# Negative light speed

1. Mar 1, 2010

### renerob

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

2. Mar 2, 2010

### Matterwave

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

3. Mar 2, 2010

### dreiter

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

4. Mar 2, 2010

### Nabeshin

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

Hope this clarifies.