Why is the speed of light a constant in space?

First of all, it's not just light that this applies to: it's all electromagnetic waves that propagate through the electromagnetic field.

From radio waves, microwaves, infrared, visible light, ultraviolet and X-rays to gamma rays, they all travel at the same constant velocity in space.

A Fundamental Reason

A fundamental reason for a speed limit is illustrated by one of the most famous thought experiments created by Albert Einstein (1879 to 1955).

What would you see if you were able to travel, in space, parallel to a beam of light?

Classical intuition suggested that you might observe a frozen electromagnetic wave.

Einstein was convinced that this was impossible for very good reasons.

Even the Mathematics indicated that light in a vacuum must travel at a constant velocity and never stop moving.

The Mathematics of James Maxwell

James Clerk Maxwell (1831 to 1879) looked at the results of myriad experiments that Michael Faraday (1791 to 1867) had created in the eighteen hundreds and produced a set of fundamental equations that showed how electric and magnetic fields behaved in the electromagnetic field.

The end result of these equations was that the velocity of propagation of electromagnetic waves was a constant in the vacuum of space, defined by just two physical constants. The end result was;

The two constants below the 1, represent values that quantify the ease with which an electric or a magnetic field can be established in space and they are values that can be measured experimentally.

They are known as the permittivity and permeability of a medium and the little 0s represent the values applicable to a vacuum.

So, mathematically speaking, the velocity of electromagnetic waves in space is constant. But it took the genius of Einstein to realise what a game changer this was.

The Laws of Physics

There is a tenet in physics that states that the laws of physics must be the same for all observers in inertial reference frames (no forces acting).

Albert Einstein realised that for the laws of electromagnetism, defined by Maxwell, to be the same for all observers;

1) Light can never be seen as a stationary set of oscillating electric and magnetic waves.

2) All inertial observers (moving at constant speed in the same direction) measure the same speed for light in the vacuum of space.

Note the emphasis on 'the speed of light in a vacuum' and 'observers in inertial reference frames'. This is because, the speed of light in different mediums, although constant for a given medium, may be lower than c and accelerating observers complicates things.

Einstein's View

Maxwell's equations implied a fixed speed for electromagnetic waves and Einstein proposed that this speed must be same for all inertial observers.

He realised that this had a major implication in physics: regardless of what velocity observers were travelling through space, they would always measure the velocity of light to be c as defined by the above equation based on permittivity and permeability.

This must be fundamental to the nature of the universe and it has profound consequences.

Regardless of your velocity, you will measure light coming towards you as c. And, it does not matter whether the source of the light is moving towards you or away from you, even at velocities close to c, you still measure the light's velocity (in space) as c.

This contradicted classical physics, which stated that the velocities of source and observer must be added and you couldn't possibly measure the same velocity for light from a source moving towards you as one moving away from you.

However, there was experimental evidence to backup Maxwell's constant speed of light and Einstein's understanding of the ramifications of this.

Experimental Evidence

Experimental evidence backed up the constant speed of light in the famous Michelson-Morley experiment, first performed between April and July in 1887, when the speed of light reaching earth from different directions and different times of the year, were taken.

In fact, Michelson (1852 to 1931) and Morley (1838 to 1923) were trying to prove that the speed of light wasn't constant to verify a theory that in order for light to travel in the vacuum of space, it needed a transport medium that existed throughout the whole of space.

They assumed that the speed of light would be influenced by the motion of the earth moving through this medium and affect the measured velocity of light as it propagated through what was called the 'aether'.

This is like the production of sound waves, which require a medium such as air to propagate and won't propagate without it. It was believed that light needed some sort of medium to propagate in space.

No matter what they did, the velocity of light travelling in the vacuum of space from any direction, taken at any time of year, was always the same constant value.

This result also showed that the propagation of light in a vacuum did not require a medium of any sort to be present and strongly undermined the classical luminiferous aether hypothesis.

Major Consequences for Time and Distance

Einstein also realised that making the velocity of light a constant must affect how an observer measures time and distance in someone else's reference frame.

Another of Einstein's thought experiments indicated that time must be relative to observers in their own inertial reference frames.

The thought experiment involved an observer on a railway embankment and an observer sitting at the mid-point of a train, on the roof, in a thunder storm.

The train is racing along at a speed of c/2, or 50% of c. Relativity has always been taught using impossibly fast trains as in the early part of the 20^th century, the fastest means of travel was the train!

Lightning hits each end of the train at exactly the same moment in time as the mid-point of the train draws level with the train spotter on the embankment.

The train spotter sees the lightning flashes from either end of the train at exactly the same time as the light has the same distance to travel from each point of impact to train spotter. Remember that the lightning strikes occur as the train's mid-point draws level with the anorak on the embankment.

Because the speed of light is a constant, the thrill seeker sitting on the mid-point of the train's roof, sees the light from the front of the train strike before the flash from the rear.

This is because the train is moving towards the front strike at c/2 and away from the rear strike at c/2, which means that the light has less distance to travel from the front strike to the adrenalin junky than the light coming from the rear as the train is moving away from it and so the light must have further to travel to the brave individual on the roof.

This thought experiment involves 2 common events, the two lightning flashes. One observer sees them at exactly the same time, but the other sees them at different times as the light has different distances to travel for the roof-passenger to see them.

The outcome of this thought experiment is that observers moving relative to one another do not agree on whether spatially separated events occur simultaneously.

Einstein decided that two observers moving relative to one another cannot both agree on what events happen simultaneously showing that there cannot be a universal notion of absolute time – it has to be relative.

Once simultaneity becomes relative, measurements of both time and distance must depend on the observer’s state of motion.

Special Relativity

There was experimental evidence that the speed of light was a constant and this was backed up by Maxwell's mathematical explanation of how electromagnetism behaves and propagates through space.

Classical physics failed to describe what happens when things are moving at close to light speed, like sub-atomic particles.

To explain all the repercussions of this, Einstein produced his theory of special relativity, which had to be employed when working with things that travelled at close to light speed.

Because of the impact on time and distance of a constant velocity for light, Einstein had to use a mathematical framework that merged space with time and space-time, originally proposed by one of Einstein's professors, Herman Minkowski (1864 to 1909), became the only way to solve what are called relativistic problems in physics.

Spacetime provides a framework in which all observers moving in their own inertial reference frames can have their perceptions of common events mapped onto this single, framework.

And this is all because of the the fact that the velocity of EM waves is a fundamental constant of nature.

When velocities are all well below that of light, special relativity reduces to classical physics and we don't notice what an impact a constant velocity to light has on the universe and spacetime is not necessary to work out solutions.

Does any of this affect us in normal life?

Without Einstein's special relativity (no acceleration involved) and his general relativity (acceleration accommodated) you would never hear that welcoming sentence; 'you have reached your destination'.

This is because the two theories are used to ensure that the atomic clocks on the GPS satellites are kept correctly synchronised to earth-bound clocks and relativistic effects are eliminated.

Without continuous corrections to keep earth-bound and satellite clocks synchronised, errors start to manifest themselves very quickly, such that within a day or two, GPS reported locations become wrong by kilometres and get worse all the time, making the system totally unusable.

Key Takeaways

Fundamental rule: The laws of physics and therefore, the laws of electromagnetism, are the same for everyone.

In Theory: James Clerk Maxwell's mathematics produced a value for the speed of light in a vacuum that was constant and the maths indicated that stationary electromagnetic waves were not possible.

Evidence: Michelson and Morley showed that regardless of how and when they measured the speed of light coming from space, it was always measured as a constant value.

A Universal Constant: No matter if you are moving towards or away from a light source, the speed of light reaching you will always be measured as c. The same applies if the light source is also in motion towards or away from you.

Minkowski Spacetime and Einstein's Special Relativity: As c is a fundamental constant of the universe, problems involving speeds approaching c require solutions using the mathematical framework of Minkowski spacetime and Einstein's special relativity for observers moving at constant velocity (speed and direction) in inertial reference frames (no forces in action).

When Velocities are Low: Special relativity reduces to classical physics.

And lastly, without special and general relativity corrections, we would not have a viable GPS system to guide us round the planet.

© 2026 Kieron Conway - All rights reserved.

--

(stay tuned to this blog for a future article on how GPS works)

----

Liked this article? Check out:

modern-physics-journey.com 

where you can read all about an exciting new science series: A Journey into Modern Physics, available in three parts, from Amazon on-line shops. (Some of the above article may appear in this series).

Also, the web-site has an index of blog articles published to date, for easy access to an article that might interest you. You can access the index using the link above.

--------