Ciro Santilli @cirosantilli 40

We are at a point in history where the electrical telegraph is well established.

But people don't want to press letters one by one on a switch. They want to talk!

The first phones appear to have used telegraph lines directly.

Also wired phones don't require modulation, which likely made their development much easier than wireless voice transmission. You just send the signal as a voltage differential directly obtained from the air pressure: how the telephone works.

Let's show that this definition is equivalent to the orthogonal group is the group of all matrices that preserve the dot product.

Note that:and for that to be true for all possible $x$ and $y$ then we must have:i.e. the matrix inverse is equal to the transpose.

Conversely, if:is true, then

These matricese are called the orthogonal matrices.

TODO is there any more intuitive way to think about this?

Some cool ones:

Persistent undo history:

One of the things Ciro Santilli really likes, see: Linux Kernel Module Cheat.

If computational physics simulates physics, emulators simulates computers.

Experiments explained by QED but not by the Dirac equation: