Popper's experiment realized again—but what does it mean?

January 27, 2015 by Lisa Zyga feature
Illustration of Popper’s experiment realized with randomly paired photons in a thermal state. In the second set-up, there is no “slit B” for the photon on the right. The new results show that this photon is not affected by a measurement on the left photon (which does travel through a slit), in agreement with Popper’s prediction. Credit: Tao Peng, et al. ©2015 EPLA

(Phys.org)—Like Einstein, the philosopher Karl Popper was a realist who was deeply bothered by some of the odd implications of quantum mechanics. Both Popper and Einstein disliked the idea in Heisenberg's uncertainty principle, for instance, that precisely measuring one property of a particle means that the particle's conjugate property is completely undetermined. This idea undermines the basic principle of common-sense realism: that every particle's properties must have precise pre-existing values, which do not depend on being measured.

Both Popper and Einstein proposed thought experiments critiquing the . But while Einstein, Podolsky, and Rosen's EPR experiment is quite famous, Popper's experiment is not as widely known.

Popper first published his proposed experiment in 1934, and in 1999, physicists Yoon-Ho Kim and Yanhua Shih realized Popper's experiment for the first time. In what came as a surprise to many, their results agreed with Popper's predictions, yet are not generally considered to be a true violation of the uncertainty principle, as Popper believed. The findings ignited a great deal of critique, both of Popper's original ideas and how they might be realized and interpreted.

Now in a new study published in EPL, Shih and coauthors at the University of Maryland in Baltimore and Oakland Community College in Waterford, Michigan, have again realized Popper's experiment using a different approach. Once again, their results agree with Popper's predictions, yet still do not violate the uncertainty principle. However, the researchers explain that the results do reveal a concern about nonlocal interference, as the observations suggest that a pair of particles is instantaneously interfering with itself, even across large distances.

Tale of two particles

The uncertainty principle places a fundamental limit on the precision with which conjugate pairs of physical properties of a particle, such as position and momentum, can be determined simultaneously. Mathematically, this means ΔpyΔyh, where p and y are the momentum and position of a particle, and h (Planck's constant) is the minimum amount of uncertainty required.

In his thought experiment, Popper considered what the uncertainty principle might mean for a pair of particles that are entangled, meaning the particles have both position-position correlation and momentum-momentum correlation with one another. The EPR thought experiment, published in 1935, similarly considered the case of an entangled pair of particles.

Theoretically, if the position or momentum of one entangled particle is measured, then this knowledge can be used to instantly determine the position or momentum of the second particle—and with exact certainty, since the particles' properties are inherently correlated.

Popper wondered what would happen if the position of one of the entangled particles were restricted within a narrow slit, Δy. Even if no slit were placed in the way of the second particle, the position of the second particle would also be restricted within the narrow range of Δy, due to the position-position correlation between the two particles. In this case, would the second particle be "diffracted" by a nonexistent "slit" in order to preserve ΔpyΔy ≥ h?

Popper believed no, that the diffraction of the second particle would happen only in the case of a real slit, and not by a nonexistent "slit." He predicted that, without the use of a real slit, the second particle should not be diffracted at all. However, this prediction means that the second particle's position has a precise value, so that ΔpyΔy < h. Popper considered this result a violation of the uncertainty principle, as it seems to show that the uncertainty is less than the minimum requirement.

Illustration of the new experimental set-up used to realize Popper’s experiment. One entangled photon travels through slit A, while the other travels a different path through slit B, which can be adjusted to be the same width as slit A or wide open. Credit: Tao Peng, et al. ©2015 EPLA

"Perhaps, deep in his mind, Popper could never accept the idea that the measurement of the position of particle one can simultaneously affect the position of particle two," Shih told Phys.org.

Ghost imaging experiments

Popper's experiment can be realized in a couple different ways. Kim and Shih's 1999 experiment was performed using entangled photon pairs generated by a spontaneous parametric down-conversion (SPDC) source, which is commonly used to generate entangled particles.

In that experiment, Kim and Shih first confirmed that the entangled photon pair can achieve a position-position correlation by means of a technique called ghost imaging. In ghost imaging, the two can "image" a real slit from the path of photon one onto the path of photon two at a distance, creating a so-called "ghost slit" for photon two. While photon one is restricted within the real slit, photon two must also be instantly restricted within the ghost slit.

Kim and Shih then compared their measured diffraction pattern from the ghost slit with the diffraction pattern of a real slit in place of the ghost slit. It was a surprise to everyone, including Kim and Shih, that their experiment agreed with Popper's prediction: the diffraction pattern from the ghost slit was much narrower than that of the real slit. Just as Popper had predicted, particle two was not affected by a measurement made on particle one.

In the new paper, Shih and his coauthors observed the same results, but instead of using , they used randomly paired photons in a thermal state. Using similar ghost imaging technology, the physicists confirmed that the randomly paired photons in the thermal state are able to produce a ghost slit at a distance. They then compared their measured diffraction pattern from the ghost slit with the from a real slit in place of the ghost slit. It was a surprise again to find that the new experimental results agree with Popper's prediction.

The important difference with the second experiment is that the randomly paired photons do not have any pre-prepared entanglement, which means they are considered to be a classical system. This raises the question, how could a classical system produce the same result as a quantum system? Shih is confident that they will soon find an answer.

Uncertainty preserved, nonlocality in question

Although it may seem like the above two experiments violate the uncertainty principle because the results show a smaller-than-required degree of uncertainty, Shih and his coauthors explain that no violation has occurred due to the fact that the experiments involve photon pairs rather than individual photons. The scientists argue that Popper's original thought experiment was based on a misunderstanding of the proper context of the uncertainty principle: it governs the behavior of single particles only, not the "correlation" of two particles.

In a sense, the entire thought experiment is flawed, at least in terms of what it means for the uncertainty principle. However, the researchers think that Popper's (and EPR's) concern about the nonlocal correlation of distant is still reasonable and fundamentally sound.

As the researchers explain, the results of Popper's experiment still reveal some important insight because the observations suggest that the photon pair is interfering with itself instantaneously in a phenomenon called nonlocal interference. In the new experiment, the randomly paired photons have two different yet indistinguishable probabilities to be simultaneously annihilated at two distant photodetectors. The observations are the result of the superposition of these two probability amplitudes. In this sense, the physicists explain that the same question that faced Popper and Einstein is still facing physicists today, which is, how long does it take for nonlocal interference to occur?

"Assuming the two random photons are separated by one light-year, how long does it take to force particle two to appear at a certain position after observing the first particle?" Shih said. "In the view of quantum mechanics—specifically, the theory of two-photon interference—a precise measurement of the position of a particle would instantly determine the position of its entangled twin at a distance. As a believer of realism and relativity, Popper had to ask these questions, similar to Einstein, Podolsky, and Rosen."

In addition to the analysis in this latest paper, there have been a wide variety of other criticisms of Popper's experiment over the past few decades. Although Popper's experiment may not have been the perfect way to test the uncertainty principle, it has nevertheless succeeded in opening new perspectives on a model of the world that may never be completely understood.

Explore further: Duality principle is 'safe and sound': Researchers clear up apparent violation of wave-particle duality

More information: Tao Peng, et al. "Popper's experiment with randomly paired photons in thermal state." EPL. DOI: 10.1209/0295-5075/109/14003

Yoon-Ho Kim and Yanhua Shih. "Experimental Realization of Popper's Experiment: Violation of the Uncertainty Principle?" Found. Phys., 29 (1999) 1849 http://link.springer.com/article/10.1023%2FA%3A1018890316979

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not rated yet Jan 27, 2015
So entanglement is an illusion then.....
Whydening Gyre
1 / 5 (1) Jan 27, 2015
Spin states.
Can they determine the ratio in diffraction tween the real and ghost?
GoogleGiggle
not rated yet Jan 27, 2015
Until we can understand what the mechanism and essence of the interaction or "connection" between particles is we will never be able to answer a question like this or what the universe is and how it works on the smallest - and largest levels. Physicists talk about "wave" and "forces" and "energy" but what does that really mean beyond a number?
Losik
Jan 27, 2015
This comment has been removed by a moderator.
tadchem
2 / 5 (4) Jan 27, 2015
Yet another brick in the tower of QM...
The wave-particle 'duality' is an artifact of the models which we seek to apply to a real-world object.
The truth is that a more mathematically consistent model would be more general and allow a quantity to be *interpreted* as either a 'wave-like' or a 'particle-like' depending on the selected coordinate system ('frame-of-reference' or 'point-of-view') and the matehmatical operators employed in the analysis of the model system. This also preserves the Principle of Rerlativity.
Look to tensor field calculus.
russell_russell
5 / 5 (4) Jan 27, 2015
Ideas evolve. New approaches emerge.
You are reminded of the search for loopholes in Bell's theorem - both versions.
And the research to test every potential loophole science conjures up and imagines.
The testing never stops. That is the science we trust.

arom
Jan 27, 2015
This comment has been removed by a moderator.
rufusgwarren
5 / 5 (2) Jan 27, 2015
common sense
ElectronSpinDensityPlots
not rated yet Jan 27, 2015
what about a "four-square" window (or, a 9 square window, with 3 by 3 slits);
then to differentiate predicted patterns to observe:
1. use a non-coherent beam or non-FEL beam and shoot through the four square window; we should expect to see, minimally, a bright spot pattern of 4 principal diffraction spots/4-spot pattern, with smaller bright scattered points arranged about the 4 principal bright spots (approximately in a square matrix pattern? not sure)
vs. 2. using a coherent beam or FEL beam to shoot through the four square window, we should expect to see a pattern of "opposing corners" bright, with the alternate corners dark; checker board pattern?
what about translating the beam either very very slightly to the left or very very slightly to the right while maintaing the laser beam perpendicular to the plane of a 2-slit experiment AND simultaneously NOT closing either of the two slits/different from Feynman? would we see appearance, disapperance, apperance,etc of spot pattern?
ElectronSpinDensityPlots
not rated yet Jan 27, 2015
...sorry, coherent beam to maintain incoming photons as in-phase, in which electric vector is either right, coupled with up magnetic vector or left, coupled with down magnetic vector, not paired with electric vector left, coupled with magnetic vector up & electric vector right paired with magnetic vector down; there could be some interesting experimentally observable phenomena with the four-square window and coherent vs. non-coherent beams...
Whydening Gyre
3.7 / 5 (3) Jan 27, 2015
common sense

@Rufus
I don't have much - but I know it when I see it...:-)
Whydening Gyre
1 / 5 (1) Jan 27, 2015
...sorry, coherent beam to maintain incoming photons as in-phase, in which electric vector is either right, coupled with up magnetic vector or left, coupled with down magnetic vector, not paired with electric vector left, coupled with magnetic vector up & electric vector right paired with magnetic vector down; there could be some interesting experimentally observable phenomena with the four-square window and coherent vs. non-coherent beams...

@Electron
Altho your input might seem somewhat disorganized and haphazard to some - you are on a right track... (at least one of them...:-)
Mimath224
5 / 5 (1) Jan 27, 2015
@Whydening Gyre genuine question; could the experiemental results involving entangled and non-entangled enitities be an extension of duality? That is, would one expect a similar result for particle-particle, wave-wave no matter if entangled or not? What I'm trying to get my thick skull around is that,say 2 free e ‾ , if some experiment produced different results in similar experiements would that not imply that e ‾ are in fact different. I thought all e ‾ were identical (apart from spin in a bound state)
Whydening Gyre
3.3 / 5 (3) Jan 28, 2015
@Whydening Gyre genuine question; could the experiemental results involving entangled and non-entangled enitities be an extension of duality? That is, would one expect a similar result for particle-particle, wave-wave no matter if entangled or not? What I'm trying to get my thick skull around is that,say 2 free e ‾ , if some experiment produced different results in similar experiements would that not imply that e ‾ are in fact different. I thought all e ‾ were identical (apart from spin in a bound state)

Good question.
In my mind, entanglement is just an illusion in that ALL like particles are "entangled" (Feynman"ish", I know) to some degree or another. The fact that they are "prepared" for observation sets the stage. Put the two particles in completely different scenarios and the won't appear to be anything at all.
Electrons are each unique. We just don't have the equipment and/or methodology to see such tiny differences -
(cont)
Whydening Gyre
3.3 / 5 (3) Jan 28, 2015
(cont)
Kinda like this hmmm, same charge out to 10 decimal points. Must be identical. What about at a million decimal points?

As to duality? I suppose you could say that a wave at a given magnification point could be viewed as a bigger particle.

And - if two electrons are both in a bound state they will exhibit the same characteristics, for the most part (like 99.99999999999999999999...). But I still think we haven't made it down to the precision level necessary to see any incredibly minute differences. So we just call it a quantum, calculate what we know of, and move on...:-)

Anyway.. just my mental meandering inside my OWN thick skull...
Prob'ly wrong...
I'm sure someone better informed (which isn't all that tuff to be) will come along and point out how so...:-)
Seeker2
not rated yet Jan 28, 2015
The universe is, I presume, in free fall. Therefore any action must be matched by some counter action. It's just a matter of finding the counter action. Notice this reaction must be instantaneous because the universe cannot be out of balance for any finite length of time - not subject to any uncertainty principle. For entangled particles we know the actual counter action. Newton was not slain by Heisenburg, apparently.
Doug_Huffman
not rated yet Jan 28, 2015
Good observation. ALL particles were entangled in the initial cosmological interaction.
Mimath224
not rated yet Jan 28, 2015
@Whydening Gyre, yes , I understand. According to another science/math blog everything, including us, is entangled but in varying degrees.
But it seems to me that on the question identity on the quantum scale it becomes increasingly more difficult to separate on entity from another.'Common sense', if such exist, suggests that there must be an ultimate entity yet the quantum scale often defies 'Common sense', so I wonder if such an entity is required at all. This would then inply what you post; that given the right conditions we might find that members of the same 'class' are indeed all different in some minute way.
robert_e_french
5 / 5 (1) Jan 29, 2015
This may not be very politically correct, but why not take these results as a refutation of Heisenberg's original principle, at least if this is taken in terms of limitations on knowledge, which I believe is the way in which both Popper and Heisenberg originally interpreted the principle.
Losik
Jan 29, 2015
This comment has been removed by a moderator.
EnsignFlandry
not rated yet Jan 30, 2015
Quantum mechanics must be modified if we are to have a theory of quantum gravity.
Losik
Jan 30, 2015
This comment has been removed by a moderator.
Whydening Gyre
1 / 5 (1) Jan 30, 2015
@Whydening Gyre, yes , I understand. According to another science/math blog everything, including us, is entangled but in varying degrees.
But it seems to me that on the question identity on the quantum scale it becomes increasingly more difficult to separate on entity from another.

They're just getting closer.
'Common sense', if such exist, suggests that there must be an ultimate entity yet the quantum scale often defies 'Common sense', so I wonder if such an entity is required at all.

Common Sense says...
Hmm. You mean a beginning and an end... in a circle there is neither...

This would then inply what you post; that given the right conditions we might find that members of the same 'class' are indeed all different in some minute way.

As long as the universe keeps "adding", correct...
Ain't it great...? :-)
I do worry, tho, about the "if and when" point we find the end of Pi. :-)
(That would spell the end point of this Universe and the start of...)
TiagoTiago
not rated yet Jan 31, 2015
Just to confirm; when they remove the real slit, the ghost slit also disappears?

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