As it is said in the previous sub-chapter, there are 4 fundamental forces acting in our Universe: gravity, strong nuclear, weak nuclear and electromagnetic. One of the challenges which physicists have faced during the recent years has precisely been to unify these forces in one single primordial force, in an epoch in which the Universe was ruled by the high energies.
Cooling and Symmetry Break-Up
It is thought that the division of the initial force into 4 distinct forces (what is called the symmetry break-up) was due to the cooling of the Universe. Stephen Hawking compares this symmetry break-up to the one that occurs when a ball swings around a roulette at a high speed. When the roulette slows down, the ball tends to fall in one of its 37 holes, which may represent 37 different states in which the ball can exist. Therefore, one may say that, in the beginning, 4 balls were swinging around the "Universe" roulette showing similar behaviours and moving at identical velocities. When the "Universe" roulette slowed down (lost energy), the 4 balls ended up falling in 4 holes of the roulette, each one with its own particular characteristics.
The Electroweak Force
The electromagnetic and the weak nuclear forces were the first ones that theoreticians achieved to unify. The messenger particle of the electromagnetic force (the photon) has an infinite range of action, in opposition to the messenger particles of the weak nuclear force (W+, W- and Z0, commonly known as intermediate bosons) which are weakened in space due to their large masses. For that reason, they lose intensity just after crossing a very short distance.
But it's presumed that the field responsible for the weakening of the intermediate bosons (called the field of Higgs) vanishes under high-energy conditions. This way, such a high-energy environment provokes the disappearance of the distinctions that set apart the weak nuclear force from the electromagnetic force, which are merged into an unified force - the electroweak force. The intermediate bosons were observed for the first time in 1983, under the high energies generated at the CERN (Centre Européen de Recherche Nucléaire), in Geneva. It was then proven the unification between these two forces.
Bird-eye view of the CERN in Geneva, where in 1983 was proven the existence of the electroweak force
The GUT Force
Following this achievement, scientists looked for reaching the unification between the electroweak and the strong nuclear forces, into the so-called GUT (Grand Unified Theory). The strong nuclear force is the most powerful of the 4 mentioned forces but has a short range of action, as the weak nuclear force.
Some experiences in big particle accelerators (like the CERN) indicate that the strong nuclear force is weakened under high-energy conditions, so then the quarks (particles that are joined by this force) and the gluons (messenger particles of the mentioned force) behave themselves almost as free particles. On the other hand, both the electromagnetic force and the weak nuclear force become more powerful under such high energies.
Therefore, there must be a determined energy level, a very high one, where elecroweak forces (weak nuclear + electromagnetic) and the strong nuclear force have the same intensity and, thus, assume themselves as being different aspects of a single force. The GUT also predicts that the quarks and electrons were essentially different aspects of the same particles in the primordial Universe, so then another unification is obtained.
The Ultimate Unification
To finish, we miss unification between the gravity and the GUT force, which shall be achieved at even higher energies. The first step for attaining this purpose is to combine the general relativity (which describes the gravity and is deterministic) with the principle of uncertainty of the quantum theory (which describes the other 3 forces). That reconciliation is eased by the conception of a particle that, similarly to the other bosons, obeys to that principle and is the responsible for the transportation of the gravity force - the graviton.
Superparticle with Multiple Personality
The graviton is a particle with a spin of 2, contrasting with the other bosons, which have a spin of 1. Therefore, for unifying gravity (spin of 2) with GUT force (spin of 1), it's necessary to conceive a theory proposing that particles with different spins may be seen as different aspects of a single super-particle. This way, not only the gravity and the GUT force are unified, but also is achieved an unification between the fermions (with fractional spins) and the bosons (gravity and GUT, with whole numbered spins).
One theory that quite successfully explains a Universe where the symmetry is actually certified is the superstrings theory which is not, nevertheless, experimentally proved. According to this theory, the particles are not infinitely tiny dots in space, but rather unidimensional entities (similar to lines) which may be closed strings (as rings) or open strings (as segments). This way, the story of each particle (its evolution across the time dimension) is represented by a bidimensional surface which may take the shape of a ribbon, in the case of the open strings, or the shape of a tube, in the case of the closed strings. Two particles that are attracted or repelled by one force (gravity, electromagnetic of nuclear) may so be seen as two strings united by a third string, which represents the messenger of the force. At the points where the 3rd string touches the other two, the behaviour of these ones is affected by the intervention of the force.
The several particles can be described as string vibrations. The vibrations of the open strings (which have sizes comparable to the Planck's extent - 10-32 mm - below which any observation becomes impossible) generate bosons with spin of 1. The open strings can become closed strings, but in that case they generate different particles, as the gravitons (bosons with a spin of 2).
Closed string (MoonRunner Design UK)
The 10 Dimensions Universe
The superstring theory predicts that the Universe originally existed in 10 dimensions. But at 10 dimensions the Universe should be unstable, because it existed in a state of false vacuum, different from the inferior state of energy. The analogy that can be made is the water retained by a dam: apparently it exists in a state of low energy (because it's relaxed), but its truly inferior state of energy would only be achieved if the water could surpass the dam for freely flow heading the sea. As it is unstable at 10 dimensions, the Universe was probably partitioned in two - one with 4 dimensions (ours) and another with 6 dimensions. It was eventually that violent split that gave birth to the Big Bang, according to the physicist Michio Kaku. The 6 dimensional universe would have been, meanwhile, contracted to an astonishingly small size, several quadrillions inferior to that of an atomic nucleus.
Stephen Hawking defends, on the other hand, that in primordial times, all the dimensions should be very folded. Let's imagine a hair (thread-shaped): it actually contains 3 dimensions: height, largeness and length. Nevertheless, as it is seen by us, it looks like an unidimensional line, stripped from any largeness or length, since the measures of both assume very little values (almost unobservable), opposing to measures of its height, which can reach several centimetres. Therefore, for some reason, 4 dimensions (the time dimension and the 3 space dimensions) would have been unfolded, while the other 6 would have been kept folded as before. It's possible that there are universes where more than 4 dimensions were unfolded or universes where more dimensions were kept folded. Is it possible the existence of life in such universes?
The 5th Dimension
How is it possible to imagine a universe with 5 dimensions (4 space dimensions and 1 time dimension)? To be able to establish a comparison between our Universe and one 5 dimensional universe, let's imagine that someone living in a 3 dimensional universe (2 space dimensions and 1 time dimension) would face the existence of a universe like ours.
The 3 dimensional universe would be comparable to an enormous flat sheet (bidimensional) where the events would occur along a time dimension (as in our universe). Let's imagine that a ball crosses the flat sheet: an observer would see it not as a sphere, but as a circle that would arise from nothing, it would expand and contract again until it would finally disappear. In a 4 dimensional universe, a cube can be represented on a paper sheet as a large square (the front side), containing a smaller square (the back side), where each apex of the large square is linked to an apex of the smaller square by an edge. It is perfectly possible for us to visualize all the sides of the cube (turning it around), in the same way as it is possible for us to see the entire representation of the cube, even if the edges are opaque. To one creature living in a 3-dimensional universe that's not possible. Unless that creature breaks the edges of the exterior square, it will never be able to see the interior (smaller) square, or in other words, the representation of the 4-dimensional universe cube.
Therefore, by analogy, the representation of a cube existing in a 5-dimensional universe shall be given to us by a cube inside another cube. If the sides of the exterior cube are opaque, it's obvious that we'll never be able to see the interior cube, unless we break it. Even more difficult is to imagine a universe that is also endowed with 5 dimensions, but where there are 2 time dimensions and 3 dimensions. Or are there universes where the dimensions assume forms other than space or time?
3 dimensional cube, its 2 dimensional representation, a 3 dimensional representation of a 4 dimensional cube
Searching for the Last Theory
Should this be the ultimate theory about reality? Possibly not. And is there any unified theory about reality? There are 3 possible answers to this question:
An even more metaphysical question is: Why did cosmos make such an effort for existing? Is the unified theory so imperative that originates its own existence?