Ah-ha! Not that I actually believed that, hence why I went out to find out why I was told that many years ago. It was because they didn’t want to explain the quantum wave to us, or decoherence (wow a word spell check doesn’t know about). Basically in the double slit experiment you start with a setup like below, 2 half silvered mirrors, and 2 full mirrors. What a half silvered mirror does is reflect half the photons and transmit the other half.

If you think about light as a wave for a second, you will understand why only one detector gets any light. When going through a half silvered mirror, light gets ‘polarized’, i.e. the light in one direction is exactly half a wavelength offset from the light going in the other direction. When the two paths are recombined at the second half silvered mirror, all the light in one direction ends up being ‘in phase’, where the waves are added together, and all the light in the other direction is ‘out of phase’ where the waves perfectly negate each other.

Now think of light as a particle, this experiment gets funny. In the image below, we’ve removed the 2nd half silvered mirror and now the detectors each see half the light. This leads us to believe that somehow the second half-silvered mirror is causing the different paths to interfere with each other. Now can you see how we were lead to believe that light ‘knows’ it’s path before it takes it? Fucking great guys. I can’t even start to describe how many fallacies we’ve been taught about physics in high school or even some university classes. Do they not think we can understand the concepts? Why the hell were we there to begin with? I remember another specific one having to do with how lift is created around a wing. How do you think it works (I bet you’re wrong – Vlad don’t answer this one)

OK – so now it’s going to get tricky. So this experiment works when thinking about light as a wave, but is very different from a particle point of view. After all, a particle cannot take both paths. So how can the existence of the other path (supposedly the one it knew not to go down) matter to what the particle does? How can this work with light as a particle without it predicting what will happen in the future?

This all has to do with Schroedinger’s cat. Basically until the ‘reality’ of the situation has been observed, the properties of the experiment are part of a wave function. In our experiment, the particle exists partially on each path simultaneously. So just like we don’t know weather the cat is alive or dead until we open the box, we don’t know which path the particle took until it hits the photometer. Quantum mechanics says that once the system is observed, the wave-probability function decoheres and is collapsed into one state or the other. This is made more clear by use of Quantum Entaglement (now I’m getting into areas that are new to me, I’ll try to explain them the best I can).

In Quantum entanglement the physical properties can have correlations. For example if there are two particles in a single quantum state such that when one is observed to be spin-up, the other is always observed to be spin-down, and vice versa. It is impossible to predict, according to quantum mechanics, which case it will be. So for our example, the property is weather the light hits detector one or detector two. What we do know however, is that if we measure detector one and find photons, then we will certainly NOT find photons at detector two.

Now there are many theories and corrections and arguments about decoherence, but I stumbled upon one which is all too familiar, due to pop-culter/television and figured I’d share the details with you.

“In the many world interpretation of quantum mechanics, when an observer ‘opens the box’ (referring to Schroedinger’s cat) he becomes entangled with the cat and the universe (or at least the part of the universe containing the observer and cat) is split into two separate universes, one containing an observer looking at a box with a dead cat, one containing an observer looking at a box with a live cat.”

Now that would be a lot of information for the universe to track. But I guess it is the Universe.

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    9 replies to "Ah-ha! So light does not technically ‘know’ its future"

    • vladimyr
    • tmscase

      omfg that’s funny!

    • anonymous

      decoherence – NOT A WORD.

      incoherence – WORD I THINK YOU MEANT.

      Spelling Nazi.

    • tmscase

      Umm actually it’s decoherence. It has to do with the decoherence of the quantum wave. Incoherence doesn’t even make sense. Who the hell are you?

    • spendocrat

      I don’t like how often Scroedinger’s cat is used, since it seems to trick so many people into believing that quantum effects can happen on a macro scale.

      the “observer” doesn’t have to be sentient or even alive. Two molecules can observe eachother and cause decoherence (sometimes!)

    • spendocrat

      Dear AC: Please check your facts before posting nonsense to LJ!

    • tmscase

      ah yes – just an analogy for ease of understanding no?

    • spendocrat

      Supposed to be, but as with so many technical analogies, it lends itself to people drawing bad inferences due to the details of the analogy.

      Quantum is weird, and sometimes it’s just better for people to have it presented to them as “this is how it works, it’s weird, as has no analogy to your everyday life”.

      Aside: This happens so often with computer topics that I rarely bother to try for analogy anymore.

    • tmscase

      Lol I know exactly what you mean!

      Thanks fer da comments.

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