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:"

Comments

hah

[Anonymous Coward]

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.

Re:hah

[BlueUnderwear]

> Actually, we won't be sending out noise for that much longer. Digital radio/communication with encryption and compression should be completely entrpic (random). In other words, no distinct signal coming from us unless they know the DTV CoDec. And we're not even that noisy a radiation source.

So we better stop those SETI type projects. Or else an alien ship might destroy the earth to punish us for violating their equivalent of the DMCA...

Re:hah

[bentini]

Actually, we won't be sending out noise for that much longer. Digital radio/communication with encryption and compression should be completely entrpic (random). In other words, no distinct signal coming from us unless they know the DTV CoDec. And we're not even that noisy a radiation source.

Re:optical detection sounds hard

[Mark Hood]

> 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

Re:How do ET aim at a moving target?

[Christopher Thomas]

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.

Inverse Square Law

[SEWilco]

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". A laser is following the geometry applicable to parallel lines (depending upon accuracy of construction), not a sphere.

Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. This comes from strictly geometrical considerations. The intensity of the influence at any given radius r is the source strength divided by the area of the sphere.

Using patters of light to indicate intelligence?

[AtariDatacenter]

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. ;)

Don't think so

[Dr_Cheeks]

Um, really. I know the beam spreads out a bit, but is it according to the inverse square law?

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.

Re:Accuracy

[FTL]

[...] stars cans be made to explode into supernova. [...] Your signal can be detected from any point around the supernova & traces last for some time.

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.
--

Re:What known source in space gives off laser beam

[ozbird]

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.

SETI is against intergalactic law

[Junior J. Junior III]

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:

• Any scientists involved in the study of Astronomy, Cosmology, or Astrophysics.
• Anyone using or distributing the Circumvention Device known as SETI-At-Home
• Anyone involved in the industry which manufactures the devices which enable the SETI-At-Home program to run and function. This would include computer manufacturers, retailers, end-users, and research and design engineers.

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.

Re:What known source in space gives off laser beam

[onion2k]

Laser. Light Amplification by Stimulated Emission of Radiation. Lasers ain't natural. If you find lasers, you find life.

So in summary

[L41N14L]

They're looking for aliens firing lasers at Earth. I can see how that'd be useful.

optical detection sounds hard

[tantrum]

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.

Accuracy

[Omniarch]

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.

How do ET aim at a moving target?

[ThinWhiteDuke]

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