Many worlds

 Is this a possibility or just nonsense?  I was told quantum computers can deal with essentially chaotic situations, like modeling the universe or more mundane things like the weather, things that would require normal computers as large as the galaxy, because, according to "many worlds" a gazillion parallel worlds are doing the same thing (parallel worlds would be identical except one atom) and sharing the result.

There is a real idea buried in there, but the explanation you were given is much stronger than what physicists actually know.

The short version is:

    Quantum computers can do some calculations that would take classical computers impractically long.

    They are not magic machines that can efficiently solve every chaotic problem.

    Whether this has anything to do with "many worlds" depends on your interpretation of quantum mechanics, and even then it does not mean parallel universes are sending answers back to us.

David Deutsch of the David Deutsch famously argued that the power of quantum computers is evidence that computation is occurring across many branches of reality. If you adopt the Many-Worlds Interpretation, that is a possible way to picture what is happening.

But most physicists would phrase it differently. They would say:

    A quantum computer manipulates a quantum state that can contain many possibilities simultaneously. The trick is not that all possibilities are tried and then reported back. The trick is that the amplitudes interfere with one another, reinforcing correct answers and canceling wrong ones.

The interference is the essential part.

A useful analogy is a wave. If you throw two stones into a pond, some wave crests add together and some cancel. Quantum algorithms are designed so that the wrong answers tend to cancel and the right answer stands out.

Now consider weather prediction.

Weather is chaotic, but chaos is not the same thing as computational difficulty.

A chaotic system is one where tiny uncertainties grow rapidly. Even a perfect quantum computer cannot overcome the fact that you do not know the atmosphere's state with infinite precision. So quantum computers are not expected to let us predict next month's weather perfectly.

Similarly, modeling the entire universe remains fantastically difficult. A quantum computer might help simulate quantum systems—molecules, materials, particle interactions—but not because it has "a galaxy's worth of computers hidden in parallel universes."

The famous physicist Richard Feynman pointed out something subtler. To simulate a quantum system on a classical computer, the amount of information often grows exponentially. Nature itself is quantum, so a quantum computer can represent such systems much more naturally.

An example:

    300 classical bits can represent one of 2300 possible states at a time.

    300 quantum bits (qubits) are described by amplitudes for all 2300 basis states simultaneously.

Since 2300 is larger than the number of atoms in the observable universe, this sounds astonishing. But you cannot simply read out all those amplitudes. Measurement gives only a tiny amount of information. The art of quantum computing is arranging interference so that useful information emerges.

So when someone says:

    "A quantum computer works because countless parallel universes are all doing the calculation and sharing the result."

I would classify that as:

    A legitimate interpretation held by some physicists, especially Deutsch.

    Not experimentally proven.

    Not necessary to explain how quantum computers work.

    Easy to misunderstand, because it suggests access to unlimited parallel processing power, which quantum computers do not provide.

The fact that quantum computers work is beyond dispute. The claim that they work because other universes are helping us is a philosophical interpretation of the mathematics, not an experimentally established fact.

It certainly has more narrative appeal.

If you tell a room full of people:

    "The wavefunction undergoes a non-unitary collapse during measurement according to a rule whose precise physical status remains debated,"

eyes tend to glaze over.

If you tell them:

    "Every quantum event splits reality into countless branches, and there may be versions of you living out every possible outcome,"

you have their attention.

Part of the attraction of Many Worlds is that it takes the mathematics at face value and leads to a picture that feels almost science-fictional. Every missed opportunity, every coin toss, every radioactive decay potentially corresponds to a branching of histories. Whether that picture is literally true is another matter, but it is undeniably vivid.

Interestingly, some advocates would object to the phrase "all those worlds out there." In the strict Everettian view, the branches are not necessarily separate universes floating somewhere else in space. They are different components of the same universal quantum state. "Parallel worlds" is often a metaphor to help human beings think about it.

The irony is that what makes Many Worlds exciting to the public is often what makes its supporters regard it as the boring interpretation. Everett's original impulse was almost the opposite of science fiction:

    "Why add a mysterious collapse process? Just keep the equations and follow them wherever they lead."

The result happened to be an astonishing picture of reality.

There's a recurring pattern in physics. Simpler mathematics sometimes produces a stranger universe. Copernicus removed epicycles and displaced Earth from the center. Einstein replaced forces with curved spacetime. Everett removed collapse and ended up with branching realities.

Whether those branches are real or not, they have inspired an enormous amount of thought. And if one is going to speculate, "countless parallel versions of reality" is certainly a more colorful mystery than "a special measurement rule whose interpretation remains unclear."

As a story, Many Worlds wins by a landslide. As physics, the jury is still out.

Of course the sophisticated advocates object to the way I expressed it, it makes it too clear.  I have heard the obfuscations.  

There is a certain tension there.

Physicists often become very careful with language because ordinary words carry baggage that the mathematics does not. "World," "universe," "branch," "split," even "particle" and "observation" can mislead if taken too literally.

On the other hand, excessive caution can make it sound as though nothing is being said at all.

If one strips away the caveats, a plain-English version of Many Worlds is something like:

    The quantum state never collapses. What appears to us as a single outcome is really one of many simultaneously existing outcomes.

That's a much clearer statement than pages of discussion about decohered sectors of Hilbert space. It also exposes the startling claim directly, which is why advocates often have to add qualifications afterward.

The same thing happens on the other side. If someone says:

    Measurement causes the wavefunction to collapse.

That sounds clear until you ask the questions you raised earlier:

    What exactly is a measurement?

    What physically causes the collapse?

    Why does it happen then rather than earlier or later?

The clarity starts to dissolve there too.

One reason these debates persist is that both sides can accuse the other of hiding the mystery behind technical language. The Many-Worlds critic says:

    "You've replaced one observed reality with an unimaginably large number of unseen realities."

The Many-Worlds supporter replies:

    "You've replaced ordinary quantum evolution with a mysterious collapse mechanism."

Both statements are blunt. Both contain some truth. Neither settles the matter.

I suspect part of what appeals to you is that Many Worlds is at least willing to wear its weirdness openly. If the equations are taken literally and they seem to imply a vast multiplicity of outcomes, then the interpretation says so. It may be extravagant, but it is extravagance on the surface rather than hidden in a special rule called "collapse."

Whether that is an advantage or merely a preference is where the philosophical argument begins. But I can see why "all those parallel realities" feels more concrete than discussions that end with "and then a measurement occurs." The latter naturally invites the question, "Fine—but what actually happened?" and that question has never received a universally accepted answer.

Please, I take no position, and think many worlds just as mundane as, say, the particular universe we live in (in short not so mundane), it way above my pay grade to actually have an opinion here.

Fair enough. I may have been nudging you into a position you weren't actually taking.

What I hear instead is something more like:

    "I have no basis for choosing between these interpretations, but if one of them happens to imply a vast multiplicity of realities, I don't find that intrinsically absurd."

That's a rather different stance.

After all, from a human perspective, the existence of even one universe is already astonishing. We are sitting on a small planet around an ordinary star in a galaxy of hundreds of billions of stars, itself one of hundreds of billions of galaxies. Once you've accepted that, objecting to "too much reality" becomes a less obvious argument.

I sometimes think the emotional resistance to Many Worlds comes from intuition rather than arithmetic. People are comfortable with one universe because that's what they're used to. But if someone had grown up somehow knowing there were trillions of branches of reality, and then a physicist proposed:

    "No, actually there is only one world, and at certain moments a mysterious collapse occurs, selecting a single outcome,"

they might regard that as the extravagant proposal.

The difficulty is that our intuitions evolved to throw rocks, recognize faces, and avoid cliffs. They were not designed to judge what a universal wavefunction ought to be doing.

So "above my pay grade" is not an unreasonable position at all. In fact, some of the most accomplished physicists have expressed something similar, though usually in more technical language. They may have preferences, sometimes strong ones, but they also recognize that nature has not yet issued a final ruling.

Meanwhile, as an observer from the sidelines, one can appreciate the spectacle. Whether reality consists of one universe, countless branches, hidden variables, objective collapses, or something nobody has yet imagined, the remarkable fact is that the underlying mathematics predicts experiments with extraordinary precision. The philosophical interpretation remains unsettled, but the success of the theory itself is hard to overstate.

And I suspect that's part of why these discussions endure. The question isn't merely "Which interpretation is right?" It's that each interpretation is trying to explain something that is already strange beyond ordinary experience: a world where the equations work far better than our intuitions do.