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Thread: Speed of Light

  1. #1 Speed of Light 
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    This question is something to do with causality so here goes.

    I understand the fact that if something travels faster than the speed of light its cause is preceeded by its effect.

    But my question is why does this mean that you cannot travel faster? Can any comparison to the speed of sound be drawn from this?
    Such as if you break the sound barrier for example you see the object before you hear the sound, why can't light be perceived as this also?

    Please let me know what you think!

    If you need a further elaboration just ask.

    Barry


    Thinking of the question is greater than knowing the answer...
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  3. #2  
    Forum Professor river_rat's Avatar
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    The standard problem is this. Suppose you are engaged in mortal combat with the mighty bob, a duel of the old 10 paces and fire variety. Now as a miracle stroke of luck you manage to get the shot off first and hit bob between the eyes, killing him outright. However, I am standing next to bob and in distress i rush over to my faster-then-light (tm) fax machine and send a message to myself just before the shot occurred (which is possible as these two events are space-like separated and thus a faster then light signal can travel between them). I receive this message and push bob out the way before the bullet hits him, sadly it hits me right in the head and kills me outright.

    Now who sent the message back?


    As is often the case with technical subjects we are presented with an unfortunate choice: an explanation that is accurate but incomprehensible, or comprehensible but wrong.
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  4. #3  
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    Quote Originally Posted by river_rat
    Now who sent the message back?
    Bob does, it reads "thanks mate, sorry about the headache"
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  5. #4 Confused 
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    I'm very confused by this. Please excuse me if the following sounds very stupid, but I know close to nothing about science. I've only taken an interest very recently, thanks to SF literature. So the things I'm guessing everyone else finds obvious... are a little confusing. If anyone has a link to a more elaborate discussion of these ideas, could you please post them?

    The standard problem is this. Suppose you are engaged in mortal combat with the mighty bob, a duel of the old 10 paces and fire variety. Now as a miracle stroke of luck you manage to get the shot off first and hit bob between the eyes, killing him outright. However, I am standing next to bob and in distress i rush over to my faster-then-light (tm) fax machine and send a message to myself just before the shot occurred (which is possible as these two events are space-like separated and thus a faster then light signal can travel between them). I receive this message and push bob out the way before the bullet hits him, sadly it hits me right in the head and kills me outright.

    Now who sent the message back?
    As far as I understand, the only relevance is that you want to send a message to yourself a minute ago. Bob sounds like a nice guy, but he's just a good excuse to send yourself a message. I don't get how that's a FTL paradox? I know that time slows down as you speed up, but isn't this a time travel paradox?

    Or do you mean that anything traveling faster than light traverses time and simultaneously creates a separate space-time continuum (one where the bullet didn't hit Bob)?

    I read another similar paradox, which I'm not understanding, if anyone could help me out?

    Imagine a spaceship leaving Earth at a large fraction of the speed of light. As we all know, according to relativity, the spaceship experiences 'time dilation'; that is, to observers on Earth, the spaceships clocks seem to run slow, say by a factor of 2-1. So when a year has passed
    on Earth, the Earth astronomers will observe that the spaceship's clocks say only six months have passed. Remember, this effect is _real_; is will appear even after correcting for propagation delays caused by the finite speed of light.

    But according to relativity, the spaceship's reference frame is just as valid as Earth's. In the ship's frame, the _ship_ is standing still, and Earth is travelling away from them at a large fraction of lightspeed. So the ship observers will observe _Earth_'s clocks as slowed by the same factor of 2-1. Again, this is a _real_ effect (to the extent that the word 'real' has any meaning)_not_ an illusion caused by the finite speed of light.

    How can the ships clocks and the Earths clocks run slower than each other? This is one of the paradoxes of relativity, and I can't really explain it myself. But if you do the math, that's the result you get. Due to the finite speed of light, we never get causality violation in this situation. But what if we can communicate, or teleport, _faster_ than the speed of light?

    For ease of argument, let's suppose we have an instantaneous communicator, an 'ansible'.

    Now, a year after the spaceship leaves Earth, Mission Control fires up their ansible and sends an instantaneous message to the spaceship. 'Hi, how are you?' Due to time dilation, the message arrives after six months have passed on the ship. The ship uses their ansible to reply instantaneously.

    'Were fine, how are you?' But according to the ship's observers, only three months have passed on Earth in the six months they've been in space. So the ship's reply arrives three months after the ship leaves Earth.

    The reply arrives nine months before the message was sent!
    I don't understand why this is a problem of only two speeds and not three? I see "Earth" traveling at a speed slower than light, the ship traveling faster than light, and the message traveling at an infinite speed (since it's delivered instantaneously)?

    I found this in an archive for a now-abandoned mailing list. This is one of five or six posts on FTL paradoxes, all using this same example.

    It's one thing to say you have one thing traveling faster than light (you're only breaking one rule there), another to say that you have a THIRD thing (radio wave, or whatever this "instantaneous communication device" is using to communicate) that can travel at INFINITE speed?

    I understand that the device mentioned above is only an example, but is there any math which would demonstrate a violation of causality if the message was traveling, say, twice as fast as the ship (since it would have to be traveling somewhat faster, or it would perpetually lag behind unless the ship slowed down to receive it), instead of "instantaneously"?

    As I said, please excuse me if I'm sounding stupid here, but it seems absurd to say that you have that you have a ship traveling faster than light... and also, AT&T have installed the perfect phone line.
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  6. #5 Re: Speed of Light 
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    Quote Originally Posted by Barry Flannery
    But my question is why does this mean that you cannot travel faster? Can any comparison to the speed of sound be drawn from this?
    The speed of light is simply the fastest speed a particle can travel through free space. It has little to do with the particle and everything to do with the permeability and permittivity of space itself.

    Such as if you break the sound barrier for example you see the object before you hear the sound, why can't light be perceived as this also?
    You are talking about two completely different concepts, hence you shouldn't expect to see the same results.

    Sound waves have more to do with the 'compression' of particles in a medium caused by a transfer of energy. Sound waves will have different speeds through different mediums.
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