Superfluidity

Here is a paper I wrote in March of 2018 about another intriguing physical phenomenon: superfluidity. I hope you find it as cool as you would have to get to observe this phenomenon, which is usually close to absolute zero! Please enjoy, and don’t hesitate to comment in the forum if you have any questions. That way more people, including me, can learn from your question!


“Superfluidity” describes a property of liquid matter, i.e. the property of having zero viscosity, or immeasurably low, to be precise (1). This means that if a superfluid were stirred, it would cycle in endless vortices, conserving  one hundred percent of its kinetic energy.  And if a hole were made in the bottom of the vessel, then the superfluid would flow out very quickly compared to other fluids, e.g. honey. The rate of flow of course depends on the size of the hole, so for comparisons of viscosity assume that the holes are the same size. If one fills a cup with honey and then pokes a hole in it, then the honey will eventually flow out, but very slowly.  A superfluid, on the other hand, would flow out the highest possible rate, only limited by the size of the hole.  (3). To understand this an understanding of viscosity is needed. The fundamental principle behind viscosity is friction: the resistance that one object has to moving over another. In the case of liquids, which lack structure, it is the friction between the molecules or atoms of the liquid itself that causes viscosity. (2). In the case of a superfluid, there is no such internal friction due to special molecular or atomic makeup. That is the fundamental definition of a superfluid, but these materials can exhibit many other strange properties. (1). Along with the property of zero viscosity comes lower density and a significant change in specific heat. But is this property of zero viscosity that produces the strangest effects.

                Superfluidity and superconductivity are normally only exhibited at extremely low temperatures.  This is because the particles that condense must always be indistinguishable. The Debroglie wavelengths of indistinguishable particles must have a high degree of overlap. Debroglie wavelength equals Planck’s constant divided my momentum, mass times velocity. Since large particles, e.g. baseballs have high mass, their Debroglie wavelengths will be infinitesimally small, which is it is so unlikely that large particles will exhibit wave-like behavior. For Debroglie wavelengths to overlap, they need to be very long, achieved by cold temperatures, and it helps to have a high packing density of the particles. (8). Cold temperatures are very costly to generate, so this requirement limits the practical applications of superfluidity and superconductivity. Some materials used to make the superconductors are also very expensive, e.g. niobium. (9).

                But, there exist so called “high temperature superconductors” that exhibit superconductivity at a balmy eighty kelvins. The BCS theory does not account for the existence of these, but they will probably prove very useful in the future, as they do exist. One current use of superconductors is in particle accelerators, such as CERN. (10).

                Superfluids also exhibit extremely high thermal conductivity, according to some sources infinitely high (12). Heat is transmitted so quickly that thermal waves are created. This is possible due to the fact that the particles of a superfluid are in the same quantum state, i.e. if one particle moves, they all are moved. Heat is conducted when excited particles bump into each other. If a particle bumps into superfluid particle it will move at the same time as all of the other molecules in the superfluid, transmitting the heat from one side of the superfluid to the other instantly. (11).

                Having no internal friction allows for some spectacular displays, e.g. if a vessel is filled with liquid helium-4 which is then cooled below 2.17 kelvin, condensing into a superfluid, it will flow up the walls of the vessel and out of the vessel. This effect is caused by minor differences in temperature and atmospheric pressure inside and outside the vessel. These tiny differences are enough to move the superfluid against the force of gravity because friction does not hinder its flow. (3).

                A vessel with tiny, molecule-sized holes in the bottom would hold liquid helium-4, but if it were cooled below 2.17 kelvins, then it would immediately begin flowing through those holes, again as a consequence of its immeasurably low viscosity. (3).

                Just a month after the discovery of superfluidity, in 1932, another odd effect was observed accidentally bay British physicist Jack Allen. He had a long, thin tube sticking above a liquid helium bath and packed with fine emery powder. He shined a flashlight on the apparatus and the emery powder absorbed the light, slightly elevating the temperature of the superfluid. As long as the light shone, a fountain of liquid helium emitted from the end of the tube above the bath. The heat creates a back pressure that forces the superfluid helium up. (13).

                As stated above, a superfluid could be used to create perpetual motion. Of course, however, no one is deceived that this means that a surplus energy could be produced and thereby unlimited energy; a superfluid can only conserve the energy that it is given and no more, it does not produce any of its own. However, it is a special property of helium that it never settles into solid state, not even at absolute zero; it always remains a liquid. This is because the helium atoms are so weakly attracted to one another that the slight jiggling caused by the quantum uncertainty principle is enough to keep them apart, at least at standard pressure. (3). It would be naive to think that infinite energy could be harvested from the quantum uncertainty principle; as physicists learn more about quantum mechanics, they will probably learn how no energy created or destroyed.

                Superfluidity was first demonstrated in two studies published in Nature in 1938 by the Brittish duo John Allen and Don Meisner, and independently by Russian physicist Pyotr Kapitza.  Alan and Meisner measured the flow of liquid helium-4 through long, thin tubes and found that it flowed with zero viscosity at temperatures below 2.17 degrees kelvin, almost absolute zero. Kapitza made comparable observations on the flow of liquid helium-4 between two glass discs. He also hypothesized a connection between superfluidity, the resistanceless flow of atoms or molecules, and superconductivity, the resistanceless flow of electrons, which had been discovered some years earlier in 1911 by Dutchman Heike Kamerlingh Onnes. These two concepts were at the frontier of physics back then, and it wasn’t until 1957 that Bardeen, Cooper, and Schriefer (BCS) devised a complete theory to link the phenomena.  (4).

                However, it was very soon after the discovery of superfluidity that an explanation was offered: Bose-Einstein condensation. This is the process whereby  particles known as bosons condense to form a single, quantum state. (4). A boson is a particle whose spin, or intrinsic angular momentum, is zero or an integer. The only other possible class of particle is the fermion, a particle with half-integer spin.  Spin dictates the energy distribution of a particle. Bosons obey Bose-Einstein statistics and fermions obey Fermi-Dirac statistics. Bose-Einstein statistics allows for an unlimited number of particles to occupy a single energy level, unlike fermi-dirac statistics that follow the Pauli exclusion principle, which dictates that no two associated fermions can occupy the same quantum state. (5). Imagine two fermions, say electrons. Let p equal the probability that electron 1 occupies state a and electron 2 occupies state b. Let p1 equal the probability that electron one occupies state a and let p2 equal the probability that electron 2 is in state b. Electrons are indistiguishable, so it is impossible to tell which electron occupies which state. So  p1 and  p2 must be arranged like this:

p=p1p2-p1p2

The above relationship describes a wave function, and if both electrons occupy the same state, a or b, the wave function will vanish. Physicists draw from this the Pauli exclusion principle. The same equation can be used for bosons, except the minus sign must be changed to a plus sign. It is their ability to condense together in unlimited numbers occupying the same energy state that allows bosons to form bose-einstein condensates. (6). But liquid helium-4 atoms are composed of six fermions (two protons, neutrons, and electrons) and no bosons! Liquid helium-4 atoms can form a bose-einstein condensate only because an even number, e.g. 6, of interacting fermions can form a composite boson. This allows liquid helium-4 atoms to condense into the lowest possible energy state and become a superfluid. (4).

                The BCS theory also offers an explanation for superconductivity. An electron moving through a lattice of superconducting material with attract the lattice toward it causing a ripple in the direction of its motion. An electron moving in the opposite with be attracted to this disturbance and the two electrons are coupled together forming what is called a Cooper pair. These cooper pairs can also act like bosons and condense into a state of zero electrical resistance called superconductivity. (7).

                After WW II, large quantities of the light isotope helium-3 became available  because it was part of the manufacturing process of tritium, to be used in hydrogen bombs. It would seem in the light of the information thus far presented that helium-3, having an odd number of fermions (two, protons, electrons, and a neutron), could not condense into a superfluid. But it might be possible, according to the BCS theory, for the helium-3 atoms themselves to form Cooper pairs and thus become a superfluid. The theoretical properties of this hypothetical superfluid helium-3 were explored in the 1960s, before the actual discovery of this superfluid at temperatures below .003 kelvins in 1972. (4).

                The spin quantum number (S) and the orbital quantum number (L) of Cooper pairs characterize two types of angular momentum. Normal BCS superconductors have S=0 and L=0, but superfluid helium-3 has S=1 and L=1. These non-zero quantum numbers cause helium-3 superfluid to break certain basic symmetries of normal liquid state, namely rotational and time reversal symmetries, entailing a non-trivial topology for the Cooper pairs. A variation of the BCS theory was required to understand this, marking the beginning of unconventional superconductivity, but the strange behaviors of unconventional superconductors were only just beginning to be discovered. (4).

                Superfluid helium-3 has two phases, A and B (in the absence of a magnetic field). The B phase exists over a much wider range of temperatures and pressures. It can also exist in many different excited states due to its lack of rotational symmetry, and these states are classified according to the total angular momentum of the cooper pairs (J), with the possibilities J=0,1, or 2. One remarkable feature of the J=2 state of the B phase is its ability to transmit transverse sound waves, something that was previously only thought possible in rigid solids. (4).

                Since the discovery of superfluid helium-3, numerous other unconventional superconductors have been discovered, e.g. cuprates. But only one other superconducting material has been found to have two superfluid phases, namely UPt3. (4).

                Even today, physicists are finding new superfluid properties that defy understanding. The most recent developments in the study of superfluidity involved helium-3 in ultr-light aerogels that was shown to exhibit never before seen phases of superfluids, that are currently being studied. (4).

                Superfluidity was just one of the amazing and counter-intuitive discoveries about quantum mechanics made in the twentieth century, and scientists continue to learn more about it in the twenty-first century. Scientists are a long way from fully understanding the phenomena harnessing its full potential.


References:

  1. Schmitt, Andreas. (2014). “Introduction to Superfluidity.” Springer. https://arxiv.org/pdf/1404.1284.pdf. Date-accessed: 4/18/2018.
  2. “What is viscosity?” (n.d.) Princeton. https://www.princeton.edu/~gasdyn/Research/T-C_Research_Folder/Viscosity_def.html. Date-accessed: 4/18/2018.
  3. Minkel, J. R. (2009). “Strange but True: Superfluid Helium can Climb Walls.” Scientific American. https://www.scientificamerican.com/article/superfluid-can-climb-walls/. Date-accessed: 4/18/2018.
  4. Halperin, William P. (2018). “Eighty Years of Superfluidity.” Nature. https://www.nature.com/articles/d41586-018-00417-7?error=cookies_not_supported&code=4fe886f1-804a-495e-a2a6-830b84b16621#ref-CR10. Date-accessed: 4/18/2018.
  5. Nave, R. (n.d.). “Spin Classification.” HyperPhysics. http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/spinc.html#c3. Date-accessed: 4/18/2018.
  6. Nave, R. (n.d.). “Pauli Exclusion Principle.” Hyperphysics. http://hyperphysics.phy-astr.gsu.edu/hbase/pauli.html#c2. Date-accessed: 4/18/2018.
  7. Nave, R. (n.d.). “Cooper Pairs.” Hyperphysics. http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/coop.html#c1. Date-accessed: 4/18/2018.
  8. Nave, R. (n.d.). “Wave Nature of Electron.” Hyperphysics. http://hyperphysics.phy-astr.gsu.edu/hbase/debrog.html#c3. Date-accessed: 4/18/2018.
  9. Cooley, Lance. Pong, Ian. (2016). “Cost drivers for very high energy p-p collider magnet conductors.” Fermilab. https://indico.cern.ch/event/438866/contributions/1085142/attachments/1257973/1858756/Cost_drivers_for_VHEPP_magnet_conductors-v2.pdf. Date-accessed: 4/19/2018.
  10. “Superconductivity.” (2018). CERN. https://home.cern/about/engineering/superconductivity. Date-accessed: 4/19/2018.
  11. “Properties of fuperfluid.” (n.d.). http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Rodney_Guritz%20Folder/properties.htm. Date-accessed: 4/19/2018.
  12. “Infinite Thermal Conductivity.” (n.d.). https://superfluidsiiti.weebly.com/index.html. Date-accessed: 4/19/2018.
  13. “Superfluidity II- The Fountain Effect.” (2006). Nature Publishing Group. https://www.nature.com/physics/looking-back/superfluid2/index.html#. Date-accessed: 4/19/2018.

Bell’s Theorem

This is a paper that I wrote in September of 2017 about Bell’s theorem, a very impactful and interesting discovery in physics. I wish I understood more about it. In explaining Bell’s theorem, I also elucidated some basic concepts in physics such as locality and how light works. I apologize for referencing YouTube videos, the faux pas of citation, but, hopefully, you can excuse this singular error and enjoy the content of the paper. Want to talk more about Bell’s theorem? Head on over to the forum.


John Stewart Bell Was born on 28 June 1928, or 6/28/28, in Belfast, Northern Ireland. He died of cerebral hemorrhage at 77 on 1 October 1990 at Belfast. Bell worked for CERN as a particle physicist, but accomplished his most important work in his off time as a hobby: developing his theorem. Bell’s thesis was contradictory to Einstein and was that reality must be nonlocal. This has been supported by many experiments after him, but has been challenged by others, and remains controversial (1). To understand his wonderful and amazing discoveries, one must comprehend also some crucial underlying physics that the following paragraphs first explain.

            Realism as a physical theory is not defined with invariance. It is concerned with the essence of scientific knowledge. Scientific realists believe in the epistemic, having faith in information an observer receives through scientific processes (2). Bell’s theorem contests this.

            Locality states that no information or particles can travel with superluminous speed. Bell’s Theorem contests this also (3).

            Now an explanation for light waves must be provided. It is helpful to first break down the term electromagnetic field. It is a word combing the terms electric field and magnetic field. An  electric field can be imagined as a plane with many vectors on it, each representing a point in space. These are force vectors exerting a force on any charged particle in space, in the direction of the vector and proportional to the length of the vector and the charge of the particle. Now imagine another vector field like the previous one. This represents the magnetic field. Only when a charged particle is moving across it, does it act with force perpendicular to the direction of the particle’s motion and the magnetic field, with a strength proportional to the length of the magnetic field vector, the particle’s charge, and its velocity. Maxwell’s equations describe the interplay between these two fields. When an electric field is circular, i.e. the vectors point in such a way to form a circle, a magnetic field will increase in strength perpendicular to that plane. And conversely, a loop of magnetic field created a change a change in the electric field perpendicular to the plane of the loop. The result of this is electromagnetic radiation, electric and magnetic fields oscillating perpendicular to each other and to the direction of propagation (4).

            The electric and magnetic field components are most easily described separately for mathematical purposes. And it would make the mathematical representation even more convenient if the light represented were horizontally polarized. Polarization refers to the direction that a field is oscillating in. For example, vertical polarization describes a field oscillating up and down (4).

            The electric field component of electromagnetic waves can be mathematically modeled by a cosign function, a variable t for time, a variable a for amplitude, a variable p for phase shift (or where the function is at time t), and a variable f for frequency. That would look like this: a(cos(360ft+p)) (4).

            Every valid wave in a vacuum solves Maxwell’s equations. They are linear equations comprised of combinations of derivatives that mathematically modify the electric and magnetic fields so that they equal zero. Every valid wave in a vacuum gives zero when entered into Maxwell’s equations. Therefore  a valid wave, 0, plus another valid wave, 0, gives yet another valid wave, 0! The third valid wave in physics is called a superposition, or sum, of the original two waves. The superposition of two waves depends for its characteristics on the amplitude and phase shift of the original two vectors. If the original two waves have different phase shift, instead of oscillating up and down or left side to right side, the superposition will oscillate in an ellipse. If the two original vectors have the same amplitude and are ninety degrees out of phase with each other, the superposition will oscillate circularly, in what is known as circular polarization (4).

            Every wave can be described  as a superposition of two oscillating vectors, one on the vertical axis, and the other on the horizontal axis. Although, all waves could also be described with respect to perpendicular diagonal axes. The attitude of the perpendicular axes you choose is known as your choice of basis. Depending on the application, it might be more convenient to choose one basis over another (4).

            What has been presented above is the classical understanding. Most of it translates directly in the quantum world. Classically, the energy of a wave is considered to be the square of the amplitude. Theoretically, this should result in an infinite number of possible energy levels for waves. This seems intuitive, but physicists now know that the energy of a wave is always a discrete multiple of the smallest possible unit of energy. Imagine a staircase. Each step represents an energy level. Every wave is on a specific step and nowhere in between. The height of each step represents the smallest possible increase of decrease in the energy of a wave. This smallest amount is known as Planck’s constant, or h. Every wave has an energy equal to an integer multiple of h times its frequency. This means that there is minimum energy level that a photon can have and if it somehow loses energy at that level, it ceases to exist (4).

            Energy, then, comes in discrete packets of different sizes, but there is a minimum size and all other sizes are a multiple of this minimum one. Now, different frequencies of light only form when the the right size packet is available. When one arrives, they zoom off with it and a light ray is formed. The higher the energy of the packet, the higher the energy of the wave that will take it away. That’s why yhe hotter a fire is, the brighter it is and why its color shifts towards violet as the temperature increases. In normal ambient conditions, only little packets are available, so humans don’t get blinded or cooked to death! These little packets are called quanta, hence quantum mechanics (5).

            An electromagnetic wave at the minimum possible level is known as a photon. The reason photons in themselves can have different energies is because of the third variable in the equation for the energy of an electromagnetic wave: frequency. A different photon can exist at any possible frequency (4).

            In quantum mechanics, the superposition of two perpendicular oscillating vectors to describe any electromagnetic wave must have a new definition. This is because in classical understanding a photon would be a superposition of two vectors with a fractional modulus, and the quantum understanding knows this to be impossible because photons carry the minimum possible energy at their frequency. Classically, the squares of the moduli of the 2 vector components of each wave, tell what percentage of that waves energy can be found in a given direction. However in quantum understanding, a photon must have all of its energy in one direction because its energy cannot be subdivided. So, the amplitudes of the component vectors give the probability that the photon can be found in a certain direction. If the probability is fifty percent for a given direction, half of the time a photon of a certain frequency will be in that direction, and half of the time it will not (4).

            Now the reader is prepared to delve into Bell’s theorem. Proof of Bell’s Theorem involves the use of what are called polarizing filters. A polarizing filter either blocks light from passing through it, or polarizes it in one direction determined by its attitude. What follows is a description of an easy demonstration of Bell’s Theorem and not the actual experiment which is quite complex (5).

            Imagine one vertically polarizing filter. All light photons oscillating in the vertical direction will be let through one hundred percent of the time. Photons oscillating at a forty-five degree angle from vertical will only pass through fifty percent of the time. Now imagine a second philter is placed on top of the first. As the second filter is rotated away from vertical towards ninety degrees away from vertical, less and less light is let through until at ninety degrees away from vertical, light passes through both filters zero percent of the time, provided the filters are perfect. This is because all light that passes through the first filter is vertically polarized, meaning that it has zero percent chance to pass through a horizontally polarized filter (5).

            Now imagine that the second filter is angled at ninety degrees, vertical being zero degrees. Add a third filter in between at forty-five degrees, and more light passes through than before! Twenty five percent of the light passing through the first filter to be exact. This is because the filter in the middle angled at forty-five degrees lets fifty percent of the vertically polarized light through, and that fifty percent becomes polarized at forty-five degrees. Fifty percent of the light polarized at forty-five degrees passes through the third filter at ninety degrees. This seems natural and intuitive (3).

            Many people  have speculated that quantum mechanics isn’t intrinsically probabilistic as shown by how photons pass through a polarizing filter, but that there are some “hidden variables” that man has yet to grasp that describe a fundamental state photons that actually determines whether a photon will pass through a filter or not, not probability (3).

            Bell’s Theorem rests on what happens when a filter at 22.5 degrees, B, is placed on top of a filter at zero degrees, A, and is below a filter on top of the other two at forty-five degrees, C. Based on previous demonstrations, it would be reasonable to expect that seventy-five percent of the vertically polarized light would pass through B and C, because without B, fifty percent of the vertically polarized light passes through, and 22.5 degrees falls halfway between zero and forty-five degrees. Actually, only fifteen percent get blocked at B and another fifteen percent at C (3)!

            To disprove hidden variable theory, first it is necessary to assume it is true. Imagine 100 photons that do have a mysterious hidden variable that answers these following questions. Would a photon pass through A? Would a photon pass through B? Would a photon pass through C? Assume all photons start out being vertically polarized and therefore all pass through A. Fifteen percent get blocked at B, so eighty-five make it through. Another small about, about fifteen percent of eighty-five, get blocked at C. That is much less than the 50 that would get blocked if B weren’t in the middle. So experiments contradict hidden variable theory (3).

            Except, there’s a loophole: if passing through one filter affects how a photon will interact with future filters, then the phenomenon is easily explainable (3).

            But there is a way to circumvent that loophole. It is called the Einstein Podolsky Rosen, or EPR, experiment that was published May 15, 1935 in Physical Review. It uses entangled pairs of photons to measure the probabilities of photons passing through different combinations of filters A, B, and C at the same point in space. Basically, it proves that it is impossible for reality to be locally real (6).

            However, experiments up until 2015 couldn’t prove this unequivocally due to flaws in the equipment used and the experimental setup. But in 2015, this result became unequivocal with a loophole free experiment (3).

References

  1. https://www.youtube.com/watch?v=i1TVZIBj7UA
  2. https://plato.stanford.edu/entries/qt-epr/
  3. https://www.youtube.com/watch?v=MzRCDLre1b4
  4. https://www.youtube.com/watch?v=zcqZHYo7ONs
  5. https://plato.stanford.edu/entries/scientific-realism/