Because the optical synchrotron radiation from the Crab Nebula has been observed to be 40 to 60% linearly polarized and as much as 0.03% circularly polarized, and its 1415-MHz radiofrequency radiation to be as much as 0.05% right circularly polarized (5), it is clear that circularly polarized ultraviolet synchrotron radiation must also be present.

The argument of Bailey et al. (equally applicable to their mechanism) that circularly polarized synchrotron radiation would be ineffective for an enantioselective radiolysis of amino acids is based on the conclusion of Mason (reference 13 in the report) (6) that the "Kuhn-Condon zero sum rule" for circular dichroism bands precludes such effects. This apparent misinterpretation of the Kuhn-Condon rule has recently been corrected (7).

Edward Rubenstein
Department of Medicine,
Stanford University School of Medicine,
Stanford, CA 94305, USA
E-mail: exr{at}leland.stanford.edu
William A. Bonner
Department of Chemistry,
Stanford University,
Stanford, CA 94305, USA
E-mail: wbonner{at}leland.stanford.edu
George S. Brown
Department of Physics,
University of California, Santa Cruz,
Santa Cruz, CA 95064, USA
E-mail: gsbrown{at}cats.ucsc.edu

REFERENCES

1. J. Bailey, et al., Science 281, 672 (1998) [Abstract/Free Full Text] .
2. J. A. Roberts, Nature 308, 318 (1984) [Medline].
3. J. D. Jackson, Classical Electrodynamics (Wiley, New York, 1975), p. 678.
4. A. G. Lyne, Nature 308, 605 (1984) .
5. R. N. Manchester and J. H. Taylor, Pulsars (Freeman, San Francisco, 1977), pp. 57-68.
6. S. F. Mason, Nature 389, 804 (1997) [CrossRef] [Medline] .
7. W. A. Bonner, E. Rubenstein, G. S. Brown, Orig. Life Evol. Biosphere, in press.

Response: Rubenstein et al. state that "circularly polarized synchrotron radiation will be emitted along the axis parallel to the magnetic field." This is correct, however, only for low electron energies (cyclotron radiation, as observed astronomically in the AM Herculis binaries or Polars, which have high optical circular polarization). In the highly relativistic case (synchrotron radiation), relativistic beaming means that radiation is not emitted parallel to the magnetic field. The radiation is beamed into a narrow cone around the direction of particle motion (that is, perpendicular to the magnetic field), and predominantly linear polarization is produced.

The quoted figure of 0.03% circular polarization for the optical emission from the Crab Nebula is an upper limit. Circular polarization was not observed (1). Subsequent more sensitive work measured small levels (about 0.1%) of circular polarization, but the investigators attributed these results to birefringence resulting from aligned grains in the interstellar medium; they found nothing intrinsic to the nebula (2).

The 0.05% polarization at 1415 MHz is a small polarization at a frequency a million times less than the circular dichroism bands of interest. The available observations at optical wavelengths (within a factor of 2 of the ultraviolet circular dichroism bands) do not show significant circular polarization. The lack of circular polarization instrumentation in space, however, means that there have been no direct observations at the wavelength of interest.

Rubenstein et al. also dispute the conclusion by Mason (3) that broad-band circularly polarized light cannot be enantioselective. While there may not be complete cancellation, as Mason has stated, there is still likely to be substantial reduction in the enantiomeric excess that can be generated if the spectrum covers bands of alternating sign, as is likely to be the case with a spectrum such as that of the Crab Nebula, which extends into the far ultraviolet and x-ray regions of the spectrum. The problem is much less serious for our mechanism, which uses scattered starlight, because the spectra of most stars falls off steeply below 200 nm, which limits absorption to the longest wavelength circular dichroism band.

Jeremy Bailey
Anglo-Australian Observatory,
Post Office Box 296,
Epping, New South Wales, 2121, Australia
E-mail: jab{at}aaoepp.aao.gov.au

REFERENCES

1. J. D. Landstreet and J. R. P. Angel, Nature 230, 103 (1971) .
2. P. G. Martin, R. Illing, J. R. P. Angel, Mon. Not. R. Astron. Soc. 159, 191 (1972) [Web of Science].
3. S. F. Mason, Nature 389, 804 (1997) .