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Thread: Speed of light question

  1. #1 Speed of light question 
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    if there were two waves of light traveling in opposite directions toward each other, and i was on one of these waves and when the two waves passed each other,then i measured the speed of the wave i was NOT on, wouldn't the speed measured be two times the speed of light?

    if that is true, than is Einstein wrong?
    or
    if the measured speed is the speed of light, how can that be?
    does that mean that one of the light waves stopped moving when i measured the speed?

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  3. #2  
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    you have to composate the speed that you are going at as well its like in a car crash with two cars going in opposite direction from each other at 30 miles an hour the force at the crash is like a car going at 60miles per hour and ramming you in the back but niether car was going that fast it was thier combined speed that made it like they crashed at twice thier speed. The faster you go the more energy you have the more force you have the worse the crash is going to be.


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  4. #3 Re: Speed of light question 
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    [quote="roflwaffle123"]if there were two waves of light traveling in opposite directions toward each other, and i was on one of these waves and when the two waves passed each other,then i measured the speed of the wave i was NOT on, wouldn't the speed measured be two times the speed of light?

    if that is true, than is Einstein wrong?
    or
    if the measured speed is the speed of light, how can that be?
    does that mean that one of the light waves stopped moving when i measured the speed?

    First off, massive objects cannot travel at the speed of light and the speed of light is not a valid frame of reference from which to measure things.

    So let's say that you are moving at 99.9999% of the speed of light and meet a light wave head on.

    You would measure the light wave as traveling at the normal speed of light relative to you. Why? because velocities don't add up the way we normally think of them as adding up.

    Normally, if you had an object going a speed of u in one direction, and another going v in the opposite direction, you would expect the speed at which each measure the other as moving with respect to himself as being equal to u+v.

    However, Relativity tells us that the correct answer is



    Now when v and u are small compared to the speed of light, this answer comes out to be really really close to what you get when you just add v and u. But as u and v get larger, it starts to diverge, and the answer will never be larger than c.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  5. #4 Re: Speed of light question 
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    Quote Originally Posted by Janus
    Shouldn't the equation read; ?
    Wise men speak because they have something to say; Fools, because they have to say something.
    -Plato

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  6. #5 Re: Speed of light question 
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    Quote Originally Posted by Arcane_Mathematician
    Quote Originally Posted by Janus
    Shouldn't the equation read; ?
    No, because then you get results larger than u+v

    example:

    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  7. #6  
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    whoops! sorry bout that. I was thinking of a different equation then, and I'm pretty sure it was related to: But I just can't place it :?
    Wise men speak because they have something to say; Fools, because they have to say something.
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  8. #7  
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    Quote Originally Posted by Arcane_Mathematician
    whoops! sorry bout that. I was thinking of a different equation then, and I'm pretty sure it was related to: But I just can't place it :?
    You're thinking the the Lorentz factor



    Which is used in time dilation and length contraction.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  9. #8  
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    Yeah!, crap, I'm not the most well versed in relativity
    Wise men speak because they have something to say; Fools, because they have to say something.
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  10. #9  
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    Leave all math out of it. If you cannot derive the Lorentz transformations from first principles, they'll only confuse you.
    The two basic postulates of Special Relativity are:
    1_Space and time (and all associated/derived quantities) are all variant depending on frames of reference since there is no one preferred frame of reference.
    2_The speed of light, c , is the only invariant, which means no matter your frame of reference, stationary, moving at 100km/hr or moving at c , you will always measure the speed of light in a vacuum as c. (Disregard accelerated frames of reference for now as that is the realm of General Relativity.)
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  11. #10  
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    Quote Originally Posted by MigL
    Leave all math out of it. If you cannot derive the Lorentz transformations from first principles, they'll only confuse you.
    The two basic postulates of Special Relativity are:
    1_Space and time (and all associated/derived quantities) are all variant depending on frames of reference since there is no one preferred frame of reference.
    2_The speed of light, c , is the only invariant, which means no matter your frame of reference, stationary, moving at 100km/hr or moving at c , you will always measure the speed of light in a vacuum as c. (Disregard accelerated frames of reference for now as that is the realm of General Relativity.)
    The two postulates usually stated for special relativity are:

    1) The laws of physics take the same form in all inertial reference frames.
    2) The speed of light is the same in all intertial reference frames.

    One can also, more rigorously, derive special relativity simply from the invariance of the Minkowski inner product. So, it is not only c that is invariant, but in fact it is the Minkowski innerproduct and the associated norm that are invariant. From that one derives the Lorentz transformations, which are the transformations that preserve the Minkowski inner product and a forward direction of time.
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  12. #11  
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    You're right DrR, I paraphrased for simplicity of understanding.
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  13. #12  
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    "Inertial reference frames", as well as "Minkowski spacetime" are abstract, "mental" frames and spacetime, which are used in a mathematical tool, i.e. – in "special relativity theory".

    But in reality there aren’t reference frames, there are concrete material objects that exist (and move sometimes) in "material" spacetime. To think that abstract spacetime is something real would be "correct" in 1905.

    But, e.g., after the appearance of accelerators, where the particles can move with speeds near speed of light – and any particle "have own reference frame" - to think that every particle transforms critically whole Spacetime in our Universe seems as "not too plausible".

    As well as 2-th postulate becomes be correct if will be as "The measured speed of light is the same in all intertial reference frames".

    More – see http://arxiv.org/abs/0812.2819 , V5, section 2.2.1.

    Cheers
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  14. #13  
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    yes , one of the light waves would stop completely.

    because it would see that you were trying to measure its speed.

    the light wave stops to prevent you from finding out that
    nothing can travel faster than the speed of light
    is a farce , so it simply stops.

    similar events can be observed as you discuss the speed of light in forums , intelligence seems to come to a full stop somehow.
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  15. #14  
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    Here is the simple answer:

    No. The light would pass at the same speed it always passes. The speed of light.
    "If you wish to make an apple pie from scratch, you must first invent the universe". - Carl Sagan
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  16. #15  
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    I think if what you are asking is if the lightxlight in opposite directions goes twice as fast as light then no sir you are wrong. both beams of light when near each other would slow down to the speed of one beam of light. The speed of light is 299 792 458 m/s. Nothing can go faster then this. it is a universal speed limit. However if you had a train that went 99.99.99 percent of this speed something very weird would happen. lets say a kid runs on the train. there speed pluse 99.99.99 Percent of the speed of light would make them go faster then light right? wrong. everything inside the train would slow down to near no movement the closer you got to the speed of light. because the inside of th train is going very slow well everything outside is going very fast you could hypothetically travel 100 years into the future within a few days on this light train. now imagine that the light train is a beam of light. and two light trains went near each other. both frams of refinance would slow down nearly stopping time temporarily.
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