
Interstellar Communication via Gravitational Lens - vackosar
http://www.centauri-dreams.org/?p=35486
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kurthr
TLDR: You’re reading that right — one-tenth of a milliwatt radio is enough to
create error-free communications between the Sun and Alpha Centauri through
two FOCAL antennas. Maccone’s paper assumes two 12-meter FOCAL antennas.

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JPLeRouzic
Actually the proposal is a bit more complicated than using a 12 meter large
antenna, it is sending this antenna at 550 AU (the FOCAL in "FOCAL antenna").
Geoffrey A. Landis wrote:

"The gravitational field of the sun will focus light from a distant source to
a focal point at a minimal distance of 550 Astronomical Units from the sun.

A proposed mission to this gravitational focus could use the sun as a very
large lens, allowing (in principle) a large amplification of signal from the
target, and a very high magnification."

[https://arxiv.org/abs/1604.06351](https://arxiv.org/abs/1604.06351)

~~~
JPLeRouzic
Some people as Louis Friedmann says that only moving 10 meters away from the
focal point (at 550 au) would reduce the gain by 10 dB.
[https://www.youtube.com/watch?v=eEZ6zYZ6lDQ](https://www.youtube.com/watch?v=eEZ6zYZ6lDQ)

That said I like people proposing big ideas.

~~~
kurthr
What's interesting about this is that with closed loop control (using multiple
antennas on the very signal they are receiving) one might well be able to
achieve that 10m accuracy.

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credit_guy
Maybe their math is correct, but the presentation here doesn't seem right.
"Not so — there is no focal ‘point’ here but a focal line. As we move through
and past 550 AU, we take advantage of the fact that the focal line extends to
infinity."

Light is reversible, even if the space is curved. If all light from the "focal
line" gathers at the focal point on the other side of the star, where does it
go in reverse?

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dandelany
> If all light from the "focal line" gathers at the focal point on the other
> side of the star...

It doesn't - the light on the focal line may be from any source that falls on
the same line on the opposite side of the star. But since the chances of more
than one light source (distant star) falling on this exact same line are
pretty minuscule, it's still a (potentially) useful lens

~~~
credit_guy
I don't understand what you are trying to say. There is no such thing as a
focal line. More generally, it's not possible for light from several points to
focalize at the same point because you can't revert those rays. I guess Randal
Monroe explains it better than me here:

[https://what-if.xkcd.com/145/](https://what-if.xkcd.com/145/)

However, I can see one way to read what they mean. If you take the Sun's
"focal point", the point where parallel lines from one side intersect after
being bent by its gravity, and move a tiny away from the Sun, then you form
the image of a point that resides on the line going from you through the Sun
all the way to infinity on the other side. You need to move very little for
the opposite to move a lot, so for measurement purposes you are still at 590
AU from the Sun. This way with very tiny moves you could survey the planets in
a different solar system, and in particular you could resolve them to multi-
pixels.

