How to explain Einstein's Special theory of Relativity.

Vorde

What is a ray of light? A group of photons.

What is the speed of a ray of light? The speed of the constitute photons. (Here is where one might differentiate between phase and group velocity).

What is the speed of the constitute photons? C.

John Huang

Based on your answer, the speed of a ray of light is c but the speed of a pulse of light could be different. However, I think you will deny your own answer if you think about your answer to your first question ONCE AGAIN.

You mentioned about a very important question, "what is a ray of light?". But your answer MISSED one major factor and one minor factor. First of all, we need one thing besides photons to define a ray of light. The minor one is, to understand MMX clearly we should know a key concept, which was named "first space" by Zhizhong Cai in year 2010.

So, could you modify your answer to your own important question with more detail, like to DEFINE a ray of light? My answer to the speed of a ray is relative to the definition of a ray of light and I need someone to confirm it. Could you help?

Vorde

First of all, I don't think you understand properly the difference between group velocity and phase velocity, but regardless, for this conversation all we need to consider is phase velocity.

Second, and pardon the language, what the hell is 'first space'? It sounds like baloney to me, so I'm going to ignore it unless you can back that phrase up with some sources.

Third of all, I don't need to define a 'ray of light', I'm taking about photons: which is what light is actually made of. If you want to claim that a 'ray of light' acts differently than photons that's fine, but it doesn't matter because photons are what exist so that's all we need to worry about, and photons travel at c.

ghwellsjr

Is the issue that you're concerned about that if you use the LT to get from S' to S using a speed of 0.866c, you cannot use the LT the other way, to get from S to S' using the same speed?

DaleSpam

I don't know what would make you think that. Please identify the phrase in that document that led you to that mistaken idea and hopefully I can help clarify your misunderstanding. You can also Google both "phase velocity" and "group velocity" to get a broader understanding if you are still confused.

Furthermore, both group velocity and phase velocity are defined for QM wavefunctions just as well as they are defined for classical waves. So the quantum/classical distinction you are trying to draw is irrelevant wrt wave velocity.

Btw, the MMX used an unmodulated beam of light, so there was no group velocity, only phase velocity. Also, the medium is non-dispersive, which removes a lot of complications. For the purpose of the MMX discussion, the applicable definition of velocity of the continuous beam is the phase velocity.

DaleSpam

I have corrected your statement. As discussed over and over and over again, the LT is part of SR, so it always supports SR and never anti-SR. Your assertion to the contrary is flat wrong and has been corrected multiple times already.

If you believe otherwise then please provide a mainstream scientific reference that supports your claim that any prediction of the LT ever violates SR. If you cannot produce such a reference and you repeat this claim then you are engaging in speculation which is against the forum rules.

John Huang

No. As I understand, we use LT to get from the measurement of S to calculate the expected measurement of S' and the inverse LT for the reverse purpose.

John Huang

Yes, it is fine to say light is actually made of photons, but, how do photons make up light?

More precisely, if we define a ray as a thread of light then how do hpotons make up a ray? That is what I mean, we need a definition for a ray, a thread of photons.

In the English version of the 6/30/1905 paper, Einstein used the term {Any "ray of light" moves ...} for his second postulate. If you do not have a definition for the "ray of light" how do you understand the speed of a "ray of light"?

Mentz114

None of this is relevant. It is straightforward to measure the speed of light. See

http://en.wikipedia.org/wiki/Fizeau_experiment

DaleSpam

The important thing is that the speed is invariant, not that light happens to travel at that speed. We now know that any massless particle will travel at the invariant speed, and light was simply the first such phenomenon that we encountered.

Vorde

This is pure terminology. And you are talking nonsense. Light is just photons moving at c. If you want to call a group (or thread, if that floats your boat) of photons by another name, that is fine, but it will still move at c.

If you cannot realize this, than it is meaningless to continue this argument as you will get nowhere.

Also DaleSpam just said a really good point, it's the invariance of the speed of the light that matters, not the speed itself.

John Huang

When we focus on just a a single line in a beam of light, not a group of different frequencies of rays in a beam of light, then what are you going to name that single line of light? Since the name "ray" is the best one available for a single line in a beam of light I adoppted it for that purpose.

I guess I should say a SINGLE line in a ray of light to make things clear. I will use a new term S-ray for that purpose. Mathematically, a ray is a straight line extending from a point, my S-ray is for a SINGLE line in a ray of light.

It is good to think about a helpful term. My new term "story" for "all events related to an object" did correct my confusing idea of "event period", what I should use is "events period" or even better, "time period between two events". Now I know that in physics, the term "world line" is for the purpose of my adopting of the new term "story". However, the "world line" is for spacetime and I am trying to let the "story" stay in space and time.

Yes, the velocity of an S-ray is defined as its phase velocity in physics. However, that definition is unable to cover the S-ray emitted from a moving (or also spinning, like the source of light in the MMX) source of light. That is why I like to know what is the proper way to define the speed of an S-ray. Do you have any idea?

John Huang

I think it is good for physicists to study a SINGLE line in a ray of light, an S-ray, not only a ray includes a group of S-rays. Now, let me try to define the speed of an S-ray. Let me start from the definition of the speed of a ray.

A ray is emitted from a source. A ray can be emitted from the location of the source to all possible directions. A ray always has a starting point but when physicists talking about the speed of a ray they don't even mention about the starting point. Am I correct?

It looks fine (but actually not) when the source of the ray is not spinning. When the source of the ray spins, what will happen to the ray? Ths photons in the ray will go to different directions. How do you define the speed of that ray in that situation?

John Huang

Thanks. It will not harm if we try to understand how photons organize themselves to make a ray of light. That is what I am trying to do and I hope someone will provide his or her idea about it to help me.

Vorde

You are inventing a term and then asking us how it would act. It does not make sense to think of light as rays in this situation. The ray theory of light was disproved a long time ago.

You need to understand this. Rays of light is a convenient term to describe light as it moves and reflects/refracts but it is not how light actually works. So if you want to understand how light behaves in order to answer your original questions you need to accept and think about light as individual photons because that is what they are.

These mathematical entities you are describing (rays) do not have any physical significance here. Light of any form are just photons. Light that is emitted in a way that makes it look like a ray is simply a group of photons emitted in a linear order. Light emitted outward from a circling emitter are just photons being emitted in a spiral pattern. The speed of any of these photons is c.

You could argue that this thing you are calling a ray might travel at a speed different that c. But this thing doesn't have any physical significance, the only things that exist physically are photons and they travel at c.

Mentz114

This is not relevant to SR but you should read Feymann's book "QED - The Strange Theory of Light and Matter "

http://www.amazon.co.uk/QED-Strange-.../dp/0140125051

DaleSpam

That is a bad distinction to make as it is self contradictory. Events are already elements of spacetime, so a set of events will also be part of spacetime. Using nonstandard terminology doesn't change that.

It doesn't matter if you are talking about the speed of a photon, a classical EM wave (which is an approximation to photons), or a ray (which is an approximation to classical waves), the speed is defined by the group or phase velocity as defined in the page I linked to previously. As also mentioned previously, in the MMX the relevant speed is the phase velocity. Rays, waves, and photons all use the same definition.