What Is the Bose-Einstein Condensate and What Does It Tell Us About Matter?
Breathe deep. Have a drink of water. Step on the ground. In those three actions, you have had an interaction with a gas, a liquid, and a solid, or the traditional three phases of matter. These are the forms that you have daily encounters with, but a fourth fundamental state of matter exists in the form of plasma, or highly ionized gas. Nevertheless, just because these are the main forms of matter does not mean that others do not exist. One of the strangest changes in matter is when you have a gas at low temperatures. Normally, the colder something gets, the more solid something becomes. But, this matter is different. It is a gas that is so close to absolute zero that it begins to display quantum effects on a larger scale. We call it the Bose-Einstein Condensate.
Now this BEC is made of bosons, or particles that do not have a problem occupying the same wave function with one another. This is the key to their behavior and a big component as to the difference between them and fermions, which do not want to have their probability functions overlapped like that. As it turns out, depending on the wave function and the temperature, one can get a group of bosons to begin to act like a giant wave. Moreover, the more and more you add to it the greater the function becomes, overruling the particle identity of the boson. And believe me, it has got some weird properties that scientists have made extensive use of (Lee).
Closing in on the Wave
Take for example the Casimir-Polder Interaction. It is somewhat based off the Casimir effect which is a crazy but actual quantum reality. Let’s be sure we know the difference between the two. Simply put, the Casimir effect shows that two plates that seemingly have nothing between them will still come together. More specifically, it is because of the amount of space that can oscillate between the plates is less than the space outside of it. Vacuum fluctuations arising from virtual particles contribute a net force outside the plates that is larger than the force inside the plates (for less space means fewer fluctuations and less virtual particles) and thus the plates meet up. The Casimir-Polder Interaction is similar to this effect, but in this instance it is an atom approaching a metal surface. The electrons in both the atoms and the metal repel each other but in the process of this a positive charge is created on the surface of the metal. This in turn will alter the orbitals of the electrons in the atom and actually create a negative field. Thus, the positive and negative attract and the atom is pulled to the surface of the metal. In both cases, we have a net force attracting two objects that seemingly should not come into contact but we find through quantum interactions that net attractions can arise from apparent nothingness (Lee).
Okay, great and cool right? But how does this relate back to BEC’s? Scientists would like to be able to measure this force to see how it compares to theory. Any discrepancies would be important and a sign that revision is needed. But the Casimir-Polder Interaction is a small force in a complicated system of many forces. What is needed is a way to measure before it is obscured and that is when BEC’s come into play. Scientists put a metal grating onto a glass surface and placed a BEC made of rubidium atoms on it. Now, BEC’s are highly responsive to light and can actually be pulled in or pushed away depending on the intensity and color of the light (Lee).
And that’s the key here. Scientists chose a color and intensity that would repeal the BEC and shine it through the glass surface. The light would pass the grating and cause the BEC to be repealed, but the Casimir-Polder Interaction begins once the light hits the grating. How? The electric field of the light causes the charges of the metal on the glass surface to begin to move. Depending on the spacing between the gratings, oscillations will arise that will build upon the fields (Lee).
Okay, stay with me now! So light shining through the gratings will repel the BEC but the metal gratings will cause the Casimir-Polder Interaction, thus an alternating pull/push will occur. The Interaction will cause the BEC to come to the surface but will reflect off of it because of its speed. Now it will have a different speed from before (for some energy was transferred) and thus a new state of the BEC will be reflected in its wave pattern. We will thus have constructive and destructive interference and by comparing that across multiple light intensities we can find the force of the Casimir-Polder Interaction! Phew! (Lee).
Bring in the Light!
Now, most models show that BEC’s must form under cool conditions. But leave it to science to find an exception. Work by Alex Kruchkov from the Swiss Federal Institute of Technology has shown that photons, the nemesis of BEC’s, can in fact be induced into becoming a BEC, and at room temperature! Confused? Read on!
Alex built upon the work of Jan Klaers, Julian Schmitt, Frank Vewinger, and Martin Weitz, all from the University of Germany. In 2010 they were able to make photon act like matter by placing them between mirrors, which would act like a trap for the photons. They began to act differently because they could both escape and began to act like matter, but years after the experiment no one was able to duplicate the results. Kind of critical if it is to be science. Now, Alex has shown the mathematical work behind the idea, demonstrating its possibility of a BEC made of photons under room temperatures as well as pressure. His paper also demonstrates the process to create such a material and all the temperature fluxes that occur. Who knows how such a BEC would act like, but since we don’t know how light would act as matter it could be an entire new branch of science (Moskvitch).
Revealing Magnetic Monopoles
Another potential new branch of science would be research into monopole magnets. These would be with only a north or a south pole but not both at once. Seems easy to find, right? Wrong. Take any magnet in the world and split it in half. The juncture where they split will take the opposite pole orientation to the other end. No matter how many times you split a magnet you will always get those poles. So why care about something that likely doesn’t exist? The answer is fundamental. If monopoles exist, they would help explain charges (both positive and negative), allowing much of fundamental physics to be firmly rooted in theory with better backing.
Now, even though such monopoles are not present we can still mimic their behavior and read the results. And as you can guess, a BEC was involved. M.W. Ray, E. Ruokokoski, S. Kandel, M. Mottonen, and D.S. Hall were able to create a quantum analog to how a monopole would act using simulations with a BEC (for attempting to create the real deal is complicated – too much for our level of tech, so we need something that acts like it in order to study what we are aiming for). So long as the quantum states are nearly equivalent, the results should be good (Francis, Arianrhod).
So what would scientists look for? According to quantum theory, the monopole would exhibit what is known as a Dirac string. This is a phenomenon where any quantum particle is attracted to a monopole and through the interaction would create an interference pattern in the wave function it displays. A distinct one that could not be mistaken for anything else. Combine this behavior with the magnetic field for a monopole and you got an unmistakable pattern (Francis, Arianrhod).
Bring in the BEC! Using rubidium atoms, they adjusted their spin and alignment of the magnetic field by tuning the velocity and the vortices of the particles in the BEC to mimic the monopole conditions they desired. Then, using electromagnetic fields, they could see how their BEC reacted. As they got to the desired state that mimed the monopole, that Dirac string popped up as predicted! The possible existence of monopoles lives on (Francis, Arianrhod).
Arianrhod, Robyn. "Bose-Einstein condensates simulate transformation of elusive magnetic monopoles." cosmosmagazine.com. Cosmos. Web. 26 Oct. 2018.
Francis, Matthew. “Bose-Einstein Condensates Used to Emulate Exotic Magnetic Monopole.” ars technia. Conte Nast., 30 Jan. 2014. Web. 26 Jan 2015.
Lee, Chris. “Bouncing Bose Einstein Condensate Measures Tiny Surface Forces.” ars technica. Conte Nast., 18 May 2014. Web. 20 Jan. 2015.
Moskvitch, Katia. “New State of Light Revealed with Photon-Trapping Method.” HuffingtonPost. Huffington Post., 05 May 2014. Web. 25 Jan. 2015.
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© 2015 Leonard Kelley