Observer effect

No fair! You changed the outcome by measuring it! The observer effect refers to the fact that merely observing or measuring some physical feature of a system will change that feature. Although it technically applies to particle and quantum physics, it can be appropriated to 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 bouncing a photon of light off it, thus changing its speed and altering its path. This occurs because the observer becomes a part of the system to be measured.

It is sometimes confused with Heisenberg's uncertainty principle. While interactions between photons, and subsequent changes in momentum are used to rationalize and explain the uncertainty principle, the principle itself refers to a fundamental limit to which we can measure the values of complementary pairs, most famously position and momentum. Essentially, even if all of quantum mechanics were wrong, the uncertainty principle would still hold. Some like to say not even God can accurately measure both position and momentum. In contrast, the observer effect is often used to describe the collapse of a wave function to a particular value. 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.