How is 10G Ethernet physically possible?
Chasing this answer down took a few different links, but it appears to boil down to this:
1. 4 differential pairs (8 wires total, but only 4 lanes).
2. 800 Mega Symbols a second.
3. Using PAM16, 16 symbols are used which translates into 4 bits per baud per lane.  

Given that information you come up with 4 bits*800 Mhz*4 lanes which results in 12800 Mb/s or 12.8 Gb/s. Due to encoding of error correction and other overhead, they only expect you to get 10 Gb/s from it.

Notice that the wires themselves are only changing symbols or amplitude at a frequency of 800 MHz. That's pretty meh, in terms of switching speeds for transistors.



Now that's all for 10Gb Ethernet. How they do it for 100Gb Ethernet is a little more mind boggling. For that, it appears that they actually do pump up the frequency to 10.3 GHz or 25GHz. WHAT THE HECK? See here for that table. The difference in frequency is due to how many copper pair data lanes you choose to have. Whether or not anyone has actually made this 25GHz ethernet in copper would be interesting to know. It's possible that they've only spec'ed it out. When you start getting to those frequencies, your cables either need to be really short or you just switch over to fiber optics where you can send hundreds of beams of light down a single fiber. In that way, you don't have to go at crazy speeds, you just parallelize your data at the source and deparallelize it at the destination.

References if you want to look into it more:
https://en.wikipedia.org/wiki/10_Gigabit_Ethernet#Copper
https://en.wikipedia.org/wiki/Pulse-amplitude_modulation
http://www.cablinginstall.com/articles/print/volume-15/issue-7/features/technology/twisted-pair-options-for-10-gigabit-ethernet.html
https://en.wikipedia.org/wiki/Differential_signaling