# Can existing quantum computers be considered evidence for parallel universes?

In this video ( http://www.youtube.com/watch?v=bJpIclDmi2M ) Max Tegmark , a MIT cosmologist says that if we build a quantum computing successfully it will be a evidence that Parallel Universes exists, because in a quantum computer the computations will happen in many other parallel universes including ours (parallel computations) .

Well, we don't have a very useful quantum computer it, but i can see that we already built some functional quantum computer and as the times goes by they a getting better. For example : 'Coherent superposition of an ensemble of approximately 3 billion qubits for 39 minutes at room temperature. The previous record was 2 seconds' http://www.theverge.com/2013/11/14/5104668/qubits-stored-for-39-minutes-quantum-computer-new-record.

So we already have the evidence that parallel universes do exist?

The question is whether quantum computing necessarily implies that the many worlds interpretation of quantum mechanics is correct. This issue is dealt with in the question How much is quantum computation changing the interpretation of quantum theory, and, if at all, how?, though I suspect the discussion there is too deep for non-physicists.

The simple answer is that no-one knows for sure and different physicists will disagree. However the impression I get as an outsider is that the success of quantum computing does not imply that the many worlds interpretation is correct.

So Tegmark's class 3 multiple universes may or my not exist, but building a working quantum computer will not settle the issue.

The debate over evidence for the many worlds interpretation is largely misconceived. Suppose that you take quantum mechanical equations of motion seriously and apply them to all physical systems, including macroscopic systems like detectors. When you do this, you find that it implies the existence of multiple versions of all those systems. Those multiple versions are bound together in layers, each of which resembles the universe described by classical physics to some approximation. In addition, taking this approach allows you to explain what is going in single particle interference, the EPR experiment and that sort of thing, see "The Fabric of Reality" and "The Beginning of Infinity" by David Deutsch for popular accounts and for more technical stuff, see

http://arxiv.org/abs/quant-ph/0104033

Theories that purport to explain those experiments without many worlds have to modify the equations of motion such as the Schrodinger or Heisenberg equations. These are often called interpretations, but they ought to be recognised and judged for what they are: alternatives to quantum mechanics. Some of these alternatives are completely ad hoc, like saying the wavefunction collapses (all the universes go away except one of them), but giving no equation of motion for collapse. These are not serious scientific theories since they make no specific independent predictions. They should be dismissed out of hand, but for some reason that nobody has explained they are not so dismissed. Other theories make non-ad-hoc modifications that may be independently testable but they do not solve any major problem with the theory (e.g. - the GRW theory), so the motivation for proposing them is a bit of a mystery.

In principle, an alternative to quantum theory might predict that other universes don't exist and that a quantum computer would not work. I am not aware of any alternative that makes a specific prediction about this issue. So it is a bit difficult to say why a quantum computer would be relevant to judging the issue.

• I'm not sure how this helps in answering the question. It seems to me that you're just making a point of interpretations not being theories by citing from arXiv (which is a great source, yet not peer reviewed). Please let me know how I misunderstood your answer. – ChrisR Feb 12 '15 at 1:12
• The question is about whether a working quantum computer is relevant to judging the multiverse theory. It is not relevant because the alternatives have not been used to make clear predictions about whether the computer should work or not. Some of them can't be used to make any predictions, for others the issue is unclear. The papers have pay walled versions that are peer reviewed. The first explains the multiverse. The second explains some specific experiments involving entanglement. The multiverse solves problems other theories don't, which is how it should be judged. – alanf Feb 12 '15 at 10:23

People have strong opinions about the interpretations of quantum mechanics. As alanf points out, quantum mechanics already implies many worlds, provided that you think that quantum mechanics should hold for arbitrarily large closed systems. We have no evidence for that, but we haven't found any counterexamples yet, and neutron stars are pretty big. I suppose some strong holdouts might concede to a weaker "many quantum computers" interpretation, but hold that the universe as a whole remains in a defined state.

Many worlds comes straight out of the "sum over outcomes" prescription, which is just fancy words that mean when you multiply a vector ($\psi$) by a matrix ($H$), the result is a sum of different possibilities. Unfortunately many people try to envision physical splitting or somehow picture many "classical worlds". Had DeWitt called it the "Universe is in a superposition of many states in the Hilbert space" interpretation instead, there might be less resistance to the idea, but that is not how it happened.