# 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?

Gold Member
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. :)

$$E=m(2)^2$$
$$2=\pm \sqrt{\frac{E}{m}}$$