Propagation of light in a vacuum

[slow]

Hi. Reading some recent threads I have found basic questions about light, fields, etc. For example, why something like light, associated with the electric and magnetic fields, can exist in a vacuum. And I noticed that every time I've been asked that kind of question, I answered "how" the phenomenon behaves and not "why" it happens.

The first thing we learn about light is that it travels in a vacuum in a straight line. We learn it from primary school. Why does light do this ? Then the little book kept in my brain was opened in the chapters of the induction, which operates perpendicularly. But in the case of light in vacuum, $\vec{E}$ and $\vec{B}$ are both transverse, then the mutual perpendicularity is not determining the direction of propagation. Then the little book was opened in Poynting's theorem, which links the power flow in the direction of propagation with $\vec{E}$ and with $\vec{B}$ . But that also does not explain "why" the propagation is rectilinear, since the plane containing $\vec{E}$ and $\vec{B}$ could rotate and the Pointyng vector can do nothing to avoid it .

The barrel of a gun has a helical groove inside. Then the bullet goes spinning and the gyroscopic effect keeps the trajectory straight. If someone asks why the bullet travels rectilinearly in a vacuum, we can give an effective and convincing response. There is a why linked with something proven in the experience. The axis of a gyro maintains the direction while the rotation persists, if nothing intervenes to twist it. We have a "why" in the case of the bullet. It is a physical "why", that is, linked to verifiable experiences.

I need help in the case of light in a vacuum, because I can not find a physical "why", something linked to verifiable experiences.

 

[muscaria]

For example, why something like light, associated with the electric and magnetic fields, can exist in a vacuum.

A Vacuum state is the lowest energy configuration (ground state) of a field. The field is "always" there, so to speak. Light is an excitation of the EM field. Consider the following analogy of a sound wave propagating through a fluid: the lowest energy configuration of the fluid would typically be that of a homogeneous/evenly distributed fluid (same density everywhere) - this is the "vacuum state" of the fluid. If we excite the fluid so as to produce small ripples "which live" on the surface of the fluid, or sound waves, these are the analogy of light waves in the EM field.
So using this analogy, do you see how your question becomes self-evident?
As a side note, you can spend a long time delving into a "why" question: it will always recede into another why.

 

[muscaria]

The first thing we learn about light is that it travels in a vacuum in a straight line. We learn it from primary school. Why does light do this ?

Have you ever heard of Fermat's principle of shortest time? Least action principle?

 

[anorlunda]

Have you ever heard of Fermat's principle of shortest time? Least action principle?

This thread is marked B level. Your two posts are far too advanced for B level answers.

 

[muscaria]

This thread is marked B level. Your two posts are far too advanced for B level answers.

Fair enough.. Apologies to the OP if that was confusing.

 

[slow]

Hello, muscaria and anorlunda. Thanks for making you present.

For muscaria, the matter to which you have dedicated your answer allows me to suppose that I have started the thread inappropriately, because I have not sufficiently highlighted what really interests me. I will try now.

Why does light propagate rectilinearly in a vacuum? I seek to respond physically "why". And I do not want an inexhaustible succession of answers in chains until I reach the frontier of pure philosophy. I gave the example of the bullet to show that an answer linked to verifiable facts is enough for me, like building a gyroscope and noticing that it is opposed when we try to deviate the axis. I purposely omitted the spin of the photon in the initial message, since I can not physically or conceptually compare spin with a mechanical gyroscope.

 

[Vanadium 50]

hen the mutual perpendicularity is not determining the direction of propagation

Sure it is. There is one line that is perpendicular to both E and B; that's the direction of propagation.

 

[muscaria]

In addition to Vanadium's post.

But that also does not explain "why" the propagation is rectilinear, since the plane containing ⃗E\vec{E} and ⃗B\vec{B} could rotate and the Pointyng vector can do nothing to avoid it .

Why would the plane rotate though?
Note that the change in time of one field (E say) is related to the curl of the other (B) and vice versa.

 

[anorlunda]

This picture illustrates what @Vanadium 50 said. Note the arrow labelled direction.

 

[slow]

Take for example the case of $\vec{E}$ inducing $\vec{B}$. How does $\vec{B}$ know that its obligation is to appear in a plane perpendicular to the $\vec{v}$ that existed an infinitesimal time before? If it do not know, it could appear in another plane, one of the infinite planes containing $\vec{E}$, suggested by the curved arrow around the axis determined by $\vec{E}$ . It could know, if the induction intervened $\vec{v}$. But $\vec{v}$ intervenes when there is a charge involved in the phenomenon and, according to the available knowledge, there is no charge involved in the propagation in a vacuum. So, how does $\vec{B}$ know that it should appear in a plane perpendicular to $\vec{v}$ ?

 

[anorlunda]

Photons can be accurately described only by quantum mechanics.

You can not use classical analysis methods in the quantum domain.

 

[Vanadium 50]

How does ⃗B\vec{B} know that its obligation is to appear in a plane perpendicular to the ⃗v\vec{v} that existed an infinitesimal time before?

How does a thermos know to keep hot things hot and cold things cold?

 

[slow]

Photons can be accurately described only by quantum mechanics.

You can not use classical analysis methods in the quantum domain.

You have highlighted the difficulty that concerns me. If you ask me why light in a vacuum propagates in a straight line, I can not give a direct answer. In the charge I can not think, the spin is not comparable with a mechanical gyroscope, etc. I really do not find a way to respond in conceptual terms.

 

[slow]

How does a thermos know to keep hot things hot and cold things cold?

You're right. Maybe I am demanding too much explanation, when the really important thing is that something functions and we have the essential mathematics to calculate what can be exploited technologically. It seems that some philosophical genes have been filtered in my DNA, accompanied by some romantic genes.

 

[Dale]

Why would the plane rotate though?

Are you talking about circular polarization? I am not sure if this is helpful for this thread or if you should ask in a different thread.

 

[Dale]

If you ask me why light in a vacuum propagates in a straight line, I can not give a direct answer.

Well, light obeys Maxwell’s equations, and Maxwell’s equations in a vacuum take the form of a wave equation, and the wave equation has solutions called plane waves which propagate in a straight line. There are also other solutions such as dipole waves.

 

[slow]

Are you talking about circular polarization? I am not sure if this is helpful for this thread or if you should ask in a different thread.

You're right, the circular polarization implies the presence of angular momentum in the propagation. There is also presence of angular momentum in a gyroscope. But I do not want to compare both cases, because angular momentum has dimensions of action, a very abstract and very general term, as entropy is abstract and general, which transcends thermodynamics and appears, for example, in information theory and in the black hole theory. To this is added that the non-polarized light also goes through the vacuum in a straight line ... Mysteries of physics ...

 

[Dale]

Mysteries of physics .

Did you miss my second post?

https://www.physicsforums.com/threads/propagation-of-light-in-a-vacuum.941496/#post-5954235

 

[slow]

Did you miss my second post?

https://www.physicsforums.com/threads/propagation-of-light-in-a-vacuum.941496/#post-5954235

I have read your second note and I appreciate it. I have not commented on it because the first one is closer to the way in which I worry about rectilinear propagation.

 

[Dale]

I have read your second note and I appreciate it. I have not commented on it because the first one is closer to the way in which I worry about rectilinear propagation.

Circular polarization is also a prediction of Maxwell’s equations. I am not sure what is causing your concern. Do you have trouble understanding that light follows Maxwell’s equations or do you have trouble understanding that circularly polarized plane waves are a solution to Maxwell’s equations.

 

[muscaria]

Are you talking about circular polarization? I am not sure if this is helpful for this thread or if you should ask in a different thread.

Not at all. Just that the OP seemed to be suggesting that the plane could rotate and I was asking him why he thought that may be so - I was trying to get him to appreciate that if the change in time of one field was proportional to the curl of the other, the plane could not rotate. Hence, the flow of energy (Poynting vector) follows a straight line.
This was in response to:

But that also does not explain "why" the propagation is rectilinear, since the plane containing ⃗E\vec{E} and ⃗B\vec{B} could rotate and the Pointyng vector can do nothing to avoid it .

To which I asked:

Why would the plane rotate though?
Note that the change in time of one field (E say) is related to the curl of the other (B) and vice versa.

 

[slow]

Circular polarization is also a prediction of Maxwell’s equations. I am not sure what is causing your concern. Do you have trouble understanding that light follows Maxwell’s equations or do you have trouble understanding that circularly polarized plane waves are a solution to Maxwell’s equations.

Hi, Dale. None of the two details you mention cause me a problem. It is only my need to find answers that survive inside and outside the abstract realm. I call abstract realm what is formulated mathematically, linking terms by theorems.

 

[Dale]

None of the two details you mention cause me a problem

Then I don’t understand the question. If you understand the laws hat govern light and you understand how those laws lead to circularly polarized plane waves then everything is answered. What else remains?

It is only my need to find answers that survive inside and outside the abstract realm. I call abstract realm what is formulated mathematically, linking terms by theorems.

It is well established by experiment too. So it is more than just abstract. Isn’t that “inside and outside the abstract realm”

 

[slow]

Then I don’t understand the question. If you understand the laws hat govern light and you understand how those laws lead to circularly polarized plane waves then everything is answered. What else remains?

It is well established by experiment too. So it is more than just abstract. Isn’t that “inside and outside the abstract realm”

It's true, Dale. Everything has been treated and it has been useful for me. I'll read the new notes appear without adding mine notes.

 

[tech99]

View attachment 221524 Take for example the case of $\vec{E}$ inducing $\vec{B}$. How does $\vec{B}$ know that its obligation is to appear in a plane perpendicular to the $\vec{v}$ that existed an infinitesimal time before? If it do not know, it could appear in another plane, one of the infinite planes containing $\vec{E}$, suggested by the curved arrow around the axis determined by $\vec{E}$ . It could know, if the induction intervened $\vec{v}$. But $\vec{v}$ intervenes when there is a charge involved in the phenomenon and, according to the available knowledge, there is no charge involved in the propagation in a vacuum. So, how does $\vec{B}$ know that it should appear in a plane perpendicular to $\vec{v}$ ?

If we look at a radiated E-field being created by an accelerating charge, why should it not initially lie in the plane of acceleration? What is there to make it rotate, or bend forward or back? The accompanying B-field is created by the E-field as it passes an observer and is in-phase with it with zero time delay, so why should it not remain at 90 degrees to the E-field from which it comes? There is nothing in a vacuum to alter its direction.

 

[LitleBang]

Hi. Reading some recent threads I have found basic questions about light, fields, etc. For example, why something like light, associated with the electric and magnetic fields, can exist in a vacuum. And I noticed that every time I've been asked that kind of question, I answered "how" the phenomenon behaves and not "why" it happens.

The first thing we learn about light is that it travels in a vacuum in a straight line. We learn it from primary school. Why does light do this ? Then the little book kept in my brain was opened in the chapters of the induction, which operates perpendicularly. But in the case of light in vacuum, $\vec{E}$ and $\vec{B}$ are both transverse, then the mutual perpendicularity is not determining the direction of propagation. Then the little book was opened in Poynting's theorem, which links the power flow in the direction of propagation with $\vec{E}$ and with $\vec{B}$ . But that also does not explain "why" the propagation is rectilinear, since the plane containing $\vec{E}$ and $\vec{B}$ could rotate and the Pointyng vector can do nothing to avoid it .

The barrel of a gun has a helical groove inside. Then the bullet goes spinning and the gyroscopic effect keeps the trajectory straight. If someone asks why the bullet travels rectilinearly in a vacuum, we can give an effective and convincing response. There is a why linked with something proven in the experience. The axis of a gyro maintains the direction while the rotation persists, if nothing intervenes to twist it. We have a "why" in the case of the bullet. It is a physical "why", that is, linked to verifiable experiences.

I need help in the case of light in a vacuum, because I can not find a physical "why", something linked to verifiable experiences.

Is it possible that a charged particle has an electric field and that field basically extends out to infinity. if we move(accelerate) the particle it's field obviously must move with it. From where the old field was to where the new field begins is the photon. In other words the particles own field would be the aether?

 

[slow]

Is it possible ...

Hello, LitleBang. I have read your note and I can not refer it to something I know.

 

[LitleBang]

No because it's just a curious thought of mine. Although it makes an interesting kind of logic.