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Talk:Theory of relativity (view source)
Revision as of 23:06, August 29, 2011
, 23:06, August 29, 2011→Uncertainty: new section
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== Uncertainty ==
Uncertainty priciple does not actually say that. Simply put, virtual particles are called virtual for a reason: They arent real. And no one, anywhere, in science really would consider them to *be* real. Its more like a useful tool than a real occurence, the same way a ruler isnt actually measuring a physical entity called "Inch", but is actually an intermediary to allow us to act upon our enviornment in a meaningful way. It would be like expecting to find inch-square chunks of reality any time you measured it out. But that just doesnt happen.
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From [http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html] (math.ucr.edu) <blockquote>
Suppose that we are trying to calculate the probability (or, actually, the probability amplitude) that some amount of momentum, p, gets transferred between a couple of particles that are fairly well- localized. The uncertainty principle says that definite momentum is associated with a huge uncertainty in position. A virtual particle with momentum p corresponds to a plane wave filling all of space, with no definite position at all. It doesn't matter which way the momentum points; that just determines how the wavefronts are oriented. Since the wave is everywhere, the photon can be created by one particle and absorbed by the other, no matter where they are. If the momentum transferred by the wave points in the direction from the receiving particle to the emitting one, the effect is that of an attractive force.
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The moral is that the lines in a Feynman diagram are not to be interpreted literally as the paths of classical particles. Usually, in fact, this interpretation applies to an even lesser extent than in my example, since in most Feynman diagrams the incoming and outgoing particles are not very well localized; they're supposed to be plane waves too.
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The uncertainty principle opens up the possibility that a virtual photon could impart a momentum that corresponds to an attractive force as well as to a repulsive one. But you may well ask what makes the force repulsive for like charges and attractive for opposite charges! Does the virtual photon know what kind of particle it's going to hit?
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It's hard even for particle physicists to see this using the Feynman diagram rules of QED, because they're usually formulated in a manner designed to answer a completely different question: that of the probability of particles in plane-wave states scattering off of each other at various angles. Here, though, we want to understand what nudges a couple of particles that are just sitting around some distance apart—to explain the experiment you may have done in high school, in which charged balls of aluminum foil repel each other when hanging from strings. We want to do this using virtual particles. It can be done.
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[[User:Waterdeep|Waterdeep]] 19:06, 29 August 2011 (EDT)