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 –10^{500} 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, before 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.

**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.

**Explore further:**
Detection of mini black holes at the LHC could indicate parallel universes in extra dimensions

## spencerpencer

## spencerpencer

What if there were a bunch of universes at the beginning, but, just like the theory that says the position of a particle can't be known until it's "observed," maybe all the different possible universes (including those without the possibility of life or even atomic cohesion) collapsed into one when the one capable of sustaining life evolved to the point where something inside it could observe it. OR! Maybe something outside our universe observed the possibilities and they all crunched down into this certain one.

Of course that's as provable as the Botzman's Brains theory. Just a thought.

## julianpenrod

## jlewis

That's kind of a weasel interpretation of the concept and doesn't actually have anything to do with the Everett multiverse concept which was constructed explicitly to handle superposition of quantum states before an observation causes a collapse - such as a single photon going through two slits. That model is still contested and has no evidence of being real.

## jlewis

You're falling into a classic trap here - observers don't have to be alive or sentient. The word Schrodinger used simply means something that takes a measurement and it got translated into 'observer' which has extra content. When two electrons interact - they measure each other - that's an 'observation'.

## Mimath224

## rpaul_bauman

If they were infinite or less Big Bangs and infinite time or at least a long time ago, we would SEE IT NOW !!!!!!

## Torbjorn_Larsson_OM

Further, since many argues -without any evidence - that predicted consequences like multiverses are untestable, Lim has done a piece of advocating. (Which really doesn't suit his review, but it is sometimes seen.)

I agree with Lim. Maybe collisions doesn't exist (they don't in all models or for all parameter values), but we don't know that yet. Multiverses are like EM bias effects, an open consequence that you can dismiss in most cases, or measure in others.

## inkosana

## Torbjorn_Larsson_OM

I think you are confusing physical, (in principle) observable structures - observable universes (infinite in forward time) or pocket universes (also infinite in spatial directions) - with subjective divisions. The observable universe is, as the term implies, observed while we have also observed that the universe is (much) larger than that. [Planck legacy archives.]

@jlewis, rpaul_bauman: "either the universe is expanding at a finite rate and so has a definite size"; "If they were infinite or less Big Bangs and infinite time or at least a long time ago, we would SEE IT NOW"

Wrong! You are probably confused about what "big bang" expansion is. It happens everywhere at the same rate at the same time, independent of the (finite or infinite) size of the expanding universe. And we do see it now, we have seen the expansion since the last century. [Planck legacy archives]

## Torbjorn_Larsson_OM

? The MWT theory is one of the few realist theories predicting quantum mechanics.

It is the other theories that have quantum properties being not realized between observations (with the possible exception of the wavefunction itself).

## inkosana

## HTK

where is your evidence?

## jsdarkdestruction

What?

## HTK

You do know what light is, right?

## Noumenon

This is actually factually incorrect. The mulitiverse idea is the basis of an interpretation of QM, rather than being derived from it.

One must presuppose mathematical idealism in regarding the wave-function as a physical entity of itself, as there is no experimental justification for such a premise.

## billpress11

There's no exact spot that the Big Bang happened. In fact, the Big Bang happened everywhere in the Universe.

http://www.univer...-happen/

Researchers at the University of Kansas working with an international team at the Large Hadron Collider have produced quark-gluon plasma—a state of matter thought to have existed right at the birth of the universe—

Read more at: http://phys.org/n...html#jCp

the universe began by expanding from an infinitesimal volume with extremely high density and temperature.

http://www.ugcs.c.../BigBang

## Noumenon

This is a common misconception even among physicists who should know better; In the Copenhagen interpretation of 'wave-function collapse', it is not that conscious mind 'reaches out' and 'does something' to collapse the state-vector,.... this is deliberately to portray the C.I. as an absurdity and is premised by a realist pov.

What is occuring imo is that the underlying-reality is projected and conformed to our conceptual structure. The 'collapse' is a collapse into conceptual form and through experiment, conceptual values. Afterall, the Hilbert space representation, the vector basis for the vector-state (wave-function) is indeed dependent upon macro-scopic experimental arrangement for possible observation.

## Noumenon

You have fallen into a problematic trap yourself. The deterministic evolution of the Scrodinger equation, and therefore Decoherence, does not in fact ever collapse the wave-function! It can't, ever (**).

Only an observer, a conscious one, can do this. If another observer then forms a new wave-function encompassing this observer and his equipment, then again this new wave-function description is collapse by the new conscious observer. Upon an actual experimental observation the superposition is lost, and a single value is obtained.

**apart from adding a non-linear term to specifically accomplish this; as in non-standard objective collapse theories.

## Noumenon

The MWT interpretation, is a realist one in the sense of mathematical idealism. As Wheeler stated, it carries too much 'metaphysical baggage'.

What does "realized between observations" mean? Physical theories are merely mathematical models that link Observables together in a way that allows for predictions,.. and nothing more. It you desire to know more than this you desire metaphysics.

## TheGhostofOtto1923

"There are other possible solutions to the "Wigner's friend" thought experiment, which do not require consciousness to be different from other physical processes [or even to exist at all, which it doesnt]. Moreover, Wigner actually shifted to those interpretations (and away from "consciousness causes collapse") in his later years. This was partly because he was embarrassed that "consciousness causes collapse" can lead to a kind of solipsism, but also because he decided that he had been wrong to try to apply quantum physics at the scale of everyday life (specifically, he rejected his initial idea of treating macroscopic objects as isolated systems)"

## TheGhostofOtto1923

"Murch says that the microwaves used to probe the superconducting circuit can be thought of as its environment because they are the predominant thing interacting with it. By monitoring the environment, the fluctuations in the microwaves become a known quantity rather than a source of unknown noise.

"That enables the quantum state to remain pure, as Murch and his demonstrated — a finding that has a practical consequence."

http://www.nature...-1.13899

-Consciousness is an illusion.

## TheGhostofOtto1923

And so the gods that wrote them cannot be the omniscient, morally impeccable paragons they describe themselves to be in their books. And you cannot trust that these lying incompetents are able to grant your wishes, perform miracles just for you, punish your enemies with eternal fire, or reward your loyalty with eternal bliss.

This leaves only the philo-invented deistic gods which by design cannot be disproved.

And they have nothing whatsoever to do with the creatures you dream of.

## Noumenon

I just explained above that I reject that notion of 'consciousness causes collapse'. If you're going to critique ones post, at least try to understand it. The reason for the collapse is because the observer is outside the quantum description.

## Noumenon

The brain is not any different than other physical processes,.... but thought is .

## inkosana

## Noumenon

Yes, a weak/partial measurement. How does this refute what I have posted above, ...in your own words because I'm not reading your links anymore.

Only a crank would say that. It is quite literally the most immediate observable phenomena.

Of course the basis of consciousness is the physical brain which is subject to physical laws,... I have never stated otherwise. However the WAY the brain evolved to process and synthesize sense experience to form 'understanding' is itself synthetic and an artificial synthesis. Obviously given that the brain evolved a means of operating on macro-experience, how can it be equipped to process quantum experience?

You don't even believe that minds exist, so how can I have a rational discussion with you?

## Noumenon

As stated above in principal, another observer could construct a quantum wave function of the first observer and equipment and system,... but that 2nd observer would then be outside the wave-function description in question.

## inkosana

## Noumenon

I'm relating standard QM, in which none of the above is controversial, except to a crank.

## inkosana

What you claim is "standard quantum mechanics" is only "logical" to crackpots. On whose principle can "another" observer construct a quantum wave function of the first observer and equipment and system"? Bohr's hallucinations? Heisenberg's uncertainties? Dirac's autism? Do you have to cast a Voodoo-spell, or throw dice? LOL! Instead of flinging insults around, please answer my question: What action must the "other observer" take to "construct" a wave function of the first observer and equipment and system?

## Noumenon

Obviously it is not practical to obtain such a description for a macroscopic object, like an observer and his equipment in addition to the quantum system under investigation,.... which is why I stated "in principle".

My comment was in relation to decoherence,.... which is that the quantum system loses its superposition coherence via interaction with the environment (if its not an isolated system),.... that decoherence, or theoretically the evolution of the deterministic Schrodinger equation in interaction with the environment,... does NOT cause wavefunction collapse to occur. Therefore the original point was that 'quantum interaction' is distinct from 'observation'.

How decoherence resolves to macroscopic observation is on going research via meso-scopic objects.

## Noumenon

When an experimentalist conducts a quantum experiment his theoretical calculations of expectation of course does not take into consideration the atoms in his body and equipment. He must isolate the system under study to avoid decoherence, because decoherence is loss of quantum superposition and thus loss of the application of QM under study. Therefore he is outside the isolated system.

However, another experimentalist, in principle, could construct theoretical calculations of expectation (a single wavefunction) for the first observer, the first equipment, and the original system under study. This 2nd observer would be outside that wavefunction description and would upon observation "collapse the wavefunction" (not a physical wave).

Schrodinger has written about this using his cat analogy.

## Noumenon

## Noumenon

## inkosana

Why is this obvious? Any macroscopic object (even Jupiter) has a de Broglie wavelength, Therefore it is a coherently-moving single wave.

How can a wave-solution which must be a coherent-wave become decoherent? Not possible!

## inkosana

This is nonsense. How can an experimentalist construct a reality for the first observer solely through theoretical calculations? This is why Schroedinger played the fool with this concept by means of his cat experiment. You can only have two separate waves superposing. Two cats not a single cat superposing with itself.

## inkosana

## inkosana

## Noumenon

That's correct, if boundary conditions apply in the given case, and follows from what I have stated. The free electron form of the Dirac equation does not include a potential term.

No, because the Hamiltonian in the Schrodinger equation includes the description of the energy of the system. You're regarding the wavefunction as a physical wave,... while instead it serves to give probabilities for particular observations. No energy is lost by 'wavefunction collapse'.

## Noumenon

In QM 'coherence' refers to the superposition of the wavefunction,... i.e. the possibility that constructive and destructive interference and thus quantum-strangeness occurs. An analogy is Fourier analysis where an arbitrary wave can be decomposed into pure sin/cos waves.

.... There is no sense in continuing here as you're arguing from the premise that the wavefunction under discussion is a physical EM wave. This is not close to what I mean by referring to 'wavefunction',.. so it is quite impossible to have a coherent discussion.

A single entity is represented by a superposition wavefunction,.... multiple entities are represented by a single superposition wavefunction. In the case of multiple entities, the basis space becomes a tensor product and we are then referring to phase-space, ...not ordinary space in which a physical wave evolves.

## inkosana

## inkosana

## Noumenon

The Schrodinger equation given its use of a Hamiltonian is compatible with energy conservation.

In the case where there is uncertainty in energy of the system, the probabilities for the possible observed energies, derived from the wavefunction, do not change in time.

## inkosana

That means that the SAME superposed wave cannot collapse into different energies. It can only have the same single energy into which it can collapse when measuring its energy.

## rpaul_bauman

## Osiris1

## Noumenon

Unless one is saying she can't possibly be correct on account of her tits, ... they are no more being chauvinistic than she is, in trying to discredit the notion that BH's exist.

There should always be some independent physical justification for invoking more dimensions, imo. If Archimedes was a string-theorist he would have said , 'grant me enough degrees-of-freedom and a parallel universe in which to stand, and I could explain everything, whether it exists or not'.

## Noumenon

The Dirac equation is Lorentz invariant and is commensurate with all such effects of SR,... however whether the electron is observed as a particle (photo electric effect, Compton scattering) or a wave (diffraction through crystals) is dependent upon the experimental arrangement. The underlying reality (apart from observation) must be neither because it can't be both at the same time. So, you would need to specify what exactly you mean by 'electron length'. Do you mean wave-packet length or wave-length. In the particle conceptual form it has zero extension.

## inkosana

## kochevnik

Living is simply harvesting zero point energy, which biological shapes are optimized to do

## kochevnik

## Noumenon

You don't give me a reason for this reaction. I don't mean that space and time are themselves physical, but rather they should be justified in some way by a physical system. As mentioned above, for example in GR Einstein was careful to Define time operationally (in a 'instrumentalist' manner) to be another physical system, i.e, a clock, and Defined space to be a physical measuring rod.

## Noumenon

In the other what, parallel universe? In MWI there is no collapse.

## inkosana

## Laborious_Cretin

## kochevnik

## inkosana

## someone11235813

## Egleton

Occam's razor and the quantum erasure experiment supports this interpretation.

People proclaim confidently that mind is an illusion. This is an act of faith in the existence of their Materialist illusion.

Only Mind exists.

History is adjusted to support the observed facts. It would seem that history is bending itself into a multi dimensional pretzel to offer some self consistent explanation of our creation . In the end I suppose it will faill some logic test and go "poof" and it will all be your fault for asking too many "but why? " questions.

## Egleton

## antialias_physorg

The possibility of reading out parallel states (if some sort of trans-universe measurement is possible). At the most basic level this could give you computers that would be able to zcompute (or look up) anything instantly.

At the most far reaching level it could tell us something about the nature of reality (which is always something worth knowing...because like in any game: the one rule that you don't know WILL eventually kill you).

## someone11235813

So you are saying that if there is such a thing as parallel universes that we will have access to them in some way? I was under the impression that they were like whatever came 'before' the creation even for our current Universe, which is not something that that is accessible even in principle.

## antigoracle

## antialias_physorg

Access in the way that we may be able to measure what is going on..not in the way of actually stepping into one.

The parallel universes talked about in the article exist alongside us (in the 'bulk' https://en.wikipe...nd_bulk) - not sequentially (although there may be that kind, too)

## Noumenon

At present.

IOW, one would have to start with the premise that rather than mathematical models of physics representing merely a predictive link between observables, ... they in fact model reality as it exists between observables. The former represents the proper scientific method, while the former enters metaphysics via mathematical idealism.

## Noumenon

## inkosana

## Noumenon

Science is fundamentally based on observation which is all that is available to us wrt knowledge of reality, therefore physical laws are merely mathematical models that link observations together in a way that allows for predictions,.. and nothing more.

It you desire to know what Reality is like in between observations, then de facto, you desire metaphysical knowledge.

## inkosana

Not quite! Science is the logic therefore physical laws are merely mathematical models that link observations together in a way that allows for predictions,.. and nothing more.

It you desire to know what Reality is like in between observations, then de facto, you desire metaphysical knowledge.

## inkosana

## Noumenon

They have been substantiated with empirical verification, therefore they're valid.

## inkosana

Bohr was so deluded that he stated: If some-one thinks he understands quantum mechanics then he does not understand it. This has become the motto of modern theoretical physics which you also adhere to. One must believe that It is impossible to understand physics and therefore, if someone claims it is possible, this person is doing metaphysics. Can you not see how absurd this is?

## Noumenon

He said 'if one is not shocked by QM, then he does not understand it.'

Feynman said "no one understands QM",... by which he meant, no one has an intuitive understanding of QM, despite that it allows for predictive knowledge.

## inkosana

He was wrong, since quantum mechanics follows logically from Maxwell's equations as can be seen in my book! His intuition was impaired!

## donjoseph

Interesting stuff for a short excursion JMHO

FYI EC is the geometer responsible for Calabi-Yau geometry

## Mimath224

Depends how one looks at it. In 1931 Dirac came to the conclusion that 'anitelectrons' were mass of the 'hole' in the 'Dirac sea' being the mass of the electron but + charge. About a year later later Carl Anderson discovered a particle that Dirac had predicted.

## inkosana