MAKING TIME Travel Easier

Time travel this term always fascinates me s a student of science and technology, so here I am again writing something about it which considered as a subject of intense research during last twenty years. Here we discuss some perspective of time travel and how it can be achieved. This time travel concept again goes back to Einstein and his special theory of relativity in a remarkable paper written in 1905, when he was a cleric the Swiss patent office, Einstein showed that the time and position at which one thought an event occurred, depended on how one was moving. 

This meant that time and space were inextricably bound up with each other. The times that different observers would assign to events would agree if the observers were not moving relative to each other. But they would disagree more, the faster their relative speed. So one can ask, how fast does one need to go, in order that the time for one observer, should go backward relative to the time of another observer.

The answer to this question is we that the relative speed must be greater than that of light but Einstein's paper of 1905 seemed to rule out time travel into the past. It also indicated that space travel to other stars was going to be a very slow and tedious business. If one couldn’t go faster than light, the round trip to the nearest star would take at least eight years, and to the center of the galaxy, at least eighty thousand years. If the spaceship went very near the speed of light, it might seem to the people on board that the trip to the galactic center had taken only a few years. But that wouldn't be much consolation if everyone you had known was dead and forgotten thousands of years ago when you got back. That wouldn't be much good. But there is some method which shows us a ray of hope in the dark sea of time travel.

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 spaceship 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 shortcut, 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 need 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 the 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 a 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 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 electromagnetic 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 vacuums 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 cannot measure them directly with a particle detector.

However, one can observe their effects indirectly. One way of doing this is 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 antiparticles. 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, Where vacuum fluctuations can have any wavelength. The reduction in the number of virtual Particles between the plates means that they don't hit the plates so often and thus don't exert As much pressure on the plates, as the virtual particles outside. There is thus a slight force Pushing the plates together. This force has been measured experimentally. So virtual particles Actually exist and produce real effects.

Because there are fewer virtual particles, or vacuum fluctuations, between the plates, they have a lower energy density, than in the region outside. But the energy density of empty Space far away from the plates must be zero. Otherwise, it would warp space-time, and the Universe wouldn't be nearly flat. So the energy density in the region between the plates must be negative. We have experimental evidence from the bending of light, that space-time is curved, and Confirmation from the Casimir effect, that we can wrap it in the negative direction. So it might seem possible, that as we advance in science and technology, we might be able to construct a Wormhole, or warp space and time in some other way, so as to be able to travel into our past.

According to string theory, which is our best hope of Uniting General Relativity and Quantum Theory, into a Theory of Everything, space-time ought to have ten dimensions, not just the four that we experience. The idea is that six of these ten dimensions are curled up into a space so small, that we don't notice them. On the other hand, the remaining four directions are fairly flat and are what we call space-time.

If this picture is correct, it might be possible to arrange that the four flat directions got mixed up with the six highly curved or warped directions. What this would give rise to, we don't yet know. But it opens exciting possibilities. The conclusion is that rapid space-travel, or travel back in time, can't be ruled out, according to our present understanding. They would cause a great logical problem but science fiction fans need not lose heart. There's hope in string theory.