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

Another Speed Of Light Travel Scenario

  1. Oct 18, 2005 #1

    -Job-

    User Avatar
    Science Advisor

    If i were travelling on a train going at the speed of light and looking at the window on my right side, photons emitted by objects on the right side of the train would still collide with my retina, causing me to see something, probably a blur or continuous flash because i'm traveling so fast. A more interesting scenario is when the train is travelling in space, with two very big objects on the right side very far away. Here's a picture:
    Code (Text):
    [COLOR="White"]....................................................[/COLOR]O
    |-|
    |-|
    |-|
    |-|
    [COLOR="DarkGreen"]( )
    (x)
    ( )[/COLOR]
    |-|
    |-|
    |-|
    |-|
    [COLOR="White"]....................................................[/COLOR]O
    Where the train is shown in green, x is were i am, and the Os are the two very large objects. Suppose the Objects have the same size, are as far away from eachother as they are from the rails of the train (this distance being about a light year) and that they are big enough so that to me, sitting in the train, they look like two small moons. The train starts in the middle of the tracks so that in the beginning i see these two objects as being of the same size. If the train starts to move up at the speed of light, with me looking over my shoulder at the two objects, as mentioned above the photons reaching me from these objects would still reach my eyes, though rather than two objects i might just see the line created by the blur of these objects.
    Finally, suppose i had suffered some brain damage in the optical regions of my brain such that i was only able to see for a very short interval of time T every S seconds. Suppose T is very, very small, and S = 5. Then, every 5 seconds i would have a clearer view of the two objects, in fact, we may say that there is a sufficiently small T such that, every S seconds i would be able to see the two objects very clearly. Under this scenario, even though i am travelling at the speed of light, i would see one of the objects get bigger and the other get smaller, apparently going against the stipulation that time comes to a halt at when you reach the speed of light. Any thoughts on this?

    EDIT: Even if i hadn't the brain damage, assuming the two objects are the only visible ones on the side of the train, would they be blurred at all?
     
    Last edited: Oct 18, 2005
  2. jcsd
  3. Oct 18, 2005 #2

    pervect

    User Avatar
    Staff Emeritus
    Science Advisor

    Whoa! You can't be on a train travelling at the speed of light - not if you are applying relativity.

    If you arrive at a paradox, it is because your initial assumptions (that you were on a train travelling at an impossible velocity) were bad.
     
  4. Oct 18, 2005 #3

    -Job-

    User Avatar
    Science Advisor

    Oh, come on, how can i argue against that? Isn't there something about the validity of irrefutable theories? So much work making the picture look pretty for nothing. :smile:
     
    Last edited: Oct 18, 2005
  5. Oct 18, 2005 #4

    pervect

    User Avatar
    Staff Emeritus
    Science Advisor

    If you are going to "refute" relativity, it is going to be by experiment, because the theory is mathematically self-consistent. What you are doing is akin to the old mathematical "paradox" that shows 1=2, i.e.

    1*0 = 2*0

    divide both sides by zero and you get

    1=2

    Sorry, but you can't divide by zero, nor can a material body travel at the speed of light if one is using the special theory of relativity.

    This is well-known, see for instance the sci.physics.faq
    Sci.physics.faq "I am going at the speed of light and turn on my headlights"

     
  6. Oct 18, 2005 #5

    -Job-

    User Avatar
    Science Advisor

    I'm not trying to go against GR, just the traditional word-of-mouth idea that if you were travelling at the speed of light time would stand still, which actually comits the same error (how is the person travelling at the speed of light?). I'm making the same erroneous assumptions because i'm starting from the given scenario.
    I think a better question is what does it mean that time comes to a halt for a given photon and what are the implications of this?
     
  7. Oct 18, 2005 #6

    JesseM

    User Avatar
    Science Advisor

    Photons don't have valid rest frames, because one of the postulates of relativity is that the laws of physics should work the same way in all inertial frames, so if light moves at c in some frames, it must move at c in all valid frames. So, it doesn't really make sense to talk about what things look like from a photon's point of view. You can take the limit of what an object would see as it approached a velocity of c relative to most of the rest of the universe--say, relative to the rest frame of the galaxy--and in this limit it is true that clocks in the galaxy approach being stopped, and the length of the galaxy along the direction the object is travelling approaches zero, so the time it sees to cross the galaxy approaches zero too. So you could sort of get the idea from this that a photon would see all clocks frozen, and that it would see lengths compressed to zero along the direction it's travelling so its entire path through spacetime would be a point which takes zero time to cross. Unfortunately a lot of things don't have a well-defined limit as you approach c, like the velocity of other objects moving at c--depending on how you take the limit you could either conclude they're still moving at c or that they're at rest. So there's really no way to give any sort of answer to the question of what one photon would look like to another photon.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?