Why is the fundamental difference between radiation and sound is?

Question

 Sound needs a physical medium through which to travel whereas radiation does not 

Answer 1

In electromagnetic radiation, you have a transverse wave consisting of an electric field vibrating in phase with a magnetic field, with the electric field pointing perpendicular to the magnetic field.  The direction of travel of the wave is perpendicular to both fields (i.e. a transverse wave).  Suppose you have an electromagnetic wave traveling along this page toward the right.  The electric field vibrates up and down the page and the magnetic field vibrates in and out of the page, with the magnetic field pointing up from the page when the electric field points up the page, and the magnetic field pointing into the page when the electric field points down the page. 

 

A sound wave is longitudinal, meaning the vibration of particles of matter occur along the same direction in which the wave travels.  Consider a sound wave traveling to the right through some medium.  The vibrations of the particles will be in the left-right direction.

 

In the classical theory of electromagnetic radiation, they did believe that electromagnetic waves propagated through a medium called the ether.  However, with the advent of the quantum theory of radiation, there is no longer any need to believe in the existence of the ether.