Brief Answers to the Big Questions

by Stephen Hawking

One can show that to create a wormhole one needs to warp space–time in the opposite way to that in which normal matter warps it. Ordinary matter curves space–time back on itself, like the surface of the Earth. However, to create a wormhole one needs matter that warps space–time in the opposite way, like the surface of a saddle. The same is true of any other way of warping space–time to allow travel to the past if the universe didn’t begin so warped that it allowed time travel. What one would need would be matter with negative mass and negative energy density to make space–time warp in the way required. Energy is rather like money. If you have a positive bank balance, you can distribute it in various ways. But, according to the classical laws that were believed until quite recently, you weren’t allowed to have an energy overdraft. So these classical laws would have ruled out us being able to warp the universe in the way required to allow time travel. However, the classical laws were overthrown by quantum theory, which is the other great revolution in our picture of the universe apart from general relativity. Quantum theory is more relaxed and allows you to have an overdraft on one or two accounts. If only the banks were as accommodating. In other words, quantum theory allows the energy density to be negative in some places provided it is positive in others. The reason quantum theory can allow the energy density to be negative is that it is based on the Uncertainty Principle...