Newton’s tree, which grew at Woolsthorpe Manor, his Lincolnshire home, was
blown down around 1820, nearly a century after his death. A team led by Madan
Thangavelu of Wye College near Ashford, Kent, is now collecting DNA from the
presumed clones and subjecting them to DNA fingerprinting.

DNA fingerprints are banded patterns that vary according to differences in
the length of sections of “junk DNA”, where certain sequences are repeated over
and over. Fingerprinting a few of these regions will not prove that any of the
trees is genuine, as there could be differences elsewhere in the genome. But the
technique should identify obvious imposters.

A tree at Woolsthorpe Manor has the best-documented link to Newton’s
original. Richard Keesing, a physicist at the University of York, went to
Woolsthorpe with a copy of a sketch from 1820 showing the fallen tree. “I stood
looking through a camera and walked slowly away from the house, trying to get
the same view, when I bumped into an apple tree,” he says. Keesing believes the
tree is a natural clone, rerooted from the fallen original.

Samples from five alleged clones have already been tested against the
Woolsthorpe tree. Most, including two in Cambridge, at the Isaac Newton
Institute and Trinity College, seem from preliminary results to have strong
claims to be genuine. But one fingerprint, from a tree in the President’s Garden
at the Massachusetts Institute of Technology, seems suspiciously unlike the
others. “It is consistently different across a good many sampling points,” says
Thangavelu.

Magnetic fields are known to change the electrical resistance of materials by deflecting electrons-a phenomenon called magnetoresistance. Now Anja Sparenberg, Bart van Tiggelen and Geert Rikken from the Max Planck Institute for Solid State Physics in Grenoble, France, have shown that the same effect occurs for photons of light.

The Grenoble team measured the transmission of light through a clear plastic disc containing europium fluoride. Although the effect was small, they found that the transparency of the plastic decreased in proportion to the square of the strength of a magnetic field across the disc.

When in motion, charged particles such as electrons create their own magnetic fields. When an electron encounters an external magnetic field, the two fields interact, deflecting the electron. But photons of light are uncharged. Even so, says Sparenberg, “the photons in the magnetic field behave as if they were carrying electrical charge”.

Sparenberg says the explanation lies in the molecular properties of the disc. Light is scattered by atoms that get in its way. The magnetic field could change the electrons’ orbits and alter the amount of scatter. Europium has shown the strongest effect so far (Physical Review Letters, vol 79, p 757).

Photonic magnetoresistance is one of several discoveries that draw parallels between the behaviour of photons and electrons. Last year, the Grenoble team found that the Hall effect-in which the paths followed by current flowing in an electrical conductor are bent by a magnetic field-has an equivalent for light. Called the photonic Hall effect, the phenomenon is also thought to be caused by the magnetic field affecting the matter that the light is travelling through.

Ultimately, photonic magnetoresistance could be used for switching in fibre-optic communications. But it will be some time before the discovery can be put to use. “The effect is still very small,” says Sparenberg. “We are looking for materials that will display a larger effect than europium fluoride.”

Sajeev John of the University of Toronto, who researches light scattering, is cautiously optimistic about the potential applications. “It is interesting that you can manipulate photons in this way,” he says. “With optics, you can get much more precision than you can with electrical devices.”