Here is my third article on the Multiverse topic. You can find the first two here and here. This article is going to be a little bit longer than the previous ones, but I encourage all the interested readers to go through an entire article because the most interesting part will be hidden at the end.
Today I’m talking about the Brane Multiverse which is one of several types of parallel Universes based on the solution called String Theory. This theory itself is a very interesting approach and it has a lot of its own distinctive properties which certainly need to be explained in a freestanding article and I will definitely write such an article in the future. In this series, however, I will focus only on the properties which should necessarily be explained taking the question of corresponding Multiverse into consideration. As always, I give you a reference to Brian Greene’s book “The Hidden Reality” which provides a very clear explanation of all these types of Multiverse I describe in my articles. And I also want to provide the quote from Timothy Ferris’ review in New York Times where he reports that “If extraterrestrials landed tomorrow and demanded to know what the human mind is capable of accomplishing, we could do worse than to hand them a copy of this book.”
Ok, let’s start with some basics of the String Theory which we will need to know about while reading to this article and the next two. I guess most of the readers might already be familiar with these concepts, yet I must provide a brief description to make things clear. The string theory is a solution whose ultimate purpose is a unification of all the physics under a single framework. We have two major XX century theories that are very precise and elegant descriptions of physical phenomena on various scales. On the one hand, we have Einstein’s General Relativity which operates extremely precisely on the large scales of stars, galaxies, galaxy clusters and the like. On the other hand, we have quantum mechanics which is even more precise description of the behavior of matter on atomic and subatomic scales.
However, combining the equations of these theories we find profound inconsistencies between the two in some situations. You might wonder why we should care – after all, atoms and large structures are completely different things – maybe these two theories just work perfectly in their own fields and there is no sense in attempting to find a unified theory. But as it turns out there are situations demanding a theory that would be capable to work with both tiny structures, perfectly described by quantum theory, and huge masses, perfectly described by General Relativity. One example is well known thing called singularity which is sitting at the centers of black holes. Whilst singularity is a ‘place’ with a vanishingly small size its mass is enormous. Such singularities are entities wherein the laws of physics break down and we certainly need a new approach to overcome this. The String Theory is one of the main candidates for the unification.
It proposes that all the elementary particles we know of – such as quarks and electrons – aren’t actually elementary. According to the string approach all these particles have something else at their cores – extraordinarily tiny vibrating filaments of energy which physicists call strings. These entities may have the size that is known in physics as the Planck size which approximately equals 10 to the power of -35 meters. This size is billions of billions times smaller than of an electron which may give you an idea of why we still can’t experimentally test their existence.
These strings vibrating in different patterns produce various particles such as quarks, electrons, photons, gluons and such. And this is exactly where the String Theory first attained a considerable attention: its solutions claim that one of the modes which vibrating strings produce corresponds to the graviton – the particle which carries gravity – that is still purely theoretical and has never been directly observed.
This unification is indeed very elegant and it has been constructed on powerful mathematical procedures. But there is a cost. The solutions of the string approach don’t quite work unless we allow for something wholly unfamiliar – extra spatial dimensions. Yes, String Theory requires our Universe to have 10 spatial dimensions which along with one dimension of time gives us 11-dimensional framework. Those dimensions are thought to be so compactly compressed that we have no opportunity to detect them experimentally. Nevertheless, their shapes determine the way strings can vibrate, thus leading to the particle properties we observe. This is a topic certainly requiring a separate article so I’ll write it in the future. In the case of Brane Multiverse we need to know what ‘branes’ refer to and how this concept has appeared in the String Theory.
It all has to do with the fact that there were 5 various types of the String Theory until in 1995 Edward Witten showed that they all can be conveyed into one another by shifting the value of what is known as the coupling constant. That was basically one of the most serious steps in the String Theory development and after Witten’s presentation this unified model was dubbed M-theory (there are several guesses to what the letter ‘M’ stands for: Mother-theory, Matrix-theory, Membrane-theory or even M as a reversed W for Witten). Initially strings had been supposed to be 1-dimensional objects, but after M-theory emerged that became pretty clear that the theory predicts the existence of objects with higher dimensionality. These objects were given the name branes with a corresponding coefficient showing the dimensionality of an object, for example 2-brane is referred to a 2-dimensional object whereas 3-brane is a 3-dimensional one.
The next important thing here is that neither strings nor branes are necessarily small. Their size depends upon the amount of energy they possess. That is, inject a noticeable amount of energy into a string and its length will get a considerable boost. A highly developed civilization would presumably be able to boost the size of a string with no actual limits. And as we all know the energy density in the early Universe was so intense that it might have stretched both strings and branes to enormous size. This is where the concept of Brane World comes into play. If a 3-brane got such an incredible boost of energy it could have easily been stretched to the size of our observable Universe. If this is true we all live inside a 3-brane and everything we know of – Orion nebula, Crab nebula, entire Milky Way, Andromeda and all the other distant galaxies – is located there as well. It does not mean that the Universe is itself a 3-brane, but in this case the 3-brane locks entire Universe within itself. It is important to mention that I’ve spoken here about a 3-brane and our 3-dimensional Universe just for the sake of possibility of imagining this, but as we’ve already seen the actual number of spatial dimensions concerned by the String Theory is equal to 10. So if this Brane World scenario is actually true we live inside a 10-brane.
Now, when we are aware of the Brane World scenario let us turn to the question of Multiverse. As we can see, according to this scenario we live inside a 10-dimensional object while the actual number of spatial dimensions could be any higher – such a higher-dimensional space is referred to as “bulk” or “hyperspace”. There could be other branes of various dimensionality in that hyperspace and they can be located any close to our own world – even millimeters apart, just in other dimensions. And their total amount might be any large, up to infinity – it is a Brane Multiverse.
And here is a striking implication which I promised you at the start of this article. You might wonder why we aren’t able to see those other brane worlds if they are just millimeters apart from us. It has to do with the types of strings allowed by the String Theory. There are basically 2 types – open strings that can be represented as pieces of fiber and closed ones which have looped shape. Here a significant question arises – could strings fly off the brane or not? A famous American string theorist Joe Polchinski once realized that it is determined by the behavior of the strings’ ends. As he showed it is simply impossible for an open string to escape a brane, the interconnections between those strings and branes just don’t allow that. On the other hand, a loop-shaped string can easily do it. And here is the surprise – all the force-carrier particles except of the graviton are represented by open strings (that is why we are not able to see the other branes, since photons aren’t allowed to move from one brane to another), whereas gravitons are closed ones as determined by the value of their spin. Spin of the particles carrying non-gravitational forces is equal to 1 and only graviton is considered to have spin 2 and be represented by a loop-shaped string, thus it could escape brane’s grip and fly off the Universe. As you might guess this is one of the proposed explanations to the question of why the force of gravity is so weak in comparison with other forces. As we see, its carrier particles just can escape our Universe.
And what’s important about it is that this prediction could even be tested someday, but I’ll leave this for one of the next articles.