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Optical SETI
R3 writes "BBC News is running this story, about SETI's renewed efforts to find ETs who might be flashing us with light, instead of radio-waves:"
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Flash lights from nearest planet millions of lightyears away (boy their arms must get tired). A few million years later, humans discover the light. Too late, aliens died.
> wouldn't we have to depend on the fact that
:)
> the other civilization know that we are right
> here, and that we are able to pick up their
> signals?
Yes, that's true if we assume they're trying to contact US specifically. If they've just got a huge laser on the top of their local Everest-sized mountain & are relying on their planet's rotation to turn it into a beacon we have a better chance of spotting it.
If we try to pick up radio we can (in theory) spot their TV signals, satellite communications & Star Wars ABM radars
> No matter what kind of transmission ET is using
> he will have to hope that we are able to pick
> up the signals
Of course - and that's why the Seti League (http://www.setileague.org/) advocate lots of smaller dishes. Their argument is that while an Arecibo size dish can look further, this comes at the cost of seeing a smaller area of the sky. Their favourite statistic is 'even if we're looking on the right frequency at the right time, there's a 99.999% chance that when the call comes in, we'll be looking the wrong way'.
Of course, they're talking about radio waves, but the same argument holds for optical SETI.
Mark
Keeper of the Wedding Shenanigans Home Page
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I vaguely recall that an experiment to bounce laser light off the surface of the Moon determined that the light's "footprint" had spread out to several km over the distance between the Moon and Earth. Your mileage may not vary.
"How many light bulbs does it take to change a person?" --BMcC-->
Lasers are just a concentrated light beam, and aren't immune from the effects of the inverse square law - so as the distance was doubled, the area that the laser would hit would be squared and thus the intensity would be greatly reduced.
Go you big red fire engine!
Any sufficiently advanced technology is indistinguishable from a rigged demo
--Andy Finkel (J. Klass?)
Yes, light does obey the Inverse Square Law. A laser does not because the light has been manipulated to make the photons travel parallel to each other, thus it does not "spread its influence equally in all directions".
Your laser beam diffracts as it passes through any finite-sized aperture (the laser mirrors count as apertures for this). This causes the resulting beam to diverge (think back to the single-slit diffraction experiment in high school physics class).
A diverging beam can be thought of as a cone. The area of a cone's cross-section goes up as the square of the distance from the origin, so the intensity of the laser light will indeed go down as the inverse square. In a real laser beam, the distribution vs. angle is gaussian instead of uniform, but the same principle applies.
Using a wider beam would result in a narrower cone, but your aperture would have to be at least 10 km wide to have a spot size as small as a planet at a distance of 10 light-years. So, any practical laser would spread out like a cone that covers a lot more area than the target.
What do you think? Are we (Terrans) currently able to target a laser beam on a planet 1, 10, 100 light years away?
Short answer: Yes, but it would be expensive.
Long answer:
Stars and planets have (relatively) easily-plotted courses. Spend a few months with a big telescope, and you can do any fine-tuning you need to in your model of the planet or parent star's trajectory. Put a big beam splitter in front of the telescope, fire a huge, very expensive laser at the beam splitter, and you can send the laser beam to the target system while using the telescope to make sure it's going in the right direction.
Target _system_?
Well, the problem is that your laser's aperture is small enough that diffraction prevents you from focusing it on something as small as a planet over interstellar distances. So you'll probably end up bathing most of the inner system of the destination star in weak laser light. Your laser has to be quite bright to be picked up (even if you pulse it), which means very, very expensive.
Alternatively, you can build an array of many lasers in space, and pull evil tricks to keep them all in phase with each other. This gives you a very large synthetic aperture, which would let you target the laser at a single planet. Of course, you'd need a synthetic aperture optical telescope of comparable size to _track_ the planet, but if you can build the laser, the telescope is within reach also.
This is "stupidly expensive", as opposed to merely "insanely expensive", but it could be done. We'd have to do something similar if we wanted to easily launch sailcraft over interstellar distances.
A laser sends out a cone of light, not a straight line. At the distances at which you normally use that laser pointer of yours, you don't notice this, but if you were sending a message to a different solar system you'd take this into account. (Either that or you'd have some explaining to do when your grant comes up for review.)
You'd also take into account the fact that your planet moves, if indeed it does in your frame of reference. There is no such thing as being stationary in space (i.e. there exists no absolute velocity, Mr. Michelson), and in any case an advanced alien civilization or a high school student would be smart enough to calculate the direction of a laser beam as a function of relative locations. Given reasonably precise astronomical tools (beyond what we have currently built AFAIK but IANAA) it shouldn't be hard to figure out from a reasonably distant solar system that the third planet around our star revolves through a certain orbit, and that our star is moving through the galaxy with a certain velocity relative to the alien body. Yes, that body might indeed rotate, but if the transmitter were actually on the surface it would be simple enough not to broadcast when the target body is beyond the horizon... or use multiple transmitters, or just launch the transmitter into orbit. If the aliens really were so dumb as to not figure that out, would we really want to communicate with them? They'd probably be OS zealots anyway, or maybe just wanting to sue Adobe for using their patented ROT13 encryption.
Now suppose that within, say, 30 ly of Earth has picked up on our radio transmissions and has used some simple triangulation to figure out that our planet is the source of these transmissions. If they wanted to make contact with us, they could send us a message encoded in a laser beam. Yes, they might have to do some simple math, but I think they could do it. And that beam, if it exists, could be visible today.
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Wow. It's been in front of us all along, and we haven't seen it. They ARE communicating with us via regular flashes of light. And it takes a very advanced level of technology to create.
;)
They're called pulsars.
Ok Mr. AC, what unshakeable foundation, what absolute truth do you posess that informs you that your perceptions of reality aren't complete hallucinations? Inductive reasoning? A gut feeling? I don't know if you've tried to alter and test your mind, but I have a feeling that you haven't. Go ahead and live within your Windows box of a mind. I've personally had experience on hallucinogens that were more real that my day to day experiences. But I guess you'll just have to experience that for yourself.
LS
There is a fine line between being a cultivated citizen and being someone else's crop. - A. J. Patrick Liszkie
I'm scratching my chin at the profundity of your statement.
You're looking at the syntax of my words, and not the meaning. Perhaps I should have said "aware" instead of real. But doesn't the beginning of my statement, that no one has an unshakeable foundation of truth, mean essentially what you are saying?
I post in all seriousness about a touchy subject, and get modded down as flamebait and only ACs respond. Fuck, I hate anonymous cowards...
There is a fine line between being a cultivated citizen and being someone else's crop. - A. J. Patrick Liszkie
Shouldn't we better start to emit that kind of laser beams ourselves? In case another civilization has a similar kind of SETI project running?
I mean, what are the chances of aliens sending exactly the kind of radiation we are detecting? How can we be sure they are sending any radiation at all?
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It's some of each, or realy neither... the duality of light. First we thought one thing, then disproved that and went with another, then Planck and Einstein had fun poking at the photoelectric effect and Einstein settled on photons. (Millikan's experiments helped Einstein win the Nobel prize, and Millikan got one of his own later)
So what's the answer? Is light a wave, or is light a flow of particles? Well, the bottom line is that it's neither one. Light is are you ready? a "quantum vector field." That phrase doesn't give you much of a mental picture, does it? I actually kind of know what a quantum vector field is, and it doesn't give me any mental picture. The fact is that the true nature of light defies mental picturing, because it's not quite like anything we can lay our hands on. Under certain conditions, such as when we shine it through narrow slits and look at the result, it behaves as only a wave can. Under other conditions, such as when we shine it on a metal and examine the spray of electrons that comes off, light behaves as only particles can. This multiple personality of light is referred to as "wave-particle duality." Light behaves as a wave, or as particles, depending on what we do with it, and what we try to observe. And it's wave-particle duality that lies at the heart of the Heisenberg uncertainty principle.
Wheeeee
If only the Independence Day aliens had though of shooting us with a laser from outer space - then Will Smith would never have whipped their butts.
Don't have my copy of The Guide to hand though, so I can't verify this I'm afraid - anyone else know?
For example - take a laser pointer. Hold it about 30 cm (approx 1 foot) away from a surface and make a rough guess what sort of area the beam is covering. Let's say a circle with a 1mm radius to make things simple. Now stand 3m (about 10 feet) away. You're now 10 times as far away, and 10^2 is 100. Is the beam now hitting an area 100 times in size? Following the inverse square law, the area that the beam hits should go from about 3.1 square millimetres to 301 square millimetres (or 10mm radius). If it is then your laser pointer is broken.
I'm afraid I can't remember the physics of why lasers work this way, but they do.
Erm, no. First of all, that laser beam would only sweep a circle, not a sphere. So unless we just happened to be exactly on the plane of their planet's rotation, we'd miss it. Secondly, even if we did happen to be lined up perfectly, we'd never notice it since the beam's path (not the photons) would sweep past Earth at a velocity far exceeding the speed of light. Consider that if they are 100 light years from us, then their laser would be painting a circle that is 2*Pi*100ly in circumference every one of their days. Assuming their day is the same as hours (24 hours), that would mean that at our distance their beam would be sweeping 70,000,000,000km a second. How long would Earth stay in such a beam? Not long enough for your detector to see more than a single photon. And that's assuming we just happen to be lined up.
No, the only way that laser communication would work is if it is directed straight at us.
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Blowing up a star to say "Hello there!" (or "First post!", or "All your base...") seems like a rather expensive way to communicate. Plus you then have to travel to another star to send your next message.
Green Peace would have a cow if we started blowing up neighbouring stars for fun.
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Lasers ain't natural. If you find lasers, you find life.
Not so - there are natural lasers; all you need are the right conditions.
Examples from space: ultraviolet lasers, Microwave lasers (masers) and near infrared lasers.
It's not that hard. Even a laser beam spreads out over distance. If we wanted to beam a message to a planet circling a star 22 light-years distant, we would design the laser so that the divergence covers the habitable zone around the star (~ 4 AU's). Then we just lead it enough to account for stellar drift, and fire our laser. The calculations required aren't all that difficult -- we know the stellar drift for most of the nearby stars, and can make a pretty good estimate of how big the habitable sphere of a given star will be. Actually, building a laser that ONLY diverges 4 AU's over 22 light-years is probably beyond our capabilities. Having more divergance makes it easier to hit our target, but it also means that we have to put out a lot more energy to maintain the same intensity at the far end.
Why is it that the proponents of "one nation under God" are so eager to get rid of "liberty and justice for all"?
Or at least hope they take the time and effort to flash every star system within, say, a 50 light-year radius. But this strikes me as a specific example of a more general problem: our SETI efforts are directed towards detection rather than transmission; we seem to be doing a lot more listening than talking. If the aliens take the same attitude, then it's obvious no-one will ever contact anyone else.
Of course, a lot more effort is necessary for detection than for transmission, but AFAIK there aren't any transmissions going out specifically related to contact (our own EM noise probably doesn't count, simply because it doesn't have the oomph to travel interstellar distances). Are there any major efforts underway to ensure continuous transmission of "Hi there!" messages from good ol' Mother Terra?
Just makes you think, though... What if they're using 1000 bit encryption on a scrambled frequency that changes every few nanoseconds... How would we ever detect them?
You see? You see? Your stupid minds! Stupid! Stupid!
Earthlings:
This is the Intergalactic Police. Intercepting and attempted decoding of encoded signals is a violation of the Digital Milky Way Copyright Act (DMCA). This is a Class II Felony according in your sector according to Intergalactic Planetary Law.
We have your planet surrouned. You will agree to hand over all conspirators and contributors in this crime to our awaiting vessel.
We consider a conspirator or contributor to be:
If you do not respond and comply to this writ within 48 hours, your planet will be subject to immediate seizure for processing into raw materials according to Andromedean Law (ref. Victims of Crime Restitution Act, Article III, Section 2, Paragraph 500.23.
You have no chance to live. Make your time.
You see? You see? Your stupid minds! Stupid! Stupid!
Laser. Light Amplification by Stimulated Emission of Radiation. Lasers ain't natural. If you find lasers, you find life.
http://twitter.com/onion2k
They're looking for aliens firing lasers at Earth. I can see how that'd be useful.
I thought that we had enough problems with discovering meteorites close to the earth, not to mention the fact that we have never really seen a planet outside of our solar system.
It should be a lot easier to detect a laser beam directed directly towards the earth, than spotting a planet, but wouldn't we have to depend on the fact that the other civilization know that we are right here, and that we are able to pick up their signals?
No matter what kind of transmission ET is using he will have to hope that we are able to pick up the signals. For how long have we been able to pick up that kind of signals, fifty years, maybe. I don't think that the aliens would bother to communicate with such a primitive rase as the humans.
umm.. that was all for now. lunch time.
Surface (or near-surface) dwelling beings would be exposed to a tremendous amount of 'visible' light. Most of the rest is either too long of a wavelength (infrared light just doesn't have the resolution of our visible wavelengths, never mind radio), or too short, and would either be filtered out by the atmosphere (UV and the higher wavelengths like Xray or gamma), or would kill anything with body chemistry similar to ours.
Someone did a paper on this a long while back, basically his conclusion was that the wavelengths that we call 'visible' light are just a natural result of our chemistry.
Besides, just imagine a primitive race that *could* see UV very well. It's not like they can just start a fire that produces copious amounts of UV radiation :)
Endless arguments over trivial contradictions in books written by ignorant savages to explain thunder in the dark.
How accurate would you have to be to aim a laser directly at the earth from who knows how many light years away? Personally, if I was an alien, I would stick to radio, or some other non-targeted communication if I was trying to contact any other life in the universe.
We can't stop here! This is bat country!
ok, let's assume ET is 100 light years away. While the laser beam travels, earth revolves around the sun and the sun itself moves within the milky way. Of course aliens could compute the movements and target where earth will (should) be when the beam reachs us. But chances are that gravity wells would slightly bend the path or dust clouds would slow the light. Again truly superior aliens could take all of this into account. But reliance on such a huge collection of initial conditions would make the system pretty chaotic.
What do you think?
Are we (Terrans) currently able to target a laser beam on a planet 1, 10, 100 light years away?
I will fight for the right to be right
It would be nice to be sure of anything the way some people are of everything.
I am not a subject-matter expert, but I was doing some reading today about various forms of communication and transmission media, and the pros/cons of each...
For beings from another planet to contact us with focused optics, they cannot use a 'broadcast' approach (one source sent out to many users, a la radio stations). If they were trying to use a focused laser, this implies they'd have to know where we are in the first place (and would have known for the years the light had to travel)... or, if from far enough away, they would have had to sent the light to us before we existed!
I dunno, it seems to me that anyone out there would be far more likely to send a broadcast signal that finds us by chance, rather than a focused one, given the very short time we've had the technology to send/receive these sorts of communication. Thoughts?