Physics Question #5131

Matthew, a 15 year old male from Darwin, Australia asks on December 4, 2010,

Why is quantum physics used to try and explain the future predicting behaviour of light as is shown in Wheeler's delayed choice experiment? Einstein's theory of special relativity shows that light does not exist in time, so surely this is a big clue as to what is really happening. If not, then could you please explain to me the consequences of a particle (or wave) that is outside of time interacting with 4 dimensional space?

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The answer

William George Unruh answered on December 5, 2010

Einstein's theory does not say that light exists outside of time. It says that there are many different times, and that if you were a light-like observer, you would not experience anything like time. But that is vastly different from saying it is outside of time, since according to other observers, light's behaviour takes place in time. One problem with this question is that it is still tied to an old view of time-- Newton's time-- in which time is monolithic.

Quantum mechanics is used to describe light because it is currently used to describe all matter. Wheeler's delayed choice thought experiments are to show that quantum mechanics has many strange features, and that trying to pretend that one can think of light as either a wave or a particle is simply incompatible with how quantum mechanics works. Many people imagine they can pretend that the universe carries on in the same classical way as Newton described it, but Wheeler's descriptions show this does not work.

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