Proving Einstein Wrong with ‘Spooky’ Quantum Experiment
by Jesse Emspak, Live Science Contributor | March 26, 2015
Credit: agsandrew | Shutterstock
Quantum mechanics is one of the best-tested theories in science, and it’s one of the few where physicists get to do experiments proving that Einstein was wrong.
That’s what a team at Griffith University and the University of Tokyo in Japan did this week, showing that a weird phenomenon — in which the measurement of a particle actually affects its location — is real.
Back in the 1920s and 1930s, Albert Einstein said he couldn’t support this idea, which he called “spooky action at a distance,” in which a particle can be in two places at once and it’s not until one measures the state of that particle that it takes a definite position, seemingly with no signal transmitted to it and at a speed faster than light. When the particle takes its definite position, physicists refer to this as its wave function collapsing.
Quantum mechanics is one of the best-tested theories in science, and it’s one of the few where physicists get to do experiments proving that Einstein was wrong.
That’s what a team at Griffith University and the University of Tokyo in Japan did this week, showing that a weird phenomenon — in which the measurement of a particle actually affects its location — is real.
Back in the 1920s and 1930s, Albert Einstein said he couldn’t support this idea, which he called “spooky action at a distance,” in which a particle can be in two places at once and it’s not until one measures the state of that particle that it takes a definite position, seemingly with no signal transmitted to it and at a speed faster than light. When the particle takes its definite position, physicists refer to this as its wave function collapsing.
The phenomenon was outside of contemporary experience in physics and seemed to violate the theory of relativity, which posits that the speed of light is an absolute limit on how fast any information can travel. Einstein proposed that theparticle isn’t in a superposition state, or two places at once; but rather it always has a “true” location, and people just couldn’t see it. [How Quantum Entanglement Works (Infographic)]
Using a single photon (particle of light), the Australian and Japanese researchers ran an experiment showing that measuring a property of a quantum particle in one place will affect what one sees in another place. That is, they showed that superposition and collapsing wave function are real phenomena.
Alice and Bob
The phenomenon is demonstrated with a thought experiment in which a light beam is split, with one half going to Alice and the other to Bob. Alice then indicates if she detected a photon and if so what state it is in — it might be the phase of the wave packet that describes the photon. Mathematically, though, the photon is in a state of “superposition,” meaning it is in two (or more) places at once. Its wave function, a mathematical formula that describes the particle, seems to show the photon has no definite position.
“Alice’s measurement collapses the superposition,” meaning the photons are in one place or another, but not both, Howard Wiseman, director of Griffith University’s Centre for Quantum Dynamics, who led the experiment, told Live Science. If Alice sees a photon, that means the quantum state of the light particle in Bob’s lab collapses to a so-called zero-photon state, meaning no photon. But if she doesn’t see a photon, Bob’s particle collapses to a one-photon state, he said.
“Does this seem reasonable to you? I hope not, because Einstein certainly didn’t think it was reasonable. He thought it was crazy,” he added, referring to the fact that Alice’s measurement looked like it was dictating Bob’s.
The paradox was partially resolved years later, when experiments showed that even though the interaction between two quantum particles happens faster than light (it appears instantaneous), there is no way to use that phenomenon to send information, so there’s no possibility of faster-than-light signals. [10 Implications of Faster-Than-Light Travel]
Splitting photons
The team at Griffith, though, wanted to go a step further and show that the collapsing wave function — the process of Alice “choosing” a measurement and affecting Bob’s detection — is actually happening. And while other experiments have shown entanglement with two particles, the new study entangles a photon with itself.
To do this they fired a beam of photons at a splitter, so half of the light was transmitted and half was reflected. The transmitted light went to one lab and the reflected light went to the other. (These were “Alice” and “Bob” of the thought experiment.)
The light was transmitted as a single photon at a time, so the photon was split in two. Before the photon was measured, it existed in a superposition state.
One lab (Alice) used a laser as a reference, to measure the phase of the photon. If one thinks of light as a repeating sine wave, phase is the angle one is measuring, from 0 to 180 degrees. When Alice changed the angle of her reference laser, she got varying measurements of the photon: Either her photon was in a certain phase or it wasn’t present at all.
Then the other lab (or Bob) looked at their photons and found the photons were anti-correlated with Alice — if she saw a photon he did not, and vice versa. The state of Bob’s photon depended on what Alice measured. But in classic physics that shouldn’t happen; rather, the two particles should be independent of one another.
Quantum computing
Akira Furusawa, professor of applied physics at the University of Tokyo and one of the co-authors on the study, said the experiment helps explore different kinds of quantum information processing — and with it, communications and computing.
“Usually there are two types of quantum information processing,” he said. “There’s the qubit type, the digital information processing, and there’s continuous variable, a sort of analog type of quantum information. We are trying to combine them.” Conventional processing often relies on counting photons, but this kind of measurement of single photons is more efficient, he said.
Wiseman said one application is in the security of communications.
“Our experiment is a more rigorous test of the properties of such states than has ever been done before, in the sense that we don’t have to trust anything that is happening in Alice’s laboratory. This could be useful for communicating secrets when not all the parties are trusted.”
The experiment is described in the March 24 issue of the journal Nature Communications.
Do parallel worlds ever cross paths? (Photo: Martin Brigden/flickr)
Quantum mechanics, though firmly tested, is so weird and anti-intuitive that famed physicist Richard Feynman once remarked, “I think I can safely say that nobody understands quantum mechanics.” Attempts to explain some of the bizarre consequences of quantum theory have led to some mind-bending ideas, such as the Copenhagen interpretation and the many-worlds interpretation.
Now there’s a new theory on the block, called the “many interacting worlds” hypothesis (MIW), and the idea is just as profound as it sounds. The theory suggests not only that parallel worlds exist, but that they interact with our world on the quantum level and are thus detectable. Though still speculative, the theory may help to finally explain some of the bizarre consequences inherent in quantum mechanics, reports RT.com.
The theory is a spin-off of the many-worlds interpretation in quantum mechanics — an idea that posits that all possible alternative histories and futures are real, each representing an actual, though parallel, world. One problem with the many-worlds interpretation, however, has been that it is fundamentally untestable, since observations can only be made in our world. Happenings in these proposed “parallel” worlds can thus only be imagined.
MIW, however, says otherwise. It suggests that parallel worlds can interact on the quantum level, and in fact that they do.
“The idea of parallel universes in quantum mechanics has been around since 1957,” explained Howard Wiseman, a physicist at Griffith University in Brisbane, Australia, and one of the physicists to come up with MIW. “In the well-known ‘Many-Worlds Interpretation’, each universe branches into a bunch of new universes every time a quantum measurement is made. All possibilities are therefore realised – in some universes the dinosaur-killing asteroid missed Earth. In others, Australia was colonised by the Portuguese.”
“But critics question the reality of these other universes, since they do not influence our universe at all,” he added. “On this score, our “Many Interacting Worlds” approach is completely different, as its name implies.”
Wiseman and colleagues have proposed that there exists “a universal force of repulsion between ‘nearby’ (i.e. similar) worlds, which tends to make them more dissimilar.” Quantum effects can be explained by factoring in this force, they propose.
Whether or not the math holds true will be the ultimate test for this theory. Does it or does it not properly predict quantum effects mathematically? But the theory is certain to provide plenty of fodder for the imagination.
For instance, when asked about whether their theory might entail the possibility that humans could someday interact with other worlds, Wiseman said: “It’s not part of our theory. But the idea of [human] interactions with other universes is no longer pure fantasy.”
What might your life look like if you made different choices? Maybe one day you’ll be able to look into one of these alternative worlds and find out.
Time is an emergent phenomenon that is a side effect of quantum entanglement, say physicists. And they have the first experimental results to prove it
When the new ideas of quantum mechanics spread through science like wildfire in the first half of the 20th century, one of the first things physicists did was to apply them to gravity and general relativity. The results were not pretty.
It immediately became clear that these two foundations of modern physics were entirely incompatible. When physicists attempted to meld the approaches, the resulting equations were bedeviled with infinities making it impossible to make sense of the results.
Then in the mid-1960s, there was a breakthrough. The physicists John Wheeler and Bryce DeWitt successfully combined the previously incompatible ideas in a key result that has since become known as the Wheeler-DeWitt equation. This is important because it avoids the troublesome infinites—a huge advance.
But it didn’t take physicists long to realise that while the Wheeler-DeWitt equation solved one significant problem, it introduced another. The new problem was that time played no role in this equation. In effect, it says that nothing ever happens in the universe, a prediction that is clearly at odds with the observational evidence.
This conundrum, which physicists call ‘the problem of time’, has proved to be a thorn in flesh of modern physicists, who have tried to ignore it but with little success.
Then in 1983, the theorists Don Page and William Wootters came up with a novel solution based on the quantum phenomenon of entanglement. This is the exotic property in which two quantum particles share the same existence, even though they are physically separated.
Entanglement is a deep and powerful link and Page and Wootters showed how it can be used to measure time. Their idea was that the way a pair of entangled particles evolve is a kind of clock that can be used to measure change.
But the results depend on how the observation is made. One way to do this is to compare the change in the entangled particles with an external clock that is entirely independent of the universe. This is equivalent to god-like observer outside the universe measuring the evolution of the particles using an external clock.
In this case, Page and Wootters showed that the particles would appear entirely unchanging—that time would not exist in this scenario.
But there is another way to do it that gives a different result. This is for an observer inside the universe to compare the evolution of the particles with the rest of the universe. In this case, the internal observer would see a change and this difference in the evolution of entangled particles compared with everything else is an important a measure of time.
This is an elegant and powerful idea. It suggests that time is an emergent phenomenon that comes about because of the nature of entanglement. And it exists only for observers inside the universe. Any god-like observer outside sees a static, unchanging universe, just as the Wheeler-DeWitt equations predict.
Of course, without experimental verification, Page and Wootter’s ideas are little more than a philosophical curiosity. And since it is never possible to have an observer outside the universe, there seemed little chance of ever testing the idea.
Until now. Today, Ekaterina Moreva at the Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, Italy, and a few pals have performed the first experimental test of Page and Wootters’ ideas. And they confirm that time is indeed an emergent phenomenon for ‘internal’ observers but absent for external ones.
The experiment involves the creation of a toy universe consisting of a pair of entangled photons and an observer that can measure their state in one of two ways. In the first, the observer measures the evolution of the system by becoming entangled with it. In the second, a god-like observer measures the evolution against an external clock which is entirely independent of the toy universe.
The experimental details are straightforward. The entangled photons each have a polarization which can be changed by passing it through a birefringent plate. In the first set up, the observer measures the polarization of one photon, thereby becoming entangled with it. He or she then compares this with the polarization of the second photon. The difference is a measure of time.
In the second set up, the photons again both pass through the birefringent plates which change their polarizations. However, in this case, the observer only measures the global properties of both photons by comparing them against an independent clock.
In this case, the observer cannot detect any difference between the photons without becoming entangled with one or the other. And if there is no difference, the system appears static. In other words, time does not emerge.
“Although extremely simple, our model captures the two, seemingly contradictory, properties of the Page-Wootters mechanism,” say Moreva and co.
That’s an impressive experiment. Emergence is a popular idea in science. In particular, physicists have recently become excited about the idea that gravity is an emergent phenomenon. So it’s a relatively small step to think that time may emerge in a similar way.
What emergent gravity has lacked, of course, is an experimental demonstration that shows how it works in practice. That’s why Moreva and co’s work is significant. It places an abstract and exotic idea on firm experimental footing for the first time.
Perhaps most significant of all is the implication that quantum mechanics and general relativity are not so incompatible after all. When viewed through the lens of entanglement, the famous ‘problem of time’ just melts away.
The next step will be to extend the idea further, particularly to the macroscopic scale. It’s one thing to show how time emerges for photons, it’s quite another to show how it emerges for larger things such as humans and train timetables.
Coffee cools, buildings crumble, eggs break and stars fizzle out in a universe that seems destined to degrade into a state of uniform drabness known as thermal equilibrium. The astronomer-philosopher Sir Arthur Eddington in 1927 cited the gradual dispersal of energy as evidence of an irreversible “arrow of time.”
But to the bafflement of generations of physicists, the arrow of time does not seem to follow from the underlying laws of physics, which work the same going forward in time as in reverse. By those laws, it seemed that if someone knew the paths of all the particles in the universe and flipped them around, energy would accumulate rather than disperse: Tepid coffee would spontaneously heat up, buildings would rise from their rubble and sunlight would slink back into the sun.
Original story reprinted with permission from Simons Science News, an editorially independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.
“In classical physics, we were struggling,” said Sandu Popescu, a professor of physics at the University of Bristol in the United Kingdom. “If I knew more, could I reverse the event, put together all the molecules of the egg that broke? Why am I relevant?”
Surely, he said, time’s arrow is not steered by human ignorance. And yet, since the birth of thermodynamics in the 1850s, the only known approach for calculating the spread of energy was to formulate statistical distributions of the unknown trajectories of particles, and show that, over time, the ignorance smeared things out.
Now, physicists are unmasking a more fundamental source for the arrow of time: Energy disperses and objects equilibrate, they say, because of the way elementary particles become intertwined when they interact — a strange effect called “quantum entanglement.”
“Finally, we can understand why a cup of coffee equilibrates in a room,” said Tony Short, a quantum physicist at Bristol. “Entanglement builds up between the state of the coffee cup and the state of the room.”
Popescu, Short and their colleagues Noah Linden and Andreas Winterreported the discovery in the journal Physical Review E in 2009, arguing that objects reach equilibrium, or a state of uniform energy distribution, within an infinite amount of time by becoming quantum mechanically entangled with their surroundings. Similar results by Peter Reimann of the University of Bielefeld in Germany appeared several months earlier in Physical Review Letters. Short and a collaborator strengthened the argument in 2012 by showing that entanglement causes equilibration within a finite time. And, in work that was posted on the scientific preprint site arXiv.org in February, two separate groups have taken the nextstep, calculating that most physical systems equilibrate rapidly, on time scales proportional to their size. “To show that it’s relevant to our actual physical world, the processes have to be happening on reasonable time scales,” Short said.
The tendency of coffee — and everything else — to reach equilibrium is “very intuitive,” said Nicolas Brunner, a quantum physicist at the University of Geneva. “But when it comes to explaining why it happens, this is the first time it has been derived on firm grounds by considering a microscopic theory.”If the new line of research is correct, then the story of time’s arrow begins with the quantum mechanical idea that, deep down, nature is inherently uncertain. An elementary particle lacks definite physical properties and is defined only by probabilities of being in various states. For example, at a particular moment, a particle might have a 50 percent chance of spinning clockwise and a 50 percent chance of spinning counterclockwise. An experimentally tested theorem by the Northern Irish physicist John Bell says there is no “true” state of the particle; the probabilities are the only reality that can be ascribed to it.
Quantum uncertainty then gives rise to entanglement, the putative source of the arrow of time.
When two particles interact, they can no longer even be described by their own, independently evolving probabilities, called “pure states.” Instead, they become entangled components of a more complicated probability distribution that describes both particles together. It might dictate, for example, that the particles spin in opposite directions. The system as a whole is in a pure state, but the state of each individual particle is “mixed” with that of its acquaintance. The two could travel light-years apart, and the spin of each would remain correlated with that of the other, a feature Albert Einstein famously described as “spooky action at a distance.”
“Entanglement is in some sense the essence of quantum mechanics,” or the laws governing interactions on the subatomic scale, Brunner said. The phenomenon underlies quantum computing, quantum cryptography and quantum teleportation.
The idea that entanglement might explain the arrow of time first occurred to Seth Lloyd about 30 years ago, when he was a 23-year-old philosophy graduate student at Cambridge University with a Harvard physics degree. Lloyd realized that quantum uncertainty, and the way it spreads as particles become increasingly entangled, could replace human uncertainty in the old classical proofs as the true source of the arrow of time.
Using an obscure approach to quantum mechanics that treated units of information as its basic building blocks, Lloyd spent several years studying the evolution of particles in terms of shuffling 1s and 0s. He found that as the particles became increasingly entangled with one another, the information that originally described them (a “1” for clockwise spin and a “0” for counterclockwise, for example) would shift to describe the system of entangled particles as a whole. It was as though the particles gradually lost their individual autonomy and became pawns of the collective state. Eventually, the correlations contained all the information, and the individual particles contained none. At that point, Lloyd discovered, particles arrived at a state of equilibrium, and their states stopped changing, like coffee that has cooled to room temperature.
“What’s really going on is things are becoming more correlated with each other,” Lloyd recalls realizing. “The arrow of time is an arrow of increasing correlations.”
The idea, presented in his 1988 doctoral thesis, fell on deaf ears. When he submitted it to a journal, he was told that there was “no physics in this paper.” Quantum information theory “was profoundly unpopular” at the time, Lloyd said, and questions about time’s arrow “were for crackpots and Nobel laureates who have gone soft in the head.” he remembers one physicist telling him.
“I was darn close to driving a taxicab,” Lloyd said.
Advances in quantum computing have since turned quantum information theory into one of the most active branches of physics. Lloyd is now a professor at the Massachusetts Institute of Technology, recognized as one of the founders of the discipline, and his overlooked idea has resurfaced in a stronger form in the hands of the Bristol physicists. The newer proofs are more general, researchers say, and hold for virtually any quantum system.
“When Lloyd proposed the idea in his thesis, the world was not ready,” said Renato Renner, head of the Institute for Theoretical Physics at ETH Zurich. “No one understood it. Sometimes you have to have the idea at the right time.”
In 2009, the Bristol group’s proof resonated with quantum information theorists, opening up new uses for their techniques. It showed that as objects interact with their surroundings — as the particles in a cup of coffee collide with the air, for example — information about their properties “leaks out and becomes smeared over the entire environment,” Popescu explained. This local information loss causes the state of the coffee to stagnate even as the pure state of the entire room continues to evolve. Except for rare, random fluctuations, he said, “its state stops changing in time.”
Consequently, a tepid cup of coffee does not spontaneously warm up. In principle, as the pure state of the room evolves, the coffee could suddenly become unmixed from the air and enter a pure state of its own. But there are so many more mixed states than pure states available to the coffee that this practically never happens — one would have to outlive the universe to witness it. This statistical unlikelihood gives time’s arrow the appearance of irreversibility. “Essentially entanglement opens a very large space for you,” Popescu said. “It’s like you are at the park and you start next to the gate, far from equilibrium. Then you enter and you have this enormous place and you get lost in it. And you never come back to the gate.”
In the new story of the arrow of time, it is the loss of information through quantum entanglement, rather than a subjective lack of human knowledge, that drives a cup of coffee into equilibrium with the surrounding room. The room eventually equilibrates with the outside environment, and the environment drifts even more slowly toward equilibrium with the rest of the universe. The giants of 19th century thermodynamics viewed this process as a gradual dispersal of energy that increases the overall entropy, or disorder, of the universe. Today, Lloyd, Popescu and others in their field see the arrow of time differently. In their view, information becomes increasingly diffuse, but it never disappears completely. So, they assert, although entropy increases locally, the overall entropy of the universe stays constant at zero.
“The universe as a whole is in a pure state,” Lloyd said. “But individual pieces of it, because they are entangled with the rest of the universe, are in mixtures.”
One aspect of time’s arrow remains unsolved. “There is nothing in these works to say why you started at the gate,” Popescu said, referring to the park analogy. “In other words, they don’t explain why the initial state of the universe was far from equilibrium.” He said this is a question about the nature of the Big Bang.
Despite the recent progress in calculating equilibration time scales, the new approach has yet to make headway as a tool for parsing the thermodynamic properties of specific things, like coffee, glass or exotic states of matter. (Several traditional thermodynamicists reported being only vaguely aware of the new approach.) “The thing is to find the criteria for which things behave like window glass and which things behave like a cup of tea,” Renner said. “I would see the new papers as a step in this direction, but much more needs to be done.”
Some researchers expressed doubt that this abstract approach to thermodynamics will ever be up to the task of addressing the “hard nitty-gritty of how specific observables behave,” as Lloyd put it. But the conceptual advance and new mathematical formalism is already helping researchers address theoretical questions about thermodynamics, such as the fundamental limits of quantum computers and even the ultimate fate of the universe.
“We’ve been thinking more and more about what we can do with quantum machines,” said Paul Skrzypczyk of the Institute of Photonic Sciences in Barcelona. “Given that a system is not yet at equilibrium, we want to get work out of it. How much useful work can we extract? How can I intervene to do something interesting?”
Sean Carroll, a theoretical cosmologist at the California Institute of Technology, is employing the new formalism in his latest work on time’s arrow in cosmology. “I’m interested in the ultra-long-term fate of cosmological space-times,” said Carroll, author of “From Eternity to Here: The Quest for the Ultimate Theory of Time.” “That’s a situation where we don’t really know all of the relevant laws of physics, so it makes sense to think on a very abstract level, which is why I found this basic quantum-mechanical treatment useful.”
Twenty-six years after Lloyd’s big idea about time’s arrow fell flat, he is pleased to be witnessing its rise and has been applying the ideas in recent work on the black hole information paradox. “I think now the consensus would be that there is physics in this,” he said.
Not to mention a bit of philosophy.
According to the scientists, our ability to remember the past but not the future, another historically confounding manifestation of time’s arrow, can also be understood as a buildup of correlations between interacting particles. When you read a message on a piece of paper, your brain becomes correlated with it through the photons that reach your eyes. Only from that moment on will you be capable of remembering what the message says. As Lloyd put it: “The present can be defined by the process of becoming correlated with our surroundings.”
The backdrop for the steady growth of entanglement throughout the universe is, of course, time itself. The physicists stress that despite great advances in understanding how changes in time occur, they have made no progress in uncovering the nature of time itself or why it seems different (both perceptually and in the equations of quantum mechanics) than the three dimensions of space. Popescu calls this “one of the greatest unknowns in physics.”
“We can discuss the fact that an hour ago, our brains were in a state that was correlated with fewer things,” he said. “But our perception that time is flowing — that is a different matter altogether. Most probably, we will need a further revolution in physics that will tell us about that.”
Original story reprinted with permission from Quanta Magazine, an editorially independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.
and the University of Tokyo in Japan did this week, showing that a weird phenomenon — in which the measurement of a particle actually affects its location — is real.
