Thread: Can someone please explain this optical SETI page to me like I'm a kid?

1. Can someone please explain this optical SETI page to me like I'm a kid?

It's short, but if TLDR. I'm mainly interested in this paragraph:

If the reader observes the antenna diameter given in columns "A" and "B" on the left of the table under the heading "OPTICAL" (near-infrared), if will be seen that the transmitting telescope aperture (uplink) is constrained to be only 22.5 cm in diameter. Why was this assumption made?

Even if one assumed that ETIs would have difficulty with the point-ahead targeting of nearby stars, since the beams would be smaller than the zones of life around these stars, why would one cripple the long range performance of the transmitter. One could simply build the largest diffraction limited transmitter affordable for ones civilization and just defocus it when targeting nearby stars. In this way, the inverse-square law would not apply, and the energy density of the beam would be essentially independent of distance out to thousands of light years!
Last edited by parallaxicality; 2019-Aug-03 at 05:57 PM.

2. Let me rephrase this as clearly as I can:

1. Why would Project Cyclops assume that aliens would limit their transmitters to just 22.5 cm in diameter?

2. How would limiting your transmitter to such a small area compensate for the problem of the beams being too narrow?

3. Why would limiting your diffraction cause the beam to widen, and why would the inverse-square law not apply?

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Originally Posted by parallaxicality
Let me rephrase this as clearly as I can:

1. Why would Project Cyclops assume that aliens would limit their transmitters to just 22.5 cm in diameter?
It drops out of their assumption that 1" beams are what the transmitter is designed for. They don't assume that is a limitation, it is just the parameter that drops out of their attempts to do a fair comparison of systems at different wavelengths. See page 48 of the Project Cyclops report: "We have assumed a limiting beam width of 1 sec of arc at all wavelengths."

Originally Posted by parallaxicality
2. How would limiting your transmitter to such a small area compensate for the problem of the beams being too narrow?
Larger systems produce narrower beams. I think I gave you links to the diffraction limit in a previous post.

Originally Posted by parallaxicality
3. Why would limiting your diffraction cause the beam to widen, and why would the inverse-square law not apply?
See above. And the inverse square law doesn't apply because this is a collimated beam. The inverse square law applies to isotropic radiators.

4. Sorry; I don't mean to be testy. I've been put into a situation where I have to research something I don't understand and not so much at the end of my tether as searching for it in baggage claim.

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Originally Posted by parallaxicality
Sorry; I don't mean to be testy. I've been put into a situation where I have to research something I don't understand and not so much at the end of my tether as searching for it in baggage claim.
Don't see any testiness here.

My brief reading of the original report section on comparisons gave me the impression that they were trying to make a fair comparison but they had to make a lot of assumptions about it to make this comparison. This leads to the slightly strange constraints in the report.

The optical SETI page you linked to doesn't seem to make this link and instead starts making claims about the reasons for the constraints that are at odds with the original paper. The paper did not conclude that aliens were inept, it concluded that for the scenario given microwave SETI was a better option. Altering your assumptions or the hardware considerations might change that conclusion dramatically.

I have to say that the optical SETI page you gave as a reference raises several flags for me. It comes across as partisan and presents what looks very much like a cherry picked set of statements to argue against. My understanding, as a bystander rather than a part of the community, is that a big part of the non-optical focus of SETI was not about energy budgets or things like that - it was simply that it was easier to search wider areas quicker with existing hardware (when SETI was just starting and budgets were small) in the RF part of the spectrum. Optical searches are slower and take more time unless you invest in very specialised hardware, which is expensive. Time on optical telescopes was also generally harder to get. So it wasn't "Boo, optical is rubbish!" it was simply "RF is easier and more accessible for us".

6. Originally Posted by Shaula
And the inverse square law doesn't apply because this is a collimated beam. The inverse square law applies to isotropic radiators.
Are you saying they can produce a beam with a zero dispersion angle? A typical laser, at least the "1 watt" laser I have has a dispersion angle of about 0.7 deg.

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Originally Posted by George
Are you saying they can produce a beam with a zero dispersion angle? A typical laser, at least the "1 watt" laser I have has a dispersion angle of about 0.7 deg.
Nope, I am just saying that the source is not isotropic. There will still be a radial distance dependency on the power received (which will have an r-squared in it) but it will not be the traditional inverse square law

Re-reading the source paragraph my comment may be irrelevant because I actually don't think that was what it was saying.

8. Originally Posted by Shaula
Nope, I am just saying that the source is not isotropic. There will still be a radial distance dependency on the power received (which will have an r-squared in it) but it will not be the traditional inverse square law.
I'm curious how this might work. Is the gaussian distribution such that the dispersion angle varies across the radius of the beam?

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Originally Posted by George
I'm curious how this might work. Is the gaussian distribution such that the dispersion angle varies across the radius of the beam?
Maybe I am not saying what I mean particularly well.
When I say the traditional inverse square law I am talking about: https://en.wikipedia.org/wiki/Inverse-square_law
Versus this: https://en.wikipedia.org/wiki/Gaussi...eam_parameters

So at a given range the beam diameter for a Gaussian beam is linearly related to the distance yielding an inverse square law with a different constant of proportionality to that for an isotropic radiator.

I read the paragraph as saying that a collimated beam (especially one with a defocused beam and hence low divergence) would put more power on the target than that of an isotropic radiator. Re-reading the paragraph I hadn't taken note of the claim that the intensity was distance independent which I don't think is practical other than, perhaps, for some of the self-focusing helical beam states.

10. Originally Posted by Shaula
Maybe I am not saying what I mean particularly well.
When I say the traditional inverse square law I am talking about: https://en.wikipedia.org/wiki/Inverse-square_law
Yep.

Versus this: https://en.wikipedia.org/wiki/Gaussi...eam_parameters
So at a given range the beam diameter for a Gaussian beam is linearly related to the distance yielding an inverse square law with a different constant of proportionality to that for an isotropic radiator.
That looks interesting but...

Using a Wo = 4mm (narrowest beam width)
Wavelength - 480 nm (blue laser)

Zo = 11,630 lyrs, which is where the surface intensity drops by root 2 (~ 70%). I hate it when I'm off by about 11,630 lyrs., though this is closer than some estimates I've done elsewhere.

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Optics are getting better for this kind of thing:

http://www.space.com/are-aliens-flas...ser-beams.html
http://www.spacedaily.com/reports/Re...light_999.html
https://www.nextbigfuture.com/2019/0...es-better.html

“What we’ve done is shown from a scientific perspective and empirically that these optics can be built” using an inexpensive, abundantly available material that is immune from the internal stresses that can change the shape of X-ray mirrors made of glass, the more traditional mirror-making material...

12. Originally Posted by George
Yep.

That looks interesting but...

Using a Wo = 4mm (narrowest beam width)
Wavelength - 480 nm (blue laser)

Zo = 11,630 lyrs, which is where the surface intensity drops by root 2 (~ 70%). I hate it when I'm off by about 11,630 lyrs., though this is closer than some estimates I've done elsewhere.
Yikes! I fouled the units. This example has a root 2 reduction in beam width after about 100 meters.

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