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What does light travel in?

  1. Jul 28, 2006 #1
    Hi I actually happen to have two questions and hope someone can help me out :)

    1) Light as been established as waves, but how is light a wave? Many of the waves I have seen is of a sineoid shape. How do people know that light is in that form?

    2) What does light travel in? Soundwaves travel through air. In a vaccuum, soundwaves do not exist. However, light does. How is that done?

    Thanks for any help o:)
  2. jcsd
  3. Jul 28, 2006 #2


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    Sound does not travel "through" air. Sound is an oscillation in the positions of the atoms/molecules that make up the air. If you take away the atoms/molecules, there's nothing there to oscillate, and hence no sound.

    Light is similarly, an oscillation in the amplitude of an electric (and magnetic) field. If you "remove" all electric fields from a certain region of space there can be no light there. On the other hand, removing atoms/molecules is naturally, quite useless if you want to have lightlessness.
    Last edited: Jul 28, 2006
  4. Jul 28, 2006 #3


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    The simplest answer (Planck - 1900, Einstein - 1905) is that light consists of particles (photons) with wave properties. Elementary particles (electrons, etc.) also have wave properties.
  5. Jul 28, 2006 #4


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    A large number of very intelligent people in the late 19th century, using arguments similar to the ones you are making, argued that light must travel in a medium that they called the ether (modern spelling aether). Experiments to detect the movement of the earth through the ether turned up no effect. A few people figured out how to get that experimental result by supposing that matter is made of stuff that is connected by electric fields, and since electric fields travel at the speed of light, matter must therefore be compressed as it speeds up. Same with clocks. Einstein figured out that as long as you're going to make these (very natural) assumptions, you might as well get rid of the ether. That is, Einstein said that if you can't detect it, you might as well assume that it's not there.

    And that's where we've been for about 100 years. There are still very intelligent people who believe in the ether. Some of them have very high reputations such as the late David Bohm and his coworkers such as Basil Hiley. But the majority of the very intelligent people in the physics establishment reject the existence of the ether, and frankly will laugh at you for bringing it up. If there does turn out to be an ether, a lot of people will have some 'splaining to do.

  6. Jul 29, 2006 #5
    Einstein considered the ether superfluous for his explanation of MMX - and he believed one could not detect uniform motion wrt the ether - but he left no doubt in this 1920 address a Leden that an ether was necessary to explain inertia as well as the propagation of light
    Last edited: Jul 29, 2006
  7. Jul 29, 2006 #6


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    Will you get of this Einstein 1920 kick? Over and over it's been explained that Einstein's use of "aether" (as he spelled it) referred to his spacetime with its atributes of reality (curvature). Einstein was then at the beginning of his long, frustrating effort to derive electromagnetism and gravitation from a "unified field" theory of spacetime, and this 1920 address was his war cry on that project. He NEVER INTENDED to give aid and comfort to 19th century ether ideas, and reading that kind of thing back into his words is BAD HISTORY because it ignores the situation surrounding the address.
  8. Jul 29, 2006 #7
    Absolutely correct. I can only add that people like yogi fail to read (and comprehend) the paper to the end, Einstein makes quite clear what he meant by "aether". Here is the complete last paragraph:

    "Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an aether. According to the general theory of relativity space without aether is unthinkable; for in such space there not only wonld be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this aether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it."

    Most "aetherists" conveniently "forget" the last two sentences.
    Last edited: Jul 29, 2006
  9. Jul 29, 2006 #8

    I've heard of "dark matters" and "dark energy" in some articles that I can not remember the name of at the moment. Do these "dark matters" and "dark energy" explain what ether is?
  10. Jul 30, 2006 #9


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    No. Modern physics does quite well without any reference to ether. It is only a very small number of people who believe in ether now.

  11. Jul 30, 2006 #10


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    Einstein may have chosen his words more carefully in the Leden address had he realized how twisted they would later become. His intent was to introduce the gravitational field [spacetime] as the background upon which the universe is embedded.
  12. Jul 30, 2006 #11
    It is not Leden but Leiden.
  13. Jul 30, 2006 #12
    Yes - you should all read all of the address as I have many times - and then maybe you will stop putting words in Einstein's mouth - Most Relativists seem to go ballistic whenever the word ether arises. As I said in my post - Einstein did not consider the ether as something to which motion could be applied - but he clearly said it was necessary for the propagation of light.

    What Einstein meant by ponderable media is not clear - it could mean that it is massless - it could mean that it did not have characteristics that could support transverse vibration - but if that is the case, then why is it necessary for the propagation of light --- Every reasonable interpretation of the words used is contrary to the clear objective that Einstein was attempting to get across when he used the words - Einstein is clearly trying to correct the impression that his theories dispensed with the need for an ether - not just in the 1920 Leiden address, but also in many of his other writings and adresses - in particular the same type of description is used later in his tribute to Faraday where he reassert the need for an ether to explain inertia -
  14. Jul 30, 2006 #13
    The Leiden address was 4 years after his publication of GR - and just how do you know that is what Einstein had in mind.
  15. Jul 30, 2006 #14
    Here is a line by Einstein from his tribute to Faraday:

    "every attempt to deny the physical reality of space collapses in the face of the law of inertia. For if acceleration is to be taken as real, then space must also be real within which bodies are conceived as accelerated"
  16. Jul 30, 2006 #15
    MeJennifer - thanks for the spelling correction - I think I have also seen it as Leyden - probably wrong

    A few more lines from the 1920 address:“...to deny the ether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics to not harmonize with this view. For the mechanical behavior of a corporal system hovering freely in empty space not only depends upon relative positions (distances) and relative velocities, but also on its state of rotation, which physically may be taken as a characteristic not appertaining to the system itself. In order to be able to look upon the rotation of the system, at least formally, as something real, Newton objectivises space. Since he classes his absolute space together with real things, for him rotation relative to absolute space is also something real. Newton might no less well have called his absolute space “Ether”; what is essential is merely that beside observable objects, another thing, which is not perceptible, must be looked upon as real, to enable acceleration or rotation to be looked upon as something real.

    It is true that Mach tried to avoid having to accept as real something which is not observable by endeavoring to substitute in mechanics a mean acceleration with reference to the totality of the masses of the universe in place of an acceleration with reference to absolute space. But inertial resistance opposed to relative acceleration of distant masses presupposes action at a distance; and as the modern physicist does not believe that he may accept this action at a distance, he comes back once more, if he follows Mack, to the ether, which has to serve as medium for the effects of inertia. But this conception of the ether to which we are lead by Mack’s way of thinking differs essentially from the ether conceived by Newton, by Fresnel and by Lorentz. Mack’s ether not only conditions the behavior of inert masses, but is also conditioned in its state by them ..."
  17. Jul 30, 2006 #16
    Leyden is an older spelling. :smile:
  18. Jul 30, 2006 #17
    When you get down to it, a probability space. The thing that's waving is the probability of measuring it at proposed values.

    So, the real question is, what's causing the probability to wave?
  19. Jul 30, 2006 #18
    OK - I'll buy that -I never could spell worth a hoot anyway.

    In furtherance of my argument as to Einstein's view in 1920, here is a quote from Wickipedia:

    "Unexpected as it is for physicists to have differing opinions on such a fundamental concept as the ether, it must be acknowledged that Einstein was the one who changed his opinion. Shortly before his lecture in Leyden in 1920 he confessed in the paper: Grundgedanken und Methoden der Relativitätstheorie in ihrer Entwicklung dargestellt: "Therefore I thought in 1905 that in physics one should not speak of the ether at all. This judgement was too radical though as we shall see with the next considerations about the general theory of relativity. It moreover remains, as before, allowed to assume a space-filling medium if one can refer to electromagnetic fields (and thus also for sure matter) as the condition thereof ".
  20. Jul 30, 2006 #19
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