The presence of parallel universes may appear like something cooked up by science fiction authors, with slight relevance to current theoretical physics. But the notion that we are living in a “multiverse” that comprises of an infinite number of parallel universes has long been thought a scientific probability – however it is still a matter of strong debate between physicists. The race is now on to discover a way to test the theory, and also searching the sky for marks of interaction with other universes.
It is essential to have in mind that the multiverse view is not in fact a theory; it is rather an outcome of our modern understanding of theoretical physics. This difference is critical. We have not flapped our hands and said: “Let there be a multiverse”. But the impression that the universe is maybe one of infinitely many is resultant of modern theories like quantum mechanics and string theory.
The many-worlds interpretation
You may have heard the thought experiment of Schrödinger’s cat, a scary animal that exists in a closed box. The action of opening the box lets us to follow one of the probable future histories of our cat, including one in which it is mutually dead and alive. The reason this appears so awkward is simply because our human instinct is not acquainted with it.
But it is completely potential according to the bizarre rules of quantum mechanics. The reason that this can occur is that the space of opportunities in quantum mechanics is enormous. Mathematically, a quantum mechanical state is a sum (or superposition) of all promising states. In the situation of the Schrödinger’s cat, the cat is the superposition of “dead” and “alive” states.
But how do we understand this to make any real sense at all? One popular way is to think of all these likelihoods as book-keeping devices so that the only “objectively true” cat state is the one we see. Though, one can just as well select to admit that all these options are true, and that they occur in different universes of a multiverse.
The string landscape
String theory is one of our most, if not the most hopeful path to be capable of unifying quantum mechanics and gravity. This is extremely hard because gravitational force is so hard to define on small scales like those of atoms and subatomic particles – which is the science of quantum mechanics. But string theory, which states that all ultimate particles are made up of one-dimensional strings, can define all identified forces of nature at once: gravity, electromagnetism and the nuclear forces.
Though, for string theory to work mathematically, it needs at least ten physical dimensions. Since we can only see four dimensions: height, width, depth (all spatial) and time (temporal), the additional dimensions of string theory must then be concealed by some means if it is to be accurate. To be capable of using the theory to clarify the physical phenomena we see, these additional dimensions have to be “compactified” by being curled up in such a way that they are too small to be observed. Possibly for each point in our large four dimensions, there are six extra blurry directions?
A difficulty, or some would say, a feature, of string theory is that there are many ways of performing this compactification –10500 options is one number typically touted about. Each of these compactifications will outcome in a universe with different physical laws – such as dissimilar masses of electrons and unlike constants of gravity. Though there are also forceful oppositions to the methodology of compactification, so the subject is not quite settled.
But given this, the apparent question is: which of this landscape of likelihoods do we live in? String theory itself does not deliver a mechanism to find that, which makes it inadequate as we can’t test it. But luckily, an idea from our research of early universe cosmology has changed this problem into a feature.
The early universe
In the very early universe, just after the Big Bang, the universe experienced a period of speeded expansion called inflation. Inflation was invoked initially to clarify why the current observational universe is nearly even in temperature. Though, the theory also foretold a spectrum of temperature variations around this equilibrium which was later proved by numerous spacecraft such as Cosmic Background Explorer, Wilkinson Microwave Anisotropy Probe and the PLANCK spacecraft.
While the precise details of the theory are still being passionately debated, inflation is extensively accepted by physicists. Though, an outcome of this theory is that there must be other parts of the universe that are still accelerating. Though, due to the quantum variations of space-time, some parts of the universe certainly not reached the final state of inflation. This means that the universe is, at least according to our present understanding, forever inflating. Some parts can consequently end up becoming other universes, which could turn in to other universes etc. This mechanism creates an infinite number of universes.
The cosmic microwave background. Scoured for gravitational waves and signs of collisions with other universes. NASA / WMAP Science Team/wikimedia
By uniting this situation with string theory, there is likelihood that each of these universes owns a different compactification of the extra dimensions and therefore has different physical laws.
Testing the theory
The universes foreseen by string theory and inflation live in the same physical space (contrary to the many universes of quantum mechanics which are present in a mathematical space), they can overlap or crash. In fact, they predictably must collide, leaving probable signs in the cosmic sky which we can attempt to look for.
The precise details of the signs depend closely on the models – ranging from cold or hot spots in the cosmic microwave background to unusual voids in the scattering of galaxies. Yet, since impacts with other universes must happen in a specific direction, a general anticipation is that any signatures will break the consistency of our visible universe.
These signs are vigorously being pursued by researchers. Some are searching for it directly through tracks in the cosmic microwave background, the afterglow of the Big Bang. Though, no such signs are yet to be seen. Others are searching for indirect support such as gravitational waves, which are waves in space-time as enormous objects pass through. Such waves could directly prove the presence of inflation, which eventually strengthens the support for the multiverse theory.
Whether we will ever be capable of proving their existence is difficult to foresee. But given the massive effects of such a discovery it should certainly be worth the search.
This post was written by Usman Abrar. To contact the writer write to firstname.lastname@example.org. Follow on Facebook