How does light travel Through a vacuum

[Darrenmackenz]

Hi I am new, I've read the FAQ's and none of them seem to help.
Its a classic question I know but I can't seem to find a full answer.

I understand that light is part of the electromagnetic spectrum
I also understand that light acts as both a particle and a wave.
I think I understand the basics that, light as it travels through a vacuum oscillates between a magnetic field and a electric field at 90 degrees to each other.
How does this all, get a photon from point A to point B in a vacuum if there is no "ether" with which a photon can travel within?

Thanks

 

[LostConjugate]

A photon is only a change in the position of an electrostatic source. It is the electrostatic force that travels through the vacuum.

 

[Drakkith]

A photon is only a change in the position of an electrostatic source. It is the electrostatic force that travels through the vacuum.

I think its an electromagnetic force, not an electrostatic. A photon has both electric and magnetic fields.

 

[Darrenmackenz]

I'm going to need more information than that, how exactly does a photon or wave both move in a vacuum?

 

[mikeph]

I think I understand the basics that, light as it travels through a vacuum oscillates between a magnetic field and a electric field at 90 degrees to each other.
Thanks

I don't know if it's a misunderstanding or a grammatical error, light doesn't travel between a magnetic field and electric field. Light is itself an oscillation of electric and magnetic fields. The oscillation propagates according to Maxwell's equations. And a vacuum means devoid of mass, not necessarily electromagnetic fields.

How do you define an ether and what gap in our understanding requires it?

 

[Darrenmackenz]

It's a grammatical error sorry, I get that light itself oscillates between magnetic and electric. Maybe you could explain to me how the oscillation propergates according to maxwell.
I referred to an ether in the context of a medium in which light could travel through. Sound for example travels through air as it's medium. I understand that light does not need a medium in which to travel, I just don't fully understand why.
You say vacuum means devoid of mass which makes sense but not necessary electromagnetic fields, what would cause such a field in vacuum?
Thanks for helping me understand this.

 

[mikeph]

If you combine Maxwell's equations for the case of no charge, the result is a wave equation, and one solution of that equation is an electromagnetic wave. It's difficult to convince a sceptical reader using only words and no equations, but it should be demonstrated in some popular textbooks (Griffiths for example should be in every physical sciences library).

Sound travels through air as it's medium- by this you mean, sound itself is a vibration in the air molecules, correct? A sound wave is fully characterised by the displacement of molecules from their equilibrium point. In the same way, a light wave is fully characterised by the displacement of the electric and magnetic fields from their equilibrium points.

Anything that couples to the electromagnetic field will create a field. Put a charge in empty space and it automatically comes with an electromagnetic field, not just where the charge is, but everywhere. It's the same as the gravitational field, and me asking you "how does the Earth orbit the sun? Why can the gravitational field cause the mutual interaction between a star and planet when there is a huge vacuum between them?" The vacuum simply means no matter is there. The fact that no matter is between two objects does not stop them from interacting.

 

[Dale]

Hi I am new, I've read the FAQ's and none of them seem to help.
Its a classic question I know but I can't seem to find a full answer.

I understand that light is part of the electromagnetic spectrum
I also understand that light acts as both a particle and a wave.
I think I understand the basics that, light as it travels through a vacuum oscillates between a magnetic field and a electric field at 90 degrees to each other.
How does this all, get a photon from point A to point B in a vacuum if there is no "ether" with which a photon can travel within?

Thanks

Do you understand Maxwells equations, in particular Faraday's law of induction? Do you understand that Faraday's law works even in vacuum? I.e. Do you understand that vacuum does not act as a "shield" for electric and magnetic fields?

If you understand those then light going through vacuum is an unavoidable logical consequence.

 

[Darrenmackenz]

Ok, air molecules who's equilibrium is being displaced is already present before a sound is even made.
Is there already an electromagnetic field present before light is even introduced?
If there isn't does light provide it's own field with which to travel?

 

[harrylin]

Hi I am new, I've read the FAQ's and none of them seem to help.
Its a classic question I know but I can't seem to find a full answer.

I understand that light is part of the electromagnetic spectrum
I also understand that light acts as both a particle and a wave.
I think I understand the basics that, light as it travels through a vacuum oscillates between a magnetic field and a electric field at 90 degrees to each other.
How does this all, get a photon from point A to point B in a vacuum if there is no "ether" with which a photon can travel within?

Thanks

Hi Darren, welcome to Physicsforums.

There is no positive evidence that "there is no ether", and a photon in transit can be regarded as a kind of wave packet. Thus it's a concept that you can use. As a matter of fact, many famous physicists (incl. Einstein) concluded that vacuum cannot be nothingness, which implies some kind of ether.

However, there is no evidence of something like "ether particles", and as far as we can tell, no linear velocity wrt an ether can be established by measurements (it would even invalidate relativity theory). That may explain why many people conclude that there is no ether.Harald

 

[saim_]

It's a grammatical error sorry, I get that light itself oscillates between magnetic and electric. Maybe you could explain to me how the oscillation propergates according to maxwell.
I referred to an ether in the context of a medium in which light could travel through. Sound for example travels through air as it's medium. I understand that light does not need a medium in which to travel, I just don't fully understand why.
You say vacuum means devoid of mass which makes sense but not necessary electromagnetic fields, what would cause such a field in vacuum?
Thanks for helping me understand this.

The "medium" that your intuition might require is the electromagnetic field. Oscillation of air particles produces sound. Oscillation of EM field produces EM waves like light. Electromagnetic field is just as physical as the air around you.

 

[HallsofIvy]

Hi I am new, I've read the FAQ's and none of them seem to help.
Its a classic question I know but I can't seem to find a full answer.

I understand that light is part of the electromagnetic spectrum
I also understand that light acts as both a particle and a wave.
I think I understand the basics that, light as it travels through a vacuum oscillates between a magnetic field and a electric field at 90 degrees to each other.
How does this all, get a photon from point A to point B in a vacuum if there is no "ether" with which a photon can travel within?

Thanks

Why would a particle (photon) need to travel "within" anything? It is only when you think of light as a wave that you need something to "wave". "Vacuum" means there is no matter. There are still fields such the gravitational field and electromagnetic field at every point in space. A light wave is a "ripple" in the electromagnetic field.

A photon is only a change in the position of an electrostatic source. It is the electrostatic force that travels through the vacuum.

I think its an electromagnetic force, not an electrostatic. A photon has both electric and magnetic fields.

No to both of these. A light wave is a ripple in the electromagnetic field. It does not "oscillate between a magnetic field and an electric field". At every point there is both a magnetic force vector and an electric force vector- at right angle to one another.

 

[Darrenmackenz]

Do you understand Maxwells equations, in particular Faraday's law of induction? Do you understand that Faraday's law works even in vacuum? I.e. Do you understand that vacuum does not act as a "shield" for electric and magnetic fields?

I think I understand it, an electric field can be induced by a magnetic one moving at 90 degrees to it. The basics of a dynamo, and the opposite is the basics of a motor.
Right?
So your saying light induces it's own electric and magnetic fields by oscillating them, this then becomes self propergating.

 

[mikeph]

Is there already an electromagnetic field present before light is even introduced?

Yes.

So your saying light induces it's own electric and magnetic fields by oscillating them, this then becomes self propergating.

No. Light is just an oscillation in a pre-existing electromagnetic field.

Do sound waves create air particles? Do water waves generate water?

 

[Darrenmackenz]

Yes.



No. Light is just an oscillation in a pre-existing electromagnetic field.

Do sound waves create air particles? Do water waves generate water?

Ok that all works in my head.
Here's another question, how did the pre existing field get where it is, i.e. the void in space? left over from the big bang?

 

[jambaugh]

Ok that all works in my head.
Here's another question, how did the pre existing field get where it is, i.e. the void in space? left over from the big bang?

It is built into the structure of space. One can speculate about what lies deeper but for now it is sufficient to describe how the "vacuum" behaves and w.r.t. classical moving charges, that is what Maxwell's Equations describe.

 

[Darrenmackenz]

It is built into the structure of space. One can speculate about what lies deeper but for now it is sufficient to describe how the "vacuum" behaves and w.r.t. classical moving charges, that is what Maxwell's Equations describe.

Humour me, what's your speculation? I'm interested in what people think.

 

[mikeph]

Electroweak interaction.
http://en.wikipedia.org/wiki/Electroweak_interaction

 

[Darrenmackenz]

So mikey, you think its an after effect of the cooling of the universe after the big bang? I didn't know that there were theories that the electromagnetic force and the weak nuclear force were once unified or could be unified if the universe heated up.

 

[Drakkith]

Look up Timeline of the Big Bang on wikipedia.

 

[jambaugh]

Humour me, what's your speculation? I'm interested in what people think.

I think that there lies a chain of infinite regress... it's http://en.wikipedia.org/wiki/Turtles_all_the_way_down"
One can invent a speculative foundation to "explain" E-M and then ask how to "explain" the foundation, inventing lower and lower levels and getting farther and farther from anything connected to empirical science.

I could say its about the flow and rotation of tiny angels and demons. What would that tell us about nature? Nothing. What's more, quantum theory as I interpret it is phenomenological and trying to paint it into some ontological world picture is less than useless.

This is not to say models are useless. I'm happy to consider models but consider them as scaffolding for predictions and not to be taken seriously as assertions about nature. For examples see: "www.ihpst2005.leeds.ac.uk/papers/Silva.pdf"[/URL].

 

[Darrenmackenz]

I think that there lies a chain of infinite regress... it's http://en.wikipedia.org/wiki/Turtles_all_the_way_down"
One can invent a speculative foundation to "explain" E-M and then ask how to "explain" the foundation, inventing lower and lower levels and getting farther and farther from anything connected to empirical science.

I could say its about the flow and rotation of tiny angels and demons. What would that tell us about nature? Nothing. What's more, quantum theory as I interpret it is phenomenological and trying to paint it into some ontological world picture is less than useless.

This is not to say models are useless. I'm happy to consider models but consider them as scaffolding for predictions and not to be taken seriously as assertions about nature. For examples see: "www.ihpst2005.leeds.ac.uk/papers/Silva.pdf"[/URL].[/QUOTE]

The paper was interesting, it's good to know the thought process that people like maxwell went through.
And I think your correct in some respect, that you can keep digging too deep into something and lose perspective, but if you don't push the boundaries there would be no progress. Some times you can be too conservative.

 

[Atomicjuicer]

In case anyone else finds this via search engine (as I did) I would like to continue the conversation with some remarks of my own.

Hopefully that's ok.

Many equation lovers seem to belittle the value of conceptual or descriptive language in physics - please don't.

In my mind, these electromagnetic fields through which light propagates helpfully solve the "ether" issue I have been trying to comprehend for years. Space may appear vacuum like with regards to concentrations of mass but clearly it is not an utter void with regards to these fields.

So, with a better understanding of what light is and how it moves, I'm more curious about the relationship between these field lines and mass.

Any comments?

 

[Popper]

Please exuse this rather long post. I believed that I could explain and describe the OP's question in a very understandable way as well as comment on and clarify other posts. I'm in a good mood today and wanted to do something both useful and fun for myself. :tongue:

I understand that light is part of the electromagnetic spectrum

That is correct.

I also understand that light acts as both a particle and a wave.

We say that photons display a wave-particle duality, which means that in some circumstances photons behave like a particle while in other circumstances they behave like a wave, but they do never behave like a particle and a wave at the same time.

I think I understand the basics that, light as it travels through a vacuum oscillates between a magnetic field and a electric field at 90 degrees to each other.

For the most part, that’s true. There are some instances where there is a time varying EM field in which the E field is not perpendicular to the B field though. There are also instances where the direction of propagation is not at right angles to the EM field too. But if you have a simple electromagnetic wave then what you say is true.

How does this all, get a photon from point A to point B in a vacuum if there is no "ether" with which a photon can travel within?

Think of what you just said in a very literal sense. You can throw a baseball through the air or in a vacuum without a medium through which it moves, right? Think of the EM field the same way.

What is going on is that the presence of an electromagnet field does not require a medium to propagate through since the electric and magnetic fields are what are propagating through the vacuum. Things like sound or water waves are actually the displacement of matter which is doing the propagating. In fact scientists only knew of those kinds of waves before EM theory was fully understood and thus assumed that if light was propagating then it too had to have a medium through which it was doing the propagating. It’s much easier to think of the EM fields as photons moving through space just as, say, a baseball would and in turn think of the fact that you can throw a baseball through space without the need of a medium.

A photon is only a change in the position of an electrostatic source. It is the electrostatic force that travels through the vacuum.

I don’t understand what you mean by this. Can you clarify it for me please? I don’t see a context in which it would make any sense. Are you speaking in terms of quantum field wherein an EM field is composed of photons? If so then it can only be said that if the, previously static, source of an electric field changes position that there is a time varying EM field present which is composed of photons. I.e. it creates photons. It cannot, however, be said to be the definition of a photon. Also it is not the force that travels through space but the field. The electromagnetic force requires the presence of a charge.

Maybe you could explain to me how the oscillation propergates according to maxwell.

According to Maxwell’s laws, where there is a time-varying electric field there will also be a time-varying magnetic field. These fields coexist in what can otherwise be a vacuum. They occur in a manner in which the disturbance does not remain at the location of the source of the varying fields. I.e. the disturbances move through space.

Recall how science works – We make observations about nature and then attempt to summarize what we observe in a quantitative fashion so that we can precisely describe[/I[ what we are observing. We call the resulting descriptions The laws of physics. However the description is not in itself an explanation. What we logically and/or mathematically deduce from. the description is what an explanation in science is all about. In the present case we’re concerned with the laws of electromagnetism, aka Maxwell’s equations. For a vacuum they are named as follows (let “@” represent the partial derivative sign)

Coulomb’s Law: div D = 0
Ampere’s Law: curl H = 0
Faraday’s Law: curl E + @B@t = 0
No monopoles: div B = 0

These equations can be manipulated to form another equation in which one contains only E and the other only B as well as the value 1/sqrt[ u_0 e_0 ] where u_0 = permeability of free space and e_0 = permittivity of free space. This is given the symbol c, i.e. c = 1/sqrt[ u_0 e_0 ] which is the speed of light in a vacuum.

If you want to see and follow the derivation then you must have an understanding of vector differential equations. If you have such skills then please see http://www.scientainment.com/tmm/images/waveq.pdf

If you combine Maxwell's equations for the case of no charge, the result is a wave equation, and one solution of that equation is an electromagnetic wave.

I find this comment confusing. You say that “one” solution is an EM wave. Do you know of another solution, i.e. a non-EM wave?

There is no positive evidence that "there is no ether", …

That’s because it’s impossible to prove a negative. But its logically sound to state that there is no either. The ether was defined as that which propagates the EM field. Since then it has been shown that the propagation of an EM field is not accomplished using an ether. Therefore it directly and logically follows that there is no justification for the suggestion that an ether exists. There is also no use for such a concept.

As a matter of fact, many famous physicists (incl. Einstein) concluded that vacuum cannot be nothingness, which implies some kind of ether.

That is incorrect. Particle physicists define the vacuum as the lowest possible energy density. In such a vacuum particle-antiparticle pairs materialize and then annihilate each other at random. However this is in no sense an ether.

A light wave is a ripple in the electromagnetic field. It does not "oscillate between a magnetic field and an electric field".

I believe that he was thinking of is the fact that an EM wave can consist of an electric field oscillating along one direction and the magnetic field oscillating along another, perpendicular, direction.

So your saying light induces it's own electric and magnetic fields by oscillating them, this then becomes self propergating.

That is not a good way to envision it. Don’t think of light as inducing EM fields which then propagate. Think of it as light is a propagating time-varying electromagnetic field.

Is there already an electromagnetic field present before light is even introduced?

Yes.

This response will be misleading if not properly understood. Any radio transmitted which is electrically neutral can emit an EM wave. It’s quite possible for an electrically neutral atom that is in an excited state to transition to a state of lower energy by emitting of a photon. You can also have a charged particle located inside a charged sphere where the total charge is zero. Set the charge inside oscillating and it will emit an EM wave. Look at your uncharged hand. If you see it then it’s emitting light and there is no electric field present. There are any number of examples of this. What mike is thinking of is the fact that at subatomic level there are fields which don’t cancel – I just wanted to be crystal clear on this point.

 

[saim_]

...the relationship between these field lines and mass.

Which field lines and which mass?

 

[Popper]

No to both of these. A light wave is a ripple in the electromagnetic field. It does not "oscillate between a magnetic field and an electric field". At every point there is both a magnetic force vector and an electric force vector- at right angle to one another.

Not at every point. E.g. consider a localized oscillating source. Then in the near zone the fields have a radial component.

See Classical Electrodynamics – Third Edition John David Jackson, John Wiley & Sons, (2001), page 408.

 

[Dale]

Many equation lovers seem to belittle the value of conceptual or descriptive language in physics

I have actually seen far more of the reverse. People who belittle the value of the equations. I think it is required in order to be a competent crackpot, although some non crackpots also take that position.

 

[Popper]

I think its an electromagnetic force, not an electrostatic. A photon has both electric and magnetic fields.

In classical electrodynamics charges "have" electromagnetic fields. I the quantum theory, photons are the quanta of this field. The EM field is "composed" of photons (I'm not entirely certain that phycistis know what that means). Conversely I suppose you can say that photons are in some sense "composed" of fields. But I myself would never say it nor think that way about it.

 

[Atomicjuicer]

Which field lines and which mass?

The fields through which light propagates on its way from the sun to the Earth through space for example. Mass of a planet.

I'm curious whether gravity affects light because these fields are influenced by mass.

Sorry if this is a confusing question. I'm at work so ill try to rephrase later!

 

[WannabeNewton]

Yes gravity affects light.

 

[DennisN]

How does this all, get a photon from point A to point B in a vacuum if there is no "ether" with which a photon can travel within?

My reply may not be satisfactory, but it is intended to make you ask yourself why you think an ether is needed at all (I've heard this em/etherargument many times before).

If you throw a ball out into empty space, the ball will travel forever in uniform motion (unless it's disturbed) - this is Newton's First Law.

My point is: We don't demand an ether for a ball to travel from point A to point B. So why should we demand it for a photon? (this is not a perfect argument - it is intended to make you think about your question).

In modern physics, there is no ether; there is no universal (or preferred) frame of reference. There are fields, e.g. the electromagnetic field, extending throughout space.

 

[davenn]

[Quote by mikeph]

[Quote by Darrenmackenz]

Is there already an electromagnetic field present before light is even introduced?[/quote]

Yes. [/quote]

This response will be misleading if not properly understood. Any radio transmitted which is electrically neutral can emit an EM wave. It’s quite possible for an electrically neutral atom that is in an excited state to transition to a state of lower energy by emitting of a photon. You can also have a charged particle located inside a charged sphere where the total charge is zero. Set the charge inside oscillating and it will emit an EM wave. Look at your uncharged hand. If you see it then it’s emitting light and there is no electric field present. There are any number of examples of this. What mike is thinking of is the fact that at subatomic level there are fields which don’t cancel – I just wanted to be crystal clear on this point.

thanks for that popper, I would need mikeph to give some good back up to his response
cuz I would have responded like you did. ... for a transmitter be it RF or say light ... how can there be a pre-existing EM field when the unit hasnt even been turned on ?

There cannot be any pre-existing EM field

Dave

 

[lss1]

I don't know much about this, but here's what I do know:

Light is created by one of two things (possibly more, but I don't know of them): a changing magnetic field or a changing electric field. Basically, if you move a magnet, you will produce a changing electric field, which will produce a changing magnetic field, which will produce a changing electric field, and so on until something absorbs the energy. You can do the same thing if you turn a current in a wire on and off again and again.
You can prove this in a thought experiment. Have a bunch of neutral particles, each composed of one positive and one negative charge, in a plane. Then, introduce an electric field. The definition of an electric field is the direction a positively charged particle (or constituency) will take in the presence of said field. Therefore, if you introduce an electric field into this plane, all of the positive charges in the particles will line up facing the same way in each particle as will the negative charges (in the opposite way, of course). Once they all line up, the positive charges will be facing the negative charges, which means they will attract. This then creates a magnetic field. I'm not sure how a changing magnetic field creates a changing electric field, but trust that it does, by a relation of reciprocity.
Here is where my knowledge fails me. I looked through my notes, and they say that the medium for electromagnetic waves is the changing magnetic field. However, when I tried to look it up on the Internet, it says that electromagnetic waves do NOT need a medium to propagate. This is very disturbing to me, so I encourage you to look it up and share the results. I will stick with my magnetic field idea but will stand corrected if need be.

I hope I have been of some help to you, or to anybody else.