Light-based interstellar communication with star-bright LED's?

[szputnyik]

As we know, radio signals degrade heavily over interstellar distances, but the light of even distant stars seem to reach us in pristine condition.

I'm wondering, would it be a logical (or even possible) approach, to construct a huge LED light in solar orbit, that can shine as bright as a star, and send binary "Li-Fi" data over it, for example to a previously launched probe to Alpha Centauri. If the probe has a camera that is at least as good as the human eye, it could see the blinking artificial star in the distance, and decode the binary code sent over it.

Additionally, unless we make the LED emit light in a strongly directional beam towards the probe, this could attract the attention of potential intelligent aliens in nearby star systems.

[jwenting]

no, light signals would degrade just like radio signals do. After all, radio signals and light signals are identical except for their wavelengths.

No difference whatsoever.

[AngelLestat]

A LED sign to bright as star??

Why not use Laser instead??

That is the possibility that scientist are studying. The big problem of comunication at those distances is bandwidth and error corection algoritms.

[szputnyik]

I suggested an LED because it can turn on and off in nanoseconds, useful if you don't want to send binary data at morse-code speeds.

[Ralathon]

As we know, radio signals degrade heavily over interstellar distances, but the light of even distant stars seem to reach us in pristine condition.

I'm wondering, would it be a logical (or even possible) approach, to construct a huge LED light in solar orbit, that can shine as bright as a star, and send binary "Li-Fi" data over it, for example to a previously launched probe to Alpha Centauri. If the probe has a camera that is at least as good as the human eye, it could see the blinking artificial star in the distance, and decode the binary code sent over it.

Additionally, unless we make the LED emit light in a strongly directional beam towards the probe, this could attract the attention of potential intelligent aliens in nearby star systems.

You misunderstand the degradation of radio signals.

Yes, the radiowaves send out by earth become indistinguishable from background noise within a few lightyears. But that's because we never intended to send those signals to other stars, they aren't designed to remain coherent over interstellar distances.

If we want to, we could quite easily send radio messages that remain readable over incredible distances.

[R0cketC0der]

I suggested an LED because it can turn on and off in nanoseconds, useful if you don't want to send binary data at morse-code speeds.

You won't get much transmission speed no matter what kind of transmission you use. There will be a latency of at least four years when transmitting data at interstellar distances.

[AngelLestat]

I suggested an LED because it can turn on and off in nanoseconds, useful if you don't want to send binary data at morse-code speeds.

Also Laser.

One test that Nasa have done with a 2,5w laser to transmit a video from the ISS to one observatory at earth

OPALS transmitted the video in 3.5 seconds instead of the 10 minutes that conventional radio would have required.

http://www.nbcnews.com/science/space/hello-world-hdtv-sent-laser-beam-space-n124956

[Idobox]

You misunderstand the degradation of radio signals.

Yes, the radiowaves send out by earth become indistinguishable from background noise within a few lightyears. But that's because we never intended to send those signals to other stars, they aren't designed to remain coherent over interstellar distances.

If we want to, we could quite easily send radio messages that remain readable over incredible distances.

It's still easier to keep things focused with shorter wavelengths, and light has a much, much shorter wavelength.

Imagine we have a 10MW emitter (Very big), and receiver that needs 10^-15W (very high end receiver), to transmit to Alpha centauri with 10mm rf, the Friis equation tells us we need an antenna gain of 174dB. We can do that with 2 antennas, or antenna arrays, with 87dB gain each. A parabolic dish at that frequency needs to be 87m (assuming .8 efficiency)

Now, if you use visible light, let's say 500nm, and the same power and sensitivity (I'm a RF engineer, not a photo guy, so I have no idea what kind of value is reasonable), you now need 260dB of gain, 130dB on each side, but it means reflectors only .7m in diameter, a decent telescope.

And alpha centauri is only the closest star. If you try to communicate with the galactic core, the numbers get reaaly ugly.

RF: 250dB, 8km

light: 335dB, 60m on each side

And for the RF, it's assuming a single dish antenna. An array the same size has the same beamwidth, but not the same gain, so it would need to be larger, until the combined area of the dishes is comparable to the area of the single dish.

60m optical telescope is significantly larger than today's largest telescope (10m), but when we want to talk with the galactic core, we should be able to make mirrors that large in orbit or something. Worst case, you can also use telescope arrays (although they're essentially the same as RF arrays, they're often called interferometric telescopes instead)

The thing is, when you reach MW levels, RF is much cheaper to produce than light, but the reflectors get expensive very, very fast.

[edit]

Also, you have a much higher bandwidth with laser/light. As in petabytes/s compared to GB/s

[Aghanim]

I suggested an LED because it can turn on and off in nanoseconds, useful if you don't want to send binary data at morse-code speeds.

There is a way around it you know, like using a seed laser and modulate it with electro-optic modulator and feed it to a laser amplifier, or if you have a powerful enough laser diode bar you could modulate the drive power directly

[Tommygun]

You misunderstand the degradation of radio signals.

Yes, the radiowaves send out by earth become indistinguishable from background noise within a few lightyears. But that's because we never intended to send those signals to other stars, they aren't designed to remain coherent over interstellar distances.

If we want to, we could quite easily send radio messages that remain readable over incredible distances.

I was wondering, does the signal have to be focused on a specific location or can it be transmitted omnidirectional?

[YNM]

As we know, radio signals degrade heavily over interstellar distances, but the light of even distant stars seem to reach us in pristine condition.

Not really, there's ISM, the atmosphere, and on intergalactic distances, expansion of universe.

I'm wondering, would it be a logical (or even possible) approach, to construct a huge LED light in solar orbit, that can shine as bright as a star

Seriously ? Many stars are brighter than Sun, yet you can't see them, more so in daytime...

and send binary data over it, for example to a previously launched probe to Alpha Centauri. If the probe has a camera that is at least as good as the human eye, it could see the blinking artificial star in the distance, and decode the binary code sent over it.

How long would the whole project takes ? "One Eternity Later" ?

Additionally, unless we make the LED emit light in a strongly directional beam towards the probe, this could attract the attention of potential intelligent aliens in nearby star systems.

Seriously, even if we suddenly have 2 sun, the difference in magnitude is only -0.75 (remember, smaller is brighter), based on the pogson equation. Not significant.

EDIT : Just to give some figures, using the idea Idobox came up with (10 MW visual (540 nm) laser), the laser's luminosity is only 2.600104*10^(-20) of Sun's luminosity, which gives an absolute magnitude of +53.7925232 (assuming Sun's absolute magnitude to be +4.83). From alpha Centauri system, 1.34 pc away from Sun, assuming the laser's light take a journey that long, the visual magnitude of the laser is only +49.42805 (go claim anything, no telescope (yet, hopefully) can see that !). Assuming the user of the receiver telescope is a human, the telescope's objective diameter would have to be a staggering 3,059,211.043 meters. Good luck on achieving that !

Edited July 3, 2014 by YNM

[lajoswinkler]

You misunderstand the degradation of radio signals.

Yes, the radiowaves send out by earth become indistinguishable from background noise within a few lightyears. But that's because we never intended to send those signals to other stars, they aren't designed to remain coherent over interstellar distances.

If we want to, we could quite easily send radio messages that remain readable over incredible distances.

True.

One of the things that even new Cosmos failed to explain (which is shameful) is the fact that unintentionally emitted signals become indistinguishable from thermal noise within at most few ly, and most of them in less than 1 ly distance. General public thinks aliens are listening our radio and watching TV shows from the 60's, but Earth is actually a radio-silent planet.

[Ralathon]

True.

One of the things that even new Cosmos failed to explain (which is shameful) is the fact that unintentionally emitted signals become indistinguishable from thermal noise within at most few ly, and most of them in less than 1 ly distance. General public thinks aliens are listening our radio and watching TV shows from the 60's, but Earth is actually a radio-silent planet.

Well, there are a few signals going out from time to time. Radio astronomers regularly bounce radiopulses from other planets and moons to accurately measure rotation and velocity. These pulses should be detectable from a very large distance. Signals send to space probes like the Voyagers or New Horizons should also be detectable from a few dozen lightyears if you're willing to try hard enough.

[Aghanim]

There is another very important issues on interstellar communication, and that is how do you point your transmitter to the correct location, like Earth

http://news.discovery.com/space/project-icarus-interstellar-communications-120206.htm

No one is seriously considering using omnidirectional antenna for interstellar communications, right?

[Nuke]

There is a way around it you know, like using a seed laser and modulate it with electro-optic modulator and feed it to a laser amplifier, or if you have a powerful enough laser diode bar you could modulate the drive power directly

like those diode pumped solid state lasers, i figure you could switch such a laser fast enough to be useful in communication.

i think another part of it is picking a laser with a wavelength in one of the host star's dark bands to make pickup and filtering a lot easier.

[Everten P.]

Hey asking the dumb question but wouldn't it it take thousands of years for light to go interstellar?

[magnemoe]

Well, there are a few signals going out from time to time. Radio astronomers regularly bounce radiopulses from other planets and moons to accurately measure rotation and velocity. These pulses should be detectable from a very large distance. Signals send to space probes like the Voyagers or New Horizons should also be detectable from a few dozen lightyears if you're willing to try hard enough.

However this is rare, you have a few high focus and high power signals you sends from time to time, in some directions.

As both Earth and the probe moves its unlikely that the next signal can be intercepted 10 light year out.

Might explain the WOW signal or the "Jeb what is you doing" signal it was called locally.

Lasers might be better then communicate with probes too.

[Aethon]

Scientists are currently looking for pulsed laser signals from intelligent life. Check out O(ptical)SETI : http://www.planetary.org/explore/projects/seti/optical-seti.html

Here's a paper called : SEARCH FOR NANOSECOND OPTICAL PULSES FROM NEARBY SOLAR-TYPE STARS : https://www.princeton.edu/~willman/observatory/oseti/oseti_apj_preprint.pdf

We can build optical nanosecond pulse LASERs that can outshine the sun by four orders of magnitude.

[YNM]

Scientists are currently looking for pulsed laser signals from intelligent life. Check out O(ptical)SETI : http://www.planetary.org/explore/projects/seti/optical-seti.html

Here's a paper called : SEARCH FOR NANOSECOND OPTICAL PULSES FROM NEARBY SOLAR-TYPE STARS : https://www.princeton.edu/~willman/observatory/oseti/oseti_apj_preprint.pdf

We can build optical nanosecond pulse LASERs that can outshine the sun by four orders of magnitude.

http://what-if.xkcd.com/13/

To outshine the Sun, you'd need something more than giving the YAL-1 to every human in the world. The article did also talk about the NIF, but of course, it assumes everyone has one. The actual single output of NIF lasers is in the orders of terrawats, while the Sun gives three hundred yottawats. Outshined by at least 8 orders of magnitude.

[phoenix_ca]

If you have need of interstellar communication, why in the world (or galaxy) would you bother to communicate with light? It's probably safe to assume you'd have FTL travel, so...just stick your message on a ship and send it. It'll get there faster.

[shynung]

If you have need of interstellar communication, why in the world (or galaxy) would you bother to communicate with light? It's probably safe to assume you'd have FTL travel, so...just stick your message on a ship and send it. It'll get there faster.

Better yet, make a ship filled with a massive data storage facility, load whatever message you need into the disks, and send it off. Similar to large mail planes.

[NovaSilisko]

Better yet, make a ship filled with a massive data storage facility, load whatever message you need into the disks, and send it off. Similar to large mail planes.

There's a fun thought. FTL communication is impossible, but FTL travel is. What do you do? Interstellar mail trucks!

[magnemoe]

There's a fun thought. FTL communication is impossible, but FTL travel is. What do you do? Interstellar mail trucks!

Plenty of fictional settings where this is common. Tou transmit data to the ship going to the jump point or safe warp distance and it send as soon its in the target system at least for things who are important, huge files with no hurry have to wait to the ship land or dock.

[Nuke]

ftl might very well turn out to be impossible. when you look at the alternatives, you really dont have much choice. if you send a generation ship (or a sleeper ship, or a relativistic ship, whatever) to set up a colony somewhere, it would be useful to maintain lines of communication with those colonies. it wouldn't really be communication as we understand it. it would be more like transmitting everything you could possibly send (for example news, the entire contents of wikipedia, scientific papers, music, movies, software, etc), as fast as you can send it, and let the other side sort through it all. there wouldn't be much in terms of dialog.

i figure before we have working ftl drives (in the event they do work) we will have sent relativistic probes all over the place doing flybys of nearby star systems, and laser communication is probibly the only way to relay the data back to earth. then when we have ftl, flying hard drives.

Edited July 5, 2014 by Nuke

[NovaSilisko]

ftl might very well turn out to be impossible.

Personally, I'm quite convinced it's entirely impossible. The laws of physics that we know - and have verified to several decimal places of precision - come to a general conclusion that FTL travel is indeed completely impossible. Even the much-hyped "warp drive" still runs headlong into the problems of causality violation. Hell, as far as physics are concerned (and for reasons I'm not 100% clear on), FTL is the same as time travel...