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 Zophar's Message Domain speed of light

 09-10-2007, 12:16 PM #11 you Senior Member   Join Date: Apr 2007 Posts: 109 Re: speed of light > As for why spacetime has a hyperbolic geometry, I'm not sure > anyone could tell you. It just does. > The Minkowski metric might have something to do with it (ds^2 = dx^2 + dy^2 + dz^2 - dt^2)

 09-10-2007, 03:14 PM #12 SpaceTiger Senior Member   Join Date: Feb 2003 Posts: 4,548 Re: speed of light > The Minkowski metric might have something to do with it > (ds^2 = dx^2 + dy^2 + dz^2 - dt^2) The metric is a means of specifying the geometry of spacetime, so that's just a more mathematically precise way of saying what I said in the first paragraph. The version you give sets c=1, but when not using natural units, the last term is cdt^2. To see that one's speed is limited to c, try computing the interval, ds, in the case, dx/dt>c, dy/dt=0, and dz/dt=0. The unanswerable question right now is why the universe obeys general relativity (for which Minkowski space is a limiting case). Until we have a more unified theory of the fundamental forces, that will have to remain a mystery.

 09-10-2007, 08:39 PM #13 Danoz Senior Member   Join Date: Oct 2003 Posts: 4,903 Re: speed of light I wish I weren't so confused by that. Special theory of relativity never made any sense to me at all. What you're saying here about infinite mass seems to make sense, but why is time relative? Why does your speed have anything to do with the time that passes? If speed and time are inexorably intertwined, but speed has limits... does time even exist at the speed of light? See. I'm confused.

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 09-11-2007, 03:53 AM #14 SpaceTiger Senior Member   Join Date: Feb 2003 Posts: 4,548 Re: speed of light > Special theory of relativity never made any sense to me at > all. What you're saying here about infinite mass seems to > make sense, but why is time relative? If light appears to be moving at the same speed, regardless of how fast I'm moving, something funny has to be going on. It's like that slasher flick effect where, no matter how fast the delirious teenage girl is running, the psycho always seems to be catching up to her. The only way to explain this, it turns out, is to say that time has to be moving at a different rate depending on how fast I'm moving. Of course, as the name implies, all of this is relative and there is no such thing as "absolute" time, but you can still compare the measurements of clocks in different reference frames. So to go back to the slasher flick analogy, if you're the teenage girl, you can run as fast as you want, but you'll always see the slasher moving towards you at the same rate. However, someone standing to the side watching the chase will see the slasher moving at this rate as well (like light, slashers always move at the same speed). As you speed up to try and get away from the slasher, the observer will see you approach (but never reach) his speed. You can't run faster than a slasher, it's bad for ticket sales. However, the observer does see you running away from the slasher, so his rate of gain will have to be less than you observe. Although you are delirious with fear, your eyes aren't deceiving you, your clock is just running more slowly than that of the curious bystander, an effect called time dilation. Observations of this effect were first made at the end of the 19th century when physicists measured the speed of light at different points in the earth's orbit. Long story short: don't even both running, the laws of physics (and horror movies) are already conspring against you.

 09-11-2007, 07:32 AM #15 pipes Senior Member     Join Date: May 2003 Posts: 1,883 Re: speed of light so say I had a spaceship and saw some star a lightyear away if I can make it point b and took my ship from point a to that b in like a minute would I go back in time or would I just watch the star go through a year in a minute?

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 09-11-2007, 11:18 AM #16 SpaceTiger Senior Member   Join Date: Feb 2003 Posts: 4,548 Re: speed of light > so say I had a spaceship and saw some star a lightyear away > if I can make it point b and took my ship from point a to > that b in like a minute would I go back in time or would I > just watch the star go through a year in a minute? If point b is at the star, then you can't reach it in a minute.

 09-11-2007, 11:48 PM #17 you Senior Member   Join Date: Apr 2007 Posts: 109 Re: speed of light > If point b is at the star, then you can't reach it in a > minute. Assuming that all world lines are time-like. He's asking what he'd see if he were on a world line that's space-like

 09-12-2007, 01:14 AM #18 SpaceTiger Senior Member   Join Date: Feb 2003 Posts: 4,548 Re: speed of light > Assuming that all world lines are time-like. He's asking > what he'd see if he were on a world line that's space-like To put what I already said in technical terms, there are no space-like world lines. However, I suspect that most folks here haven't taken courses in modern physics, so I don't think there's anything to gain from repeating my explanations in jargon.

 09-12-2007, 09:52 PM #19 you Senior Member   Join Date: Apr 2007 Posts: 109 Re: speed of light > To put what I already said in technical terms, there are no > space-like world lines. You don't seem to hear when we say "what if" In any case, if you explained it in jargon, I probably would've already heard it before. Special Relativity and I have an special relationship, in that I never remember any proofs

 09-12-2007, 11:15 PM #20 SpaceTiger Senior Member   Join Date: Feb 2003 Posts: 4,548 Re: speed of light > You don't seem to hear when we say "what if" Perhaps someone can answer a similar question: What if I created a pointer in Fortran 77? What would happen then?

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