Courageous Inmate Sort
Jedi Master
As far as I remember and understand, they were talking about accessing parallel universes in the sense that when a qubit is in a state of superposition (1 and 0 at the same time) it exists in more than one universe at the same time, and the more qubits you have, the more parallel universes are needed. The "talking to" part is probably some way of saying that the quantum computer can somehow make use of such superpositions and quantum entanglement (Einstein's "spooky action at a distance") to make its calculations. If I remember correctly the quantum effects are achieved by cooling the CPU to very low temperatures.
As far as I know this explanation is based on the many worlds interpretation of quantum mechanics. David Deutsch is a proponent of this theory, was involved in the theoretical development of the quantum computer and has said that “Quantum computation […] will be the first technology that allows useful tasks to be performed in collaboration between parallel universes”.
The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wavefunction collapse.[2] This implies that all possible outcomes of quantum measurements are physically realized in some "world" or universe.[3] In contrast to some other interpretations, such as the Copenhagen interpretation, the evolution of reality as a whole in MWI is rigidly deterministic.[2] Many-worlds is also referred to as the relative state formulation or the Everett interpretation, after the physicist Hugh Everett who first proposed it in 1957.[4][5] The formulation was popularized and named many-worlds by Bryce DeWitt in the 1960s and 1970s.[1][6][7][2]
In many-worlds, the subjective appearance of wavefunction collapse is explained by the mechanism of quantum decoherence. Decoherence approaches to interpreting quantum theory have been widely explored and developed since the 1970s,[8][9][10] and have become quite popular. MWI is currently considered a mainstream interpretation along with the other decoherence interpretations, collapse theories (including the Copenhagen interpretation),[11] and hidden variable theories such as Bohmian mechanics.
The many-worlds interpretation implies that there is a very large—perhaps infinite[12]—number of universes. It is one of many multiverse hypotheses in physics and philosophy. MWI views time as a many-branched tree, wherein every possible quantum outcome is realised.[13] This is intended to resolve the correlation paradoxes of quantum theory, such as the EPR paradox[14][2]:118 and Schrödinger's cat,[1] since every possible outcome of a quantum event exists in its own universe.
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The many-worlds interpretation shares many similarities with later, other "post-Everett" interpretations of quantum mechanics which also use decoherence to explain the process of measurement or wavefunction collapse. MWI treats the other histories or worlds as real since it regards the universal wavefunction as the "basic physical entity"[20] or "the fundamental entity, obeying at all times a deterministic wave equation".[21] The other decoherent interpretations, such as consistent histories, the Existential Interpretation etc., either regard the extra quantum worlds as metaphorical in some sense, or are agnostic about their reality; it is sometimes hard to distinguish between the different varieties. MWI is distinguished by two qualities: it assumes realism,[20][21] which it assigns to the wavefunction, and it has the minimal formal structure possible, rejecting any hidden variables, quantum potential, any form of a collapse postulate (i.e., Copenhagenism) or mental postulates (such as the many-minds interpretation makes).
The unreal/real interpretation[edit]
According to Martin Gardner, the "other" worlds of MWI have two different interpretations: real or unreal; he claims that Stephen Hawking and Steven Weinberg both favour the unreal interpretation.[26] Gardner also claims that the nonreal interpretation is favoured by the majority of physicists, whereas the "realist" view is only supported by MWI experts such as Deutsch and Bryce DeWitt. Hawking has said that "according to Feynman's idea", all the other histories are as "equally real" as our own,[27] and Martin Gardner reports Hawking saying that MWI is "trivially true".[28] In a 1983 interview, Hawking also said he regarded the MWI as "self-evidently correct" but was dismissive towards questions about the interpretation of quantum mechanics, saying, "When I hear of Schrödinger's cat, I reach for my gun." In the same interview, he also said, "But, look: All that one does, really, is to calculate conditional probabilities—in other words, the probability of A happening, given B. I think that that's all the many worlds interpretation is. Some people overlay it with a lot of mysticism about the wave function splitting into different parts. But all that you're calculating is conditional probabilities."[29] Elsewhere Hawking contrasted his attitude towards the "reality" of physical theories with that of his colleague Roger Penrose, saying, "He's a Platonist and I'm a positivist. He's worried that Schrödinger's cat is in a quantum state, where it is half alive and half dead. He feels that can't correspond to reality. But that doesn't bother me. I don't demand that a theory correspond to reality because I don't know what it is. Reality is not a quality you can test with litmus paper. All I'm concerned with is that the theory should predict the results of measurements. Quantum theory does this very successfully."[30] For his own part, Penrose agrees with Hawking that QM applied to the universe implies MW, although he considers the current lack of a successful theory of quantum gravity negates the claimed universality of conventional QM.[31]
Many-worlds interpretation - Wikipedia
en.wikipedia.org
Deutsch sees quantum superpositions and the Schrödinger equation as evidence for his many worlds quantum multiverse, where everything physically possible occurs in an infinite branching of alternate histories. Deutsch argues that a great deal of fiction is close to a fact somewhere in the multiverse.[4] Deutsch extols the usefulness of the concept of fungibility in quantum transactions, his universes and the particles they contain are fungible in their interactions across the multiverse structure. Deutsch explains that interference offers evidence for this multiverse phenomenon where alternate histories affect one another without allowing the passage of information, as they fungibly intertwine again shortly after experiencing alternate events. According to Deutsch, our perspective on any object we detect with our senses is just a single universe slice of a much larger quantum multiverse object.
The Beginning of Infinity - Wikipedia
An interesting discussion between D. Deutsch and Seth Lloyd about how real are the parallel universes in relation to our own:
Are parallel universes equally real?
David Deutsch and Seth Lloyd
07.09.97
Seth Lloyd endorses the description given by quantum mechanics of a universe that is constantly branching into different "worlds," but insists that he and the world he inhabits are real in a way that his "twins" in parallel worlds are not. When the universe splits into decohering branches ("quantum decoherence"), the split is irrevocable, and, for all intents and purposes, the other branches and their corresponding worlds don't exist: "The branch of the universe in which pigs fly exists only when pigs actually fly."
Our "twins" in parallel universes are just as real as we are, says David Deutsch. The notion that our world is somehow more real is derived not from experience (since each "world" seems equally real to its inhabitants), nor from quantum mechanics (in an absolute sense there are no splits at all, and no moment when your unseen counterparts can no longer affect you - it just gets very expensive to measure the effects), but ultimately from the pernicious influence of a sterile branch of philosophy.
Seth Lloyd 1 of 3
Dear David,
I am writing to relate to you a strange thing that happened to me a few weeks ago in the MIT bookstore. I was standing in front of a bookshelf, trying to decide whether to buy your new book or Roger Penrose's. Now as you know, neurons are notoriously sensitive cells, capable of amplifying very small effects. As a result of a tiny quantum-mechanical fluctuation, a few extra transmitter chemicals jumped across one of my synapses and bound to the receptors on a neuron. This extra stimulation pushed the neuron over its threshold and caused it to fire, triggering a burst of neural activity and causing me, on impulse, to buy your book, The Fabric of Reality. Upon reading this book, I discovered that you espouse the so-called "many worlds" interpretation of quantum mechanics, in which every quantum fluctuation causes the world to split into different parts, each one of which is equally real. You seem to be implying that in another equally real world there is another me, equally real, who is currently reading Penrose's book. How dare you imply this! I really bought your book, I really like it, and I really prefer to be reading it rather than Penrose's book. Who are you to say that imposter in the other world is just as real as me? I await your response.
Yours sincerely,
Seth Lloyd
Deutsch responds
David Deutsch 1 of 3
Dear Seth,
I'm glad you enjoyed The Fabric of Reality, but are you sure that you chose it randomly? Most readers, I'm sure, buy it as a result of rational thought, in which case most of their counterparts in parallel universes buy it too. But to the extent that your decision did depend on random events, there are indeed other, equally real, versions of you in other universes, who chose differently and are now enduring the consequences.
Why do I believe this? Mainly because I believe quantum mechanics. Just write down the equation describing the motion of those fateful transmitter molecules, and their effect on you and on the environment. Notice that their "randomness" consists in their doing two things at once: crossing that synapse and not crossing it; and that the effect on you was likewise that you did two things at once: buy my book and buy Penrose's. Such effects spread out, making everything do many things at once, which is what we mean by saying that there are "parallel universes."
Furthermore, the universes affect each other. Though the effects are minute, they are detectable in carefully designed experiments. There are projects underway - close to your heart, I know, as well as mine - to harness these effects to perform useful computations. When a quantum computer solves a problem by dividing it into more sub-problems than there are atoms in the universe, and then solving each sub-problem, it will prove to us that those sub-problems were solved somewhere - but not in our universe, for there isn't enough room here. What more do you need to persuade you that other universes exist?
Yours sincerely,
David
Lloyd disagrees
Seth Lloyd 2 of 3
Dear David,
Thank you for your response: I feel sorry for the other me in the parallel universe who accidentally bought Penrose's book and is debating him instead. But since I'm really debating you and not Penrose, I don't understand why you refer to my shadowy twin in the parallel universe as equally real. Call me old-fashioned, but I like to refer to things in this universe as real: I really had toast, not cereal, for breakfast; I really drank tea, not coffee; and I really am writing this letter to David, not Roger. The toast, the tea, and this letter are real in a way that my Penrose-reading twin is not: The toast can satisfy my hunger, the tea my thirst, and our correspondence my curiosity, but nothing that my parallel twin can do can make any difference to me, to you, or to anyone in this universe that we really do inhabit.
The reason that my twin can make no difference lies in the physical phenomenon of decoherence. As you know, decoherence is a process that robs quantum states of their ability to interfere with one another. The hotter and more intrusive the environment of a quantum system, the more rapidly decoherence takes place. I'm a mammal, you're a mammal. The mammalian brain is a hot, wet place. Within a tiny fraction of a second after the quantum fluctuation had taken place that set in motion the chain of events that caused me to buy your book rather than Penrose's, the universe in which the neuron fired and my hand reached to D had separated irrevocably from the universe in which the neuron didn't fire and my hand reached to P. (The tendency of the brain to decohere its contents is the reason Penrose's claims for the importance of quantum coherence for consciousness should be regarded with skepticism.) Once decoherence has taken place, my twin can no longer interfere with me, and has ceased to be real in the way that you, who can argue with me, are.
Quantum computers, in contrast to our brains, operate coherently. I am comfortable with the notion of a quantum system such as a photon being many places at once - my eye wouldn't function unless each photon passed through all points in the lens simultaneously. Why then can't a quantum computer effectively perform many computations simultaneously without the universe splitting into many parts?
Yours sincerely,
Seth
Deutsch replies
David Deutsch 2 of 3
Dear Seth,
So you admit that photons, atoms, and quantum computations have invisible, differently behaving counterparts, but you still cling to the belief that you exist in only one copy. I don't think this makes sense, because you are made of atoms, and if they have invisible counterparts, so must you.
Look inside a quantum computer during a computation. The particular set of values you see in its registers corresponds to one particular computation - but you agree that, in fact, it was performing vast numbers of other computations at the same time, computations of which you detected no trace. Now, in the example you gave, quantum mechanics describes you as performing two different computations (viz., reading two different books). Yet you maintain that when you check your own memory and remember reading The Fabric of Reality, then that is the only reading that has been going on. Your excuse for treating these two cases differently, when their quantum-mechanical descriptions are perfectly analogous, is that the quantum computer gives you an output which depends logically on vast numbers of invisible computations, so you must accept that they really happened. By contrast, the other computations that quantum mechanics says that you performed (such as reading Penrose's book) can never affect you because of decoherence, so you prefer to believe that they never happened. But decoherence is just a matter of degree. There is never a moment after which an object's invisible counterparts cannot affect it any longer. It just gets too expensive to set up the apparatus that would demonstrate their existence. To claim that if something is too expensive to measure, it doesn't exist, is surely just a perverse form of solipsism.
Shoot a photon out into an empty region of the night sky. Unlike decoherence, this is a truly irrevocable act: If you change your mind, the laws of physics say you'll never retrieve the photon. Yet you wouldn't deny that it still exists, would you? The proper criterion for whether something exists is not whether it can still affect us, but whether it figures in our best explanation of what affects us. To deny the existence of that photon, or of those invisible computations, or invisible universes, is to renounce explanation of what we do see.
Yours sincerely,
David
Lloyd counters
Seth Lloyd 3 of 3
Dear David,
You champion the existence of multiple mes and assert their equal claim to reality; I insist on my prior claim, since I am really debating you while the other mes are not. Rather than imitating my daughter and repeating "me me me" until you give up, let me try to first find where we agree and then where we differ. I think that we agree that that quantum description of the universe (the "wave function") is constantly branching into different "worlds," in one of which we are debating now, in another of which I am debating Penrose, and in others (I hope the plurality) of which we are amicably sipping margaritas on the beach. Each one of these "worlds" seems equally real to its inhabitants, even if these multiple realities are mutually exclusive. In addition, quantum mechanics picks out no one of these "worlds" or branches as special.
So far, so good. Now, as I understand your position, you assign all of these "worlds" an equal measure of reality and claim they all exist. Here we differ. Arguing with philosophers has made me wary of putting too great a burden of meaning on the words "exist" and "real." But the conventional use of those words refers to objects and events in our particular branch of the universe. The cup of tea that I am drinking exists; we are really debating. In contrast, the other branches or "worlds" are exactly the parts of the wave function in which the cup of tea does not exist and we are not really debating. These other "worlds" do not exist in the way that this one does. For better or worse, I suspect that most people will believe that the branch of the universe in which pigs fly exists only when pigs actually fly.
We differ on more than semantics, however. From your clear and elegant discussion of quantum computation, I can tell that we differ also as to the point at which the universe branches into different "worlds." You argue that one must accept the existence of the other branches because these branches could interfere with our branch in the future. For decoherence, however, the issue is not whether the branches could interfere with each other, but rather whether they actually do interfere with each other at some time in the future. Decoherent branches, by definition, do not interfere with each other: As a result, when the universe splits into two decohering branches, the split is irrevocable, and for all intents and purposes the other branches and their corresponding "worlds" do not exist.
In a quantum computation, as you point out, the branches actually do interfere with each other, allowing the computation of quantities, such as factors of large numbers, that are properties of all the branches taken together but not of any branch in particular. (This performance of the computation in many branches is at bottom no stranger than the single photon that, at your suggestion, I shot into the air last night occupying many points of space at once.) But the very fact that the quantum computer gives us an answer that depends on all the branches at once means that the universe has not really split: Those branches were part of our world, not of other worlds! The universe splits if and only if its branches decohere.
Thanks for a great debate, better than the one I had with Borges on the same subject. I met him at a garden party in Cambridge in the last year of his life, and asked whether he had quantum mechanics in mind when he wrote his wonderful evocation of a branching universe, "The Garden of Forking Paths." His answer: "No."
Yours,
Seth
Deutsch concludes
David Deutsch 3 of 3
Dear Seth,
Our disagreement is certainly about more than just semantics; however, your latest reply suggests that our positions may be closer than your forthright opening remarks might have indicated.
Our key area of agreement is, as you say, that what quantum mechanics describes is not a single universe but something that "is constantly branching into different 'worlds,' in one of which we are debating now, in another of which I am debating Penrose."
You also say: "The very fact that the quantum computer gives us an answer that depends on all the branches at once means that the universe has not really split: Those branches were part of our world, not of other worlds! The universe splits if and only if its branches decohere." I entirely agree! Reality consists of a multiverse, an enormous entity which, on a gross scale, has a structure that resembles many copies of the universe of classical physics, but which is, on a sufficiently fine scale, a single, unified system. In an absolute sense, there are never any splits at all.
Our key point of disagreement is that, despite the fact that "quantum mechanics picks out no one of these 'worlds' or branches as special," you still want to believe that "these other 'worlds' do not exist in the way that this one does."
If you are right, it surely follows that the thing that singles out our own branch as more real than all the others is nowhere to be found in quantum mechanics. Nor is it, of course, anywhere to be found in our experience, since, as you say, "each one of these 'worlds' seems equally real to its inhabitants." So where is it to be found?
It is found in (or rather, demanded by) philosophy - and in particular, I believe, by the sterile philosophy of positivism and related doctrines that have been impeding scientific progress for the last seventy-odd years. You say: "Arguing with philosophers has made me wary of putting too great a burden of meaning on the words 'exist' and 'real.'" There may lie your problem: I think you have been arguing with the wrong philosophers!
Yours sincerely,
David
Are parallel universes equally real?
Seth Lloyd endorses the description given by quantum mechanics of a universe that is constantly branching into different "worlds," - Our "twins" in parallel universes are just as real as we are, says David Deutsch.www.wired.com
