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  • Is it possible for an electron to be in two places at once?

  • Or for a cat to be both alive and dead at the same time?

  • According to quantum mechanics, both scenarios are possible

  • and at the heart of this conceptual dilemma lies the wave function,

  • the mathematical formula that we use to describe quantum objects.

  • To see the wave function at work, let's take the case of an electron

  • shooting out of the back of a cathode ray tube,

  • a device found in the back of old fashioned TVs.

  • The wave function for this electron is just the description

  • of where the electron is and what it's doing.

  • For example, how fast it's going.

  • To predict where the electron will be in the future,

  • you just plug the wave function into Schroedinger's wave equation,

  • the quantum equivalent of Newton's laws of motion.

  • But here's where it gets tricky:

  • The wave function isn't a regular number.

  • It's a complex function that relies on imaginary numbers,

  • such as the square root of negative one.

  • And it gets even worse...

  • The wave function can only give you a statistical prediction

  • about where the electron will be in the future.

  • It's a statement about probabilities, nothing more.

  • If you took two electrons with the exact same wave function

  • and looked to see where they are, you might find that one is over here

  • and the other is over here.

  • And worse yet, once you observe an electron, detecting it in this place

  • and not in this place,

  • the wave function immediately and irreversibly changes.

  • Physicists say that it collapses from a broad range of possibilities

  • to one specific answer.

  • Think about that for a second.

  • The act of measuring the electron is what gives it its position.

  • And this brings us back to the cat

  • that's both alive and dead at the same time.

  • As it's usually told,

  • a cat and a vial of poison are sealed into a box.

  • Then a quantum event happens with a probability of 50 percent

  • that breaks or does not break the vial of poison

  • and kills or does not kill the cat.

  • So, is the cat still breathing?

  • The wave function reflects both possibilities.

  • In the standard interpretation of quantum mechanics,

  • both possibilities are real.

  • The cat is truly both alive and dead

  • until an observer opens up the box.

  • At this point, the kitty collapses

  • into either an alive or a dead state.

  • This is absurd, of course.

  • Cats can't both be alive and dead at the same time.

  • So how do we get out of this paradox?

  • One of two ways:

  • Either the wave function doesn't really exist,

  • it's just a reflection of our knowledge.

  • Or the cats are both alive and dead, but in two parallel universes.

  • For Scientific American's Instant Egghead, I'm Michael Moyer.

Is it possible for an electron to be in two places at once?

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What Is the Wave Function? - Instant Egghead #50

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    葉大 谷翔平 posted on 2017/06/08
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