# If sound travels through matter what medium does light travel through? [duplicate]

So sound is a wave and is basically just vibrations, an atom vibrates causing another next to it vibrate and so on until it finally reaches our ears to become sound.

If that's normally how waves behaves, what about light? I understand it's also a particle but something must have caused its starting point to influence its next point in space thereby allowing it to travel through space. what is the cause and effect relationship that allows light to propagate through space? I ruled out matter because most visible light is blocked by matter.

• "Why don't electromagnetic waves require a medium?": physics.stackexchange.com/q/19670/9887 Sep 3, 2014 at 22:57
• You essentially are asking whether there is a luminiferous aether, there isn't, see the linked question by Alfred Centauri. Sep 3, 2014 at 23:07
• Am I supposed to accept the fact that there is no cause and effect relationship of light at t = 1 that affects where light will be located in space at t = 2?
– Mike
Sep 3, 2014 at 23:10
• Well, since EM waves propagate, the EM field configuration at one point very well influences the EM field configuration at other point. It is just the case that there is no carrier of these waves - they evidently need no aether, but are oscillations of the EM field itself. (Side note: Cause and effect become, when viewed through the lens of relativity (and electromagnetism is the archetypical special relativistic phenomenon), notions you will want to think very carefully about before using them.) Sep 3, 2014 at 23:32

The process of light propagation is described by the Maxwell equations.

$$\nabla\cdot{\bf D} = \rho$$ $$\nabla\cdot{\bf B} = 0$$ $$\nabla\times{\bf E} = - {{\partial{\bf B}}\over{\partial t}}$$ $$\nabla\times{\bf H} = {\bf J} + {{\partial{\bf D}}\over{\partial t}}$$

These equations say (in simple terms) that: change in the electric field is causing a change in magnetic field, while change in magnetic field is causing a change in the electric field.

The original source of electromagnetic waves is some oscillating charge (for instance an electron) which has an electric field around it. This field is changing (because the charge is oscillating). Therefore (according to the fourth equation) a magnetic field $H$ is formed. But the creation of magnetic field is in fact a change in magnetic field. This leads (according to the third equation) to creation of new electric field $E$. But this change in $E$ leads to $H$, which leads to $E$, etc.

• This made a lot of sense to me and is exactly what I was looking for. Is it safe to assume that the source of light (or when a light source is created) it is either a sudden creation of an electric field or an magnetic field that triggers this process?
– Mike
Sep 4, 2014 at 0:49
• @kotsumu The process is triggered by changes in the electromagnetic field (notice the time derivatives - this is the change over time). A "sudden creation" of the field is in fact a change in time, so yes, it can also trigger the process. But usually there is already some stationary charge existing (inside an antenna) whose stationary electric field does not create waves; then it starts oscillating back and forth and the changes in the field trigger the radiation.
– mpv
Sep 4, 2014 at 7:06