Hi, Jigsaw,
I'll answer what I can, and leave the really hard questions to others...
First of all, movies are entertainment, not science education. Rarely do they stick very closely to scientific accuracy, especially movies like this one (which I have not seen, BTW).
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On 2002-02-23 22:50, Jigsaw wrote:
1. Stars emit FM signals? Is it plausible that he'd be the only radioastronomer in the world to think of listening on FM wavelengths? Has anybody actually checked for FM signals from outer space?
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The important thing here is to distinguish FM
frequencies, which are simply the part of the electromagentic spectrum between 88 and 108 MHz, from the FM
modulation scheme, which is a way of encoding meaningful information (e.g. "Roll Over Beethoven") onto a carrier wave.
While stars may emit some radiation at FM frequencies (that would vary according to the type and size of the star), they would not encode any information in the form of frequency modulation.
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2. Stars emit microwaves? Don't laugh, I didn't know that.
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Again, stars emit EM radiation over a wide range of frequencies; depending on the type of star there may be more or less microwave energy. I don't believe either radio or microwave energy makes up much of our sun's output.
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3. ...aren't there lots of different telescopes all listening, and wouldn't lots of other people have heard it, too, anyway, without needing to get a panicky phone call, "Hey, man, turn on your telescope, there's something really hot on 107 MHz"? ...
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While any given radiotelescope may have the ability to tune into a 107MHz signal (depending on its dish characteristics and equipment), it's not the case that every scope (or even a lot of them) would be tuned to that frequency much of the time. Worse, RTs are highly directional, so the other scopes would have to be aimed at exactly the same place in the sky. This is quite unlikely unless there was some special reason for it.
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4. ... Does the military not use wavelengths like 107 MHz? Anyway, can you pick up local radio stations on, say, the Arecibo array? In the movie, they've given him a really huge dish...
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The RF spectrum is divided into ranges that are assigned to various uses... like CB radio, FM radio, TV broadcast, cellular phones, etc. The military owns big chunks of the spectrum, and woe to you if you should interfere with them.
However, there's no reason that the military couldn't broadcast in the FM band if they wanted to, or accidentally if their equipment was malfunctioning.
You can be sure that radiotelescopes would be designed specifically to
exclude the wavelengths used for terrestrial broadcasting... that's just annoying noise to an astronomer.
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5. ... How could they be talking if the star is 14 light years away? How long would an FM signal take to get from here to Wolf 336? Is that plausible?
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You've answered your own question -- it would take a signal 14 years to get there, and another 14 years for the answer to come back. Kind of a slow process, unless you use Asimov's "back fence" method... both parties simply yak away at full speed and let the other party garner as much information as they can. After a few centuries you might actually be able to make sense out of it, and begin to ask questions and get answers.
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6. ... he goes around to all the folks in his customer service area and rearranges all their TV satellite dishes into a "phased array" and runs the thing out of his attic, with computers and stuff. Can you do that?
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Sounds barely possible but unlikely. When you build a phased array you need to control the spacing between the elements very tightly. If the various dishes were randomly located, it might be possible to "crunch" the data and get useful results, but I'm dubious.
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7. ... a "cooling jacket for a low noise amp". So there's a big yellow metal tank like an oxygen tank connected to it, so I'm assuming that it's liquid nitrogen? Yes? No? Do they use liquid nitrogen for that? The kid writes his name with his fingertip in the frost on the canister-thingie, which looks like it's made of Pyrex like a beaker. Is that plausible, that a "cooling jacket" with liquid nitrogen would be made out of Pyrex? Also, wouldn't the kid's fingertip freeze?
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The key element needed to be chilled in a low-noise receiver is not an amp on a shelf somewhere, but the actual detector, located at the focus of the telescope dish. In the FM band, though, there's no real need for even that, as it's far from the microwave frequencies produced by thermal radiation.
The kid's fingertip would probably be ok after a brief touch to the outside of a container of LN2, especially if it was at least minimally insulated. The frost demonstrates that it's not very well insulated, but he might get away with it.
He'd regret it if he dunked his pinkie, though.
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8. While he's demonstrating his attic equipment for the kid, he supposedly picks up a signal from Voyager 2. You can do that, in your attic, with a backyard satellite dish and a PC? Don't laugh, I didn't know that.
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No. Voyager has an extremely weak transmitter and it's a long way off. No way anything less than a very large dish would be able to pick it out of the noise.