The precise nature of what we call ‘light’ has bewildered and confused scientists for centuries.  In the 16th century Newton believed light was particulate in nature and considered it to be made up of what he called ‘corpuscles’.  In this model, he could easily imagine particles streaming out of candle flames, bouncing off mirrors like tiny rubber balls, and heating surfaces on their impact.

Then came the discovery of interference, where light passing through double slits would create patterns that could only be explained in terms of wave motion, such as ripples on a pond.  So now light was understood as a wave.  Waves bounce off surfaces much like particles, explaining reflection.  And as an added benefit, waves also undergo refraction when passing through different depths of water or different densities of air.  This is something that particles couldn’t account for.  The light-wave model was looking good!

Later again came the discovery of the photoelectric effect.  Light is shone on a metallic surface causing electrons to be ejected.  But instead of their ejection speed being based on intensity, it is based on frequency of the incoming wave.  Stranger still, light below a certain cutoff frequency causes no electrons to be ejected, even when the light intensity was great.

This lead to the concept of photons, in which waves are somehow bundled up in discrete packages, each of which carries a certain amount of energy based on its frequency.

Now light was either wave or particle, depending on the situation.  Or perhaps it was both at same time.  How to make sense of this?


What about aether?

First let’s step back a bit.  Following the discovery of the wave nature of light came the concept of ‘luminiferous ether’ (or ‘aether’ as it’s now called – changed when chemistry took over the old word).

The aether described a stiff elastic material that permeated all of space.  The idea being, when a charged particle oscillated it would create a disturbance that would propagate as a waving motion through the surrounding aether.  Then when that disturbance hit another charged particle it would cause that particle to likewise oscillate.  Thus the delayed transmission of a radio signal was effected across seemingly empty space.  The same principle also applied to electromagnetic signals of higher frequency, such as visible light.

The aether concept was of course officially abandoned following the famous ‘null result’ of the Michelson-Morley interferometer described earlier.  As pointed out however, the experiment and its replications were problematic because they were done within refractive mediums, rather than an ‘aether medium’ (a vacuum).  Still, let us assume at least one of them was done properly.

In order to explain the M-M experimental outcome a number of explanations followed.  These included ideas that the interferometer arms contracted as they moved through the aether, to the aether being dragged along by Earth’s gravity.  Then finally came the now accepted explanation of Special Relativity (SR) in which the speed of light adjusts itself to match the moving target.

Of all the explanations, the SR idea is the least likely, and arguably the most ridiculous.  Because it requires effect to precede cause. That is, it requires the light to adjust its speed in advance of hitting a target, which it could not know of the existence of, let alone know its relative speed.

Because of that and many other problems with SR as detailed earlier, a number of non-mainstream theorists have attempted to resurrect the aether in order to account for the wave nature of light.  Another reason for wanting an aether was to provide a fixed frame of reference to measure velocity against, and this would allow motion-based time dilation to occur. [1]

So does such a medium exist?  It seems unlikely.  The main problem with aether is that it would cause a drag on any charged particle that moved through it, even when moving at a constant velocity.  And so the motion of all objects in the universe would quickly come to a halt.  Now it could be argued that the aether allows for neutral particles to move through it without resistance.  But we can’t say that about charged particles, otherwise the ‘rippling’ of the aether caused by the motion of one charged particle would not affect another at a distance.  In short, light would travel but could never be detected!


So how then?

But then how could light propagate without a medium?  If light is a wave it must have a medium to exist in, right?  In fact this is not true.  And here lies the point of confusion everyone is having.  Thing is, there are two type of waves: disturbance waves and matter waves.  Their differences are outlined as follows.

Disturbance waves

A disturbance wave involves small local movements in a (solid, liquid, or gas) medium that propagate themselves into the surrounding portions of the medium.  Sound is a disturbance wave.  So are ripples on a pond or the plucking of a guitar string.  A disturbance wave involves small (up/down, left/right, or forward/back) movements but no net movement of material.

Matter waves

Matter waves are different.  These types of waves are composed of a steady stream of moving material that fluctuates in intensity.  For example, if we take a fire hose and periodically vary its flow rate you will see pulses of water emerging from the hose and moving away.  To someone on the receiving end, these pulses will look and feel just like waves.  That is, they will have a frequency, wavelength and velocity.

Here’s a better example.  Turn on a garden hose and move it left and right.  You’ll notice the water coming out in a something like a sine-wave pattern.  But this water wave is not going through a medium (OK it’s going through air but we can also do this in a vacuum).  So this is also a matter wave.

Here’s a animation showing the water wave example. To be done properly the hose also needs to be swiveled in order to prevent the wave becoming wider as it moves away.

By moving a hose side-to-side, water will come out in a wave pattern – here the nozzle is also being rotated to prevent the wave from widening.  This simple demonstration refutes the notion that a surrounding medium is required for waves to propagate.

The other important difference between a disturbance wave and a matter wave is that a disturbance wave’s speed is fixed by the medium it’s in.  Whereas a matter wave can move at any speed and its speed will increase as the speed of its source increases.


Light as a matter wave

How does this relate to light?  In the preceding chapter we looked at electric fields and hypothesized that they consist of a type of substance generated within charged particles and ejected from all sides at the speed of light.  When this substance hits another charged particle it applies a constant force.  Or at least it will be constant if the source and target remain motionless relative to each other.

But suppose the source particle were not standing still but oscillating.  The implication should be obvious.  Instead of receiving a constant force, the target will experience a fluctuating force – fluctuating in intensity and direction.  In short the target will experience a wave!

So that answers the mystery.  Light is a matter wave consisting of a fluctuating electric field.  It requires no elastic aether medium to propagate through because it is the ‘medium’ being transmitted.

This also explains why a Michelson-Morley interferometer will get the same result regardless of its net velocity, because there is no medium to slow light down.  Instead the velocity of light will be a constant, relative to the equipment generating and reflecting it.

Here is an animation explaining the process:

Here an electron moves up and down in a sinusoidal motion.  Its generated field radiates equally in all directions but, due to the motion of the charge, the field pattern becomes distorted.   Some of the field ends up moving directly to the right, and only these parts are shown.  As a result, a similarly charged particle far to the right will experience a fluctuating field matching the motion of the source.  These fluctuations will cause the target particle to oscillate in the same up/down direction as the source (the force directions will be discussed in a later chapter).  And thus the transmission of light has been achieved across empty space.


Some fundamentals

In order to formalise things, let’s make a few statements:

Definition of light:  Light is a fluctuating electric field.

We could add to that by saying the fluctuating field is generated by an oscillating charged particle(s), although in theory it could be generated by other means and still be light.  Note also that it says ‘electric’ rather than ‘electromagnetic’.  As pointed out earlier a magnetic field is just an electric field experienced at a different velocity than c.  So adding the suffix ‘magnetic’ is redundant.

Replace: “The speed of light is a constant for all observers”  with:

The speed of light is a constant relative to its source.

Some might object to this due to something called The de Sitter Effect.  This is discussed in a supplemental chapter:

The de Sitter Effect (<-- click to read)


More to come

In order to properly associate light with the wave model it will be necessary to explain the experimental observations that suggest other ideas.  This is going to take a while but the first of these has to do with photons and the Photoelectric Effect as discussed here:

The Photoelectric Effect

Another important effect has to do with how light expands while passing though openings.  While this is something that occurs with waves, the way that it happens with light turns out to be rather different:

Diffraction of Light

[1] In SR, velocity is often measured relative to Earth, which means it is being treated a special reference frame, and this is forbidden by SR.


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Copyright © 2014 Bernard Burchell, all rights reserved.