Chown's Cosmos: The Sun at night
The 'Super-K' detector is built 3,000 feet down in a mine beneath Hida in Japan. This is one of its most famous images. Marcus Chown explains
This picture of the Sun is hardly high-definition. But, in its own way, it is extraordinary. Why? Because it was taken at night. It was taken looking down through the Earth. And it was taken not with light but with neutrinos.
Neutrinos are ghostly subatomic particles which are created in abundance by the sunlight-generating nuclear reactions in the core of the Sun. To them solid matter is as transparent as a pane of glass.
Hold up your hand. You would never know it but about a 100 million million neutrinos are passing through every square centimetre of your flesh every second. That’s why it is possible to image the Sun on the other side of the Earth by looking down through almost 13,000 kilometres of rock.
This picture was obtained by the Japanese Super-Kamiokande neutrino detector, situated in the Kamioka metal mine in the Japanese Alps. While sunlight takes about 30,000 years to work its way out from the centre to the surface of the Sun, neutrinos take just two seconds.
Once at the surface, it is only another eight-odd minutes of free-flight before they get to the Earth. Consequently, neutrinos reveal what the core of the Sun is like “now”.
Since the Sun’s light was made at the height of the last Ice Age, for all we know its nuclear fires could have gone out 29,000 years ago. However, solar neutrinos, on account of being in the heart of the Sun just over eight minutes ago, tell us all is well with the Sun and there is no need to worry. For now.