if the theory can be shown to reduce in an appropriate limit to a previous theory that accounts for all that data. By contrast, for researchers investigating possible quantum theories of gravity, any quantum constraints on their theories can only be theoretical, since we have no experimental data at all on quantum aspects of gravity. But to me, the fact that a quantum theory of gravity has to account for all of the classical experimental data on gravity is an experimental constraint (which any such theory would presumably meet by reducing to GR in the classical limit). > Such a theory is not "useful", for many definitions of useful. No theory is useful for all definitions of useful. > Before launching the GPS constellation, we launched the experimental Timation satellites to act as a proof of concept. As I understand it, the Timation satellites did not keep independent time using clocks aboard the satellites. They just re-broadcast time signals from the ground. One of the major difficulties with getting GPS operational was the additional complexity involved in having independent clocks on the satellites that would keep their own time, with only periodic updates from the ground. And one of the major issues with that was that many non-scientists involved in the GPS project did not understand relativity and did not believe that the "natural" clock rates of the clocks on the satellites would be different from those of ground clocks, so some kind of adjustment would have to be made. And there was no data from previous projects to convince them since the relative clock rates of previous satellites vs. ground clocks had not been measured. If the non-scientists in the GPS project had not given in to the scientists who wanted to include the capability for adjusting the clock frequency in the initial set of GPS satellites, those satellites would indeed have been useless.

heavenlyblue 5y ago
Citation needed for the whole paragraph you wrote here :)