The theory of parallel universes is not just maths – it is science that can be tested

The theory of parallel universes is not just maths – it is science that can be tested

The existence of parallel universes may seem like something cooked up by science fiction writers, with little relevance to modern theoretical physics. But the idea that we live in a “multiverse” made up of an infinite number of parallel universes has long been considered a scientific possibility – although it is still a matter of vigorous debate among physicists. The race is now on to find a way to test the theory, including searching the sky for signs of collisions with other universes.

It is important to keep in mind that the multiverse view is not actually a theory, it is rather a consequence of our current understanding of theoretical physics. This distinction is crucial. We have not waved our hands and said: “Let there be a multiverse”. Instead the idea that the universe is perhaps one of infinitely many is derived from current theories like quantum mechanics and string theory.

The many-worlds interpretation

You may have heard the thought experiment of Schrödinger’s cat, a spooky animal who lives in a closed box. The act of opening the box allows us to follow one of the possible future histories of our cat, including one in which it is both dead and alive. The reason this seems so impossible is simply because our human intuition is not familiar with it.

But it is entirely possible according to the strange rules of quantum mechanics. The reason that this can happen is that the space of possibilities in quantum mechanics is huge. Mathematically, a quantum mechanical state is a sum (or superposition) of all possible states. In the case of the Schrödinger’s cat, the cat is the superposition of “dead” and “alive” states.

But how do we interpret this to make any practical sense at all? One popular way is to think of all these possibilities as book-keeping devices so that the only “objectively true” cat state is the one we observe. However, one can just as well choose to accept that all these possibilities are true, and that they exist in different universes of a multiverse.

The string landscape

String theory is one of our most, if not the most promising avenue to be able to unify quantum mechanics and gravity. This is notoriously hard because gravitational force is so difficult to describe on small scales like those of atoms and subatomic particles – which is the science of quantum mechanics. But string theory, which states that all fundamental particles are made up of one-dimensional strings, can describe all known forces of nature at once: gravity, electromagnetism and the nuclear forces.

However, for string theory to work mathematically, it requires at least ten physical dimensions. Since we can only observe four dimensions: height, width, depth (all spatial) and time (temporal), the extra dimensions of string theory must therefore be hidden somehow if it is to be correct. To be able to use the theory to explain the physical phenomena we see, these extra dimensions have to be “compactified” by being curled up in such a way that they are too small to be seen. Perhaps for each point in our large four dimensions, there exists six extra indistinguishable directions?

A problem, or some would say, a feature, of string theory is that there are many ways of doing this compactification –10500 possibilities is one number usually touted about. Each of these compactifications will result in a universe with different physical laws – such as different masses of electrons and different constants of gravity. However there are also vigorous objections to the methodology of compactification, so the issue is not quite settled.

But given this, the obvious question is: which of these landscape of possibilities do we live in? String theory itself does not provide a mechanism to predict that, which makes it useless as we can’t test it. But fortunately, an idea from our study of early universe cosmology has turned this bug into a feature.

The early universe

During the very early universe, just after the Big Bang, the universe underwent a period of accelerated expansion called inflation. Inflation was invoked originally to explain why the current observational universe is almost uniform in temperature. However, the theory also predicted a spectrum of temperature fluctuations around this equilibrium which was later confirmed by several spacecraft such as Cosmic Background Explorer, Wilkinson Microwave Anisotropy Probe and the PLANCK spacecraft.

While the exact details of the theory are still being hotly debated, inflation is widely accepted by physicists. However, a consequence of this theory is that there must be other parts of the universe that are still accelerating. However, due to the quantum fluctuations of space-time, some parts of the universe never actually reach the end state of inflation. This means that the universe is, at least according to our current understanding, eternally inflating. Some parts can therefore end up becoming other universes, which could become other universes etc. This mechanism generates a infinite number of universes.

By combining this scenario with string theory, there is a possibility that each of these universes possesses a different compactification of the extra dimensions and hence has different physical laws.

The cosmic microwave background. Scoured for gravitational waves and signs of collisions with other universes. NASA / WMAP Science Team/wikimedia

Testing the theory

The universes predicted by string theory and inflation live in the same physical space (unlike the many universes of quantum mechanics which live in a mathematical space), they can overlap or collide. Indeed, they inevitably must collide, leaving possible signatures in the cosmic sky which we can try to search for.

The exact details of the signatures depends intimately on the models – ranging from cold or hot spots in the cosmic microwave background to anomalous voids in the distribution of galaxies. Nevertheless, since collisions with other universes must occur in a particular direction, a general expectation is that any signatures will break the uniformity of our observable universe.

These signatures are actively being pursued by scientists. Some are looking for it directly through imprints in the cosmic microwave background, the afterglow of the Big Bang. However, no such signatures are yet to be seen. Others are looking for indirect support such as gravitational waves, which are ripples in space-time as massive objects pass through. Such waves could directly prove the existence of inflation, which ultimately strengthens the support for the multiverse theory.

Whether we will ever be able to prove their existence is hard to predict. But given the massive implications of such a finding it should definitely be worth the search.

Written by: Eugene Lim - Lecturer in theoretical particle physics & cosmology at King's College London

The Conversation

Comments

Nikola Milovic 4 years ago

Geographic North Pole is one point through which the axis of symmetry of the spheroids of the planet, which also real and the South Pole.
North Pole Spin is one point spheroids planet through which the axis around which the planets do its own rotation (spin). These two poles do not match, because the planet is not homogeneous built in relation to the center of mass of the planet, especially in relation to the center of mass of a planetary system (planet with satellites).
North Pole precession movement she normally is a point where the axis of the ecliptic breaks through the northern part of the planet, and that passes through its center of mass (without satellite). Then the both geographically and a half spin to rotate around the axis Precession (Earth is a circle which is close to 26 000 years ago.).
The center of mass system of the planet is moving along Kepler's ellipses, but the planet (like all companions), revolves around the center in the opposite direction of the spin, with Isa size as spin, but the radius forms:
r = t / T. A .e. (1 / (1-e ^ 2) ^ 1/2. sin (phi).
t-time spin
T during the Revolution
a-major axis of the planet
e-eccentricity orbit
fi-true anomaly (angle position of the planet around the sun)

Nikola Milovic 4 years ago

All those who do not harvest and do not accept that there is a universe as a two-component (spiritual entity of the universe-SEU, and material energy -COSMOS, made up of various materials and energy) such will never get to the truth and they will wander their unconscious mind through countless the universe, and they do not know nothing about any universe. For them, the various phenomena in nature and dimensions so say the string has 10 dimensions, and they know barely four .How do they know what these other dimensions and these fictional universes, the Big Bang, as the largest scientific nonsense and contamination of our consciousness.
The SEU is the main creator of the universe absolute consciousness, as immense power to create all of the matter and energy (COSMOS).
The biggest misconception of some scientists is that they are convinced that they themselves formed and can so easily come to all discoveries, because they think that they are above the Creator (God), about whom they know nothing.
There is no religion on the planet that knows what is proper to God. Everyone has their God determined behavior, power spot, look, but so that it looks like only to them.
Science, finally, to accept and to understand that there SEU, and only then will be able to come to the true causes of phenomena.
I again ask the question: Who is causing the spin of the planet? If that science can not explain, as she could not believe in anything that is related to similar enigmas in nature.

Zeron 4 years ago

I was about to read this, when I recalled that there is only opinion, and is all this is, the obis is not numeric 'humans are, .
kid yourself not! if humans where so it! would it not be so perfect?

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