|MadSci Network: Physics|
Dear Jamie, I'm not familiar with the original question/answer, so I'll have to work with what you've asked here. First, let's clear up a misunderstanding. There is no such thing as negative energy. Energy is what is known as a positive definite quantity-- that means it can only have positive values. Other positive definite quantities include length, mass, volume. What would negative mass mean? For example, let's consider the equation E = m c^2. Can you imagine what negative mass is? I certainly can't! It's got to be positive. Also, any number times itself is positive (c^2) [except for imaginary numbers, but c is quite real!], so that means that E has to be positive as well. Next, what's the difference between an electron and a positron? A positron is the anti-matter version of an electron. Matter/anti-matter pairs differ in only one way. They have the same mass, the same spin, the same everything, except for charge. Matter/anti-matter pairs have the same magnitude of charge, but opposite in sign. Thus, electrons have charge -1, and positrons have charge of +1. What is the difference between real and virtual? This is very subtle. The short answer is that virtual particles are terminated in Feynman diagrams. Pretty meaningless, eh? What that really means is that virtual particles are created and destroyed in a very very short period of time. Real particles are created and last for some macroscopic period of time. That is the only difference. To make the virtual electrons and positrons real, the electric field must be large enough so the energy density of the field is equal to or greater than the equivalent mass of the electron/positron pair. The mass of an electron is (in my favorite units, megaelectron Volts) 0.511 MeV/c^2. That means the energy of the electric field must be greater than twice that (since there is an electron and a positron to make) or 1.022 MeV. The electric field seperates them, since they have opposite charge, and things with opposite charges move in opposite directions in an electric field. Once the electron and positron are far enough apart, the will continue to exist until they meet their anti-matter counter part and annihilate. As you might imagine, the electron lives longer than the positron simply because the universe is made of matter-- there are more electrons for the positron to interact with than there are positrons for the electron to interact with. Finally, as for the Casimir effect, I'm not sure I understand your question. The Casimir effect is simply that there will be a tiny force applied to two conductive plates when they are close together. Once they are far apart, the vacuum is unable to muster enough strength to apply any force.... I hope this helps! If you want futher information, I recommend Introduction to Elementary Particles, by Griffiths.
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