# Constant speed of light

#### rahuljayanthb

hi all, this is a very basic question.
apart from maxwell's equation, why else is the speed of light a universal constant?

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#### Jack21222

hi all, this is a very basic question.
apart from maxwell's equation, why else is the speed of light a universal constant?
It just is. I don't think there is a "why" involved. If it wasn't a constant, we'd have different laws of physics.

#### ghwellsjr

Gold Member
hi all, this is a very basic question.
apart from maxwell's equation, why else is the speed of light a universal constant?
Because anytime anyone attempted to measure it, they always got the same answer, so much so, that we have assigned a value to it and now it is used as a universal standard from which we derive the standard for the meter, rather than using a platinum bar with marks inscribed on it defined to be one meter apart.

#### Naty1

No one has a good complete theory why the speed of light is constant, nor why the charge of the electron is what it is, nor why we have only four fundamental forces, nor for the mass of any fundamental particles....they are all experimentally determined, that is, measured....

#### lightarrow

hi all, this is a very basic question.
apart from maxwell's equation, why else is the speed of light a universal constant?
Because it cannot be overtaken. Let's assume it's not constant, that is, there exist a frame of reference moving at a speed v, in which light speed has another value c' = c-k, with k = positive speed.
You can then take another frame, moving at a speed v with respect to the previous one; since *the physical situation is exactly identical to the first*, you will find a new light speed c'' = c'-k = c-2k and so on with another frame...At a certain point you will find a frame which is overtaking light; but this is impossible, so c must be invariant (frame-independent).

Now you could ask why light speed cannot be overtaken. Imagine an infinite value: you will never be able to reach it. Yes, c is not, actually, an infinite value, but it's impossible to reach it, as if it was: light "rapidity" *is* infinite. Rapidity is a quantity, analogous to speed, which however has the advantage not to be bound to space and time in the way it is speed, so sometimes is easier to understand what's happening looking at rapidity instead of speed.

The definition of speed as space/time was given because it is simple and because, at those times, scientists didn't think to the possibility that space and time could be linked, at high speeds (near c). At very high speeds, the very concept of "speed" loose its intuitive meaning. An example: you are in a starship moving at a speed extremely near c, with respect to the Earth. In that ship, you will see *all the visible universe* in just some second. Do you intuitively associate a finite number with this? I can't. It would be more intuitive an *infinite* speed here. What is more intuitive, in this respect, is rapidity, since, as I wrote, rapidity increases from zero to infinite, going from 0 speed to light speed.

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