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What would you see.

  1. Jul 7, 2008 #1
    Hi, what would you see if you were traveling at 0.5c? And how would others see you?

    Length Contraction- I would see everything shorter and other's would see me shorter?

    Time Dilation- I would be slower than other? Why? Shouldn't it be like length contraction?

    Mass- ?
     
  2. jcsd
  3. Jul 7, 2008 #2

    HallsofIvy

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    "What you would see" depends upon what you look at! If I were in an enclosed space ship, so that everything I can see is moving at the same speed I am, then everything would look perfectly normal. Of course, "moving at the same speed", or "moving" at all, is relative- You can't just say "traveling at 0.5c". You must specify relative to what I am moving at 0.5 c. If I were moving at 0.5c relative to another space ship and could watch it for a moment, I would see it moving at 0.5c relative to me and so would see it with the usual space contraction of relativity. If I were to look at the entire universe around me, different stars would be moving at different speeds relative to me and I suspect it would look exactly like I see it now- I suspect there are already stars movingat 0.5c relative to me and so I am moving at 0.5c relative to them.
     
  4. Jul 7, 2008 #3

    JesseM

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    Both time dilation and length contraction are relative, if that's what you're asking. You'd measure other's clocks slowed down relative to yours just as they measure your clock slowed down relative to theirs.

    Note that in relativity there is a difference between what is measured in your frame (by rulers and clocks at rest relative to you) and what you actually see with your eyes, which depends on how long it takes light from different points on an object and different events to catch up to your position (for example, the Doppler effect causes a clock to appear to be ticking faster when it's coming towards you, even though in your frame it is 'really' slowed down). Both length contraction and time dilation deal with what is measured, not with what is seen visually.
     
  5. Jul 7, 2008 #4
    I would see everything shorter and other's would see me shorter?

    the reason this is possible is that there is a loss of simultaneity. look up 'relativity of simultaneity'.
     
  6. Jul 8, 2008 #5
    Hello granpa.

    Quote:-

    --the reason this is possible is that there is a loss of simultaneity. ---

    This is incorrect.

    Matheinste
     
  7. Jul 8, 2008 #6
    That would make sense just from the equation.


    I mean to a observer at rest to you.

    For mass would the same happen? And why not for time?
     
  8. Jul 8, 2008 #7

    JesseM

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    What do you mean "why not for time"? Like I said already, time dilation is symmetrical, each person measures the other person's clock to run slower than their own.
     
  9. Jul 8, 2008 #8
    Let's say we have a clock that keeps time by counting waveforms radiated continuously at 299.7925MHz. Each waveform corresponds to one meter in the clock's local frame, and propogates isotropically at c (passing through any other inertial reference frame at the same observed speed).

    Given that any observer would be familiar with the clock's operation, would such a clock appear to run slow in both approaching and receding inertial reference frames?

    Regards,

    Bill
     
  10. Jul 8, 2008 #9
    Hmm then what happens in the twin paradox?
     
  11. Jul 8, 2008 #10
    Hello Bright Wang

    Quote:-

    ---Hmm then what happens in the twin paradox? ----

    In the twins paradox, which is not really a paradox as it is easily explained,the motions of the twins are not symmetrical. There are many threads in this forum explaining this very well.

    Matheinste.
     
  12. Jul 8, 2008 #11
    In the twin "paradox" we compare the elapsed time of two clocks. In the case of two objects in relative movement we compare the light signals from the distant clock with the observer's clock.
     
  13. Jul 8, 2008 #12
    so is the time dilation symmetrical?
     
  14. Jul 8, 2008 #13
    Quote:-

    ------In the twin "paradox" we compare the elapsed time of two clocks. In the case of two objects in relative movement we compare the light signals from the distant clock with the observer's clock.-----

    Yes but when the twins, in the "paradox" reunite no distant clock is involved. This paradox however can tend to complicate the basic fact of time dilation being symmetrical for observers moving with uniform relative motion to each other. Until this fundamental fact is understood it can cause confusion because of arguments about the role of acceleratuion creep in. I would say to Bright Wang keep it simple to start with and don't get sidetracked by the twins paradox, it opens up the usual endless debates.

    Matheinste.
     
  15. Jul 12, 2008 #14

    aaj

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    This is incorrect. Your sentence would have been correct if the word 'measure' had been used instead of 'see'.

    As to what you would see, you would actually see other objects rotated and not length contracted. This is a visual effect and is known as 'Terell rotation'. Length contraction is actually invisible to the eye because of the finite speed and resulting propogation delays of light signals from different parts of an object.
     
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