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Getting to the speed of light

  1. May 17, 2009 #1
    Hello World!

    I'm new to the forums and have been pondering something for about a year now. I made up this theory to travel at the speed of light. I'm only in eighth grade and will be taking Advanced Physics next year so I don't know much right now but here it goes :smile: My theory is this:

    You take a conveyor belt that can move extremely fast. Lets say 1000 mph. now take another conveyor belt that travels really fast and stack it on top of the first one. So now relative to the ground, your traveling 2000 mph (I'm pretty sure anyway). Continue by stacking conveyor belts until your very close to the speed of light. Now put a fast train on top of all the conveyor belts then get in.

    Would you be traveling at the speed of light. Please explain. You could also consider that the conveyor belts are very light weight.
  2. jcsd
  3. May 17, 2009 #2
    No, you wouldn't.

    The specifics of your example don't really matter. At low speeds you use what is called "Galilean Relativity": the idea that you can add up or subtract speeds. If you are in a car going 75 MPH relative to the ground, and a car passes you going 80 MPH relative to the ground, you would see that car going 5 MPH past you. (80-75). Likewise, if you were on a train going 40 MPH and you were on top of it running 5 MPH relative to the train, to an outside observer you would appear to be running 45 MPH relative to the ground.

    When things get close to the speed of light, this adding doesn't work anymore. No matter how fast you chase a beam of light, it will always be going the same speed. This implies that no matter how much you try, you will never go the speed of light. It's the 'cosmic speed limit' so to speak. You just can't go that fast!

    So with your conveyor belts, you were adding up the speeds using Galilean relativity. But it doesn't work near the speed of light. The rules change, so to speak.
  4. May 17, 2009 #3
    Oh, so if I was going 99% the speed of light, would light still be passing me at the speed of light?
  5. May 17, 2009 #4


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    Yes. Weird huh? The speed of light is constant no matter how fast you're going relative to whatever frame of reference you want to think of.
  6. May 17, 2009 #5
    Whoa, that is cool. I seriously cant wait to take physics next year!
  7. May 18, 2009 #6
    You could simplify the problem (as indeed is often posed as a teaser in physics textbooks) and say that you're going to stand on the conveyor belt with a torch and then switch the torch on whilst pointing it in the direction of motion.

    If the conveyor belt is going at 100mph then the light coming out of the torch should be c + 100mph. Er... except it doesn't.

    An observer on the ground would measure the speed of light coming out of the torch as c, and the observer on the conveyor belt would also measure it as c. If you think that's freaky wait until you've done more stuff on relativity.

    In the meantime, consider this teaser :

    A lighthouse is positioned in the middle of a huge dark circular room. The turret atop the lighthouse spins faster and faster until the speed of the moving spot of light on the wall reaches the speed of light. Then the lighthouse turret is cranked up to spin even faster. What happens to the speed of the spot of light on the wall ?
  8. May 18, 2009 #7
    Thats a good one :P
  9. May 18, 2009 #8
    Hehe, actually the spot of light on the wall isn't a physical thing in itself, so there is no reason why it cannot travel faster than the speed of light.
    For the same reason, some oscilloscopes can rightly claim writing speeds greater than the speed of light (since the spot on the screen isn't an actual "thing")

    Now, let's raise the question up a notch.

    I position one guy against the wall and give him a laser gun. I place a second guy diametrically across from him against the wall at the other side.

    I say to the first guy "when you are illuminated by the spot of light from the lighthouse, shoot across at the other guy"
    I then say to the second guy "when you are illuminated by the spot of light from the lighthouse - duck !!!"

    I then spin the lighthouse turret around so the spot travels faster than the speed of light.

    Now the *second* guy will get information about the *first* guy pulling the trigger in less time than it takes light to travel across the gap, so he will be able to duck in time.

    But isn't that against the laws of relativity ?
  10. May 18, 2009 #9
    Yes, right?
  11. May 18, 2009 #10
    Not exactly. It only works because you "pre-arranged" everything beforehand by telling the two guys all the relevant information *before* you set the lighthouse going.

    So the information already existed for the two guys before the lighthouse started spinning - it didn't need to travel anywhere.
  12. May 18, 2009 #11
    You guy forgot E=MC^2 of einstein... ^^
    No u can't get to the speed of light cause according to Einstein's special theory of relativity, objects gain mass as they accelerate to greater and greater speeds. Now, to get an object to move faster, you need to give it some sort of push. An object that has more mass needs a bigger push than an object with less mass. If an object reached the speed of light, it would have an infinite amount of mass and need an infinite amount of push, or acceleration, to keep it moving. No rocket engine, no matter how powerful, could do this. In fact, as far as we know, nothing can exceed the speed of light.
  13. May 19, 2009 #12
    Thats true but there are many things moving much less than the speed of light so I thought that they could add up.
  14. May 19, 2009 #13

    Ok if that theory is accurate how do you explain this:

    If one particle of light leaves its destination then another leaves 1 second after, the first beam of light will see the other beam of light travelling past it, if that is so then how do you explain this? the beam of light is already travelling at the speed of light so is the other beam....if what your saying is true, then the other beam of light would be travelling faster than the speed of light, say for instance 2x the speed of light or whatever.

    What your saying is a highly improbable but not impossible.
  15. May 19, 2009 #14
    No, in SR you just cant use a frame moving at c.
    Any other frame - yes, but not moving at c.
    You can not reach c, and you cant (even theoretically) speak about 'what would I see if I was a photon'
  16. May 19, 2009 #15
    I dont understand your responce because you stated if you were moving at the speed of light you would see a beam of light moving past you at the speed of light, how can you if you are already travelling at the speed of light?

    If what you are saying is an experiment conducted using light then how did they explain the dilemma that came out of the experiment? and if they can't explain it, i believe they call that a theory ;)

    :D i love Physics! :D
  17. May 19, 2009 #16
    Please read my post carefully
    I stated the contrary, I stated that you CANT move at speed of light.
    And you CANT use any reference frame moving at c
  18. May 19, 2009 #17
    Oh ok, so why cant we use a frame moving at C?
  19. May 19, 2009 #18
    Physically, for the same reason why you cant reach speed of light. If you cant reach it, there are no observers (even theoretically) moving at c

    Pure mathematically if you try to calculate what you would observe moving at c, you get /0 in almost every formala
  20. May 19, 2009 #19
    Adopting a frame moving at c would cause dividing-by-zero type absurdities. The first of which being that space would contract 100%. How can you be moving at c if theres nowhere to go? xD
  21. May 19, 2009 #20
    Ok i understand not being able to move an object at c but just because you can't observe it does'nt mean its not happening.

    But you do agree moving at c then another beam of light moving at c 1 second later, they would be both travelling at the speed of light but one beam of light behind at 1 second delay, your previous theory says the light would catch up (irrespective weather there is an observer) light is light, nothing changes in respects to how fast it travels.

    My point being is the theory of light with an observer light then overtaking you is flawed.
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