Another Revolution in Optics?
July 4, 2005
A 50-Fold Increase in Optical Resolution
The once-stodgy branch of physics known as optics must have shown little change from the time Chester More Hall invented the achromatic lens around 1735 though the19th century. The reinstatement of the wave theory of light and its linkage to electromagnetic radiation would have altered the theoretical underpinnings of optics during the latter half of the 19th century, but optical construction still depended upon the laborious grinding and figuring to optical surfaces. In the second half of the 20th century, automated lens grinding techniques and international competition permitted mass production, and much lower prices for microscopes and telescopes. Also, during the past few decades, active optics and non-linear optics have expanded our optical horizons. Now comes a new invention, featured in tonight's Science News (Nanoantennas Superfocus Light), that could possibly trigger another revolution in optics by permitting a 50-fold improvement in optical resolution from features that are 0.25 microns to features that are 0.005 microns (5 nanometers) across. This means that in 10 or 15 years, a 400X optical microscope might be replaced by a 20,000X optical microscope--a size domain currently accessible only to electron microscopes. Similarly, a 1,000X clinical microscope would be able to muster 50,000X, spotting objects the sizes of individual protein molecules. Given oil immersion and techniques such as digital image enhancement, it might be possible to identify features even smaller than 5 nanometers.
One problem with such improved magnifications is that the image becomes dimmer the more it's magnified. This new approach might require time exposures along with image processing to fully exploit its capabilities.
Mapping Planets Around Other Stars
If this nanoantenna approach can be used to magnify images produced by primary telescope mirrors, the possibility suggests itself (to me) that it might be used to 50-fold the resolutions of telescopes. This may be problematical, applying only to microscopes if the optical nanoantennas must be placed within a few nanometers of what they're examining. However, if these nanoantennas can somehow magnify the images produced by the primary telescope mirrors, then great improvements in telescopic resolution would be also be feasible. It's interesting to consider what size telescopes would be required to required to map the surfaces of planets around neighboring stars. (NASA has a space-based planet-finding mission planned for 2012.)
Resolution versus Light-Gathering Power
The resolution of a telescope is given by 0.1 second of arc/aperture (in meters). A telescope with a 100 mm (4-inch) aperture will have a resolving power of one arc-second, or about one part in 200,000. For example, the Mt. Palomar telescope, with an aperture of 200 inches, has a resolving power of 1/50th arc-seconds, or one part in 10,000,000. That means that it can resolve two light sources that are one kilometer apart at a distance of 10,000,000 kilometers.
The nearest stellar system, at a-Centauri, is about 4.3 light-years or 43 trillion kilometers away. This means that the Mt. Palomar (Hale) telescope can, in principle, resolve two light sources that are 4.3 million kilometers apart in the Centauri system. (In practice, it can't do that well because of atmospheric blurring, aggravated by the fact that the Hale Telescope isn't equipped with active optics to reduce twinkle.)
A telescope is currently under construction (the Magellan telescope, World's largest telescope begins with a spin - New Scientist) that is 5 times the diameter of the Mt. Palomar telescope, increasing its resolving power to about 1,000,000 kilometers.
The European Union is currently contemplating the construction of a telescope (the Overwhelmingly Large Telescope, or OWL) that would be 4 times the size of the Magellan telescope, improving its resolving power to 250,000 kilometers. If it were possible to 50-fold this resolution, we would be looking at a resolution of about 5,000 kilometers, or with image enhancements, to possibly spot continents on planets light-years away. If we could improve that resolution by a factor of 50, we would be looking at resolutions of the order of 100 kilometers, or sufficiently detailed that we might be able to spot major river deltas on a distant planet. (Linear features can be detected well below Dawes Limit.)
To go beyond this may require space-based telescopes that, in the absence of gravity, could conceivably even larger. Also, interferometric telescopes might be constructed with baselines measured in kilometers and resolutions in the tens or hundreds of kilometers.... fine enough to reveal continental details.
The ultimate interferometric telescope in the Solar System might utilize a one-billion kilometer baseline. Without assuming a 50-folding of its resolving power, it could support a resolution of one centimeter at a-Centauri.
More reasonably, if we imagined a space-based interferometric telescope with a 100-kilometer separation, it should be able to resolve 250-kilometer features at the distance of the Centauri system. (Of greater interest may be oxygen-detection measurements that will strongly suggest whether or not life exists on distant planets.)
The second leg of the planetary observation tripod is that of light-gathering power. Extrasolar planets are going to be many orders of magnitude fainter than planets here in the Solar System. Of the eight other planets, three of them, Uranus, Neptune and Pluto, are too faint to discern with the naked eye. The most distant planet that is plainly visible in the night sky is Saturn, which, at its closest approach is about 1.25 billion kilometers from us. The pupil of the human eye has a diameter of the order of 5 millimeters. The Magellan telescope, with its 25-meter diameter, will have an aperture that is about 5,000 times that of the naked eye. This means that a planet like Saturn that is 5,000 times as far away as Saturn or about 6.25 trillion kilometers from Earth would have the same apparent brightness as Saturn. For the OWL, that number would rise to 25 trillion kilometers. Clearly, we shouldn't have any trouble spotting planets around nearer stars with the OWL, and should probably be able to visually observe them with the Magellan telescope.
Separating the Planet from Its Star
The third leg of the tripod is that of being able to see dim, dim planets in the glares of their parental stars. That may be a function of resolution. Given resolutions of a few million kilometers, and the blocking techniques that are used to see the solar corona, it may soon be feasible to spot at least Jupiter-sized planets from the Solar System. (Actually, one such planet has already been observed.) And given that, it shouldn't take long to begin identifying smaller, rockier planets.
Other Stars without Leaving the Solar System
I suspect that it won't be terribly long before ideas begin to percolate about studying the topography of extra-solar planets from our Solar System. That would seem to be cheaper, quicker, and technically less-demanding than trying to send probes to other stellar systems. In any case, the information gathered in this way would be of vital importance in determining what to examine in planning an interstellar mission.