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Theory of relativity (view source)
Revision as of 19:14, September 7, 2018
, 19:14, September 7, 2018→Experimental and Observational Evidence Confirming Relativity: assumptions
It is important to be aware that it is fairly rare for an experiment to '''prove''' a theory. In general, experiments can only '''refute''' a theory. They can also '''be consistent with''' a theory. When enough experiments, especially experiments that investigate a wide variety of phenomena, are shown to be consistent with a theory, it lends credence to that theory. When no other plausible theory can explain those observations, we can say that they '''validate''' the theory. The Mercury observations are just one phenomenon. By themselves they couldn't validate or prove GR. And there was at one time another competing theory—the Newcomb-Hall exponent-fudging theory. That one didn't hold up for things other than Mercury. No other theory has come up explaining the phenomenon; GR has withstood the test of time. That goes a long way toward validating GR. There are many other observations and experiments, covering a wide variety of phenomena, described below—bending of light, gravitational time dilation, gravitational waves, geodetic precession, Shapiro effect, etc. It is these widely disparate observations, and the lack of any alternative theories explaining any of them, that lead people to say that GR is "experimentally validated".
It is important to be aware that it is fairly rare for an experiment to '''prove''' a theory. In general, experiments can only '''refute''' a theory. They can also '''be consistent with''' a theory. When enough experiments, especially experiments that investigate a wide variety of phenomena, are shown to be consistent with a theory, it lends credence to that theory. When no other plausible theory can explain those observations, we can say that they '''validate''' the theory. The Mercury observations are just one phenomenon. By themselves they couldn't validate or prove GR. And there was at one time another competing theory—the Newcomb-Hall exponent-fudging theory. That one didn't hold up for things other than Mercury. No other theory has come up explaining the phenomenon; GR has withstood the test of time. That goes a long way toward validating GR. There are many other observations and experiments, covering a wide variety of phenomena, described below—bending of light, gravitational time dilation, gravitational waves, geodetic precession, Shapiro effect, etc. It is these widely disparate observations, and the lack of any alternative theories explaining any of them, that lead people to say that GR is "experimentally validated".
While the experimental tests for General Relativity are rather esoteric, those for Special Relativity are fairly straightforward. So much so that the Michelson-Morley experiment could be said to have single-handedly established the case for SR—the logical syllogism leading from Michelson-Morley to SR and the Lorentz transform was fairly clear once people saw it.
When analyzing an experiment to see whether it validates a theory, one must be careful not to assume the theory in one's reasoning. For establishing Special Relativity with the Michelson-Morley experiment, the assumptions are:
#Newtonian and Galilean mechanics.
#Galilean relativity, that is, the notion that there is no absolute frame of reference.
#The universality of the speed of light.
#And, of course, proper calibration of the equipment.
#:The third one was the observation that Michelson and Morley made.
#:
#:
#:For the derivation of E=mc², these assumptions are added:
#:
#Conservation of energy.
#Conservation of momentum.
#:All of the assumptions listed above constitute SR.
#:
#:It would be nearly inconceivable to derive GR without SR. Gravitational time dilation, for example, requires a lot of information coming from SR.
#:So, for the experiments listed below for GR, the assumptions are the 6 above plus:
#The Equivalence Principle.
#:So the experiments below for GR are really just establishing that the Equivalence Principle is valid, and that GR follows logically from that.
The most famous experiment, and the one that is commonly cited in textbooks as the experiment that established the case for relativity,<ref>Though relativity did not actually originate from this experiment</ref> was the [[Michelson-Morley experiment]]. This showed that all observers will obtain the same measured value for the speed of light (3x10<sup>8</sup> meters per second) no matter what their state of motion. This is the first of the two fundamental principles:
The most famous experiment, and the one that is commonly cited in textbooks as the experiment that established the case for relativity,<ref>Though relativity did not actually originate from this experiment</ref> was the [[Michelson-Morley experiment]]. This showed that all observers will obtain the same measured value for the speed of light (3x10<sup>8</sup> meters per second) no matter what their state of motion. This is the first of the two fundamental principles: