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Physicist Trying To Send a Signal Back In Time
phil reed writes "University of Washington physicist John Cramer is attempting to send a signal back through time."
From the article: "We're going to shoot an ultraviolet laser into a (special type of) crystal, and out will come two. lower-energy photons that are entangled," Cramer said.
For the first phase of the experiment, to be started early next year, they will look for evidence of signaling between the entangled photons. Finding that would, by itself, represent a stunning achievement. Ultimately, the UW scientists hope to test for retrocausality — evidence of a signal sent between photons backward in time.
The test will involve sending one of the photons down 10 miles of fiber optic cable, delaying it by 50 microseconds, then testing a quantum-mechanical aspect of the delayed photon. Due to quantum entanglement, the non-delayed photon would need to reflect the measurement made 50 microseconds later on the delayed photon. In order for this to happen, some kind of signal would need to be sent 50 microseconds back in time from the delayed photon to the non-delayed photon. (Confusing? Quantum physics is like that.)
Quoting from:
http://en.wikipedia.org/wiki/Quantum_entanglement
Although two entangled systems appear to interact across large spatial separations, no useful information can be transmitted in this way, so causality cannot be violated through entanglement.
The slashdot editor's brains seem to be traveling back in time though.I actually graduated in quantum information, this is no news and it is wrong.
I explain my opinion:
- Entanglement has been observed, pairs of fotons and spin of electrons can be correlated in a manner impossible to describe in classical physics.
- The experiment described does not even measure entanglement, as you could achieve the same result classically:
Say I have a black ball and a white ball, I put one at random in a closed box, the other one in another box. Say the boxes are put 1000 miles away from each other, from the content of one of the boxes I can predict which ball is in the other one, as I can check later.
The point is that they are not choosing in which state (of polarization) the light will be in the moment they measure the first time. So they aren't going to send any message ever this way. To do it they would require a classical channel wich works as we expect...
For the proof of entanglement one must implement physically the Bell's system or the Greenberger-Horne-Zeilinger one (I have no link), and SURPRISE! it has already been done.
Actually the experiment is designed properly. The thing is, they are already going to misinterpret the results. Quantum entaglement means that at the moment of setting wavefunction of one of the particles, the wavefunction of second particle is immediately changed to "second" possible state
I believe they are hypothesizing actual signalling to occur as follows. Call the two detectors Ap (for prompt arm) and Ad (for delayed arm), and the two photons Pp and Pd for the same reasons.
Ap and Ad are not the same. Ap has some capacity to respond to the photon in two different ways. I don't know what they're planning, but conceptually some kind of double-slit apparatus followed by a two-layer detector that has one layer capable of determining which slit the particle passed through, followed by another layer that is sensitive only to photons in interference maxima that have classically very low probabilities. So if you detect the photon in layer 1 it is behaving as a particle, in the layer 2 it is behaving as a wave.
On the other end, at Ad, rather than giving photon Pd a "choice" of what to do, you have two different detector systems: one that is an interferometer, one that is a localized particle detector. One or the other gets switched into the beam "after" the photon has been detected at Ap. With correct placement of the detectors it should be possible to give the term "after" an absolute meaning.
The claim is that the results of the measurement of Pp by Ap will necessarily reflect the choice made by the experimenter at Ad. So if Pp is detected "as a particle" it will be "because" the experimenter has chosen to detect Pd "as a particle" some time "later", and similarly if Pp is detected "as a wave". The heavy use of scare quotes is due to my respect for relativity and disbelief in strong quantum ontologies.
I hope I have made this seem plausible, although it is all wrong.
The perfect linearity of quantum reality ensures that when one gets down to the detailed computations there is an exact balance between terms that wipes out any possibility of transmission of information by this means. This experiment is testing this aspect of reality, and if no one has been able to explain to them "exactly" why it won't work it is because no one has bothered to do the detailed analysis of their apparatus that would be required. When detector efficiencies are folded into the mix the analysis can become quite complex, and you really need to do that if you want to test causality in this manner. If you want to simply demonstrate that the conventional interpretation of QM predicts no knowable information will be transmitted the analysis is much easier.
So this is a pretty ordinary test of the linearity of quantum reality, and as they say, it is virtually certain that no transmission of information will occur. Unfortunately, given the truly terrible standard of communication demonstrated by this article it is likely that that fact will never be clearly understood by the public.
Blasphemy is a human right. Blasphemophobia kills.
IAAP, and this point of view (that in standard quantum mechanics no information is transmitted superluminally) is entirely correct. This will just be one more shoddy example of science reporting in a very very long line. The only question here is who got their basic facts wrong?
The physicist in question really ought to know better. Did he lie to the reporters in order to get press for his experiment?
The newspaper ought to have done some basic fact-checking; reading Wikipedia would be enough to figure things out in this case. Did they lie to the public to make the story more interesting?
So let's do some digging. The physicist in question is a proponent of the "transactional interpretation" of quantum mechanics (not coincidentally invented by this same guy). In this interpretation, particles may send signals back in time that "handshake" with other particles in the past; however, they do so in such a way that ordinary causality is always correct. See, for example, Cramer's paper at http://www.npl.washington.edu/npl/int_rep/qm_nl.ht ml where he says:
So that seems to answer that question. However, he goes on to muddy the water by suggesting that quantum mechanics as verified by every experiment to date is actually very slightly wrong, that quantum theory is actually slightly nonlinear. In that case, the delicate conservation of our usual notion of causality will break down and superluminal signals become possible again. Virtually nobody believes this is the case, but I suppose that shouldn't stop us from checking just to be sure. After all, sometimes what nobody believes still turns out to be true.
The blame here (as so very often) must fall on the reporters. Let's examine some of their shoddy work:
That's not a problem with quantum theory; it's a problem with what you think is weird.
Like the twin paradox, this is not a paradox at all. Quantum mechanics predicts something. EPR say, "Hey, that sounds weird and wrong." Experiment verifies quantum mechanics. Once again, the problem is with what is perceived as being "normal", not with quantum theory.
This one is the core conceptual problem with the whole article. It should read:
If one of the entangled photon's trajectories is measured to be up, the other one, no matter how distant, if measured will be measured to be down.
That doesn't sound very weird at all, which is why reporters persist in getting it wrong. People like to think quantum mechanics is weirder than it is; it adds some kind of mystical aura to the whole thing. But the universe is plenty weird and interesting even when you get all your facts right. I hope eventually the popular writing on quantum theory will reflect that.