PDA

View Full Version : Is this plausible? The Arrival, with Charlie Sheen, FM signa



Jigsaw
2002-Feb-24, 03:50 AM
Hey, this is great! I'm watching this video and I'm sitting there wondering how plausible it is, and it just so happens I know the perfect place to ask. /phpBB/images/smiles/icon_biggrin.gif It's "The Arrival", with Charlie Sheen, from 1996. Here's its IMDB listing. http://us.imdb.com/Title?0115571

Total non-astronomy person here, so speak slowly and use words of one syllable, please. (Thanking you in advance for not laughing.)

The movie is a pretty straightforward "the aliens are already among us and the government is covering it up" flick. The central premise is that Charlie is a serious professional radioastronomer who is using his telescope time to search the FM bands for SETI signals from the stars. He's presented as a maverick for doing this, as they're supposed to be searching microwave wavelengths. So, questions.

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?

2. Stars emit microwaves? Don't laugh, I didn't know that.

3. So, Charlie and his buddy get the signal for 40 seconds and they're frantically trying to call some other observatories to get confirmation. Does it really work like that? "Quick, phone so-and-so and see if they heard it, too"? IRL, 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"? Which gets back to whether it's plausible that he'd be listening for FM, and whether other people wouldn't be listening for FM, too.

4. So Ron Silver, in the thankless role of the "You-Know-He's-the-Bad-Guy Executive", tells him, "It's a radio burst from a blah-blah-blah military something", and Charlie says defiantly, "Not at one-oh-seven megahertz, it isn't". So I thought 107 MHz was, like--radio. My Sony boombox out in the kitchen goes up to 108 MHz. So how could Charlie be so certain that he wasn't just picking up the local Dance Party station? He does mention that it was "moving in sidereal time", and I do kinda know what that means. But, the way he said it, it sounded like the number of MHz had something to do with it, was the important thing--he didn't say, "It was moving in sidereal time". 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, like the size of the one in that Sam Neill movie that just came out, about the Australian radiotelescope and Apollo 11, "The Dish", that's what it's called.

5. This signal is supposedly coming from a star called Wolf 336, which is 14 light years away. Later on he gets another signal from it, but it gets mixed up in an atmospheric "bounce" from a Mexican radio station. So he says, "Hey, the first signal was definitely from the star because it moved in sidereal time, but this one is definitely from Earth. Hey, what if they're talking?" So I go, "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?"

6. He finds the second signal because after he gets fired from his radioastronomy job, he gets hired on as a satellite dish installer, and then 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?

7. Up in his attic, he has tubes and things installed hanging out of a big canister-thingie with dry-ice smoke coming out of it, and he tells the handy neighborhood kid (who is there in his role as "audience surrogate") that that's 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?

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.





<font size=-1>[ This Message was edited by: Jigsaw on 2002-02-23 23:03 ]</font>

Donnie B.
2002-Feb-24, 01:09 PM
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).



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?


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.



2. Stars emit microwaves? Don't laugh, I didn't know that.


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.



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"? ...


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.



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...


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.



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?



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.



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?


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.



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?


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.



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.


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.

Jigsaw
2002-Feb-25, 02:31 AM
Thank you. Yeah, I did know that it was probably "Holy Hollywood!" Time. I like my science fiction to have at least a little "science" in it.

BTW, later on Charlie Sheen does witness the aliens using a gigantic widget to send FM signals "back home", and he picks it up on his Walkman headset.

Kaptain K
2002-Feb-25, 07:34 AM
FWIW - If the tank was yellow, it was liquid helium. Liquid nitrogen would be in a brown tank.

Donnie B.
2002-Feb-25, 04:59 PM
Ha! I wondered about the tank color, but I wasn't sure, so I glossed over it.

Liquid Helium in an open beaker?

HA Hahahahahahaha!!

It would be gone like that!

LN2 would last at least a little while; it's often used in Physics demonstrations and such. But LHe is nasty stuff to work with, much colder, and would evaporate explosively in an open container. Not to mention far too expensive to waste like that.

Low-temperature Physics labs keep the stuff in very high-quality dewars (basically super-duper thermos bottles) and dole it out with an eyedropper (figuratively speaking).

Kizarvexis
2002-Feb-26, 01:13 AM
<<<<<<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... >>>>>>

<<<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.>>>

When I was in the US Army Infantry we carried a backpack radio, AN/PRC-77 or something like that, that could reach into the commecial band. I could run the freq selector all the way up to near the top and pick up the local NBC station in Ft. Campbell, KY.

Kizarvexis

Jigsaw
2002-Feb-26, 01:35 AM
During the exciting conclusion at the big dish, they showed a yellow tank just like the one up in the attic (he uses it to freeze three of the aliens, spraying it like a fire extinguisher), and it quite definitely said "Liquid Nitrogen" on the side.



<font size=-1>[ This Message was edited by: Jigsaw on 2002-02-25 20:36 ]</font>

PAV629
2002-Feb-26, 01:33 PM
yeh i remeber the ending also.....jigsaw is right the tank did say liquid nitrogen on it......i actually enjoyed this movie......wasnt to bad........the movie seems to play right into those conspiracy theorists who preach that ETs are already here and in cohoots with the govt......

Carpe Diem
2002-Apr-22, 02:10 AM
[quote]
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?


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.

[quote]

Actually thats not necessarily true - I have put my finger in Liquid Nitrogen before. Obviously not for a long time but you can dip it in and out quickly. Because your hand is so hot (above the boiling temp for N2) it turns the nitrogen around it into gas and creates an insulating layer. This obviously doesn't work for long before the cold gets through anyway. But it was kinda cool. NB - if you ever try it - don't leave your finger in there!!!

Jim
2002-Apr-22, 04:09 AM
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?

---------------------------------------------

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.

Yeah, me, too, but for another reason. The dishes would have to be aimed at the same area in the sky for Sheen's purposes. However, for satellite TV, the dish needs to be aimed at the satellite.

Seems the dish company would be getting a lot of complaints from customers who couldn't get the WWF Pay-Per-View they ordered.

<font size=-1>[ This Message was edited by: Jim on 2002-04-22 00:11 ]</font>

Donnie B.
2002-Apr-22, 11:47 AM
Carpe:
Right, a short exposure would not be disastrous. It takes half a minute or so to freeze a rose-blossom solid; a finger might take even longer as it has circulation to help it stay warm.

Jim:
You are correct, sir... home satellite dishes are not steerable. Another example of a Hollywood script writer's notion of science... it only holds up as long as you don't think about it very long.

radioastronomer
2005-Jan-05, 06:02 PM
There are a number of problems with this movie:

There are two real sources of noise that limits the radio astronomer's ability to search for very weak signals, the galactic halo noise and our atmosphere. The Galactic noise halo interferes with us below 1GHz and noise due to earth's atmosphere interferes with us above about 10GHz.

This pretty much keeps all SETI searches (at least radio ones) between 1 and 10GHz. Inside of these two frequencies, the noise drops even closer to the 2.7K cosmic microwave background between about 1.4 and 7GHz. This is why many of the SETI searches and much of our radio astronomy research is centered around the frequencies that the OH (hydroxyl) and H (hydrogen) molecule masers emit. This is the so-called water hole you may have read about (H - OH). Tell me scientists don't have a sense of humor. :-)

This is also why it would be assumed that any interstellar transmissions would be within the 1 to 7GHz frequencies as well.

Most large dish antennas, such as the one shown in the movie, are microwave receivers with very low noise front ends. As a previous poster pointed out, the first amplifier is the critical one with regards to noise. Not some silly box in an attic. Also these antennas have specific feeds that would not allow you to pick up an FM broadcast anyway.

On to the backyard dishes: The dish positioners on these antennas are designed to only sweep out the arc that the geosynchronous satellites are located on. They also have a feed/front end that is not designed to receive an FM broadcast in the MHz range. So unless he added an entirely new FM feed assembly, X/Y drive (or alt/az etc.), and control circuits that did not interfere with the normal operation of the dish from the home owner, it just would not work.

Also a dish antenna is not the best antenna for receiving an FM broadcast in the MHz range. He would have done better with a tuned yagi I think.

Lastly, if this race was so advanced in their technology, why would they not be using something like a microwave burst frequency hopping 8-ary encrypted signal.

oynaz
2005-Jan-06, 09:51 PM
Just a note.

Once upon a time, when I still went the Danish variant of High School, our teacher demonstrated the the weird properties of liquid helium. So it is possible to fool around with it a bit before it evaporates.

mid
2005-Jan-07, 02:59 PM
As a handy piece of guidance, I was told back at college that their liquid nitrogen cost about as much as milk, and you should avoid wasting it in a similar manner. Meanwhile, liquid helium works out roughly that of a passable whisky, and the same applies.

Of course, it might just have been made up to justify the "spill that flask of liquid He and you owe the tutor a bottle of Malt" rule...

gethen
2005-Jan-07, 03:04 PM
Where are our manners? :wink: Welcome to the BABB, radioastronomer.
I saw this movie in the theater when it was released and I guess I wasn't paying attention. After reading Jigsaw's post, I guess I'll have to rent it and watch more closely.

radioastronomer
2005-Jan-09, 07:04 PM
Where are our manners? :wink: Welcome to the BABB, radioastronomer.
I saw this movie in the theater when it was released and I guess I wasn't paying attention. After reading Jigsaw's post, I guess I'll have to rent it and watch more closely.

Thank you! :-)

radioastronomer
2005-Jan-09, 07:17 PM
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).



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?


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.



2. Stars emit microwaves? Don't laugh, I didn't know that.


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.



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"? ...


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.



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...


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.



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?



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.



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?


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.



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?


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.



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.


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.


I am still learning how to reply so please be patient. :-)

Actually a small dish with a really good front end (extremely low noise figure) can recieve an amazing amount of "stuff" out there. If the dish were tuned to the correct frequency, you had the proper ephemeris, and you narrowed the reciever down to about 0.8Hz, you may be able to detect the carrier from a spacecraft such as the Voyagers.

I though I would add something about noise temperature since I briefly mentioned it:

Astronomers use temperature to represent the strength of detected radiation. Any body with a temperature above -273 deg C (approximately absolute 0) emits electromagnetic radiation (EM). This thermal radiation isn't just in the infrared but is exhibited across the entire electromagnetic spectrum. (Note: it will have a greater intensity (peak) at a specific area of the EM spectrum depending on its temperature). For example, bodies at 2000 K (Kelvin), the radiation is primarily in the infrared region and at 10000 K, the radiation is primarily in the visible light region. There is also a direct correlation between temperature and the amount of energy emitted, which is described by Planck's law.

When the temperature of a body decreases, two things happen. First, the peak shifts in the direction towards the longer wavelengths and second, it emits less radiation at all wavelengths.

This turns out to be extremely useful. When a radio astronomer looks at a particular location of the sky and exclaims that it has a noise temperature of 1500 K, he/she isn't declaring how hot the body (nebulae, etc) really is, but is providing a measurement of the strength of the radiation from the source at the observed frequency. For example, radiation from an extra solar body may be heated from a nearby source such as a star. If this body is radiating at a temperature of 500 K, it exhibits the same emissions across all frequencies that a local test source does. The calculated noise figure will be the same across all frequencies. (Note: this does not take into account other sources of radiation such as synchrotron radiation).

A problem for radio astronomers is that not only the observed source emits thermal radiation; the local environment (ground, atmosphere, etc) and the equipment (antenna, amplifiers, cables, receiver, etc) being used to make the measurements also emit thermal radiation. To accurately observe and measure the distant sources, the radio astronomer must subtract all of the local environment and detection equipment noise additions.

Back in 1963, Arno Penzias and Robert Wilson were working with a horn antenna trying to obtain the high efficiency possible for the Telstar project. This antenna was also going to be used for radio astronomy at a later date. They pointed it to a quiet part of the sky and took measurements. When they subtracted all of the known sources of noise, they found approximately 3 K left over. They worked very diligently to eliminate/describe this noise source and were unable to. This mysterious source of noise seemed to be there no matter where they pointed the antenna. What they had discovered was the microwave background produced from the Big Bang. This 3 (closer to 2.7) K microwave background originated approximately 300,000 years after the Big Bang itself had occurred. It has been determined that when these signals originated, the universe had already cooled down to around 3000 K.