Subject: Connections between models and reality

Posted by Bill Barroway on 3/11/2004
In Reply To:Connections between models and reality Posted by Doug Cardell on 3/11/2004

Message:

As long as there are mathematical relations we can make measurements of "flows" that are instantaneous. For example a pitot tube is used to measure aircraft speed. Pitot tubes do this by measuring air pressure. Now one would think that this is direct and instantaneous, because there are no two closely timed measurements. One just measures pressure. But there are two measurements -- static and total pressure, from which dynamic pressure can be inferred. The aircraft's speed is calculated from the dynamic pressure. So the speed of the aircraft is obtained this way by calculating the
*difference* between two pressures. The qualification to the earlier discussion I wish to make here is the point that speed measured this way is not a measurement of two quantities separated in time. It still is a difference measurement, and measuring difference is still a necessary process to the measurement of rates, aka "flows".

What's necessary is a mathematical relation, aka a mathematical model, to relate the differences in pressure to speed. For the pitot tube you can find this at the following NASA site -- and notice that there are problems with making measurements with this technique both at high and low speeds. One is experimentally induced (accuracy of the strain gage), the other has to do with the model not accounting for the shockwave at supersonic speeds.

http://www.grc.nasa.gov/WWW/K-12/airplane/pitot.html

So the gremlin in the back of my head retorts "What about light shift?
Doesn't measuring the speed of a receding star by measuring the red-shift of its light count as a direct and instantaneous measurement?"

And my response is still "NO", but I won't drag this issue on by explaining.

bb