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Is light speed fixed?

  1. Aug 17, 2007 #1
    i brought this topic up in another forum and as it was a debate forum i didnt get much of an answer but, rather, a lengthy redundant argument between everyone.

    so im giving it a try here

    i dont have much knowledge with physics in general, and relativity is especially rough for me. that said it really interests me so if anyone can help me out with this question... and, maybe, dumb it down a bit :smile: id be in debt.

    if you are on a ship shooting through space at the speed of light and you aim a flashlight towards the front of the ship and turn it on will it ever reach the front?

    im under the impression that it will not because a beam of light cant travel at twice the speed of light. that is assuming that the speed of light is fixed.

    if im incorrect there then just ignore the rest of this:

    does the speed of our planet, our solar system, our galaxy come in to play with this? if we are spinning though the universe at, say, .5 light speed then wouldnt the speed of light from our perspective be only half of the actual speed of light? or does it affect it etc etc.

    also, as a complete different request, can you recommend a book about relativity and/or time that is for the lay person.
    time is a very interesting subject to me only because of the fact that not only can i not fathom the concept, but i dont think i can fathom fathoming the concept.


    and this a really great place. i plan to be more active now that i've found it again. i signed up a year or two ago and just remembered it. i might not be too participatory as im not very informed on the topics but ill definately be an observer.
  2. jcsd
  3. Aug 17, 2007 #2


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    You can't be on a ship which is travelling at the speed of light. This is because the ship has mass and thus it would require an infinite amount of energy to accelerate it to the speed of light.

    If you are on any real ship which is travelling at any constant allowed velocity (any velocity less than the speed of light) you will measure the beam of light to be moving at a speed equal to exactly the speed of light. This is exactly the same speed that, strangely enough, any other observer will claim that the beam of light was travelling at--i.e., the speed of light.

    Thus, the speed of light is a constant--independent of reference frames... as is well-known and experimentally well-verified.
    Last edited: Aug 17, 2007
  4. Aug 17, 2007 #3


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    There happens to be a FAQ on just this question.


    The simple answer is that spaceships can't move at the speed of light - at least not in the context of relativity.

    As per the FAQ (which I will quote in part - read the whole FAQ at the link above for more information)

    Assuming false statements (such as the impossible assumption of a spaceship moving at the speed of light in a relativistic theory) in a hypothetical question is a surefire route for confusion.

    Note that pre-relativity, it was not realized that objects could not go faster than the speed of light, and the question makes sense. In fact, Einstein pondered this question before he discovered relativity. However, if one asks the question in the context of what relativity predicts, the question makes no sense - it makes assumptions that are incompatible with relativity.

    If you want a specific example of the sort of trouble bad hypotheticals can cause, consider that one can logically prove that if 2+2=5 (a false hypothetical) that I am the King of England. (Details on request, the original proof is due to Lewis Carroll.)
  5. Aug 27, 2007 #4
    hi! i hav just joined ths fabulous place lets see if i could do any help

  6. Nov 26, 2007 #5
    100% cannot be reached by a physical object, but suppose you reach something like 99.9% and use your flashlight. As I see it, anything that flashlight hits in front of you will obviously be hit faster(goes out at 100% lightspeed) than if it was just going out at the .1% speed difference between you and 100% lightspeed.
  7. Nov 27, 2007 #6


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    The speed of light from a flashlight going at 99.9 of the speed of light will still be 'c'. While flashlights have not be tested in this manner, the velocity of gamma ray emissions from relativistic particles have been measured and found to be equal to 'c'.

    See for instance This faq

    You'll probably need a library to access the original articles, the abstracts of which can be found at:

  8. Nov 27, 2007 #7
    I know, but what I'm saying is, it would appear the flashligh should seem to be going extremely slow to anyone inside of the spaceship(especially if they were going even faster arbitrarily closer to c) which should not be the case as far as I know(those inside should measure and feel it going at c relative to themselves, aka, using it as an ordinary flashlight without problem ).
    Last edited: Nov 27, 2007
  9. Nov 27, 2007 #8


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    You seem to be saying two different things at the same time there. Let me say it a different way: to someone inside a fast-moving spacecraft, a flashlight works the same as it did before they left earth - they measure the photons to be traveling at C. And to someone on the ground who can also see the beam, the photons would also appear to be traveling at C.
  10. Nov 27, 2007 #9
    Yeah, I gave it some more thought and it's due to time dilation, if the ship was transparent and you pointed a laser foward within it(going arbitrarily close to c), those outside would see it extremely slowly inching foward(if the ship is long enough and going fast enough it may seem to take months or even a year for it to reach the front for those outside), but for those inside it would seem near instantaneous.

    If the light got out of the ship those inside would see it going ahead of the ship arbitrarily far at tremendous speed(c), but those outside would see it slowly inching ahead of the ship.
    Last edited: Nov 27, 2007
  11. Dec 4, 2007 #10
    Stpehen Hawking does a really good job, I think, in explaining physics to the layman in his A Brief History of Time.
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