You might ask why the speed of light is a universal constant. Specifically, why the speed of light is the same for all observers irrespective of the relative motion of the light source. The most straightforward answer is that this is simply how it measures. That is, when we go out and measure the speed of light, it always measures the same (299 792 458 m/s in a vacuum) whether the light source is beside us, travelling towards us, or travelling away from us. This applies to all particles (not just photons) that travel at the speed of light. That the speed of light plays some fundamental role in the fabric of the universe appears to be consistent with experimental observation. This postulate of Einstein’s special theory of relativity withstands the scrutiny of some of the brightest minds - not to mention the calculations of some of the most complex and accurate instrumentation - humanity can throw at it.
But there is another way to think about it. And it involves going back to basics (if you will, the flip-side of complex calculations and instrumentation). Let’s consider a lightbulb hanging from the ceiling of a room. We can determine the lightbulb’s location within that room by setting up a coordinate system using three points of reference (say we use the lightbulb’s distance from the ceiling, its distance from one wall, and its distance from another wall). If we know the dimensions of the room then these coordinates will determine the exact position of the lightbulb within that room. But the lightbulb exists in that room regardless of how we choose to measure or relate to its position. We impose a construct of coordinates so we may understand how the lightbulb relates to us (as a sentient being within that local environment). A deeper understanding is that the lightbulb simply exists. To relate to it we choose to define its position within our experience of the three-dimensional space in which we find it.
The same goes with the speed of light. A photon is what it is. When it travels through a vacuum we can determine its speed by measuring the distance it travels through space and dividing it by the time taken. To do this we may impose a construct (like we did with the example of the lightbulb) through which we can understand how a beam of light (or, more accurately, a beam of electromagnetic radiation) relates to us. The basis of that construct is an agreeably consistent, three-dimensional Euclidean space with time that reliably tempos in one direction - to which we apply some universally agreed measure of space and time (a lifetime of being told where we need to be and at what time tends to reinforce such a construct). All well and good for the most part but keep in mind that this is a perception of reality and not reality itself. And that this construct is learnt, not innate. If the speed of light appears unchanging regardless of the relative velocity of the observer’s frame of reference, then this means that there is a problem with the tools we have at our disposal. Either the instrumentation we use to measure the speed of a photon is faulty or inaccurate (the speed of light is, after all, ridiculously fast) or the construct on which we base our measurement is wrong. A photon exists. It just “is”. Our attempts to define it (and measure it) revolutionised our (deeper) understanding of space and time.
For the hundred years since Einstein’s general theory of relativity found its mathematical construct we have understood the speed of light to be a universal constant. One of a select number of physical constants that determines the laws of physics in the known universe (https://en.wikipedia.org/wiki/Physical_constant). The question we should ask is not why the speed of light is a universal constant (although it is worthwhile continuing to check its validity using clever experiments) but how much things would change should the speed of light, or the value of any other physical constant for that matter, be different from that which we have calculated.
One of my favourite skits from the wonderful Blackadder series is the one where Edmund Blackadder tries to teach Baldrick how to count.
It’s funny because nothing Baldrick says is actually wrong. Blackadder creates an expectation (if you will, a basic mathematical construct which we share with Blackadder) that Baldrick is unable to meet.
In truth, some things are not nearly as obvious as they first appear (http://scienceblogs.com/goodmath/2006/06/17/extreme-math-1-1-2/).
While others are not as they seem.
While others are not as they seem.