Closely related to time travel, is the ability to travel rapidly from one position in space, to another. As I said earlier, Einstein showed that it would take an infinite amount of rocket power, to accelerate a space ship to beyond the speed of light. So the only way to get from one side of the galaxy to the other, in a reasonable time, would seem to be if we could warp space-time so much, that we created a little tube or wormhole. This could connect the two sides of the galaxy, and act as a short cut, to get from one to the other and back while your friends were still alive. Such
wormholes have been seriously suggested, as being within the capabilities of a future civilization. But if you can travel from one side of the galaxy, to the other, in a week or two, you could go back through another wormhole, and arrive back before you set out. You could even manage to travel back in time with a single wormhole, if its two ends were moving relative to each other.
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.
This says that certain quantities, like the position and speed of a particle, can’t both have well defined values. The more accurately the position of a particle is defined, the greater is the uncertainty in its speed, and vice versa. The uncertainty principle also applies to fields, like the electro-magnetic field, or the gravitational field. It implies that these fields can’t be exactly zeroed, even in what we think of as empty space. For if they were exactly zero, their values would have both a well-defined position at zero, and a well-defined speed, which was also zero. This would be a violation of the uncertainty principle. Instead, the fields would have to have a certain minimum amount of fluctuations. One can interpret these so called vacuum fluctuations, as pairs of particles and anti particles, that suddenly appear together, move apart, and then come back together again, and annihilate each other. These particle anti particle pairs, are said to be virtual, because one can not
measure them directly with a particle detector.
However, one can observe their effects indirectly.
One way of doing this, is by what is called the
Casimir effect. One has two parallel metal plates, a
short distance apart. The plates act like mirrors for
the virtual particles and anti particles. This means
that the region between the plates, is a bit like an
organ pipe, and will only admit light waves of certain
resonant frequencies. The result is that there are
slightly fewer vacuum fluctuations, or virtual
particles, between the plates, than outside them,