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Since the Sun is a gravitational lens with as focal length of 550 AU for visible light, with an immense amplification factor, shouldn't it light up objects hanging out there?

We should get solar sails up there to finally boost them somewhere interesting, opposite to the star that is emitting the light.

For neutrinos, I've heard that the focal length is 110 AU. Wouldn't a massive neutrino bombardement trigger interesting nuclear reactions (maybe that's not an astronomy question)?

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  • $\begingroup$ I wonder if Jupiter is massive enough to be a "useful" gravitation lens. $\endgroup$
    – Keith Thompson
    Dec 13, 2011 at 21:42
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    $\begingroup$ Where do you get the focal length of 550 AU? I don't think it can be characterized by a single focal length. A device that's characterizable by a single focal length deflects rays less if they're near the axis. The opposite is true for the sun. Also, what do you mean by "an immense amplification factor?" Do you mean high light-gathering power? For neutrinos, I've heard that the focal length is 110 AU. Where in the world did you get this? This is wrong. $\endgroup$
    – user4552
    Sep 2, 2013 at 16:24

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There will not be any kind of "massive bombardment" and it won't "light up" things out there.

However, using the Sun as a gravitational lens for a radio telescope is a real possibility. There was a lecture at the SETI Institute on 11/25/2009 titled "Deep Space Flight and Communications: SETI, KLT and Astronautics in a 2009 book" by Claudio Maccone, Co-Vice Chair of the SETI Permanent Study Group, International Academy of Astronautics. Here is a link to the page where you can download a .zip file of the presentation and you can see the video of the talk here.

The problem is that you have to choose exactly the one target that you want to image and then launch a space craft with, say, a radio telescope out to between 550 and 1000 AU on exactly the other side of the Sun from that object. For example, you might use this to image the black hole at the center of our galaxy, or if you got a SETI signal from a star you might deploy a satellite to examine it closely.

The reason it won't light up objects is because the gravitational field of the sun does not act like a perfect lens. In particular, at the distance of 550 AU only radio waves that just miss the limb of the sun will focussed to that point. At a greater distance, say 700 AU, there will be a ring further away from the sun where all rays will be focussed. The width of the ring that is focussed depends on the size of the radio dish on the satellite. The big advantage for signal gathering capabilities is that it is like having many many dishes of that size forming a ring around the sun!

UPDATE: For any of you in the south San Francisco Bay Area, there is a special colloquim at SLAC tomorrow (Friday, Dec 2, 2011 at 11AM). See the announcement and details here.

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  • $\begingroup$ How long would it take radio waves to be received on earth from 550AU? About 3 days 4 hours? Does that sound right? $\endgroup$
    – tomh
    Nov 16, 2015 at 12:18
  • $\begingroup$ @tomh - According to wolframalpha.com/input/… it would take 3 days 4 hours 14 minutes 12.63 seconds :-) But note that this is the time from when the radio waves just graze the Sun to when they are received at the radio telescope. So that number doesn't mean much. The waves have been travelling in space for many many years (depending on the distance in light years of the source). $\endgroup$
    – FrankH
    Nov 17, 2015 at 21:03
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A lens will focus light, and the sun's gravity does indeed focus light from objects behind it. The issue is that all stars aside from the sun are very far away so even with focusing, you will still not get anything like the equivalent of the sun's output. And there won't be massive bombardment of anything - despite having a focus, there just aren't enough particles available to be focused.

Also, that focal length. 110 AU - that's a very long way out. Pluto is about 40 AU out. Voyager 1 is about the right distance - it has been travelling for over 30 years.

And finally, at that distance out - a spacecraft may be at the focal point for light from a star, but the sun moves, the spacecraft moves...focused light from that star isn't going to be at that point for long.

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  • $\begingroup$ In fact I'm mostly interested in the question, if it possible to brighten up objects far out? Do you think this is possible? $\endgroup$
    – draks ...
    Nov 22, 2011 at 12:00
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    $\begingroup$ @draks... just take a flashlight with you. $\endgroup$
    – John Dvorak
    Feb 15, 2016 at 15:09
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Individual objects can act as gravitational lenses, this is referred to as microlensing. If you watch a group of stars for long enough, some will get brighter for a short period of time.

Gravitational lensing is more closely associated with much more massive objects such as galaxies and galaxy clusters. The Sun is simply too small to do much to light passing by other than give it a slight deflection, here's Wikipedia's take on it, light deflection by the Sun.

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    $\begingroup$ Don't get me wrong: I'm not staring at the sun at 1AU, but 550AU. $\endgroup$
    – draks ...
    Nov 22, 2011 at 12:01
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How does 200 metre resolution at Alpha Centauri grab you? http://www.cesr.fr/~pvb/gamma_wave_2005/presentations/optics/Koechlin.pdf

  • Wherever in space there are intelligent creatures like us, they will be driven to explore and understand our universe, just as we do. We and they wish to see to the farthest depths of space with the greatest clarity allowed by the laws of nature. To this end, we build, at great expense, ever more powerful telescopes of all kinds on Earth, and now in space.
  • As each civilization becomes more knowledgeable, they will recognize, as we now have recognized, that each civilization has been given a single great gift: a lens of such power that no reasonable technology could ever duplicate or surpass its power. This lens is the civilization's star. In our case, our Sun.
  • The gravity of each such star acts to bend space and thus the paths of any wave or particle, in the end creating an image just as familiar lenses do.
  • This lens can produce images which would take perhaps thousands of conventional telescopes to produce. It can produce images of the finest detail of distant stars and galaxies.
  • Every civilization will discover this eventually, and surely will make the exploitation of such a lens a very high priority enterprise.
  • One wonders how many such lenses are being used at this moment in time to scan the universe, capturing a flood of information about both the physical and biological realities of our time. 6
  • Frank Drake, 1999, from his Foreward to Claudio’s book.
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