Why is light speed constant?

[JCJ Barnard]

I have my own theory:

Light is a wave, this wave expand at the front and fade away at the back.

It’s the same principal as if you take let say 2 blocks.
You place a block at the front and take away a block at the end.
If you repeat this over and over again it looks like the 2 blocks move forward.

When a light wave is created it has no speed at all not even if it’s created by a moving object.
The rate of the expansion at the front and fading away at the back is what we see as the speed of light.

[ajb]

I have my own theory:

Well, you should not really post ill-founded theories here.

 

Light is a wave, this wave expand at the front and fade away at the back.

Yes, classically we understand light as electromagnetic waves.

 

 

The rate of the expansion at the front and fading away at the back is what we see as the speed of light.

The speed of light 'falls out of' Maxwell's equations... well this is the closest to 'why' as I can think of.

 

The speed of light, as measured by any inertial observer is written into the causal structure of space-time. The first clues of this are indeed found in Maxwell's equations.

[Strange]

I have my own theory:

 

Light is a wave, this wave expand at the front and fade away at the back.

 

It’s the same principal as if you take let say 2 blocks.

You place a block at the front and take away a block at the end.

If you repeat this over and over again it looks like the 2 blocks move forward.

 

When a light wave is created it has no speed at all not even if it’s created by a moving object.

The rate of the expansion at the front and fading away at the back is what we see as the speed of light.

 

 

1. This sounds more like group velocity rather than phase velocity, which is what "the speed of light" refers to.

 

2. You don't answer the "why" question from the title.

 

3. For this to be a theory, you would need a mathematical model and some evidence consistent with that model.

[geordief]

 

The speed of light, as measured by any inertial observer is written into the causal structure of space-time. The first clues of this are indeed found in Maxwell's equations.

I don't suppose you could expand slightly on that ...... What is "causal structure" ?

 

Is something "causing" something else ?

[fiveworlds]

Why is light speed constant??

 

It isn't. You are referring to the theoretical speed of light in a vacuum. Perfect vacuums don't exist. (That we know of)

Edited January 2, 2016 by fiveworlds

[Sensei]

Indeed, speed of light, while passing through medium varies from the one in vacuum

https://en.wikipedia.org/wiki/Speed_of_light#In_a_medium

[Strange]

Why is light speed constant??

 

It isn't. You are referring to the theoretical speed of light in a vacuum. Perfect vacuums don't exist. (That we know of)

 

As he is, as you correctly deduce, referring to the speed of light in a vacuum then it is constant and, more importantly, invariant.

 

The presence or otherwise of a perfect vacuum is totally irrelevant.

[dimreepr]

Why is the speed of light a constant? Just because...

[StringJunky]

Why is the speed of light a constant? Just because...

It's a postulate that happens to hold true on investigation.

[dimreepr]

Indeed but the question was why; perhaps you have a better explanation?

[StringJunky]

Indeed but the question was why; perhaps you have a better explanation?

Like an axiom, there is no 'why' in postulate. It just is.

[dimreepr]

Or just because.

[StringJunky]

Or just because.

Yes.

[geordief]

Is it possible for a photon to find a path through ,for example a pane of glass that did not entail any interaction with any of the atoms making up the glass?

 

I realise that this might be vanishingly improbable but if it did happen would the proton traverse the region at if it was traveling in a vacuum?

 

Have I misunderstood the whole process?

 

I was under the impression that light traveled at c "between collisions" and that it was the collisions that apparently caused the speed to go down whereas ,in actuality light had 2 speeds only -c or zero.

[swansont]

Is it possible for a photon to find a path through ,for example a pane of glass that did not entail any interaction with any of the atoms making up the glass?

 

I realise that this might be vanishingly improbable but if it did happen would the proton traverse the region at if it was traveling in a vacuum?

 

Have I misunderstood the whole process?

 

I was under the impression that light traveled at c "between collisions" and that it was the collisions that apparently caused the speed to go down whereas ,in actuality light had 2 speeds only -c or zero.

Not from a classical perspective — you have a field with a spatial extent, so it can't not interact with the medium, which has a different permittivity and permeability (which affect electric and magnetic fields). From a QM perspective you could say it's possible, but the odds are vanishingly small. There are a lot of atoms with which it might interact, even for a small amount of matter (which would then have a correspondingly small effect on the light) A microgram of material is still going to be somewhere of order 10^15 atoms.

[Sensei]

There are even mediums which change refraction index, thus speed of light in medium, after applying to medium external electric field, or external magnetic field.
It's done f.e. placing electromagnet near medium, and passing light through medium, when electromagnet is turned on, and when electromagnet is turned off, and comparison between them.

[geordief]

So is a photon a term restricted to quantum mechanics and a field a term that only has applicability in classical physics?

 

Is a photon what you get when you examine a wave of light at high resolution?

[Strange]

So is a photon a term restricted to quantum mechanics and a field a term that only has applicability in classical physics?

 

Quantum theory deals with fields that are, well, quantized. Roughly, the quantized perturbations of the fields are "particles".

Here is an overview: http://www.preposterousuniverse.com/blog/2013/06/20/how-quantum-field-theory-becomes-effective/

And a more detailed work-through the maths: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/fields-and-their-particles-with-math/ball-on-a-spring-classical/

 

 

Is a photon what you get when you examine a wave of light at high resolution?

 

I don't think so, no.

[Sensei]

Is a photon what you get when you examine a wave of light at high resolution?

 

If you shine light on f.e. metallic sodium, potassium, calcium, magnesium,

If light has correct color (frequency,wavelength,energy),

electrons are ejected, and accelerated accordingly to photon energy minus energy needed to ejection.

If color is wrong (too low frequency/too low energy) nothing happens.

 

In classic physics energy of light is same when we shine f.e. double intensity of red light, as half intensity of blue/violet light. Blue/violet light has approximately double frequency of red light.

But if it's done with photoelectric effect, light with wrong color is not ejecting electrons. Even though intensity is very high.

 

Energy of photon is E=h*f

If we multiply it by quantity of photons (intensity),

we get total energy of light.

If you sum all energies of all photons all frequencies, you have total energy of f.e. white light, or multi-color light.

[cjmg85]

The way I always saw it was like this. Light is energy moving at such a great speed that it never even has the chance to interact with anything long enough to either accelerate or decelerate it in a vacuum. The only place that this does not hold true is beyond the event horizon, where it is trapped in a never ending free fall around a singularity. Of course to the light, it is just following a straight line as it always does.

[ajb]

Light is energy moving at such a great speed that it never even has the chance to interact with anything long enough to either accelerate or decelerate it in a vacuum.

It is better to think of light, so electromagnetic radiation or photons, as possessing a property that we call energy. Light is not 'pure energy'.