SkiesOfBordom said:
really... ok then i guess i am wrong then, or you don't understand what i am talking about as I have a hard time explaining things. however, you don't have to put in a mean way...
Sorry, I didn't intend to be mean, but I can understand why it seemed mean. We just have a problem on this site with new members trying to spread their (incorrect) personal theories about physics, so I'm a little sensitive. Let me explain why your understanding is wrong...
It's true that you can only shove a limited number of electrons per second through a wire before it melts. It's also true that you can send an (essentially) unlimited number of photons through a fiber optic cable simultaneously.
On the surface, this would appear to mean that fiber optic cables can carry an immensely larger "volume" of data per unit time.
The problem with that conclusion is that it assumes that information occupies space, or that the quantity of information is somehow directly related to the number of electrons or the number of photons used in transmission. This is what's wrong with your understanding -- sending more information does not imply pushing more electrons through a wire, or more photons through a fiber optic cable.
Consider a signal that's just a clean, pure sine wave of a specific frequency. If you pass this signal through a wire, the electrons just oscillate back and forth inside the wire; if we ignore thermal motion, the electrons don't actually move from one end of the wire to the other at all. Passing this same signal through a fiber optic cable just means sending light of one specific frequency.
Now, consider sending two signals at once. You add another sine wave to your existing one, the new one of a different frequency. When you pass this signal through the wire, the net movement of the electrons is
still zero. Their oscillations are now more complex, but they still do not move any more energetically. Passing the signal through the fiber optic cable might mean just passing two light beams through it at once (assuming it's multi-mode and all that), but there are other ways of accomplishing it, too.
The bottom line is that you can stuff as many different sine waves of different frequencies as you want through a copper wire, as long as your sine waves are all of frequencies less than some tens of gigahertz. You can continue to stuff the sine waves in with frequencies that get closer and closer, ad infinitum. In that sense, the "information bandwidth" of a copper wire is
infinite.
The only problem is that your receiver has to become more and more sophisticated in order to distinguish all these sine waves of very very slightly different frequencies, and unfortunately it'll have to start looking at the signal for a longer and longer period of time to distinguish a sine wave of one frequency from one with a very close neighboring frequency. This effect works against you, so you'll have to make some trade-offs between how you use bandwidth and how you design your receiver. The conclusion of all this is that it's the practical concerns about designing a
receiver that limit how much data you can practically send over a copper wire. You could theoretically send an infinite amount of information over a copper wire, but you could not build a receiver to capture it all.
The
exact same argument applies to signals sent over fiber optic media. In other words, the "theoretical information capacity" of fiber is also infinite, since nothing's stopping you from sending many many many trillions of different wavelengths through it at once. On the oither hand, you'll never be able to develop a receiver which can decipher the resulting signal.
The speed of propagation, by the way, is not so much of a concern. Light travels at about one foot per nanosecond, and electric signals propagate through wires at about half that. It's true that the light signal propagates more quickly to its destination. However, if you're sending a continuous stream of data from one place to another -- say, from a broadcasting station to a bunch of different TVs -- it doesn't really matter if the data gets there a bit earlier, or a bit later. It still comes in the same quantity of data per second.
- Warren
hmm... well i ment under normal un-vaccuum'ed environments.
