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You can't exceed the speed of light

  1. Aug 25, 2009 #1
    "You can't exceed the speed of light"

    Ya so...forgive my ignorance but this question has been bugging me.

    Why is there a cap on the speed a particle/organism can travel through space?

    If there were a supernova on Alpha Centauri just at the second you were clipping your toenails, but both those things happened at the same time; the only thing keeping us from knowing that was that information hindered by the speed cap of photons kept us from realizing it. But that doesn't mean that the two things didn't happen at the same time, in kind of a universal objective space-time. Information will always exist independently of one's subjective capacity to receive it. Speed of light or not, that supernova happened just as you were clipping your toenails. Just because your great-great-great-great-great-great-great-great-grandson was the only one to hear about it, once information in the form of light particles reached him, doesn't mean it didn't happen when it happened. So the only real limit in space is our means of information retrieval, which at the moment is dictated by the fastest known moving thing - photons. I don't see how that has anything to do with time, or aging.
  2. jcsd
  3. Aug 25, 2009 #2
    Re: "You can't exceed the speed of light"

    Hi there,

    To answer your question simply, take a look at Maxwell's equation. They will show and explain the reasons why the speed of light is the upper limit.

    Secondly, things happening at the "same" time is irrelevant here. You need to look at the special relativity theory. You will then see that simultanous is a matter of frame. Things can happen at the "same" time in your frame, but not in the next one.

  4. Aug 25, 2009 #3
    Re: "You can't exceed the speed of light"

    Just a slight correction. Alpha Centauri is only a little over four light years away, so you'd know about it before your grandchildren.

    We'll probably never fully know why light speed has the value it has, but we've noticed some attributes of the universe that seem to be closely related. Classically, like fatra2 said, Maxwell derived the speed of light from some relationships between magnetic and electric fields. Obviously that doesn't explain it "fully", but it gives a bigger picture. Then there's also some interesting things we're only now realizing about information transfer non-classically (i.e. between a given pair of particles). For example, they don't seem to care as much about light speed. Its as if they don't mind breaking the limit as long as they obey it on the average (a large group of particles will obey it generally), and don't introduce any paradoxes. See quantum entanglement.
  5. Aug 25, 2009 #4
    Re: "You can't exceed the speed of light"

    Maxwell equations give nothing about a "limit". The limit is due to the fact that particles gain more mass upon speeding, the fact that Einstein discovered through special relativity, where he assumed that how much ever fast we were moving, we will keep seeing the speed of light as it's, not like when you drive your car and a car passes you, the relative speed between you and it is something smaller than the relative speed between that car and someone watching it from the street standing there, and therefore if it hits you, it won't cause a lot of damage, but if it hits him (even if he was in the car), the damage would be more, because the "difference" in speed is greater. So what relativity says, is that the difference between your speed and the speed of light is always constant.

    Some details:

    at rest, let's call the mass at rest m(0), and its speed v and the speed of light c, then if you're standing still, and the mass is moving in front of you, you'll see that its mass is


    sqrt means square root, v^2 is the square of v. If you think of this equation, you'll find that the closer the particle gets to the speed of light, the higher its mass will be, and so according to Newton's second law (F=ma, F fource, m mass, a acceleration), you'll find that we need more force to accelerate the particle, and here the competing starts, you accelerate more, and the particle gets heavier and heavier, and therefore theoretically you'll need infinite force to accelerate the particle to the speed of light, which happens only when if we (impossible, but some fancy argument) stop time, because you'll be changing momentum of this particle during 0 seconds (F=P/t, P=mv, P is momentum, another form of Newton's second law), a very small time. For this reason we say that in the frame of photons (where photons live) time stops.

    I answered your question and tried to give some deeper idea about relativity.

    YES, we are very ignorant, we know nothing about this universe, the only way we learn things here is through light, and so even if we discover some intelligent life in some other galaxy, we won't be able to communicate, because our signal will be received by them at least few years later.

    Hope this gives you what you want :) if you have any other questions just ask.

    Good luck :)
  6. Aug 25, 2009 #5


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    Re: "You can't exceed the speed of light"

    If you are going to accept relativity (no speed faster than light) accept all of it. Whether or not two things, at different places, happen at the same time (simultaneity) is dependent on the frame of reference. There is no "universal objective space-time".

    Yes, and what the theory of relativity really says is that information cannot be transmitted faster than the speed of light

    Then read a good, introductory, book on relativity.
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