The paper must be wrong. Inter-branch communication is impossible. I searched briefly for a published rebuttal, and found this guy on quora who claims that Plaga himself eventually came around to his proposal being mistaken. That’s hearsay about Plaga, but the quora response also purports to explain the exact error, and it seems sensible at a glance. (I didn’t scrutinize either the paper or the quora rebuttal; I’m more confident about the wrongness than about what exactly the error is.)
Thanks to finding this. As I understood, the rebuttal says that if you excite the ion in one branch, it becomes entangled with that branch and stops being non-entangled with anything. It is easy to see from energy conversation: if I have excited the ion I spent some energy, but if I see the ion excited in another brach, this energy would come from nowhere, which is prohibited by conversation law.
But also I think there still can be way to communicate: I think that instead of exciting ion, the senders have just to measure it (and it should be prepared in a complex state from the beginning). And the receiver will have to measure if ion remains in quantum state or is in one of eigenstates. This makes the whole setup more complex and may require more ions measurements to send just one but of data.
No matter what basis they measure in, the receiver will observe results consistent with the ion being in whatever state it was already in before the senders even did anything. This is a result of the linearity of quantum mechanics. If the overall wavefunction is a sum of two nearly-orthogonal vectors, then the evolved wavefunction is the sum of each vector evolved separately, and the terms in this sum will also be nearly-orthogonal. In equations:
U(|W1⟩+|W2⟩)=U|W1⟩+U|W2⟩
If |W2⟩ wasn’t there, then |W1⟩ would still evolve to U|W1⟩ and see the exact same outcomes. To get communication, there would have to be significant amplitudes for the universe’s state to spontaneously shift from being in one world to the other. (i.e. even if world 1 is initially the only world, it still has some amplitude to end up in world 2). This is not realistic for the physics of macroscopic objects. We don’t see, either theoretically or experimentally, large amplitudes for a dead cat to turn into a live one, etc, even if the initial decision to kill the cat or not was made by measuring polarization of a single photon.
EDIT: Also, it is a well known fact in QM that “one does not simply measure whether a system is an eigenstate or a superposition”. If you measure a spin of up for an electron, you do not know whether it was actually spinning up, or it was spinning left and you happened to measure the “up” component of the left spin.
But if you’re just concerned about energy conservation, such a complicated fix is not needed anyways: There are many systems that have multiple quantum states with identical energy, momentum, angular momentum, etc, yet are still orthogonal (i.e. perfectly distinguishable by measurement).
So the real reason it doesn’t work is linearity, not energy-conservation or anything like that.
In Plaga model, the trapped ion is isolated from both branches and thus orthogonality equation is not directly applicable to it.
“one does not simply measure whether a system is an eigenstate or a superposition”
We can’t do it simply, but with some efforts we can do it. We can measure was the ion in superposition or not if we repeat the experiment many times in exactly the same settings. Eigenstate will give always 1. Superposition state will give mix of 0 and 1 measurements.
The fix is needed mostly not because energy conversation, but because if we transfer any definite state, the sender destroys isolation of the ion by becoming in superposition in it (as was discussed in Quora objection to Plaga model). Here I suggest to use the mere fact of destruction of ion’s quantum state as a way to transfer information.
If the ion is isolated, that means you take a tensor product of its state with the state of the environment. If u,v are orthogonal, then a⊗u and a⊗v are still orthogonal.
Why does your “repeated measurement” method not also work to use entangled qubits to send signals faster than light? (Since measuring one qubit also collapses the state of the other.)
Or, maybe just tell me the density matrix for the ion that you expect the reciever to see if the sender sends a 0, and also the density matrix for if they send a 1?
Plaga’s article has equations and was published in a scientific journal—Foundations of Physics, 1997. The journal had small impact factor at the time and its editor was Carlo Rovelli. There is no public retraction or refutation of it, except an obscure Quora post where someone said that Plaga doesn’t endorse this anymore. Plaga had around 10 astrophysics articles in 90s.
He (or a person with the same name) works now on “LLM security in Germany”—not a really bad sign as H.Everett also turned to defense industry later.
This is extremely weak signal compared to understanding the technical argument, the literature is full of nonsense that checks all the superficial boxes. Unfortunately it’s not always feasible or worthwhile to understand the technical argument. This leaves the superficial clues, but you need to be aware how little they are worth.
Interesting coincident: Yesterday a new preprint appeared on the same topic
Quantum observers can communicate across multiverse branches Maria Violaris
It is commonly thought that observers in distinct branches of an Everettian multiverse cannot communicate without violating the linearity of quantum theory. Here we show a counterexample, demonstrating that inter-branch communication is in fact possible, entirely within standard quantum theory. We do this by considering a Wigner’s-friend scenario, where an observer (Wigner) can have quantum control over another observer (the friend). We present a thought experiment where the friend in superposition can receive a message written by a distinct copy of themselves in the multiverse, with the aid of Wigner. To maintain the unitarity of quantum theory, the observers must have no memory of the message that they sent. Our thought experiment challenges conventional wisdom regarding the ultimate limits of what is possible in an Everettian multiverse. It has a surprising potential application which involves using knowledge-creation paradoxes for testing Everettian quantum theory against single-world theories
If memory serves, the journal Foundations of Physics was long known as a place for people to publish wild fringe theories that would never get accepted by more mainstream physics journals.
I remember back in 2007, this was common knowledge, so it was big news that (widely respected physicist) Gerard ’t Hooft was due to take over as editor-in-chief, and people in the physics department were speculating about whether he would radically change the nature of the journal. I don’t know whether that happened or not. But anyway, 1997 is before that.
The paper must be wrong. Inter-branch communication is impossible. I searched briefly for a published rebuttal, and found this guy on quora who claims that Plaga himself eventually came around to his proposal being mistaken. That’s hearsay about Plaga, but the quora response also purports to explain the exact error, and it seems sensible at a glance. (I didn’t scrutinize either the paper or the quora rebuttal; I’m more confident about the wrongness than about what exactly the error is.)
Thanks to finding this. As I understood, the rebuttal says that if you excite the ion in one branch, it becomes entangled with that branch and stops being non-entangled with anything. It is easy to see from energy conversation: if I have excited the ion I spent some energy, but if I see the ion excited in another brach, this energy would come from nowhere, which is prohibited by conversation law.
But also I think there still can be way to communicate: I think that instead of exciting ion, the senders have just to measure it (and it should be prepared in a complex state from the beginning). And the receiver will have to measure if ion remains in quantum state or is in one of eigenstates. This makes the whole setup more complex and may require more ions measurements to send just one but of data.
No matter what basis they measure in, the receiver will observe results consistent with the ion being in whatever state it was already in before the senders even did anything. This is a result of the linearity of quantum mechanics. If the overall wavefunction is a sum of two nearly-orthogonal vectors, then the evolved wavefunction is the sum of each vector evolved separately, and the terms in this sum will also be nearly-orthogonal. In equations:
U(|W1⟩+|W2⟩)=U|W1⟩+U|W2⟩
If |W2⟩ wasn’t there, then |W1⟩ would still evolve to U|W1⟩ and see the exact same outcomes. To get communication, there would have to be significant amplitudes for the universe’s state to spontaneously shift from being in one world to the other. (i.e. even if world 1 is initially the only world, it still has some amplitude to end up in world 2). This is not realistic for the physics of macroscopic objects. We don’t see, either theoretically or experimentally, large amplitudes for a dead cat to turn into a live one, etc, even if the initial decision to kill the cat or not was made by measuring polarization of a single photon.
EDIT: Also, it is a well known fact in QM that “one does not simply measure whether a system is an eigenstate or a superposition”. If you measure a spin of up for an electron, you do not know whether it was actually spinning up, or it was spinning left and you happened to measure the “up” component of the left spin.
But if you’re just concerned about energy conservation, such a complicated fix is not needed anyways: There are many systems that have multiple quantum states with identical energy, momentum, angular momentum, etc, yet are still orthogonal (i.e. perfectly distinguishable by measurement).
So the real reason it doesn’t work is linearity, not energy-conservation or anything like that.
In Plaga model, the trapped ion is isolated from both branches and thus orthogonality equation is not directly applicable to it.
“one does not simply measure whether a system is an eigenstate or a superposition”
We can’t do it simply, but with some efforts we can do it. We can measure was the ion in superposition or not if we repeat the experiment many times in exactly the same settings. Eigenstate will give always 1. Superposition state will give mix of 0 and 1 measurements.
The fix is needed mostly not because energy conversation, but because if we transfer any definite state, the sender destroys isolation of the ion by becoming in superposition in it (as was discussed in Quora objection to Plaga model). Here I suggest to use the mere fact of destruction of ion’s quantum state as a way to transfer information.
You mean linearity equation?
If the ion is isolated, that means you take a tensor product of its state with the state of the environment. If u,v are orthogonal, then a⊗u and a⊗v are still orthogonal.
Why does your “repeated measurement” method not also work to use entangled qubits to send signals faster than light? (Since measuring one qubit also collapses the state of the other.)
Or, maybe just tell me the density matrix for the ion that you expect the reciever to see if the sender sends a 0, and also the density matrix for if they send a 1?
Plaga’s article has equations and was published in a scientific journal—Foundations of Physics, 1997. The journal had small impact factor at the time and its editor was Carlo Rovelli. There is no public retraction or refutation of it, except an obscure Quora post where someone said that Plaga doesn’t endorse this anymore. Plaga had around 10 astrophysics articles in 90s.
He (or a person with the same name) works now on “LLM security in Germany”—not a really bad sign as H.Everett also turned to defense industry later.
This is extremely weak signal compared to understanding the technical argument, the literature is full of nonsense that checks all the superficial boxes. Unfortunately it’s not always feasible or worthwhile to understand the technical argument. This leaves the superficial clues, but you need to be aware how little they are worth.
Interesting coincident: Yesterday a new preprint appeared on the same topic
Quantum observers can communicate across multiverse branches
Maria Violaris
https://arxiv.org/abs/2601.08102
Scott Aronson already wrote in FB that it is wrong.
If memory serves, the journal Foundations of Physics was long known as a place for people to publish wild fringe theories that would never get accepted by more mainstream physics journals.
I remember back in 2007, this was common knowledge, so it was big news that (widely respected physicist) Gerard ’t Hooft was due to take over as editor-in-chief, and people in the physics department were speculating about whether he would radically change the nature of the journal. I don’t know whether that happened or not. But anyway, 1997 is before that.