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...humans. They are both relatively easy to reclaim from the rocks in which they’re found, and are easy to work. Gold is extremely unreactive and doesn’t tarnish like most other metals. Consequently, gold jewellery can survive essentially unchanged for thousands of years. Gold is also extremely heavy, with a density of 19.4 g cm-3. The density of lead, by comparison, is...

...human head. Inside the head, a symbol for the parameter theta is visible. A smaller figure stands on a ladder leaning against the head. This person is actively reaching inside to "extract" information. As a result of this extraction, a stream of disorganised numbers (like 1, 5, 7, 6, 2) is shown tumbling out of the head and falling downward. A large arrow points from the...

...scans the skies, hoping to hear from other intelligent species. But how can you be sure you've tuned in to aliens and not just got interference? Andy Norton explains...[An alien in a phone box] Phone home ... yeah, right ... by Jinterwas Astronomers working at the Arecibo radio telescope in Puerto Rico have detected a weird radio signal, spotted when pointing their