Does Making A Quantum Measurement Really Destroy Information?
“The act of measurement and observation seems like it shouldn’t affect the outcome, as it’s a truly absurd idea that watching a system can alter its properties. But in the quantum Universe, this not only occurs, but was demonstrated before the theory was even fully understood. If you measure a particle’s spin along one direction, you destroy any previously-obtained information about the other two directions. Even if you had measured them previously and known them exactly, the act of making that new measurement fundamentally erases (or randomizes) any information you had gained earlier.
When many physicists first hear Einstein’s quip about how “God doesn’t play dice with the Universe,” this is the first experiment they should think of as a counterexample. No matter how well you think you understand reality — no matter how precisely or accurately you measure it in a variety of ways — the act of making any new measurement will intrinsically randomize some of the information you had pinned down just prior to your measurement. Making that new measurement really does destroy old information, and all you need is a magnet and some particles to prove that this is true.”
If you’ve learned about quantum mechanics before, you’ve probably realized that reality is in a superposition of possible outcomes, and that the act of measuring fundamentally “chooses” one possible outcome, with many possible interpretations. But what’s far less-well appreciated is that making that measurement can also destroy information that you previously knew.
This isn’t just true; the experimental verification is almost 100 years old. Here’s the classic experiment that proved that the act of measuring really can destroy information.
