Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Speed of photons and its effect on us

  1. Jun 6, 2013 #1
    If something is travelling at the speed of light, its time stops to a relative outsider.
    But photons (basically light) are travelling at the speed of light... So does that mean that to me the photons are stationary? But if they are stationary, then they're not being reflected off an object and coming to my eyes. So then how do we see?

    Also, it's my first post here. So if I didn't do something right, let me know.
    And I'm definitely not as knowledgeable as you guys, so use "baby words" please?
     
  2. jcsd
  3. Jun 6, 2013 #2

    WannabeNewton

    User Avatar
    Science Advisor

    Hi mohit. You can't apply the Lorentz transformations from an observer's frame to that of a photon because a photon has no rest frame. The Lorentz transformations take you from one rest frame to another so it makes no sense to apply it to a photon. As such the result that "time stops for a photon relative to an observer" is not really valid.
     
  4. Jun 6, 2013 #3

    Mentz114

    User Avatar
    Gold Member

    It is hard to reconcile 'stationary' and 'travelling at the speed of light'. So you must choose one of these and as you say, experience shows that light is not stationary.

    Of course, by definition, everything that has mass can be said to be stationary in some frame, but this is not true of massless stuff like light.
     
  5. Jun 6, 2013 #4
    Hm. I think I might be more confused than I was before I asked this question.
    Thanks for your answers though! I'll read up on Lorentz transformations.
     
  6. Jun 6, 2013 #5

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    Well, your original question was "But photons (basically light) are travelling at the speed of light... So does that mean that to me the photons are stationary?"
    Do you not see that "traveling at the speed of light" and "stationary" are contradictory?
     
  7. Jun 6, 2013 #6
    That's exactly why I was confused. I know they're contradictory but I didn't know why. I was unaware of Lorentz transformations and how you can't compare things with different frames of motion.
    Also I can't wrap my brain around why so many laws can't be applied to light. I know all of physics pretty much breaks down when travelling at those speeds but it's still hard for me to imagine why.
     
  8. Jun 6, 2013 #7

    WannabeNewton

    User Avatar
    Science Advisor

    No one said this. The Lorentz transformations can always be applied to boost from the rest frame of an inertial observer to that of another inertial observer. They can't be used to boost from the rest frame of an inertial observer to that of light because it makes no sense to define an inertial rest frame for light.
     
  9. Jun 6, 2013 #8

    ghwellsjr

    User Avatar
    Science Advisor
    Gold Member

    I'm not going to address all your points, some of them have already been addressed by others. I just want to show you how the Proper Time of an object traveling at some speed relative to you changes with speed and why we can't talk about the Proper Time for a photon. To do this, I have made a spacetime diagram showing you in blue and a bunch of green objects traveling away from you at different speeds, every 10% of the speed of light up to the last red object which is traveling at 99.9% of the speed of light. The speed of light in this diagram is one foot per nsec. I have drawn in a black line at the Coordinate Time of 10 nsecs so that you can easily see how far each object has traveled in that amount of time. You can see that they each travel 1 foot farther than the previous one.

    The dots on each line show one nsec increments of Proper Time and I have shown ten increments for you (in blue) and for all the green objects but for the last one in red I have shown only one increment because if I had shown ten, the diagram would be ten times larger. If I had shown an object traveling at 99.99%c it would have had its first increment of Proper Time at 70 nsec and 70 feet. At 99.999%c these numbers would be over 223 and at 99.9999%c they would be over 707. As the speed of an object approaches that of light, the first increment approaches infinity so we don't want to make a drawing of an object traveling too close to it.

    I have shown a photon as the thin red line extending out from the red object. Proper Time does not apply to it.

    attachment.php?attachmentid=59332&stc=1&d=1370527545.png

    Any questions?
     

    Attached Files:

  10. Jun 6, 2013 #9

    PeterDonis

    User Avatar
    2016 Award

    Staff: Mentor

  11. Jun 6, 2013 #10

    Dale

    Staff: Mentor

    Classically speaking Maxwells equations apply to light. I am not sure what other laws you think should apply.
     
  12. Jun 6, 2013 #11

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    No, time in a moving objects frame of reference has nothing to do with how we see it moving. It has no effect upon us. If ball bouncing vertically were to pass you, horizontally, at near the speed of light, you would see it going up and down very slowly by that would not afffect how fast it was passing.

     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Speed of photons and its effect on us
  1. A photon watching us (Replies: 17)

  2. Photon speed (Replies: 20)

  3. A photon and its speed (Replies: 1)

Loading...