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Question about Light Speed

  1. Nov 7, 2008 #1
    Hi there,
    I have a question for you pros on this site.

    If an observer were to reach the speed of light, would time dilate to a stop?

    Now I have another question about the nature of matter and light.

    I've made this observation:
    Matter has 3 physical dimensions (length, width, height) and one "movement" dimension - time
    Light has 2 physical dimensions (electricity and magnetism) and one "movement" dimension - out in all directions in time. Light by nature is 2 dimensional - projections in your eye or on a screen - any image in fact - is 2 dimensional.

    If you want to get from matter to light you have to multiply the matter by c^2 to get that amount of light. So you have 4d matter, 3d light... now say you want 2 dimensions (one dimension of space, one dimension of time) wouldn't you just multiply the equivalent matter by c^2^2 and then if you wanted just 1 dimensional material (no spacial dimensions and one dimension of time) you just multiply the equivalent matter by c^2^2^2, or c^8

    Just been having these ideas and I've read through Einstein's special theory - still not clear on how much time would mathematically dilate if one were to actually reach light speed.
  2. jcsd
  3. Nov 7, 2008 #2


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    A massive particle cannot accelerate to the speed of light.

    A ray of light travels a path of zero proper time.

    Light as a wave can travel in all three spatial dimensions, so it isn't more two dimensional than matter. But you have the right intuition about multiplying by c.

    For massive particles at rest E=mc2

    Light has no rest mass, but it has momentum "p", so the equation for light is E=pc.

    Of course particles can have momentum too, so the full equation for massive particles is E2=(mc2)2+(pc)2.

    To work out how many "c"s you need, use the idea that "c" converts between space and time, ie. ct and x have the same dimensions.
  4. Nov 7, 2008 #3


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    An observer can't so the question of what would happen is meaningless. You cannot assume a situation that cannot happen in relativity and then ask what relativity would say about that situation!

    You are using the word "dimensions" in two different ways. The first is the standard use but I have no idea what you mean by "dimensions" in the second case.
    No it isn't. In the first place "light" is not the same as "image". In the second place, a convex mirror will produce a "real" three dimensional image. A sculpture is a three dimensional image.

    That makes no sense at all! I guess you are referring to e= mc2. That has nothing at all to do with "get from matter to light".

    Multiplying one number by another number has nothing at all to do with dimensions.

    Odd. I would think that you would have learned very early that one can't actually reach light speed.
  5. Nov 7, 2008 #4
    There are many subtlies with special and general relativity....it took perhaps twenty years for many of them to be uncovered and understood...so don't feel bad if some seem unclear to you yet...also it takes time to learn standard terminology so others understand you better.

    After reading material two or three or in my case even more times, you'll begin to understand....

    As noted by others, light moves through four dimensional spacetime. So do we: the cool part is that it can be said we all move though spacetime at the speed of light...always....this has been discussed on other threads here...

    In simple terms, yes...I would add to posts above that photons traveling at the speed of light do not age...as measured by their "proper time"...if an original photon from just after the big bang reaches an observer, it is unchanged from way back when....maybe 13B years or so!!! unless maybe it has been polarized or red/blue shifted a bit....but it's still the same photon....But everything in relativity depends on the observer, on their frame of reference.

    You have it backwards. via E=mc^2, if you have some energy (say light) and want to convert it to an equivalent mass, it would be M = E/c^2....so you divide an amount of energy by c^2 to see what the equivalent mass would be....

    Light has kinetic energy, it has momentum, but no mass...these are relativistic, not absolute...so they,too, are observer dependent....
    Last edited: Nov 7, 2008
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