8 Are there other universes?
Our universe is a very unlikely place. Alter some of its settings even slightly and life as we know it becomes impossible. In an attempt to unravel this "fine-tuning" problem, physicists are increasingly turning to the notion of other universes. If there is an infinite number of them in a "multiverse" then every combination of settings would be played out somewhere and, of course, you find yourself in the universe where you are able to exist. It may sound crazy, but evidence from cosmology and quantum physics is pointing in that direction.
11 What's so weird about prime numbers?No, this is not a scientific question and a solution will not affect the internet.
The fact you can shop safely on the internet is thanks to prime numbers – those digits that can only be divided by themselves and one. Public key encryption – the heartbeat of internet commerce – uses prime numbers to fashion keys capable of locking away your sensitive information from prying eyes. And yet, despite their fundamental importance to our everyday lives, the primes remain an enigma. An apparent pattern within them – the Riemann hypothesis – has tantalised some of the brightest minds in mathematics for centuries. However, as yet, no one has been able to tame their weirdness. Doing so might just break the internet.
17 What's at the bottom of a black hole?No, relativity teaches that we can never know what happens inside a black hole. M-theory cannot tell us anything about it.
It's a question we don't yet have the tools to answer. Einstein's general relativity says that when a black hole is created by a dying, collapsing massive star, it continues caving in until it forms an infinitely small, infinitely dense point called a singularity. But on such scales quantum physics probably has something to say too. Except that general relativity and quantum physics have never been the happiest of bedfellows – for decades they have withstood all attempts to unify them. However, a recent idea – called M-Theory – may one day explain the unseen centre of one of the universe's most extreme creations.
20 Is time travel possible?Time travel to the past is science fiction. Simple thought experiments show that the concept does not make sense.
Time travellers already walk among us. Thanks to Einstein's theory of special relativity, astronauts orbiting on the International Space Station experience time ticking more slowly. At that speed the effect is minuscule, but ramp up the velocity and the effect means that one day humans might travel thousands of years into the future. Nature seems to be less fond of people going the other way and returning to the past, however some physicists have concocted an elaborate blueprint for a way to do it using wormholes and spaceships. It could even be used to hand yourself a present on Christmas Day, or answer some of the many questions that surround the universe's great unknowns.