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Quantum oddities could help explain how chemical reactions get started in the freezing near-vacuum of interstellar space.

Space is cold, really cold. That should mean very little happens, and yet there are stars, planets and people. Now an ultracold experiment could help explain how chemical reactions get started in the freezing near-vacuum of interstellar space.

It seems that, far from staying aloof, some atoms actually find it easier to get cosy with each other at temperatures close to absolute zero, which is -273.15 °C, or 0 kelvin. The reason could be to do with that well-known mischief-maker, quantum physics, which causes particles to behave like waves and to exist in myriad states at once.

Textbooks show atomic nuclei surrounded by electrons in different energy levels, or shells. Atoms form bonds by sharing or exchanging electrons, usually so that pairs of electrons fill the outermost shell. So closed-shell atoms, whose outer shells are full, are expected to be fairly complacent and resistant to change. In contrast, so-called radicals have a single unpaired electron and are anxious to give it away or steal another electron to keep it company.

However, this simple picture had never been tested at a fraction of a degree above absolute zero. Now Wade Rellergert at the University of California, Los Angeles, and his colleagues have found that calcium, which has a pair of electrons in its outer shell, is more reactive at low temperatures than the radical rubidium. "The reaction rate can go flying up when things are very cold," says team member Scott Sullivan, also at UCLA.

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