MadSci Network: Physics

Re: my question is regarding question 982969482.Ph

Date: Wed Apr 25 14:33:19 2001
Posted By: Benn Tannenbaum, Post-doc/Fellow, Physics and Astronomy, University of California, Los Angeles
Area of science: Physics
ID: 987793769.Ph

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

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

I hope this helps! If you want futher information, I recommend Introduction
to Elementary Particles, by Griffiths.

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