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Thread: The Speed of Light Paradox

  1. #1 The Speed of Light Paradox 
    Forum Freshman sp1der's Avatar
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    I recently had a very in-depth conversation with a dear friend of mine where we discussed the chances of humans ever achieving light speed.
    Over the course of this discussion we also realized that this entire concept is an utter paradox.

    Speed of Light = 299, 792, 458 m/s
    The Length of the Universe = Theoretically infinite

    So, what's the point in traveling such speed if we will never be able to truly utilize it? Now, I do realize the scientific breakthrough it would be
    and the concept is just so interesting to me I would love to see that discovery within my lifetime. (which most likely won't occur)

    But, here's the real point of my discussion and where the actually paradox comes into play:
    Scientists say that the speed of light is so fast that if we were to travel anywhere we would
    get there instantaneously. Makes sense, right? But, how can that be true if the Universe is infinite?

    So, my question is if such events were possible, would humans feel the journey across the Universe or would it feel instant?
    Would it all be one big blur?
    What would it feel like if the human body could survive such high G's?


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  3. #2  
    KJW
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    Humans will never be able to directly travel at the speed of light. In principle, we could travel arbitrarily close to the speed of light, but regardless of how close to the speed of light we do travel, the speed of light will always be faster than us by the same amount as it is now.


    There are no paradoxes in relativity, just people's misunderstandings of it.
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  4. #3  
    Brassica oleracea Strange's Avatar
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    Firstly, it isn't possible to travel at the speed of light, so some of your conclusions are moot.

    So, what's the point in traveling such speed if we will never be able to truly utilize it?
    If we could travel at near the speed of light, it would still be useful. For example, we could get to the nearest star in a few years rather than decades.

    Scientists say that the speed of light is so fast that if we were to travel anywhere we would
    get there instantaneously. Makes sense, right?
    No, it doesn't make sense. At best, you are mixing up frames of reference. If you were to travel to the nearest star (about 4 light years away) it would take you just over 4 years at almost-the-speed-of-light - as measured by someone on earth.

    Form "your" point of view, the distance would appear to be less and so it would take less time.

    What would it feel like if the human body could survive such high G's?
    You haven't mentioned acceleration so it is not possible to know what sort of Gs you are talking about.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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  5. #4  
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    Quote Originally Posted by Strange View Post
    If you were to travel to the nearest star (about 4 light years away) it would take you just over 4 years at almost-the-speed-of-light - as measured by someone on earth.

    Form "your" point of view, the distance would appear to be less and so it would take less time.
    If you traveled at 90%c how long would that journey take for you by your onboard clock?

    Quote Originally Posted by KJW View Post
    Humans will never be able to directly travel at the speed of light. In principle, we could travel arbitrarily close to the speed of light, but regardless of how close to the speed of light we do travel, the speed of light will always be faster than us by the same amount as it is now.
    Is there such a thing as a source of light that moves wrt an observer at c? (I imagine not ;any source would have mass and so be unable to travel at c )
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    Quote Originally Posted by geordief View Post
    If you traveled at 90%c how long would that journey take for you by your onboard clock?
    In that case, you would see the distance reduced by a factor of 2.29 (1) and so it would be 4/2.29 light years at 0.9c = 1.94 years.

    (1) RELATIVITY CALCULATOR

    Is there such a thing as a source of light that moves wrt an observer at c? (I imagine not ;any source would have mass and so be unable to travel at c )
    I think you are right.
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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  7. #6  
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by geordief View Post
    If you traveled at 90%c how long would that journey take for you by your onboard clock?
    In that case, you would see the distance reduced by a factor of 2.29 (1) and so it would be 4/2.29 light years at 0.9c = 1.94 years.
    If the answer is 2.15 light years ,it will be the first time I have got one of those questions right
    (1) RELATIVITY CALCULATOR
    Suppose he skimmed the Earth at the same moment an observer on Earth was judging the star in question to be 4 light years away ,what distance would the astronaut judge the star to be as he sped past?

    If the answer is 2.15 ly's then it will be the first time I will have got one of those questions right

    So not confident....at all!
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  8. #7  
    Brassica oleracea Strange's Avatar
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    Not sure where you get 2.15. If an observer on the Earth measure the distance to be 4 ly, then the person passing at 0.9c will measure it as 4/2.29 = 1.75 ly. (Unless I have got it wrong, which is entirely possible. I am going to double check ...)

    ... no. seems to be correct.

    Your answer would be correct if the person were passing at 0.843c
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    Quote Originally Posted by Strange View Post
    Not sure where you get 2.15. If an observer on the Earth measure the distance to be 4 ly, then the person passing at 0.9c will measure it as 4/2.29 = 1.75 ly. (Unless I have got it wrong, which is entirely possible. I am going to double check ...)

    ... no. seems to be correct.

    Your answer would be correct if the person were passing at 0.843c
    Oh well (I divided 1.94 by 0.9)

    If a monkey can type Hamlet ,my day will come
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  10. #9  
    Brassica oleracea Strange's Avatar
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    You had me worried/confused for a moment there!

    speed = distance / time
    So distance = speed x time = 1.94 x 0.9 = 1.746
    Without wishing to overstate my case, everything in the observable universe definitely has its origins in Northamptonshire -- Alan Moore
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    Quote Originally Posted by Strange View Post
    You had me worried/confused for a moment there!

    speed = distance / time
    So distance = speed x time = 1.94 x 0.9 = 1.746
    So,I was almost there?(I just divided instead of multiplying?)
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  12. #11  
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    Quote Originally Posted by geordief View Post
    Quote Originally Posted by Strange View Post
    You had me worried/confused for a moment there!

    speed = distance / time
    So distance = speed x time = 1.94 x 0.9 = 1.746
    So,I was almost there?(I just divided instead of multiplying?)
    Exactly. Don't worry, I always have to double-check these basic things myself! (But that could just be age...)
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  13. #12  
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by geordief View Post
    Quote Originally Posted by Strange View Post
    You had me worried/confused for a moment there!

    speed = distance / time
    So distance = speed x time = 1.94 x 0.9 = 1.746
    So,I was almost there?(I just divided instead of multiplying?)
    Exactly. Don't worry, I always have to double-check these basic things myself! (But that could just be age...)
    I sometimes wonder if time doesn't also dilate as we get older
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  14. #13  
    Forum Freshman sp1der's Avatar
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    No, it doesn't make sense. At best, you are mixing up frames of reference. If you were to travel to the nearest star (about 4 light years away) it would take you just over 4 years at almost-the-speed-of-light - as measured by someone on earth.

    Form "your" point of view, the distance would appear to be less and so it would take less time.

    This makes incredible sense. I never thought of it from such perspective.

    You haven't mentioned acceleration so it is not possible to know what sort of Gs you are talking about.

    I was just adding some additional thoughts sorry if it was confusing.
    By the way is there any way to calculate how many G's the speed of light has?

    Last edited by sp1der; October 27th, 2017 at 08:40 AM. Reason: formatted text incorrectly
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  15. #14  
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    Quote Originally Posted by sp1der View Post
    By the way is there any way to calculate how many G's the speed of light has?[/COLOR]
    G is used to measure acceleration not speed. (Imagine sitting in a car: you are pressed back into your seat by the "g-force" as the car accelerates. When it is moving at a steady speed, you feel no pressure.)

    Light doesn't accelerate so, none.
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    Quote Originally Posted by Strange View Post

    Light doesn't accelerate so, none.
    If it was moving through a continuously "transparency decreasing" medium could it be said to accelerate?
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  17. #16  
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    could it be said to accelerate?

    Once light photons are created they are already traveling at light speed. Thus, the acceleration simply isn't there. I feel like this '"transparency decreasing" medium' would have no affect. But I'm not sure you may need to be more specific.
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    Well light travels slower than c depending on the medium.For instance photons take hundreds(?) of years to make their way out from the centre of the Sun.

    Their speed must vary as they encounter different regions,I imagine.
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  19. #18  
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    Quote Originally Posted by geordief View Post
    Well light travels slower than c depending on the medium.
    Nah.

    Their speed must vary as they encounter different regions,I imagine.
    Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted.
    There's a difference between the speed of light (a constant) and the speed at which it propagates.
    As light it's always at c. It's the absorption and emission which "slows it down" (but it's not "travelling light" per se while it's doing that bit).
    "[Dywyddyr] makes a grumpy bastard like me seem like a happy go lucky scamp" - PhDemon
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by geordief View Post
    Well light travels slower than c depending on the medium.
    Nah.

    Their speed must vary as they encounter different regions,I imagine.
    Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted.
    There's a difference between the speed of light (a constant) and the speed at which it propagates.
    As light it's always at c. It's the absorption and emission which "slows it down" (but it's not "travelling light" per se while it's doing that bit).
    Thanks
    That is the "phase velocity" of light that I was confusing with the speed of individual photons,perhaps.
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  21. #20  
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    Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted.
    There's a difference between the speed of light (a constant) and the speed at which it propagates.
    As light it's always at c. It's the absorption and emission which "slows it down" (but it's not "travelling light" per se while it's doing that bit).
    Very interesting..and explained extremely well I might add
    Last edited by sp1der; October 27th, 2017 at 11:01 AM. Reason: numerous typos
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  22. #21  
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by geordief View Post
    Well light travels slower than c depending on the medium.
    Nah.

    Their speed must vary as they encounter different regions,I imagine.
    Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted.
    There's a difference between the speed of light (a constant) and the speed at which it propagates.
    As light it's always at c. It's the absorption and emission which "slows it down" (but it's not "travelling light" per se while it's doing that bit).
    That's incorrect.

    https://youtu.be/CiHN0ZWE5bk?t=4m53s
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  23. #22  
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    Yes I was wondering whether someone would draw attention to that.

    As I understand it, it can't really be described as true absorption and re-emission, since if it were, there would be scattering, which there isn't. The explanation I have been given is that the electric vector of the radiation couples to the medium via its polarisability, and this slows down the phase velocity, though not, I think, the front velocity, which determines the rate of transmission of information.

    But this area is fraught with scope for misunderstanding, so I'd be very happy to hear from a real physicist.
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  24. #23  
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    Quote Originally Posted by exchemist View Post
    Yes I was wondering whether someone would draw attention to that.

    As I understand it, it can't really be described as true absorption and re-emission, since if it were, there would be scattering, which there isn't. The explanation I have been given is that the electric vector of the radiation couples to the medium via its polarisability, and this slows down the phase velocity, though not, I think, the front velocity, which determines the rate of transmission of information.

    But this area is fraught with scope for misunderstanding, so I'd be very happy to hear from a real physicist.
    I always end up talking to you about this and you never seem to remember. Very bizarre!

    https://en.wikipedia.org/wiki/Refrac...ic_explanation
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  25. #24  
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    Quote Originally Posted by Secular Sanity View Post
    Quote Originally Posted by exchemist View Post
    Yes I was wondering whether someone would draw attention to that.

    As I understand it, it can't really be described as true absorption and re-emission, since if it were, there would be scattering, which there isn't. The explanation I have been given is that the electric vector of the radiation couples to the medium via its polarisability, and this slows down the phase velocity, though not, I think, the front velocity, which determines the rate of transmission of information.

    But this area is fraught with scope for misunderstanding, so I'd be very happy to hear from a real physicist.
    I always end up talking to you about this and you never seem to remember. Very bizarre!

    https://en.wikipedia.org/wiki/Refrac...ic_explanation
    Haha, indeed! It's my old age I think. But it is a topic that comes round quite often.
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  26. #25  
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    Quote Originally Posted by exchemist View Post
    Haha, indeed! It's my old age I think. But it is a topic that comes round quite often.
    My first time around . I hope I can keep up.
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