Lensing: The Gravitational Imperative

by Paul Gilster on February 19, 2008

We usually think of gravitational lenses in terms of massive objects. When light from a distant galaxy is magnified by a galactic cluster between us and that galaxy, we get all kinds of interesting magnifications and distortions useful for astronomical purposes. But gravitational lensing isn’t just about galaxies. It happens around stars as well, as we saw recently with the discovery of a solar system with planets analogous to Jupiter and Saturn in our own system. That find was made with the help of a single star crossing in front of another, the resulting magnification allowing the signature of two planets around the closer star to be seen.

Interestingly enough, some of the earliest work on solar sails in interstellar environments came out of the attraction of taking advantage of the Sun’s own gravitational lens. Push some 550 AU out and you reach the point where solar gravity focuses the light of objects on the other side of the Sun as seen from a spacecraft. Note two things: At 550 AU, electromagnetic radiation from the occulted object is boosted by a factor of roughly 108. Secondly, gravity-focused radiation does not behave like light in a conventional optical lens in one important sense. The light does not diverge after the focus as the spacecraft continues to move away from the Sun. Indeed, the focal line extends to infinity.

The Italian aerospace company Alenia Spazio (based in Turin) began investigations into inflatable sail technologies as long ago as the 1980s. Since then, physicist Claudio Maccone has continued to investigate a mission he calls FOCAL, a probe to the gravity focus. Maccone sees such a mission as inevitable, for it takes advantage of an asset every technological civilization will ultimately want to exploit. Here’s how he puts it in his 2002 book The Sun as a Gravitational Lens: Proposed Space Missions:

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… This lens is the civilization’s star; in our case, our Sun. The gravity of each 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….Every civilization will discover this eventually, and surely will make the exploitation of such a lens a very high priority enterprise.

Maccone is the FOCAL mission’s most eloquent spokesman; his continuing travels and presentations on its behalf are part of the discovery process for our own civilization as we begin to see the possibilities opening up in nearby interstellar space. Another part of the discovery process is the distribution of news like the recent Hubble Space Telescope findings, which have demonstrated 67 new gravitationally lensed galaxies. Clearly, we are only beginning to understand the power of lensing to help us make sense out of even the most distant parts of the cosmos.

Hubble’s latest finds are part of a survey of a single 1.6 square degree field of sky with various space-based and Earth-based observatories. A team of European astronomers using Hubble’s Advanced Camera for Surveys (ACS) identified the new lenses, which were found around massive elliptical or lenticular galaxies without spiral arms or discs.

Here again we run into the issue of scale. Many of the gravitational lensing observations made thus far have involved not just single galaxies but whole clusters. Jean-Paul Kneib (Laboratoire d’Astrophysique de Marseille) notes the difference:

“We typically see the gravitational lens create a series of bright arcs or spots around a galaxy cluster. What we are observing here is a similar effect but on much smaller scale – happening only around a single but very massive galaxy.”

An Einstein ring

Four of the discovered lenses have produced ‘Einstein rings,’ which occur when a complete circular image of a background galaxy is formed around a foreground galaxy. Finding such lenses isn’t easy, and involves going through a catalog of more than two million galaxies by eye to identify possible candidates. Given the odd shapes that lensed galaxies can assume, filtering out a genuine lensing event from the observation of an unusually shaped galaxy takes time. But the European team now plans to train robot software on the lenses thus far found, hoping to identify still more.

Image: An Einstein ring can be seen in this image from the COSMOS project. An Einstein ring is a complete circle image of a background galaxy, which is formed when the background galaxy, a massive, foreground galaxy, and the Hubble Space Telescope are all aligned perfectly. Credit: NASA, ESA, C. Faure (Zentrum für Astronomie, University of Heidelberg) and J.P. Kneib (Laboratoire d’Astrophysique de Marseille).

The universe, it would seem, continually tries to tell us about itself through lensing inherent in the effect of mass upon spacetime. We’re a long way from such an infrastructure, but imagine the consequences for astronomy of having a wide variety of observational tools moving on spacecraft within the Sun’s gravity focus to examine targets ranging from the earliest galaxies to nearby stars and their planets. Such an outcome would depend, of course, upon our mastering propulsion technologies that can reach such distances within a single human lifetime. But it’s clear that the benefits of going interstellar won’t be found just around other stars. There’s plenty of work to do within 1000 AU of home.

{ 27 comments }

James M. Essig February 19, 2008 at 11:12

Hi Folks;

The plenty of work to do within 1,000 AU of home could be accomplished by nuclear electric propulsion schemes. I could see first generation fusion rockets getting us there in a timely manner. Other possibilities obviously include nuclear fission reactor powered electron or ion rockets. The required Isp would be greatly reduced compared to the maximum levels of perhaps as high as 3,000,000 for very effiecient fusion rockets to our stellar niegboors with 1/3 C terminal velocity.

By the way, I like photos of Einstein rings. I like to call them Mother Nature’s eyes.

Thanks;

Jim

Hans Bausewein February 19, 2008 at 13:09

I suppose lensing will also work in the other direction: put a radio source in the 550 AU focus and create a 100 million times amplified transmitter.

Maybe an idea for METI?

Or communicating with an Alpha Centauri probe?

Administrator February 19, 2008 at 13:31

Hans, communications come immediately to mind, a topic Dr. Maccone has explored as well. I must say that the METI notion never entered my thoughts on this, but of course you’re right — the focus would make such possible.

andy February 19, 2008 at 14:01

In Stephen Baxter’s novel Manifold:Space the gravitational focus was used to host a galactic transportation system.

AlfaCentavra February 19, 2008 at 14:23

Hubble Telescope 2.4m (diameter) in the 550 AU focus – it`s 100 million times amplifier – will be have power like telescope 24000m (diameter).

Hans Bausewein February 19, 2008 at 14:37

I should read more fiction

AlfaCentavra February 19, 2008 at 15:49

I suppose lensing will also work on gravitational waves and gravitational field of stars (black holes). Gravitational
pull most close stars, like Alpha Centauris A and B (total mass = 2 mass Sun) will be 822.56 times more strong than gravitational pull of Sun in 550 AU focus!

Alpha Centauri distance = 49681440000000km (4,3 light years).
550 AU = 82500000000km (0,000872 light years or 7,639 light hours).
It`s 100 million times amplifier in the 550 AU focus.

AlfaCentavra February 19, 2008 at 17:04

Analysis of the radio tracking data from the Pioneer 10 and 11 spacecraft at distances between 20–70 AU from the Sun has consistently indicated the presence of a small but anomalous Doppler frequency drift. The drift can be interpreted as due to a constant acceleration of (8.74 ± 1.33) × 10−10 m/s2 directed towards the Sun. Although it is suspected that there is a systematic origin to the effect, none has been found.
The nature of this anomaly could be the gravitational pull of stars, when the Pioneer 10 and 11 spacecraft is going more close to the border Suns focus in 550 AU.

Adam February 20, 2008 at 4:13

Hi AlfaCentavra

You might be right – I do wonder about what the effect of focussed gravity waves would be. I did see a paper or two a few years ago that was wondering what happens at the Sun’s neutrino focus – out near Uranus’ orbital radius. Could be some interesting effects if Franklin Felber’s relativistic repulsive gravity affects neutrinos as he predicts. Neutrinos have such huge gamma factors their repulsion effect should be huge, relative to their tiny rest masses (~0.5 eV.)

Just a thought.

James M. Essig February 20, 2008 at 5:56

Hi Folks;

One thing that occurred to me is that a gamma ray burst might be amplified many times due to a chance accultation by an intermeadiate star, blackhole, or other body with disasterous results. Supposedly, as I am sure some of the readership has heard, Eta Carena could go supernova at any time. If models of gamma ray burst that suggest that such bursts originate from supernova are correct, then if Eta Carena was occulted by another stellar body, Earth could be in for a real bad time, and likewise, any ETI civilization in the way. Even unmagnified by such an occultation, such a burst could potentially fry the Earth side of the planet even at 7,500 lightyears if only in terms of the damage or destruction of the atmospheric chemisty that maintains the habitability of our ecosystem. Luckily, the probability of Eta Carena projecting a gamma ray burst beam at Earth is small when it goes supernova and much less is the probability that it will be occulted by a gravitational lensing body at the suitable distance such that if the beam was pointed at us, it would be significantly magnified. Still, just one more potential danger we need to be aware of as we consider all of the stars that will produce gamma ray burst, assumming this model is correct, in the future.

Such occultation should not only be able to produce magnfied gravitational wave flux densities at Earth as suggested by AlfaCentavra, but should also be able to produce magnified nuetrino burst should a cosmic phenomenon producing such be appropriately occulted by a stellar body or other object. Luckily, neutrinos interact only very weakly with baryonic matter. Still, this is a neat prospect if only a remote chance occurance for observational purposes.

Thanks;

Jim

david February 20, 2008 at 10:01

Something I’ve at times dreamed about – having not just a telescope but a colony, or several of them, at that distance from the sun. Colonies that could build not just the small telescopes that a probe might carry but telescopes with lenses in the hundreds of meter range. Something that would really let us peer within other solar systems.

For a single probe I wonder how many near by stars would it be able to focus on. And how much detail would be possible.

As for actually getting there. Maybe in 20 or 30 years time we will be willing to invest the resources into it. In getting there we will need to do better than the voyager spacecraft. At the 3.5AU per year that voyager I is traveling it would take it 157 years to get 550AU out. DS1 I seem to recall reading somewhere would have a final velocity of 4.5k/s if it burned all its fuel. It would take that probe 581 years (without any gravitational assists). If you increased the fuel to 100 times what the DS1 had and used the same gravitational assists that voyager 1 used you might get a final velocity of 20AU per year. It would still be over a 30 year journey.

However if we were willing to invest in the research and the development propulsion methods giving 100,000 isp might be possible relatively soon (compared to 157 years anyway). I’m just not sure we humans are willing to make the commitment.

Can’t help but wonder just what sort of detail a telescope at that location would see if focused on the centauri system. If we could detect saturn like planets at 5000LY then imagine what sort of detail we might see at 4.37LY.

Administrator February 20, 2008 at 10:12

The view of Alpha Centauri from a FOCAL-style mission would presumably be spectacular. Eventually, to exploit a gravitational lens like this we’d need a fleet of telescopes moving about beyond 550 AU to target various stars, positioning themselves with the needed precision to get the lensing effect. But I would imagine a sufficiently advanced culture would put an operation like that into effect, learning a great deal about any target system before thinking about sending a probe in that direction. Surely this is in the toolbox of any Type II civilizations. As Dr. Maccone puts it, the ultimate lens is available for the taking.

Hans Bausewein February 20, 2008 at 15:17

Maybe Alpha Centauri is not the most logical target.

rough calculation:
1 AU = 8 light minutes, so 550 AU = 4400 light minutes = +- 0.2 ly

If we have the technology to go with sufficiently large telescopes to 550 AU, we probably also can go with a small probe to Alpha Centauri.

But it’s nice to hava a goal in between.

Administrator February 20, 2008 at 15:36

I don’t know, Hans. The gap beween 550 AU and 270000 AU (to Centauri A and B) is a mighty big one!

george scaglione February 20, 2008 at 15:46

paul,yes sir! i thought exactly the same thing! but hans,keep thinking that is ALWAYS a good idea! thank you paul,thank you hans, your friend george

Hans Bausewein February 20, 2008 at 15:57

Oops, forgot the hours. 24 times off.

It was 73 light hours, not days, so 0.008 ly
Yes, that’s only a tiny fraction of 4.3 ly (0.2 %)

Kevin February 25, 2008 at 16:47

It would be a very inflexible observatory. How does one train such a lens on more than one object? At a radius of 550 AU you’d have to drive the sail many AUs to look at a second object even just a few degrees away from the first.

Administrator February 25, 2008 at 18:13

I have to disagree, Kevin. A single observatory would still be able to view carefully chosen targets as it refined the technology for this kind of work. But think long-term: A culture with deep space propulsion capabilities can aim for a network of such observatories circling its star and using this amazing lens for observations that would be unavailable from any other source. I’m going to assume that a sufficiently advanced technology will almost certainly take this step, as it’s a logical way to push the envelope on astronomy to the max.

andy February 25, 2008 at 18:45

To take an example, if we want to scan, say, Alpha Centauri A and B, which have an average separation of 17.59 arcseconds, it involves moving the scope about 7 million km to aim at the other star in the system.

The habitable zones of both stars are approximately 1 AU in radius, which at a distance of 4.36 light years means the angular diameter would be 1.5 arcseconds across, which for a scope at 550 AU would mean moving 600,000 km to traverse the HZ.

Assuming we don’t constrain the orbits of habitable planets, we need to work out the area through which our telescope should sweep to ensure it can observe planets at any orientation within the habitable zone. If my calculations are correct, this turns out to be about 300 billion square kilometres, or about 550 times the total surface area (including oceans) of the Earth, per star.

So to conduct a good survey of the habitable zones of both of the Alpha Centauri stars, it would seem you need to have scopes over an area about 1100 times the surface area of the Earth…

All this assumes the scope is at 550 AU. Obviously as you go outwards you have to move further to re-aim the scope.

Of course, if you know where the planets are to begin with (and for that you’d need some pretty good astrometric measurements), things become rather less daunting, but on the face of it, I’d say Kevin’s got a good point here.

Administrator February 25, 2008 at 20:50

I’ve sent both comments (Kevin’s and andy’s) on to Dr. Maccone in hopes of a comment for further clarification. He may be traveling at the moment, but if not, I’ll hope to have more on this shortly.

ljk February 26, 2008 at 10:56

GRAVITAS: Portraits of a Universe in Motion

Authors: John Dubinski, John Kameel Farah

(Submitted on 25 Feb 2008)

Abstract: GRAVITAS is a self-published DVD that presents a visual and musical celebration of the beauty in a dynamic universe driven by gravity. Animations from supercomputer simulations of forming galaxies, star clusters, galaxy clusters, and galaxy interactions are presented as moving portraits of cosmic evolution. Billions of years of complex gravitational choreography are presented in 9 animations – each one interpreted with an original musical composition inspired by the exquisite movements of gravity.

The result is an emotive and spiritually uplifting synthesis of science and art. The GRAVITAS DVD has been out for two years now but I am now making the DVD disk image freely available for personal and educational use through a bittorrent download. Download and burn at your leisure. The animations are also downloadable in various video formats.

Comments: Link to animations and burnable DVD image at this http URL

http://www.galaxydynamics.org/gravitas.html

Subjects: Astrophysics (astro-ph); Popular Physics (physics.pop-ph)

Cite as: arXiv:0802.3664v1 [astro-ph]

Submission history

From: John Dubinski [view email]

[v1] Mon, 25 Feb 2008 17:44:52 GMT (10kb)

http://arxiv.org/abs/0802.3664

Administrator February 27, 2008 at 9:59

Dr. Maccone does seem to be traveling, as I feared, so I won’t try to make his arguments for the FOCAL mission as much as reiterate my own thought that exploiting the 550 AU gravitational focus will demand propulsion breakthroughs that would allow these observatories to move as necessary to do their work. A non-trivial task, to be sure, with today’s technology, but we’re always working toward better solutions, and once we do have the capability of moving such spacecraft effectively (and over the necessary distances), a series of observing stations could do astronomy at unprecedented levels. No one is arguing this is near-term, but an initial FOCAL mission as proof of concept is a natural follow-on to projects like Innovative Interstellar Explorer. Long-term, there is much to do at 550 AU and beyond.

ljk February 27, 2008 at 14:44

Galactic globular clusters contribution to microlensing events?

Authors: Fabiana De Luca, Philippe Jetzer

(Submitted on 26 Feb 2008)

Abstract: In this note we perform an analysis of the large set of microlensing events detected so far toward the Galactic center with the purpose of investigating whether some of the dark lenses are located in Galactic globular clusters. We find that in four cases some events might indeed be due to lenses located in the globular clusters themselves. We also give a rough estimate for the average lens mass of the subset of events being highly aligned with Galactic globular cluster centers and find that, under reasonable assumptions, the deflectors could most probably be either brown dwarfs, M-stars or stellar remnants.

Comments: 11 pages, 3 figures, accepted for publication in International Journal of Modern Physics D

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0802.3827v1 [astro-ph]

Submission history

From: Philippe Jetzer [view email]

[v1] Tue, 26 Feb 2008 15:12:48 GMT (346kb)

http://arxiv.org/abs/0802.3827

ljk February 28, 2008 at 7:14

Gravitational Lens Systems to probe Extragalactic Magnetic Fields

Authors: D. Narasimha, S. M. Chitre

(Submitted on 27 Feb 2008)

Abstract: The Faraday rotation measurements of multiply-imaged gravitational lens systems can be effectively used to probe the existence of large-scale ordered magnetic fields in lensing galaxies and galaxy clusters.

The available sample of lens systems appears to suggest the presence of a coherent large-scale magnetic field in giant elliptical galaxies somewhat similar to the spiral galaxies.

Comments: 11 pages, 1 figure

Subjects: Astrophysics (astro-ph)

Journal reference: Current Science, Vol 93, 10th December 2007, 1506-1513

Cite as: arXiv:0802.4044v1 [astro-ph]

Submission history

From: Delampady Narasimha [view email]

[v1] Wed, 27 Feb 2008 16:55:06 GMT (21kb)

http://arxiv.org/abs/0802.4044

Daniel Medicis February 28, 2008 at 22:40

Gentlemen,

Reading through the posts, a thought comes to mind – if we could drop a telescope far away enough from the sun to take advantage of gravitational lensing, don’t we already have that opportunity with nearby stars, i.e. use Alpha Centauri as a lens for something in it’s distant path? Apologies if this seems a bit of a simple approach to this question.

Administrator February 29, 2008 at 9:25

This is a good point, Daniel, and you’re right — we live in a universe that is filled with potential lensing effects due to the effect of mass on spacetime. The fact that the gravity-focused radiation from the Sun remains along the focal axis (i.e., the focal line extends to infinity for separations greater than 550 AU) is interesting, in that it implies other stars can also be used in a similar way, since we would be able to take advantage of that focused radiation. Here’s the problem (and now I’m quoting Greg Matloff): “…the off-axis gain decreases with the inverse square root of the off-axis distance.” In other words, there is a spot-radius, or distance from the center line of the image, where the image intensity gain falls by a factor of 4, and it’s quite narrow. Matloff points out that a telescope at 2200 AU from the Sun would work with a spot radius of 11 kilometers. I take this to mean that the farther we are away from the gravitational focus of another star, the narrower that spot radius is going to be (I think I’ve got that right, but I’m just an amateur at this kind of optics, and hope anyone will jump in if corrections are needed).

For now, it appears that the best use of other stars is in ‘micro-lensing,’ where a star moving in front of another, more distant one creates lensing effects (changes in light intensity in particular) that can demonstrate the presence of planets around the nearer star. That’s just occurred with the discovery of a Jupiter and Saturn-class planet orbiting a star about 5000 light years from Earth:

http://www.centauri-dreams.org/?p=1719

So we have much to learn about how we can use distant objects for lensing, but it’s clear that this science is developing rapidly. I hope we see a FOCAL-style mission set out one day to explore its possibilities.

ljk March 10, 2008 at 23:15

LensPerfect: Gravitational Lens Massmap Reconstructions Yielding Exact Reproduction of All Multiple Images

Authors: D. Coe, E. Fuselier, N. Benitez, T. Broadhurst, B. Frye, H. Ford

(Submitted on 9 Mar 2008)

Abstract: We present a new approach to gravitational lens massmap reconstruction. Our massmap solutions perfectly reproduce the positions, fluxes, and shears of all multiple images. And each massmap accurately recovers the underlying mass distribution to a resolution limited by the number of multiple images detected.

We demonstrate our technique given a mock galaxy cluster similar to Abell 1689 which gravitationally lenses 19 mock background galaxies to produce 93 multiple images. We also explore cases in which far fewer multiple images are observed, such as four multiple images of a single galaxy. Massmap solutions are never unique, and our method makes it possible to explore an extremely flexible range of physical (and unphysical) solutions, all of which perfectly reproduce the data given. Each reconfiguration of the source galaxies produces a new massmap solution. An optimization routine is provided to find those source positions (and redshifts, within uncertainties) which produce the “most physical” massmap solution, according to a new figure of merit developed here.

Our method imposes no assumptions about the slope of the radial profile nor mass following light. But unlike “non-parametric” grid-based methods, the number of free parameters we solve for is only as many as the number of observable constraints (or slightly greater if fluxes are constrained). For each set of source positions and redshifts, massmap solutions are obtained “instantly” via direct matrix inversion by smoothly interpolating the deflection field using a recently developed mathematical technique. Our LensPerfect software is straightforward and easy to use and is made publicly available via our website.

Comments: 17 pages, 18 figures, accepted by ApJ. Software and full-color version of paper available at this http URL

Subjects: Astrophysics (astro-ph)

Cite as: arXiv:0803.1199v1 [astro-ph]

Submission history

From: Dan Coe [view email]

[v1] Sun, 9 Mar 2008 18:39:12 GMT (2604kb)

http://arxiv.org/abs/0803.1199

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