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Time question

  1. Mar 19, 2009 #1
    [This was moved from a thread in Philosophy Forum. -MIH]

    ..an example that is commonly used in physics and my response for it.

    Is light actually passing him at 300,000,000 m/s? I would say no. Let's say he starts moving at 299,999,999 m/s. This will slow down thinking, perception, any measuring equipment, etc. So even though light is passing by him at only 1 m/s he will perceive or measure that it passed at the speed of light since he is measuring everything so much slower.

    Guess what I'm trying to say is that his perception doesn't matter because reality is that light is passing him at 1 m/s.

    Hmm...second thought. I guess relativity is a function of cognitive thought? I.e. inanimate objects would never perceive anything and therefore relativity never comes into play.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 19, 2009 #2


    Staff: Mentor

    First, what do you mean by "reality is that light is passing him at 1 m/s". In what sense is that "real".

    Second, relativity is not about perception or optical illusions. It is about effects that remain even after you account for the invariant finite speed of light.
  4. Mar 20, 2009 #3
    Thanks for your reply Dale.

    I guess by "reality" I mean a point of view that isn't affected by any forces(anything that "dilates time"). A good example is to try looking at relativity from the point of view of light(ignore gravity for now). If an object is moving at 299,999,999 m/s, from the POV of light it is overtaking the object by 1 m/s. My problem is that we take the perspective of someone being affected by their velocity and say that what they're viewing is reality. I understand it is reality for them and their frame of reference but that is not what is actually happening in the larger picture. Don't we need to discern between these two "realities" of what is being perceived and what is happening from a neutral viewpoint? Basically, should we not be incorporating something into the equations to account for the slowdown in perception if we're going to use these examples? If it was me moving at 1/2 the speed of light before the trip I would think to myself "Hmm, everything I bring onto my ship will be occurring twice as slow, if I want to get accurate measurements I should make sure my clocks/equipment work twice as fast as normal to account for the slowdown." In addition I should also try to remind myself that I am aging twice as slow and that I am thinking twice as slow relative to people on Earth, just to get a better picture of what I'm experiencing.

    Maybe it's due to my ignorance on this subject but what effects are leftover from relativity?
  5. Mar 20, 2009 #4


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    The whole point of relativity is that there is no preferred frame of reference. Your frame of reference is exactly as valid is Jimmy's. i.e. It is just as valid to say that Jimmy is receding from you at 200,000,000m/s.

    You cannot do this. The speed of light is not a valid frame of reference. You will get non-sensical results.

    There is no such thing as an absolute or "neutral" PoV.
    There is no larger picture.
    There is no "perceived as opposed to actual" reality.
    Last edited: Mar 20, 2009
  6. Mar 20, 2009 #5


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    Okay, here are a couple of problems with your viewpoint:

    1. The time dilation formula is
    [tex]t = \sqrt{\frac{t`}{1-\frac{v^2}{c^2}}}[/tex]

    Note, that at 2,999,999,999m/s (assuming c=3,00,000,000 m/s) The time dilation factor is then 38730.

    300,000,000/38730 = 7745m/s not 1 m/s.

    At 0.5c the factor is 1.33 not 2.

    2. So far you've based your analysis just on light passing the ship going in the same direction as the ship.

    Assume you have two light sources one in front, and one behind. The ship is running away from one light and running towards the other. There is no way to say the it is the slower running time on the Ship that causes the occupants to measure the speed of the light they are meeting is moving at 300,000,000 m/s relative to them.

    There is no "larger picture". There is what the occupants of the ship measure, and the there is what those for who the ship is moving at almost c measure. And neither set of measurements is better or more "real" than the other

    That's what Relativity is all about, that time and space are frame dependent, and there is no absolute preferred frame from which which can judge reality.
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