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Nothing can travel faster than the speed of light

  1. May 6, 2004 #1
    It's said that nothing can travel faster than the speed of light, can someone explain exactly why this is thought? I know photons have no mass and in a vacuum obviously no resistance, but those aren't the only factors that apply to speed, are they? What about the "propulsion" of light?

    I also read something about that objects decrease in length...or something of the likes...and decreasing to 0 at c. This may be all wrong, but I can't find where I read that. (T'was on howstuffworks, but I can't find the piece) If that's true, I'd appreciate it if someone could explain that to me.
    Last edited by a moderator: Jan 3, 2014
  2. jcsd
  3. May 6, 2004 #2


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    Your questions all relate to special relativity. I suggest you google it or go to the library.
  4. May 7, 2004 #3
    Part of the explanation is that, as one approaches c. the amount of energy required to accelerate you grows exponentially greater for a linear increase in velocity. The Energy required approaches infinity as one approaches the speed of light.

    At least, this is a standard response to this question.

    I am fairly new to special relativity as well. I have Dr. Einstein's book, Relativity. It has helped me to understand these concepts - as has fraternizing on this board.

    BTW, whether supra-light-speed travel is possible or not is a matter of hot debate, even among physicists. But, if you have a background in Newtonian mechanics, you should probably take the step to Relativity and grok that before speculating further.

    Hope this helps.
  5. Jul 9, 2004 #4
    Hi there NanakiXIII,
    First things first, the article your refering to can be found at:
    As demonstrated in the article, if you blast away from earth on a rocket ship, an observer on earth will see that time appears to slow down for you. Length contraction is necessary to balance this effect. Otherwise, a person observing the rocket the rocket travel a certain distance in a certain amount of time would disagree with the person on the rocket about his velocity since he says he travelled the same distance in less time. Even relativity doesn't allow for you to say your moving at a specific speed relative to me yet I say you're travelling at a different speed relative to me. Only if the person on the rocket observes himself to travel less distance (hence all the lengths parallel to the direction of his velocity are decreased) can this be avoided.
    As OneEye said, the reason an object with mass can't accelerate to the speed of light is that as we approach c our relativistic mass increases and it takes more energy to accelerate. You can never get that last little push. The restriction is built into the fabric of spacetime itself.
  6. Jul 17, 2004 #5
    Why can't objects travel at light speed? Maybe because if they could light would not be able to reach them. Therefore how could they exist relative to another mass?
  7. Jul 17, 2004 #6


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    What does light have to do with existence? Are you suggesting that two blind people would not exist "relative to one another"?

    In fact, what do you mean by "existance relative to"?
  8. Jul 18, 2004 #7
    Well, I was thinking that light gave information from one planet to another. If light could not reach planet 2 from planet 1 how would planet 2 know of the existence of planet 1? If it does not know of the existence of planet 1, what is the point of planet 1 existing? It may as well not exist. Or say that planet A is 5 million miles from planet B and 10 million miles from planet C. You can say that planet C is twice as far from planet A than planet B is. Light will take twice as long from A to C than from A to B. Call this a ratio '2 : 1'. This is RELATIVE. Now say that planet C is moving away at lightspeed. Light is sent to planet C by planet A but can never reach C. Call this a ratio 'undefined : 1'. This is NOT RELATIVE (How much bigger is something undefined to 1? There is no relative answer). So in this case A and B and C do not exist relative to each other. This is just my thinking though. I could be wrong.

    note: Light is the interaction of charges. For planets in which charges exist to interact they must be limited to below light speed. There is no other reason for the existence of mass and planets than the interaction of charges....
    Last edited: Jul 18, 2004
  9. Jul 18, 2004 #8


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    Notice that the two planets may be spacelike related at one time (light cannot get from one to the other) but as time passes their future light cones will intersect and Planet 1 can learn about Planet 2's PAST. This is, in fact how we know what we do know about distant galaxies.
  10. Jul 18, 2004 #9
    Apply this to humans... all your massive body counts for nothing. It's only the charges in you that matter really... in your brain, the way you control your body through the nerves, all interactions of +ve and -ve.... producing feedback according to charge interactions... Look at rock. It doesn't 'mean' anything to you. It's just a piece of mass. Sculpt the rock according to charge interactions and suddenly it 'means' something to you... get it?
  11. Jul 19, 2004 #10
    could someone possibly give me a link to a site explaining light cones, or if you dont feel too tired expliain it yourselves, because i have never properly understood the concept.

    Thanks K_
  12. Jul 19, 2004 #11
    Build a plane that can reach a speed of light. But still light is greater by the same amount of speed!
    Do you know what is greater than the speed of light?
    "It is your thinking. But since it have no mass you can't comapre it with speed of light."
  13. Jul 19, 2004 #12


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    Imagine that you are in the space ship traveling with a constant 1 g acceleration. All the time you are accelerating you are performing an experiment to measure the speed of light. Your measurement will not vary from c. How can I say this? We are living on the earth at a constant 1g acceleration and measure c as the speed of light. Thus this is our constant condition.

    Now, since you never get any closer to the speed of light, you will never lose contact with your starting point which is constantly communicating with you at the speed of light. That starting point must be considered our reference point for measurement of velocity. Initially, using Newtons laws to compute your velocity, you will find good agreement that the actual distance you have traveled from home. But as your speed increasing there will appear a noticeable error, your velocity will be lower then that predicted by Newton. The home base, knowing that you are maintaining a constant 1g acceleration, will be thinking that the ship is gaining mass. But you, will know that the food suxs and in fact the crew has lost weight, and the ship has not changed in the least.

    Only if you use the formulas of Special Relativity will your calculations agree with reality. Since Newtons laws are simply LOW VELOCITY approximations to the formulas of Lorentz (and Special Relativity) you must use them to obtain the correct velocity and distance travelers when you velocity with respect to your starting point exceeds about .1c. Of course if you do any local measurements you will find that Newtons laws work fine.

    Key points... Locally you will never record a variation in c, your shipboard flash lights will never cease to function as you expect.

    ... Since, locally, you never exceed or even approach the speed of light, you cannot run away from any source light signals that have been in your frame of reference.

    ... As far as you, the traveler is concerned; no matter how much fuel you expend, you will get NO CLOSER to the speed of light. Since, it is always measured as c.
  14. Jul 20, 2004 #13
    Nothing can travel faster than the speed of light. An important point to understand is that it is also true that nothing can travel slower than the speed of light.

    There is only one speed in space-time, the speed of light.

    The speed of light is constant in space-time. Motion through space-time is an interaction of motion through space with motion through time. When people typically speak of moving at the speed of light, they mean moving at maximum motion through space, and at the minimum rate of motion through time.

    The speed of light is significant because it is light that enables space and time to interact as space-time. It is not possible to move through space at a speed that people typically mean when they say "faster than the speed of light" because time cannot become negative.

    Time, as we know it now bound up in space-time, began after the Big Bang, and can only go forward. Space, therefore, can never exceed this upper bound.
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