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"Measured" velocities?

  1. Aug 31, 2014 #1
    I was reading Isaac Asimov's introductory physics text "Understanding Physics". In the chapter on Relativity, he mentions in a footnote:

    I'm not sure what he means by this. Could anyone elaborate a little on this? I'll really appreciate it!
     
  2. jcsd
  3. Aug 31, 2014 #2
    That's rubbish; if the impossible were suddenly possible, and something were to approach us at such a ludicrous speed we would just see it after it had passed us, like we do with the sound of supersonic objects.
     
  4. Aug 31, 2014 #3

    PeterDonis

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    I'm not sure what he means either; it's been a *long* time since I read this book. :wink: But I *think* he may have been referring to the fact that it's not really possible to construct a theory of interacting tachyons (objects that move faster than light), and if you can't interact with them, you can't measure them. I can't be sure, though.
     
  5. Aug 31, 2014 #4

    PeterDonis

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    This is true provided the object can emit light (or at least something that we can detect). But that may not be possible; see my post in response to the OP.
     
  6. Aug 31, 2014 #5

    Orodruin

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    I would hope that he is referring to that the distance between objects can grow faster than the speed of light (i.e., expanding universe). But as it stands I would say this is impossible to tell.
     
  7. Aug 31, 2014 #6
    Yeah, there's no way to follow the logic, he's postulating something that is not detectable, so it ain't physics.
     
  8. Aug 31, 2014 #7

    D H

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    It's a bit hard to tell what Asimov was writing about with what is perhaps a quote out of context. That said, what Asimov wrote is wrong, but not for the reasons cited above. What's wrong is that we can and do see objects moving faster than the speed of light. Extremely distant objects have a recession velocity greater than the speed of light. And yet we do see them.
     
  9. Aug 31, 2014 #8

    PeterDonis

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    Hm, yes, it's possible Asimov was referring to this. I don't know if I still have my copy of the book to check the context.
     
  10. Aug 31, 2014 #9

    Matterwave

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    A recessional velocity is not a real velocity in the sense of the word "velocity" I would argue though...and nowhere do we measure their velocities either, we just measure their redshifts, and it happens to be z>1, which, if we naively plug into the non-relativistic Doppler shift formula gives us a v>c.

    @OP: objects which do not interact at all are hidden to us since by definition we can never observe them. Such objects are beyond the realm of physics.

    But if they DO have some interactions (perhaps even just gravitationally, like the dark matter for example) then we might be able to start talking about physics.
     
  11. Sep 1, 2014 #10
    Just in case anybody wants to see it in full context for themselves, you can find it on Page#104, Volume 2 of the book. (a pdf version of the book is easily available on the internet). I remember a few pages back another instance where he puts emphasis on the word "measured velocity" for some reason.

    Thanks for your response everyone!
     
    Last edited: Sep 1, 2014
  12. Sep 1, 2014 #11
    That's the first thing that came to my mind too. But yeah, can't say for sure. It's too vague.
     
  13. Sep 1, 2014 #12

    Dale

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    Hold on, Asimov is a science fiction writer. Did he also write a textbook? That is odd. Without context the comment sounds like a misunderstanding that may have been a plot element so he believes it is true.
     
  14. Sep 1, 2014 #13
    Asimov wrote a lot of material on factual science too and he was a PhD in biochemistry. This textbook I'm talking about should have anyone believe he were a PhD in Physics, though. Yeah, it's introductory with minimal math, but he delivers it with such clarity that it's not difficult to see that it's coming from a man of deep understanding.

    I would say he was the most scientifically well-read sci-fi writer. So if it's coming from him, I'd take it seriously. That's not to say he can't be wrong, though.
     
    Last edited: Sep 1, 2014
  15. Sep 1, 2014 #14

    Dale

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    I always liked his science fiction.

    I read the thing in context and I think he is simply wrong. The context is the velocity addition formula, and superluminal particles are certainly consistent with the formula. If it were in a context like showing why charge is only compatible with a massive particle, then that might have been valid.
     
  16. Sep 1, 2014 #15

    Doc Al

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    Asimov was extremely prolific. We're talking hundreds of books, including several popular science classics. Some of his stuff was excellent.
     
  17. Sep 1, 2014 #16

    PAllen

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    But a recession velocity is NOT a relative velocity. It is a growth of proper distance in a given foliation, which makes it unambiguously a separation speed - which can trivially exceed c. As I am sure you agree, relative velocity at a distance is undefined (or, at least, fundamentally ambiguous) in GR. [But if you define it via comparing vectors after parallel transport, it is always < c for timelike vectors].
     
  18. Sep 1, 2014 #17
    It's, largely, Feynman's Lectures on Physics, simplified for high school students (and almost as old!) Asimov does reference Feynman, at least. Not a bad thing, by the way, I remember being entranced by it when I encountered it in my Public Library at the age of 14. Just be careful though, he was a chemist, so he may not get all the physics right. Actually, is there anything else that does as good a job for 14 year olds? I certainly haven't seen anything. It's about time a physics professor had a go at doing something like this. It's a bit embarrassing to be outdone by a chemist? :devil:
     
  19. Sep 1, 2014 #18
    On the subject of causality, and just to point out I'm not knocking the guy, many years ago I enjoyed his "paper" on "resublimated thiotimoline", http://en.wikipedia.org/wiki/Thiotimoline ;)
    I don't think his teachers saw the funny side, though.
     
  20. Sep 1, 2014 #19

    PeterDonis

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    Understanding Physics isn't quite a textbook; it's more like a book for the serious lay person, who isn't going to get a degree in physics but wants to understand it at a level beyond the usual "pop science" presentation. Asimov wrote *lots* of books and articles about science along these same lines; for example, I first learned about biochemistry and the Krebs cycle from his book Life and Energy. So yes, he was a science fiction writer, but he wasn't *just* a science fiction writer.
     
  21. Sep 1, 2014 #20

    PeterDonis

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    Is the context anywhere online?

    Yes, that's true; but I still wonder if he had some (quite possibly mistaken) idea about tachyons in the back of his mind. The paper by Bilianuk and Sudarshan about the possibility of faster than light particles was published in 1962, four years before this book (although the term "tachyon" wasn't coined until 1967, according to Wikipedia).
     
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