I have a question concerning the scientific experiment proposed in the following video, (25:00-29:00) titled the "EPR Thought Experiment", which bears resemblance to the quantum eraser experiment.

The Quantum Conspiracy: What Popularizers of QM Don't Want You to Know. Ron Garret, Google Tech Talks (YouTube), 12 January 2011.

I had an email exchange about a year ago with Ron Garret, in the exchange he said the following:

Let’s make sure we’re on the same page about what Copenhagen actually say, because there’s considerable disagreement. I think most people think of the Copenhagen interpretation as the idea that measurement causes a physical phenomenon known as “collapse of the wave function” which is non-linear and irreversible, i.e. it causes a quantum superposition to change into a probabilistic mixture of classical states, and that there is an physical difference between these two states. That idea can definitely be ruled out experimentally in any number of ways, the EPRG thought experiment from my talk being, of course, my favorite example.

My issue here is it seems as though if his experiment were to be performed, it would violate known laws of physics. As photons entangled with different states of the other particles wouldn't interfere any longer, am I misunderstanding something?

Is this experiment physically plausible in accordance with the known laws of physics?

Does this refute the Copenhagen interpretation and/or quantum indeterminacy?

  • 2
    $\begingroup$ @Aniket: when making edits, please do not remove quote blocks of actual quotes, doing so does not improve readability. $\endgroup$ – Kyle Kanos Sep 23 '15 at 13:58
  • $\begingroup$ Without watching the video: No, it wouldn't. If such a claim is being made, then it's probably total nonsense. The "collapse of the wavefunction" is merely another name for the Born rule. The irreversibility of a measurement follows directly from the definition of a measurement and the validity of the Born rule follows more or less directly from a density matrix treatment of an irreversible measurement process. All of this has been known since the 1960s or so, there is simply a bunch of folks out there who didn't get the memo (or they don't care to read it). It's just like perpetual motion. $\endgroup$ – CuriousOne Sep 23 '15 at 14:12
  • $\begingroup$ @CuriousOne: Yes this is just what I was thinking. My bs detector was going haywire during that entire segment. I'm hoping someone with my knowledge then myself can explain why this claim above is incorrect. $\endgroup$ – Hybrid Sep 23 '15 at 14:19
  • $\begingroup$ DanielSank and some of the others could walk you trough the details, if they care. They will also be able to explain that the Born rule is just a special case of quantum mechanical measurement. So called weak measurements work in a more general way than the original Copenhagen interpretations suggests, but they don't actually disprove the Copenhagen interpretation, they merely refine it (at least in my opinion). $\endgroup$ – CuriousOne Sep 23 '15 at 14:25
  • 2
    $\begingroup$ Please include all relevant information into the question. A YouTube link can rot and I am not going to watch a video to understand your question. $\endgroup$ – ACuriousMind Sep 24 '15 at 0:37

Garret's presentation of what he calls the "Einstein-Podolsky-Rosen-Garret" paradox, in the 25:00 - 29:00 range of the video you link to, is not sound.

Garret proposes a source of entangled particles which produces the state $$ \newcommand{\up}{|\!\uparrow⟩}\newcommand{\down}{|\!\downarrow⟩} |\Psi⟩=\frac{\up\down+\down\up}{\sqrt{2}} $$ and then sends the particles to distant locations, to be measured at spatially-separated events. His protocol then asks you to 'measure on the left, and look for interference on the right', which can be succinctly phrased as measuring on the $\{\up,\down\}$ basis on the first mode, and on the $$ \newcommand{\plus}{|+⟩}\newcommand{\minus}{|-⟩} \left\{\plus=\frac{\up+\down}{\sqrt{2}},\minus=\frac{\up-\down}{\sqrt{2}}\right\} $$ basis on the second mode. The measurement probabilities in this case are easily seen to be $$ \newcommand{\bup}{⟨\uparrow\!|}\newcommand{\bdown}{⟨\downarrow\!|} \newcommand{\bplus}{⟨+|}\newcommand{\bminus}{⟨-|}\newcommand{\bpm}{⟨\pm|} \left|\bup\bpm|\Psi⟩\right|^2\frac12, $$ and analogously for $\bdown$, so if you measure on the left there is no interference on the right.

Garret then claims that this can be used for superluminal communication, and this is where he is incorrect.

So far, there's nothing that Alice, who is in control of the first mode, can do to alter the outcome at all - she definitely cannot control which of the two outputs ($\up$ or $\down$) she will get. The only choice she has is whether to measure her system prior to the interference step, or to let the two arms interfere and then measure. What Garret apparently doesn't realize is that even if Alice does let her system produce interference, the other system will not produce interference either. Alice has no way to make Bob's side of the system display interference without sending him classical information at subluminal speeds.

Let me sketch that calculation as it is important to the argument. Suppose both Alice and Bob measure on the $|\pm⟩$ basis, with Alice obtaining $|a⟩=|\pm⟩$ and Bob obtaining $|b⟩=|\pm⟩$. The probability for this outcome is then \begin{align} |⟨a|⟨b|\Psi⟩|^2 &=\frac18\left|(\bup+a\bdown)(\bup+b\bdown)(\up\down+\down\up\right|^2 \\&= \frac18\left|a+b\right|^2. \end{align} Thus if Alice gets $\plus$ it is certain that Bob will get $\plus$, and ditto for $\minus$, so it seems that Bob does observe interference. However, just because Alice decides that she wants to run her system through the 'recombine' step that doesn't mean that she gets to control which outcome she gets. She will get $\plus$ as often as she does $\down$, which means that so will Bob, and what that looks like to Bob is simply no interference.

Alice's actions, then, have no effect on what Bob observes, and therefore this channel cannot be used for superluminal communication.

This does sort of have a bearing on the Copenhagen Interpretation as described by Garret,

most people think of the Copenhagen interpretation as the idea that measurement causes a physical phenomenon known as “collapse of the wave function” which is non-linear and irreversible, i.e. it causes a quantum superposition to change into a probabilistic mixture of classical states, and that there is an physical difference between these two states.

but it doesn't rule CI out. In particular, this understanding of the Copenhagen Interpretation does force Bob's system to change instantaneously as soon as Alice measures her side of their entangled pair, but Bob's system somehow contrives to (partially) "hide" this inner change of state from any possible measurement, in a way which exactly prohibits superluminal communication.

This bothers some people (it somehow imagines Nature as having an extra set of supernatural powers which it purposefully denies us) but it is not inconsistent with the laws of physics. In particular, this "EPRG" argument does not rule out the Copenhagen Interpretation as "scientifically untenable", as Garret appears to claim.

I'm afraid I won't have time to critique whatever it is he says in the second half of the video, though. Quantum interpretations are a tricky business, and if you make technical mistakes on the underlying mechanics then there's no telling how much of a basis the resulting arguments will have.

  • $\begingroup$ Thank you very much, Ron appears to argue on the comment section of his blog(blog.rongarret.info/2015/02/31-flavors-of-ontology.html) and within his paper(flownet.com/ron/QM.pdf) that this is the case. He than states the he invented a machine for faster-than-light communication and it would function if Copenhagen were true on the comment section of that blog post. $\endgroup$ – Hybrid Sep 23 '15 at 17:03
  • $\begingroup$ He said: "I spent ten years NOT HAVING an adequate explanation of QM. I asked dozens of card-carrying physicists to explain it to me, including Freeman Dyson. None of them knew the answer. I entertained wild fantasies of making a breakthrough myself and winning the Nobel prize. I got a U.S. patent on a gadget that would let you communicate faster than light if Copenhagen were true." I lol'd at the last sentence and had to look into this further, hence the question. Thanks for the coherent answer. $\endgroup$ – Hybrid Sep 23 '15 at 17:09
  • $\begingroup$ I can't comment fully without watching the whole video and reading all that material, and I don't want to so I won't. However, his characterization of "if Alice measures and that destroys Bob's interference, then you can communicate FTL", without any mention that in that setup there is never any interference to begin with and that FTL comms are therefore ruled out, is at the very best irresponsibly misleading. $\endgroup$ – Emilio Pisanty Sep 23 '15 at 17:12
  • 1
    $\begingroup$ Think of it this way: CI is fully consistent with QM (that's what it means to be an interpretation), and QM provably forbids FTL comms. Therefore CI does not allow FTL comms. $\endgroup$ – Emilio Pisanty Sep 23 '15 at 17:13
  • $\begingroup$ Here's another way to think about it, from the lecture by Coleman: there's no interaction Hamiltonian between the photons. Where are you going to get a superluminal signal? $\endgroup$ – Robin Ekman Oct 7 '16 at 15:33

My issue here is it seems as though if his experiment were to be performed, it would violate known laws of physics.

The picture and words in the section of the video you cited seem a little too vague to uniquely specify a specific experiment. But in the whole first 40 minutes of the video you see that all he is claiming is that we don't see departures from the Schrödinger equation, which is correct. And the opposite (seeing departures from Schrödinger) would be violating the known laws of physics. The Schrödinger equation is the law of physics, the Born rule is an attempt to use the wavefunction of one system to describe the state of a device that measures the aggregate statistics of an ensemble of identically prepared subsystems. When you write the Schrödinger equation for D+S1+S2+...+Sk where D is the device that measures the aggregate statistics of a whole ensemble and S1+S2+...+Sk is the whole ensemble itself and all the things it interacts with too, then you do get the Born rule. It is a hack only in that you focus on just one of the many identically prepared systems. It isn't a hack in the sense that the results are wrong.

The whole first 2/3 of the video is just saying that a measurement is a process by which the state of the device becomes entangled with the newly created eigenstate of the observable. And it is created in the sense of being entangled with the newly created branch of the entanglement of the devices and the objects. This isn't a radical departure from quantum mechanics, it is only a modification of some oversimplifications of some, rather old fashioned, descriptions.

As photons entangled with different states of the other particles wouldn't interfere any longer, am I misunderstanding something?

The whole video is saying there are proto measurements, and that entanglement is just another example of a protomeasurement. A protomeasurement is upgraded to a full irreversible measurement solely when the different states become entangled with too many other things to be reversible in a purposeful way or even on accident. It's the thermodynamic kind of irreversibility.

The entanglement of two things is just a protomeasurement, then the entanglement gets passed to more and more things until eventually you have an entanglement between two different collections that are so numerous and complicated that reversing it just isn't going to happen. At that point, the entanglement is effectively irreversible and you can call it (each of the branches of the entanglement) a measurement result.

The whole video is just saying that measurements are just an outcome of entanglements.


Everyone criticizing my work here has completely missed the point: the EPRG paradox is intended to be a straw man. Of course it doesn't actually work. The whole point of the talk is to explain why it doesn't work, because on the traditional pedagogy of quantum mechanics it seems like it should work. If you're going to criticize someone's work, you really ought to read it first. Particularly section 4 of the paper.

  • $\begingroup$ Sorry, Ron, but no. If you broadcast a talk to the winds, the talk needs to stand on its own - you cannot make misleading statements on a talk (and particularly such a talk) and then fix them deep in a paper. That said, I will have a closer look on Monday (as I am travelling at the moment). $\endgroup$ – Emilio Pisanty Oct 8 '16 at 1:15
  • 3
    $\begingroup$ Ron, someone has suggested that this post does not constitute an answer to the question, and if the community agrees, your post will be deleted. It would be great to have your answer here but it does need to be an actual answer. If you can edit this post to make it more clearly and directly answer the question (I mean, even if it is an answer, make it more obvious to everyone that it is) , that would definitely help. $\endgroup$ – David Z Oct 8 '16 at 4:58
  • $\begingroup$ (To clarify a bit on David's comments, the site standard is to use answer posts for answers to the original question, as opposed to comments on other posts. You will be able to comment everywhere after a bit of use of the site, for spam-fighting reasons. In that light, it would be best if you could expand this post to address the questions raised by Hybrid.) $\endgroup$ – Emilio Pisanty Oct 8 '16 at 8:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.