The colours emitted by an atom are a dead giveaway for its identity, but now American physicists have worked
out how to make any atom impersonate any other—by hitting it with a specially
crafted pulse of light.
The technique realises “an
aspect of the alchemist dream to make different elements or materials look
alike,” study author Andre Campos and colleagues from Princeton University describe
in the journal Physical Review Letters,“albeit for the duration of a control laser
For 200 years scientists have identified chemicals by the light they
emit. You can recognise a sodium lamp from its orange glow. It’s the same
orange you get by holding salt to a flame, a standard trick of chemistry. Astronomers
can deduce the components of stars or dust clouds from their particular light fingerprint.
Now, the Princeton team show that, in theory at least, these
fingerprints can be forged.
As the Danish physicist Niels Bohr and others worked
out in the early 20th century, atoms and molecules emit light when the
electrons around them shuffle about between different levels of energy. An electron
falling from a high energy level to a lower one gives out the colour of light
that exactly matches the energy difference.
It’s a bit like a dog’s squeaky toy tumbling down the
stairs, making a different sound depending on the height of the steps.
What Campos’s team realised was that they could use
lasers to excite an atom into a state of any energy, at least temporarily, by
hitting it with a specially designed light pulse. When the electron fell back
down, it would then emit whatever light colour the physicists chose.
To illustrate their idea, the researchers worked out
how to make hydrogen “look like” argon
This means that in principle astronomers detecting what
they assume to be a cloud of argon gas might be mistaken.
[I]t could be a
little green man with a fancy laser trying to mislead us by making hydrogen
atoms look like argon atoms,” study co-author Denys Bondar told the American
In fact, two atoms or
molecules can be made to look alike, by applying specific pulse shapes. In
general, this means that simply measuring the light given out by a material is
not enough to know what it’s composition. You need to know the input light,
vealing a surprising
flexibility in the behaviour of light and matter, the study might also be put
to use for designing special materials with weird properties—perhaps molecules
that fluoresce at will at different colours.
It might also be useful helping in biology, where it
is a major challenge to distinguish very similar molecules in certain mixtures.
Specific laser pulses will cause different molecules to emit different colours,
and so provide a means to tell between them.
Though the team haven’t done the actual experiment
yet, they point to recent work in Nature which show the technology to do it should be available. On to the
Cathal O'Connell is a science writer based in Melbourne.
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