the problem is, if everybody sees the speed of light to be ~3x10^8 m/s (hereby called "C") regardless of their own velocity relative to eachother, we have a problem with standard newtonian mechanics.<br /><br />In newtonian mechanics, when you throw something from a moving reference frame (like a moving truck), you add the velocities in a straightforward fashion. If a roadside observer says the truck is going 30mph, and you throw the ball at 30 mph forward, from the back of that truck, the roadside observer will see the ball going 60 mph (you in the truck see it going at 30 mph, and the observer go 30 mph in the other direction, you can consider yourself stationary).<br /><br />Now, lets use a laser. In the truck you fire the laser, and see the light receed at C. No problems so far, same as above (throw the ball 30 mph, see the ball go 30 mph).<br /><br />But experiments show that the observer on the roadside will <i>also</i> see the laser travel at "C". It's not C + 30 mph, as with the ball.<br /><br />To reconcile this, take C as a constant, light always travels at that speed in a vacuum (this is <i>observed</i> and basically not up for debate), no matter who watches it, and how they're moving.<br /><br />Then to understand how two frames moving at different speeds can see the same speed for light, one must treat time and distance as alterable. They are no longer absolute. What I say is a meter, and is a second, will be different from yours, if we move relative to eachother. I can take your meter stick, and begin moving. I'll say it's a meter, and you measure it from your stationary lab...and it won't be the same size. We can do the same with clocks.<br /><br />In order for C to be constant for everyone, the length of a second and a meter must change accordingly for the people moving.<br /><br />Taking this change in time and distance, you end up with some interesting consequences (time dilation and length contraction for one). Such as increased inertia with speed, an <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>