Collision detection is perhaps a bit mysterious. First, why would two computers both transmit at the same time if they had first listened to the wire to discover that no one is currently transmitted?

It is possible that two computers might begin transmitting their packets at exactly the same time, even though they had both first listened respectfully and (correctly) ascertained that there were no packets on the wire. It is also possible that the two senders started within a very short time of each other and the second didn't have time to detect the first bit of the first sender on the wire before it started transmitting.

After all, the speed of light, though very great, is finite and these wires can be up to about 1500 meters long. Light travels about 300,000 kilometers/sec in a vacuum (186252 miles/sec) although it travels only about 50% or 75% of that in copper due to resistance of the electrons. Dividing 1500 meters by 300,000,000 meters/sec we get 0.000005 sec, which is an incredibly long 5 microseconds. That means that if the signals traveled at their maximum rate possible and two computers were their furthest distance apart on the wire (1500 meters), there would be a lapse of 5 microseconds from the time that one computer began signaling and the other detected it.

Thus, if they decided independently to start and it just so happened that this occurred within a timeframe of 5 microseconds or less, they would interfere.