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by Thomas Hinks on June 17th, 2005

Thomas Hinks

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The instance of blue shift clearly shows that light, though still coming at speed 'c'; has it's frequency increased. It is therefore striking at a higher speed and could be variable. So how can light's speed be constant?

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Answers. 7 helpful answers below.

  • by John Rambo on October 8th, 2009

    John Rambo

    Light would still travel at c for an observer on that planet/star. Remember, light always travels at c for ALL observers. So to you, light moves at c towards you, but the planet moves a NLS towards you, so in your reference frame the light would be moving away from the planet slowly, but for someone on the planet light would be racing away from them at c, thus light staying a constant. That simply means that time dilation will take place to even out the odds.

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  • by chudsosoft on June 23rd, 2005

    chudsosoft

    Frequency and speed are totally unrelated. Light (sorta) travels in waves. Think of the classic sine wave. Frequency measures how many of these waves go by in a certain amount of time, eg. how close together the waves are. The object travelling toward us didn't make its light travel faster. Rather, it pushed the waves closer together, increasing the frequency.

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  • by IrnRose on November 7th, 2009

    IrnRose

    doppler shift is also from the POV of the observer not the light source, so your motion needs to be accounted for as well.

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  • by Moongrim on November 8th, 2009

    Moongrim

    Light speed is constant.

    Yours isn't.

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  • by guardian on October 8th, 2009

    guardian

    I agree, I think that their is something that we are missing and when we do take a space ship to almost light speed we will find that ..."oooh thats why that does that but then because of this it makes it all work out fine"... I have read many books on it and I am still not convinced that doesn't make my idea right, I just am not convinced yet.+3

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  • by iwnit on November 8th, 2009

    iwnit

    1) Light waves do not work like sound waves or water waves. They don't need a propagation medium.

    Also, the oscillation does not take place in the direction of propagation, but in a perpendicular direction. What oscillates is not the photon either, but the related electric and magnetic *field*.

    For this reason, when the frequency of light increases, the light is still going at the same speed (the speed of light), it is just the frequency of the wavefront observed that is changing.

    "In optics and physics, a wavefront is the locus (a line, or, in a wave propagating in 3 dimensions, a surface) of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described."
    Source and further information:
    http://en.wikipedia.org/wiki/Wavefront


    2) "Motion along the line of sight

    Assume the observer and the source are moving away from each other with a relative velocity v (v is negative if the observers are moving toward each other). Let us consider the problem in the reference frame of the source.

    Suppose one wavefront arrives at the observer. The next wavefront is then at a distance λ=c/fs away from him (where λ is the wavelength, fs is the frequency of the wave the source emitted, and c is the speed of light). Since the wavefront moves with velocity c and the observer escapes with velocity v, the time observed between crests is
    t = λ/(c-v) = c /((c-v)*fs) = 1/((1-β)*fs)
    where β = v/c is the velocity of the observer in terms of the speed of light (see beta (velocity)).

    Due to the relativistic time dilation, the observer will measure this time to be
    t = t0/γ
    where
    γ = 1/sqrt(1-β^2)
    is the Lorentz factor. The corresponding observed frequency is
    f0 = 1/t0 = γ*(1-β)*fs = sqrt((1-β)/(1+β))*fs

    The ratio
    fs/f0 = sqrt((1+β)/(1-β))
    is called the Doppler factor of the source relative to the observer. (This terminology is particularly prevalent in the subject of astrophysics: see relativistic beaming.)

    The corresponding wavelengths are related by
    λ0/λs = fs/f0 = sqrt((1+β)/(1-β))
    and the resulting redshift
    z = (λ0-λs)/λs = (fs-f0)/f0
    can be written as
    z = sqrt((1+β)/(1-β)) -1
    In the non-relativistic limit (when v<<c) this redshift can be approximated by
    z ~ β = v/c
    corresponding to the classical Doppler effect."
    Source and further information:
    http://en.wikipedia.org/wiki/Relativistic_Doppler_effect


    3) "Light as electromagnetic radiation:
    Light is a form of electromagnetic radiation and as such can be described in classical electromagnetism by the electromagnetic wave equation, which is derived from Maxwell's equations. According to this theory, the speed of light in vacuum is related to the electric constant ε0 and the magnetic constant μ0 by the equation c^2 = 1/(ε0*μ0). In Gaussian units, the speed of light fixes the ratio between electrostatic and electromagnetic units.

    An important consequence of Maxwell's equations is that the speed of light in vacuum is independent of the frequency and wavelength of the waves, unlike many other types of waves in physics, including light travelling through a transparent material such as water or glass. However, for light in vacuum, it has been experimentally verified to a very high degree of precision (see "Variations with time and frequency", below).

    Light as photons:
    It is also possible to describe light as a stream of particles, called photons. In this case, the energy E of each photon is related to the frequency f of the light by the Planck constant h: E = hf. The speed of light is simply the speed at which these particles travel but, if it is independent of the frequency, it must also be independent of the energy of the particles. This is paradoxical in Newtonian mechanics, where the kinetic energy of a particle depends on its speed: the paradox is resolved in relativistic mechanics if the photon has no rest mass. The interaction of photons with matter is described by quantum electrodynamics (QED). Here the speed of light enters the theory via the dimensionless fine structure constant α."
    Source and further information:
    http://en.wikipedia.org/wiki/Speed_of_light

    Further information:
    http://en.wikipedia.org/wiki/Variable_speed_of_light

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  • by alfred schrader on November 7th, 2009

    alfred schrader

    This Doppler shift was first observed by astronomer Edwin Hubble. I am also an astronomer and I discovered that light from very distant sources slows down.
    The Doppler shift is a phantom. The universe is simply swirling at random. As for time corrections of photons (or light particles) is unlikely. I discovered the Graviton Particle which besides creating gravity & inertia, it is also the source of time itself... Time and gravity are both the result of the same particle...Alfred Herman Schrader

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