Detecting a Gravity Field without Making a Logical Error
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Original Question
Hello everyone. Happy 4th of July.
Gravitometers measure a local gravity field by measuring a force on a test mass.
To detect the presence of a gravity field we use this argument.
If there is a gravity field, then the gravitometer will give a non-zero reading. The gravitometer gives a non-zero reading, therefore there is a gravity field.
This is a logical error (Affirming the Consequent)! So how is it possible that we can use gravitometers, or other scientific instruments, to provide data from which we can make conclusions? In this example the presence of gravity.
For the purposes of the logical analysis, assume that the experiment is done on a stationary, non-accelerating platform.
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Answers
3You can use more precise instrumentation. For example, a device that will only give a non-zero reading in the presence of a gravity field. Then you can say, while be logically consistent
If the device gives a non-zero reading, then there is a gravity field.
The device gives a non-zero reading.
Therefore, there is a gravity field.
As Dr Bo said, precise instrumentation means that P implies Q if and only if P is true, and Q is true if and only if P is true. Thus, you can make a valid deduction.
Using scientific instruments to make measurements isn't an argument of logic. The instrument is used based on our understanding of physics and it's use isn't to prove that understanding but rather to make measurements.
When the device was created they wouldn't have just tried it out and then assumed that the results were correct and therefore that meant it works, the test results would have been predicted and by being correct the device was deemed to work.
It seems that science denial is in vogue with the rise of social media and this question seems like the sort of thing which would come up in a discussion with flat earthers where no science is offered in favour of flat earth, and instead attempts are made to discredit science using logical fallacies, word games and ignorance.
A gravitometer isn't used to determine that gravity exists, it is used only to measure it on the assumption that the law of universal gravitation is correct. All observations* are in accordance with this law, and thus the assumption that it is correct remains reasonable and is the basis for the engineering of structures which would otherwise fail.
*When I say all observations I mean those within science and discount any ignorance or dishonesty based objections by science deniers.
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Perhaps I should clarify why I am asking this question.
The question does not have to be specific to gravity, it can be extended to any instrument designed to detect a phenomenon.
If phenomenon A (gravity) is present, then effect B (weight) will be observed.
Effect B (weight) is observed, therefore phenomenon A (gravity) is present.
This is a formal logical error, so could many conclusions in science be fallacious?