1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How does light travel Through a vacuum

  1. Jul 26, 2011 #1
    Hi Im new, Ive read the FAQ's and none of them seem to help.
    Its a classic question I know but I cant 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
     
  2. jcsd
  3. Jul 26, 2011 #2
    A photon is only a change in the position of an electrostatic source. It is the electrostatic force that travels through the vacuum.
     
  4. Jul 26, 2011 #3

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    I think its an electromagnetic force, not an electrostatic. A photon has both electric and magnetic fields.
     
  5. Jul 27, 2011 #4
    I'm going to need more information than that, how exactly does a photon or wave both move in a vacuum?
     
  6. Jul 27, 2011 #5
    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?
     
  7. Jul 27, 2011 #6
    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 refered 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.
     
  8. Jul 27, 2011 #7
    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.
     
  9. Jul 27, 2011 #8

    Dale

    Staff: Mentor

    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.
     
  10. Jul 27, 2011 #9
    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?
     
  11. Jul 27, 2011 #10
    Hi Darren, welcome to Physicsforums. :smile:

    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.

    Best,
    Harald
     
  12. Jul 27, 2011 #11
    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.
     
  13. Jul 27, 2011 #12

    HallsofIvy

    User Avatar
    Science Advisor

    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.

    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.
     
  14. Jul 27, 2011 #13
    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.
     
    Last edited by a moderator: May 19, 2013
  15. Jul 27, 2011 #14
    Yes.

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

    Do sound waves create air particles? Do water waves generate water?
     
  16. Jul 27, 2011 #15
    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?
     
  17. Jul 27, 2011 #16

    jambaugh

    User Avatar
    Science Advisor
    Gold Member

    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.
     
  18. Jul 27, 2011 #17
    Humour me, what's your speculation? I'm interested in what people think.
     
  19. Jul 27, 2011 #18
  20. Jul 27, 2011 #19
    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.
     
  21. Jul 27, 2011 #20

    Drakkith

    User Avatar
    Staff Emeritus
    Science Advisor

    Look up Timeline of the Big Bang on wikipedia.
     
  22. Jul 27, 2011 #21

    jambaugh

    User Avatar
    Science Advisor
    Gold Member

    I think that there lies a chain of infinite regress.... it's http://en.wikipedia.org/wiki/Turtles_all_the_way_down" [Broken]
    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].
     
    Last edited by a moderator: May 5, 2017
  23. Jul 28, 2011 #22
     
    Last edited by a moderator: May 5, 2017
  24. May 19, 2013 #23
    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?
     
  25. May 19, 2013 #24
    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:

    That is correct.

    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.

    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.

    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.

    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.

    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 [Broken]

    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?

    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.

    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.

    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.

    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.

    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.
     
    Last edited by a moderator: May 6, 2017
  26. May 19, 2013 #25
    Which field lines and which mass?
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook