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Is particle speed affected by source?

  1. Apr 27, 2012 #1
    I have some basic understanding that:
    Sound travels through air medium, and sound is in [pure wave form] (Please, correct [...] if wrong). Sound speed is not affected by its source.

    Some heavy object fired by its source. The object doesn't need medium to travel. The object is in [pure particle form] (Please, correct [...] if wrong). The object's speed is affected by its source.

    Light is in wave/particle form. Light doesn't need medium to travel. Light speed is not affected by its source.

    But, I don't know this.
    What about particles? Particles also are in wave/particle form. And they also don't need medium to travel. Is particle speed affected by its source?
    Last edited: Apr 27, 2012
  2. jcsd
  3. Apr 27, 2012 #2
    The reason light travels at c from all reference frames is because it's massless. (Well, technically, you can't have just one of these two criteria and not the other.) Massive particles' waves don't need a medium, but their speed is affected by their source. Think about all this in terms of time dilation and hopefully you'll understand me.
  4. Apr 27, 2012 #3


    Staff: Mentor

    When you are talking about QM these things are uncertain. If you have a particle with a well defined mass (eg an electron) and if the source produces those particles with a well defined momentum distribution, then yes, the particles speed is affected by it's source.
  5. Apr 27, 2012 #4
    Yes, if a particle is ejected from e.g. a gun at velocity v relative to the gun at rest in frame S and the gun has a velocity of u relative to another frame S', then the particle velocity w in S' is given by:
    [tex]w = \frac{u +v}{1+uv/c^2} [/tex]

    For a photon where v=c the velocity w is always c for any value of u. For small u<<c and v<<c, the equations aproximates to the Newtonian w ≈ u + v.

    Note that the relativistic velocity addition equation does not treat massive and massless particles differently. It is only concerned with their relative velocities.
  6. Apr 28, 2012 #5
    Thanks guys.

    Ok, so my main concern is I want to understand what is actually "motion".

    Light's speed is not affected by source, as Whovian says and I also think that it is because photon doesn't have mass.
    We have some examples like sound waves, light waves/matters, electron waves/matters, Heavy object matters. I had read somewhere that heavy object also have wave/particle duality, is this right? I am trying to connect all this thing by their characteristics.

    Sound: waves - medium required - mass less - speed not affected.
    Light: waves/matters - no medium - mass less - speed not affected.
    Electron: waves/matters - no medium - with mass - speed affected.
    Heavy Object: matters - no medium - with mass - speed affected.

    I am actually trying to understand the relationship between above content. Is this good idea?
  7. May 2, 2012 #6

    yes...it's called DeBroglie wavelength:


    Matter, mass, has an associated wavelength.

    This summary of yours is ok. It's a 'good idea' if it helps you learn. I would mention that an electron IS a 'heavy object', so you can have just three categories.

    You should probably remember that where a medium is required for transmission, the medium determines the speed of propagation. So with sound, for example, speed in water is different than speed in air.

    Light [electromagnetic waves] is unique in that in a vacuum [no transmission medium]everyone measures its speed as 'c'. In other words, no matter how fast you are going, light always whizzes past you at 'c'. This takes some getting used to because it's not obvious from our everyday experience!
  8. May 3, 2012 #7
    Thanks Naty1.
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