Duh. But according to albert einstein, THE INTRIGUING notion that time might run backwards when the Universe collapses has run into difficulties. Raymond Laflamme, of the Los Alamos National Laboratory in New Mexico, has carried out a new calculation which suggests that the Universe cannot start out uniform, go through a cycle of expansion and collapse, and end up in a uniform state. It could start out disordered, expand, and then collapse back into disorder. But, since the COBE data show that our Universe was born in a smooth and uniform state, this symmetric possibility cannot be applied to the real Universe.
Physicists have long puzzled over the fact that two distinct "arrows of time" both point in the same direction. In the everyday world, things wear out -- cups fall from tables and break, but broken cups never re- assemble themselves spontaneously. In the expanding Universe at large, the future is the direction of time in which galaxies are further apart.
Many years ago, Thomas Gold suggested that these two arrows might be linked. That would mean that if and when the expansion of the Universe were to reverse, then the everyday arrow of time would also reverse, with broken cups re-assembling themselves.
More recently, these ideas have been extended into quantum physics. There, the arrow of time is linked to the so-called "collapse of the wave function", which happens, for example, when an electron wave moving through a TV tube collapses into a point particle on the screen of the TV. Some researchers have tried to make the quantum description of reality symmetric in time, by including both the original state of the system (the TV tube before the electron passes through) and the final state (the TV tube after the electron has passed through) in one mathematical description.
Murray Gell-Mann and James Hartle recently extended this idea to the whole Universe. They argued that if, as many cosmologists believe likely, the Universe was born in a Big Bang, will expand out for a finite time and then recollapse into a Big Crunch, the time-neutral quantum theory could describe time running backwards in the contracting half of its life.
Unfortunately, Laflamme has now shown that this will not work. He has proved that if there are only small inhomogeneities present in the Big Bang, then they must get larger throughout the lifetime of the Universe, in both the expanding and the contracting phases. "A low entropy Universe at the Big Bang cannot come back to low entropy at the Big Crunch" (Classical and Quantum Gravity, vol 10 p L79). He has found time-asymmetric solutions to the equations -- but only if both Big Bang and Big Crunch are highly disordered, with the Universe more ordered in the middle of its life.
Observations of the cosmic microwave background radiation show that the Universe emerged from the Big Bang in a very smooth and uniform state. This rules out the time-symmetric solutions. The implication is that even if the present expansion of the Universe does reverse, time will not run backwards and broken cups will not start re- assembling themselves.
Barker's a friggin homosexual.
In one of the wildest developments in serious science for decades, researchers from California to Moscow have recently been investigating the possibility of time travel. They are not, as yet, building TARDIS lookalikes in their laboratories; but they have realised that according to the equations of Albert Einstein's general theory of relativity (the best theory of time and space we have), there is nothing in the laws of physics to prevent time travel. It may be extremely difficult to put into practice; but it is not impossible.
It sounds like science fiction, but it is taken so seriously by relativists that some of them have proposed that there must be a law of nature to prevent time travel and thereby prevent paradoxes arising, even though nobody has any idea how such a law would operate. The classic paradox, of course, occurs when a person travels back in time and does something to prevent their own birth -- killing their granny as a baby, in the more gruesome example, or simply making sure their parents never get together, as in Back to the Future. It goes against commonsense, say the sceptics, so there must be a law against it. This is more or less the same argument that was used to prove that space travel is impossible.
So what do Einstein's equations tell us, if pushed to the limit? As you might expect, the possibility of time travel involves those most extreme objects, black holes. And since Einstein's theory is a theory of space and time, it should be no surprise that black holes offer, in principle, a way to travel through space, as well as through time. A simple black hole won't do, though. If such a black hole formed out of a lump of non-rotating material, it would simply sit in space, swallowing up anything that came near it. At the heart of such a black hole there is a point known as a singularity, where space and time cease to exist, and matter is crushed to infinite density. Thirty years ago, Roger Penrose (now of Oxford University) proved that anything which falls into such a black hole must be drawn into the singularity by its gravitational pull, and also crushed out of existence.
But, also in the 1960s, the New Zealand mathematician Roy Kerr found that things are different if the black hole is rotating. A singularity still forms, but in the form of a ring, like the mint with a hole. In principle, it would be possible to dive into such a black hole and through the ring, to emerge in another place and another time. This "Kerr solution" was the first mathematical example of a time machine, but at the time nobody took it seriously. At the time, hardly anybody took the idea of black holes seriously, and interest in the Kerr solution only really developed in the 1970s, after astronmers discovered what seem to be real black holes, both in our own Milky Way Galaxy and in the hearts of other galaxies.
This led to a rash of popular publications claiming, to the annoyance of many relativists, that time travel might be possible. In the 1980s, though, Kip Thorne, of CalTech (one of the world's leading experts in the general theory of relativity), and his colleagues set out to prove once and for all that such nonsense wasn't really allowed by Einstein's equations. They studied the situation from all sides, but were forced to the unwelcome conclusion that there really was nothing in the equations to prevent time travel, provided (and it is a big proviso) you have the technology to manipulate black holes. As well as the Kerr solution, there are other kinds of black hole time machine allowed, including setups graphically described as "wormholes", in which a black hole at one place and time is connected to a black hole in another place and time (or the same place at a different time) through a "throat". Thorne has described some of these possibilities in a recent book, Black Holes and Time Warps (Picador), which is packed with information but far from being an easy read. Now, Michio Kaku, a professor of physics in New York, has come up with a more accessible variation on the theme with his book Hyperspace (Oxford UP), which (unlike Thorne's book) at least includes some discussion of the contribution of researchers such as Robert Heinlein to the study of time travel. The Big Bang, string theory, black holes and baby universes all get a mention here; but it is the chapter on how to build a time machine that makes the most fascinating reading.
"Most scientists, who have not seriously studied Einstein's equations," says Kaku, "dismiss time travel as poppycock". And he then goes on to spell out why the few scientists who have seriously studied Einstein's equations are less dismissive. Our favourite page is the one filled by a diagram which shows the strange family tree of an individual who manages to be both his/her own father and his/her own mother, based on the Heinlein story "All you zombies --". And Kaku's description of a time machine is something fans of Dr Who and H.G. Wells would be happy with:
consists of two chambers, each containing two parallel metal plates. The intense electric fields created between each pair of plates (larger than anything possible with today's technology) rips the fabric of space-time, creating a hole in space that links the two chambers.
Taking advantage of Einstein's special theory of relativity, which says that time runs slow for a moving object, one of the chambers is then taken on a long, fast journey and brought back: Time would pass at different rates at the two ends of the wormhole, anyone falling into one end of the wormhole would be instantly hurled into the past or the future .
And all this, it is worth spelling out, has been published by serious scientists in respectable journals such as Physical Review Letters (you don't believe us? check out volume 61, page 1446). Although, as you may have noticed, the technology required is awesome, involving taking what amounts to a black hole on a trip through space at a sizeable fraction of the speed of light. We never said it was going to be easy! So how do you get around the paradoxes? The scientists have an answer to that, too. It's obvious, when you think about it; all you have to do is add in a judicious contribution from quantum theory to the time travelling allowed by relativity theory. As long as you are an expert in both theories, you can find a way to avoid the paradoxes.
but I'm going to have to give this one to Barker....as much as I loved DW's (and especially the little Cloud sprite), I think the simple look of Barker's matched the render better.
Take it like a man 
