Einstein, Hawking, and Time Machines
Relativity and Spacetime
Albert Einstein's Theory of Relativity is complex and touches upon many different areas of interest. For the sake of this report, however, we will narrow it down to one subcategory: gravity and its effect on spacetime.
Until Einstein's General Theory of Relativity was published in 1916, gravity was perceived only as a force of attraction between objects based on the mass of the objects. Einstein, however, had come across certain anomalies that led him to formulate a new definition for gravity. To understand this definition, one must first grasp the concept of spacetime.
Spacetime is a reference to the direct relation between space and time. Rather than time and space being independent of one another, they are considered to be part of the same "fabric", a four-dimensional grid, and are affected as such. Spacetime is illustrated using embedded diagrams, such as the one below, where the plane represents spacetime and the sphere in the centre represents and object with mass and gravity.
With that in mind, Einstein suggested that gravity is not a force of attraction based on mass, but rather a curvature of spacetime which affects the paths of objects moving through it. Below is an example of one of the methods that was used to prove this theory:
This illustration, although terribly out of proportion, demonstrates the path of light from a distant star being curved by the gravitational field of the sun. An observer on Earth would perceive the star to be in a different location than it actually is. When objects with strong gravitational fields to bend light and distort images of objects behind them, the result is called an Einstein Ring.
Above left is an image of the true position of light-emitting objects in space. Above right shows the distortion which would occur if there was a massive black hole between the observer and the images. The gravitational field of the black hole acts to bend the light around it, a process similar to diffraction except that it occurs within spacetime.
Another effect of gravity on light travelling through spacetime is the gravitational lens. An object with a sufficient gravity field can act as a kind of "lens" in space, producing multiple distorted images of objects behind it.
Another phenomenon that sprang from Einstein's ideas of gravity was the gravitational wave. A gravitational wave is a wave that propagates at the speed of light and uses spacetime as its medium. This means that instead of moving through spacetime, as with most energy and matter, gravitational waves warp spacetime as they move through it. Gravitational waves are generated by supermassive accelerating objects, such as binary star or black hole systems. These systems involve two stars (or two black holes, or a black hole and a star) which orbit around each other. Binary systems experience something called inspiralling, in which the bodies gradually move closer together. In doing so, they lose gravitational potential energy, and release it in the form of gravitational waves. This type of wave exists everywhere in the universe, but is undetectable in our area because there are no bodies massive enough to generate strong waves.
One might ask what all this has to do with black holes. Black holes, as they are extremely massive but relatively small in size, cause great curvatures in spacetime. A long distance from a black hole, objects could orbit normally without being "sucked in", but the closer to the singularity, the more severe the distortion in spacetime becomes. Remember that space and time are linked; so an object falling into the singularity would experience not only a change in space, but also a change in time.
Einstein proposed the theory of time dilation. It states that speed and time are connected, just as speed and mass are connected (E=mc^2). An object gains mass as it increases its speed, and so the force required to continue its acceleration increases. As speed approaches c, mass and force required approach infinity; this is why it is impossible for anything with mass to travel at the speed of light.
In a similar way, time is affected by acceleration as well. To an observer, a moving clock would appear to tick slower than a stationary clock. The principle of time dilation is that time does not flow at a fixed rate. It tends to slow as it approaches the speed of light. This will be illustrated in the tour.
Quantum Field Theory
In the early 1970's, reknowned physicist Stephen Hawking produced a theory based on Quantum Mechanics and the Uncertainty Principle.
Uncertainty Principle
The more precisely the position of an object is defined,
the less precisely its speed can be defined,
and vice versa.
This principle is applicable to particles as well as to electro-magnetic fields. It says that even in "empty space", these fields cannot have values equal to zero; if the position was zero, the speed would be uncertain, and if the speed was zero, the position would be uncertain. Therefore, Hawking suggested, and proved, that vacuum fluctuations occur.
A vacuum fluctuation involves a pair of virtual particles, one particle and one antiparticle, that exists in virtual space. These virtual particles have the ability to defy the Law of Conservation of Energy for a very short time. The pair will materialize in real space (using up energy), then instantaneously destroy each other (restoring the stolen energy). This is important to black holes in that it determines their lifespan. The curvature of spacetime around a black hole is so extreme that at the moment a virtual particle pair materializes, one particle is drawn into the singularity and the other is radiated away from the black hole. In order for the radiated particle to enter real space, it steals a bit of energy from the black hole, thus reducing its energy and mass.
However, combined with Einstein's theories, there is another application for vacuum fluctuations...
Theoretical Time Travel
Stephen Hawking realized that attempting time travel by going through a wormhole or attaining the speed of light was impossible, but he suggested another approach in his Lecture on Space and Time Warps. He suggested that in order to travel through time, we must find a way to negatively warp spacetime. This would save us the trouble of travelling to the nearest black hole or inventing a Ferrari that defies the Laws of Relativity. If spacetime could be warped in the opposite way from the way gravity warps it... well, something neat would happen.
It has been proven that spacetime can be warped in such a way, at least on a very small scale. This is known as the Casimir Effect. It was demonstrated by placing two metal plates (which act to limit the frequencies of virtual particles, kind of like polarized lenses) in close proximity to one another. The energy density between the plates should be slightly less than the density outside the plates, and a force pushing the plates together should be detected. This proves the vacuum fluctuation theory. It also proves the existence of an area with a negative energy density, since the density outside the plates must be equal to zero (or else it would warp spacetime) and the density inside the plates is less than that.
Now, what does it all mean? Not a whole lot right now. Being able to travel through negative spacetime and into the past is just a speculative theory, but it's an interesting one at least.
Back to the Future... I mean, Table of Contents!
