negative light speed

[renerob]

If e=mc^2, then (e/m)1/2= either +c or -c. What is the physical meaning of minus c?

[Matterwave]

e=mc^2 shouldn't be used to try to find "c". "c" is a defined constant of 299792458 m/s.

[dreiter]

Mathematically -c exists, but this is a non-physical result, and is therefore ignored when using the e=mc2 equation. :)

[Nabeshin]

e=mc^2 shouldn't be used to try to find "c". "c" is a defined constant of 299792458 m/s.

Precisely. Imagine the equation like this:
E=m(2)^2
2=\pm \sqrt{\frac{E}{m}}

Hope this clarifies.