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Q & A: Measurements

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Most recent answer: 10/22/2007
what are some fun facts about measurements in physics.
- boonu (age 16)

Hello! One of the most interesting things about measurements that has been debated and discussed to this day is something called measurement theory or the measurement problem. Measurement theory is a big issue in the area of quantum mechanics -- quantum mechanics deals with the physics of very small things such as atoms, subatomic particles, molecules, etc. It turns out that these small objects behave in very strange ways that you would never normally expect. I won't try to explain all the details of it right now because it gets pretty complicated and confusing, but I'll give you an analogy that will illustrate how bizarre quantum mechanics is and why to this day there is no good definition of a measurement.

Let's say that you have two jelly beans, one red and one green, put together in a bag. You drop both of them into a tube that leads to a closed box that is divided into two parts -- one part is empty and the other has a monster that only likes to eat red jelly beans (and he growls when he eats one). That is, the two jelly beans have a 50/50 chance of passing through the empty side or the side with the monster, and you can't see which side they pass through since the box is closed. However, you *can* see what comes out of the box and you can hear the monster growl when he eats a red jelly bean. So you start your experiment by dropping the bag containing the two jelly beans into the box, and you hear the monster growl. You notice that the red jelly bean has been eaten so that only the green jelly bean has survived and has made it out of the box. Now you repeat the experiment and this time you hear nothing, BUT the strange thing is that the green jelly bean has disappeared and only the red one comes out of the box! You scratch your head and wonder what happened to the green one because there was nothing inside the box that could have made it disappear, jot some notes in your lab book, and walk back home in a very puzzled state of mind...

Of course, this is only an analogy that would never actually happen with jelly beans, but similar things do happen when you start dealing with very small objects. In our story with the monster, the monster (the "measurement") did not actually have to interact with the bag of jelly beans in order for the contents of the bag to change; similarly in quantum mechanics you don't have to directly interact with a system in order to affect it.

If you're not completely confused by all this, then I haven't done a good job of explaining it. For more information on measurement theory, check out .

(published on 10/22/2007)

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