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  1.  
    At the risk of appearing like a brown-nosed snivelling toady, this is my favourite post of the day.
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      CommentAuthorAngus
    • CommentTimeJun 3rd 2013
     
    If you are referring to post 241 in the Galaxy Classification thread, I'd say "vain, conceited snob" rather than "brown-nosed snivelling toady". But surely you meant something else.

    By the way, this is my favourite post of the day.
  2.  
    Posted By: alsetalokinDark matter, humbug. I've discovered something much better, it-doesn't-matter, or IDM for short. IDM pervades the known Universe, yet doesn't interact in any way with anything, except to make it more dull. Many terrestrial objects, people and spacetime events are composed entirely


    This one.
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      CommentAuthorAngus
    • CommentTimeJun 3rd 2013
     
    Ah - properly quoted, I see. Much improved.

    242 is still my favourite.
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      CommentAuthorTrim
    • CommentTimeJun 5th 2013
     
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      CommentAuthorDuracell
    • CommentTimeJun 5th 2013 edited
     
    Posted By: TrimNS

    They have phoned me at last.

    Mysterious radio waves emitted from nearby galaxy


    From the article:Yet it does seem to be moving – and fast: its apparent sideways velocity is four times the speed of light. Such apparent "superluminal" motion has been seen before in high-speed jets of material squirted out by some black holes. The stuff in these jets is moving towards us at a slight angle and travelling at a fair fraction of the speed of light, and the effects of relativity produce a kind of optical illusion that makes the motion appear superluminal.


    Hmmm ... the "nearby" galaxy M82 is currently 12 million light years away from here, so how long ago were those radio waves emitted? Was it at least 12 million years ago because they are only "travelling at a fair fraction of the speed of light", or was it only 3 million years ago because their "apparent sideways velocity is four times the speed of light"?

    ETA: D'oh! I presume it must be the former ...
    • CommentAuthorloreman
    • CommentTimeJun 5th 2013
     
    Pardon this poor scientific illiterate for asking, but how does "motion appear super luminal"?
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      CommentAuthorDuracell
    • CommentTimeJun 5th 2013 edited
     
    If 2 objects were moving towards each other at more than 50% of the speed of light then wouldn't the motion of one of those objects appear to be super luminal to an observer on the other object?
    • CommentAuthorloreman
    • CommentTimeJun 5th 2013 edited
     
    But how could it "appear" superluminal? What does superluminal look like?
  3.  
    Posted By: loremanPardon this poor scientific illiterate for asking, but how does "motion appear super luminal"?
    Simple example - if you had a laser bright enough to see its reflection off the moon from here, then with a simple flick of the wrist you could have the spot appear to traverse the entire moon equator from one side to the other in milliseconds, and that would appear to be as fast as a superluminal beam. The numbers are probably off, but you get the idea.
  4.  
    Posted By: DuracellIf 2 objects were moving towards each other at more than 50% of the speed of light then wouldn't the motion of one of those objects appear to be super luminal to an observer on the other object?
    That's the core idea which makes special relativity non-intuitive, and the answer's No. The addition of velocities in SR isn't Galilean; it's not simply v1+v2. That was 1905, and everything common sense and intuitive has slowly been leaking out of physics ever since. Now we have a big puddle of failed common sense on the floor, and up where the physics is, a big shiny ball of what-the-fuck-over.
    • CommentAuthorloreman
    • CommentTimeJun 5th 2013 edited
     
    But the motion doesn't "appear" superluminal. That's a conclusion drawn from subluminal observations.
  5.  
    Posted By: loremanBut themotiondoesn't "appear" superluminal. That's a conclusion drawn from subluminal observations.
    Sure it does. The spot from the beam appears to traverse a huge distance in a tiny time. Think lighthouse. Even better, think cosmic lighthouse - a pulsar. We can see them from thousands of light years away, and some of them rotate so fast that the apparent velocity of the beam around an imaginary circumference is many orders of magnitude greater than c at that sort of distance.
    • CommentAuthorloreman
    • CommentTimeJun 5th 2013
     
    Posted By: Andrew Palfreyman
    Posted By: loremanBut themotiondoesn't "appear" superluminal. That's a conclusion drawn from subluminal observations.
    Sure it does. The spot from the beamappearsto traverse a huge distance in a tiny time. Think lighthouse. Even better, think cosmic lighthouse - a pulsar.


    But that's not the motion itself that appears superluminal. As you say, that's a conclusion based on some sublight observations.
  6.  
    Imagine you could build a screen circumferentially around a pulsar R=1000 LY distant. The circumference is 2*PI*R light YEARS all the way around, and the beam gets around this racetrack in, say, 0.1 seconds. That's about 10^12 times faster than c. But it's apparent motion, of course.
    • CommentAuthorloreman
    • CommentTimeJun 5th 2013
     
    You can't see superluminal motion though.
  7.  
    That's because formally that's a semantic null statement. Put otherwise, it doesn't exist in the "real" world. Theoretically, in TachyonVille, it can, but then you would have to multiply yourself and everything else by the square root of minus one. Nobody knows how to do that. In that world, everything would travel slightly faster than c up to infinity. I can't imagine it.

    ETA: Actually, you could see the effects of it though. You would simply record the time it took to traverse between two measuring posts.
    • CommentAuthorloreman
    • CommentTimeJun 5th 2013
     
    Yes, you can see the effects which lead to the conclusion that there's been superluminal motion. However, the motion itself does not appear superluminal. And it's goodnight from me, off up woody hill to blanket bay.
    • CommentAuthorloreman
    • CommentTimeJun 5th 2013
     
    Yes, you can see the effects which lead to the conclusion that there's been superluminal motion. However, the motion itself does not appear superluminal. And it's goodnight from me, off up woody hill to blanket bay.
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      CommentAuthorDuracell
    • CommentTimeJun 5th 2013
     
    Posted By: Andrew Palfreyman
    Posted By: DuracellIf 2 objects were moving towards each other at more than 50% of the speed of light then wouldn't the motion of one of those objects appear to be super luminal to an observer on the other object?
    That's the core idea which makes special relativity non-intuitive, and the answer's No. The addition of velocities in SR isn't Galilean; it's not simply v1+v2. That was 1905, and everything common sense and intuitive has slowly been leaking out of physics ever since. Now we have a big puddle of failed common sense on the floor, and up where the physics is, a big shiny ball of what-the-fuck-over.


    OK (and yes - I know I will probably regret asking this!), but if 2 objects were travelling towards each other at 95% of the speed of the light, then from the point of view of an observer on one of the objects, what speed would the other object appear to be travelling towards her at?