Starship Asterisk*

by **Qev** » Wed Aug 15, 2007 4:58 am

Well, I consider lasers and masers to be the same thing, but yeah, that's what I meant. I wasn't aware of the coherence issue over distance... good to know.

I think Eta Carinae's been discovered to be a natural UV laser, hasn't it?

by **Chris Peterson** » Tue Aug 14, 2007 8:13 pm

Qev wrote:I'm curious how astronomers go about detecting naturally-occurring stellar lasers. Are they detecting the coherence of the laser light, or simply the strong peak emission frequency in the stellar spectrum?

As noted earlier, laser light loses its coherence quickly with distance, so that cannot be used as an indicator. Natural lasers (which are rare compared with natural masers) are identified by detecting a narrow emission band with a predicted Doppler broadening profile, and at too high an intensity to be explained without some amplification process. I believe all the natural lasers that have been seen are either in the infrared (around 10um) and are the result of CO2 transitions, or in the far infrared (169um) from a hydrogen transition.

by **Qev** » Tue Aug 14, 2007 7:42 pm

I'm curious how astronomers go about detecting naturally-occurring stellar lasers. Are they detecting the coherence of the laser light, or simply the strong peak emission frequency in the stellar spectrum?

by **Chris Peterson** » Tue Aug 14, 2007 1:25 pm

Dr Evil wrote: Wrong wrong wrong

Coherence is whenphotons are "in step" and have a definite phase relation. Unrelated to the point you are making.

The question I was addressing suggested that the coherence of a laser could be used as a signature of an artificial source. I merely pointed out that coherence would be lost by the time the light reached another system, so that could not be used. This is not wrong, regardless of whether there are other reasons as well that laser communications might be difficult.

Lasers will not be detected in another solar system, much less galaxy because of "divergence". Divergence is when the the beam spread out over distance.

1/R affects all electromagnetic radiation. That doesn't mean it can't be detected, only that the signal gets weaker with distance. A bright, modulated laser beam can certainly be detected even if it has diverged- it's just a question of the sensitivity of the receiver.

by **Dr Evil** » Tue Aug 14, 2007 9:24 am

[quote="Chris Peterson]
Not necessarily. First of all, there's no way that laser light traveling between stars will stay coherent. Coherence is a property that is lost pretty quickly with distance from the cavity. And simply being nearly monochromatic isn't a guarantee that the source is artificial- natural lasers probably exist, as well as natural narrowband filters. If the sender wants to be sure their signal is recognized as artificial, it's important to modulate it in some way. Low speed data could be transfered with simple AM over huge distances.[/quote]

Wrong wrong wrong

Coherence is whenphotons are "in step" and have a definite phase relation. Unrelated to the point you are making. Lasers will not be detected in another solar system, much less galaxy because of "divergence". Divergence is when the the beam spread out over distance.

Take for example this these lasers http://www.dragonlasers.com which are pretty much the best the average person can get. They have a divergence of 1.2mRad.

This means for every meter the beam travels, it will spread by 1.2mm. go ahead and multiply 1.2mm by the number of meters required to travel any where outside this solar system then tell me that a laser can be seen!!!

Sure, you can spend A LOT of money and have a much smaller divergence but given the distances the beam will travel, useless.

by **Chris Peterson** » Wed Aug 01, 2007 10:49 pm

akwaugh wrote:Ref. todays picture and comment regarding the effect of shining a laser at the galactic centre. Forgetting for the moment the distances and hence time frames involved and given that we, with our limited technology, are able to detect water molecules in distant atmospheres, is it not the case that no matter how much the light from the sun "blends" with the laser light, it should be possible for a sufficiently advanced civilisation to distinguish between coherent (laser) light and non-coherent (sun) light?

Not necessarily. First of all, there's no way that laser light traveling between stars will stay coherent. Coherence is a property that is lost pretty quickly with distance from the cavity. And simply being nearly monochromatic isn't a guarantee that the source is artificial- natural lasers probably exist, as well as natural narrowband filters. If the sender wants to be sure their signal is recognized as artificial, it's important to modulate it in some way. Low speed data could be transfered with simple AM over huge distances.

by **iamlucky13** » Wed Aug 01, 2007 5:49 pm

As for inter-galaxy warfare, when viewed from our Galaxy's center, no causalities are expected.

Causality aside, this was the part that caught my attention. Would there be casualties viewed from somewhere else? Do casualties obey relativity?

by **owlice** » Wed Aug 01, 2007 3:32 pm

tmneff wrote:"As for inter-galaxy warfare, when viewed from our Galaxy's center, no causalities are expected. "

And here I thought causality was at the core of astrophysics.

Nice to know I'm not the only language PITA who looks at APOD!

(It's been fixed, BTW.)

by **Qev** » Tue Jul 31, 2007 7:32 pm

A powerful laser will usually be visible (especially in the dark), since even the cleanest air outdoors is loaded with dust and water droplets that will scatter light from the beam. Only in a high vacuum or other 'perfect' environment, would the beam would be practically invisible.

As for the colour of the laser light, I imagine they'd be using a frequency close to whatever frequencies they'd be observing at, since the atmosphere is going to scatter or refract different frequencies of light by differing amounts.

by **ta152h0** » Tue Jul 31, 2007 7:27 pm

that was the point, to quantify how visible is the laser beam thru the earths atmosphere. Pass the brew, ice cold.

by **BMAONE23** » Tue Jul 31, 2007 7:22 pm

Since most Laser Light isn't naked eye visible, I would presume that the image was taken through a series of filters that would mave the light visible.

by **rower** » Tue Jul 31, 2007 6:36 pm

Why is the laser beam visible from the side view in the picture? Without dust in the atmoshpere it should not be able to be seen from a side view no matter how powerful. Is it drawn in, or am I incorrect?

by **iamlucky13** » Tue Jul 31, 2007 6:05 pm

Andy Wade wrote:I was wondering about this one, but thought it was because (1) we had a space telescope already, and (2) that the movements of the occupants and various docking procedures would 'knock' the telescope.

Additionally, the Hubble and other space telescopes have their own pointing priorities, while NASA likes to keep the ISS in certain orientations relative to the sun to help maintain proper temperatures. I imagine this would quite frequently conflict with observations. Also, the ISS orbit was chosen for accessibility to the Russians, and the I believe the Hubble orbit was chosen for observational opportunities. Of course, the future JWST won't be anywhere near either of those.

There would be a few small reasons to want a telescope on the ISS, like accessibility for maintenance, but the vibrations of people moving around alone would make it less useful than even a modest independent telescope.

Skylab and the Russian Saylut space stations were equipped with telescopes. Since then robotic space telescopes have eliminated the need.

I'm slightly confused though...how did this question arise from today's APOD?

by **tmneff** » Tue Jul 31, 2007 3:39 pm

"As for inter-galaxy warfare, when viewed from our Galaxy's center, no causalities are expected. "

And here I thought causality was at the core of astrophysics.

by **rigelan** » Tue Jul 31, 2007 1:40 pm

For the telescope that requires very cold temperatures, such as the infared telescope, it would be better to be a bit further away from the space station and its heat radiation. But it wouldn't need to be too far. It might be able to be a full mile away easily. Who knows.

by **makc** » Tue Jul 31, 2007 11:49 am

nothing to thank me for, it was copied from apod in question.

by **harry** » Tue Jul 31, 2007 11:23 am

Hello Markc

My son thanks you for the above link.

by **makc** » Tue Jul 31, 2007 10:42 am

akwaugh wrote:Ref. todays picture and comment regarding the effect of shining a laser at the galactic centre... it should be possible for a sufficiently advanced civilisation to distinguish between coherent (laser) light and non-coherent (sun) light?

That's the idea.

by **craterchains** » Tue Jul 31, 2007 10:10 am

It would be handy to park the telescope close to the ISS though I am not sure that would be a good orbit for the telescope.

by **akwaugh** » Tue Jul 31, 2007 9:20 am

Ref. todays picture and comment regarding the effect of shining a laser at the galactic centre. Forgetting for the moment the distances and hence time frames involved and given that we, with our limited technology, are able to detect water molecules in distant atmospheres, is it not the case that no matter how much the light from the sun "blends" with the laser light, it should be possible for a sufficiently advanced civilisation to distinguish between coherent (laser) light and non-coherent (sun) light?

(With apologies if this is a daft question - I'm a newbie!)

by **Andy Wade** » Tue Jul 31, 2007 6:51 am

ta152h0 wrote:http://antwrp.gsfc.nasa.gov/apod/ap070731.html

Has anybody ever thought of equiping the International Space Station with a telescope ?

I was wondering about this one, but thought it was because (1) we had a space telescope already, and (2) that the movements of the occupants and various docking procedures would 'knock' the telescope.

by **ta152h0** » Tue Jul 31, 2007 4:42 am

http://antwrp.gsfc.nasa.gov/apod/ap070731.html

Has anybody ever thought of equiping the Iinternational Space Station with a telescope ?

Starship Asterisk*

LASER beam (APOD 31 July 2007)

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