Have Australians and their photons legitimized time travel?

I've received links to this story approximately from 7 people so I will write a short blog post although I don't claim that it's a well-deserved honor for the authors. At any rate, the popular science media were full of the news that physicists managed to simulate time travel with photons, showed that there is nothing wrong with closed time-like curves at the quantum level, and so on.



See e.g. The Daily Mail – to be sure that at least one URL works for more than 30 days – and the press release at the University of Queensland, Doctor Who meets Professor Heisenberg.

All these wonderful things were not invented by the journalists. There is actually an article in Nature Communications

Experimental simulation of closed timelike curves

by Martin Ringbauer, a PhD student (!) and a lead author, and collaborators.




The journalists are adding several additional layers of silliness, nonsense, and hype. But the paper in Nature Communications is bad enough. In fact, I think that even the abstract is more than enough for anyone who has a clue about these matters to say "Holy cow".




Let me repost the whole abstract:

Closed timelike curves are among the most controversial features of modern physics. As legitimate solutions to Einstein’s field equations, they allow for time travel, which instinctively seems paradoxical. However, in the quantum regime these paradoxes can be resolved, leaving closed timelike curves consistent with relativity. The study of these systems therefore provides valuable insight into nonlinearities and the emergence of causal structures in quantum mechanics—essential for any formulation of a quantum theory of gravity. Here we experimentally simulate the nonlinear behaviour of a qubit interacting unitarily with an older version of itself, addressing some of the fascinating effects that arise in systems traversing a closed timelike curve. These include perfect discrimination of non-orthogonal states and, most intriguingly, the ability to distinguish nominally equivalent ways of preparing pure quantum states. Finally, we examine the dependence of these effects on the initial qubit state, the form of the unitary interaction and the influence of decoherence.

Pretty much everything written above is a completely unconstrained and unlimited nonsense. I will discuss some issues separately but it may be useful to describe the primary "methodological blunder" that acts as the source for this nonsense.

It may happen that a PhD student does an important experimental or theoretical work that should be published in a prestigious journal – although this study isn't an example. But what is virtually impossible is for a pure experimenter to write a good paper about cutting-edge and marginally speculative topics that are of purely theoretical nature.

And be sure that the topics that these authors wanted to study but they have failed are absolutely theoretical in character. Experimenters just don't have a clue – and they may be forgiven for having no clue because it's not their job.

But it's very clear which recent credible research direction is pointing towards a possible legitimization of some kinds of wormholes – and the ensemble of legalized wormholes may be extended in the future. It's the Maldacena-Susskind ER-EPR correspondence that offers a dictionary between the ordinary quantum entanglement on one side (EPR); and non-traversable wormholes (the ER bridges) on the other side.

Needless to say, the Maldacena-Susskind paper isn't being cited by this Nature Communications salvo of pseudoscience written by Mr Ringbauer et al. I would bet that they're not aware of the ER-EPR correspondence and they perhaps don't even know who Maldacena and Susskind are! Nevertheless, Mr Ringbauer, his collaborators, and the editors and referees working for Nature think that Mr Ringbauer et al. are somewhat able to do a meaningful research of these purely theoretical questions, anyway.

It isn't the case.

You know, experimenters who are cutting-edge theorists at the same moment are extremely rare. Enrico Fermi was the ultimate template of this category. You could find some more recent counterparts but the closer you approach the present era, the more questionable the status of these people as "Fermi's peers" would become.

It's not necessarily due to all the people's being intrinsically worse than Fermi. A part of the problem is that physics has gotten more complex, extensive, and demanding than it was 70 or 80 years ago. There is a greater body of knowledge one has to master to get to the top. And this implies that people are increasingly specialized in average. Some people are more specialized, others are broader, but the statistical distribution has shifted towards a greater specialization in the recent 70 or 80 years.

If you realize these things and if you acknowledge some basics of the scientific method, you should agree that an experimenter who is doing "real science" should restrict his or her interpretations to reproducible actions with his or her devices, and reading of the displays to find the results from the measurement apparatuses. If there is some higher-level interpretation of what the experiment is measuring, the experimenters should better carefully follow the procedures prescribed by theorists in previous, sufficiently acceptable and accepted papers. If experimenters who have no reasons to think that they're top theorists at the same moment pretend to create their own theory on the run, the results can't be good.

In other words, every interpretation of our or experimenters' observations is theory-laden or theory-dependent. One can't deduce the deep truth "directly" and the theoretical framework in which the experiments are being interpreted should be functional, tested – and that almost always means that it was discovered by a theorist sometime in the past. The experimenters just can't interpret what they're seeing without such a good theoretical framework, and the further they deviate from modestly saying that "they see the number 5 on the LCD display", the greater risk of writing complete nonsense they take. To claim that they directly (?) see a photon talking to his older self means to be intoxicated.

Now, if you want to make important and valid statements about the ability of quantum mechanics or quantum gravity to legitimize some wormholes, you should be a deep theorist. And in fact, the theory is completely enough.

My first criticism was purely sociological in nature. It's just hopeless for an experimenter – especially an experimental PhD student – to try to revolutionize some cutting-edge theoretical concepts that remain partly controversial among the theorists themselves. He just doesn't have a chance to write anything that makes sense.

My second criticism is somewhat less sociological but it is related: experiments are really completely useless for conceptual questions about the relationships between wormholes and quantum mechanics. Ringbauer claims to have done some real experimental work involving photons. However, it is damn obvious – and it is extremely bad if it is not obvious to him, to his adviser, to the editors, or to the referees – that the theories we know are enough to predict the results of any low-energy experiment that an Australian PhD student may do with photons in 2014.

It means that it is utterly useless to do these experiments. We know what properties of the photons will be measured with what probabilities, anyway. If some of these experiments are relevant for the viability of the extremely audacious concepts such as wormholes or time travel, then it's all about the interpretations of these known experimental results. And one has to be a competent and intelligent enough theorist for these interpretations to make any sense. That's why you shouldn't be surprised at all that nothing in that paper makes sense at all.

Now, let me discuss the content of the individual sentences in the abstract separately and a little bit more technically:

Closed time-like curves are among the most controversial features of modern physics. As legitimate solutions to Einstein’s field equations, they allow for time travel, which instinctively seems paradoxical.

The fact that they're "controversial" is probably the reason why the PhD student found it irresistible to talk about them. It's perfectly OK to be intrigued. Those things are exciting and the word "exciting" isn't far from a synonym of "controversial" in this case. However, the editors shouldn't have allowed the publication of a self-evidently crackpot paper just because the topic is "controversial".

The closed time-like curves may arise as solutions to Einstein's field equations but it's not guaranteed that all solutions to these equations are physical and may materialize in Nature. Various additional constraints on initial conditions and the implications these constraints have on the time evolution may render many solutions to Einstein's equations unphysical. And I think it's the case here.

Closed time-like curves allow you not only to castrate your grandfather before he sleeps with your grandmother for the first time. There are various other links of these animals with instabilities and other problematic properties that show that the life in the presence of closed time-like curves can't be life like usual and that Nature has various other tools to avoid these problematic features of the spacetime geometry. Most of the detailed subclasses of closed time-like curves may be rigorously excluded because they contradict mathematical logic. It is utterly misleading to say that the problems with these concepts are just "instinctively" paradoxical. Physics isn't about instincts. Physics is about the evidence and solid calculations and logical argumentation.

The very fact that Nature allowed these physics questions to be discussed at the level of "instincts" is unfortunate.

However, in the quantum regime these paradoxes can be resolved, leaving closed timelike curves consistent with relativity.

I don't think that this statement may be correct in any sense. At the end, if we define the presence of closed time-like curves operationally enough, the definition will inevitably agree with the classical definition in the classical limit. And those effects are banned for the same reason as in classical physics. One may really use the classical arguments in the classical limit.

Outside the classical limit, there may be effects that are formally analogous to wormholes (and perhaps even to closed time-like curves) but they may only be allowed if they qualitatively differ (when it comes to their practical implications) from what we would call wormholes in classical physics. So if quantum mechanics or quantum gravity allows some wormholes, it's only those that cannot be proven impossible by a valid classical argument. The non-traversable Einstein-Rosen bridges may be OK. But closed time-like curves with an observer who may "sail around" are always a problem. Again, one needs to be a deep theorist to meaningfully discuss whether a relationship between two objects may be described as a "Planckian wormhole" or not. It is completely counterproductive for an impressionable student to vent his "instincts" about these questions in a journal.

If quantum mechanics changes something about these restrictions, it adds new restrictions which may be of a different character than any of those that are relevant for the classical limits. But classical physics is "included" as a limit of the quantum theory, and so must be all the restrictions that follow from the classical arguments.

The study of these systems therefore provides valuable insight into nonlinearities and the emergence of causal structures in quantum mechanics—essential for any formulation of a quantum theory of gravity.

If we talk about the observables, it is an absolutely critical and universal fact that all observables in any quantum theory (with real values) are expressed by linear Hermitian operators. It's true for generators of transformations such as the Hamiltonian or the angular momentum, too. Nothing will ever change about this universal postulate of quantum mechanics – or other universal postulates of quantum mechanics. If you get uncertain about the linearity of quantum mechanics whenever an intoxicated student writes some nonsense in Nature, then you might also get uncertain whether the Earth is round or flat every time you hear about this question. You just shouldn't. People who doubt the linearity of quantum mechanics are totally and utterly deluded and incompetent.

(On Strings 2014, Joe Polchinski tried to criticize the Papadodimas-Raju picture of the black hole interior by saying that the picture violates the linearity of operators in quantum mechanics. But it doesn't. Joe is completely wrong. One of the numerous mistakes he is doing is to try to "adjust the reference microstate" before each operation. But that's totally wrong. If quantum mechanics is used in any meaningful sense at all, we must fix our association of observables before the experiment and keep it fixed! The microstate gives us a dictionary for the field operators in the black hole interior and those are completely linear.)

The (Heisenberg) equations of motion that dictate the evolution of operators are nonlinear and so are the formulae for the Hamiltonian etc. but that was true even in mechanics. One doesn't need to talk about fancy things like quantum gravity – and one shouldn't use these words if he has no clue what they mean and it is self-evident that Mr Ringbauer has no idea what quantum gravity is.

Here we experimentally simulate the nonlinear behaviour of a qubit interacting unitarily with an older version of itself, addressing some of the fascinating effects that arise in systems traversing a closed timelike curve.

The writers of this sentence must know that they're high, mustn't they? Operators encoding anything that may be observed act on qubits – or any other Hilbert space – linearly. At least at the level of the fundamental laws of Nature, it's so. Any nonlinearity is an artifact of some approximations. Even if you believed that this law doesn't hold universally, what would you have to observe to become convinced (or even to convince others who are not smoking) that you have observed a "nonlinear action of an operator on a qubit"? Or another nonsensical would-be process such as the "interaction of a qubit with an older version of itself"?

A photon is an extremely simple system; it is an elementary particle. It won't show you a display with the message "Cool, I just interacted with an older copy of myself. My God. To prove it, let me tell you that my alter ego was just singing 'Oops I Did It Again' by Britney Spears." A photon will tell you something about its frequency or a bit about its polarization, or something like that. None of these things may be used to "directly deduce" that you have observed a time machine or a nonlinearity of quantum mechanics or any other of the wonderful things that Mr Ringbauer et al. claim to have observed.

A measurement occurs at a particular moment of time. It shows some interaction of objects A,B if these objects are observed in a different state than one would expect without interactions, e.g. if A is observed in a different state than the state it would be found at if the initial conditions for A were the same but B was completely absent. And vice versa. Experimentally, you may always see an interaction in between two measurements. If you see the interaction, all the objects that interacted were interacted at the same time. That's true by definition.

It's just impossible to even operationally define what it could mean for two objects at different times to interact. To put this fact modestly, a careful experimenter may interpret all of his observations in such a way that all interactions he uses to describe what he has seen are taking place at a specific time – and all the objects that interact are interacting at the same time, the time of the interaction. Even if he saw "some traces of a different time", he would interpret it as a memory (or a memory effect) and he could study the property of this memory (or memory effect).

The sentence about nonlinearity of qubits and the interaction with your obsolete doppleganger can't possibly correspond to any conceivable observation, any feature of the experiment. It is absolutely clear and provable that the PhD student is making this sci-fi stuff up because he's high.

These include perfect discrimination of non-orthogonal states and, most intriguingly, the ability to distinguish nominally equivalent ways of preparing pure quantum states.

Clearly, this young Gentleman failed to understand undergraduate quantum mechanics – and perhaps even the rudiments of mathematical logic and set theory. One "can't perfectly discriminate non-orthogonal (normalized) states" \(\ket\psi\) and \(\ket\chi\) for a simple reason. The probability that \(\ket\chi\) behaves as \(\ket\psi\) is equal to the squared absolute value of the inner product\[

P = \abs{ \bra\psi\chi\rangle }^2

\] Because the inner product is nonzero – that's what it means for the states to be non-orthogonal – the probability calculated by the Born rule above is nonzero. And the nonzero probability means that \(\ket\psi\) may behave in all respects as \(\ket\chi\) with a nonzero probability, and vice versa, which is exactly the same statement as the statement that it is impossible to perfectly discriminate between the two states! You may only err in these simple matters if you have no idea how to translate in between "plain English" (like "perfect discrimination") and "mathematical language" (like orthogonality of vectors).

The equivalence of the adjectives "perfectly mutually exclusive" (plain English) and "orthogonal" (mathematics) in front of two state vectors holds by the basic quantum rules, pretty much by definition (by the quantum definition of the word "exclusive" etc.). You can't "falsify" this separate principle experimentally because this principle is tautologically incorporated into the way how quantum mechanics interprets and predicts all observations. If you want to change something about the equivalence, you must first develop and validate (super extremely unlikely to succeed!) your own alternative non-quantum theory which uses some similar vectors in a completely different way where the equivalence doesn't hold. In your hypothetical new theory, you may define the term "mutually exclusive" in a different way. But such an alternative theory will first have to describe (almost?) all the things where quantum mechanics succeeded. You just can't claim to have switched from quantum mechanics to a totally different theory "a piece by piece".

The second miracle promised by the authors is equally logically impossible. If two initial states are exactly the same, i.e. equal as two elements of the Hilbert space up to a phase (or equal as two density matrices if we talk about mixed states), they imply exactly the same predictions because all these predictions depend on the question only – the choice of operators in the future, and we keep this choice fixed – and on the initial state only.

The sentence is exactly as logically self-contradictory as the sentence that two numbers \(x,y\) are equal to each other but different from each other, too. It's just not logically possible. You can't eat a cake and have it, too.

Finally, we examine the dependence of these effects on the initial qubit state, the form of the unitary interaction and the influence of decoherence.

In other words, to make the paper really cool, one must also include some equally nonsensical sentences involving these other sexy buzzwords like decoherence. The authors clearly don't understand them either but the more buzzwords like that are incorporated, the better.

It's sad that the quality of the publications has dropped so much. Just be sure, no one has found any legitimization of closed time-like curves and this paper doesn't make the slightest sense. But I guess that no one really cares. Science journalists may write thousands of MSM articles about this "research", anyway, and millions of people read it as if it were a scientific result.