Sean M. Carroll Pushes Many-worlds Again

New interview:

Quantum Mechanics Still Doesn't Make Sense | Sean Carroll
New Scientist

What really happens in the quantum world?

In this conversation, physicist Sean Carroll explores some of the deepest mysteries in quantum mechanics: the famous double-slit experiment, wave function collapse, the Many Worlds interpretation, entropy and the arrow of time.

Speaking to New Scientist reporter Jacklin Kwan, Carroll discusses why electrons appear to behave like waves, how observation seems to affect reality and whether the universe constantly branches into countless parallel worlds. Carroll also explains the measurement problem, the challenges of interpreting quantum theory and why physicists still debate what quantum mechanics is actually telling us about the nature of reality.

Carroll is a theoretical physicist, cosmologist and author whose work focuses on the foundations of physics, quantum mechanics, cosmology and the nature of time.

He gives a brief explanation of why he believes in many-worlds theory, if you want it straight from him.

He lays it out clearly. When you have a situation where several outcomes seem possible, you forget about making a prediction, and just assume that they all happen in parallel universes.

That's it. It does not really have anything to do with quantum mechanics or the Schroedinger equation. It is just a rule for replacing probabilities with unseen worlds.

You do not even get to say that some worlds are more likely than others. You just have to accept that all possibilities happen, and nothing can be predicted.

I'm a believer in the many worlds interpretation of quantum 16:58 mechanics. ... But really it's just a theory that says you want the answer to the reality problem. Wave functions are real. They represent reality precisely and completely. ...

When I observe the electron, I don't observe the wave function. I observe different possibilities. You can interpret that as saying that what the wave function really is is a superposition of all possible measurement outcomes. And what you're seeing when you measure it is you're picking out one of the set of possibilities with different probabilities. Right?

And so what many worlds ends up saying and it's not an assertion again it's it's comes out of the equations but it says that what you send up with is a superposition but not just of the electron the combined system of the electron and the scientist doing the measurement. There's a superposition of the electron was here and you observed it there plus the electron was there and you observed it there plus the electron was there and you observed it there and the many worlds boom they're there.

And so if you accept the picture of wave functions as superpositions of different measurement outcomes it's no work at all to accept them as superpositions of different possible worlds and then everything is just physics.

So you observe electrons, not wave functions. The wave function is just a way of representing all the possible observable outcomes. All that is correct. But then he makes the leap to say that "it's no work at all" to accept the possible outcomes as parallel worlds.

I spell all this out, because most readers here probably think I am attacking a straw man when I say how stupid many-worlds theory is. It really is stupid and nonsensical. Just listen to its most eloquent advocate.

It is no work at all to replace a scientific theory with one that makes no predictions. Just say that actual observables are not real, and that the real entities are the imaginary possibilities that no one ever sees. Just forget about making predictions, and brag about the explanatory power of being able to talk about unobservable parallel universes.

I continue to be amazed that anyone takes this nonsense seriously. It is impossible to reconcile this with a scientific worldview. It is the most anti-science philosophy.

Microsoft Qubits Questioned Again

Microsoft has been trying to build quantum computers with topological qubits, which would have durability advantages if they worked. But SciAm reports:

A top quantum computing expert assails Microsoft’s claims that it has a “topological qubit,” arguing in a new paper that the company has failed to demonstrate the technology.

University of St Andrews physicist Henry Legg argues that the “topological qubit,” a storer of quantum information that could theoretically maintain a higher fidelity than any in existence, might simply be noise.

The commentary was published today in Nature’s “Matters Arising,” the journal’s venue for formal criticism of its published papers. Legg’s response is aimed at Microsoft’s most recent Nature paper, which was published earlier this month—but it is just the latest in a string of criticism aimed at Microsoft’s Quantum division by other researchers in the field.

The company has been forced to retract certain previous peer-reviewed papers. And in the new commentary, Legg argues that their most recent Nature paper may be similarly flawed. In a response also published today by Nature, a member of Microsoft’s Quantum team argues that their measurements do justify the claim that they’ve produced a topological qubit.

The Microsoft paper has about 150 authors. I did not understand it. Maybe Microsoft has done some Nobel Prize quality research, but I doubt it. I wonder how the scientists convince Microsoft management to keep funding it. There is no possibility of a monetary payoff.

I did not know that Nature had a journal for formal criticism of its papers. That seems like a good idea.

Gil Kalai defends his Quantum Skepticism

Dr. Quantum Supremacy has running feuds with several people, including Israeli mathematician Gil Kalai who says in response:

When it comes to quantum computing, I regard the choice made by many colleagues and friends to pursue this direction of research as entirely (3000%) justified, and it has led to beautiful mathematics and science. (I hope that my own choice to pursue the skeptical direction was justified as well, though this is a more difficult call.)

For the record, I expect that scalable quantum computation — and even important milestones toward this goal — are inherently impossible. ...

One very clear and important point of disagreement between Scott and me concerns the following simple factual question:

Is it currently possible to produce (without classical-computing interference) samples of size 500K for depth-14 random circuits with 20 qubits and an XEB fidelity above 0.2? (This is something Google claimed in 2019.)

I tend to think that the answer to this question is negative; Scott strongly believes that this and much more have already been achieved experimentally. Thus these are sharp factual disagreements, and I hope they will be resolved within the next few years.

I do not know who is right about that factual question, but I am included to believe that Google did not really prove it, if Kalai is till unconvinced seven years later.

Aaronson is also wound up about some Middle East war issues. I understand why he is so pro-Israel, but I do not understand why he is so anti-Trump. At any rate, it is out of my expertise.

The Modern Crypto Primitives

The quantum crypto folks are threatening the cryptographic infrastructure that runs the world. I summarize the main functions.

Hash. Any digital message can be transformed to a 32-byte hash. It is believed that no one will ever find two messages with the same hash.

Public key pairs. A 32-byte private key can be converted to a 32-byte public key. No one can invert the operation.

Signature. A private key and a message hash can be signed to a 64-byte signature. The public key can verify the signature. No one can forge the signature without the private key.

Encryption. A 32-byte secret key can encrypt an arbitrary digital message. No one can decrypt without the secret key.

Key agreement. Two public key pairs can be combined to give a 32-byte shared secret, using the private key of one and the public key of the other. Both ways give you the same shared secret. No one can get it from just the public keys.

These functions are quite efficient and underlie the security of nearly everything on the internet today, notably https and ssh. They also underlie Bitcoin. There is broad agreement that they cannot be cracked with conventional Turing computers for the foreseeable future. There are widely available free libraries to do all these things.

Quantum computers threaten the public keys, making it possible to deduce the private key. Google predicts this will happen on what they call Q-Day, maybe as early as 2029. I doubt it will ever happen.

There are post-quantum public key pairs that are resistent to quantum algorithms, but they are much bigger and not as well tested.

Microsoft and Google are hard at work replacing their public key pairs with post-quantum ones. Some high-value targets want to switch now, because of the possibility that transmissions are being recorded now so that the Chinese can decrypt ten years from now. For a typical user ordering an online product, this is not a concern.

This reminds me a little about how IPv6 was designed to replace IPv4 internet protocol in 1998. Most of you reading this are probably still using IPv4. The reasons for change are different, but it shows how the networks are slow to change, if the old system is working just fine.

Einstein Objecting to using Observables

Einstein's unhappiness with quantum mechanics was already clear in 1926, within a year of the theory being formulated.

Heisenberg told this story:

In the spring of 1926, I was invited to address this distinguished body [University of Berlin] on the new quantum mechanics, ...

[Einstein said] "What you have told us sounds extremely strange. You assume the existence of electrons inside the atom, and you are probably quite right to do so. But you refuse to consider their orbits, even though we can observe electron tracks in a cloud chamber. I should very much like to hear more about your reasons for making such strange assumptions."

“We cannot observe electron orbits inside the atom," I must have replied, "but the radiation which an atom emits during discharges enables us to deduce the frequencies and corresponding amplitudes of its electrons. After all, even in the older physics wave numbers and amplitudes could be considered substitutes for electron orbits. Now, since a good theory must be based on directly observable magnitudes, I thought it more fitting to restrict myself to these, treating them, as it were, as representatives of the electron orbits."

"But you don't seriously believe," Einstein protested, "that none but observable magnitudes must go into a physical theory?"

[Heisenberg] "Isn't that precisely what you have done with relativity?" I asked in some surprise. "After all, you did stress the fact that it is impermissible to speak of absolute time, simply because absolute time cannot be observed; that only clock readings, be it in the moving reference system or the system at rest, are relevant to the determination of time."

"Possibly I did use this kind of reasoning," Einstein admitted, "but it is nonsense all the same. Perhaps I could put it more diplomatically by saying that it may be heuristically useful to keep in mind what one has actually observed. But on principle, it is quite wrong to try founding a theory on observable magnitudes alone. In reality the very opposite happens. It is the theory which decides what we can observe.

Einstein is credited with abolishing the aether, absolute space, absolute time, etc., but maybe that is not how he thought about it at all. He went on to try to develop unified field theories that were completely detached from observation.

Generating Quantum Randomness

I posted about someone claiming to prove quantum randomness, and now the journal Nature has published an article on Experimental randomness amplification. The article is paywalled, but here is the 2024 preprint.

In this context, randomness is defined as being fundamentally unpredictable, which means that the laws of physics forbid the prediction of its values. ...

Conventional random number generators, rooted in classical physical processes, grapple with a foundational concern — the potential for adversaries to predict their outputs by scrutinizing the microscopic degrees of freedom, thereby eroding their essential unpredictability.

Quantum-mechanical processes, on the other hand, feature innate randomness and therefore offer a natural ground to build such devices.

This is foolishness. There is no law of physics forbidding prediction.

When you measure X-component of electron spin, then the wave function collapses, and the Y-component has 50-50 chances. Heisenberg uncertainty prohibits measuring the X and Y components at the same time. So the theory is sometimes not able to predict spin. But that is not quite saying that prediction is forbidden. Maybe there is some way to predict, and we do not know how yet.

This paper does not even talk about spin. It merely assumes that you are doing Bell test experiments. Under some assumptions, you can make some random choices, and get even more random outputs.

This method could be used to generate random numbers for practical purposes like cryptography, but I do not think it is any better than tossing coins, or pointing a webcam at a lava lamp.

The Evidence for Quantum Supremacy

A newly revised paper reviews the outstanding evidence for quantum supremacy:

A brief history of quantum vs classical computational advantage
Ryan LaRose

In this review article we summarize all experiments claiming quantum computational advantage to date. Our review highlights challenges, loopholes, and refutations appearing in subsequent work to provide a complete picture of the current statuses of these experiments. In addition, we also discuss theoretical computational advantage in example problems such as approximate optimization and recommendation systems. Finally, we review recent experiments in quantum error correction -- the biggest frontier to reach experimental quantum advantage in Shor's algorithm.

So has it been proved, or not? No, not really.

It seems at this moment in history we are just on the boundary between quantum and classical computational advantage, and in the near future we expect the status of computational advantage to continue shifting between quantum and classical. We hope that this brief history helps to propel readers to the research frontier and develop new ideas which advance both classical and quantum computation.

There is still no convincing experiment that quantum computers are possible.

I would have predicted that by 2026, quantum computing would be proved possible, or investor money would dry up. I was wrong. Investor enthusiasm for QC is stronger than ever. The Trump administration just instested $2 billion into QC, mostly in startup companies. There are about five of these companies worth about $10B apiece. Most of them went public in sneaky financial maneuvers where they did not have to disclose all their risks.

New Scientist magazine just posted a video on Quantum Computers Are More Dangerous Than You Think:

On Q-Day, your privacy will be at stake. This is the moment when quantum computers break the encryption protecting the modern world, bank transactions become readable, private messages get exposed and even state secrets become vulnerable.

For years it sounded like sci-fi, something that was decades away from happening, if it happened at all. But now, research suggests that we may be hurtling towards Q-Day at a rapid speed.

In this video, New Scientist uncovers why many experts think the countdown to Q-Day may already have begun, and explains how quantum computers work and why these machines could both threaten the security of the modern world and unlock breakthroughs that could change our lives. Special thanks to Quantum Motion for letting us film at its facilities.

If and when the countdown to Q-day starts, I think that we will have about ten years. Only the computer security companies, NIST, and the US Dept of War need to prepare now.

For most people, SSL/TLS/SSH is just a way of getting assurance that a web site is real, and that no one is stealing a password or credit card number. If Q-day hits, you will just update your browser and not notice the difference.

The Bitcoin blockchain would have to be restructured, and that is feasible as long as there is a consensus on how to do it. A consensus could take a couple of years.