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“”No fair! You changed the outcome by measuring it!
The observer effect refers to the fact that merely observing or measuring some parameter will change that parameter. Usually applied to particle and quantum physics, it does have real world (macroscopic) instances: for example, measuring tyre pressure normally involves releasing a little air, thus changing the pressure, by however small an amount.
Detecting an electron requires that it interact with some other particle or force and thus its path is changed.
It is sometimes confused with Heisenberg's uncertainty principle. While interactions between photons, and subsequent changes in momentum are used to rationalise and explain the uncertainty principle, the principle itself refers to measurement (and is a fundamental part of wave mechanics, you can't have wavelike properties without uncertainty coming along for the ride), not the acts of observation. Experimental findings from 2010-2012 suggest that the observer effect doesn't play a part in quantum uncertainty, and that a model of the uncertainty principle that assumes the observer effect plays a significant role overestimates the degree of uncertainty in measurements.