But it is different in the real world, outside a vacuum; for instance, light not only bends but also slows ever so slightly when it passes through glass or water. Still, that's nothing compared with what happens when Hau shines a laser beam of light into a BEC: it's like hurling a baseball into a pillow. She is able to manipulate light this way because the density and the temperature of the BEC slows pulses of light down.
She recently took the experiments a step further, stopping a pulse in one BEC, converting it into electrical energy, transferring it to another BEC, then releasing it and sending it on its way again. Hau uses BECs to discover more about the nature of light and how to use "slow light"—that is, light trapped in BECs—to improve the processing speed of computers and provide new ways to store information.
Not all ultracold research is performed using BECs. In Finland, for instance, physicist Juha Tuoriniemi magnetically manipulates the cores of rhodium atoms to reach temperatures of trillionths of a degree F above absolute zero.
The Guinness record notwithstanding, many experts credit Tuoriniemi with achieving even lower temperatures than Ketterle, but that depends on whether you're measuring a group of atoms, such as a BEC, or only parts of atoms, such as the nuclei.
It might seem that absolute zero is worth trying to attain, but Ketterle says he knows better. Bright idea: Wolfgang Ketterle in his M. T lab hopes to discover new forms of matter by studying ultracold atoms. Richard Howard Where's the coldest spot in the universe? Post a Comment. In physics, this distribution is called the Boltzmann distribution. Physicists working with Ulrich Schneider and Immanuel Bloch have now realised a gas in which this distribution is precisely inverted: many particles possess high energies and only a few have low energies.
This inversion of the energy distribution means that the particles have assumed a negative absolute temperature. The meaning of a negative absolute temperature can best be illustrated with rolling spheres in a hilly landscape, where the valleys stand for a low potential energy and the hills for a high one.
The faster the spheres move, the higher their kinetic energy as well: if one starts at positive temperatures and increases the total energy of the spheres by heating them up, the spheres will increasingly spread into regions of high energy.
If it were possible to heat the spheres to infinite temperature, there would be an equal probability of finding them at any point in the landscape, irrespective of the potential energy.
If one could now add even more energy and thereby heat the spheres even further, they would preferably gather at high-energy states and would be even hotter than at infinite temperature. The Boltzmann distribution would be inverted, and the temperature therefore negative. At first sight it may sound strange that a negative absolute temperature is hotter than a positive one.
This is simply a consequence of the historic definition of absolute temperature, however; if it were defined differently, this apparent contradiction would not exist. This inversion of the population of energy states is not possible in water or any other natural system as the system would need to absorb an infinite amount of energy — an impossible feat! However, if the particles possess an upper limit for their energy, such as the top of the hill in the potential energy landscape, the situation will be completely different.
In their experiment, the scientists first cool around a hundred thousand atoms in a vacuum chamber to a positive temperature of a few billionths of a Kelvin and capture them in optical traps made of laser beams. The surrounding ultrahigh vacuum guarantees that the atoms are perfectly thermally insulated from the environment.
The laser beams create a so-called optical lattice, in which the atoms are arranged regularly at lattice sites. In this lattice, the atoms can still move from site to site via the tunnel effect, yet their kinetic energy has an upper limit and therefore possesses the required upper energy limit. Temperature, however, relates not only to kinetic energy, but to the total energy of the particles, which in this case includes interaction and potential energy. The system of the Munich and Garching researchers also sets a limit to both of these.
The physicists then take the atoms to this upper boundary of the total energy — thus realising a negative temperature, at minus a few billionths of a kelvin. I f spheres possess a positive temperature and lie in a valley at minimum potential energy, this state is obviously stable — this is nature as we know it.
If the spheres are located on top of a hill at maximum potential energy, they will usually roll down and thereby convert their potential energy into kinetic energy.
The energy limit therefore renders the system stable! Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search.
I'm having a discussion with someone. He said that this isn't true, because my theory violates energy-time uncertainty principle. See that it's lowest energy state is still non-zero. By the third law of thermodynamics, a quantum system has temperature absolute zero if and only if its entropy is zero, i. But it has nothing to do with all molecules standing still, which is impossible for a quantum system as the mean square velocity in any normalized state is positive.
It doesn't matter what the ground state energy is. It's true that all molecules in the substance would stand perfectly still at absolute zero [well, they don't have exact positions by the uncertainty principle, but the probability distribution of position would be perfectly stationary].
But so what? Why would that make absolute zero impossible? Nevertheless, there is no process that can get a system all the way to absolute zero in a finite amount of time or a finite number of steps. There's just no way to get that last little bit of energy out. This is one aspect of the third law of thermodynamics, as discussed in some but not all thermodynamics textbooks.
It seems likely that I misunderstood. By "stand perfectly still", I guess you meant "have a fixed and definite position, and a fixed and definite velocity equal to 0". If that's what you meant, then "standing perfectly still" is indeed impossible because of the Heisenberg Uncertainty Principle. But "standing perfectly still" is not expected or required to happen at absolute zero. In physics, certain systems can achieve negative temperature; that is, their thermodynamic temperature can be expressed as a negative quantity on the kelvin scale.
A substance with a negative temperature is not colder than absolute zero, but rather it is hotter than infinite temperature. As Kittel and Kroemer p. For a temperature to be definable and measurable the distribution of the kinetic energies of the molecules in the medium under discussion should be known.
The process of cooling involves removing thermal energy from a system. When no more energy can be removed, the system is at absolute zero, which cannot be achieved experimentally. Absolute zero is the null point of the thermodynamic temperature scale, also called absolute temperature. If it were possible to cool a system to absolute zero, all motion of the particles comprising matter would cease and they would be at complete rest in this classical sense.
Microscopically in the description of quantum mechanics, however, matter still has zero-point energy even at absolute zero, because of the uncertainty principle.
The uncertainty principle assures that molecules cannot stay perfectly still and continue being in a certain position , i. Certainly not all molecules of the material, this would be necessary to define a 0K temperature. The solution with the vibrational degrees of freedom that molecules may have is not conclusive , though sufficient as proof for that the specific material that displays these vibrational modes cannot go to 0K.
It is the HUP that is general for all materials. I wonder why the measurement postulate has not been mentioned so far. Consider a cubical microcrystal of sodium chloride containing 64 atoms 4 on each side.
If we cool it off so it is as close to absolute zero as possible, then we can represent its state as a superposition of pure states. One of those states is the ground state. If we then measure its energy, is there not some finite probability that it will be found in its ground state? The atoms will not be stationary.
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