- Popper's experiment
Popper's experiment is an experiment proposed by the 20th century philosopher of science

Karl Popper , to test the standard interpretation (theCopenhagen interpretation ) of Quantum mechanics.cite book

last = Popper

first = Karl

authorlink = Karl Popper

title = Quantum Theory and the Schism in Physics

publisher = Hutchinson

date= 1982

location = London

pages = 27-29] cite journal

author =Karl Popper

title = Realism in quantum mechanics and a new version of the EPR experiment

journal = Open Questions in Quantum Physics, Eds. G. Tarozzi and A. van der Merwe

volume =

issue =

pages =

year = 1985] Popper's experiment is similar in spirit to the thought experiment of Einstein, Podolsky and Rosen (TheEPR paradox ). For some reasons, it did not become as well known. Currently, the consensus is that the experiment was based on a flawed premise, and thus its result doesn't constitute a test of quantum mechanics. The experiment does remain important, however, from a historical point of view, and also in that it exemplifies the pitfalls that one comes across in trying to make sense out of quantum mechanics.**Background**Quantum theory is an astoundingly successful theory when it comes to explainingor predicting physical phenomena. There are various interpretations of quantum mechanics that do not agree with each other. Despite their differences, they are nearly experimentally indistinguishable from each other. The most widely accepted interpretation of quantum mechanics is the

Copenhagen interpretation put forward byNiels Bohr . The spirit of the Copenhagen interpretation is that thewavefunction of a system is treated as a composite whole, so disturbing any part of it disturbs the whole wavefunction. This leads to the counter-intuitive result that two well separated, non-interacting systems show a mysterious dependence on each other. Einstein called thisspooky action at a distance . Einstein's discomfort with this kind of spooky action is summarized in the famous EPR argument.cite journal

author = A. Einstein, B. Podolsky, and N. Rosen

title = Can the quantum mechanical description of physical reality be considered complete?

journal = Phys. Rev.

volume = 47

issue =

pages = 777–780

year = 1935

doi = 10.1103/PhysRev.47.777] Karl Popper shared Einstein's discomfort with quantum theory. While the EPR argument involved a thought experiment, Popper proposed a physical experiment to test the Copenhagen interpretation of quantum mechanics.**Popper's proposed experiment**Popper's proposed experiment consists of a source of particles that can generate pairsof particles traveling to the left and to the right along the x-axis. Themomentum along the y-direction of the two particles is entangled in such away so as to conserve the initial momentum at the source,which is zero. Quantum mechanics allows this kind of entanglement. There are two slits, one each in the paths of the two particles.Behind the slits are semicircular arrays of detectors which can detect theparticles after they pass through the slits (see Fig. 1).Popper argued that because the slits localize the particles to a narrowregion along the y-axis, from the

uncertainty principle they experience large uncertainties in the y-components of their momenta.This larger spread in the momentum will show up as particles beingdetected even at positions that lie outside the regions where particleswould normally reach based on their initial momentum spread.Popper suggests that we count the particles in coincidence, i.e., we count only those particles behind slit B, whose other member of the pair registers on a counter behind slit A. This would make sure that we count only those particles behind slit B, whose partner has gone through slit A. Particles which are not able to pass through slit A are ignored.

We first test the Heisenberg scatter for both the beams of particles going to the right and to the left, by making the two slits A and B wider or narrower. If the slits are narrower, then counters should come into play which are higher up and lower down, seen from the slits. The coming into play of these counters is indicative of the wider scattering angles which go with narrower slit, according to the Heisenberg relations.

Now we make the slit at A very small and the slit at B very wide. According to the EPR argument, we have measured position "y" for both particles (the one passing through A and the one passing through B) with the precision $Delta\; y$, and not just for particle passing through slit A. This is because from the initial entangled EPR state we can calculate the position of the particle 2, once the position of particle 1 is known, with approximately the same precision. We can do this, argues Popper, even though slit B is wide open.

We thus obtain fairly precise "knowledge" about the y position of particle 2 - we have "measured" its y position indirectly. And since it is, according to the

Copenhagen interpretation , our "knowledge" which is described by the theory - and especially by the Heisenberg relations - we should expect that the momentum $p\_y$ of particle 2 scatters as much as that of particle 1, even though the slit A is much narrower than the widely opened slit at B.Now the scatter can, in principle, be tested with the help of the counters. If the

Copenhagen interpretation is correct, then such counters on the far side of slit B that are indicative of a wide scatter (and of a narrow slit) should now count coincidences: counters that did not count any particles before the slit A was narrowed.To sum up: if the Copenhagen interpretation iscorrect, then any increase in the precision in the measurement of our "mere knowledge" of the particles going through slit B should increase theirscatter.

Popper was inclined to believe that the test would decide against the Copenhagen interpretation, and this, he argued, would undermine Heisenberg's uncertainty principle.If the test decided in favour of the Copenhagen interpretation, Popper argued, it "could" be interpreted as indicative of

**action at a distance**.**The debate**Many viewed Popper's experiment as a crucial test of quantum mechanics, and there was a debate on what result an actual realization of the experiment would yield.

* In 1985, Sudbery pointed out that the EPR state, which could be written as $psi(y\_1,y\_2)=\; int\_\{-infty\}^\{infty\}\; e^\{iky\_1\}e^\{-iky\_2\}dk$, already contained an infinite spread in momenta (tacit in the integral over k), so no further spread could be seen by localizing one particle. A. Sudbery:"Popper's variant of the EPR experiment does not test theCopenhagen interpretation", "Phil. Sci.":52:470-476:1985] A. Sudbery:"Testing interpretations of quantum mechanics", "Microphysical Reality and Quantum Formalism":470-476:1988] Although it pointed to a crucial flaw in Popper's argument, its full implication was not understood.

* Kripps theoretically analyzed Popper's experiment and predicted that narrowing slit A would lead to momentum spread increasing at slit B. Kripps also argued that his result was based just on the formalism of quantum mechanics, without any interpretational problem. Thus, if Popper was challenging anything, he was challenging the central formalism of quantum mechanics. cite journal

author = H. Krips

journal = Brit. J. Phil. Sci.

title = Popper, propensities, and the quantum theory

volume = 35

issue =

pages = 253–274

year = 1984

doi = 10.1093/bjps/35.3.253]* Redhead analyzed Popper's experiment with a broad source and concluded that it could not yield the effect that Popper was seeking. cite journal

author = M. Redhead

journal = Karl Popper: Philosophy and Problems, edited by A. O'Hear (Cambridge)

title = Popper and the quantum theory

volume =

issue =

pages = 163–176

year = 1996] "However, it has been demonstrated that if one uses a converging lens with a broad source, the kind of setup Popper was looking for, can be realized." Fact|date=December 2007**Realization of Popper's experiment**

thumb|200px|left|Fig.4 Results of the photon experiment by Kim and Shih, aimed at realizing Popper's proposal. The diffraction pattern in the absence of slit B (red symbols) is much narrower than that in the presence of areal slit (blue symbols).Popper's experiment was realized in 1999 by Kim and Shih using a SPDC photon source.cite journal

author = Y.-H. Kim and Y. Shih

journal = Found. Phys.

title = Experimental realization of Popper's experiment: violation of the uncertainty principle?

volume = 29

pages = 1849–1861

year = 1999

doi = 10.1023/A:1018890316979] Interestingly, they did not observe an extra spread in themomentum of particle 2 due to particle 1 passing through a narrow slit. Rather, the momentum spread of particle 2 (observed in coincidence with particle 1 passing through slit A) was narrower than its momentum spread in the initial state. This led to a renewed heated debate, with some even going to the extent of claiming that Kim and Shih's experiment had demonstrated that there is no non-locality in quantum mechanics. cite journal

author = C. S. Unnikrishnan

journal = Found. Phys. Lett.

title = Is the quantum mechanical description of physical reality complete? Proposed resolution of the EPR puzzle

volume = 15

pages = 1–25

year = 2002

doi = 10.1023/A:1015823125892]* Short criticized Kim and Shih's experiment, arguing that because of the finite size of the source, the localization of particle 2 is imperfect, which leads to a smaller momentum spread than expected. cite journal

author = A. J. Short

journal = Found. Phys. Lett.

title = Popper's experiment and conditional uncertainty relations

volume = 14

pages = 275–284

year = 2001

doi = 10.1023/A:1012238227977] "However, Short's argument implies that if the source were improved, we should see a spread in the momentum of particle 2."* Sancho carried out a theoretical analysis of Popper's experiment, using the path-integral approach, and found a smililar kind of narrowing in the momentum spread of particle 2, as was observed by Kim and Shih. cite journal

author = P. Sancho

journal = Found. Phys.

title = Popper’s Experiment Revisited

volume = 32

pages = 789–805

year = 2002

doi = 10.1023/A:1016009127074] Although this calculation did not give them any deep insight, it indicated that the experimental result of Kim-Shih agreed with quantum mechanics. It did not say anything about what bearing it has on theCopenhagen interpretation , if any.**What is wrong with Popper's proposal?**The fundamental flaw in Popper's argument can be seen from the following simple analysis. cite journal

author = T. Qureshi

journal = Am. J. Phys.

title = Understanding Popper's Experiment

volume = 53

pages = 541–544

year = 2005] cite journal

author = T. Qureshi

journal = arXiv:quant-ph/0505158

title = On the realization of Popper's Experiment

volume =

pages =

year = 2005]The ideal

EPR state is written as $|psi\; angle\; =\; int\_\{-infty\}^\{infty\}|y,y\; angle\; dy\; =\; int\_\{-infty\}^\{infty\}|p,-p\; angle\; dp$, where the two labels in the "ket" state represent the positions or momenta of the two particle. This implies perfect correlation, meaning, detecting particle 1 at position $x\_0$ will also lead to particle 2 being detected at $x\_0$. If particle 1 is measured to have a momentum $p\_0$, particle 2 will be detected to have a momentum $-p\_0$. The particles in this state have infinte momentum spread, and are infinitely delocalized. However, in real world, correlations are always imperfect. Consider the following entangled state$psi(y\_1,y\_2)\; =\; A!int\_\{-infty\}^infty\; dpe^\{-p^2/4sigma^2\}e^\{-ipy\_2/hbar\}\; e^\{i\; py\_1/hbar\}exp\; [-\{(y\_1+y\_2)^2over\; 16Omega^2\}]$

where $sigma$ represents a finite momentum spread, and $Omega$ is a measure of the position spread of the particles. The uncertainties in position and momentum, for the two particles can be written as

$Delta\; p\; \_\{2\}\; =\; Delta\; p\; \_\{1\}\; =\; sqrt\{sigma^2\; +\; \{hbar^2over\; 16Omega^2,~~~~\; Delta\; y\_1\; =\; Delta\; y\_2\; =\; sqrt\{Omega^2+hbar^2/16sigma^2\}$

The action of a narrow slit on particle 1 can be thought of as reducing it to a narrow Gaussian state: $phi\_1(y\_1)\; =\; frac\{1\}\{(epsilon^22pi)^\{1/4\}\; \}\; e^\{-y\_1^2/4epsilon^2\}$. This will reduce the state of particle 2 to $phi\_2(y\_2)\; =\; !int\_\{-infty\}^infty\; psi(y\_1,y\_2)\; phi\_1^*(y\_1)\; dy\_1$.The momentum uncertainty of particle 2 can now be calculated, and is given by

$Delta\; p\_\{2\}\; =\; sqrt\{frac\{sigma^2(1+epsilon^2/Omega^2)+\; hbar^2/16Omega^2\}\{1+4epsilon^2(sigma^2/hbar^2+1/16Omega^2)$

If we go to the extreme limit of slit A being infinitesimally narrow ($epsilon\; o\; 0$), the momentum uncertainty of particle 2 is $lim\_\{epsilon\; o\; 0\}\; Delta\; p\_\{2\}\; =\; sqrt\{sigma^2+\; hbar^2/16Omega^2\}$, which is exactly what the momentum spread was to begin with. In fact, one can show that the momentum spread of particle 2, conditioned on particle 1 going through slit A, is always lessthan or equal to $sqrt\{sigma^2\; +\; hbar^2/16Omega^2\}$ (the initial spread), for any value of $epsilon,\; sigma$, and $Omega$.

**Thus, particle 2 does not acquire any extra momentum spread than what it already had.**This is the prediction of standard quantum mechanics.**Thus, the basic premise of Popper's experiment, that the**Copenhagen interpretation implies that particle 2 will show an additional momentum spread, is incorrect.On the other hand, if slit A is gradually narrowed, the momentum spread of particle 2 (conditioned on the detection of particle 1 behind slit A) will show a gradual increase (never beyond the initial spread, of course). This is what quantum mechanics predicts. Popper had said

...if the Copenhagen interpretation is correct, then any increase in the precision in the measurement of our mere knowledge of the particles going through slit B should increase their scatter.

This clearly follows from quantum mechanics, without invoking theCopenhagen interpretation .**Popper's experiment and faster-than-light signalling**The expected additional momentum scatter which Popper wrongly attributed to the

Copenhagen interpretation can be interpreted as allowing faster-than-light communication, which is known to be impossible, even inquantum mechanics . Indeed some authors have criticized Popper's experiment based on this impossibility of superluminal communication in quantum mechanicscite journal

author = E. Gerjuoy, A.M. Sessler

journal = Am. J. Phys.

title = Popper's experiment and communication

volume = 74

pages = 643–648

year = 2006

doi = 10.1119/1.2190684 arxiv|quant-ph|0507121] cite journal

author = G. Ghirardi, L. Marinatto, F. de Stefano

journal =

title = A critical analysis of Popper's experiment

volume =

pages =

year = 2007 arxiv|quant-ph|0702242] . Every attempt to use quantum correlations for faster-than-light communication is known to be flawed because of theno cloning theorem in quantum mechanics. One will putatively try to signal 0 and 1 by narrowing the slit, or not narrowing it. However in order to investigate the scattering of each singlequbit , one needs to have many identical copies of it. Due to unitarity in quantum mechanics, if one tries to copy a qubit they will produce an entangled "pseudo-copy" that will collapse at the very moment the original qubit is measured. So the result of Popper's experiment cannot be used for faster-than-light communication.**References**

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