quote:

*Originally posted by realmswalker*

its such a basic tenet of modern physics...but i dont get why it is this way...!?!

why cant any thing go faster than light? why did einstein set that as the constant and what not?

i mean its illogical to think that if you have an engine with infinite output and you kept pushing on something, that it couldnt overcome the speed of light eventually....

idk

Ok, I'll try and outline this briefly, but keep in mind that this explanation will be rather simplified and therefor probably imprecise in certain details. Ok? Here goes:

The basic idea that we're starting with here, is that the speed of light is apparently always the same, no matter from where you are observing it, or how fast you as an observer are moving. This may be hard to accept, but so far, all experiments done to test this idea seem to confirm it. So let's just accept that as fact for the moment (if this is actually true, and the implications of it not being true might be an interesting topic in another thread).

So now, imagine two lamps, of identical build, fastened to the roofs of two identical rooms. On the floor, opposite from the lamps, there are two light detectors. One of the rooms is stationary from the observers point of view. So now, in the stationary room, the lamp shines a beam of light to the detector on the floor below it. If the observer measurs how long it took the beam to reach the detector, he'll see that it is obviously the height of the room, divided by the speed of light.

Now the same experiment is conducted in the second room, while it is moving sideways at a constant speed relative to the observer. The beam of light now travels a longer distance, namely the diagonal of a rectangle with the height of the room, and the width being the distance travelled during the time it takes. So now, if we accept that the speed of the light was the same, and the distance was larger, and the time from our point of view was the same, the only way to explain it is if the time in the room was running slower that for the observer. Clear so far?

Now we'll keep imagining that experiment, while speeding up the moving room. You'll see that the angle at which the beam of light is travelling gets flatter and flatter, the closer we get to the speed of light. The distance the beam travels gets longer and longer, so the local time in the room gets slower and slower. If we continue this up until the room is moving at the speed of light, time will stop completely in the room. If you've followed this far, you've understood a major part of relativity.

Now, analogue to time slowing down, things moving at high speeds also tend to get heavier (actually, more massive. There are similar thought experiments that show why this is so, but they're quite a bit more complex that the time slowing one we did above). This increase of mass doesn't happen linearly. At first up to about half the speed of light, the gain in mass is very small, after that, it keeps getting quicker. For the last percent of lightspeed, the increase in mass is very very large. Now along with an increase in mass, comes an increase in inertia. You'll notice this if you try pushing a cart. The more mass is on the cart, the harder you have to push. Now if our object would actually reach the speed of light, it's mass would increase to infinite. This means that the closer we get to the speed of light, the more energy we'd need to keep speeding things up. The actually get to lightspeed, we'd need an infinite amount of energy, which in a finite universe is more than can ever be available. The conclusion of all this, is that objects without mass can move at the speed of light, because their mass stays at zero. Anything with a non-zero mass cannot, because it would take an infinite amount of energy to get it to those speeds.