Download Quantum physics: Hot and cold at the same time Date:April 9

Quantum physics: Hot and cold at the same time
Date:April 9, 2015Source:Vienna University of Technology
Summary:Temperature is a statistical concept. Very small systems, consisting of a small number of
particles, are not usually described statistically. Scientists have now measured how quantum systems reach a state
with well defined statistical properties -- and surprisingly, they found out that quantum systems can have several
temperatures at once. The connection between small quantum systems and large systems obeying the laws of
classical physics is one of the big open questions in physics.
Temperature is a very useful physical quantity. It allows us to make a simple statistical
statement about the energy of particles swirling around on complicated paths without having to
know the specific details of the system. Scientists from the Vienna University of Technology
together with colleagues from Heidelberg University have now investigated, how quantum
particles reach such a state where statistical statements are possible. The result is surprising: a
cloud of atoms can have several temperatures at once. This is an important step towards a deeper
understanding of large quantum systems and their exotic properties.
Statistics Helps where Things get Complicated
The air around us consists of countless molecules, moving around randomly. It would be
utterly impossible to track them all and to describe all their trajectories. But for many purposes,
this is not necessary. Properties of the gas can be found which describe the collective behaviour
of all the molecules, such as the air pressure or the temperature, which results from the particles'
energy. On a hot summer's day, the molecules move at about 430 meters per second, in winter, it
is a bit less.
This statistical view (which was developed by the Viennese physicist Ludwig Boltzmann)
has proved to be extremely successful and describes many different physical systems, from pots
of boiling water to phase transitions in liquid crystals in LCD-displays. However, in spite of
huge efforts, open questions have remained, especially with regard to quantum systems. How the
well-known laws of statistical physics emerge from many small quantum parts of a system
remains one of the big open questions in physics.
Hot and Cold at the Same Time
Scientists at the Vienna University of Technology have now succeeded in studying the
behaviour of a quantum physical multi-particle system in order to understand the emergence of
statistical properties. The team of Professor Jörg Schmiedmayer used a special kind of microchip
to catch a cloud of several thousand atoms and cool them close to absolute zero at -273°C, where
their quantum properties become visible.
The experiment showed remarkable results: When the external conditions on the chip
were changed abruptly, the quantum gas could take on different temperatures at once. It can be
hot and cold at the same time. The number of temperatures depends on how exactly the scientists
manipulate the gas. "With our microchip we can control the complex quantum systems very well
and measure their behaviour," says Tim Langen, leading author of the paper published in
"Science." There had already been theoretical calculations predicting this effect, but it has never
been possible to observe it and to produce it in a controlled environment.
The experiment helps scientists to understand the fundamental laws of quantum physics
and their relationship with the statistical laws of thermodynamics. This is relevant for many
different quantum systems, maybe even for technological applications. Finally, the results shed
some light on the way our classical macroscopic world emerges from the strange world of tiny
quantum objects.
http://www.sciencedaily.com/releases/2015/04/150409143037.htm
Potential energy Potential energy is stored energy.
Potential energy is the energy that exists by virtue of the relative positions
(configurations) of the objects within a physical system.
This form of energy has the potential to change the state of other objects around it, for
example, the configuration or motion.
Various forms of energy can be grouped as potential energy.
Each of these forms is associated with a particular kind of force acting in conjunction
with some physical property of matter (such as mass, charge, elasticity, temperature etc).
For example, gravitational potential energy is associated with the gravitational force
acting on object's mass; elastic potential energy with the elastic force (ultimately electromagnetic
force) acting on the elasticity of a deformed object; electrical potential energy with the
coulombic force; strong nuclear force or weak nuclear force acting on the electric charge on the
object; chemical potential energy, with the chemical potential of a particular atomic or molecular
configuration acting on the atomic/molecular structure of the chemical substance that constitutes
the object; thermal potential energy with the electromagnetic force in conjunction with the
temperature of the object.
For an example of gravitational potential energy, consider a book placed on top of a table.
To raise the book from the floor to the table, work must be done, and energy supplied. (If
the book is lifted by a person then this is provided by the chemical energy obtained from that
person's food and then stored in the chemicals of the body.) Assuming perfect efficiency (no
energy losses), the energy supplied to lift the book is exactly the same as the increase in the
book's gravitational potential energy.
The book's potential energy can be released by knocking it off the table.
As the book falls, its potential energy is converted to kinetic energy.
When the book hits the floor this kinetic energy is converted into heat and sound by the
impact.
A temperature below absolute zero: Atoms at negative absolute temperature are the
hottest systems in the world
Date:January 4, 2013Source:Max-Planck-Gesellschaft
Summary:On the absolute temperature scale, which is used by physicists and is also called the Kelvin scale, it
is not possible to go below zero – at least not in the sense of getting colder than zero kelvin. According to the
physical meaning of temperature, the temperature of a gas is determined by the chaotic movement of its particles –
the colder the gas, the slower the particles. At zero kelvin (minus 273 degrees Celsius) the particles stop moving and
all disorder disappears. Thus, nothing can be colder than absolute zero on the Kelvin scale. Physicists have now
created an atomic gas in the laboratory that nonetheless has negative Kelvin values. These negative absolute
temperatures have several apparently absurd consequences: although the atoms in the gas attract each other and give
rise to a negative pressure, the gas does not collapse – a behavior that is also postulated for dark energy in
cosmology.
Temperature as a game of marbles: The Boltzmann distribution states how many particles
have which energy, and can be illustrated with the aid of spheres distributed in a hilly landscape.
At positive temperatures (left image), most spheres lie in the valley at minimum potential energy
and barely move; they therefore also possess minimum kinetic energy. States with low total
energy are therefore more likely than those with high total energy – the usual Boltzmann
distribution. At infinite temperature (centre image) the spheres are spread evenly over low and
high energies in an identical landscape. Here, all energy states are equally probable. At negative
temperatures (right image), however, most spheres move on top of the hill, at the upper limit of
the potential energy. Their kinetic energy is also maximum. Energy states with high total energy
thus occur more frequently than those with low total energy – the Boltzmann distribution is
inverted.What is normal to most people in winter has so far been impossible in physics: a minus
temperature. On the Celsius scale minus temperatures are only surprising in summer. On the
absolute temperature scale, which is used by physicists and is also called the Kelvin scale, it is
not possible to go below zero – at least not in the sense of getting colder than zero kelvin.
According to the physical meaning of temperature, the temperature of a gas is determined
by the chaotic movement of its particles – the colder the gas, the slower the particles. At zero
kelvin (minus 273 degrees Celsius) the particles stop moving and all disorder disappears. Thus,
nothing can be colder than absolute zero on the Kelvin scale. Physicists at the LudwigMaximilians University Munich and the Max Planck Institute of Quantum Optics in Garching
have now created an atomic gas in the laboratory that nonetheless has negative Kelvin values.
These negative absolute temperatures have several apparently absurd consequences: although the
atoms in the gas attract each other and give rise to a negative pressure, the gas does not collapse
– a behaviour that is also postulated for dark energy in cosmology. Supposedly impossible heat
engines such as a combustion engine with a thermodynamic efficiency of over 100% can also be
realised with the help of negative absolute temperatures.
In order to bring water to the boil, energy needs to be added. As the water heats up, the
water molecules increase their kinetic energy over time and move faster and faster on average.
Yet, the individual molecules possess different kinetic energies – from very slow to very fast.
Low-energy states are more likely than high-energy states, i.e. only a few particles move really
fast. 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 inverted Boltzmann distribution is the hallmark of negative absolute temperature;
and this is what we have achieved,” says Ulrich Schneider. “Yet the gas is not colder than zero
kelvin, but hotter,” as the physicist explains: “It is even hotter than at any positive temperature –
the temperature scale simply does not end at infinity, but jumps to negative values instead.”
A negative temperature can only be achieved with an upper limit for the energy
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. The researchers in
Immanuel Bloch’s and Ulrich Schneider’s research group have now realised such a system of an
atomic gas with an upper energy limit in their laboratory, following theoretical proposals by
Allard Mosk and Achim Rosch.
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.
At negative temperatures an engine can do more work
If 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. “If the spheres are at a negative temperature, however, their kinetic
energy will already be so large that it cannot increase further,” explains Simon Braun, a doctoral
student in the research group. “The spheres thus cannot roll down, and they stay on top of the
hill. The energy limit therefore renders the system stable!” The negative temperature state in
their experiment is indeed just as stable as a positive temperature state. “We have thus created
the first negative absolute temperature state for moving particles,” adds Braun.
Matter at negative absolute temperature has a whole range of astounding consequences:
with its help, one could create heat engines such as combustion engines with an efficiency of
more than 100%. This does not mean, however, that the law of energy conservation is violated.
Instead, the engine could not only absorb energy from the hotter medium, and thus do work, but,
in contrast to the usual case, from the colder medium as well.
At purely positive temperatures, the colder medium inevitably heats up in contrast,
therefore absorbing a portion of the energy of the hot medium and thereby limits the efficiency.
If the hot medium has a negative temperature, it is possible to absorb energy from both media
simultaneously. The work performed by the engine is therefore greater than the energy taken
from the hotter medium alone – the efficiency is over 100 percent.
The achievement of the Munich physicists could additionally be interesting for
cosmology, since the thermodynamic behaviour of negative temperature exhibits parallels to socalled dark energy. Cosmologists postulate dark energy as the elusive force that accelerates the
expansion of the universe, although the cosmos should in fact contract because of the
gravitational attraction between all masses. There is a similar phenomenon in the atomic cloud in
the Munich laboratory: the experiment relies upon the fact that the atoms in the gas do not repel
each other as in a usual gas, but instead interact attractively. This means that the atoms exert a
negative instead of a positive pressure. As a consequence, the atom cloud wants to contract and
should really collapse – just as would be expected for the universe under the effect of gravity.
But because of its negative temperature this does not happen. The gas is saved from collapse just
like the universe.
http://www.sciencedaily.com/releases/2013/01/130104143516.htm