Quantum theory is strange. With it’s particle-wave duality, and spooky action at a distance, it is a difficult theory to wrap your head around. But long before we were able to test some of its strangest implications, we used hypothetical thought experiments. One of the most famous was the delayed-choice experiment proposed by John Wheeler in 1978. 

The delayed choice experiment looks at one of the strangest behaviors of quantum systems, specifically that they can sometimes interact like particles, and sometimes like waves. For example, light can behave like a wave, and waves of light can overlap to create interference patterns that can be seen as ripples of bright and dark on a screen. Light can also behave as particles called photons, and a single photon can trigger a detector, similar to the way a baseball can be used to drop someone in a tank of water.

It would seem, then, that a quantum system such as light is sometimes a wave, and sometimes made of particles. But if that’s the case, you should be able to trick the system. What if you set up a wave experiment for light, and then changed it to a particle experiment after the light has already been released? Or what if you randomly chose a particle or wave experiment long after light began its journey. If the light really has to be either a particle or a wave, then it somehow has to choose before the experiment is decided. What would happen if it chooses wrongly? This is what Wheeler called the delayed-choice experiment, because the choice of experiment is only decided after the light is already committed.

Wheeler’s idea was to imagine a “cosmic interferometer.” Suppose light from a distant distant quasar were to be gravitationally lensed by closer galaxy. As a result, light from a single quasar would appear as coming from two slightly different locations. Wheeler then noted that this light could be observed in two different ways. The first would be to have a detector aimed at each lensed image, thus making a particle measurement. The second would be to combine light from these two images in an interferometer, thus making a wave measurement. According to quantum theory, the results of these two types of experiments (particle or wave) would be exactly as we’ve observed in their standard form. But the light began its journey billions of years ago, long before we decided on which experiment to perform. Through this “delayed choice” it would seem as if the quasar light “knew” whether it would be seen as a particle or wave billions of years before the experiment was devised.

It took nearly 30 years before the experiment was successfully done using an interferometer in a lab. The results were exactly what Wheeler predicted. Even though the choice is delayed, the outcome is just what quantum theory predicts. This was considered solid proof for most scientists, but a few argued that because the experiment was done entirely in the lab, some subtle interaction might have given light a clue as to the outcome. So there has been some effort to do the experiment at much greater distances. Now a team has succeeded in doing it between a lab on Earth and a satellite in space.

Their setup is similar to other delayed choice experiments, but with a spacey twist. Light passes through an interferometer in the lab, but instead of simply measuring the result, the combined beams travel out into space, where they strike a mirror on a satellite in low Earth orbit. Only after the light bounces back to the lab is the outcome measured. The choice of measurement isn’t made until the light beam is well beyond the lab. So the lab can’t affect the outcome. As expected, the result matches Wheeler’s prediction.

So quantum theory is right. Quantum systems don’t choose to be particles or waves to fit the experiment. They have both particle-like and wave-like properties at the same time. It’s strange, but it’s not magic.

Paper: Francesco Vedovato, et al. Extending Wheeler’s delayed-choice experiment to space. Sci. Adv. e1701180 (2017)

The post Quantum Thought Experiment Works In Space appeared first on One Universe at a Time.

Our fate is written in the stars, so the old stories go. It makes for thrilling drama, but it isn’t the way the Universe works. But there’s an interesting effect of quantum mechanics that might leave an opening for a starry fate, so a team of researchers decided to test the idea. 

The idea stems from a subtle effect of quantum physics demonstrated by the Einstein-Podolsky-Rosen (EPR) experiment. One of the basic properties of quantum objects is that their behavior isn’t predetermined. The statistical behavior of a quantum system is governed by the laws of quantum theory, but the specific outcome of a particular measurement is indefinite until it’s actually performed. This behavior manifests itself in things such as particle-wave duality, where photons and electrons can sometimes behave like particles and sometimes like waves.

One of the more subtle effects related to this property is known as entanglement, when two quantum objects have some kind of connection that allows you to gain information about object A by only interacting with object B. As a basic example, suppose I took a pair of shoes and sent one shoe to my brother in Cleveland, and the other to my sister in Albuquerque. Knowing what a prankster I am, when my sister opens the package and finds a left shoe, she immediately knows her brother was sent the right one. The fact that shoes come in pairs means they are an “entangled” system.

The difference between shoes and quantum entanglement is that the shoes already had a destined outcome. When I mailed the shoes days earlier, the die was already cast. Even if I didn’t know which shoe I sent to my brother and sister, I definitely sent one or the other, and there was always a particular shoe in each box. My sister couldn’t have opened the box to find a slipper. But with quantum entanglement, slippers are possible. In the quantum world, it would be like mailing the boxes where all I know is that they form a pair. It could be shoes, gloves or socks, and neither I nor my siblings would know what the boxes contain until one of them opens a box. But the moment my brother opens the box and finds a right-handed glove, he immediately knows our dear sister will be receiving its left-handed mate.

If all of this sounds really strange, you’re not alone. Even quantum physicists find it strange, and they have confirmed the effect countless times. It’s such a strange thing that some have argued that quantum objects must have some kind of secret information that lets them know what to do. We may not know what the outcome might be, but the two quantum objects do.

The key to doing the EPR experiment is to ensure that entangled objects are measured in a random way. That way the system is truly indefinite until one of the objects is measured. This is usually done by letting a random number generator decide the measurement after the experiment has begun.  But if you really want to be picky, you could argue that while the experiment is being set up, there is plenty of time for the system to know what is going on. Technically, the experiment, random generator, and scientist are all “entangled” as a single system, so the outcome may be pre-biased. What looks like a random choice made after the experiment started may not actually be random. This is known as the setting independence problem.

A light cone diagram showing the range of influence possible for the cosmic EPR experiment. Credit: Johannes Handsteiner, et al.

To address this issue, the team used distant stars to roll the dice for their experiment. Rather than using a local random generator, the team took real-time observations of two stars. One star is about 600 light years away, and the other is about 1,900 light years away. They took observations of each star at particular wavelengths to ensure the light wasn’t influenced by local effects such as Earth’s atmosphere, and use the observations as their random number generator. It would take hundreds of years for the quantum objects of experiment to entangle with these distant stars, so this solves the independence problem. What they found was that Bell’s inequality was violated in their experiment, just as it is in similar experiments, meaning that the system can’t have any hidden information to bias the outcome. So once again the EPR experiment shows there aren’t any hidden variables within the system.

Now it is true that this new experiment doesn’t fully solve the independence problem. Perhaps the experiment, scientists and the entire region of stars within hundreds of light years conspired to ensure the system had inside information. That’s theoretically possible, but it would have had to been given to the experiment at least 600 years ago. As the authors note, the experiment would have been given insider information about the time the Gutenberg Bible was being printed.

So we can safely assume that there aren’t any hidden variables within the system, and quantum theory acts just as we’d expect.

Paper: Johannes Handsteiner, et al. Cosmic Bell Test: Measurement Settings from Milky Way Stars.  arXiv:1611.06985 [quant-ph] (2017)

The post Starry Fate appeared first on One Universe at a Time.

There has been a lot of digital ink spilled over the recent paper on the reactionless thrust device known as the EMDrive. While it’s clear that a working EM Drive would violate well established scientific theories, what isn’t clear is how such a violation might be resolved. Some have argued that the thrust could be an effect of Unruh radiation, but the authors of the new paper argue instead for a variation on quantum theory known as the pilot wave model. 

One of the central features of quantum theory is its counter-intuitive behavior often called particle-wave duality. Depending on the situation, quantum objects can have characteristics of a wave or characteristics of a particle. This is due to the inherent limitations on what we can know about quanta. In the usual Copenhagen interpretation of quantum theory, an object is defined by its wavefunction. The wavefunction describes the probability of finding a particle in a particular location. The object is in an indefinite, probabilistic state described by the wavefunction until it is observed. When it is observed, the wavefunction collapses, and the object becomes a definite particle with a definite location.

While the Copenhagen interpretation is not the best way to visualize quantum objects it captures the basic idea that quanta are local, but can be in an indefinite state. This differs from the classical objects (such as Newtonian theory) where things are both local and definite. We can know, for example, where a baseball is and what it is doing at any given time.

The pilot wave model handles quantum indeterminacy a different way. Rather than a single wavefunction, quanta consist of a particle that is guided by a corresponding wave (the pilot wave). Since the position of the particle is determined by the pilot wave, it can exhibit the wavelike behavior we see experimentally. In pilot wave theory, objects are definite, but nonlocal. Since the pilot wave model gives the same predictions as the Copenhagen approach, you might think it’s just a matter of personal preference. Either maintain locality at the cost of definiteness, or keep things definite by allowing nonlocality. But there’s a catch.

Although the two approaches seem the same, they have very different assumptions about the nature of reality. Traditional quantum mechanics argues that the limits of quantum theory are physical limits. That is, quantum theory tells us everything that can be known about a quantum system. Pilot wave theory argues that quantum theory doesn’t tell us everything. Thus, there are “hidden variables” within the system that quantum experiments can’t reveal. In the early days of quantum theory this was a matter of some debate, however both theoretical arguments and experiments such as the EPR experiment seemed to show that hidden variables couldn’t exist. So, except for a few proponents like David Bohm, the pilot wave model faded from popularity. But in recent years it’s been demonstrated that the arguments against hidden variables aren’t as strong as we once thought. This, combined with research showing that small droplets of silicone oil can exhibit pilot wave behavior, has brought pilot waves back into play.

How does this connect to the latest EM Drive research? In a desperate attempt to demonstrate that the EM Drive doesn’t violate physics after all, the authors spend a considerable amount of time arguing that the effect could be explained by pilot waves. Basically they argue that not only is pilot wave theory valid for quantum theory, but that pilot waves are the result of background quantum fluctuations known as zero point energy. Through pilot waves the drive can tap into the vacuum energy of the Universe, thus saving physics! To my mind it’s a rather convoluted at weak argument. The pilot wave model of quantum theory is interesting and worth exploring, but using it as a way to get around basic physics is weak tea. Trying to cobble a theoretical way in which it could work has no value without the experimental data to back it up.

At the very core of the EM Drive debate is whether it works or not, so the researchers would be best served by demonstrating clearly that the effect is real. While they have made some interesting first steps, they still have a long way to go.

Paper: Harris, D.M., et al. Visualization of hydrodynamic pilot-wave phenomena, J. Vis. (2016) DOI 10.1007/s12650-016-0383-5

The post Doing The Wave appeared first on One Universe at a Time.

China recently launched a satellite to test quantum entanglement in space. It’s an interesting experiment that could lead to “hack proof” satellite communication. It’s also led to a flurry of articles claiming that quantum entanglement allows particles to communicate faster than light. Several science bloggers have noted why this is wrong, but it’s worth emphasizing again. Quantum entanglement does not allow faster than light communication. 

This particular misconception is grounded in the way quantum theory is typically popularized. Quantum objects can be both particles and waves, They have a wavefunction that describes the probability of certain outcomes, and when you measure the object it “collapses” into a particular particle state. Unfortunately this Copenhagen interpretation of quantum theory glosses over much of the subtlety of quantum behavior, so when it’s applied to entanglement it seems a bit contradictory.

The most popular example of entanglement is known as the Einstein-Podolsky-Rosen (EPR) experiment. Take a system of two objects, such as photons such that their sum has a specific known outcome. Usually this is presented as their polarization or spin, such that the total must be zero. If one photon is measured to be in a +1 state, the other must be in a -1 state. Since the outcome of one photon affects the outcome of the other, the two are said to be entangled. Under the Copenhagen view, if the entangled photons are separated by a great distance (in principle, even light years apart) when you measure the state of one photon you immediately know the state of the other. In order for the wavefunction to collapse instantly the two particles must communicate faster than light, right? A popular counter-argument is that while the wavefunction does collapse faster than light (that is, it’s nonlocal) it can’t be used to send messages faster than light because the outcome is statistical. If we’re light years apart, we each know the other’s outcome for entangled pairs of photons, but the outcome of each entangled pair is random (what with quantum uncertainty and all), and we can’t force our photon to have a particular outcome.

The reality is more subtle, and vastly more interesting. Although quantum systems are often viewed as fragile things where the slightest interaction will cause them to collapse into a particular state, that isn’t the case. Entangled systems can actually be manipulated in a variety of ways, and you can even manipulate them to have a specific outcome. I could, for example, create pairs of entangled photons in different particular quantum states. One state could represent a 1, and the other a 0. All my distant colleague needs to do is determine which quantum state a particular pair is in. But to do this my colleague would need to make lots of copies of a quantum state, then make measurements of these copies in order to determine statistically the state of the original. But it turns out you can’t make a copy of a quantum system without knowing the state of the quantum system. This is known as the no-cloning theorem, and it means entangled systems can’t transmit messages faster than light.

Which brings us back to the experiment China just launched. The no cloning theorem means an entangled system can be used to send encrypted messages. Although our entangled photons can’t transmit messages, their random outcomes are correlated, so a partner and I can use a series of entangled photons to generate a random string we can use for encryption. Since we each know the other’s outcome, we both know the same random string. To crack our encryption, someone would need to make a copy of our entangled states, which can’t be done. There are ways to partially copy the quantum state, which would still improve the odds of breaking the encryption, but a perfect copy is impossible.

So entanglement doesn’t give us faster than light communication, but it may make it a bit easier to keep our secrets secret.

The post Quantum Entanglement: Slower Than Light appeared first on One Universe at a Time.

According to general relativity, if you gather together enough mass into a small enough space, you can create a black hole. No matter what kind of matter you use (cars, protons, old issues of National Geographic) the black hole you get will have only three properties: mass, electric charge, and rotation (angular momentum). This is known as the no-hair theorem, because the material properties of any object (referred to as “hair” because a physicist named John Wheeler once coined the phrase “a black hole has no hair”) become unmeasurable (hence unknowable) as the object collapses into a black hole. While it seems a simple enough idea, it’s caused all manner of problems for theoretical physicists. 

To begin with, the no-hair theorem is in direct conflict with another principle of physics, namely that information about an object can’t simply disappear. In physics, information about an object tells us what’s going on. Since events are caused by what happened before them, and allow us to predict what will happen next, the amount of information we have about a system must be conserved. But a black hole violates this rule. Once an object enters a black hole, all information about it effectively disappears.

In classical relativity there’s no way around this problem. It’s generally thought that quantum gravity would solve the issue, but even that path has been plagued with problems. One of the thing quantum gravity predicts is that black holes should leak a small amount of energy over time due to Hawking radiation. A popular idea has been that perhaps Hawking radiation isn’t simply random, but carries information about what has fallen into the black hole. However this approach led to another problem known as the firewall paradox. Basically, Hawking radiation is caused by quantum fluctuations in spacetime. In order to carry information they must also create a firewall of superheated particles near the black hole’s event horizon. This violates the central idea of relativity known as the equivalence principle.

Arguments over these ideas and their theoretical implications have raged for years, but recently Stephen Hawking and his colleagues have devised a possible solution. It starts with a subtle property of quantum theory.

In classical physics, a “vacuum” is simply a region of space in which there is nothing. In quantum theory “nothing” is hard to define. Because of things like the Heisenberg uncertainty principle a vacuum is filled with a sea of quantum fluctuations that average out to zero. Usually it is assumed that there is just one vacuum state in quantum theory, however there is a way to have an infinite number of quantum vacuum states.

Imagine a vacuum of space with a single photon, but make the energy of the photon so tiny that it’s essentially zero. In classical physics this would just reduce to the standard vacuum, however in quantum physics it would reduce to a unique vacuum state. Since you can do this in basically an infinite number of ways, you can create an infinite number of vacuum states. Normally this would just be theoretical mumbo-jumbo, since all these quantum vacuum states would yield the same physics in the end. But with black holes it could solve the information paradox.

The idea of Hawking and his peers is that a black hole is surrounded all these unique vacuum states, forming a kind of quantum hair (or soft hair, as they call it) around the black hole. By itself the soft hair looks just like a classical vacuum, but it can contain the information of all the stuff that fell into a black hole. The Hawking radiation emitted by the black hole is random (thus preventing the firewall paradox), but it interacts with the soft hair of the quantum vacua, releasing the information they contain (thus solving the information paradox).

If this model is right, then it means information isn’t lost after all. It’s just hidden in a quantum vacuum, waiting to be released by Hawking radiation.

Paper: Stephen W. Hawking, Malcolm J. Perry, and Andrew Strominger. Soft Hair on Black Holes. Phys. Rev. Lett. 116, 231301 (2016)

The post Hair Of The Dog appeared first on One Universe at a Time.

In 1937  Pyotr Kapitsa and John F. Allen discovered a strange behavior of ultracold liquids known as superfluidity. A superfluid is a fluid with no viscosity, basically a frictionless liquid. Without viscosity, the fluid has no way to dampen its motion. Because of this, superfluids have some pretty unusual behaviors. If a bit of superfluid is suspended in an open container, it will creep up along the walls, then drip down to a lower container. It can flow through tiny pores that regular liquids can’t, and can create fountains that could flow forever. This seeming defiance of gravity and common sense is due to the fact that its behavior is rooted in quantum physics. Though it is not a truly quantum state such as a Bose-Einstein condensate, it shares some commonality with it. In the lab, superfluids are only seen at temperatures barely above absolute zero. The most common example, helium-4, becomes superfluid when cooled below 2.17 K. So it might seem odd that superfluids are also found in the hot interiors of neutron stars. 

A neutron star is a stellar remnant formed with a star runs out of hydrogen and heavier elements to fuse. After a star explodes as a supernova, the remaining core of the star collapses under its own weight to the point that only the pressure of nuclei can counter the force of gravity. A neutron star has a mass of about two Suns, but are only about 20 kilometers in diameter. They have a dense atmosphere of carbon only a few centimeters thick, and a thin crust of iron nuclei. In the interior of a neutron star, nuclei are pushed together ever more tightly, and reach a point where the nuclei can’t hold themselves together. As a result, individual neutrons “drip” out, and sink into the star’s core, forming a neutron fluid. As a neutron star cools, this neutron fluid transitions to a superfluid state. This happens not at a few degrees Kelvin, but at 500 to 800 million Kelvin. The interior of a neutron star is a hot superfluid sea.

When the interior transitions to a superfluid, the temperature of the star’s surface can drop dramatically, as has been seen in the neutron star of Cassiopeia A. Once the interior is superfluid, it can significantly affect the star’s behavior. Neutron stars have strong magnetic fields, and coupled with their fast rotation they radiate intense radio energy (which we can sometimes see as pulsars). Because the crust of a neutron star is magnetically pinned, its rotation gradually slows down, but the superfluid interior has no viscosity, and doesn’t slow down.  Over time this means the interior can rotate much faster than the outer crust. Eventually the difference between the two becomes so great that some of the rotation is transferred to the crust, causing it to speed up quickly. In pulsars this is seen as a glitch in the rate of its radio pulses. These glitches can be used to determine the mass of a neutron star.

Perhaps what’s most amazing about all this is how two radically different materials can share such similar properties. Ultracold helium and superhot neutrons both acting as a strange quantum fluid.

The post The Quantum Fluid Inside Neutron Stars appeared first on One Universe at a Time.

Heisenberg’s uncertainty principle is the foundational concept of quantum theory. It’s also commonly misunderstood, which leads to a great deal of confusion about what quantum theory really says about the universe. 

The uncertainty principle is often presented in terms of an observer effect. Suppose you want to measure the position of an electron. One way to do this is to shine light in the electron’s direction. When a photon scatters off the electron, you can measure how it scatters and determine where the electron is. But of course when the photon scatters off the electron it would cause the electron to scatter off in some direction. Measuring the position thus makes the electron’s motion (momentum) somewhat uncertain. Since any measurement of an electron’s position or momentum would make one or the other uncertain, there is a limit to what can be measured about the electron.

While that makes a nice intuitive picture, it’s completely wrong. The uncertainty principle isn’t a limit on what you can measure, but an inherent property of quantum objects. The reason you can’t precisely measure the position and momentum of an electron is not because you’re experiment is sloppy, but because electrons don’t have a precise simultaneous position and momentum.

Heisenberg’s uncertainty is what leads to all the strange aspects of quantum objects, such as particle-wave duality and quantum tunneling. Unfortunately, quantum systems are often portrayed as weird things that keep changing the rules to keep you in the dark, or only become real when you look at them. Such descriptions assume that quantum systems should behave like everyday objects. But the universe is far more subtle. The everyday, common sense ideas we have about the world are often only rough approximations that human-sized objects seem to follow.

To really understand the universe, we sometimes have to view things through Heisenberg’s mirror.

The post Heisenberg’s Mirror appeared first on One Universe at a Time.

Quantum theory is strange, but very real. Through countless experiments we’ve found that quantum objects have both particle-like and wave-like properties. In some experiments the particle nature dominates, while in others the wave nature dominates. Some experiments can even show the effects of both properties. This duality between particles and waves in quantum theory is deeply counterintuitive, which means often the results of quantum experiments are interpreted incorrectly.

Take, for example, recent claims that reality doesn’t exist until we measure it. The claims arise from a recent experiment published in Nature that uses a single atom to perform what is known as the delayed-choice experiment. This experiment was first proposed as a thought experiment (gedanken experiment) by John Wheeler as a way of exploring the counterintuitive aspects of particle-wave duality.

Wheeler’s idea was to imagine a “cosmic interferometer.” Suppose light from a distant distant quasar were to be gravitationally lensed by closer galaxy. As a result, light from a single quasar would appear as coming from two slightly different locations. Wheeler then noted that this light could be observed in two different ways. The first would be to have a detector aimed at each lensed image, thus making a particle measurement. The second would be to combine light from these two images in an interferometer, thus making a wave measurement. According to quantum theory, the results of these two types of experiments (particle or wave) would be exactly as we’ve observed in their standard form. But the light began its journey billions of years ago, long before we decided on which experiment to perform. Through this “delayed choice” it would seem as if the quasar light “knew” whether it would be seen as a particle or wave billions of years before the experiment was devised.

Although the quasar experiment Wheeler proposed isn’t practical, modern experimental equipment allows us to perform a similar experiment in the lab, where the decision to measure a particle or wave is done at random after the quantum system is “committed.” For example, in 2007 a delayed-choice experiment was made using laser light to create a delayed-choice double slit experiment. In this new paper, the team used an ultracold helium atom to do a similar delayed-choice interference experiment. With both experiments the results were exactly as predicted by quantum theory. So both matter and light exhibit this strange quantum effect.

While this is great work, the result isn’t unexpected. Quantum theory made a very clear prediction about this kind of experiment, and its prediction has been confirmed. Where things get fuzzy is in the interpretation. One popular way to interpret quantum theory is to presume quanta have a potential wavefunction, which then collapses into a definite state when observed. In this view the act of measurement gives reality to the quantum. In the delayed-choice experiment that would mean the quantum doesn’t become “real” until you measure it, which could be billions of years after its origin in the case of quasar light. But this is an overly simplistic take on things. Quantum objects are real, but simply have indefinite properties. These properties are defined by the experiments we do. What the delayed choice experiments really show is that quanta don’t exist as particles or waves, but are truly unique objects which can exhibit particle and wave properties in certain experiments.

While that might seem strange, it isn’t magical or mystical. The Moon wouldn’t vanish from existence if everyone closed their eyes, and reality isn’t dependent upon us observing it.

Paper: A. G. Manning, et al. Wheeler’s delayed-choice gedanken experiment with a single atom. Nature Physics, DOI: 10.1038/nphys3343 (2015)

Paper: Jacques, V. et al. Experimental realization of Wheeler’s delayed-choice gedanken experiment. Science 315, 966–968 (2007).

The post Real and Unreal appeared first on One Universe at a Time.

Yesterday I mentioned the Casimir effect, and how it could hypothetically be used to detect gravitons. But what exactly is the Casimir effect, and how do we know it’s real?

Fluctuations are limited by the conducting plates.

The Casimir effect is a great example the strangeness of quantum theory, and how even some of its strangest predictions turn out to be right. The effect was first proposed by Hendrik Casimir in the 1940s as a consequence of quantum fluctuations. The basic idea is that within quantum electrodynamics, a region of empty space actually contains quantum fluctuations of the electromagnetic field. These fluctuations are extraordinarily small, and in most cases we’d never notice them. But since they are electromagnetic, they are still affected by the presence of conducting materials. Specifically, they can be bounded by a conducting surface. So if you place two parallel conducting surfaces close to each other, the fluctuations are bound between the plates, but not outside the plates. As a result, there are less fluctuations between the plates than on either side. This means there is less pressure between the plates, and the plates are therefore pulled together.

This net attraction due to wave fluctuations is not particularly surprising. In fact you can demonstrate this effect with water waves. What makes is surprising is that according to classical electromagnetism, since the two plates are uncharged there should be no electric field between them and no force of attraction. Two plates in a vacuum are somehow attracted to each other simply because they are close together. When Casimir first calculated the effect, he used perfect “ideal” conductors. Later, more detailed calculations showed the effect for realistic conductors, and in 1997 the effect was confirmed experimentally. The most recent experiments get results to within 1% of the theoretical result. Strange as it is, the Casimir effect is very real.

Although the reality of the Casimir effect is not in doubt, its strangeness has led to much debate over what it actually means. Since it seems to show an extraction of energy from the “vacuum,” zero-point-energy fans have used it to support claims of “free energy” devices. Since the energy level between the plates is less than the average energy level outside the plates, the effect has been suggested as a solution for exotic physics such as wormholes and warp drive. It also raises difficulties in cosmology. If quantum fluctuations have real energy, then they should be affected by gravity, and that should affect the cosmological constant. According to QED the cosmological constant should be huge, but in fact it’s actually very small (assuming it’s the cause of dark energy).

But it’s important not to overstate the implications of the Casimir effect. It does raise some interesting questions about quantum gravity, but the main thing it does is demonstrate that our understanding of electromagnetism on a quantum scale is actually quite good.

Paper: H. B. G. Casimir and D. Polder. The Influence of Retardation on the London-van der Waals Forces. Phys. Rev. 73, 360 (1948)

The post Nothing But Net appeared first on One Universe at a Time.

There’s been much buzz about a new paper claiming that it’s observed light acting as both a particle and a wave at the same time. Is this legitimate research? Yes, absolutely. Did they actually observe particles and waves at the same time? Well…

Much of the hype around this paper is driven by some basic misconceptions regarding quantum objects. The popular view of quantum theory is that things like photons are sometimes particles and sometimes waves, and which one they become depends upon how you observe them. But in fact quantum objects are neither particles nor waves. They are quanta, which is a separate thing altogether. Under the right conditions quanta can demonstrate wave-like and particle-like behaviors, and there is complementarity between them so that quanta tend to lean toward one or the other in an experiment. But within the formalism of quantum theory, particle-wave duality is a property of the quanta as a whole. Thinking of quanta as particles or waves is far to simplistic when dealing with quantum theory. This is important to keep in mind when popular articles such as this hit the web.

As research areas such as quantum optics and quantum computing developed, we’ve gained tools to really start looking at sophisticated quantum interactions. It’s how we’ve been able to study things like the connection between the uncertainty principle and entropy, or study phase velocity in a quantum system. But since this kind of work isn’t easy to describe in simple terms, it gets hyped as “quantum mechanics gets simpler!” or “speed of light not absolute!” The same is the case here.

The team maintained a wave pattern while inducing particle-like interactions. Credit: L Piazza, et al.

So what’s really going on in this work? The team pulsed laser light at a tiny wire of conductive material (a nanowire). The light induced what is known as surface plasmon polaritons in the nanowire, which is basically an electromagnetic wave pattern within the electrons of material. Because of the size of the nanowire, the plasmon polaritons form a standing wave within the wire, which is where the “wave” aspect comes into the experiment. They also radiate light, which in a quantum sense means that photons are emanating from this standing wave. The team then aimed a beam of electrons at the set up. Some of the electrons collided with the emanating photons, and thus gained some energy. Since these collisions are particle-like, they gain specific (quantized) energy amounts from the induced photons.  Basically the team found a way to induce particle-like interactions while maintaining the overall wave aspect of the system at the same time.

Does this mean the team caused a specific photon or electron to behave as a particle and wave at the same time? No. The particle interactions with the electrons and the induced wave pattern in the wire are two separate aspects of the system. But their result is useful because it could allow us to study quantum interactions directly. This type of work is really useful for photonics and quantum computing, and it’s a clever way to interact with quantum systems.

But this is not an experiment that somehow violates quantum theory. We’ve known for a while that we should be able to do this kind of thing in theory. The achievement here is that they actually pulled it off.

Paper: L Piazza, et al. Simultaneous observation of the quantization and the interference pattern of a plasmonic near-field. Nature Communications 6:6407 DOI: 10.1038/ncomms7407 (2015)

The post Two For One appeared first on One Universe at a Time.

Quantum theory is often viewed as a strange and mysterious model where objects behave in illogical ways. While it’s true that quantum objects behave in ways that are counterintuitive, we actually understand the behavior quite well. In fact, many of these strange behaviors are used in modern astronomy. Take, for example, the quantization of magnetic moments.

Most atoms have a small magnetic field. This magnetic field can be approximated as a small magnet, just as the Earth’s magnetic field is sometimes treated as a magnetic. The strength of that imaginary magnet is given by the magnetic moment of the atom. With this in mind, the orientation of an atom’s magnetic field can be represented by the orientation of the magnet.

The basic setup of a Stern-Gerlach experiment. Credit: Wikipedia

Suppose, then, that we were to toss atoms through an inhomogeneous magnetic field. Individually the atoms have no particular orientation, so we would expect that the orientation of their magnetic moments are entirely random. As a result, some of the atoms would be more strongly attracted toward the north direction of the magnetic field, while others would be more attracted to the south, and everywhere in between. If the atoms really did act like tiny magnets, we would expect to see the beam of atoms spread out evenly by the magnetic field. In fact, what we see is that the atoms either move toward the north or south, and that’s it. Instead of spreading out evenly, the atoms lock into specific orientations. This experiment is named the Stern-Gerlach experiment, after the physicists who first performed it in 1922, and it demonstrates one of the basic aspects of quantum theory. When you try to measure the state of a quantum system, the results you get are often snapped to discrete results. It would be like measuring the height of a random collection of people, and finding they are all exactly either 5 ft or 6 ft tall.

The Zeeman effect.

As strange as this is, we actually use a similar effect to measure the strength of magnetic fields in the Sun. Since electrons also have magnetic moments, strong magnetic fields can cause their energy levels in an atom to shift. As a result, the emission lines an atom gives off can be shifted by magnetic fields. Emission lines can even be split slightly, which is known as the Zeeman effect. We see this effect near sunspots, which is how we know that sunspots are cooled by magnetic dampening.

That’s part of the real power of astrophysics. Once we understand a phenomena, however strange, we can use it has a tool to study the stars.

The post Snap To appeared first on One Universe at a Time.

In modern physics, matter in the universe is made up of quanta or “particles” such as electrons, protons  and neutrons. These particles can be said to interact through various forces or fields (strong, weak, electromagnetic, gravitational) for which there are corresponding “field quanta” such as photons and gluons. These quanta can are often seen as the particles that make up these fields, and while things are a bit more complicated it is the right basic idea. We have a lot of experimental evidence for these quanta, but there is one that’s often mentioned for which we have no experimental evidence. That’s the graviton.

Known particles and field quanta. Credit: AAAS

One of the basic ways in quantum field theory to is to start with a wave form and then “quantize” it using the mathematical formalism. In this way you can show, for example, how photons arise from the electromagnetic field. The same thing can be done with the gravitational field. Start with gravitational waves, and then quantize it to derive gravitons. But there are some problems with this approach. In quantum field theory all fields act within a flat background of space and time (what we call Minkowski space). Gravitational waves distort space and time itself, so to derive gravitons it’s often assumed that the gravitational waves are a fluctuation within a background of Minkowski space. It this way you can treat gravity as a field within flat space in order to quantize it.

Of course, general relativity shows that’s not how gravity works. Gravity is a product of spacetime curvature, so to quantize gravity you’d need to quantize spacetime itself. Just how that might be done is one of the great unsolved problems in physics. So it’s possible that gravitons don’t exist. But it’s generally thought that they do, since most physicists think that in the end quantum theory will be at the heart of everything. The current main approaches to quantum gravity, such as string theory and loop quantum gravity, predict the existence of gravitons with the same properties we see in the simple “quantized wave” approach.

Even if gravitons exist, it’s likely that we’d never be able to detect them. As one recent paper demonstrated, gravitons would interact so weakly with masses that you’d need something like a Jupiter-mass detector orbiting a neutron star. Even then it would take more than a decade to detect a single graviton. Even then the noise from things like neutrinos would wash out your signal. If there’s no practical way to detect gravitons, does it make sense to talk of them as a scientific model?

Perhaps. Assuming they remain within a robust model of quantum gravity, there may be indirect ways of confirming their existence. For now, however, they are purely hypothetical.

Paper: Rothman, T. and Boughn, S. Can Gravitons be Detected? Foundations of Physics 36 (12): 1801–1825 (2006).

The post And Then There’s Maude appeared first on One Universe at a Time.