Speed of Light

Part 2

Spacetime Model


Speed of Light

Electromagnetic (EM) radiations are mathematically described with a high degree of accuracy, but no one knows their constitution and why the speed of light "c" is constant at 299,792,458 m/s.

To solve these enigmas, we will proceed by an indirect method. In this chapter, we will try to understand why "c" is invariant. There is a good probability that the solution to this enigma will lead us to the constitution of EM waves.

Since the first law of the wave-particle duality shows that waves, particles, and medium have necessarily the same constitution, the knowledge of the constitution of EM waves will lead us directly to the knowledge of particles. So, the solution to the c invariance, although interesting, is not an end per se but rather a method of investigation to solve the mystery of the constitution of particles, i.e. the constitution of matter.

Note: The photon enigma is covered in Part 4 www.waves-photons.com.

This webpage is the second part of the website www.what-is-matter.com
It is strongly suggested to start reading at the begining of this website.



This section may be bypassed on first reading.

The nature of EM radiations has always been the "pet peeve" of physicists. To date, this problem has not still been solved.

  • Newton thought that light was made of particles.
  • In the 19th century, physicists favoured the wave concept. EM waves were propagated in aether, an unknown propagation medium.
  • In 1905, continuing the works of Max Planck (Nobel Prize - 1918), Albert Einstein (Nobel Prize - 1921) built a particle theory of EM radiations. The concept of aether became obsolete since photons do not need any aether to be propagated. However, the photons concept does not solve some enigmas such as the Young Slits.
  • Later, in 1922, Einstein returned to aether. He was confronted with the problem of duality since the photon is incompatible with the wave, and therefore with aether.
  • In 1959, Louis de Broglie, Nobel Prize - 1929, proposed the idea that the aether was made of neutrinos.
  • Around 1980, physicists verified once more the constant speed of light with quasars, using embedded systems and telescopes in satellites.

These recent experiments show that the propagation of EM waves and the enigma of the constant speed of light are still not solved. The aether concept would help but no one is able to give an exact definition of it. Moreover, the aether is not compatible with photons.

Finally, since 1905, the enigma of the constitution of the light has been so persistent it prompted Louis De Broglie to say: "Science will make a great step ahead the day that it can explain a simple ray of light".


Method of Investigation

One of the peculiarities of the EM wave is that it can be propagated in a vacuum. But in a vacuum, there is nothing and the EM wave cannot exist in the absence of a propagation medium.

The introduction of the photon partially solves this problem. Indeed, like any particle, the photon can move in a vacuum. However, if EM radiations behave like particles in 90% of the cases, they also behave like waves in the remaining 10% of cases, as in Young's slits experimentation. The enigma thus remains unsolved for these 10% of cases.

In Modern Physics, lacks of understanding basic phenomena do not occur in one's, two's or three's, but in ten's: What is Mass? What is Gravitation? What is Electromagnetism? Why "c" is constant? What are quarks? ...and several enigmas such as Young Slits, EPR, strong force etc... In such a case, the only thing to do is to forget the classic schemes of physics and restart with a blank sheet, ignoring the few laws of quantum mechanics that are inconsistent with logic and good sense but bearing experimentations in mind.

Therefore, to understand the nature of EM radiations, we must return to the early 1900s, when Einstein explained the photoelectric effect. However, we must take into account all these experimentations conducted since the 1900s.

Note: The photon enigma is covered in the Part 4 www.waves-photons.com.


Separation of Media

The problem of additions of velocity suggests that we are in the presence of two distinct media:

  1. Apparent medium
    This is the visible medium that carries out the experimentations, and from where the EM wave is emitted. For example, in Fizeau's experimentation, this medium is water, in Michelson's, it is the Earth's atmosphere moved by the Earth itself...
  2. Real medium
    EM waves are propagated in a Real Medium. In the 1900s, the real meadium was called "Aether". De Broglie thought that it was a neutrinos sea. For the moment, the nature of the real medium is unknown.

This consideration leads to consider that the Apparent Medium and the Real Medium are overlaid.

speed of light - 1


Properties of the Real Medium

The Real Medium must have at least the following two properties:

  1. To be present everywhere
    Since EM waves are propagated everywhere, the Real Medium must also be present everywhere, in air, in water, and even in a vacuum. We must note that spacetime is present everywhere.
  2. To have propagation properties
    We know that any wave needs a propagation medium. Since spacetime has the properties of elasticity (Einstein), it could be a propagation medium.

This means that

Spacetime could be an excellent
candidate to support EM waves.

1/ Gravity and EM waves do not curve spacetime in the same manner. Gravitation is covered by Part 1 (www.mass-gravity.com) whereas EM waves are covered in Part 4 www.waves-photons.com. Here, the structure of spacetime has been deliberated simplified for pedagogical purposes.
2/ The real medium is not "aether". To avoid any confusion, we will use the term "real medium" instead of aether. On the other hand, we should not have any confusion between the word "motionless" used in the context of the global universe, which is correct, and the same word used in Special Relativity, which is not relevant in that study. The spacetime of the universe, sometimes called "global spacetime structure", is the one that was created about 13.7 billion years ago, and not the local spacetime of special relativity. So, in this website, the word "spacetime" will always refer to "global spacetime structure of the universe", as in Friedmann-Robertson-Walker Definition.


Constant Speed of Light

Let's imagine the emission of a beam from a laser diode A fixed on a motionless Apparent Medium. Contrary to what we think, the beam emitted from A does not reach the point L directly but via the real medium: A → A' → L' → L.

speed of light - 2

Now let's consider that the apparent medium is moving with the velocity "V" (figure below). An external observer, such as Fizeau or Michelson in 1900's, could think that the total speed will be the sum of the speed of the light, V(A-L), plus that of the apparent medium, V(L-X).

speed of light - 3

In reality, the speed of light will be constant, whatever the speed of the apparent medium because the light is not propagated in the apparent medium, as we could think, but in the real medium, which is motionless. Therefore its speed depends only on the nature of the real medium, i.e. the Global Spacetime of the Universe, and nothing else.

In concrete terms, the velocity of the light is calculated from two constants, the permittivity of free space ε0 and the spacetime permeability μ0, as demonstrated by the Maxwell Equations.


Case of two Reference Spaces

The figure below shows a photon which is emitted from an apparent medium A to another apparent medium B. The two apparent media are moving in the opposite direction. In this case, no one can explain why the speed of this photon is constant.

speed of light - 4

The next figure explains this phenomenon.

The light is not emitted as a photon but as a wave. In two words, the photon is not a particle but a mathematical object called "Vector" that represents the direction and the magnitude of an EM wave (see the notes at the end of this webpage).

So, the light is not propagated directly from A to B, but through the real medium from A' to B' (or conversely), i.e. in the Global Spacetime of the universe. As a result, the speed of light is exclusively a function of the real medium characteristics, i.e. spacetime permittivity ε0, and spacetime permeability μ0. Thus, the speed of light is always 300,000 km/s, whatever the relative velocity of the two apparent media from where the light is emitted.

speed of light - 5



  • EM waves are emitted from an apparent medium but are propagated by the real medium, which is the Global Spacetime of the universe.
  • In the Global Spacetime of the universe, the speed of light is always 300,000 km/s.

Therefore, although it amounts to the same thing, it would be more accurate to write:

"The speed of light is 300,000 km/s in spacetime"

rather than:

"The speed of light is 300,000 km/s in a vacuum"

1/ In Part 4 www.waves-photons.com, we show that the theory of photon is inconsistent with Modern Physics. Part 4 proposes an alternative to the photon, the "quantified wave", which solves all the inconsistencies of the photon concept.
2/ Under certain conditions, EM waves may move at a speed different than 300,000 km/s. For example, using Bose Einstein condensats made up with sodium atoms at -273.15 oC, Lene Vestergaard Hau, from Harvard University, USA, slowed down EM waves to 17 m/s. This phenomenon, as the existence of the photon, the Young Slits, EPR, and photons intrication, is covered in Part 4 "Electromagnetism".