The Mystery of Quantum Physics (Part 2): Spooky Action at a Distance

No matter how far apart two entangled photons are - even light years - they can affect each other instantly.

Last month we started our expedition into the weird world of quantum physics. A place where things do not exist unless you look at them, where cats can be both dead and alive at the same time. This month we will focus on how some interpretations of quantum physics suggest that everything in the universe is instantly connected with everything else, no matter how distant apart they are. (Note, it is strongly recommended that you read part one before proceeding to part two).

The year 1927 saw the start of a series of debates between two of the most knowledgeable scientists in the world at the time: Einstein, the author of the theory of general relativity and Neils Bohr, one of the early researchers in quantum theory. The first clash took place at Fifth Solvay International Conference on Electrons and Photons held in Brussels, Belgium. The attendees at this conference were few, but distinguished. Of the 29 scientists there 17 had, or would in the future, win a Nobel Prize. Marie Curie won two.

Einstein, despite being one of the founders of quantum theory, was uncomfortable with it. One of Einstein's greatest strengths as a scientist was his ability to conduct thought experiments - experiments that might be impossible to do in real life, but when done in the imagination yielded clues to the nature of physics. (One of Einstein's most productive thought experiments was to consider what the world would look like if he could ride his bike at the speed of light). Attempting these kinds of thought experiments to get at the true nature of quantum theory, however, was frustrating. The results seemed nonsensical. Objects didn't appear to be there unless you looked at them. Cats turned out to be both dead and alive at the same time. If you knew where a particle (such as an electron) was exactly, you couldn't tell anything about how it was moving.

Bohr didn't have a problem with this, however. He seemed unconcerned with the great conundrums that the theory raised, only in what the results that the equations gave. As physicist David Mermin once put it, Neil Bohr's attitude, as expressed in his famous Copenhagen Interpretation of quantum physics, was just 'Shut up and calculate!'

The attendees of the Fifth Solvay International Conference on Electrons and Photons. Einstein is seated in the first row towards the center.

A typical Einstein/Bohr encounter would start with Einstein coming up with an example to show that quantum theory was wrong or incomplete. Bohr would then spend the next evening pondering the problem and reply the following day with something to discredit Einstein's criticism. The debate came to a head in 1935 when Einstein, along with Boris Podolsky and Nathan Rosen, published a paper that described what would become known as the EPR paradox (for Einstein-Podolsky-Rosen paradox).

Spooky Action at a Distance

Einstein's thought experiment in the paper consisted of taking a particle (for our example let's use a pion) and letting it decay into two photons (light particles) that race off in different directions. Because they came from the same pion, the photons are "entangled." This means that they share a common wave function. The two photons would also have some complementary characteristics. For example, their spin: The pion had no spin to begin with, so if one photon is observed to have an up spin on its x-axis, the other, to make things equal, must have an opposite down spin on its x-axis.

According to quantum theory, however, the property does not exist until it is measured. So when you measure the first photon and find it has an up spin, that forces the other photon to instantly assume a down spin even if the second photon might be a light year away from the first. Einstein and his co-authors argued that this didn't make sense. Either the photons had carried the spin information with them when they first split up, or the first photon, when it was measured, must have communicated its spin to the second one instantly across an enormous difference at faster than the speed of light. Einstein referred to this effect as "spooky action at a distance."

Since information cannot travel faster than the speed of light, this caused Einstein to argue that the photons must have "hidden variables" that carried the spin information since the pair of photons were created. Since quantum theory didn't allow for these variables, the theory must be incomplete.

Bell and his Theorem

Einstein's "spooky action at a distance" challenge remained unresolved past both his and Bohr's deaths in 1955 and 1962 respectively. In 1964 an Irish physicist named John Bell published a paper entitled On the Problem of Hidden Variables in Quantum Mechanics. Bell initially supported Einstein and thought that there must be hidden variables. In his paper he suggested a test to see if hidden variables could account for what was being seen.

Bohr (left) and Einstein's (right) argument over the ERP paradox wasn't settled until Bell came up with this theorem and Clauer did the experiment showing that Bohr was right.

Bell's test consisted of creating a pair of entangled particles and sending them off to two people (traditionally known as Alice and Bob) who then test the particles for a complementary property like spin. The specifics are hard to understand, but Bell was able to show that over a large number of trials, the number of times that Alice and Bob's results agree should be different if the properties were pre-existing than if they were being created when the first one was measured as quantum theory says they are. Bell thought that after he published his theorem (often referred to as "Bell's Inequity" because of one of its predictions) it would be many years before anybody could set a up test to actually try it. Only a year later, however, an enterprising graduate student at Columbia University, John Clauser, was able to run a crude version of the experiment and showed that the photons obeyed what was predicted by quantum physics, not what would be expected from a hidden variables theory. Another scientist, Alan Aspect, later duplicated Clauser's experiments with greater accuracy proving that despite Einstein's qualms, there was indeed "spooky action at a distance" in the quantum world.

Bell's work opened up what was largely thought to be a philosophical issue to experimentation. His contribution was so great that Henry Stapp of the Lawrence Berkeley National Laboratory in California was later to call Bell's work on quantum theory "the most profound discovery of science".

Bohm's Interpretation

Despite his work proving quantum theory's accuracy, Bell was not a fan of the standard Copenhagen Interpretation with its dependency on observation to collapse the wave function and make a particle (or for that matter a cat) real. Bell found an interpretation created by physicist David Bohm taht made more sense to him. To understand Bohm's interpretation, it is helpful to go back to our example in part one of looking at the star Alnilam in the constellation Orion. In our discussion of the Copenhagen interpretation we saw that a photon, a particle of light, doesn't really leave Alnilam. Instead, a wave of probability makes the trip to our eyes. In Bohm's interpretation there is indeed a real photon that leaves the star guided by a "quantum potential" force that moves backwards in time like a beacon to bring the particle to us. Everything in the universe is connected in Bohm's interpretation. There is no need for a wave function to collapse on observation as needed in the Copenhagen interpretation. However, this interpretation isn't without its problems. While it is deterministic, which means that with enough information you can absolutely predict everything that will happen in the universe from the very beginning, it requires information to travel backwards in time and instantly across immense distances. For these reasons the Bohm interpretation has not had a wide following among scientists.

The "Many Worlds" Interpretation

In the Many Worlds interpretation the universe would split and Schrödinger's cat would be dead in one and alive in the other. (Credit Dc987 and Wikipedia Commons)

Perhaps the most noteworthy alternative to the standard Copenhagen interpretation among physicists that study quantum theory is entitled the "Many Worlds" interpretation. Such notable scientists as Stephen Hawking and Richard Feynman have supported the interpretation and it seems to be getting more and more popular. The interpretation is the work of Hugh Everett III a graduate student at Princeton at the time he came up with his idea which he originally called the "relative state" formulation.

Everett's idea is simply that the wave function never collapses. This puts the Schrödinger's cat thought experiment on steroids. Not only is the cat in both the alive and dead state at the same time, the scientist conducting the experiment would also be split into two with one seeing a dead cat and the other a live one. This splitting just doesn't pertain to the one "cat" experiment, however, but to every possible outcome of a quantum event for every particle in the universe. According to this interpretation, the universe is continually splitting into infinitely different versions every moment like a giant branching tree. There are parallel universes which differ only slightly from ours and others that vary wildly.

In fact, if you follow this interpretation out to its logical conclusion, everything that is possible, no matter how improbable, does exist in some version of the universe. In one you are the President of the United States. In another, you are in prison for mass murder. As strange as this seems, the idea that everything exists, however, has been cited as one of the interpretations strongest supporting ideas. Cosmologist Max Tegmark, who has created a hierarchy of multiple universe levels based on this idea, argues that it is much simpler to describe a set of universes (sometimes called a multiverse) where everything exists, rather than a single universe with particular rules. "A common feature of all four multiverse levels is that the simplest and arguably most elegant theory involves parallel universes by default. To deny the existence of those universes, one needs to complicate the theory by adding experimentally unsupported processes and ad hoc postulates: finite space, wave function collapse and ontological asymmetry. Our judgment therefore comes down to which we find more wasteful and inelegant: many worlds or many words."

Max Tegmark is a cosmologist that has come up with a hierarchy of muliple universes.

The many worlds interpretation also solves one of the most difficult philosophical questions for anybody contemplating building a time machine. If there is only one universe using a time machine, to go back in time and kill your grandfather would create a paradox. This isn't true with a many worlds multiverse. Killing your grandfather would simply spawn an alternate history in which you never appear. On another branch of history, your grandfather would have lived and you would be born. If you returned to your original branch your grandfather would still be alive. If you remained in the alternate history where you killed your grandfather you would be an alien entity with no past.

There are other interpretations of quantum physics other than Copenhagen, Bohm and Many Worlds. However, all of them seem to have some form of "weirdness" attached to them. At this point physicists are still arguing about which interpretation, if any of them, are correct. The solution will probably have to wait till some clever physicists comes up with an experiment to prove or disprove some of these interpretations.

For our conclusion, however, let's move back from ideas that cannot yet be tested to an experiment that has been duplicated already in several laboratories. It's one of the most beautiful experiments ever created: The delayed choice quantum eraser.

The Delayed Choice Quantum Eraser Experiment

In the original double slit experiment we noted if you obtained the "which way" information that told you which slit the photon passed through the interference pattern disappeared because now the photon starting acting like a particle instead of a wave. The delayed choice quantum eraser experiment, originally done by Yoon-Ho Kim, R. Yu, S.P. Kulik, Y.H. Shih, and Marlan O. Scully is set up like the double slit we saw in part one. However, just behind the slits (let's label the slits A and B) is positioned a beta barium borate crystal (BBO). This crystal, when struck by a photon, can release two lower energy photons that are entangled (sharing a single wave function). One photon (called the signal photon) is allowed to go to a detector (let's refer to this as D0) where its location can be plotted. Both the paths from slit A and slit B hit the detector D0 so as more and more photons arrive this information can be used to see if the photon is, or is not, part of an interference pattern, just like in the original two slit experiment.

The entangled partner of the signal photon (called the idle photon) heads off in a different direction. Like the signal photon it can travel two different paths (A and B), one from each slit. The experiment is designed so that paths of the idle photon are much longer than the paths of the signal photon. The means that by the time the idle photon encounters any optical equipment the signal photon has already hit and been registered by the D0 detector. The idle photon first reaches two beam splitters (We will call them BSA and BSB, one for each path). A beam splitter is an optical device that has a 50% chance of letting the photon go through it like a piece of glass and a 50% chance of reflecting it like a mirror. If the photon reflected by the beam splitter it encounters one of two detectors depending on whether it was following path A or B (We will call these detectors D3 and D4). If the photons pass through the beam splitters they instead hit mirrors that bounce them to a single final beam splitter. Here the photons from one path can go through to hit detector D1 or reflect to hit detector D2. Photons traveling the other path can do just the opposite going through to D2 or reflecting to D1.

The set up for the Delayed Choice Quantum Eraser experiment. Items marked BS are beam splitters, M are mirrors and D are detectors. (Credit Patrick Edwin Moran and Wikapedia Commons)

The result of this arrangement is that if a photon is reflected by the first beam splitter it will be detected at either D3 or D4 and the "which way" information about which slit it and its entagled partner went through can be obtained. If the photon passes through the first beam splitters, however, the final beam splitter mixes the paths up so we know that a photon has arrived at D1 or D2, but we have no idea which path it and its partner traveled through the two slit device.

All the detectors are hooked up to a device known as a "coincidence counter" which matches the arrival and position of a signal photon at D0 with a idle photon at D1, D2, D3, D4. The coincidence counter is needed to keep any spurious photons from affecting the experiment. Since not every photon that hits the BBO creates an entangled pair, there can be quite a few unentangled photons that need to be kept out of the final data.

When the data from this experiment is plotted some very interesting things can be seen. If an idle photon hits D3 or D4 so we can tell which slit it and its partner took and we can see that it does not participate in an interference pattern. If the signal photons from D0 are matched with their partners from D1 and plotted we do see a clear interference pattern because we don't know which slit the photon went through. We see the same thing when we plot D0 and D2 because we again have no idea of which slit the photon came through (though in this second case the interference pattern has been shifted a bit over from the D0/D1 pattern because of the optical properties of the last beam splitter).

Backwards in Time?

What is amazing about this experiment is that neither signal photons or the idle photons do anything much different as far as physical interaction with the lab equipment in either case of an interference pattern being formed or not. Going back to the regular double slit experiment it could be argued that the detector watching the photon going through the slit somehow physically interacted with it causing it to change from a wave to a particle. Here the decision of the whether a signal photon will interfere with itself is made based solely whether the information about which slit it went through is still available after the idle photon hits its detector. Since the idle photon hits its detector significantly after the signal photon has arrived at its detector, it would appear that the information is being "erased" and something is going back in time and changing history. Is that really what is happening?

From a strict Copenhagen interpretation point of view, probably not. The signal photon is in superposition when it arrives at the D0 detector. It has both participated in the interference pattern and not participated in the interference pattern. When it hits the detector it entangles its wave function with that of D0 also putting the detector into superposition. The wave function only collapses into one or the other situations when the idle photon arrives at its detector and the observation is complete. Of course what, or who is needed to complete the observation (man or machine) is still a matter for debate, though it would seem that the D0 detector by itself does not qualify as an observer.

Physicist Chad Orzel re-enacts the Bohr-Einstein debates with dog puppets.

For those people who believe in the Many Worlds interpretation the superposition never collapses and the universe, including the scientists doing the experiment is split four ways, one for each possible detector that the idle photon can arrive at. It would appear to the each of scientists in their own worlds as if the arrival of the idle its detector had changed the past, but in reality it would just be an effect caused by the splitting of the universe four ways.

So this is a small peek at quantum weirdness. At this point in time scientists are very sure what quantum theory works, but still argue about what interpretation we should embrace. Do our observations create the universe around us, or are we just tiny specs in a multiverse than contains every possible history? Or is there some other possibility? Nobody knows for sure.

For many years physicists avoided these questions and the weirdness of quantum physics was, as scientist and writer J.M. Jauch put it "a kind of skeleton in the closet." Recently there has been increasing interest in this area and research into the meaning of quantum physics is starting to become respectable. Hopefully this will lead to more answers to what seems to be the universe's most puzzling questions.

Back to Part 1

Copyright Lee Krystek 2010. All Rights Reserved.


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