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Rediculous questions on light speed

  1. Dec 4, 2009 #1
    just a fun couple of questions that seem interesting. but redicoulous.

    say you could, and i know you can't, but say you could make an object travel faster than the speed of light.

    what would happen when you hit the light barrier?

    and if the object was traveling at 2x light speed,and it stopped when it got to you, would it appear to move backwards when the light got to you?


    just a couple of ideas, but honestly, its not important.
     
  2. jcsd
  3. Dec 4, 2009 #2

    Redbelly98

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    A better (and realistic) question to ask is, what happens when the object's speed approaches the light barrier?

    Simple answer: it's mass increases, thus increasing it's resistance to acceleration. So less acceleration is produced by whatever force is accelerating the mass. That acceleration becomes less and less as time goes on (because of the increasing mass as the speed approches c), in such a way that the object in fact never reaches the speed of light.

    Yet another explanation: at the speed of light, the equations that describe what is going on produce meaningless answers.
     
  4. Dec 4, 2009 #3

    DrGreg

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    In another thread, someone gave a good answer to all "rediculous" questions of this sort:
    ___
    correct spelling: ridiculous
     
  5. Dec 4, 2009 #4

    Dale

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    Darn, I was going to post the exact same quote but DrGreg beat me to it.
     
  6. Dec 4, 2009 #5
    im just gonna stick a question in here if its ok? ok then..

    is there a reason why light travels at a speed which it does? now i know it has to be 'some value' but is there any explanation why its 186000mile/hr in a vacuum?
     
  7. Dec 4, 2009 #6

    arildno

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    You can't MAKE particles travel faster than light, but the question is still open whether there might exist particles that HAVE speeds greater than light speed.

    Such (as yet) hypothetical particles are called tachyons.

    Tachyons would, for example, have the fascinating property that their velocity increased if their energy decreased..
     
  8. Dec 4, 2009 #7
    As far as I know, there isn't any particular reason. The speed of light is determined by the electric permittivity and magnetic permeability of free space. But really that doesn't answer your question, because then you'd just start wondering why these two constants are set the way they are. At some level, the values of c, h, and G are simply intrinsic to our universe.

    Inflationary theory tells us that the universe settled into a specific quantum state due to vacuum fluctuations that expanded during the inflationary period. I know this explains why the universe isn't perfectly homogenous, but maybe it also determines the values of physical constants. Of course my only knowledge of cosmology comes from a senior undergrad course, so I'm not really the person to talk to about that.
     
  9. Dec 4, 2009 #8

    Dale

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    Sure, the value of any dimensionful constant is a reflection of the system of units. So it has that value because of how long we chose a mile to be and how long we chose an hour to be.
     
  10. Dec 6, 2009 #9
    I think Einstein used to theorize that as you were at the speed of light, your length would decrease to 0 and the time would stop for you.
    Now comes my theory which is obviously stupid.
    Moving at 2x the speed of light, if you could, you would be going back in time as fast as time would pass for you at the speed of 0.

    Feel free to destroy my theory, its me learning here.
     
  11. Dec 6, 2009 #10
    As far as i know there's no logical reason whatsoever, and Einstein never offered an explanation either.

    About the 'what happens at the speed of light barrier', from your own pespective you're mass stays the same so there's no reason you can't cross over the barrier. Only problem is that any light kicking around outside your spaceship will bombard the front windshield with infinite intensity, and each photon will pack an infinite punch. So even if your windshield is indestructible, the force hitting the windshield will bring the ship to a standstill. A bit like infinite air resistance - only from photons instead of air particles.
     
    Last edited: Dec 6, 2009
  12. Dec 6, 2009 #11

    DrGreg

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    Trying to accelerate to the speed of light is a bit like trying to reach the horizon (on Earth). To an outside observer it looks as though you are getting nearer, but from your own point of view it's just as far away as ever.

    Even though someone else would say you are travelling at 99.999% of c, the speed of light, from your own perspective the speed of light is still c (relative to yourself) and you are no nearer to it.
     
  13. Dec 6, 2009 #12
    - If someone else thinks you're tavelling at 99.999% of the speed of light, I think you would also notice the effect of tremendous light enegy hitting you in the face, even though that light is still travelling at c.

    So with that in mind, you can say that motion is absolute, not just relative. You would definitely know that you were getting close to c.
     
  14. Dec 6, 2009 #13

    Integral

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    Ya' Think? The fact is we ARE traveling at a large fraction of c with respect to distant galaxy's. Do you feel any wind in your face?
     
  15. Dec 6, 2009 #14
    I don't think at all, I just quote Einstein.
     
  16. Dec 6, 2009 #15

    {~}

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    The increase in light energy is actually an increase in frequency. As you begin the have a non negligible speed with respect to the normal "random" motion of stars and galaxies you will begin to notice a visible blue-shift of the sky in front of you, and a redshift to your aft if you can see beyond the glare of the engines. This due to the Doppler effect of waves.

    The chromatic effect becomes more intense at greater speeds. The once visible part of the spectrum turns to harmful radiation of higher frequencies and the lower frequencies of infrared and microwaves now become visible light. To your aft you might see x-ray and gamma sources now in the visible spectrum. You will begin to notice that you cant see anything to the sides of the ship.

    Near the speed of light all the glory of the firmament in front of you is now transmitting deadly super intense radiation from one small bright place in the sky. To your aft you aren't likely to see anything. What light is reaching you is now on the ultra low frequencies and probably need huge superconducting antennae just to detect.

    As you continue to accelerate the intensity of the bombarding radiation will become infinite, according to classical relativity anyway.

    There is a proposed plank temperature which is so hot that it makes stars seem like ice balls by comparison. But it is a limit nonetheless. Light has temperature just like matter.

    My question,
    What happens according to quantum theory as you approach the speed of light and the bombarding light hitting your spaceship reach the maximum possible temperature?
     
  17. Dec 6, 2009 #16

    {~}

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    In a sense you are right as astronomers use the principles of the Doppler shift whenever they determine the relative motions of stars and galaxies, but we are talking here of much greater speeds
     
  18. Dec 6, 2009 #17
    it's the same thing. Light travells at c
     
  19. Dec 6, 2009 #18

    {~}

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    Not obvious to everyone, besides light changing color is more wondrous to ponder than light increasing in energy.
     
  20. Dec 6, 2009 #19

    Integral

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    Then you need to read Einstein more and understand that there is no absolute frame of reference.This is one of Einstein's 2 fundamental postulates. The laws of physics are the same in every constant velocity frame of reference. You are implying that there is some measurement that can be made which will depend upon your speed. This simply is not the case.
     
  21. Apr 17, 2010 #20
    It seems that what you are asking is that if you disregarded the theory of special relativity, and travelled at twice the speed of light, then what would happen according to the theory of special relativity?

    I am unable to answer this question as the limitations specified are paradoxical. However travelling at the speed of light (according to the theory of special relativity) presents many impossibilities:
    Because of γ=1/sqrt(1-(v²/c²)), travelling at the speed of light would multiply your mass by the factor of infinity, you would infinitesimally Lorentz-contract, and an infinite amount of time would pass externally regardless of how long you sustain light speed for. If you had a mass to start with, because it becomes infinite and because of E=mc², you would require infinite energy to accelerate to this speed. (Note that all these calculations assumes that division by zero equals infinity, although I think it does in this context).
    All these things would happen at the speed of light. If you travelled at "99.999% of c", the differences are much more plausible. You would be Lorentz-contracted to the factor of 0.00002584569597, and your mass would be multiplied by 38691.16163. Also, one day at this speed would equal 106 years of time dilation.
     
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