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Thread: Goofy question about light speed

  1. #1
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    Goofy question about light speed

    Forgive the silly question, but I've been trying to puzzle it out. I recently watched Close Encounters of the Third Kind again, an old favorite. In one scene researchers who have been receiving and monitoring the five-note signal are discussing where it could be coming from. One says it is close, "less than seven seconds light travel time." Maybe not verbatim, but pretty close.

    So my question: That would mean the signal is coming from somewhere around 1.3 million miles away. How would it be possible to know how long the signal had been traveling? I could stand by a freeway and watch a car go by at 60 mph, but I have no way of knowing if it had been traveling for the last ten minutes, an hour or ten hours.

    Am I missing something obvious or did the screenwriters just take some creative liberties?

    Thanks.

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    Quote Originally Posted by FramerDave View Post
    Forgive the silly question, but I've been trying to puzzle it out. I recently watched Close Encounters of the Third Kind again, an old favorite. In one scene researchers who have been receiving and monitoring the five-note signal are discussing where it could be coming from. One says it is close, "less than seven seconds light travel time." Maybe not verbatim, but pretty close.

    So my question: That would mean the signal is coming from somewhere around 1.3 million miles away. How would it be possible to know how long the signal had been traveling? I could stand by a freeway and watch a car go by at 60 mph, but I have no way of knowing if it had been traveling for the last ten minutes, an hour or ten hours.

    Am I missing something obvious or did the screenwriters just take some creative liberties?
    I don't remember the scenes from the movie anymore (I saw it in the theater when it first came out). But if your description is correct, I would say they are taking some liberties. First of all, I don't know how you broadcast musical notes with electromagnetic waves, and then second, as you say, if it's photons I don't know why you would know how far away it is. Seven light seconds is somewhat further than the sun, so it can't be broadcast in actual sound, i.e. air pressure waves.
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    maybe the triangulated the source from several receivers....
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    Quote Originally Posted by Jens View Post
    First of all, I don't know how you broadcast musical notes with electromagnetic waves
    Radio?

    I can't remember much about the film (it is in my bottom 3 movies of all time) but could they have been measuring the time to get a reply (which would have been about 14s)?

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    Goofy question about light speed

    I thought the sun was about 8 light minutes from Earth.

    As for sending notes via EM waves, to quote Everclear, ďAM radio, AM radio...Ē

    (Iíll just wander over to the ďSong stuck in my head thread. )


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    Last edited by Extravoice; 2018-Jan-30 at 01:26 PM.
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    Quote Originally Posted by Strange View Post
    I can't remember much about the film (it is in my bottom 3 movies of all time
    I didnít much like the movie when I originally saw it in the theater, but stumbled across it on cable TV recently, and enjoyed it.


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    Quote Originally Posted by FramerDave View Post
    How would it be possible to know how long the signal had been traveling?
    I think you're right, Dave. I can't see how it would be possible. Typically, estimating distance using an electromagnetic signal measures the redshift caused by the expansion of space, er, I mean the difference in reference frames. The redshift for a distance of 7 light-seconds would essentially be undetectable.
    Everyone is entitled to his own opinion, but not his own facts.

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    Ok, thanks for confirming. I'm not missing something obvious, and it looks like the movie either took some liberties or just goofed.

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    They could tell it took 12 parsecs to get there.
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    Quote Originally Posted by Trebuchet View Post
    They could tell it took 12 parsecs to get there.
    IIRC, the many ET craft did seem to be racing each other. A turn marker at 7 light sec. would reveal that distance due to Doppler.
    Last edited by George; 2018-Jan-30 at 04:05 PM.
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    They could of bounced radar off from the craft. The moon is about 1.3 seconds so the craft is about 5 to 6 times further out. The way that they found out how far Venus was is that they bounced a radar signal from Arecibo and before that they know the ratios between the Earth-Venus and Sun so they could now exactly how away the sun was.
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    Quote Originally Posted by Cougar View Post
    I think you're right, Dave. I can't see how it would be possible. Typically, estimating distance using an electromagnetic signal measures the redshift caused by the expansion of space, er, I mean the difference in reference frames. The redshift for a distance of 7 light-seconds would essentially be undetectable.
    It's also vaguely possible if you know how different frequencies are attenuated over distance. I suspect 7 light seconds is far too short a distance for that to be significant.

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    Quote Originally Posted by Trebuchet View Post
    They could tell it took 12 parsecs to get there.
    Yes, 12 parsecs and 16 angstroms.
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    Quote Originally Posted by Jens View Post
    I don't know how you broadcast musical notes with electromagnetic waves,
    By modulating them? Pretty much like AM/FM radio.
    You have a carrier, and then superimpose a frequency on top of it - by modulating either the frequency or amplitude. This produces the actual signal. If that signal pulses between 10 and 15,000 Hz, you have sound.

    Quote Originally Posted by Jens View Post
    if it's photons I don't know why you would know how far away it is.
    They don't go into sufficient detail to tell, but there are ways.

    For all we know, they have more than one tracking station This would allow them to triangulate.


    Quote Originally Posted by Jens View Post
    Seven light seconds is somewhat further than the sun, so it can't be broadcast in actual sound, i.e. air pressure waves.
    Er. The Sun is 8 light minutes away. 7 light seconds is about 1.3 million miles - or about 5x farther than the Moon.

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    Quote Originally Posted by DaveC426913 View Post
    Er. The Sun is 8 light minutes away. 7 light seconds is about 1.3 million miles - or about 5x farther than the Moon.
    Oops. Yeah, I meant the moon. I got a bit confused between those two bright things.
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    Goofy question about light speed

    Quote Originally Posted by Jens View Post
    Oops. Yeah, I meant the moon. I got a bit confused between those two bright things.
    (Cough)Post#5(Cough)

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    Last edited by Extravoice; 2018-Jan-31 at 10:38 AM.
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    Thanks for your answers, everyone. I consider myself a fairly sharp guy, so it's gratifying to know I didn't screw up and miss something painfully obvious.

    I was just going off the scene in the movie, trying not to read too much into it. I thought of the idea of triangulating the signal but there was no mention of any other receivers picking up the signal, although obviously anyone with the right equipment and listening at the right frequency could have picked it up. I had not thought of the simple idea of bouncing a radar signal off it, but again there was no mention of it. Technically, had they done that, then the "light travel time under seven seconds" comment would have been correct, but 1) they made no mention of radar and 2) it would have been much simpler to say so than to resort to a convoluted, sciencey-sounding phrase.

    I'll just file this one away with the Kessel Run and enjoy it as a fun movie. I saw it as a seven-year-old kid when it first came out and it's always been a favorite.

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