John Gribbin Quote

Motion in space can proceed in any direction and back again. Motion in time only proceeds in one direction in the everyday world, whatever seems to be going on at the particle level. It’s hard to visualize the four dimensions of spacetime, each at right angles to the other, but we can leave out one dimension and imagine what this strict rule would mean if it applied to one of the three dimensions we are used to. It’s as if we were allowed to move either up or down, either forward or back, but that sideways motion was restricted to shuffling to the left, say. Movement to the right is forbidden. If we made this the central rule in a children’s game, and then told a child to find a way of reaching a prize off to the right-hand side (backward in time) it wouldn’t take too long for the child to find a way out of the trap. Simply turn around to face the other way, swapping left for right, and then reach the prize by moving to the left. Alternatively, lie down on the floor so that the prize is in the up direction with reference to your head. Now you can move both up to grasp the prize and down to your original position, before standing up again and returning your personal space orientation to that of the bystanders.* The technique for time travel allowed by relativity theory is very similar. It involves distorting the fabric of space-time so that in a local region of space-time the time axis points in a direction equivalent to one of the three space directions in the undistorted region of space-time. One of the other space directions takes on the role of time, and by swapping space for time such a device would make true time travel, there and back again, possible. American mathematician Frank Tipler has made the calculations that prove such a trick is theoretically possible. Space-time can be distorted by strong gravitational fields,and Tipler’s imaginary time machine is a very massive cylinder, containing as much matter as our sun packed into a volume 100 km long and 10 km in radius, as dense as the nucleus of an atom, rotating twice every millisecond and dragging the fabric of space-time around with it. The surface of the cylinder would be moving at half the speed of light. This isn’t the sort of thing even the maddest of mad inventors is likely to build in his backyard, but the point is that it is allowed by all the laws of physics that we know. There is even an object in the universe that has the mass of our sun, the density of an atomic nucleus, and spins once every 1.5 milliseconds, only three times slower than Tipler’s time machine. This is the so-called millisecond pulsar, discovered in 1982. It is highly unlikely that this object is cylindrical—such extreme rotation has surely flattened it into a pancake shape. Even so, there must be some very peculiar distortions of space-time in its vicinity. Real time travel may not be impossible, just extremely difficult and very, very unlikely. That thin end of what might be a very large wedge may, however, make the normality of time travel at the quantum level seem a little more acceptable. Both quantum theory and relativity theory permit time travel, of one kind or another. And anything that is acceptable to both those theories, no matter how paradoxical that something may seem, has to be taken seriously. Time travel, indeed, is an integral part of some of the stranger features of the particle world, where you can even get something for nothing, if you are quick about it.